Keywords: dyes, molecular dynamics simulation, microscopic structure, excited state, quantum chemical calculation, ionic liquids

RESUME

L'objectif de cette thèse est d'une part d'analyser les propriétés électroniques de plusieurs modèles de colorants potentiellement utilisables dans les cellules solaires et d'autre part de caractériser l'effet du solvant sur la structure locale du colorant D205 en solution et à l'interface du TiO2. Ces recherches ont été réalisées à l'aide de calculs de chimie quantique et de simulation de dynamique moléculaire (MD).

Dans le premier chapitre de la thèse, nous avons essayé de résoudre deux problèmes importants pour décrire et prédire les propriétés spectrales des colorants pour les DSSCs. Les géométries optimisées du colorant D205 dans l'état fondamental S0 et l'état excité S1, ont été réalisées en utilisant respectivement les méthodes DFT et TDDFT. les fonctions B3LYP and CAM-B3LYP (or M06-2X) ont été utilisées dans ce travail avec la base 6-31++G(d,p). Nous avons montré que ce niveau de calcul peut être recommandée pour la prédiction des spectres d'absorption de ces colorants. Nous avons également proposé un algorithme pour prédire de nouvelles structures moléculaires avec des paramètres spectraux électroniques améliorés et avons prédit la structure de trois nouveaux colorants pour les DSSC.

Dans la deuxième partie de la thèse, nous avons analysé l'effet de la composition du mélange de colorants D205/Acétonitrile/BmimBF4. Le D205 est un colorant organique utilisé dans les cellules solaires. Pour réaliser cette analyse, nous avons effectué des calculs de chimie quantique afin de paramétrer la partie intramoléculaire du champ de force du colorant D205. Les charges décrivant l'état fondamental et l'état excité du colorant D205 ont également été déterminées tandis que les paramètres de Lennard-Jones ont été tirés de la bibliothèque OPLS(AA). Les fonctions de distribution radiales des plus proches voisins ont été utilisées pour caractériser l'environnement autour des parties donneur, accepteur, pont et chaîne butyle du colorant D205 dans les états fondamental et excité.

Dans la troisième partie de cette thèse, des simulations conjointes de dynamique moléculaire (MD) et des calculs de théorie fonctionnelle de la densité (DFT) ont été utilisés pour étudier la structure et la dynamique d'un colorant indoline D205 ancré à l'interface solide˗liquide de petites nanoparticules de TiO2 anatase en mélange avec des solvants tels que l'acétornitrile, et des liquides ioniques à base de 1-butyl-3-méthylimidazolium avec des anions hexapluorophate et trifluorométhanesulfonate. Des calculs DFT ont été effectués pour estimer la géométrie d'équilibre d'une petite nanoparticule d'anatase Ti30O62H4 et pour déduire les paramètres d'interaction pour la liaison bidentate d'un colorant D205 avec le TiO2. L'effet de différents solvants sur la conformation du colorant D205 ancré dans l'anatase a été analysé par des simulations MD, démontrant qu'une représentation explicite du solvant est vitale pour reproduire les spectres optiques d'un colorant D205.

Mots-clés : colorants, simulation de dynamique moléculaire, structure microscopique, état excité, calcul chimique quantique, liquides ioniques

ABSTRACT

The objective of this thesis is on the one hand to analyze the electronic properties of several dye models that can potentially be used in solar cells and on the other hand to characterize the effect of the solvent on the local structure of the dye D205 in solution and at the TiO2 interface. These investigations were carried out using quantum chemistry and molecular dynamics (MD) simulation calculations.

In the first chapter of the thesis we tried to solve two problems important for describing and predicting the spectral properties of dyes for DSSCs. It was shown that the Time-Dependent DFT (TDDFT) approximation with a combination of B3LYP and CAM-B3LYP (or M06-2X) functionals with the 6-31++G(d,p) basis set can be recommended for the prediction of dye absorption spectra. We also proposed an algorithm to predict new molecular structures with improved electron spectral parameters and predicted the structure of three new dyes for DSSC.

In the second part of the thesis we analyzed the effect of the composition of the D205/Acetonitrile/BmimBF4 dye mixture. D205 is an organic dye used in solar cells. To perform this analysis, we performed quantum chemical calculations to parameterize the intramolecular part of the D205 dye force field. The charges describing the ground state and excited state of the D205 dye were also determined while the Lennard-Jones parameters were taken from the OPLS(AA) library. Radial nearest neighbor distribution functions were used to characterize the environment around the donor, acceptor, bridge and butyl chain moieties of the D205 dye in the ground and excited states.

In the third part of this thesis joint molecular dynamics (MD) simulations and density functional theory (DFT) calculations were used to study the structure and dynamics of an indoline dye D205 anchored at the solid˗liquid interface of small TiO2 anatase nanoparticles in admixture with solvents such as acetornitrile , and 1-butyl-3-methylimidazolium-based ionic liquids with hexapluorophate and trifluoromethanesulfonate anions. DFT calculations were performed to estimate the equilibrium geometry of a small Ti30O62H4 anatase nanoparticle and to deduce the interaction parameters for the bidentate bonding of a D205 dye with TiO2. The effect of different solvents on the conformation of the anatase-anchored D205 dye was analyzed by MD simulations, demonstrating that an explicit solvent representation is vital to reproduce the optical spectra of a D205 dye.

General introduction

Dye-sensitized solar cells (DSSCs) based on π-conjugated organic dyes covalently anchored on the TiO 2 surface have many attractive characteristics, such as high solar energy conversion efficiency and low cost of the components [1][2][3][4][5][6][7][8].

Dye-sensitizers are the most important component in DSSCs because they affect key processes that regulate energy conversion efficiency, such as light absorption, charge injection, dye regeneration, and charge recombination, respectively [5,9,10]. Therefore, the rapid development of DSSC technologies requires constant tuning of the chemical properties of organic dyes since the light conversion efficiency is largely determined by the photophysics of the dye [1].

Indoline-based dyes is a promising class of free-metal dyes for various applications due to their photophysical and photochemical properties. They exhibit high photon-to-current conversion efficiency combined with high molar absorption coefficients and quantum yields [11][12][13][14]. The D205 dye was built according to the scheme Donor -π-bridge -Acceptor [15,16]. Several studies suggest that the sensitizing efficiency of the D205 dye can be enhanced by chemical binding to the active layer of the TiO 2 semiconductor [17,18].

In addition to proper dye-TiO 2 interface design, the DSSC performance is also affected by the environment. Due to the standing out properties of ionic liquids (ILs), such as low vapor pressure, high conductivity, non-combustibility, chemical and thermal stability, ILs have become good electrolytes, capable of passivating the active surfaces of DSSC devices [19]. With the aim to reduce the viscosity of the electrolyte the polar molecular solvent (e.g. AN) is commonly added to the DSSC.

However, understanding the microstructure of the liquid/electrode interface requires studies of the interaction between the electrolyte and the electrode surface at the atomic level.

Current quantum chemical approaches especially time-dependent extension of the density functional theory (TDDFT) can provide a theoretical basis for describing most of the properties of individual dye molecule or dye-surface components, such as dye geometry, optical absorption spectra, and semiconductor bandgap [10,[20][21][22][23]. However, accurate prediction of the absorption spectra and solvatochromic shifts of organic dyes in liquid media and attached to the TiO 2 surface and solvated by polar liquids is still a challenging task. On the other hand, the solvent-induced conformational changes of the dye molecule due to the nature of the solvent, dye aggregation, and specific hydrogen-bonded interactions with solvent molecules must also be correctly accounted for [24][25][26].

The classical molecular dynamics (MD) simulation is a powerful computational instrument to investigate microscopic environment of a dye in the bulk and within the solvent/surface interfaces, which can guide rational dye design and save synthetic and experimental resources [27][28][29][30][31]. Moreover, MD simulations can complement the experimental results of the conformational dynamics and spectroscopic properties of the dye anchored to TiO 2 [19]. However, the local structure of the D205 dye in the mixture of ILs with molecular polar solvent as well as the phenomena and mechanisms related to the interaction between TiO 2 and dye have not been well studied, especially at the microscale and nanoscale levels.

Accordingly, the objectives of this thesis were formulated as following:

(i) first, to analyze the existing quantum chemical approaches and develop an algorithm capable to predict new molecular structures of the dyes with improved electron spectral parameters for DSSC application;

(ii) to investigates the effect of the composition of the acetonitrile/BmimBF 4 mixtures on the local environment of D205 dye in the ground and excited state;

(iii) to study the structure and dynamics of the D205 dye anchored at the solid˗liquid interface of small TiO 2 anatase nanoparticles in admixture of acetonitrile with Bmim + -based ionic liquids.

Organization of the thesis

In this thesis the complex theoretical investigation of -conjugated organic dyes in mixture and on the TiO 2 has provided. The discussion of the obtained results organized as follows.

Chapter 1 presents the proposition of the algorithm for the predicting dyes structures with the desired properties, in particular the absorption spectra. In additional to it, the validation of the existing quantum chemical method (semiempirical and TDDFT ones) to calculate the electronic properties of dyes was provided.

The Finally, conclusions, summarizing the key findings of the work, and future perspectives are outlined.

Chapter 1. Quantum-chemical methods for prediction of the spectral properties of dyes and virtual screening

The material presented in this chapter is based on the publication: In order to predict the spectral properties of organic dyes for solar cells the series of the calculations of the typical π-conjugate systems with various structural fragments, have been performed.

A significant discrepancy in the estimations of electronic excitations is demonstrated by the density functional theory for the different functionals. It is concluded that TDDFT сalculations should be performed by using both B3LYP and CAM-B3LYP (or M06-2X) functionals with the basis set 6-31++G(d,p). It was found that semi-empirical approaches especially π-electron method PPP/CIS and all valence method ZINDO/S can provide adequate estimates of excitation energies of π-conjugated dyes for moderate computer resources.

With the help of method CAM-B3LYP and using different basis set the spectral properties of indoline dye D205 were calculated. It was shown that using State Specific (SS) or Linear responce (LR) PCM model for describing spectral properties of the D205 molecule in AN media are far from the experimental results. Only the basis sets with the diffusion orbitals can predict the shape of absorption spectra, which consist of 2 bands.

An attempt was made to build a systematic procedure for the selection / prediction of organic π-conjugate molecules with certain parameters of the electronic structure suitable for DSSC application. More than four thousands of π-conjugated compounds were generated by using benzene, oxazole, oxadiazole, thiophene and coumarin fragments with nitro-, methoxy-, dimethylamino-and diethylamino-groups as substitutes. The careful screening by using semi-empirical methods (PPP/CIS and ZINDO/S) followed by application of TDDFT methods with B3LYP and CAM-B3LYP functionals allowed us to deduce the relation between structure and spectral properties of the dyes and predict for the first time a new promising candidates for DSSC application.

Introduction

Considering the unlimited variety of molecular structures based on organic dyes, the development of methods for forecasting their spectral properties based on their structure is an important problem. The modern state of computer technology has brought computational chemistry to new heights. Today, a multifaceted theoretical study of various systems from atoms and molecules to complex complexes including "heavy" transition elements has become possible.

The arsenal of theoretical methods available to chemists includes a wide variety of approaches from the construction of empirical statistical "structure-property" laws to the methods of molecular dynamics and quantum chemistry. In particular, the methods of quantum chemistry, implemented in many popular computer programs, can provide valuable information about the geometry of the ground state, charge distribution, characteristics of reactivity, etc.

However, calculations of excited states remain a difficult problem today.

This especially applies to theoretical studies of electronic excitations (UV and visible spectroscopy) of rather large organic π-conjugated dyes, which may include a significant number of non-hydrogen atoms, including carbon, nitrogen, oxygen, sulfur, etc. Dyes, built on the scheme of Donor(D)-π-bridge-Acceptor(A) are attracted the huge attention like an alternative for the Rucomplexes dyes for using in DSSC due to their easily tuning optical properties (thanks to the possibility change the different parts of dye to obtained the necessary properties), high molar extension coefficient [15,16,21,22,30,32,33].

Density functional theory (TDDFT) [34] is usually used to calculate different properties of such molecules. Dozens of TDDFT functionals are implemented in the popular quantum chemical packages Gaussian [35] and GAMESS [36]. Of course, the presence of a large number of TDDFT functionals, different in nature, also poses a problem of choice. Fortunately, the well-known B3LYP functional [36,37] and hybrid meta exchange-correlation functionals of the M06 class [38] are largely show the high accuracy in studies of the ground state of molecules.

At the same time, current studies showed that the accuracy of the description of electronic excitations within the TDDFT frameworks is not always satisfactory ( [37]). Meantime, it should be emphasized that, unlike ground state calculations, choosing a functional to describe electronic excitations is not a simple problem. In particular, it was shown that even calculations of valence and Rydberg excitations for certain molecules require different functionals [23,39,40]! Therefore, the choice of a DFT functional, and even a quantum chemical method, for calculations of excited states is an open problem. This problem is especially acute when there is a need for "virtual screening" of dyes, which is implemented on databases that include hundreds of thousands of molecules, from which it is necessary to select several hits with given spectral properties. It is clear that in this case computationally expensive ab initio methods are not applicable.

Simpler calculation schemes are needed, which also include semi-empirical methods. To justify the use of such approximate methods, it would be interesting to conduct a comparative analysis of the results of various approaches that have been developed to date.

Screening of libraries of molecules, to identify such structures that would have given properties, is intensively used in modern chemistry [41][42][43][44]. Usually, this procedure is used to find "hits" of compounds that correspond to a pharmacophore in the problem of finding structures with a certain bioactivity [44,45]. In this chapter, an attempt was made to build a systematic procedure for the selection of organic conjugated molecules with certain parameters of the electronic structure. Finding such compounds is a key point in a number of molecular electronics tasks, in particular, in the development of dye-sensitized solar cells. In this case, the most important parameters are the absorption wavelength at the maximum intensity (λ, nm), the absorption intensity parameter (oscillator strength, f).

Thus, the purpose of this chapter is to choose quantum-chemical TDDFT method and PCM model, which can accurately describe the spectral properties of organic -conjugated dyes. In addition, an attempt to build the procedure of finding hits, suitable for DSSC application was performed.

The choosing of the method for quantum chemical calculation 1.2.1. Calculations of electronic absorption spectra

To clarify the possibilities of quantum-chemical methods in the problem of describing the electron absorption spectra (EAS) of π-conjugated systems, we used a number of approaches that cover a significant part of the available approximations. However here it should be noted that the problem of describing the spectral properties of conjugated systems in general, and organic dyes in particular, is divided into two significantly different parts. The first one of them consists in choosing such a quantum-chemical approach that is capable of adequately describing the system in a vacuum (or gas phase). The obvious complexity of such a study is connected with the lack of a sufficient amount of experimental data. The second problem is an adequate description of the system in the environment. It should be noted that polarization-continuum approaches (PCM) [46,47], which are usually used to describe the non-specific effects of solvatochromism, are too model-based and, generally speaking, do not guarantee a strict correspondence to any real spectroscopic experiment.

Taking into account the above mentioned factors, in the presented work a number of approaches were used that make up a certain hierarchy of quantumchemical approximations, for which appropriate models for taking into account the non-specific effects of solvatochromism were implemented. In particular, the following methods were selected.

1. The semi-empirical Pariser-Parr-Pople method with account of configuration interaction (PPP/CI) is a fairly simple approach to the description of spectra within the framework of the π-electron approximation. Standard averaged bond lengths and angles characteristic of conjugated systems are used to describe the geometry in the PPP/CI calculation [48,49]. The electronic parameters of the PPP Hamiltonian are tabulated values.

