Anna Grabarz

Beata Jędrzejewska

Agnieszka Skotnicka

N Arul Murugan

Filip Patalas

Wojciech Bartkowiak

Denis Jacquemin
email: denis.jacquemin@univ-nantes.fr

Borys Ośmiałowski
email: borys.osmialowski@umk.pl

The Impact of the Heteroatom in a Five-Membered Ring on the Photophysical Properties of Difluoroborates

Keywords: TD-DFT, difluoroborates, functionalization, charge transfer

A series of novel BF 2 complexes, bearing a five-membered heterocyclic ring (with X=NMe, O, and S), were synthesized and characterized with a focus on the influence of atom exchange on the photophysical properties of both unsubstituted and dimethylamino derivatives. The experimental results show that the optical spectra substantially differ in both sets of dyes. In particular, the dimethylamino series are more strongly affected by heteroatom substitution, i.e., the insertion of X=O or X=S in lieu of X=NMe causes substantial bathochromic shifts of the absorption and emission bands, as well as marked changes in their topologies. In contrast, the optical spectra of the unsubstituted compounds undergo only relatively small redshifts, and no variation of band shapes is observed. Moreover, the measured absorption spectra of the unsubstituted compounds bearing X=NMe and X=O are almost identical. Interestingly, the fluorescence yields of the dimethylamino derivatives are much larger (up to one order of magnitude) than those of the corresponding unsubstituted compounds. The experimental analyses are supported by state-of-the-art quantum chemistry calculations, which satisfactorily reproduced the experimental trends and provided further insights into the observed optical signatures.

Introduction

Structure-optical property relationships are now established for many classes of dyes, and one can often safely predict the evolution of absorption spectra with chemical substitution. This is more difficult to achieve for emission, which is a key challenge in view of its use in many biomedical imaging applications. Indeed, there is a need to design new fluorescent probes, effectively exhibiting sensitivity to environmental changes, physiological conditions and/or to specific interactions ongoing in biomacromolecules. To achieve these goals, countless syntheses have been carried out to functionalize fluorophores and tune their emissive properties, which can be achieved through standard variation of the substituent(s) [START_REF] Kaya | Synthesis and characterization of fluorescent polyphenol species derived from methyl substituted aminopyridine based schiff bases: The effect of substituent position on optical, electrical, electrochemical, and fluorescence properties[END_REF][2][START_REF] Chen | Esipt fluorescent dyes with adjustable optical properties: Substituent and conjugation effects[END_REF][START_REF] Yoshida | Fluorescence enhancement of tetraazaporphyrins by the use of bulky substituent effects[END_REF][START_REF] Tsai | Fluorescence enhancement of unconstrained gfp chromophore analogues based on the push-pull substituent effect[END_REF][START_REF] Wang | Substituent effect: A new strategy to construct a ratiometric fluorescent probe for detection of al 3+ and imaging in vivo[END_REF], benzannulation [START_REF] Uppal | Synthesis, computational modeling, and properties of benzo-appended bodipys[END_REF][START_REF] Ni | Benzene-fused bodipys: synthesis and the impact of fusion mode[END_REF][START_REF] Kato | A series of π-extended thiadiazoles fused with electron-donating heteroaromatic moieties: Synthesis, properties, and polymorphic crystals[END_REF][START_REF] Golden | Boron dipyridylmethene (dipyr) dyes: Shedding light on pyridine-based chromophores[END_REF][START_REF] Kim | Synthesis of fluorescent indazoles by palladiumcatalyzed benzannulation of pyrazoles with alkynes[END_REF][START_REF] Zhang | A benzothiadiazole-based fluorescent sensor for selective detection of oxalyl chloride and phosgene[END_REF], elongation of the π-conjugated spacer separating the donor and acceptor moieties in push-pull systems [START_REF] Li | Efficient structural modification of triphenylamine-based organic dyes for dye-sensitized solar cells[END_REF][START_REF] Miguel | Triazole bridges as versatile linkers in electron donor-acceptor conjugates[END_REF][START_REF] Fihey | Multiphotochromic molecular systems[END_REF][START_REF] Grabarz | The influence of the π-conjugated spacer on photophysical properties of difluoroboranyls derived from amides carrying a donor group[END_REF][START_REF] Alcaide | Electronic and functional scope of boronic acid derived salicylidenehydrazone (bashy) complexes as fluorescent dyes[END_REF], or introduction of additional chromophores into their structure [START_REF] Flors | Energy and electron transfer in ethynylene bridged perylene diimide multichromophores[END_REF][START_REF] Lv | Polybrominated bophy dyes: Synthesis, reactivity, and properties[END_REF][START_REF] Wei | Multiple yet controllable photoswitching in a single aiegen system[END_REF]. However, the addition of several substituents may lead to undesired effects such as the decrease of the fluorescence quantum yield (Φ fl ) inherent to flexible side group that enhance the efficiency of non-radiative pathways. This explains why the most potent fluorescent molecules remain "simple and rigid" derivatives, e.g., xanthenes [START_REF] Chen | Fluorescent chemosensors based on spiroringopening of xanthenes and related derivatives[END_REF][START_REF] Astakhova | Interfacing click chemistry with automated oligonucleotide synthesis for the preparation of fluorescent dna probes containing internal xanthene and cyanine dyes[END_REF][START_REF] Pandey | A highly specific 'turn-on' fluorescent detection of mg 2+ through a xanthene based fluorescent molecular probe[END_REF][START_REF] Ma | Design strategies and progress on xanthene-based fluorescent probe for metal ions[END_REF][START_REF] More | Characteristics of new bioreductive fluorescent probes based on the xanthene fluorophore: Detection of nitroreductase and imaging of hypoxic cells[END_REF], cyanines [START_REF] Yuan | Far-red to near infrared analyte-responsive fluo-rescent probes based on organic fluorophore platforms for fluorescence imaging[END_REF][START_REF] Cheng | A near-infrared fluorescent probe for selective detection of hclo based on se-sensitized aggregation of heptamethine cyanine dye[END_REF][START_REF] Yin | Cyanine-based fluorescent probe for highly selective detection of glutathione in cell cultures and live mouse tissues[END_REF][START_REF] Nano | A rhodium-cyanine fluorescent probe: Detection and signaling of mismatches in dna[END_REF][START_REF] Wang | Screening and investigation of a cyanine fluorescent probe for simultaneous sensing of glutathione and cysteine under single excitation[END_REF], BODIPYs [26; 31-35], and coumarins [START_REF] Jung | Coumarin-derived cu 2+ -selective fluorescence sensor: Synthesis, mechanisms, and applications in living cells[END_REF][START_REF] Jung | Coumarin-based thiol chemosensor: Synthesis, turn-on mechanism, and its biological application[END_REF][START_REF] Li | A coumarin-based fluorescent probe for selective and sensitive detection of thiophenols and its application[END_REF][START_REF] Gupta | Selective naked-eye detection of magnesium (ii) ions using a coumarin-derived fluorescent probe[END_REF][START_REF] Wang | A rapid-responsive fluorescent probe based on coumarin for selective sensing of sulfite in aqueous solution and its bioimaging by turn-on fluorescence signal[END_REF]. Among these classes, BODIPY derivatives occupy a privileged position [START_REF] Ziessel | The chemistry of bodipy: A new El Dorado for fluorescence tools[END_REF], because they can be easily functionalized and are very photostable, explaining the undisputed popularity in this family [START_REF] Potopnyk | Gražulevičius, N,o π-conjugated 4-substituted 1,3-thiazole bf 2 complexes: Synthesis and photophysical properties[END_REF][START_REF] Frath | Polyanils and polyboranils: Synthesis, optical properties, and aggregation-induced emission[END_REF][START_REF] Weber | Synthesis, structure, and properties of luminescent diazaborole and indole systems[END_REF][START_REF] Zhang | Novel fluorescent n,o-chelated fluorineboron benzamide complexes containing thiadiazoles: Synthesis and fluorescence characteristics[END_REF][START_REF] Yu | Dipyrrolylquinoxaline difluoroborates with intense red solid-state fluorescence[END_REF]. However, one of the main disadvantages of regular BODIPY fluorophores is the impossibility to directly doubly-benzannulate their five-membered rings. Indeed, while single benzannulation has been achieved [START_REF] Ulrich | Chemistry at boron: Synthesis and properties of red to near-ir fluorescent dyes based on boron-substituted diisoindolomethene frameworks[END_REF][START_REF] Ni | Benzene-fused bodipys: synthesis and the impact of fusion mode[END_REF][START_REF] Skotnicka | Tautomeric equilibria in solutions of 1-methyl-2phenacylbenzimidazoles[END_REF] and has shown to be an efficient strategy for red-shifting their absorption and fluorescence maxima, double benzannulation has only been achieved at other positions [START_REF] Grabarz | Photophysical properties of phenacylphenantridine difluoroboranyls: effect of substituent and double benzannulation[END_REF][START_REF] Sheng | a]-phenanthrene-fused bf 2 azadipyrromethene (azabodipy) dyes as bright near-infrared fluorophores[END_REF][START_REF] Descalzo | Influence of the meso-substituent on strongly red emitting phenanthrene-fused boron-dipyrromethene (bodipy) fluorophores with a propeller-like conformation[END_REF][START_REF] Jiang | Synthesis and photophysical properties of long wave-length absorbing bodipy/aza-bodipy bearing a five-membered ring[END_REF]. It is also well recognized that atom exchange, leading to isoelectronic compounds, may significantly impact the photophysical signatures of dyes. In fluoroborate dyes, substituent, benzannulation, and π-extension have been extensively applied to tune the photophysical properties, but atom exchange has been relatively less studied, though fluorescent techniques are also well suited to assess the impact of the changes induced by atom-by-atom substitutions. In the classes of dyes encompassing atom-substituted BODIPYs, one should, of course mention: i) aza-BODIPY dyes -with a nitrogen atom at the meso position -that shows red-shifted optical bands compared to standard BODIPYs [START_REF] Zhao | Conformationally restricted aza-bodipy: Highly fluorescent, stable nearinfrared absorbing dyes[END_REF][START_REF] Gresser | Synthesis and characterization of near-infrared absorbing benzannulated aza-bodipy dyes[END_REF][START_REF] Min | Two similar near-infrared (ir) absorbing benzannulated aza-bodipy dyes as near-ir sensitizers for ternary solar cells[END_REF][START_REF] Diaz-Moscoso | Synthesis of a class of core-modified aza-bodipy derivatives[END_REF][START_REF] Karlsson | Origin of the red-shifted optical spectra recorded for aza-bodipy dyes[END_REF]; ii) other difluoroborates showing one or both nitrogen atoms in their N-BF 2 -N core replaced by oxygen atoms, e.g., boranils [START_REF] Frath | Facile synthesis of highly fluorescent boranil complexes[END_REF][START_REF] Frath | Chemistry on boranils: An entry to functionalized fluorescent dyes[END_REF][START_REF] Lipunova | Boron(iii) complexes with n,n'-and n,o-heterocyclic ligands: Synthesis and spectroscopic properties[END_REF][START_REF] Urban | Highly fluorescent red-light emitting bis(boranils) based on naphthalene backbone[END_REF], hydroxyphenylbenzoxazole (HBO) complexes [START_REF] Massue | Synthesis of luminescent 2-(2'hydroxyphenyl)benzoxazole (hbo) borate complexes[END_REF][START_REF] Massue | Effect of 3,5-disubstitution on the optical properties of luminescent 2-(2' -hydroxyphenyl)benzoxazoles and their borate complexes[END_REF][START_REF] Benelhadj | Solution-and solid-state luminescent borate complexes based on a substituted π-conjugated 2-(6'-hydroxy-5'-benzofuryl) scaffold[END_REF], difluoroboron β-diketonate complexes [START_REF] Xu | Aromatic difluoroboron β-diketonate complexes: Effects of π-conjugation and media on optical properties[END_REF][START_REF] D'aleo | Boron difluoride complexes of 3-hydroxyflavone derivatives: efficient bioin-spired dyes for solution and solid-state emission[END_REF][START_REF] Štefane | Ethynylene-analogues of hemicurcuminoids: Synthesis and ground-and excited properties of their boron difluoride complexes[END_REF],