2. ZINDO/S [50] is the full valence semi-empirical method, which is focused on the description of absorption spectra. But the method is not adapted to geometry optimization in the ground state. Thus, it is first necessary to optimize the geometry of the ground state, for example, by using the AM1 method, or using standard force fields of molecular mechanics (Python module rdkit [51] allows this to be done). Only then one can calculate the spectrum using the ZINDO/S method.

3. Full valence semi-empirical method AM1 in combination with CIS (AM1/CI) represents a more consistent calculation scheme than the previous one.

After all, a single parameterization is used for both the ground and excited states.

AM1/CI is a definite alternative to the ZINDO/S method.

4. Time-dependent DFT (TDDFT) calculations with functionals found in contemporary spectral studies: B3LYP, CAM-B3LYP, M06-2X, PBE1PBE, wB97XD. A brief description of the functionals can be found for instance in [52].

For TDDFT calculations we used basis set of atomic orbitals -6-31++G(d,p).

This part presents the experimental and calculated results of electronic absorption spectra (hereinafter) of some typical π-conjugated systems (Fig. 1.1), which are usually the components of the effective dyes and phosphors. These molecules were chosen due to small size to reduce the computational time for the calculations and for all them there are the experimental data in different solvents are available to comparing. In the given compounds, the donor groups are represented by methoxy-, diethylamino-and N,N-dimethyl. In the compounds 5-8 coumarin is -conjugate bridge, while the benzene is acceptor part.

The results of preliminary calculations of absorption wavelengths are given in Here, the overestimation of the absorption wavelengths for the two bands is associated with unbalanced inclusion of long-range Coulomb effects [16].

However, it has already been noted that the CAM-B3LYP method corrects this problem. Indeed, according to the presented data (Table 1.1 and Table 1.2), CAM-B3LYP gives the best agreement with the experimental data and with the PPP/CIS and ZINDO/S methods. At the same time, the closest to the experimental λ max absorption data were found in the PPP/CI method.

Solvatochromism of the dyes with π-conjugation

Another important issue is PCM estimations of solvato-and fluorochromy of π-systems. Despite the long history, which begins with the works of Onsager and Kirkwood [54] and continues in the works of Tomasi, Menucci, and others. [46,47,55], this problem is still far from being solved. Starting from PCM estimates of the equilibrium solvation of the ground state of the molecule, spectral transitions can be described as processes with non-equilibrium solvation conditions. Among a number of approaches for describing non-equilibrium solvation, currently two main approaches used [56]. The method of linear response (LR) makes it possible to estimate directly the excitation energies of the system. In the State Specific (SS) method, the energy of the system in a certain From the presented tables it is clear, that the results obtained with both methods are close to each other, but still quite far from the experimental data.

Therefore, the LR approach is used for further calculations due to its simplicity and computation time and recourses saving. In the next part, within the framework of LR, using a number of TDDFT functionals, the λ max absorption estimates were made for compounds 5-8 in various environments (Table 1.5) utilizing the method from the previous part.

According to the data in table 1.5 for acetonitrile, the obtained results generally characterize the B3LYP and CAM-B3LYP methods well. It can also be seen that the realistic results for the given functionals lie between the B3LYP and CAM-B3LYP functionals. The M06-2X results are quite close to CAM-B3LYP.

At the same time, the B3LYP functional sometimes gives overestimates, while CAM-B3LYP gives slightly underestimated estimates. Thus, it can be expected that the values will be closer to the experimental data on average. In the work [57] was shown, that these is 2 maximum for spectrum in acetonitrile: 390 nm and 525 nm. The table shows that the results, obtained for both LR and SS approaches are far from experimental one. It means, that the calculation of spectrum of D205 in polar solvent does not well describe the spectral properties.

However, there is the second peak appears in basis set with the diffuse functions on heavy atoms and hydrogen, so these results are in good agreement with the shape of spectrum. Meanwhile, the increasing of electronic level splitting from 2 to 3 not improve the results. The results obtained in vacuum on the level of theory CAM-B3LYP/6-311G are close to the spectrum in solvent, like cyclohexane with the small dielectric constant of this solvent (2.02) and equal to 527 nm and 524 nm respectively. [57]

The virtual screening of π-conjugated systems

The virtual screening is the computation procedure of looking over the set of molecules aiming to search for those with certain physical and chemical properties. This set can consist of both existing molecules as well as designed in silico. The last approach allows to perform the physical synthesis and laboratory analysis only of the promising candidates.

The semi-automatic procedure has been developed to create a library of chemical compounds from the initial set of molecular fragments (see fig. 1.2) was proposed by professor V. Ivanov, V.N. Karazin Kharkiv National University. This set includes a number of π-conjugated molecular units like oxazole, oxadiazole, thiophene, several heterocycles and coumarin fragments to be used for generation of a large number of compounds. As not all of these "molecules" are chemically meaningful or can be synthesized (due to restrictions or the difficulty of the procedure), a specially developed semi-automatic algorithm of initial fragments sequential joining was utilized. It significantly reduces a number of possible output compounds restricting by the chemical sense. In order to identify the most suitable structures, a preliminary calculation of the absorption wavelength of π-conjugated systems was carried out using semiempirical methods (the PPP/CI [48,49] and ZINDO/S [50]. For further geometry optimization at semi-empirical quantum chemistry level, we used AM1 method [58]. After it the results were compared with the TDDFT methods. In the table 1.1. was shown that the results, obtained with the semi-emperical methods are close to the TDDFT ones, so it is possible to use them for the for primary selection. (oxadiazole) and D (tiophene) are also popular components of dyes and luminophores (see for instance [53]). In addition, to be used in DSSC the anchoric group (e.g. COOH-) should be presented as well.

Combinatorial generation of possible chain structures based on fragments A-B-

C-D can be enumerated (see table 1.8). Question markthere is no exact data about the possibility to synthesis this structure.

Table 1.8. Possible structures which generated from molecular fragments from Fig. 1.6, where the "?"there is no exact information about the possibility of synthesis.

Chain Steric effects filter Possibility of synthesis

A-5B4-C-D - - A-5B3-C-D ? ? A-6B5-C-D - - A-6B4-C-D ? ? A-6B3-C-D A-6B3-C-D A-6B3-C-D A-7B6-C-D - - A-7B5-C-D ? ? A-7B4-C-D A-7B4-C-D ? A-7B3-C-D A-7B3-C-D A-7B3-C-D A-8B3-C-D A-8B3-C-D ? .... .... ....
From initially generated structures a number structures are correspond to sterically difficult structures. These structures removed from consideration.

Finally, only several structures ("hits") were selected by the group of Prof.

Kovalenko (Department of Organic Chemistry of V. N. Karazin Kharkiv National Uinversity) as perspective for synthetic work (Fig. 1.7). Corresponding wavelength for these dyes provided in table 1.9.

From table 1.9 one can note that, that the increasing of the number of thiophene rings (Ia-Ib-Ic) has effect on the bathochromic shift and the maximum transfer from 397 nm for 1 ring to 424 nm for three of them. Changing the thiophene ring to phenil decreases the wavelength of the maximum.

; (E)-2-cyano-3-(5'-(5-(6-(diphenylamino)-2-oxo-2H- chromen-3-yl)-1,3,4-oxadiazol-2-yl)-[2,2'-bithiophen]-5-yl)acrylic acid (Ib, n=2); (E)-2-cyano-3-(5''-(5-(6-(diphenylamino)-2-oxo-2H-chromen-3-yl)-1,3,4- oxadiazol-2-yl)-[2,2':5',2''-terthiophen]-5-yl)acrylic acid (Ic, n=3); (E)-2-cyano- 3-(5'-(2-(5-(7-(dimethylamino)-2-oxo-2H-chromen-3-yl)-1,3,4-oxadiazol-2-yl)- 4,5-dihydrothiazol-5-yl)-[2,2'-bithiophen]-5-yl)acrylic acid (II); (Z)-2-cyano-3- (4-(5-(2-(5-(7-(dimethylamino)-2-oxo-2H-chromen-3-yl)-1,3,4-oxadiazol-2-yl)- 4,5-dihydrothiazol-5-yl)thiophen-2-yl)phenyl)acrylic acid (III) (I) (II) (III)

Conclusions

Despite the wide range of available quantum chemical methods, calculations of electronic excitations of organic π-systems (in particular, dyes) remain a rather difficult problem. Quantitative estimates of such parameters as absorption wavelength and corresponding intensities are still far from ideal for a number of reasons. The results presented in this chapter showed that well-known semi-empirical approaches allow to describe spectral transitions with accuracy comparable with the TDDFT theory, but they are much less computation and time consuming. In particular, in a situation where it is necessary to carry out virtual screening, the PPP/CI and ZINDO methods can provide the necessary information at the cost of a small computer resources.

According to our estimates, it makes sense to use three functionals with the basis set 6-31++G(d,p) in TDDFT calculations of π-conjugated systems. Namely, the results of the B3LYP and CAM-B3LYP (M06-2X) methods give the upper and lower limits of the absorption wavelength, respectively.

With the help of selected CAM-B3LYP method the absorption spectra of indoline dye D205 was calculated both in vacuum and AN PCM model in terms of LR and SS methods. It was established that only the basis sets with the diffusion orbitals can reproduce the experimental shape of absorption spectrum with 2 band, while the increasing of electronic level splitting from 2 to 3 not improve the results.

As a results of scanning the library of -conjugated dyes the 6 promising candidates was proposed. All these molecules consist of coumarin fragment. It was shown that the maximum of the absorption spectrum increases with the increase of the amount of thiophene rings.

Chapter 2. Force field parametrization and the local structure of D205 in BmimBF 4 /AN mixture

The photophysical properties of the dyes, e.g. the position of maximum on the absorption or emission spectrum, the stoke shift depends on the composition of the mixture. For the investigation of the spectral properties experimental studies can be combined with the molecular dynamic simulations. For the theoretical studies indoline dye D205 was chosen, due to the promising efficiency.

On the first step, the parametrization of force field was done. Partial charges were calculated both in ground and excited states with the method CHelpG. The force constants for the all degree of freedom were calculated in the frame of the rigid PES scanning. MD simulation in the mixture with the different composition of Ionic liquid and molecular solvent was performed. The competition between dye, cation, anion and solvent was analyzed in the terms of nearest neighbor approach, radial distribution function and coordination numbers. It was shown that interaction between cation and anion between cation and anion of IL stronger, than with dye D205.

Introduction

Several reports were made regarding the mixture composition of the photophysical properties of several dyes. [1][2][3][4][5][6][7][8][9] As the physical chemical properties of the mixture such as the polarity, the viscosity, the dielectric constant as well as the hydrogen bond, dipole-dipole, - interactions are modulated by the change in the mixture composition, the photophysical properties of the dye are affected consequently. For instance, the position of maximum on the absorption or emission spectrum, the stoke shift, the quantum yield and the excited state relaxation times undergo a nonlinear behavior. The most striking result is that the distribution of the H m atom of the cation methyl around the electronegative atoms of the D205 is the most affected when the charge distribution is changed from that mimicking the ground state to that of the excited state. Interestingly, this behavior is used to assess about the local organization such as microheterogeneity of the mixtures.

Considerable progress towards understanding the solvation dynamics of dyes in mixtures has been made in recent years, through molecular dynamics (MD) [1,[10][11][12][13] and quantum-chemical calculations [1,12,[14][15][16][17][18][19][20]. For instance, using MD simulation the solvation dynamics of Coumarin C153 was analyzed in DMSO and water mixture spanning the entire composition range. It was shown that the solvation of C153 is predominantly determined by its interaction with the DMSO molecules, even at low 0.25 mole fraction DMSO content while the water molecules are only mildly coupled to the charge transfer in C153, resulting in a small water relaxation component. [21] In another MD study, the anomalous behavior of two alcohol-water (TBA-water and ethanol-water) binary mixtures were investigated by collecting ultrafast solvation response around the C480 dye.

It was found that in a specific mole fraction of these mixtures the average solvation time rises while in the mixture of ethanol-water, this behavior is totally absent. It was suggested that the structural transition from water like tetrahedral network to zigzag alcohol like structure is the probable reason for slow solvation dynamics. [22] In another work, in order to understand the molecular reorganization and relaxation behavior of aqueous binary mixtures, the solvation dynamics of tryptophan over a wide range of DMSO concentration was studied using MD simulation. A remarkable slow-down in the solvation dynamics around 0.10-0.20 and again around 0.40-0.50 mole fraction of DMSO was observed. The authors of this paper suggest that the slowdown is a reflection of various structural transformation in water-DMSO mixture that is driven by a cooperative aggregation of DMSO molecules aided by water in the above mentioned concentration ranges. In another MD simulation work, the solvatochromism and the dynamic stokes-shift of coumarin C153 in mixtures of benzene-acetonitrile and of benzene-methanol. The MD results allowed to interpret these dynamics in benzene-acetonitrile mixtures in terms of motions of benzene around the coumarin. More interestingly, they show that the role of benzene in the solvation process of CI53 seems to be more important than usually assumed. Furthermore, the solvation dynamics of dyes was also investigated in neat ionic liquids and in their mixtures ILs with a molecular solvent [23]. By investigating the solvation properties of solvatochromic probe dye molecules dissolved in mixtures of large number protic ionic liquids with various molecular solvents using multi-parameter solvation scales, the authors of the previously cited papers demonstrate the high solvation capability of protic ionic liquids which open the way to their use in applications which require high polarity and H bonding ability.

Experimental methods, were combined with MD simulation to investigate the solvation dynamics of 3 organic dyes (coumarine-derivative) in [pMIM][Br]/water system. It was suggested that the coumarin dyes preferably interact with the IL. [24][25][26] The authors investigate how the water in small concentration affect the solvation dynamics of the C153 in [HMIM][PF6] IL [27].

It was found that water screens the direct Coulombic interactions between cations and anions and that it facilitates rotational and translational motion. Furthermore, the relaxation of solvent environments to local minima is considerably faster in the presence of small amounts of water. The fact that it is possible to choose the IL that the dynamics or kinetics of relaxation of the solvent can be controlled (selected) by adjusting the absorption frequency is one of the main advantages of these systems.

Indoline-based dyes form a promising class of compounds for various applications because of their photophysical and photochemical properties. They reveal high photon-to-current efficiency in combination with high molar absorption coefficients and quantum yields [6,14,18,[28][29][30][31][32][33][34][35][36][37][38][39][40]. One of particularly intriguing applications of these molecules is the photosensitive dyes in dyesensitized solar cells (DSSCs) [30,41]. In these devices, molecular dyes are typically used in the mixtures of IL and molecular solvents, described below. The photophysics in the system of indoline derivated dyes in molecular solvents (MS) and in IL/MS mixtures, making use of the favorable properties of both, MSs and ILs was also investigated [42][43][44]. For this purpose, these mixtures were investigated using the spectroscopic properties of four indoline derivates (D131, D102, D149 and D205) dyes encompassing steady-state UV-vis absorption and fluorescence spectroscopy and time-resolved spectroscopy for understanding the photophysics of these dyes and its mixture composition dependence. It was shown that the solvation dynamics is crucial for the excited state dynamics of the dyes. Furthermore, the obtained relaxation times values, that describe the relaxation of these dye from the excited state to the ground, were found to decrease with dilution of the ionic liquid and the slowest one among them go through a minimum around 0.1 mole fraction of the ionic liquid. This interpreted as an indication of the the presence of different specific and non-specific interactions between the dye and the surrounding mediums.

Furthermore, the solvation of the indoline dyes was also investigated at the interface e.g TiO 2 [6,7,[45][46][47][48][49]. It was shown that D205 with the longer alkyl chains shows larger mobility and consequently higher diffusion coefficient than D149 dye with shorter alkyl chain. In the work [50] the combined solvatochromic shift and TDDFT study of ground and excited state was performed for Alexa Fluor 350 dye in various solvents like alcohols and some general ones. It was shown that if dipole moment of solute increases during the excitation, a positive solvatochromism normally observed. The value of Stokes shift varies with the solvent polarity as evidenced by the considerable change of geometry of dye in excited state upon electronic transition. It might be influenced by solvent parameters like hydrogen bond donor and acceptor and solvent polarity. With the help of experimental study the group [51] show that the behavior of the dye is dictated by solute-solvent and solvent-solvent interactions, which are effected by the solvent polarity.

It should be mentioned that in the investigation of the solvation dynamics of dyes using molecular dynamics simulation, the choice of the force field is crucial. To our best knowledge, forced field based on Amber library for describing dyes were reported, especially tuning the partial charges, what is extremely important for -conjugated systems [10]. The same class of dyes, like Alexa Fluor (rhodamine-derivative), Atto (zwitterionic dyes) and Cyanine families, their force filed parameters were described on the basis of CHARMM force field with the sequential application for high accuracy diffusion coefficient, comparatively with the existed force fields [52] e.g. UFF [53], OPLS-AA [54], Amber [55], etc. The accuracy of their application for the bigger molecules is low. However, there is the lack of the investigation of the solvation dynamics of these dyes using a force filed parametrized that is specifically derived for the indoline dyes. Consequently, in this work we aim to investigate the structural properties of dye D205 (the structural formula is shown on Fig. 2.1) in the system of ionic liquid BmimBF 4 and molecular solvent acetonitrile (AN) using combined molecular dynamics and quantum-chemical calculations approaches. Thus, mixture was chosen because it was the previous experimental works, performed in the group [42,44]. Due to the lack of the D205 structure experimental investigations, we chose the M06-2X quantum-chemical calculation method in basis 6-311+G(d) to optimize and calculate the force constants to describe the intramolecular part of the force field.

We also derive the charge distribution in the ground and excited states. Based on this force field, molecular dynamics simulations of the mixture of the D205/BmimBF4/AN were carried out.

Force Field development of D205

The force field of a polyatomic molecule is a mathematical expression that describes the dependence of the intramolecular and intermolecular potential energy of the system as a function of interatomic distances and intramolecular coordinates:

𝑈 = 𝐸 intra + 𝐸 inter , (2.1)
where the first term describe the intramolecular contributions to the potential energy), and the second -intermolecular contributions.

The structure, absorption and electronic properties of the dyes and adsorption on surfaces may be affected by the isomerism of the dye molecule. [19,45,56] For dye D205 possible 4 isomers: dihedrals ψ 1 (С8-C4-C19-C16) and ψ2 (S2-C2-C1-S1) can be in cis-and trans-positions. For all the further calculations the most energetically favorable isomer trans-trans were used (

Intramolecular force field of D205

The intramolecular potential energy can be described by the equation 2.2:

𝐸 intra = 𝑈(𝑟, 𝜃, 𝜓) = ∑ 𝑘 𝑟 2 𝑏𝑜𝑛𝑑 (𝑟 -𝑟 0 ) 2 + ∑ 𝑘 𝜃 2 𝑎𝑛𝑔𝑙𝑒 (𝜃 -𝜃 0 ) 2 + ∑ 𝑘 𝜙 2 𝑑𝑖ℎ𝑒𝑑𝑟𝑎𝑙𝑠 (𝜓 -𝜓 0 ) 2 , (2.2)
where 𝑈(𝑟, 𝜃, 𝜓) is -the sum of the potential energies of the internal coordinates of the bonds, angles and dihedral angles, respectively; k -force constant; 𝑟, 𝜃, 𝜓values at a given scan step, where the index "0" corresponds to the values in the local energy minimum.

The optimal geometry was obtained under very tight optimization convergence criteria implemented in the software and tested to be true minima by the absence of imaginary frequencies in the corresponding vibration spectra. The quantum chemical calculations of a single D205 dye molecule in gas phase were carried out using M06-2X/6-311+G(d) level of theory. After that, calculations of force constants associated with the bond lengths and angles stretching were used within the concept of rigid potential energy surface (PES) scanning. [1,[57][58][59][60][61] Using this approach, the minimum potential energy along a given internal coordinate is calculated, without simultaneous optimization of internal degrees of freedom at each scanning step, which means, that the conformer transition is impossible.

To calculate the force constants, the dye molecule was divided into separate fragments (different colors on Fig. 2.1) to reduce the computation time. In total, 20 configurations of D205 fragments with a step of 0.0005 nm for bonds, 0.5° for angles and 0.8° for dihedral angles were analyzed with the help of rigid PES scanning. As a result of this calculation, the energy profiles obtained are well described by a second-order polynomial, which is shown on Fig 2 .2. For the dihedrals the harmonic approximation was used, because the molecule is stable only for the small range of radian, so it's better describe with the same polynomial form. [62]), UFF [53] and averaged from this work For each atom were chosen corresponding types of atom from the field OPLS [54] and after it all force constant, as well as geometry parameters were averaged (table S1-S2) for the possibility to use it for another molecules, especially, indoline dyes. From fig 2.2 it is noticeable that corresponding constants from the standard force fields (UFF [53], OPLS-AA [62]) not in all cases correctly reproduced geometry behavior in this molecules.

Intermolecular force field of D205

The formula 2.3 describes the intermolecular terms.

𝐸 inter = ∑ 4є 𝑖𝑗 𝐿𝐽 ( 𝜎 𝑖𝑗 12 𝑟 𝑖𝑗 12 - 𝜎 𝑖𝑗 6 𝑟 𝑖𝑗 6 ) + ∑ 𝑞 𝑖 𝑞 𝑗 𝑟 𝑖𝑗 𝑖,𝑗 , (2.3)
Where the first term describes the Lennard-Jones potential and second -Coulomb interactions.

For the current force field model of the D205 dye, data from the OPLS [54] force field was utilized for the describing Lennard-Jones parameters.

The spectral properties of the dye depend on its excited state. For the calculation the method (CHelpG) in vacuum on the M06-2X/6-311++G(d,p) level of theory was used. The schematic representation of this process is seen on the time step was equal to 0.05 fs, during the first 5 ps and after was increased by a factor of 10 for further calculations for 5 ns. In total 4 consecutive productive runs of a duration 5 ns (A, B, C, D) were done for every system. Table 2.2 is represented the composition of all the mixtures. In the range 0-0.2 there is the small step of the ionic liquid mole fraction, because it was shown that in this range there is the specific interactions occurs for the ionic liquid/molecular solvent system. [44] Table

Results and discussions

The local structure around the dye was assessed using the radial distribution function, 𝑔(𝑟), (RDF) Due to the large number of atoms of dye, we decided to select atoms from the donor, bridge and acceptor parts of the D205.These atoms are either hydrogen atoms or electronegative ones. The numbering of these atoms is shown in Fig 1,b. As a consequence in our analysis of the locale structure around the dye, the following radial distribution were calculated:

1) The 𝑔(𝑟) of the N of the AN around the donor H (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24-25) parts of the dye D205 dye.

2) The 𝑔(𝑟) of H atoms of cation Bmim + , namely H (1) , H (4) , H (5) , H (m) and H (b) around the electronegative atoms of D205 dye: O (2) , O (3) , O (4) , S (3) .

3) The 𝑔(𝑟) of the F of the anion BF 4 -around the donor H (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24-25) parts of the dye D205 dye.

In order to capture insight into the local structure at the level of the neighbors, the nearest neighbor radial distributions, 𝑔 1 (𝑟) corresponding to the previously cited RDFs also were calculated. In order to obtained statically reliable 𝑔(𝑟) and 𝑔 1 (𝑟) , we carried out 4 subsequent calculations, named A, B, C, D , lasting each 5ns. Two situations were obtained: In the first one, as it is illustrated in Figure 2.7, the subsequent 𝑔 1 (𝑟) , describing the nearest radial neighbor distribution of the F atom of the anion around the hydrogen atom H 3-4 , are showing the same trend. As a consequence we averaged these distributions and calculated the average value using the equation:

< 𝑟 𝐻 3-4 -𝐹 (𝑛) > = ∫ 𝑑𝑟 ′ 𝑟 ′ ∞ 0
𝑔 1 (𝑟 ′ ) . This distance was taken as observable to quantify the distribution of the F atoms around these hydrogen atoms of the D205 dye. In a second situation, we found a large evolution between subsequent calculations of the 𝑔 1 (𝑟). This illustrated, in change is confirmed in the third 5 ns calculation. However, in the fourth, the large distance contribution dominates the short distance one. We decided then to characterize the distance between the O1 and H4 using two values associated with the position of the short and the large distance contributions.

Distribution of the AN molecule around the D205 dye in neat AN

In order to characterize the distribution of the solvent around the D205, we calculated the radial distribution functions, 𝑔(𝑟), of the N atom of the AN around the hydrogen atoms of the D205 dye, that were selected among donor H (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24-25) parts (parts of the dye (see Fig. 1 for the number of atoms). These 𝑔(𝑟) are given in Figure 10. The corresponding 𝑔 1 (𝑟) are given in Figure 11. To quantify the changes in the local structure around the D205 after photoexcitation, we calculated the same 𝑔(𝑟) and 𝑔 1 (𝑟) in the excited state. We recall that the photoexcitation is mimicked by changing only the charge distribution on the D205 atom while the geometry of the D205 is unchanged.

Figure 2.9. Radial distribution function of the N atom of AN around hydrogen atoms chosen in donor H (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24-25) parts of the dye D205 dye. These functions were calculated in the ground and excited states. (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24-25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

More insight into the distribution of the solvent around the various part is obtained by analyzing the shape of the corresponding 𝑔 1 (𝑟). Indeed, Figure 2.11

shows that the position of the solvent molecule, the position of which is described by the N atom, shifts to lower distance around the bridge and acceptor parts of the D205 in comparison those around the donor and chain parts. For the last one, even we notice a slight shift to higher distances. Of note is that spatial distribution of the N atoms around the bridge and acceptor parts of the D205 is reduced in the excited as compared with that in the ground state. Nearest neighbor radial distribution of the N atom of AN around hydrogen atoms chosen in the donor H (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24-25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

Distribution of the cation and anion around the D205 dye in neat BmimBF4.

In order to investigate the distribution of the cation and anion around the D205, we chose the F atom of the anion, the ring hydrogen atom of the cation, namely H (2) , H (4) and H (5) as well as the H (m) and H (b) atoms of the methyl and butyl groups.

We then calculated both the radial distribution functions and the coordination number of F atom of the anions around the hydrogen atoms belonging to the acceptor, bridge, donor and chain parts of the D205 dyes. This allows us to characterize the distribution of the anions around the dye. We calculated the radial distribution functions and the corresponding coordination number chosen hydrogen atoms of the cation around the oxygen and sulfur atoms, O (2) , O (3) , O (4) and S (3) of the D205 dye.

Distribution of the anion around the D205 dye

Figure 2.12 shows clearly that either in the ground state or in the excited state there is no structured radial distribution of the anion around the terminal butyl chain hydrogen atoms. On the contrary the other 𝑔(𝑟) are characterized by the occurrence of peaks that suggests that the anion is solvating the H atoms of the acceptor, the bridge and the donor parts of the D205. In general there is small change in the shape of these radial distribution when going from the ground to the excited state. This is confirmed by the behavior of either the corresponding coordination number (see Figure 2.13) and the nearest neighbor radial distribution functions (See Figure 2.14). Indeed, when going from the ground to excited state, the coordination number increases marginally around the H atoms of the chain, the acceptor, the donor parts of the D205 while a noticeable change occurs in that around the bridge H atoms. Of note is that the main changes in the shape of the 𝑔 1 (𝑟) distribution involving the hydrogen atoms of the acceptor and donor parts of the D205 dyes are localized at large distances and indicates that the F atoms of some anions is expelled to large distances in the excited state with respect to that in the ground states. hydrogen atoms chosen in the donor H (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24-25) parts of the D205 dye. These functions were calculated in the ground and excited states. Coordination number (CN) of the F atom of the anion BF 4 - around hydrogen atoms chosen in the donor H (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24-25) parts of the dye D205 dye. These functions were calculated in the ground and excited states. (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24-25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

Distribution of the cations around the D205 dye

Furthermore, we analyzed the distribution of cations around the H atoms of the dye and then calculated the 𝑔(𝑟) describing the radial distribution of the H atoms of the cation namely the H atoms of the cation Bmim + , namely H (2) , H (4) , H (5) , H (m) and H (b) . (See Figure for numbering) around the electronegative atoms O (2) , O (3) , O (4) , S (3) of the dye D205. These functions, their corresponding coordination number and nearest neighbor distributions are displayed in Figures 2.15, 2.16 and 2.17, respectively.

Figure 2.15 shows that there distribution of the butyl chain H atoms of the cation around the O (2) , O (3) , O (4) , S (3) atoms of the D205 is diffuse as inferred from the absence of any defined peak in the corresponding 𝑔(𝑟). The shape of the corresponding 𝑔(𝑟) is almost not affected when going from the ground to the excited states. This is also confirmed by the small change in the corresponding coordination number displayed in Figure 2.16. However, the distributions of the methyl H (m) of the cation around these electronegative atoms is well structured as indicated by the occurrence of peaks at short distances. In the excited state the distribution of the H (m) becomes around of S (3) and O (4) atoms is more diffuse. This is confirmed by the decease of the corresponding number shown in Fig. 2.16, while in the excited state the distribution of H (m) around O (2) , O (3) atoms becomes more structured as indicated by the increase of the intensity of the first peak with respect to that in the ground state. This is also confirmed by the increase of the corresponding coordination number. Small changes are observed in the shape of the 𝑔(𝑟) describing the distribution of the H (2) atom around the electronegative atoms of D205 in particular around the O (4) , S (3) atoms while a decrease in the intensity is observed for the 𝑔(𝑟) involving the O (2) , O (3) . This confirmed by the small change in the corresponding coordination number. In general, the distribution of H (5) around the electronegative atoms of the D205 follows the same trend as that of H (m) . However, the changes (increase/decrease of the intensity of the first peak as well as the coordination number) occur with less extent.

The 𝑔(𝑟) of H (4) atom around O (4) , S (3) and O (3) electronegative atom of the dye seems not bet affected when going in from the ground to the excited states. Also, this is confirmed by the small change in the corresponding coordination number.

However, although the shape of 𝑔(𝑟) of the H (4) around the O (2) atom is strongly affected when going from the ground to the excited state as indicated by the large increase of the intensity of the first peak , the corresponding coordination number is marginally increasing suggesting that the number of H (4) around O (2) is not strongly affected.

These results indicate that the main changes in the distribution of the cation around the electronegative atoms of the dye involve the methyl H (m) .

Figure 2.15.

Radial distribution function of the H (1) , H (4) , H (5) , H (m) and H (b) atoms of the cation BMIM + around electronegative atoms O (2) , O (3) , O (4) , S (3) of the dye D205 dye. These functions were calculated in the ground and excited states. Coordination number (CN) of the H (1) , H (4) , H (5) , H (m) and H (b) atoms of the cation BMIM + around electronegative atoms O (2) , O (3) , O (4) , S (3) of the dye D205 dye. These functions were calculated in the ground and excited states.