and others [42; 69-71] (see Figure 1). Notably, in contrast to standard BODIPY compounds, some of these derivatives, e.g., HBO complexes and boranils, are characterized by improved Stokes shifts (∆ ss ), usually larger than 50 nm [START_REF] Dobkowski | Substituent and solvent effects on the excited state deactivation channels in anils and boranils[END_REF][START_REF] Wu | New fluorescent imidazo[1,2-α]pyridine-bodipy chromophores: Experimental and theoretical approaches, and cell imaging exploration[END_REF][START_REF] Chen | Detection of thiophenol in buffer, in serum, on filter paper strip, and in living cells using a red-emitting amino phenothiazine boranil based fluorescent probe with a large stokes shift[END_REF], whereas the ∆ ss typically do not exceed 20 nm in standard BODIPYs [31; 41; 75]. Additionally the emission range of those compounds can be further red-shifted by extending the π-conjugated structure [76; 77]. This suggests that additional efforts to synthesize and understand the impact of atom substitution in this class of dyes are of interest. In a recent work, we have studied the impact of atom exchange directly bonded to the boron atom (OBF 2 N versus SBF 2 N) [START_REF] Jędrzejewska | Synthesis and photophysical properties of novel donor-acceptor n-(pyridin-2-yl)-substituted benzo(thio)amides and their difluoroboranyl derivatives[END_REF]. While this provided the first example of fluorophores of the BF 2 family with a S-B bond, these molecules were composed of six-membered rings only. In this context, our goal here is to synthesize NMe 2 -bearing fluoroborate derivatives containing one five-membered ring and to compare their photophysical properties with their unsubstituted analogues (Scheme 1).

One can expect that the impact of atom-exchange is stronger in five-membered ring than in their six-membered counterparts, because of the higher strain in the former compounds. We underline that atom exchange does not influence only the electronic properties but also impacts the vibrations in the five-membered ring and, in turn, can modify the vibronic couplings and hence the absorption and fluorescence band shapes [79; 80]. In more detail, we have three goals here: i) studying the effect of atom exchange on fluorophores incorporating or not a strong donor group; ii) probing the electron accepting properties of three different five-membered heterocycles; and iii) gaining insights into the electronic structure of the newly synthesized compounds using first principles calculations Scheme 1: The general structure of BF 2 -complexes of various heterocycles studied herein.

Experimental and theoretical methods

Experimental methods

The 2-phenacylheterocycles [49; 81] were first obtained and next converted to their BF 2 complexes following the procedure established for similar derivatives [50; 82]. For all presented structures NMR data ( 1 H, 11 B, 13 C, 15 N and 19 F) have been obtained (see Experimental Section at the end of this work). The fluorescence yields were determined in chloroform as follows. The fluorescence spectrum of dilute (A ≈ 0.1) solutions of the compounds were recorded by excitation at the absorption band maximum of the reference (see Table S3 in the SI). The quantum yield of the tested compounds (Φ dye ) was calculated using the following equation [START_REF] Brouwer | Standards for photoluminescence quantum yield measurements in solution (iupac technical report)[END_REF]: (Φ fl ), the radiative (k r ) and nonradiative (k nr ) rate constants were calculated using:

Φ dye = Φ ref I dye A ref I ref A dye • n 2 dye n 2
k r = Φ fl τ av k nr = 1 -Φ fl τ av

Computational details

The geometries of the six studied compounds were optimized in their electronic ground state (GS) at the B3LYP/6-311++G(d,p) level of theory. The effects of the solvent (CHCl 3 solution is used consistently with experiment, except when noted) were included by means of the linear-response variant of polarizable continuum model (LR-PCM) [START_REF] Tomasi | Molecular interactions in solution: An overview of methods based on continuous distributions of the solvent[END_REF].