The overall changes in the shape of the 𝑔 1 (𝑟) describing the radial distribution of the H atoms of the cation ring around the electronegative atoms of the D205 are shown in Fig. 2.17 and are consistent with those in the shape of the corresponding 𝑔(𝑟). Based on these distributions we calculated the corresponding average distances that are shown in Figure 2.18.

Figure 2.17. Nearest neighbor radial distribution function of the the H (1) , H (4) , H (5) , H (m) and H (b) atoms of the cation Bmim + around electronegative atoms O (2) , O (3) , O (4) , S (3) of the dye D205 dye. These functions were calculated in the ground and excited states. Average distance as determined from the earest neighbor radial distribution functions of the the H (1) , H (4) , H (5) , H (m) and H (b) atoms of the cation Bmim + around electronegative atoms O (2) , O (3) , O (4) , S (3) of the D205 dye.

These functions were calculated in the ground and excited states.

When going from the ground to the excited state (i) the average distance between the H (m) and the electronegative atoms of the D205 increases when considering the O (4) , S (3) while it decreases when considering the other ones.

(ii) the average distance between H (5) and the S (3) increases and O (3) decreases, while almost no changes are occurring when considering its average distance with respect to the O (3) and O (4) atoms.

(iii) the average distance between H (1) and the electronegative atoms, it increases systematically when considering the O atoms while no change is observed for S (3) .

(iv) the average distance between H (b) and O (3) or O (2) increases, while almost no changes are occurring when considering its average distance with respect to the O (2) and S (3) atoms (v) the average distance between H (4) and S (3) or O (4) or O (3) remains almost the same, while it increase when considering O (2) .

Distribution of the cations, anions and solvent around the D205 dye in the D205/BmimBF4/AN mixture

In this part of the thesis, we will investigate how the local distribution of the cation, anion, and solvent molecules around different part of the hydrogen atoms of the D205 is affected by the change in the mixture composition. Following the same scheme that we used for the analysis of the locale structure of the D205 in neat solvent and ionic liquid, we started our analysis by calculating the radial distribution function between different atoms of the D205 and the N atom of the AN. They are displayed in Figure 2.19. This figure shows that in the range of mole fraction of ionic liquid between 0.05 and 0.2 (high solvent content), the shape of these functions is not affected when going from the ground to excited state. However, noticeable differences in the distribution of the N atoms of the solvent are observed at low solvent content, showing that in the case of donor H (43) , bridge H (2) and the acceptor H (3-4) parts the intensity of the first peak is lower, respectively that the corresponding one in the ground state. While in the case of the chain H atoms, this intensity increases.

Due to the statistical uncertainties, we think that the behavior of these 𝑔(𝑟) at very low solvent content are difficult to interpret.

Figure 2.19.

The ionic liquid mole fraction dependence of the radial distribution function of the N atom of AN around hydrogen atoms chosen in the donor H (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24-25) of the dye D205 dye. These functions were calculated in the ground and excited states.

If now we look to these 𝑔(𝑟) through the mixture composition dependence of the corresponding coordination number (shown in Figure 2.20), we can formulate the following observations:

(i) The coordination number is not affected in the range of mole fraction range of the ionic liquid between 0.05 and 0.2.

(ii) At 0.5, in accordance with our interpretation of the behavior of the 𝑔(𝑟), the coordination number of N atom of the solvent around the donor H (43) , the bridge H (2) , the acceptor H (3-4) decrease in the excited state, while around the butyl chain H (24-25) it increases. (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24-25) parts of the dye D205 dye. These functions were calculated in the ground and excited states. (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24-25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

The mole fraction dependence of the nearest neighbor distribution of the solvent molecules around the H atoms of the dyes is given in Figure 2.21. Their behavior is in accordance with our previous analysis.

To The ionic liquid mole fraction dependence of the radial distribution function of the F atom of the anion BF4 -around hydrogen atoms chosen in the donor H (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24-25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

This affirmation can be affined by investigating the mixture content dependence of the coordination number shown in Fig. 2.24. Indeed, the difference between this parameter in the ground and exited state, is reduce when the ionic liquid is diluted. The ionic liquid mole fraction dependence of the coordination number (CN) of the F atom of the anion BF 4 -around hydrogen atoms chosen in donor H (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24- 25) parts of the dye D205 dye. These functions were calculated in the ground and excited states. The ionic liquid mole fraction dependence of the nearest neighbor radial distribution function F atom of the anion BF 4 -around hydrogen atoms chosen in donor H (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24- 25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

The investigation of the mole fraction dependence of the distribution of the cation around the electronegative atoms of the D205 is analyzed through the calculation of the same statistical distributions namely the 𝑔(𝑟), the corresponding coordination function and the 𝑔 1 (𝑟) that are displayed in the following figures: figures 2.25 for H (b) , figure 2.26 for H (m) , figure 2.27 for H (1) , figure 2.28 for H (4) and figure 2.29 for H (5) . Based on the evolution of these distributions according to both the change of the charge distribution of the D205 from the ground to the excited state, and the change of the mixture composition we can formulate the following points:

(i)

The distribution of the H b atom of the cation butyl group around the electronegative atoms of the dye becomes diffuse at low ionic liquid content (around 0.2) and at the same mole ionic liquid mole fraction the shape of these distributions is in general less affected when going from the ground to the excited state. The difference between the coordination numbers of H b around all the electronegative atoms in ground and excited state becomes smaller in lower mole fraction of ionic liquid. This is confirmed by the behavior of the corresponding 𝑔 1 (𝑟). Of note is that we don't have any explanation in the change in the shape of this distribution involving the S (3) and O (2) at 0.5 ionic liquid mole fraction.

(ii) The distribution of the H (m) of the cation methyl group is affected by the change of both charge distribution on the D205 and the mixture composition around all the electronegative atoms. In the whole mixture composition, the coordination numbers of H (m) around the O (2) and O (3) atoms is higher in the excited state than in the ground state while the opposite trend is observed for that around S (3) and O (4) .

(iii) The distribution of H (1) around the S (3) is diffuse and it is not affected neither by the change of the mixture composition nor by the change in the charge distribution. This also seen in the small difference in the corresponding coordination numbers. Although there are changes in the shape of the describing the distribution of H (1) around S (3) , the corresponding coordination number is almost not affected by the change in the charge distribution.

(iv) The distribution of H (4) atom around all the electronegative atoms is more affected by the change in the charge distribution and not by the change in the ionic liquid mole fractions. This is based on the small change of both the position and shape of the peak stay stable for the full concentration range. Minor changes of the coordination number of H (4) around S (3) were observed below 0.8 mole fraction.

(v) The distribution of H (5) atom around S (3) atom shows the changes only due to the transfer from ground to excited state. Minor changes are observed in the corresponding coordination number as well as that around O (4) . (2) , O (3) , O (4) , S (3) of the D205 dye. These functions were calculated in the ground and excited states. (2) , O (3) , O (4) , S (3) of the D205 dye. These functions were calculated in the ground and excited states. (1) atom of the cation Bmim + around electronegative atoms O (2) , O (3) , O (4) , S (3) of the D205 dye. These functions were calculated in the ground and excited states. 4) atom of the cation Bmim + around electronegative atoms O (2) , O (3) , O (4) , S (3) of the D205 dye. These functions were calculated in the ground and excited states. (4) atom of the cation Bmim + around electronegative atoms O (2) , O (3) , O (4) , S (3) of the D205 dye. These functions were calculated in the ground and excited states.

In figure 2.30, we gathered the distance average values between the H atoms of the D205 and the N and F atoms of the solvent and anions respectively. The ionic liquid mole fraction dependence of the mean distances between N of AN and F atom of the anion BF4 -around hydrogen atoms chosen in donor H (43) , bridge H (2) , the acceptor H (3-4) and butyl chain H (24- 25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

These average distances values were calculated for the ground and excited state in the while mole fraction range. Fig. 2.30 shows clearly that there is a cross over between one side the distance of the H atoms of the dye and the N atoms of AN and from the other side the distance between the same hydrogen atoms of the dye and the F atom of the anion. This cross over occurs in the mole fraction range between 0.2 and 0.4. In addition, Fig. 2.30 also shows, that the behavior of the average distance involving the H atoms of the chain part and the donor parts of the dye is almost similar to each other.

Conclusions

In this work we have provided the parameters for the intramolecular part of the force field model of indoline dye D205 as well as the charge distributions that mimic its ground state and excited states. Our quantum calculations show that there is a small change in the geometry of the dye D205 in the ground and excited states. As consequence in our molecular dynamic simulations the geometry of the D205 was not changed between the two states.

Based on the new force field model, the effect of the mixture composition of BmimBF4/AN on the local structure of D205 was analyzed in the whole mixture composition including the neat components. The is analysis was based the radial distribution function, the corresponding nearest neighbor, and the coordination numbers. Our results show that in neat ionic liquid, there is no difference between the ground and excited states in the distribution of the anion around the H atoms of the donor, bridge, acceptor, and the chain parts, while there a large difference for the distribution of the H atom of the methyl group of the cation around these atoms. Our results also show that there is a difference between the ground and excited states in the distribution of the N atom of AN around both the bridge and the acceptor parts.

The investigation of the mixture composition of the solvent and ions around the H atoms of the dye indicates that:

(i) the distribution of the solvent is only affected at low solvent content and that the coordination number of N atom of the solvent around the donor H (43) , the bridge H (2) , the acceptor H (3-4) decrease in the excited state, while around the butyl chain H (24-25) it increases.

ii)

The distribution of the anions undergoes a small change when going from the ground to the excited state.

Because of the large number of possible ways to describe the distribution of the cation around the electronegative atoms of the D205, it was difficult to come with a clear conclusion on this issue, however our results show that because the O (3) and O (4) atoms are close to each other the distribution of the cation around these atoms is quiet similar in the ground and excited states. While differences are observed the distribution of the cation around the O (2) and S (3) atoms.

Chapter 3. Structure and properties of TiO2anchored D205 dye in different solvent: TDDFT and MD simulation study

The material presented in this chapter forms the basis of publication The conformational dynamics of a dye molecule anchored to TiO2 is the key to understanding the mechanical aspects of the photovoltaic performance of dye-sensitized solar cell devices. In this chapter joint molecular dynamics (MD) simulations and density functional theory (DFT) calculations to investigate the structure and dynamics of an indoline D205 dye anchored at the solid˗liquid interface of small TiO2 anatase nanoparticle solvated with explicit acetonitrile (AN) solution was present. DFT calculations were carried out to estimate the equilibrium geometry of a small Ti30O62H4 anatase nanoparticle and to derive interaction parameters for bidentate binding of a D205 dye to TiO2. The effect of the different solvent on the conformation of the anataseanchored D205 dye and the resulting solvatochromic shift was investigated by MD simulations, demonstrating that an explicit solvent representation is vital to reproduce the optical spectra of a D205 dye.

Introduction

Dye-sensitized solar cells (DSSCs), which are based on dyes adsorbed (or covalently bonded) onto a TiO 2 surface, have many promising features, such as high efficiency of solar energy conversion and the low cost of structural elements [1]. Dye-sensitizers are the most critical component in a DSSCs part because they govern key processes, regulating the power conversion efficiency, such as light absorption, charge injection, dye regeneration, and charge recombination, respectively [2]. Therefore, the rapid development of DSSC technologies demands continuous tuning of the chemical properties of organic dyes because the light conversion efficiency is crucially governed by dye photophysics [1].

Moreover, significant efforts have still to be applied in order to overcome the major technological challenges, including the design of an anchoring moiety and a binding mode of a dye onto a TiO 2 surface [3][4][5].

It has been found that indoline dyes exhibit greater power conversion efficiency compared to available organic dye sensitizers [6]. A D205 dye has a push-pull system, in which an indoline moiety acts as an electron donor, and a rhodamine ring operates as an electron acceptor. Several lines of evidence suggested that the sensitizing efficiency of a D205 dye could be enhanced by the chemical anchoring to the TiO 2 semiconductor active layer [7][8][9]. Therefore, for practical application, a functional anchoring group, such as carboxylic one -COOH, can be introduced into the rhodanine-acceptor moiety [3].

Besides the proper design of the dye-TiO 2 interface, the environment also affects the DSSC overall performance. Due to the unique properties of ionic liquids (ILs), such as low vapor pressure, non-flammability, high conductivity, and thermal and chemical stability, ILs have become promising electrolytes for DSSC devices capable of passivation of active surfaces of DSSC devices [10]. In addition, some molecular solvents with a large dipole moment, such as acetonitrile (AN), may also be utilized as a perspective electrolyte for DSSC applications.

However, understanding the microstructure of an liquid/electrode interface requires atomic-level investigations of the interaction between an electrolite and an electrode surface [11].

Modern quantum-chemical methodologies, such as those based on density functional theory (DFT) and its time-dependent extension (TDDFT), can provide the theoretical framework to describe most of the crucial properties of the individual components of dye/semiconductor systems, such as the dye geometry, optical absorption spectra, and the semiconductor bandgap [12][13][14][15]. However, the reliable prediction of absorption spectra and solvatochromic shifts of organic dyes attached to small TiO 2 nanoparticles and solvated by polar liquids is still challenging from a computational point of view [16][17][18][19][20][21]. It is crucial to accurately compute the excited states of a dye molecule and reproduce correctly an electronic bandgap of TiO 2 semiconductor utilizing the same DFT approach [9,14,18,22,23]. On the other hand, the solvent-induced conformational changes of a dye molecule due to the polar environment, dye aggregation, and specific hydrogenbonding interactions with solvent molecules also need to be correctly accounted for [24][25][26][27].

Recently, classical molecular dynamics (MD) simulations have proven itself a powerful computational tool for studying the structure of electrolyte/TiO 2 interfaces, which may guide the rational design of a dye and save synthetic and experimental resources [9,28,29]. Moreover, MD simulations may complement experimental results of conformational dynamics and spectroscopic properties of a dye attached to TiO 2 [19]. Nonetheless, most of previous efforts were focused on electrolyte-TiO 2 interactions [9], so that phenomena and mechanisms related to TiO 2 -dye interactions have not been well understood, especially on the nanoscale.

Molecular Dynamics Simulation Setup 3.2.1 DFT Structure of D205 Dye anchored to Ti30O62H4

The structure of the indoline D205 dye attached to a Ti 30 O 62 H 4 anatase nanoparticle was first studied by the density functional theory (DFT) approach at the HSEH1PBE/TZV [START_REF] Krukau | Influence of the exchange screening parameter on the performance of screened hybrid functionals[END_REF] level of theory. The Ti 30 O 62 H 4 nanoparticle was truncated from a perfect anatase slab, and its size was sufficient to approximate the electronic structure and the bandgap of the bulk TiO 2 [14,30,31]. For simplicity, the long N-octyl tail in the D205 dye was truncated to the ethyl moiety (Figure 3 UV-vis absorption spectra of a free D205 dye and the D205-Ti 30 O 62 H 4 conjugate were calculated using the time-dependent DFT (TDDFT) at the hybrid CAM-B3LYP functional [36] and the TZV basis set [37]. To estimate a UV-vis absorption spectrum, up to 30 vertical transition S 0 →S n were calculated. To heed the spectral band broadening in solution, the excited-state transitions possessing non-zero oscillator strength were approximated by the Gaussian function. The resulting TDDFT spectra were represented as a superposition of a sum of single Gaussian bands. All DFT calculations were performed with the Gaussian 09 software package [38].