The minimal nature of the optimal GS geometries was confirmed by evaluation of the Hessian. These minimum-energy geometries were used in all subsequent molecular dynamics simulations described below. In order to determine the vertical electronic spectra we have applied the state-of-the-art polarizable-embedded resolution-of-identity secondorder coupled-cluster (PERI-CC2) method [START_REF] Schwabe | Peri-cc2: A polarizable embedded ri-cc2 method[END_REF]. In more detail, we firstly carried out molecular dynamics (MD) calculations for BF 2 -complexes in chloroform solvent and the trajectories were subjected to subsequent PERI-CC2/MM calculations. During the MD simulations, the interactions between the solute and solvent subsystems were described considering both electrostatic and van der Waals interactions, and this requires charges and force field parameters for each of the molecules. The electrostatic potential fitted charges for the difluoroborates were obtained using the B3LYP/6-311++G(d,p) level of theory and CHEPLG procedure [START_REF] Breneman | Determining atom-centered monopoles from molecular electrostatic potentials. the need for high sampling density in formamide conformational analysis[END_REF] as implemented in Gaussian 09 software. To describe the van der Waals interactions, we employed General AMBER Force Field (GAFF) [START_REF] Wang | Development and testing of a general amber force field[END_REF] and CHCL3.frcmod for BF 2 -carrying and chloroform solvent molecules, respectively.

Selected chloroform force field includes both bond length and bond angle parameters making the solvent flexible. Even though GAFF provides a flexible molecular model for solutes, we treated the difluoroborates molecules with rigid body approximation.

The simulations for BF 2 -carrying compounds in chloroform encompassed a single solute molecule and a few thousands of solvent molecules. The calculations were carried out at room temperature and 1 atmospheric pressure using the Amber16 software [START_REF]AMBER 16[END_REF]. The calculations were performed in an isothermal-isobaric ensemble, so that the density of the solute-solvent system evolves to the right value independent of the initial density.

The maintenance of the system at a specific temperature and pressure has been achieved by connecting the system to Langevin's thermostat and Barendsen's barostat. The time step for integration of equation of motion was set to 2 fs and the total time scale for the production run was 10 ns. The convergence of key properties (energies and density) in each simulation was checked. Fifty configurations from the last 5 ns simulations selected at equal intervals have been used for computing the spectra using PERI-CC2 method which describes the interaction between the solute-solvent subsystems using electrostatic embedding. The same force-fields as in the MD simulations were used to describe the solvents during the PERI-CC2 calculations. The PERI-CC2 calculations were carried out by using the Turbomole package [START_REF]TURBOMOLE V7.0 2015, a development of University of Karlsruhe and Forschungszentrum Karlsruhe GmbH[END_REF].

Since excited-state (ES) properties, e.g., electronic density changes upon electronic excitation, are not available yet at the PERI-CC2 level, we have employed the Kohn-Sham formulation of time-dependent density functional theory (TD-DFT) for their determination. The DFT and TD-DFT calculations were performed with the Gaussian 09 package [START_REF] Frisch | Gaussian 2009[END_REF], employing 11 different exchange-correlation functionals (XCF), i.e., BLYP [91; 92],

B3LYP [START_REF] Becke | Density-functional thermochemistry. iii. the role of exact exchange[END_REF], PBE0 [94; 95], M06 [START_REF] Zhao | The m06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four m06-class functionals and 12 other functionals[END_REF], BHandHLYP [START_REF] Becke | A new mixing of hartree-fock and local density-functional theories[END_REF], CAM-B3LYP [START_REF] Yanai | A new hybrid exchange-correlation functional using the coulomb-attenuating method (cam-b3lyp)[END_REF], M06-2X [START_REF] Zhao | The m06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four m06-class functionals and 12 other functionals[END_REF], ωB97X [START_REF] Chai | Systematic optimization of long-range corrected hybrid density functionals[END_REF], ωB97X-D [START_REF] Chai | Long-range corrected hybrid density functionals with damped atom-atom dispersion corrections[END_REF], LC-BLYP [START_REF] Iikura | A long-range correction scheme for generalizedgradient-approximation exchange functionals[END_REF], and M06-HF [102; 103]. Five excited states were considered. For all DFT/TD-DFT calculations, we used so-called ultrafine integration grid. The density difference plots were simulated with a 0.002 au contour threshold, with density increase (decrease) indicated by red (blue) zones. Subsequent charge-transfer parameters, namely the distance on which charge is transferred (d CT ), the amount of charge (q CT ) and the dipole moment change upon transition (µ CT = µ ES -µ GS )

were calculated using Le Bahers' procedure described in details elsewhere [104; 105]. In this procedure, the CT parameters are determined using the barycenters of density increase and decrease upon photon absorption. All calculations of transition energies were performed with the aug-cc-pVDZ basis set [START_REF] Dunning | Gaussian basis sets for use in correlated molecular calculations. i. the atoms boron through neon and hydrogen[END_REF][START_REF] Kendall | Electron affinities of the first-row atoms revisited. systematic basis sets and wave functions[END_REF][START_REF] Woon | Gaussian basis sets for use in correlated molecular calculations. iv. calculation of static electrical response properties[END_REF].

Results and discussion

The measured absorption and emission spectra of all six compounds obtained in chloroform are given in Figure 2 and the associated data are listed in Tables 1 and2. Let us first discuss the results obtained for the absorption spectra. Obviously, the absorption bands of the unsubstituted compounds (R=H) are blue-shifted compared to the R=NMe 2 dyes, a statement holding irrespective of the X heteroatom. Indeed, for all unsubstituted compounds (X=NMe, O, and S) the absorption band peaks at 350-375 nm, whereas for R=NMe 2 the span is twice as large (400-450 nm). In addition, both the shapes and the positions of the absorption bands of the unsubstituted X=NMe and X=O compounds are highly similar, whereas the corresponding NMe 2 -substituted derivatives show more pronounced discrepancies in their band topologies. Introducing the donor group also yields a significant evolution of the band shape for the thio derivative.

Table 1: Photophysical properties of examined compounds measured in chloroform.

Struct. As can be seen in Table 1, the measurements of the fluorescence quantum yields for substituted dyes (see Table 2 and the fluorescence decay profiles in Figures S10-S15

λ abs λ fl ε ∆ ss Φ a fl Φ b fl X R [nm] [nm] [M -1 cm -1 ] [cm -1 ] [%] [
in the SI). For the unsubstituted compounds, two exponential fluorescence decays were observed (See Table 2) with a major component close to 70-90 ps and another at ca. 0.9-1.8 ns. In contrast, in the NMe 2 series, the fluorescence decay profiles fit well with a single-exponential function, giving fluorescence lifetimes of 0.86, 1.59 and 1.78 ns for X=NMe, O, and S, respectively. In other words, in the amino series, the fluorescence lifetimes increase along with the NMe→O→S pattern. From the Table 2 one also notices that the radiative rate constant (k r ) increases when the compounds are substituted by electron releasing substituent NMe 2 , i.e., the enhancement of Φ fl originates in both the expected decrease of k nr (vide supra) and the increase of k r . The differences in the k r values for R=H series are not significant but the k r /k nr ratio between is two-or three orders lower than unity, indicating that the non-radiative deactivation of S 1 is efficient.

The situation is opposite in the R=NMe 2 : the k r /k nr ratio is close to unity for X=NMe derivative and becomes even higher in the X=S and X=O derivatives. in the SI whereas the corresponding spectroscopic and photophysical properties are collected in Tables S5 andS6. As can be seen in Figure S8, the UV-vis absorption spectra of the unsubstituted difluoroborates (R=H) do not display any significant variation of their absorption maxima position, when changing the solvent dielectric constant. In contrast, the absorption maxima of the dimethylamine-substituted derivatives undergo positive solvatochromism, e.g., a noticeable red-shift from 439 nm in MCH to 450 nm in DMF is found for the X=S R=NMe 2 compound (see Figure S9 in the SI). These red-shifts of the absorption maxima position along with the high molar absorption coefficients indicate that the transitions of the R=NMe 2 derivatives have a π -π * nature associated with a significant charge transfer (CT) (see Table S5). Similarly, a considerable redshift driven by a solvent dielectric constant change is also observed in the emission spectra of X=O R=NMe 2 and X=S R=NMe 2 compounds, whereas no significant shift was found in the fluorescence spectra of unsubstituted difluoroborates, indicating that the CT character of the first singlet excited state is much more marked in the dimethylamine derivatives [109; 110]. Interestingly, the emission spectra of the X=NMe R=NMe 2 dye display significantly smaller solvatochromic shifts, than their benzoxazole and benzothiazole analogues. In short, the redshifts of the emission maxima positions, as well as higher sensitivity of fluorescence spectra (with respect to absorption) to the dielectric constant of the environment are characteristics of push-pull compounds undergoing CT transitions [START_REF] Lakowicz | Principles of Fluorescence Spectroscopy[END_REF][START_REF] Bünau | Photophysics of aromatic molecules. wiley-interscience, london 1970. 704 seiten. preis: 210s[END_REF][START_REF] Al-Sehemi | A combined experimental and computational investigation on spectroscopic and photophysical properties of a coumarinyl chalcone[END_REF][START_REF] Garcia | Evaluating push-pull dye efficiency using td-dft and charge transfer indices[END_REF].

As can be seen in Table S5, the fluorescence quantum yields of the unsubstituted dyes remain very small (<1%), irrespective of the solvent dielectric constant. Obviously, such trifling values do not allow for an unambiguous interpretation of the impact of the solvent dielectric constant on the Φ fl ; the trends being however similar as in CHCl 3 . In contrast, the R=NMe 2 dyes fluoresce intensively in all studied solvents. The Φ fl of the X=S R=NMe 2 compound is systematically decreasing with increasing solvent dielectric constant, which is a usual trend in dyes exhibiting large charge separation in their excited state [113; 114]. Interestingly, the opposite is observed for X=NMe R=NMe 2 dye, in which fluorescence quantum yield increases from 14.5% (MCH) up to 56.5% (DMF). It is not easy to interpret this peculiar behavior but we hypothesize that it is related to the motion of the additional methyl group that would be somehow diminished in DMF.

Indeed, such improvement of the fluorescence quantum yield with increasing in solvent dielectric constant, also known as negative solvatokinetic effect, might be connected with interactions between the excited solute and solvent molecules, which changes the excited-state configuration and the fluorescence intensity [START_REF] Wassam | proximity effect" in radiationless transitions[END_REF][START_REF] Fery-Forgues | Photophysical properties of styryl derivatives of aminobenzoxazinones[END_REF][START_REF] Gulbinas | Charge transfer induced excited state twisting of n,n-dimethylaminobenzylidene-1,3-indandione in solution[END_REF][START_REF] Rurack | Quantum yield switching of fluorescence by selectively bridging single and double bonds in chalcones: Involvement of two different types of conical intersections[END_REF][START_REF] Asiri | Spectral characteristics of 4-(p-n,n-dimethylaminophenylmethylene)-2-phenyl-5-oxazolone (dpo) in different media[END_REF][START_REF] Jędrzejewska | Experimental and theoretical studies of the influence of solvent polarity on the spectral properties of two push-pull oxazol-5-(4h)-one compounds[END_REF]. However, the above hypothesis needs more precise studies in a wider range of solvents and is out of the scope of the current work. Eventually, for X=O R=NMe 2 , the Φ fl is similar in THF (68.9%) than in MCH (73.5%), but a lower efficiency is observed in the most polar DMF (52.5%) , as usual. We also highlight that, irrespective of the selected solvent, the unsubstituted compounds exhibit a two-exponential decay emission whereas the R=NMe 2 dyes show mono-exponential decays, but for X=NMe R=NMe 2 in MCH. This clearly suggests at a rapid internal conversion from higher excited states to the lowest vibrational energy level of the S 1 state as well as the presence of only one emitting species. In addition, the fluorescence excitation spectra were recorded for two emission wavelengths (detection at blue and red edges of the emission spectrum), as well as at the maximum of the emission spectrum for diluted and more concentrated solutions.

Figures S6 and S7 display the steady-state fluorescence excitation spectra of the tested compounds, and one can observe, on the one hand, spectra independent of the considered wavelength, and, on the other hand, a non-negligible impact of the concentration of the solution. The differences between absorption and excitation spectra of the dyes in 10 -5 M solutions show the existence of the excited molecules that differ in spatial conformations and/or solvent relaxations.

All these experimental results encouraged us to carry out computer simulations in order to shed light onto the electronic structure of these derivatives, one of our goals being to unravel the origin of the band shape differences: multiple electronic state and/or various vibronic couplings. Our results are presented in Table 3. As can be seen, the first electronic excited state is well separated from the second one, with an energy gap between them attaining ca. 1 eV for all studied molecules. In addition, the oscillator strengths, f are much larger for the S 0 →S 1 transition (from 1.07 to 1.59) than for the S 0 →S 2 transition (from 0.02 to 0.06). This clearly indicates that the observed spectra are solely due to the first electronic ES. For that excitation, the NMe 2 -substituted compounds exhibit larger oscillator strengths than their unsubstituted counterparts. We note that the predicted f values are rather consistent with measured molar absorptivities (see ε in Table 1): in both cases, significantly lower values are obtained for R=H derivatives.

Nevertheless, based on the measurements, one would have expected significantly lower oscillator strength for the X=NMe R=NMe 2 derivative. Figure 3 3), a feature that can be directly associated with an increase of the intramolecular charge-transfer character when adding the electron-donating group. As neither the presence of multiple electronic states nor explicit solvent-solute interactions can explain the presence of several maxima/shoulders in the absorption spectra of Figure 2, these band shapes should reflect the changes in the vibrational fine structure induced by chemical modifications. To confirm this hypothesis, complementary calculations were performed using CC2 method, but in gas phase (see Figure S3 in the SI). Although these calculations do not account for the solvent environment, and hence, logically yield band positions that are blue-shifted compared to the measurements and PERI-CC2 results, one notices that the experimental band shapes are reasonably well reproduced, confirming the vibronic nature of the multi-peak absorption bands. (+14 nm). The respective shifts predicted by the PERI-CC2 method are +3 nm and +15 nm, thus demonstrating the excellent agreement with experimental data. Significantly larger auxochromic effects are observed experimentally for the X=NMe→S substitution, i.e., +20 nm for R=H and +50 nm for R=NMe 2 , with again simulated shifts in good agreement (+28 nm and +40 nm). One can therefore conclude that the overall In order to obtain additional insights into the electronic structure of the dyes, we have probed the nature of the excited state properties in the Franck-Condon region. Since ES properties cannot yet be determined with the PERI-CC2 approach, we have turned towards TD-DFT. The performance of TD-DFT in predicting ES properties is known to significantly depend on the selected XCF [121; 122], and no single XCF can be considered as outperforming the others on a systematic basis. Hence, in order to select a XCF providing accurate results for the molecules under investigation, we have benchmarked eleven XCFs and compared the obtained GS and ES dipole moments to CC2 reference values. The dipoles were chosen as they are a measure of the quality of the electronic density and are also directly related to the CT character of the electronic transition.

These benchmark calculations were performed for isolated (no solvent) molecules. As can be seen in Figure 4, the errors made by DFT for GS dipole moments (µ GS ) are sub- 4, PCM-TD-B3LYP also fairly reproduces all auxochromic shifts though less accurately than with PERI-CC2, as expected. We are well aware of the inherent limits of B3LYP for describing long-range CT systems [START_REF] Laurent | Td-dft benchmarks: A review[END_REF], but the molecules considered are probably compact enough so that these limits are not relevant here.

Table 5 provides a summary of the TD-DFT ES properties computed in the Franck-Condon region together with electron density difference (EDD) plots showing the electronic reorganization accompanying the transition to the lowest electronic excited state.

Figure 4: Errors in TD-DFT dipole moments: maximum (∆ max ) and average (∆ avg ) absolute errors compared to CC2 (see also corresponding Table S1 in the SI). All results were obtained in gas phase with the aug-cc-pVDZ basis set.

As can be seen, for the R=H compounds, the electronic density changes mainly take place within the BF 2 -containing core and adjacent five-membered ring. 5 to the fluorescence quantum yields, one notices that the decrease of the d CT and µ ES -µ GS promotes the radiative transition for all three dyes of each series (R=H and R=NMe 2 ), e.g., in the R=H series the quantum yield decreases with increasing CT character. In contrast, Φ fl is much higher in the NMe 2 series, although the CT character is enhanced in the latter. We hypothesize that this result comes from the fact that in the amino dyes, the CT involves the electron lone pair of the amino nitrogen, which allows the formation of a quinoidal structure, and hence lead to the rigidification of the excited state. That conclusion is also supported by the single-exponential fit of the fluorescence decay (see Figures S10-S15 in the SI).