MD force field for all the components

A TiO 2 (001) surface was approximated by the perfect passivized anatase slab composed of 12×12×1 unit cells. The slab was cut from the perfect anatase crystal with the lattice parameters a=0.373 nm and c=0.937 nm using the VESTA software package [39]. Lennard-Jones (LJ) potential energy function. These 12-6 LJ potential parameters adopted for Ti and O atoms were subsequently validated by MD simulations in terms of the TiO 2 interactions with solvents and small ligands [41][42][43][44]. Therefore, we used these LJ parameters to simulate TiO 2 anatase, as summarized in Table 3.1. Van der Waals interactions were calculated using r cutoff =1.4 nm. Electrostatic interactions were calculated using the particle mesh Ewald (PME) technique beyond a real-space cutoff of r cutoff =1.4 nm. The coordinates of all TiO 2 atoms were kept rigidly fixed in their crystal lattice positions. For mixed non-bonded interactions between TiO 2 , the dye, and solvent molecules, the Lorentz-Berthelot combination rules were used [45]. The all-atom force field for the D205 dye was derived in this work. The DFT optimized length of the Ti-O and angle Ti-O-C bonds in the D205-TiO 2 conjugate were used to set up classical MD simulations. The FF stretching constants for the Ti-O bonds were adapted from ref. [24]. In MD simulations, the D205 dye possesses the full-length octyl chain, as shown in Figure 3.2.

Many ionic liquids have very attractive electrochemical properties for DSSC devices as electrolytes. Therefore, the structure and dynamics of most popular ILs were systematically examined with MD simulations [9,[46][47][48][49]. Mondal and Balasubramanian quantitatively predicted the physicochemical properties of the set of 1-butyl-3-methylimidazolium-based ILs, such as BmimPF 6 and BmimTFO, by using classical MD simulations, where the non-bonded interactions are described with the 12-6 LJ potential [50]. The partial charges were derived from the well-established CLaP force field, developed by Lopes et al. [51]. A good quantitative agreement with experimental measurements for the density, surface tension, enthalpy of vaporization, and ion diffusion coefficients allowed us to use these potential models for our MD simulations.

MD simulation setup

All the MD simulations were carried out in the canonical (NVT) ensemble.

Each system was firstly energy-minimized and then pre-equilibrated for 5 ns before accumulating productive MD runs. After which, the system was sampled for 100 ns. The reference temperature of 298.15 K was kept constant using the velocity rescaling weak coupling scheme [52] with the coupling constant τ T of 0.1 ps. The initial atomic velocities were generated with a Maxwellian distribution at the given absolute temperature. Periodic boundary conditions were applied to all three directions of the rectangular box. Electrostatic interactions were described by the particle mesh Ewald (PME) approach [53] using the long-range cutoff of 1.4 nm. The cutoff distance of Lennard-Jones interactions was also equal to 1.4 nm. The MD simulation time-step was set to 0.5 fs, with the neighbor list updates every 10 fs. The MD simulations were carried out by using the GROMACS 2016.3 software package [54]. Molecular graphics and visualization were performed using VMD 1.9.3 [55]. On the figure 3.3 these is the simulation box provided.

Results and Discussion

Theoretical and computational modeling is a powerful tool to investigate and characterize the structural, electronic, and optical properties of dye-TiO 2 conjugates as the main components of dye-sensitized solar cells (DSCs).

Therefore, we first performed the benchmarking of available DFT methods to select the theory level, enabling us to resemble electronic properties and photovoltaic characteristics of small-sized TiO 2 anatase nanoparticles.

Structure and Electronic Properties of Isolated Ti30O62H4 Anatase Nanoparticle

A TiO 2 anatase nanoparticle was represented by means of a Ti 30 O 62 H 4 cluster. The starting geometry for the structural optimization of the cluster was cut from the X-ray structure of the bulk anatase. It has been shown that (TiO 2 ) n clusters with n≥24 resemble electronic properties and the bandgap between occupied and unoccupied molecular orbitals of bulk anatase [12,14,21,56,57].

The geometry of Ti 30 O 62 H 4 was optimized at the HSEH1PBE/TZV theory level.

This structure was further used as a starting configuration for MD simulations.

The optical energy bandgap for Ti 30 O 62 H 4 was computed at various levels of theory in the PCM-acetonitrile solution evaluated as (i) vertical excitation energy (S 0 →S 1 ) and (ii) one-electron Kohn-Sham eigenvalues HOMO and LUMO difference, as summarized in Table 3.2. Considering the small size of the Ti 30 O 62 H 4 nanoparticle, we expect some overestimation of the energy bandgap compared to the experimental value for bulk anatase (3.2 eV) [14,31]. As can be noted from Table 3.2, the CAM-B3LYP value of 3.60 eV for the optical transition S 0 →S 1 is in reasonably good agreement with the experiment. In contrast, the B3LYP and HSEH1PBE functionals result in some underestimation of the energy gap to be at 2.73-2.91 eV, respectively.

Significant overestimation of the optical bandgap up to 3.89-4.11 eV was found by the LC-wPBE and M06-2X functionals. We also noted that the excitation energy S 0 →S 1 has only minor basis set dependence, as seen from comparing the CAM-B3LYP/TZV and CAM-B3LYP/TZVP results. It should also be pointed out that the low intensity calculated for the S 0 →S 1 oscillator strength confirms the optically symmetry forbidden character of the electronic transition in Ti 30 O 62 H 4 , as expected for an indirect bandgap semiconductor [14]. To sum up, these results

suggest that the TD-CAM-B3LYP/TZV method is one of the available reliable tools for estimating the electronic properties of small TiO 2 particles.

Structure of D205-Ti30O62H4 Conjugate in Acetonitrile

Absorption Spectra and Solvatochromic Shift of D205 Dye

Although the B3LYP exchange-correlation functional is commonly employed in investigating the absorption spectra of the organic dyes [62][63][64].

However, it is also well-recognized that it fails with excited-state calculations involving charge-transfer excitations, which is crucial for dye-TiO 2 conjugates [12,22,65,[START_REF] Jungsuttiwong | Theoretical study on novel double donor-based dyes used in high efficient dye-sensitized solar cells: The application of TDDFT study to the electron injection process[END_REF]. Therefore, TDDFT calculations were done using exchange-correlation functionals combined with the Coulomb-attenuating method (CAM-B3LYP) [36]. It has been shown that the TD-CAM-B3LYP approach performed well for indoline-based dyes [START_REF] Lambert | Characterization of high-performance organic dyes for dyesensitized solar cell: a DFT/TDDFT study[END_REF].

Figure 3.5 shows the optical spectra of a free D205 dye calculated by the TD-CAM-B3LYP/TZV method using the PCM model for the acetonitrile solution. In the region of 250-700 nm, we observe two absorption bands located at 328 nm and 423 nm, respectively. The wavelength positions of the peaks and the relative intensity (related to the oscillator strength) agree with those reported in the literature both for the experiment and for previous calculations [58]. We A crucial feature of an efficient organic sensitizer is its intramolecular charge-transfer property, so that there should be an efficient transfer of charges from the electron-rich donor dye moiety into the electron acceptor semiconductor band [17,[START_REF] Akimov | Theoretical Insights into Photoinduced Charge Transfer and Catalysis at Oxide Interfaces[END_REF][START_REF] Duncan | Theoretical Studies of Photoinduced Electron Transfer in Dye-Sensitized TiO2[END_REF]. It has been well established that, in a DSSC cell, efficient photoelectronic conversion can occur when the following criteria are met [9,17]:

(1) To achieve efficient light harvesting, the energy level of the highest occupied molecular orbital (HOMO) of a dye must be located above the HOMO energy level of TiO with BmimPF 6, BmimTFO, and AN, respectively. We found that the density of BmimPF 6 and BmimTFO depend strongly on distance calculated from the TiO 2 surface along the z-axis, as shown in Figure 3.9. At the distances from 0 up to 1.5 nm from the surface, three density peaks could be identified. For BmimPF 6 , the position of these peaks was found to be at 0.50, 0.87, and 1.28 nm, respectively.

In the case of BmimTFO, the peak positions are at 0.48, 0.64, and 0.94 nm, respectively. The position of these peaks coincides with the ordered layers of the imidazolium cations, as seen in the top panels of Figures 4a and4c. These finding are in the agreement with published results of MD simulations of similar systems, where authors reported that strong adsorption of IL ions onto charged TiO 2 surface, leading to the interfacial ions ordering and dense packing of adsorbed species [28,29,46,[START_REF] Zhou | A review of recent advances in computational and experimental analysis of first adsorbed water layer on solid substrate[END_REF]. Our MD simulations demonstrate that at distances over According to the experimental data, the mass density of AN at 298.15 K is reported to be 775.9 kg/m 3 [START_REF] Moumouzias | Excess properties of the binary liquid system propylene carbonate + acetonitrile[END_REF]. These data can be used for comparison and validation of our MD results, as shown in Figure 3.9f. One can see that the mass density of pure AN simulated at the isothermal-isobaric ensemble is in perfect agreement with the experimental values. We also noted some 5-7 % increase in the mass density of AN due to its contacts with the polar TiO 2 surface. In addition, similarly to the above-discussed density distributions of the ILs, the density plot of AN revealed up to three peaks close to the TiO 2 interface. The peak position is at 0.42 nm, 0.78 nm, and 1.21 nm, respectively. These density peaks can be attributed to the three ordered layers of AN molecules, as shown in Figure 4e.

Microscopic structure of solvent molecules at TiO2 interface

The local microscopic structure of ILs and AN at the TiO 2 interface was examined by the calculation of the radial distribution functions (RDFs). To The RDF plots show well-defined peaks at the short distance 0.4-0.5 nm from the TiO 2 surface, indicating that the atomic structure is characterized by the short-range order. The analysis of the RDF peaks calculated between BmimTFO and the TiO 2 slab shows that the anion of TFO -is coordinated onto the inorganic interface through the interactions between Ti atoms of the anatase slab and oxygen atoms of the anion. In this case, the center˗of˗mass is spread between the sulfur atom and the trifluoromethyl group as the electron˗withdrawing group. As in the case of BmimPF 6 , the Bmim + and oxygen atoms of TiO 2 coordinate by the imidazolium ring as the center of mass of the cation. Table 3 As can be noted from Table 3.4., the strongest interactions occur between the Ti atoms and the fluorine atoms of PF 6 - . The difference of the peak intensities related to the PF 6 -could be explained by the position of the phosphorous atom as a center˗of˗mass of the anion. Besides, the interactions of the Bmim + ions and the OT atom are driven by the interaction of the imidazolium ring, mimicking interactions with the aromatic π + -ring in real sytems. These results are in agreement with Malali and Foroutan [46]. It was shown that the arrangement of the Bmim + cations could be explained by the electrostatic and van der Waals interactions between the imidazolium ring and the TiO 2 surface.

The RDF plots for TiO 2 /AN are shown in Figure 3.12. The position of the first RDF peaks is summarized in Table 3.5. It could be noted from Figure 3.12 and Table 3.5 that the essential interactions occur between the nitrogen atom of the cyano group of AN and Ti atoms of anatase. This type of interaction originates from the nature of an AN molecule, where the electronegativity difference between the triple bond of carbon and nitrogen produces a dipole moment. surface in the gas phase using classical MD simulations based on the reactive force field [19]. They showed that the most stable adsorbates were bound to the substrate through bidentate coordination of their carboxylic groups to the available Ti sites of the surface. In some rare cases, tridentate coordination was also observed, in which the carboxylic anchoring is reinforced by direct contacts between the sulfur atom protruding out of the acceptor ring and the outer atoms of the anatase slab. These types of binding determined the orientation of the dye in relation to the surface (which was around 60° as already observed by other authors) [33,64,[START_REF] Agrawal | Modeling the effect of ionic additives on the optical and electronic properties of a dye-sensitized TiO2 heterointerface: absorption, charge injection and aggregation[END_REF].

During our MD sampling, we found that the D205 dye favored being faced towards the bulk IL phase. It has been shown that dye molecules anchored to TiO 2, are unlikely to undergo significant motions articulated from the anchoring point [START_REF] Vaissier | How mobile are dye adsorbates and acetonitrile molecules on the surface of TiO2 nanoparticles? A quasi-elastic neutron scattering study[END_REF]. The QENS data for two small dyes, isonicotinic acid and bis-isonicotinic acid, attached to TiO 2 nanoparticles via carboxylate groups, revealed that the dye moieties were immobile and did not rotate around the anchoring groups on the time-scale between around 10 picoseconds and a few nanoseconds [START_REF] Vaissier | How mobile are dye adsorbates and acetonitrile molecules on the surface of TiO2 nanoparticles? A quasi-elastic neutron scattering study[END_REF].

Adsorption and structure of free D205 dye at TiO2 interface

To examine the role of the covalent anchoring of the D205 dye to the Ti 2 O surface, we carried out a series of control MD simulations, in which a free dye molecule was adsorbed to the anatase surface by non-covalent interactions. It is well recognized that dye molecules bearing the protonated group -COOH has a low binding affinity to TiO 2 [3,4,[START_REF] Ma | Effect of functional group on photochemical properties and photosensitization of TiO2 electrode sensitized by porphyrin derivatives[END_REF], so that the carboxylic group of the D205 dye was modelled at the deprotonated form -COO -.

MD simulations demonstrated that a free dye, placed in a solvent at distances 0.4˗0.6 nm from the TiO 2 interface, does not diffuse to bulk solvent and favors adsorption onto the anatase surface. We observed that the adsorbed D205 molecule shows significant variations in adsorption modes, which also depends on the environment.

In BmimPF 6 , the D205 dye is found to be weakly bound to the anatase surface via the dissociated carboxylic group (Figures 3.17a

Conclusions

In this chapter, the effect of the various environments, such as 1-butyl-3methylimidazolium-based ionic liquids with hexapluorophophate and trifluoromethanesulfonate anions, and polar organic solvent, acetonitrile, on the microscopic structure and dynamics of a D205 dye bonded covalently into the It was determined that anchoring of the D205 to a TiO 2 nanoparticle induces an 860 cm -1 redshift of its long-wavelength band, as calculated by the TD-CAM-B3LYP/TZV approach using the PCM model of the acetonitrile solution. MD simulations in explicit AN revealed that the polar environment affects conformational dynamics of the TiO 2 -anchored D205 dye. Its peripheral diarylsubstituted ethylene moiety shows essential conformational flexibility, which plays a critical role in tuning electronic π-π conjugation across an indoline chromophore. These environment-induced conformational changes lead to solvatochromic shifts in UV-vis absorption spectra of the D205 dye, so that its long-wavelength absorption band is further redshifted up to 1900 cm -1 .

The analysis of the excited-state properties and electronic structure of a free D205 dye and a Ti 30 O 62 H 4 nanoparticle revealed that the alignment between HOMO and LUMO energy levels of the dye and the semiconductor suggests the favorable driving force for photoinduced dye-to-TiO 2 electron transfer. These findings were also supported by the analysis of the frontier MOs of a D205-Ti 30 O 62 H 4 conjugate, providing further evidence for favorable energetics for electron ejection from the dye to the TiO 2 conduction band. Finally, the combined MD/TDDFT approach outlined in this work opens up the computationally reasonable alternative for reliable predictions of sensitizing properties of other dye-TiO 2 structures in an explicit solution.