To further ascertain the CT character of the amino-substituted dyes, we calculated the excited-state properties in Franck-Condon region of the benzothiazole derivatives in three additional solvents: MCH, DMF and THF. The results of those calculations together with the corresponding EDD plots are presented in Table S3 in the SI. As can be seen, the above-described trends (obtained in chloroform) pertain, e.g., the S 0 →S 1 transition is associated with a much smaller µ ES -µ GS in the R=H (not exceeding 3 D) than in the R=NMe 2 dye (larger than 5.8 D). The EDD plots also tend to confirm the Table 5: Electron density difference plots and related electron transition parameters. All results were obtained at B3LYP/aug-cc-pVDZ level of theory for chloroform solution. Blue (red) color marks electron density depletion (gain) upon photon absorption. The contour value was set to 0.002 au.

Struct. EDD plots To further characterize the first excited state beyond the Franck-Condon region, we have optimized the excited-state geometries of all compounds using TD-DFT. Let us first discuss the changes in bond lengths between the two electronic states. The complete set of data can be found in Table S2 in the SI. We focus our analysis on the geometrical changes, as given by ∆ EG bond = bond ES -bond GS . A graphical representation of the general variations of geometries can be found in Scheme 2. First, based on the comparison of the sum of absolute changes of bond lengths in the series, it appears that the S 0 →S 1 excitation induces stronger effects for unsubstituted derivatives (R=H) than that for their substituted analogues (R=NMe 2 ). Indeed, the total variations, Σ∆ EG bond , are equal to 1.249 Å and 1.157 Å, for the R=H and R=NMe 2 series, respectively. This holds for the bonds belonging to the heterocyclic part of the structures, confirming that the excitation causes more dramatic changes in geometry for R=H, which is detrimental for emission yields. In contrast, except for the C11-C14 distance, the opposite trends are observed for 1,4-phenylene ring moieties. This can be easily explained by the presence of NMe 2 donor which strongly affects the electron density distribution thus also the geometry of the R=NMe 2 dyes (See Figure S2 in the SI). Second, most of the computed changes in bond distances follow a N→O→S ranking, e.g., for the C10-C11 bond, the excitation-induced changes are maximal for X=S derivatives and minimal for their X=NMe counterparts.

d CT q CT µ ES -µ GS X R [Å] [e] [D
In contrast, for the C9-N1 bond distance the above trend is reversed in both dye series (See Figure S1 in the SI). Third, for the side phenyl rings, the expected p-quinoid form

Conclusions

We have investigated the influence of heteroatom (X=O, S, and NMe) substitution in five-membered heterocyclic ring on the photophysical properties of novel BF 2 complexes containing or not a dimethylamino group (R=NMe 2 ). The experimental results

showed that the photophysical properties of the molecules bearing an electron-donating group are substantially different from these observed for unsubstituted analogues, which follows previous findings of our groups [2; 50; 82]. For instance, the NMe→O heteroatom replacement has a negligible impact on the absorption and emission properties of R=H derivatives, but induces significant red-shifts accompanied by changes of the relative intensities of absorption/emission shoulders for R=NMe 2 compounds. More pronounced changes stem from the NMe→S substitution, i.e., both R=H and R=NMe 2 optical bands are substantially redshifted. It is noteworthy that all R=NMe 2 derivatives exhibit fluorescence quantum yields significantly higher (up to one order of magnitude) than their unsubstituted counterparts. The experimental data were supported by the quantum chemistry calculations, which not only reproduced the experimental trends but also contributed to the understanding of electronic structure and excited-state properties: i) the different photophysical properties of the R=NMe 2 fluorophores likely result from the more pronounced CT character of their S 0 →S 1 transition, as evidenced by both the large inhomogeneous broadening of the absorption spectra obtained with the PERI-CC2 simulations and the dipole moment differences obtained at various levels of theories; ii) the difference between absorption band topologies of R=H and R=NMe 2 have their roots in the CT nature of the latter derivatives, that washes out the impact of vibronic couplings, as PERI-CC2 calculations show that only one electronic state contributes to the overall visible absorption band; and iii) the enhancement of fluorescence quantum yield observed for compounds with R=NMe 2 can be linked to stabilization of a more planar structure, both in the ground and the excited electronic states.

Experimental Section

In general, 2-phenacylbenzoxazoles can be obtained in the condensation of 2-aminophenol and 3,3-dialkoxy-3,3-dimercapto-1-phenylprop-2-en-1-ones [125; 126] or alkyl benzoylacetates [127]. These compounds were also conveniently prepared by subtraction of one of the methyl protons in 2-methylbenzoxazole and treatment of the formed carbanion with acylating agent such as benzoic acid ester or benzoyl chloride [81; 128-137].

All compounds discussed in the present paper were obtained in the two-step synthesis starting from 2-methylbenzoxazole and substituted benzoyl chlorides as before [START_REF] Dzvinchuk | C-mono-and dibenzoylation of 2-methylbenzimidazole with use of benzoyl chloride[END_REF]. It is worth underscoring the tautomeric equilibrium exists in these compounds.

Figure 1 :

 1 Figure 1: The common derivatives of BODIPY dyes, from left to right: aza-BODIPY, boranils, HBO complexes and difluoroboron β-diketonate complexes.

  ref where Φ ref is the fluorescence yield of the reference, A dye and A ref are the absorbance of the dye and reference samples at the excitation wavelengths, I dye and I ref are the integrated emission intensity for the dyes and references samples, n dye and n ref are the refractive indices of the solvents used for the dyes and reference, respectively. Knowing the (average) lifetime of the S 1 excited state (τ av ) and the fluorescence quantum yield

Figure 2 :

 2 Figure 2: Experimental absorption and emission spectra in CHCl 3 .

  Our next step was to check how solvatochromic effects affect the properties of R=H and R=NMe 2 derivatives, thus we measured absorption and emission properties of the examined dyes in three solvents characterized by different dielectric constant. The electronic absorption and fluorescence spectra recorded in methylcyclohexane (MCH), N,Ndimethylformamide (DMF) and tetrahydrofuran (THF) can be found in FiguresS8-S9

Figures

  Figures S4 and S5 in the SI, the position of the fluorescence peaks remains constant independently of both the λ exc and concentration in the solution for all compounds.

Figure 3

 3 Figure 3 originate from the inhomogeneous broadening due to solute-solvent interactions, whereas the vibronic couplings are neglected, so that the band widths are logically underestimated compared to experiment. Nevertheless, by comparing the R=H and R=NMe 2 derivatives, one clearly notices that the latter are much more sensitive to the solvent microenvironment, explaining the larger inhomogeneous broadening of derivatives possessing a dimethylamino group (see the σ values in Table3), a feature that can be directly

Figure 3 :

 3 Figure 3: Electronic vertical absorption spectra simulated at the PERI-CC2/aug-cc-pVDZ level of theory.

(

  Scheme 2: Schematic representation of geometry changes upon excitation in R=H and R=NMe 2 dyes.

2-( 1 , 3 -

 13 benzoxazol-2-yl)-1-[4-(dimethylamino)phenyl]ethanone -White solid, yield: 73%, Mp 199-200.4 • C; 197-198 • C [81]. In CDCl 3 solution 2-(1,3-benzoxazol-2-yl)-1-[4-(dimethylamino)phenyl]ethanone (ketimine tautomeric form) is in equilibrium with (Z)-2-(1,3-benzoxazol-2-yl)-1-[4-(dimethylamino)phenyl]ethenol (enolimine form). 1 H NMR (CDCl 3 from TMS) δ: 7.96 (m, 2H), 7.72 (m, 1H), 7.50 (m, 1H), 7.29 (m, 2H), 6.68 (m, 2H), 6.05 (s, 1H, signal of low intensity coming from low amount of the enolimine form), 4,53 (s, 1H), 3.05 (s, 6H). 13 C NMR δ: 190.0, 166.7, 161.6, 153.9, 151.3, 141.5, 131.0, 127.3, 124.8, 124.4, 124.2, 123.7, 123.4, 119.9, 111.5, 110.8, 110.6, 80.4, 40.0, 39.1, 15 N NMR (CDCl 3 from MeNO 2 ) δ: -135.6, -322.9. C 17 H 16 N 2 O 2 , Calcd. C 72.84, H 5.75, N 9.99. Found C 72.66, H 5.88, N 9.86. Some of 1-methyl-2-phenacylbenzimidazoles were synthesized according to reaction of benzoyl substituted mercaptals with N-methyl-o-phenylenediamine [138; 139]. Aforementioned synthesis, as well as other earlier described in the literature [125; 140], are characterized by low yield, therefore 1-methyl-2-phenacylbenzimidazole were prepared by treating 1,2-dimethylbenzimidazole with benzoyl chloride in the presence of triethylamine followed by thermal decomposition of formed 2-(1-methyl-1H-benzo[d]imidazol-2-yl)-1phenylvinyl benzoate. Thus, benzoyl chloride (8 mmol) was added at once to the mixture of 1,2-dimethylbenzimidazole (0.29 g, 2 mmol), triethylamine (0.81 g, 8 mmol) and diglyme (10 mL) and the resulted mixture was heated (steam bath) and stirred overnight. Addition of water (6 mL) precipitated the byproduct. Solution of morpholine (0.52 g, 6 mmol) in methanol (3 mL) was added to crude 2-(1-methyl-1H-benzo[d]imidazol-2-yl)-1-phenylvinyl benzoate and the resulted mixture was refluxed for 5 min. Reaction was quenched by addition of water (3 mL) and the precipitated solid product was recrystallized from ethanol. 1-Methyl-2-phenacylbenzimidazole -Yellow solid, yield 40%, Mp 134-135.5 • C; 119-121 • C (plates) and 149-152 • C (needles) [49], 150-151 • C and 134-134.5 • C [139], 150-152 • C [125; 140], and 150-151.5 • C [128; 141]. In CDCl 3 solution 1-methyl-2-phenacylbenzimidazole (ketimine tautomeric form) is in equilibrium with (Z)-2-(1-methyl-1H-benzimidazol-2-yl)-1-phenylethenol (enolimine form).

H 5 .

 5 57, N 9.29. The complexes were prepared based on a literature protocol [82]. For their purification DCM was used as eluent in column chromatography (SiO 2 ). 1,1-difluoro-5-methyl-3-phenyl-1,5-dihydrobenzo[4,5]imidazo[1,2-c][1,3,2]oxazaborinin-10-ium-1-uide (X=NMe, R=H) -White solid, yield 36%, Mp 255.6-256.7 • C. 1 H NMR (DMSO-d 6 from TMS) δ: 8.09 (m, 2H), 7.77 (m, 1H), 7.68 (m, 1H) 7.56 (m, 3H), 7.44 (m, 2H), 7.05 (s, 1H), 3.98 (s, 3H). 11 B NMR (DMSO-d 6 from BF 3 •Et 2 O) δ: 1.66 (t).

Table 2

 2

		: Fluorescence lifetimes (τ ), component amplitudes (α), correlation coefficients
	(χ 2 ) and radiative (k r ) and non-radiative (k nr ) rate constants for examined compounds
	in chloroform.
		Struct.	τ 1	τ 2	α 1	α 2	τ av	χ 2	k r	k nr	k r /k nr
	X	R	[ps] [ps]			[ps]		[s -1 ] × 10 8 [s -1 ] × 10 8
	NMe H	72 1630 99.345 0.654 82.2 1.668	0.29	121	0.0024
	O	H	71 1981 98.785 1.215 94.2 1.439	0.33	106	0.0031
	S	H	86 1021 99.438 0.562 91.3 1.538	1.32	108	0.0120
	NMe NMe 2	-	861	-	100 861 1.395	5.66	5.96	0.95
	O	NMe 2	-1593	-	100 1593 1.077	6.16	0.12	53.6
	S	NMe 2	-1775	-	100 1775 1.172	3.94	1.69	2.33

The fluorescence excitation and emission characteristics of all compounds have been measured in THF at different emission and excitation wavelengths. As can be seen in

Table 3 :

 3 Average vertical excitation energies (∆E), standard deviations of ∆E distributions (σ(∆E) * ) and average oscillator strengths (f ) computed at the PERI-CC2/aug-cc-pVDZ level of theory in chloroform. * FWHM=2 2 ln σ(∆E).Let us now assess the accuracy of the PERI-CC2 method in reproducing the auxochromic shifts on the absorption spectra. In Table4the experimental and simulated spectral shifts of the S 0 →S 1 absorption band maxima are listed. In that Table, we used the X=NMe R=H compound as a reference. The experimentally determined shift upon X=NMe→O replacement is trifling for R=H (+2 nm) but significant for R=NMe 2

		Struct.	∆E [eV] ∆E [nm] σ(∆E) [eV]	f
	X	R		S 0 →S 1
	NMe H	3.73	333	0.01	1.12
	O	H	3.70	335	0.01	1.12
	S	H	3.44	361	0.01	1.07
	NMe NMe 2	3.19	389	0.05	1.59
	O	NMe 2	3.07	404	0.05	1.59
	S	NMe 2	2.90	428	0.06	1.57
				S 0 →S 2
	NMe H	4.73	262	0.02	0.02
	O	H	4.66	266	0.04	0.02
	S	H	4.42	280	0.05	0.05
	NMe NMe 2	4.19	296	0.03	0.06
	O	NMe 2	4.20	295	0.02	0.06
	S	NMe 2	4.00	310	0.02	0.03

Table 4

 4

		: Excitation energy changes, w.r.t. X=NMe R=H compound, induced by chemi-
	cal modifications: comparisons between theory (PERI-CC2 and PCM-TD-B3LYP) and
	experiment.
		Struct.			S 0 →S 1
	X	R	exp. exp. PERI-CC2 PERI-CC2 PCM-TD-B3LYP PCM-TD-B3LYP
			[eV] [nm]	[eV]	[nm]	[eV]	[nm]
	NMe H	0.00 0	0.00	0	0.00	0
	O	H	-0.02 +2	-0.03	+3	0.00	0
	S	H	-0.19 +20	-0.29	+28	-0.17	+18
	NMe NMe 2 -0.43 +48	-0.54	+56	-0.37	+43
	O	NMe 2 -0.54 +62	-0.66	+71	-0.42	+50
	S	NMe 2 -0.78 +98	-0.83	+96	-0.57	+70

stantially smaller compared to the TD-DFT errors for the ES (µ ES ), which is consistent with previous benchmarks

[123; 124]

. As a consequence, the errors in the so-called excess dipole moments (µ GS -µ ES ) are mainly originating from the TD-DFT part. For the µ GS the smallest errors are reached with the ωB97X XCF (smaller than 0.2 D), although all XCFs provide satisfying results. In contrast, for the ES, ωB97X yields the largest maximum deviations (ca. 3.0 D), whereas B3LYP provides the smallest maximum error for both µ ES (1.3 D) and µ ES -µ GS (1.9 D). B3LYP also gives one of the smallest average errors for µ GS (0.3 D), µ ES (1.2 D), and µ ES -µ GS (1.3 D). In addition, this XCF accurately reproduces the impact of chemical substitution on the ES dipoles, but for a slight underestimation of the dipole increase when adding the dimethylamino group. Therefore we select B3LYP for our analysis of the ES properties in CHCl 3 solution. As can be seen in Table

  ESµ GS ) which exceed 6 D for all three dyes, whereas significantly smaller values are obtained for the R=H derivatives. Logically, the charge-transfer distances d CT are increasing upon NMe 2 substitution (up to d CT ≈ 2.7 Å). However as one can see the d CT values of X=NMe R=H (d CT = 2.310 Å) and X=O R=H (d CT = 2.110 Å) derivatives are much closer to their NMe 2 -substituted counterparts, than to X=S R=H (d CT = 1.539 Å). This can be explained by looking at the EED of R=H derivatives collected in Table 5, as one can see both X=O and X=NMe derivatives have red (accepting) lobes on the bond connecting the fluoroborate core to the adjecent phenyl ring. This indicates that d CT in fact should be larger for those two derivatives. Additionally, the R=NMe 2 compounds are also characterized by a larger transferred charge (q CT of ca. 0.50 e) than in their R=H counterparts (q CT in the 0.41-0.43 e range). In the R=H series the CT character of the electronic transition is clearly decreasing following X=NMe→O→S, which is confirmed by a systematic decrease of both the d CT (NMe→O=-0.209 Å, O→S=-0.562 Å) and µ ESµ GS (NMe→O=-0.395 D, O→S=-1.332 D) parameters. An interesting observation can be made by comparing NMe 2 -substituted compounds, i.e., d CT and µ ES -µ GS values are lowest for X=O (d CT =2.713 Å, µ ES -µ GS =6.537 D), while opposite to R=H, the highest values are obtained for the thio derivatives (d CT =2.751 Å, µ ES -µ