It was found that counterions of BmimPF 6 and BmimTFO, and molecules of polar AN interact strongly with polar TiO 2 anatase (001) surface to form the wellordered interfacial region composed of up to three solvent layers. The mass density distribution of the adsorbed BmimPF 6 and BmimTFO species allows estimating the thickness of the IL/TiO 2 interfacial region, which span a range from 0 up to 1.5 nm, respectively. Similarly, the TiO 2 /AN interfacial region was found to be in a range of 0.0-1. In perspectives, the work presented in this thesis can be the cause of these further developments:

• The methodology used for the developing new potential models for the D205 used in this work may be applied for the organic dye molecules, in particular indoline ones (e.g. D102, D149, etc) both in mixture and on TiO 2 .

• The local structure analysis of the MD simulation results in the terms of NNA can be used for the analysis D205 in other solutions, especially combination, which consist of IL and molecular solvent.

• The proposed method of finding "hits" with the further analysis can be applied for the wider library of different compounds with the aim to choose the molecules with requested properties, not only absorption spectra. It can be useful for the molecular design of new compound before its synthesis.

• MD simulation and TDDFT analysis of dyes on another surfaces or nanoparticles, like ZnO, in ternary systems containing IL-molecular solvent mixtures and dye molecules as electrolytes for dyesensitized solar cells. The microscopic environment, in terms of its polarity, viscosity and possible specific intermolecular interactions, can significantly alter the photodynamics of the dye molecule which is, in turn, reflected in the performance of the entire device.

3. 2 .

 2 Molecular Dynamics Simulation Setup .................................................... 3.2.1 DFT Structure of D205 Dye anchored to Ti 30 O 62 H 4 ............................... 3.2.2 MD force field for all the components .................................................... 3.2.3 MD simulation setup ............................................................................... 3.3. Results and Discussion .............................................................................. 3.3.1. Structure and Electronic Properties of Isolated Ti 30 O 62 H 4 Anatase Nanoparticle ..................................................................................................... 3.3.2. Structure of D205-Ti 30 O 62 H 4 Conjugate in Acetonitrile ........................ 3.3.3. Absorption Spectra and Solvatochromic Shift of D205 Dye ................. 3.3.4. Nature of Excited-States of D205-Ti 30 O 62 H 4 ........................................ 3.3.5. Mass density profiles of ionic liquids (ILs) and acetonitrile (AN) at TiO interface ............................................................................................................ 3.3.6. Microscopic structure of solvent molecules at TiO 2 interface ............... 3.3.7. Structure of surface-anchored D205 dye at TiO 2 interface .................. 3.3.8. Adsorption and structure of free D205 dye at TiO 2 interface .............. 3.4. Conclusions ............................................................................................. 3.5. References for Chapter 3 ......................................................................... Conclusions and perspectives .......................................................................... 115 Appendix A ..................................................................................................

  force field parametrization of indoline dye D205 with the further MD simulation in BmimBF 4 /AN mixture is provided in Chapter 2. Radial distribution function, corresponding nearest neighbor distribution functions and coordination numbers were used to characterize the environment around the donor, acceptor, bridge and chain moieties of the D205 dye in the ground and excited states. Chapter 3 is focused on behavior D205 on TiO 2 . There is the joint MD simulation and DFT study D205 dye anchored at the solid˗liquid interface of TiO 2 solvated with AN provided. As well the effect of the various environments, such as 1-butyl-3-methylimidazolium-based ionic liquids with hexapluorophophate and trifluoromethanesulfonate anions and acetonitrile on the microscopic structure and dynamics of a D205 dye bonded covalently into the TiO 2 -anatase surface were examined by using MD simulations in terms of radial distribution function.

Figure 1 . 1 .

 11 Figure 1.1. The structures of the analyzing dyes: methyl [1,1'-biphenyl]-4carboxylate (1), 1-methoxy-4-[(E)-2-phenylethenyl]benzene (2), 5-(4methoxyphenyl)-2-(thiophen-2-yl)-1,3-oxazole (3), N,N-dimethyl-4-[2-(4nitrophenyl)-1,3-oxazol-5-yl]aniline (4), 3-(2-phenyl-1,3-oxazol-5-yl)-2H-1benzopyran-2-one (5), 7-methoxy-3-(2-phenyl-1,3-oxazol-5-yl)-2H-1benzopyran-2-one (6), 8-methoxy-3-(2-phenyl-1,3-oxazol-5-yl)-2H-1benzopyran-2-one (7), 7-(diethylamino)-3-(2-phenyl-1,3-oxazol-5-yl)-2H-1benzopyran-2-one (8).

Figure 1 .

 1 3 depicts the sketch of the π-conjugated compound construction procedure. The active positions of these molecules are indicated on fig. 1.3, chosen from structural and chemical considerations. For the obtained systems, the geometry optimization was carried out utilizing molecular mechanics methods. Totally, the library at our disposal contains about four thousand entities, which are formed by various structural fragments, in various combinations.

Figure 1 . 3 .

 13 Figure 1.3. Molecular fragments which have been used for the construction of dyes.

Figure 1 . 4 .

 14 Figure 1.4. Scheme of formation of π-conjugated compounds from the initial set of fragments.

Figure 1 . 5 .

 15 Figure 1.5. Histogram of distribution of molecules by absorption wavelength (nm).

Figure 1 . 6 .

 16 Figure 1.6. Molecular fragments for automated generation of dyes.

Figure 1 . 7 .

 17 Figure 1.7. The promising structures of dyes I (a,b,c), II, III: E)-2-cyano-3-(5-(5-(6-(diphenylamino)-2-oxo-2H-chromen-3-yl)-1,3,4-oxadiazol-2-yl)thiophen-2-yl)acrylic acid (Ia, n=1); (E)-2-cyano-3-(5'-(5-(6-(diphenylamino)-2-oxo-2H-

Figure 2 . 1 .

 21 Figure 2.1. Molecule D205 (blackdonor part, redbridge, grey -acceptor part and purplechain), b -D205 with the selected atoms for further calculations

  Fig 2.1).a b

Figure 2 . 2 .

 22 Figure 2.2. Examples of PES profiles for C-C bond (left) and C-C-H angle (right), linesharmonic approximation from OPLS_AA[62]), UFF[53] and averaged from this work

Figure 2 . 3 .

 23 Figure 2.3. Point (1) correspond to the optimized Ground state of the dye, point

Figure 2 . 3 .

 23 Figure 2.3. Schematic representation of the dye excitation

Figure 2 . 4 . 2 . 3 .

 2423 Figure 2.4. HOMO (left) and LUMO (right) spectrum of D205 dye

Figure 2 . 5 .Figure 2 . 6 .

 2526 Figure 2.5. AN molecule

Figure 2 . 7 .

 27 Figure 2.7. Nearest neighbor radial distribution of F atoms of the anion, BF4 -, around the H atoms number 3 and 4 of the D205 dye. A, B, C, D refer to the subsequent calculations, lasting 5ns each. The average distribution of these distributions is also shown in this figure.

Figure 2 . 8 ,

 28 in the case of the nearest neighbor distribution of the oxygen atom, O 1 (see figure1for the numbering) around the H 4 of the cation Bmim + .

Figure 2 . 8 .

 28 Figure 2.8. Nearest neighbor radial distribution of O 1 atoms of the D205 dye around the H atom number 4 of the cation Bmim + . A, B, C, D refer to the subsequent calculations, lasting 5ns each.

Fig. 2 .

 2 Fig. 2.8 shows clearly a large statistical change in the shape of the 𝑔 1 (𝑟). Indeed, the first 5 ns calculations shows clearly that there are two preferential radial positions of the nearest neighbor O 1 of the D205 around the H 4 of the cation. A change in the ratio between the intensities of the two contribution is observed in the subsequent 5 ns calculation, in favor of the short distance contribution. This

Figure 2 .

 2 Figure 2.9 shows that the position of the first peak (or shoulder) in these 𝑔(𝑟) is located around 3 Å and the solvation shell expands up to 4.5 Å. Of note is when we changed the charge distribution to mick the excited state of the dye, the rising part at short distance of the 𝑔(𝑟) around the H atoms of the acceptor and bridge parts of the D205 shifts to lower distances in the excited state while the shape of the other 𝑔(𝑟) is almost similar in both states. We calculated then the corresponding coordination numbers that are displayed in Figure 2.10. This

Figure 2 . 10 .

 210 Figure 2.10. Coordination number (CN) of the N atom of AN around hydrogen atoms chosen in the donor H(43) , bridge H(2) , the acceptor H(3-4) and butyl chain H(24-25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

Figure 2 .

 2 Figure 2.11. Nearest neighbor radial distribution of the N atom of AN around hydrogen atoms chosen in the donor H(43) , bridge H(2) , the acceptor H(3-4) and butyl chain H(24-25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

Figure 2 . 12 .

 212 Figure 2.12. Radial distribution function of the F atom of the anion BF 4 -aroundhydrogen atoms chosen in the donor H(43) , bridge H(2) , the acceptor H(3-4) and butyl chain H(24-25) parts of the D205 dye. These functions were calculated in the ground and excited states.

Figure 2 .

 2 Figure 2.13. Coordination number (CN) of the F atom of the anion BF 4 -around hydrogen atoms chosen in the donor H(43) , bridge H(2) , the acceptor H(3-4) and butyl chain H(24-25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

Figure 2 . 14 .

 214 Figure 2.14. Nearest neighbor radial distribution function of the F atom of the anion BF 4 -around hydrogen atoms chosen in the donor H(43) , bridge H(2) , the acceptor H(3-4) and butyl chain H(24-25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

Figure 2 .

 2 Figure 2.16. Coordination number (CN) of the H(1) , H(4) , H(5) , H(m) and H (b) atoms of the cation BMIM + around electronegative atoms O(2) , O(3) , O(4) , S(3) of the dye D205 dye. These functions were calculated in the ground and excited states.

Figure 2 .

 2 Figure 2.18.Average distance as determined from the earest neighbor radial distribution functions of the the H(1) , H(4) , H(5) , H(m) and H (b) atoms of the cation Bmim + around electronegative atoms O(2) , O(3) , O(4) , S(3) of the D205 dye.

Figure 2 . 20 .

 220 Figure 2.20. The ionic liquid mole fraction dependence of the coordination number (CN) of the N atom of AN around hydrogen atoms chosen in the donor H(43) , bridge H(2) , the acceptor H(3-4) and butyl chain H(24-25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

Figure 2 . 21 .

 221 Figure 2.21. The ionic liquid mole fraction dependence of the nearest neighbor radial distribution function radial distribution function of the N atom of AN around hydrogen atoms chosen in the donor H(43) , bridge H(2) , the acceptor H(3-4) and butyl chain H(24-25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

  investigate the mole fraction dependence of the local structure of the anion around the H atoms of the D205, we calculated the radial distribution function, the corresponding coordination number, and the nearest neighbor distribution displayed in Figures 2.22

, 2 .

 2 23 and 2.24 respectively. These figures suggest that there is a small change in the radial distribution of the anion around the Hydrogen atoms of the dye.

Figure 2 .

 2 Figure 2.22.The ionic liquid mole fraction dependence of the radial distribution function of the F atom of the anion BF4 -around hydrogen atoms chosen in the donor H(43) , bridge H(2) , the acceptor H(3-4) and butyl chain H(24-25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

Figure 2 .

 2 Figure 2.23. The ionic liquid mole fraction dependence of the coordination number (CN) of the F atom of the anion BF 4 -around hydrogen atoms chosen in donor H(43) , bridge H(2) , the acceptor H(3-4) and butyl chain H(24- 25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

Figure 2 .

 2 Figure 2.24.The ionic liquid mole fraction dependence of the nearest neighbor radial distribution function F atom of the anion BF 4 -around hydrogen atoms chosen in donor H(43) , bridge H(2) , the acceptor H(3-4) and butyl chain H(24- 25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

Figure 2 . 25 .

 225 Figure 2.25. The ionic liquid mole fraction dependence of the radial distribution function (left) and coordination number (middle) and the nearest neighbor (right) of the H (b) atom of the cation Bmim + around electronegative atoms O(2) , O(3) , O(4) , S(3) of the D205 dye. These functions were calculated in the ground and excited states.

Figure 2 . 26 .

 226 Figure 2.26. The ionic liquid mole fraction dependence of the radial distribution function (left) and coordination number (middle) and the nearest neighbor (right) of the H (m) atom of the cation Bmim + around electronegative atoms O(2) , O(3) , O(4) , S(3) of the D205 dye. These functions were calculated in the ground and excited states.

Figure 2 . 27 .

 227 Figure 2.27. The ionic liquid mole fraction dependence of the radial distribution function (left) and coordination number (middle) and the nearest neighbor (right) of the H(1) atom of the cation Bmim + around electronegative atoms O(2) , O(3) , O(4) , S(3) of the D205 dye. These functions were calculated in the ground and excited states.

Figure 2 . 28 .

 228 Figure 2.28. The ionic liquid mole fraction dependence of the radial distribution function (left) and coordination number (middle) and the nearest neighbor (right) of the H(4) atom of the cation Bmim + around electronegative atoms O(2) , O(3) , O(4) , S(3) of the D205 dye. These functions were calculated in the ground and excited states.

Figure 2 . 29 .

 229 Figure 2.29. The ionic liquid mole fraction dependence of the radial distribution function (left) and coordination number (middle) and the nearest neighbor (right) of the H(4) atom of the cation Bmim + around electronegative atoms O(2) , O(3) , O(4) , S(3) of the D205 dye. These functions were calculated in the ground and excited states.

Figure 2 .

 2 Figure 2.30.The ionic liquid mole fraction dependence of the mean distances between N of AN and F atom of the anion BF4 -around hydrogen atoms chosen in donor H(43) , bridge H(2) , the acceptor H(3-4) and butyl chain H(24- 25) parts of the dye D205 dye. These functions were calculated in the ground and excited states.

  .1 right), mimicking structurally similar indoline D149 dye. It has been demonstrated that organic dyes used for DSSC devices were commonly anchored to TiO 2 by the carboxylic group. The latter prefers the chemical bonding to TiO 2 by the bidentate mode. The structure of the D205 dye, bonded to the surface of a Ti 30 O 62 H 4 anatase nanoparticle by the deprotonated carboxylic group -COO -, was fully optimized at the HSEH1PBE/TZV level, as shown in Figure 3.1(right). The proton dissociated from the carboxylic group was bound to the nearest uncoordinated Ti atom. This proton bonding allows keeping electroneutrality so that the total change of D205-Ti 30 O 62 H 4 conjugate was equal to zero. This procedure is commonly used to avoid DFT optimization of adsorption of openshell systems and charged dyes onto TiO 2[13, 19, 30,[32][33][34][35].

Figure 3 . 1 .

 31 Figure 3.1. The structure of a free D205 dye (left) and of the D205 dye anchored to a Ti 30 O 62 H 4 nanoparticle optimized at the HSEH1PBE/TZV theory level (right). For computational reasons, the N-octyl chain in the D205 dye was truncated to the ethyl one.

Figure 3 .

 3 2 shows a supercell organized into the two (001) layers with the size of 4.54 nm along x and y-axes, respectively.Most of MD simulations of bulk TiO 2 are commonly carried out with the empirical force-field (FF) developed by Matsui and Akaogi (MA)[40]. In the MA-FF, the non-bonded interaction parameters between Ti and O were determined by reproducing the crystal structure of various polymorphic forms of TiO 2 . The energetic interactions in TiO 2 were assumed to be completely noncovalent. The dispersion and repulsion interactions were represented by the Buckingham potential[40], whereas the electrostatic contributions were treated with the pairwise additive Coulomb potential.

Figure 3 . 2 .