In contrast for molecules bearing a NMe 2 group, the density changes involve not only the BF 2 -carrying ring but also the side NMe 2 -phenyl moiety, the amino group acting, as expected, as a donor (mostly in blue in the EDDs). The CT character of the transitions in NMe 2 derivatives is reflected by the significant values of the dipole moment differences (µ GS =6.612 D). In turn X=NMe d CT values are close to X=O, whereas µ ES -µ GS results are similar to those of X=S. Comparing the parameters of Table

  15 N NMR (CDCl 3 from MeNO 2 ) δ: -134.4, -164.1. C 15 H 11 NO 2 Calcd.

	2-Phenacylbenzoxazole -White solid, yield 59%, Mp 94-96 • C; 94-96 • C [81], 88-
	88.5 • C [126], 88-88.5 • C [128], 88 • C [129], 90-91 • C [130], 97-98 • C [125], and 93.5-
	94.5 • C [131]. In CDCl 3 solution 2-phenacylbenzoxazole (ketimine tautomeric form) is in
	equilibrium with (Z)-2-(1,3-benzoxazol-2-yl)-1-phenylethenol (enolimine form). 1 H NMR
	(CDCl 3 from TMS) δ: 12.50 (br s, 1H), 8.06 (m, 1H), 7.89 (m, 3H), 7.73 (m, 1H), 7.62
	(m, 2H), 7.50 (m, 3H), 7.46 (m, 4H), 7.33 (m, 2H), 7.29 (m, 1H), 6.21 (s, 1H), 4.64
	(s, 1H). 13 C NMR δ: 192.4, 166.3, 165.74, 160.4, 151.3, 148.8, 141.3, 139.9, 135.8, 134.1,
	133.9, 130.6, 128.9, 128.6, 128.6, 125.9, 125.0, 124.6, 124.3, 124.1, 120.0, 117.9, 110.6,
	110.2, 83.7, 39.6. C 75.94, H 4.67, N 5.90; Found C 75.87, H 4.75, N 5.83.

  CDCl 3 from MeNO 2 ) δ: -137.4, -210.4, -233.3, -253.5. C 16 H 14 N 2 O, Calcd. C 76.78, H 5.64, N 11,19. Found C 76.85, H 5.74, N 11.03. , 111.4, 110.73, 110.67, 110.4, 109.3, 74.9, 40.1, 40.0, 38.8, 30.5. 15 N NMR (CDCl 3 from MeNO 2 ) δ: -136.8, -233.8, -317.3. C 18 H 19 N 3 O, Calcd. C 73.69, H 6.53, N 14.32. C NMR δ: 191.8, 165.1, 153.8, 152.6, 136.0, 131.1, 127.5, 126.3, 125.8, 124.9, 123.6, 123.6, 122.7, 121.5, 121.3, 110.8, 88.4, 43.3, 40.0. 15 N NMR (CDCl 3 from MeNO 2 ) δ: -67.8, -317.1. C 17 H 16 N 2 OS, Calcd. C 68.89, H 5.44, N 9.45. Found C 68.95,

	1 H NMR (CDCl 3 from TMS) δ: 14.04 (br s, 1H), 8.13 (d, 2H, 3 J H,H =7.32 Hz), 7.93 (m, 2H), 7.74 (m, 1H), 7.61 (m, 1H), 7.50 (m, 2H), 7.43 (m, 4H), 7.33 (m, 1H), 7.28 (m, 1H), 7.23 (m, 3H), 5.91 (s, 1H), 4.70 (s, 2H), 3.76 (s, 3H), 3.67 (s, 3H). 13 C NMR δ: 194.0, 174.8, 154.4, 148.6, 138.4, 134.9, 134.8, 134.0, 133.14, 130.11, 129.9, 128.93, 128.87, 128.83, 128.3, 127.3, 126.2, 124.3, 122.9, 122.8, 122.7, 122.3, 122.2, 119.3, 113.9, 110.6, 109.4, 108.3, 77.0, 39.1, 30.5, 29.1. dimethylformamide (0.5 mL) and sodium hydride (0.15 g, 60% suspension in mineral oil) was refluxed and stirred magnetically overnight. The reaction was quenched by addition of water (15 mL) and the resulted mixture was extracted with ethyl acetate (20 mL). The organic layer was subsequently washed with water (2 x 10 mL) and brine (2 x 10 mL) and dried with sodium sulfate. Solid impurities were filtered off, the filtrate was concentrated on the rotary evaporator. The obtained solid was recrystallized from ethanol. 1-[4-(dimethylamino)phenyl]-2-(1-methyl-1H-benzimidazol-2-yl)ethanone -Yellow solid, 2-(1,3-benzothiazol-2-yl)-1-[4-(dimethylamino)phenyl]ethanone -yellow solid, yield 116.8In CDCl 3 solution 2-phenacylbenzothiazole (ketimine tautomeric form) is in equilibrium 77%, Mp 207-208 • C. 1 H NMR (CDCl 3 from TMS) δ: 7.99 (m, 3H), 7.86 (d, 1H,
	yield 36%, Mp 228-230 • C, and 228-230 • C [49]. In CDCl 3 solution 1-[4-(dimethylamino)-with (Z)-2-(1,3-benzothiazol-2-yl)-1-phenylethenol (enolimine form). 1 H NMR (CDCl 3
	phenyl]-2-(1-methyl-1H-benzimidazol-2-yl)ethanone (ketimine tautomeric form) is in equi-from TMS) δ: 13.81 (br s, 1H), 8.09 (d, 2H, 3 J H,H =7.36 Hz), 8.03 (m, 1H), 7.88 (m, 2H),
	librium with (Z)-1-[4-(dimethylamino)phenyl]-2-(1-methyl-1H-benzimidazol-2-yl)ethenol 7.80 (m, 2H), 7.59 (m, 4H), 7.52 (m, 4H), 7.39 (m, 1H), 7.30 (m, 1H), 6.38 (s, 1H), 4.85
	(enolimine form). 1 H NMR (CDCl 3 from TMS) δ: 8.02 (m, 2H), 7.68 (d, 1H, 3 J H,H =8.80 Hz), (s, 1H). 13 C NMR δ: 194.0, 168.1, 165.6, 163.8, 152.3, 150.4, 135.8, 135.2, 134.8, 133.9,
	7.74 (m, 2H), 7.30 (m, 1H), 7.24 (m, 1H), 7.18 (m, 2H), 6.73 (m, 4H), 6.66 (m, 2H), 5.81 131.4, 130.4, 130.0, 129.1, 128.9, 128.7, 128.5, 127.5, 126.5, 126.2, 125.9, 125.2, 124.2,
	(s, 1H), 4.61 (s, 2H), 3.78 (s, 6H), 3.03 (s, 6H), 3.01 (s, 6H). 13 C NMR δ: 191.7, 170.8, 122.8, 122.4, 121.6, 121.4, 120.0, 90.9, 43.7. 15 N NMR (CDCl 3 from MeNO 2 ) δ: -120.0.
	153.8, 153.5, 149.8, 142.2, 136.1, 131.9, 131.2, 127.5, 123.7, 122.4, 122.3, 122.0, 119.2, C 15 H 11 NOS, Calcd. C 71.12, H 4.38, N 5.53. Found C 71.20, H 4.51, N 5.42.