 32 Figure 3.2. Chemical structure of a free D205 dye and the illustration of the D205 dye, anchored covalently to TiO 2 (001) slab by the carboxylic group, as shown with ball-and-stick representation. Recently, Zhou and co-workers [41] re-parameterized the MA parameters for TiO 2 by re-fitting the Buckingham potential parameters for TiO 2 by the 12-6

Figure 3 . 3 .

 33 Figure 3.3. Example of a MD simulation box, containing the D205-Ti 30 O 62 H 4 conjugate in explicit AN solution (left). The representative snapshot of the equilibrated D205-Ti 30 O 62 H 4 conjugate (right).

  To elucidate the role of the acetonitrile environment on the structure and conformational dynamics of a TiO 2 -anchored D205 dye, we carried out classical MD simulations. The Ti 30 O 62 H 4 nanoparticle was placed in the middle of the MD box so that its geometry was rigidly fixed during MD sampling. The D205 indoline dye was covalently bound to two uncoordinated Ti surface atoms of the Ti 30 O 62 H 4 nanoparticle, as shown in Figure 3.2, right. The geometry of the D205 dye was free to relax during MD sampling.

Figure 3 .

 3 Figure 3.4 shows a series of the D205-Ti 30 O 62 H 4 structures taken at different times along the MD trajectory. Since the coordinates of the Ti 30 O 62 H 4 nanoparticle were rigidly fixed, these MD snapshots represent the time-evolution of conformational dynamics of the D205 dye during the MD sampling. It can be noted that the most conformationally labile fragments of the D205 dye were localized at the peripheral diaryl-substituted ethylene moiety (Figure 3.4).

Figure 3 . 4 .

 34 Figure 3.4. Superpositions of a series of MD snapshots of a D205 dye anchored to the Ti 30 O 62 H 4 nanoparticle taken at 10 ns time intervals. It has been shown that the spectral properties of indoline-based dyes depend on the environment [8, 19, 58-61]. Therefore, we used a series of MD-equilibrated structures of the D205-Ti 30 O 62 H 4 conjugate to study how environment-induced conformational changes can further govern the electronic structure and a solvatochromic shift in absorption spectra of a D205 dye at the TiO 2 /AN interfaces. For computational reasons, acetonitrile molecules were omitted upon TDDFT calculations.

3 . 5 .

 35 found a small redshift of the long-wavelength band upon anchoring a D205 dye to the Ti 30 O 62 H 4 nanoparticle. For the HSEH1PBE/TZV-optimized D205-Ti 30 O 62 H 4 conjugate, the two long-wavelength absorption bands were found at 328 nm and 439 nm, respectively. Thus, the long-wavelength band of the D205 dye was redshifted up to 860 cm -1 upon the bonding to TiO 2 . The averaged absorption spectrum of a D205 dye in explicit acetonitrile solution was generated from the six MD snapshots of the D205-Ti 30 O 62 H 4 conjugate taken from classical MD trajectory, some of which are shown in Fig. It can be noted an increasing redshift of the absorption maximum upon going from vacuum to the acetonitrile solution is in accordance with experimental results [58]. For the MD-relaxed D205-Ti 30 O 62 H 4 conjugate, the absorption bands were shifted to 339 nm and 479 nm, respectively. Therefore, the solvatochromic shift of the D205 at the TiO 2 /AN interface, estimated as the energy difference between the maximum of the long-wavelength absorption band of the DFToptimized and MD-relaxed D205-Ti 30 O 62 H 4 conjugates, was found to be 1900 cm -1 .

Figure 3 . 5 .

 35 Figure 3.5. Comparison of UV-vis absorption spectra for DFT-optimized and MD-relaxed D205-Ti 30 O 62 H 4 conjugate, as predicted by TD-CAM-B3LYP/TZV calculations, demonstrating a redshift of the long-wavelength absorption band in the acetonitrile solution. An analogous spectrum corresponding to the free D205 dye is also provided for comparison.

2 ; ( 2 )

 22 The HOMO energy level of a dye must be established below the redox couple of the I -/I 3 -electrolyte; (3) To enhance efficient charge injection, the energy level of the lowest unoccupied molecular orbital (LUMO) of a dye must be situated above, and close to, the conduction band of TiO 2 .

Figure 3 .

 3 Figure 3.6 compares the Kohn-Sham (KS) orbital energy diagram of a free D205 dye and the Ti 30 O 62 H 4 nanoparticle. The energy levels of HOMO and LUMO of the dye were calculated to be -7.9278 eV and -1.7712 eV, respectively, while the corresponding energy for the MOs of Ti 30 O 62 H 4 was found to be -9.5477 eV and -2.1497 eV by the TD-CAM-B3LYP/TZV calculation. Thus, the HOMO-LUMO alignment of the dye and TiO 2 suggests the favorable migration direction for a photoexcited electron from the D205 donor toward the Ti 30 O 62 H 4 acceptor.

Figure 3 . 6 .

 36 Figure 3.6. Orbital energy diagram of the Ti 30 O 62 H 4 nanoparticle and a free D205 dye calculated by the TD-CAM-B3LYP/TZV(PCM-acetonitrile) method.

Figure 3 .

 3 Figure 3.7 shows the frontier MOs and their energy in eV for the representative MD-equilibrated D205-Ti 30 O 62 H 4 conjugate calculated by the TD-CAM-B3LYP/TZV method using the PCM model for the acetonitrile solution. As can be seen, in the D205-Ti 30 O 62 H 4 conjugate, the HOMOs are predominantly located at the D205 dye, while the LUMOs are mainly delocalized over the Ti 30 O 62 H 4 nanoparticle. The calculated MO alignment in the D205-Ti 30 O 62 H 4 conjugate illustrates how an electron is transferred from a D205 moiety towards the TiO 2 semiconductor. The TD-DTF calculations clearly show that when the dye is adsorbed onto the TiO 2 surface, the HOMOs of the D205-Ti 30 O 62 H 4 conjugate are exclusively localized on the dye moieties. The electronic π-π coupling between the dye and TiO 2 is weak due to the presence of the saturated methylene group CH 2 in the anchoring moiety. In contrast, the LUMOs of the D205-Ti 30 O 62 H 4 conjugate are delocalized over the energy levels of the Ti 30 O 62 H 4 nanoparticle, which should allow electrons to transfer efficiently from the dye to the semiconductor.

Figure 3 . 7 . 3 . 3 . 5 .

 37335 Figure 3.7. Frontier molecular orbitals of a MD-equilibrated D205-Ti 30 O 62 H 4 conjugate calculated by the TD-CAM-B3LYP/TZV(PCM-acetonitrile) method. The MO energy level in eV is given in the brackets.

Figure 3 . 8 .

 38 Figure 3.8. Molecular structures of the studied ionic liquids and acetonitrile AN (a). The snapshots of the equilibrated rectangular simulation cells containing 1-butyl-3-methylimidazolium hexafluorophosphate BmimPF 6 (b) and trifluoromethanesulfonate BmimTFO (c), and AN (d) equilibrated at the TiO 2 anatase interface.

Figure 3 . 9 .

 39 Figure 3.9. shows MD snapshots and the mass density profiles calculated along the z-axis of the simulated box normal to the TiO 2 slab (Fig. 3.9b) solvated

1. 5

 5 nm, the bulk mass density of the studied ILs agrees well with the experimental values. The experimental density values of BmimPF 6 and BmimTFO are reported to be 1368 and 1297 kg/m 3 at 298.15 K, respectively [50, 73-75]. Moreover, these simulated mass densities of the ILs agree well with the density of the pure ILs, simulated for the NPT ensemble (Figures 3.9b and 3.9d).

Figure 3 . 9 .

 39 Figure 3.9. The comparison of the MD estimated mass density of pure BmimPF 6 , BmimTFO, and AN and the mass density of these solvents at the TiO 2 interface. Panels a, c, and e show snapshots of BmimPF 6 /TiO 2 , BmimTFO/TiO 2 , and AN/TiO 2 interfaces. Some solvent molecules located at the two ordered interfacial layers are highlighted. Comparison of the MD estimated and experimental mass densities of BmimPF 6 (b), BmimTFO (d), and AN (f), respectively. The mass density of pure ILs and AN were taken from the control MD simulations carried out at the NPT ensemble. The dotted lines represent the experimental mass density of the bulk ILs and AN, respectively.

  identify preferred absorption mode, the RDF's plots between some selected atoms of either BmimPF 6 or BmimTFO and all titanium and oxygen atoms of the anatase slab were calculated as shown in Figs. 3.10 and 3.11, respectively.

Figure 3 . 10 .

 310 Figure 3.10. The RDFs plots calculated between the selected atoms of BmimPF 6 and the Ti and O (OT) atoms of TiO 2 , respectively.

  .4. summarizes the RDF positions of the first two coordination shells for the BmimPF 6 and BmimTFO ˗ composed systems.

Figure 3 . 11 .

 311 Figure 3.11. The RDFs plots calculated between the selected atoms of BmimTFO and the anatase Ti and O (OT) atoms, respectively.As can be noted from Table3.4., the strongest interactions occur between

Figure 3 . 12 . 3 . 3 . 7 .

 312337 Figure 3.12. The RDFs plots calculated between AN heavy atoms and the anatase Ti (a) and OT (b) atoms, respectively.

Figure 8

 8 Figures 3.13e-f show the typical conformations of the TiO 2 -anchored D205 dye in AN. The conformation of the D205 dye was similar in many aspects to those observed in the environment of BmimPF 6 and BmimTFO (Figures 3.13ad). The rhodanine rings of the D205 dye were also located within the two interfacial layers of adsorbed AN molecules.

Figure 3 . 13 .

 313 Figure 3.13. The MD snapshots of a D205 dye anchored to the TiO 2 anatase (100) surface and solvated with BmimPF 6 (a-b), BmimTFO (c-d), and AN (e-f) are given in positions of face (right) and sideways (left), respectively.The D205 location at the TiO 2 interface is also estimated by averaging its mass density along the z-axis of the MD box. Figure3.14 shows the overlap of

Figure 3 .

 3 14 shows the overlap of the D205 mass density with the density plots of the studied ILs and AN. It can be seen that the D205 density spans a range from 0.35 nm up to 2.2 nm. Therefore, these data also suggest that the dye moiety of the TiO 2 -anchored D205 favors residing within the IL/TiO 2 interfacial region. These findings are crucial for the rational design of DSSC devices because, from the experimental point of view, the physicochemical properties of the IL/TiO 2 interfacial layers differ from those in bulk in terms of local density, viscosity, polarity, etc.

Figure 3 . 14 .

 314 Figure 3.14. The comparison of the mass density distribution of the TiO 2anchored D205 dye (shaded area) with the density of BmimPF 6 (a), BmimTFO (b), and AN (c), respectively. The mass densities were calculated along the z axis of the TiO 2 slab (see Fig. 3.3 for the axes defintion). For clarity, the D205 density was scaled by a factor of 20.The structure and conformational dynamics of the TiO 2 -anchored D205 dye play an essential role in operating a dye sensitizer for the dye˗to˗TiO 2 electron transfer process. Figure3.15 shows overlaps of initial and final conformations of

Figure 3 .

 3 15 shows overlaps of initial and final conformations of a D205 dye taken during MD simulations in BmimPF 6 and BmimTFO, respectively. One can notice that despite the rigid chemical anchoring to the TiO 2 surface, the D205 moiety remains conformational labile, and flexible. The most conformationally labile fragments of the D205 dye were localized at the octyl tail and peripheral diaryl-substituted ethylene moiety (Figure 3.15).

Figure 3 . 15 .

 315 Figure 3.15. The conformational dynamics of the TiO 2 -anchored D205 dye solvated in BmimPF 6 (a) and BmimTFO (b), respectively. The MD snapshots are shown for the initial structure (green) and the final conformation (blue) of D205 taken at the end of the 50 ns sampling. To characterize the D205 location at the TiO 2 /BmimTFO interface quantitatively, we plotted time-traces of the z-distance of some selected atoms of the dye from the TiO 2 surface (Figure 3.16). The time-traces of the z-distance of some selected atoms of D205 from the TiO 2 surface reveal that the dye molecule retains the well-defined geometry during the long-scale MD sampling. The sulfur atom of the thiazole moiety is found to be the closest to the TiO 2 slab. The location of the nitrogen atom of imidazole is around 1.09 nm, and the carbon atom of the peripheral ethenyl group is around 1.63 nm from the TiO 2 surface.

Figure 3 . 16 .

 316 Figure 3.16. The time-traces of the z-axis position of some selected atoms of the TiO 2 -anchored D205 dye plotted for the last 20 ns of the MD sampling. The system was solvated in BmimTFO. For clarity, a MD snapshot of TiO 2 -bound D205 is also shown.

  -b). The D205 chromophore is centered above the BmimPF 6 /TiO 2 interface. In contrast, the surface binding of the dye molecule occurs due to the adsorption of the rhodanine moiety flat to the BmimTFO/TiO 2 and AN/TiO 2 , respectively (Figures 3.17c-f).The dissociated carboxylic group does not interact with the anatase surface and is faced towards bulk solvent. In addition, the non-polar aliphatic octyl chain of the rhodanine moiety favors interactions with solvent molecules instead of the TiO 2 binding, as seen in Figures 3.17d and 3.17f.These findings suggest that, in the absence of the chemical anchoring of a dye onto the TiO 2 surface, the dye adsorption is characterized by significant heterogeneity in bound conformations, the degree, and the strength of dye˗to˗TiO 2 interactions. Moreover, the adsorption modes could be altered by the physicochemical properties of the environment. Altogether, MD simulations suggest that the appropriate chemical anchoring of the D205 dye to TiO 2 is crucial for its application in DSSC devices. The physical adsorption of a free dye may strongly affect the photovoltaic performance of a DSSC device due to broad variations of the distance and orientation between a dye and a TiO 2 surface, which in turn play a decisive role in the performance and rate of dye-to-semiconductor electron injection[3].

Figure 3 . 17 .

 317 Figure 3.17. The MD snapshots of a free D205 dye adsorbed onto the TiO 2 anatase (100) surface and solvated with BmimPF 6 (a-b), BmimTFO (c-d), and AN (e-f) are shown with face (right) and sideways (left) orientations, respectively.

TiO 2 -

 2 anatase surface were examined by using classical molecular dynamics simulations. The structure and conformational dynamics of the D205 dye bonded covalently into the small-sized Ti 30 O 62 H 4 -anatase nanoparticle were examined by using classical MD simulations combined with TDDFT excited-state calculations. It was investigated conformational dynamics and electronic properties of the D205 dye anchored to the TiO 2 anatase nanoparticle immersed in the AN solution as a representative electrolyte for DSSC devices. TD-CAM-B3LYP calculations of a series of MD-relaxed D205-Ti 30 O 62 H 4 conjugates were used to take into account the role of the AN environment on conformational heterogeneity of the dye chromophore.

  3 nm, respectively. MD simulations revealed that the densely ordered layers of the adsorbed IL cations and AN molecules shield the direct contact of the TiO 2 -anchored D205 chromophore with the TiO 2 surface. The rhodanine ring of the D205 is located within the first two adsorption layers of the ILs and AN. Despite the rigid chemical anchoring of D205 to the TiO 2 surface, the dye moiety remained conformational labile, and flexible. The most conformationally labile fragments of the D205 dye were localized at the octyl tail and peripheral diaryl-substituted ethylene moiety. Control MD simulations of adsorption of a free dye revealed that the D205 dye molecule prefers binding onto TiO 2 . Its adsorption is characterized by significant heterogeneity in bound conformations and the strength of dye˗to˗TiO 2 interactions. Therefore, the lack of the appropriate anchoring of the D205 dye may lead to a broad distribution of the distance and orientation between a dye and a TiO 2 surface, which may decrease the overall photovoltaic performance of a dye˗sensitized solar cell device. Finally, the TDDFT studies and MD simulation of D205 on the Ti 30 O 62 H 4anatase nanoparticle surface were performed. It was determined that anchoring of the D205 to a TiO 2 nanoparticle induces an 860 cm -1 redshift of its longwavelength band, as calculated by the TD-CAM-B3LYP/TZV approach using the PCM model of the acetonitrile solution. MD simulations in pure AN revealed that the polar environment affects conformational dynamics of the TiO 2 -anchored D205 dye. These environment-induced conformational changes effect to solvatochromic shifts in UV-vis absorption spectra of the D205 dye. The analysis of the excited-state properties and electronic structure of a free D205 dye and a Ti 30 O 62 H 4 nanoparticle revealed that the alignment between HOMO and LUMO energy levels of the dye and the semiconductor suggests the favorable driving force for photoinduced dye-to-TiO 2 electron transfer. It was found that counterions of BmimPF 6 and BmimTFO, and molecules of polar AN interact strongly with polar TiO 2 anatase (001) surface to form the well-ordered interfacial region composed of up to three solvent layers. MD simulations revealed that the densely ordered layers of the adsorbed IL cations and AN molecules shield the direct contact of the TiO 2 -anchored D205 chromophore with the TiO 2 surface. The most conformationally labile fragments of the D205 dye were localized at the octyl tail and peripheral diaryl-substituted ethylene moiety. Control MD simulations of adsorption of a free dye revealed that the D205 dye molecule prefers binding onto TiO 2 .

  ..... 119

		LIST OF ABBREVIATIONS
	ADF	angular distribution function
	AN	acetonitrile
	Bmim +	1-n-butyl-3-methylimidazolium
	DFT	density functional theory
	DSSC	dye sensitized solar cells
	HOMO	highest occupied molecular orbital
	IL	ionic liquid
	LJ	Lennard-Jones
	LR.	Linear response
	LUMO	lowest unoccupied molecular orbital
	MD	molecular dynamics
	MO	molecular orbital
	NNA	nearest neighbor aproach
	PBC	periodic boundary conditions
	PES	potential energy surface
	PME	Particle-Mesh-Evald
	RDF	radial distribution function
	RTIL	room temperature ionic liquid
	SS	State specific
	TDDFT	time dependent density functional theory
	TFO -	trfluoromethylsulfonate, triflate, CF3SO3 -

Table 1

 1

	.1. The TDDFT method is represented by two functionals -B3LYP and
	CAM-B3LYP, which, according to the previous studies, give certain limits of
	theoretical estimates of absorption spectra. Results of calculations corresponding

Table 1 .4. Calculation

 1

			of the absorption wavelength (nm) of π-systems taking
	into account the effects of the environment within the framework of the LR-
	model and SS model in acetonitrile. For TDDFT calculations, 6-31++G(d,p)
			basis set was used.
	Acetonitrile	CAM-B3LYP	M06-2X		Experimantal
		LR	SS	LR	SS
	5	325	315	316	307	323; 339, 342
	6	330	326	334	328	362
	8	373	380	370	378	433

Table 1 .5. The

 1

	The PBE1PBE

.2.3. Spectral properties of D205 With

  the goal to check the proposed approach, the indolinederivative D205 dye was chosen (figure1.2).

Figure 1.2. Structural formula of dye D205 On

  the table 1.7. the calculated the absorption wavelength maximum on different level of theory and solvation model are presented.

	Table 1.7. Calculation of the absorption wavelength (nm) of D205 in vacuum
	and in AN with LR and SS approaches
	Level of theory	Wavelength, nm Level of theory Wavelength, nm
	CAM-B3LYP/6-31G	518	CAM-B3LYP/6-	386; 541
			31++G(d,p)
	CAM-B3LYP/6-	527	CAM-B3LYP/6-	432; 576
	311G		31++G(d,p) in
			AN LR
	CAM-B3LYP/6-	510	CAM-B3LYP/6-	347; 450
	31G(d,p)		31++G(d,p) in
			AN SS
	CAM-B3LYP/6-	378; 540
	311++G(d,p)

Table 1 . 9 .

 19 Absorption wavelengths (λ, nm) and oscillator strength of the "hits" molecules obtained at the M06-2X/6-31++G(d,p) level of theory

	Structure	n	λ(nm)	Oscillator strength
	Ia	1	397	1.856
	Ib	2	410	2.201
	Ic	3	424	2.556
	II		418	2.02
	III		376	1.768

Table 2 .

 2

	Comment

1. Comparison of the dipole moment for the different states of dye No Geometry Singlet μ x , D μ y , D μ z , D μ, D intramolecular parameters in excited state use the partial charges, obtained on the Excited state, optimized on ground state level. In fig. 2.4 the HOMO and LUMO states of molecule D205 is provides. It is shown that the electron density moves

from donor to acceptor part. The -bridge part has the electron density in both cases, while no density observed around chain. This illustrate, why the calculation of ground and excited state is so important for local structure, so it is necessary to create the force field, which can reproduce well the properties of both states.

Table 3 . 1 .

 31 Non-bonded 12-6 LJ potential parameters for TiO 2 anatase[41].

	Interactions	ε ij (kJ/mol)	σ ij (nm)
	Ti-Ti	2.427	0.220
	Ti-O	1.757	0.272
	O-O	1.297	0.324

Table 3 . 2 .

 32 Electronic properties of HSEH1PBE/TZV-optimized Ti 30 O 62 H 4 nanoparticle calculated in the PCM-acetonitrile solution with various TDDFT

	levels.

Table 3 . 3 .

 33 Electronic properties of MD-relaxed D205-Ti 30 O 62 H 4 conjugate calculated by TD-CAM-B3LYP/TZV(PCM-acetonitrile).

	Transition	Excitation	Oscillator	Configuration
		energy (eV, nm)	strength (f)	(HOMO = H, LUMO =
				L)
	S 0 →S 1	2.5657 (483)	1.1342	H→L+5 (0.59844)
				H-1→L+5 (0.27803)
	S 0 →S 2	3.3330 (372)	0.0000	H→L (0.68975)
	S 0 →S 3	3.4223 (362)	0.0005	H→L+3 (0.47701)
				H→L+2 (0.24271)
				H→L+1 (-0.27776)
	S 0 →S 4	3.4414 (360)	0.0001	H→L+1 (0.63685)
				H→L+2 (0.19562)
	S 0 →S 5	3.4670 (357)	0.0021	H→L+3 (0.60009)
				H→L+2 (0.30231)
	S 0 →S 6	3.4887 (355)	0.0083	H-3→L+5 (-0.49974)
	S 0 →S 7	3.5571 (348)	0.8574	H-2→L+5 (0.32725)
				H-1→L+5 (0.37590)
				H→L+5 (-0.11016)

Nature of Excited-States of D205-Ti30O62H4

Table 3 . 4 .

 34 The peak positions of the first two coordination shells based on RDFs between TiO 2 and BmimPF 6 and BmimTFO.

	Interactions	BmimPF 6	BmimTFO
		First peak	Second	First peak	Second peak
		(nm)	peak (nm)	(nm)	(nm)
	Ti -N1	0.56	0.66	0.55	0.76
	Ti -N2	0.55	0.75	0.55	0.75
	Ti -C4	0.49	0.61	0.49	0.61
	Ti -C5	0.52	0.59	0.54	0.75
	Ti -C8	0.49	0.60	0.49	0.58
	Ti -F	0.40	0.60	-	-
	Ti -P	0.51	0.71	-	-
	Ti -S	-	-	0.48	0.66
	Ti -O	-	-	0.40	0.58
	Ti -C	-	-	0.53	0.62
	Ti -F	-	-	0.50	0.71
	OT -N1	0.55	0.79	0.55	0.75
	OT -N2	0.49	0.58	0.50	0.56
	OT -C4	0.49	0.68	0.49	0.68
	OT -C5	0.49	0.57	0.49	0.55
	OT -C8	0.49	0.69	0.49	0.75
	OT -F	0.42	0.59	-	-
	OT -P	0.52	0.72	-	-
	OT -S	-	-	0.48	0.66
	OT -O	-	-	0.43	0.58
	OT -C	-	-	0.50	0.65
	OT -F	-	-	0.42	0.71

Table A1 .

 A1 Comparison of the partial charges in ground (S0) and excited (S1) states of dye D205, ordered by the biggest S1-S0 difference.

	#	Atom	S0	S1	S1-S0
	C		0,203947	0,287585	0,084
	C		-0,03464	0,0441	0,079
	S		-0,14419	-0,09067	0,054
	N		-0,48918	-0,44391	0,045
	C		-0,04915	-0,00975	0,039
	C		-0,00579	0,028249	0,034
	C		0,137303	0,170258	0,033
	C		-0,20767	-0,18552	0,022
	S		-0,10966	-0,08969	0,020
	C		-0,26916	-0,25622	0,013
	C		-0,07814	-0,06755	0,011
	C		-0,20866	-0,19847	0,010
	H		0,128439	0,13836	0,010
	H		-0,02514	-0,01551	0,010
	H		-0,03491	-0,02543	0,009
	H		0,050379	0,058419	0,008
	C		0,011194	0,019206	0,008
	H		0,12574	0,13328	0,008
	C		-0,22657	-0,21904	0,008
	C		0,000269	0,007778	0,008
	C		-0,1029	-0,09585	0,007
	C		-0,05387	-0,04731	0,007
	C		-0,13909	-0,13308	0,006
	H		0,014305	0,019886	0,006
	H		0,007441	0,012885	0,005
	C		-0,12045	-0,11502	0,005
	H		0,051524	0,056608	0,005
	H		0,117092	0,122144	0,005
	H		0,00569	0,010668	0,005
	H		0,146085	0,150659	0,005
	H		-0,00196	0,002561	0,005
	H		0,010867	0,015243	0,004
	H		0,085242	0,089416	0,004
	C		-0,15819	-0,15456	0,004
	H		-0,00382	-0,00049	0,003
	C		-0,11519	-0,11238	0,003
	H		0,109824	0,112317	0,002
	C		-0,06614	-0,06372	0,002
	H		0,125807	0,128161	0,002
	H		-0,0891	-0,08707	0,002

Table A1 .

 A1 Comparison of the partial charges in ground (S0) and excited (S1) states of dye D205, ordered by the biggest S1-S0 difference.

	C	-0,1065	-0,10454	0,002
	C	-0,16721	-0,16528	0,002
	H	0,112792	0,114525	0,002
	H	0,094682	0,096385	0,002
	H	0,106676	0,108329	0,002
	C	-0,09283	-0,09119	0,002
	H	0,433233	0,434722	0,001
	H	0,024108	0,02558	0,001
	H	0,054642	0,056018	0,001
	H	0,104312	0,105591	0,001
	H	0,101544	0,102703	0,001
	H	0,116355	0,117038	0,001
	H	0,014838	0,015163	0,000
	C	0,187739	0,188005	0,000
	C	-0,27307	-0,27283	0,000
	H	-0,01921	-0,01925	0,000
	H	0,145286	0,14506	0,000
	H	0,010347	0,009894	0,000
	H	0,05687	0,05636	-0,001
	C	0,339361	0,338851	-0,001
	H	0,048015	0,047444	-0,001
	H	-0,05309	-0,05368	-0,001
	C	0,289936	0,28917	-0,001
	H	0,021032	0,01994	-0,001
	C	-0,12079	-0,12193	-0,001
	H	0,021422	0,020145	-0,001
	H	0,011261	0,009857	-0,001
	H	0,058312	0,056833	-0,001
	C	-0,11389	-0,11565	-0,002
	H	0,064727	0,062862	-0,002
	H	0,022929	0,021029	-0,002
	H	0,021782	0,01955	-0,002
	H	0,017379	0,014833	-0,003
	C	0,299009	0,296303	-0,003
	C	0,138952	0,136082	-0,003
	O	-0,64167	-0,64509	-0,003
	C	-0,19834	-0,20177	-0,003
	C	-0,20871	-0,21225	-0,004
	C	-0,06078	-0,06453	-0,004
	C	0,021697	0,0168	-0,005
	C	-0,08113	-0,08702	-0,006
	H	0,057466	0,05045	-0,007
	C	-0,13511	-0,14217	-0,007
	H	0,03899	0,031182	-0,008
	C	0,172488	0,164611	-0,008

Table A1 .

 A1 Comparison of the partial charges in ground (S0) and excited (S1) states of dye D205, ordered by the biggest S1-S0 difference.

	C	0,816254	0,807586	-0,009
	O	-0,58668	-0,5962	-0,010
	H	0,203241	0,193243	-0,010
	C	0,051518	0,039949	-0,012
	C	-0,11215	-0,12373	-0,012
	C	0,42904	0,415627	-0,013
	C	0,283984	0,269342	-0,015
	H	0,088574	0,073331	-0,015
	N	-0,12454	-0,14024	-0,016
	C	-0,16546	-0,18615	-0,021
	H	0,08638	0,064832	-0,022
	C	0,187893	0,163937	-0,024
	C	-0,26213	-0,28971	-0,028
	C	0,56501	0,535086	-0,030
	O	-0,45013	-0,4813	-0,031
	S	-0,25482	-0,28877	-0,034
	C	0,513482	0,477705	-0,036
	O	-0,50036	-0,53967	-0,039
	N	-0,12189	-0,16521	-0,043
	C	-0,18068	-0,30037	-0,120

ACKNOWLEDGEMENTS

My deepest gratitude is indeed due to my supervisors, Professor Abdenacer IDRISSI, Professor Oleg KALUGIN, Professor François-Alexandre MIANNAY for their patience, scientific

Conclusions and perspectives

In this thesis the theoretical investigation of dyes was performed. In the case, when it is necessary to carry out virtual screening, the PPP/CIS and ZINDO methods can provide the necessary information at the cost of a small computer resources. Among the all TDDFT method, the results of the B3LYP and CAM-B3LYP methods give the upper and lower limits of the absorption wavelength, respectively. However, the results, obtained with the method M06-2X is still reliable and close to B3LYP. Additionally, to this, the absorbance spectrum of D205 dye was calculated on the different level of theory. It was shown that only the atomic basis set with the diffusion functions can reproduce the experimental shape of the spectrum. As a result of virtual screening 6 new dye molecules were proposed as a promising candidate for the further synthesis and using in DSSC.

In the Chapter 2 with the quantum-chemical method the force field for dye D205 was parametrized. It was shown that there is a big change in the HOMO-LUMO structure and due to this it is necessary to calculate and analyze both ground and excited states. Electron density move from donor part in ground state to acceptor in excited state. The difference in dipole moments and consequently partial charges between excited state with geometry, optimized in ground state and excited state with geometry relaxation is tiny, so it is possible to use geometry, obtained on ground state. From the result of MD simulation D205 in the mixture of BmimBF 4 and AN for the range of molar fraction 0-1 there is noticeable, that interaction between electronegative atoms of D205 and selected atoms H of Bmim is weaker than between cation and anion of IL. However, these atoms can interact with AN. It was presented that in the case of acetonitrile the difference between local structure in ground and excited states observed, while for the pure BmimBF 4 the shape of rdf for the 1 st neighbor is similar. For the chain and donor part the equiprobable interactions occurs near the mole fraction 0.4, while in 0.5 the distance between D205 and BF 4 less than for D205 and acetonitrile. For the bridge part in 0.5 the interaction with AN stronger, that with anion.

Appendices