15

N NMR (A slightly modified procedure

[START_REF] Huang | The synthesis and tautomerization of ketene aminals with benzimidazoline ring[END_REF]

was used to prepare 1-

[4-(dimethylamino)

phenyl]-2-(1-methyl-1H-benzimidazol-2-yl)ethanone. A mixture of 1,2-dimethylbenzimidazole (0.29 g, 2 mmol), ethyl 4-(dimethylamino)benzoate (0.39 g, 2 mmol), dry benzene (10 mL), Found C 73.78, H 6.63, N 14.23. 2-Methylbenzothiazole (2.5 g, 16.7 mmol) was added to a suspension of NaH (2 g, 50 mmol, 60% suspension in oil) in dry THF (15 mL) stirred at room temperature for 2 h. After that time methyl benzoate (3.27 g, 21.8 mmol) or ethyl 4-(dimethylamino)benzoate (4.21 g, 21.8 mmol) was added, and the resulting mixture was heated at reflux and stirred magnetically overnight. After cooling down to room temperature, 2 M HCl aq. was slowly added, and the yellow precipitate was filtered and washed with water. Resulting solid was purified by column chromatography (SiO 2 , DMC) [142-144]. Other methods of synthesis of 2-phenacylbenzothiazoles are reported in the literature [125; 128; 134; 136; 137; 145-147]. 2-phenacylbenzothiazole -yellow solid, yield 69%, Mp 110.5-112.5 • C, 113.5-114.5 • C [134], 110 • C [129], 114 • C [125], 113-114 • C [126], 110-111 • C [128], and 116-117 • C [142].

3 J H,H =8.0 Hz), 7.45 (m, 1H), 7.36 (m, 1H), 6.67 (d, 2H, 3 J H,H =8.0 Hz ), 4.75 (s, 2H),

3.07 (s, 6H).

13

  13 C NMR δ: 164.8, 150.4, 134.4, 133.8, 132.0, 131.7, 129.2, 126.9, 125.1, 124.8, 114.3, 111.8, 81.8, 30.5.15 N NMR (DMSO-d 6 from MeNO 2 ) δ: -218.76, -242.04.19 F NMR (DMSO-d 6 from CFCl 3 ) δ: -134.14. C 16 H 13 BF 2 N 2 O, Calcd. C 64.47, H 4.40, N 9.40; Found C 64.53, H 4.46, N 9.48.3-(4-(dimethylamino)phenyl)-1,1-difluoro-5-methyl-1,5-dihydrobenzo[4,5]imidazo[1,2c][1,3,2]oxazaborinin-10-ium-1-uide (X=NMe, R=NMe 2 ) -Yellow solid, yield 39%, Mp 282-283.4 • C.1 H NMR (DMSO-d 6 from TMS) δ: 7.97 (d, 2H, 3 J H,H =9.04 Hz), 7.74 (m, 1H), 7.65 (m, 1H) 7.46 (m, 2H), 6.85 (d, 2H, 3 J H,H =9.12 Hz), 6.79 (s, 1H), 3.96 (s, 3H), 3.09 (s, 6H). 11 B NMR (DMSO-d 6 from BF 3 •Et 2 O) δ: 1.59 (t). 13 C NMR δ: 166.1, 152.7 151.2, 133.8, 131.9, 128.5, 124.6, 124.2, 120.8, 113.8, 111.7, 111.3, 77.9, ca. 40.64-39.36 (overlapped with solvent), 30.23. 15 N NMR (DMSO-d 6 from MeNO 2 ) δ: -211.89, -245.63, -319.77. 19 F NMR (DMSO-d 6 from CFCl 3 ) δ: -134.64. C 18 H 18 BF 2 N 3 O, Calcd. C 63.37, H 5.32, N 12.32; Found C 63.30, H 5.44, N 12.30. 1,1-difluoro-3-phenyl-1H-benzo[4,5]oxazolo[3,2-c][1,3,2]oxazaborinin-10-ium-1-uide (X=O, R=H) -Yellowish-green solid, yield 38%, Mp 239.2-240.8 • C. 1 H NMR (DMSOd 6 from TMS) δ: 8.12 (m, 2H), 7.94 (m, 1H), 7.72 (m, 1H) 7.65 (m, 1H), 7.59 (m, 4H), 7.55 (d, 1H, 3 J H,H =1.64 Hz), 7.22 (s, 1H). 11 B NMR (DMSO-d 6 from BF 3 •Et 2 O) δ: 1.85, (t). 13 C NMR δ: 170.8, 165.1, 148.5, 133.1, 130.1, 129.5, 127.5, 126.9, 126.4, 114.9, 112.5, 82.1. 15 N NMR (DMSO-d 6 from MeNO 2 ) δ: -213.54. 19 F NMR (DMSO-d 6 from CFCl 3 ) δ: -135.07. C 15 H 10 BF 2 NO 2 , Calcd. C 63.20, H 3.54, N 4.91; Found C 63.25, H 3.49, N 4.83. 3-(4-(dimethylamino)phenyl)-1,1-difluoro-1H-benzo[4,5]oxazolo[3,2-c][1,3,2]oxazaborinin-10-ium-1-uide (X=O, R=NMe 2 ) -Orange solid, yield 41%, Mp 298-300 • C. 1 H NMR (DMSO-d 6 from TMS) δ: 7.96 (d, 2H, 3 J H,H =9.12 Hz), 7.85 (d, 1H, 3 J H,H =7.36 Hz), 7.61 (d, 1H, 3 J H,H =7.64 Hz), 7.50 (m, 2H), 6.89 (s, 1H), 6.81 (d, 2H, 3 J H,H =9.16 Hz), 3.07 (s, 6H). 11 B NMR (DMSO-d 6 from BF 3 •Et 2 O) δ: 1.79, (t). 13 C NMR δ: 172.1 153.7, 152.4, 148.2, 130.6, 129.6, 127.0, 125.9, 119.1, 114.2, 112.1, 111.8, 77.7, ca. 40.61-39.35 (overlapped with solvent). 15 N NMR (DMSO-d 6 from MeNO 2 ) δ: -313.97. 19 F NMR (DMSO-d 6 from CFCl 3 ) δ: -133.19. C 17 H 15 BF 2 N 2 O 2 , Calcd. C 62.23, H 4.61, N 8.54; Found C 62.14, H 4.64, N 8.39.1,1-difluoro-3-phenyl-1H-benzo[START_REF] Yoshida | Fluorescence enhancement of tetraazaporphyrins by the use of bulky substituent effects[END_REF][START_REF] Tsai | Fluorescence enhancement of unconstrained gfp chromophore analogues based on the push-pull substituent effect[END_REF]thiazolo[3,2-c][1,3,2]oxazaborinin-10-ium-1-uide (X=S, R=H) -Yellow solid, yield 43% Mp 244.5-245.7 • C. 1 H NMR (DMSO-d 6 from TMS) δ: 8.23 (d, 1H, 3 J H,H =8.0 Hz), 8.13 (m, 2H), 8.00 (d, 1H, 3 J H,H =8.16 Hz) 7.69 (m, 1H), 7.59 (m, 4H), 7.51 (s, 1H). 11 B NMR (DMSO-d 6 from BF 3 •Et 2 O) δ: 1.64 (t).13 C NMR δ: 169.3, 164.9, 142.1, 133.1, 132.8, 129.5, 129.4, 128.9, 127.3, 126.6, 124.1, 117.8, 90.8. 15 N NMR (DMSO-d 6 from MeNO 2 ) δ: -186.16. 19 F NMR (DMSO-d 6 from CFCl 3 ) δ: -132.16. C 15 H 10 BF 2 NOS, Calcd. C 59.83, H 3.35, N 4.65. Found C 59.75, R=NMe 2 ) -Yellow solid, yield 44%, Mp 280-283 • C. 1 H NMR (DMSO-d 6 from TMS) δ: 8.17 (d, 1H, 3 J H,H =8.0 Hz), 7.89 (m, 2H), 7.85 (s, 1H) 7.61 (m, 1H), 7.47 (m, 1H), 7.23 (s, 1H), 6.82 (d, 2H), 3.06 (s, 1H). 11 B NMR (DMSO-d 6 from BF 3 •Et 2 O) δ: 1.54 (t). 13 C NMR δ: 168.5, 166.5, 153.5, 142.3, 129.4, 128.5, 125.7, 123.8, 118.8, 117.1, 111.9, 87.6, ca. 40.60-39.35 (overlapped with solvent). 15 N NMR (DMSOd 6 from MeNO 2 ) δ: -194.22, -314.90. 19 F NMR (DMSO-d 6 from CFCl 3 ) δ: -133.09. C 17 H 15 BF 2 N 2 OS, Calcd. C 59.32, H 4.39, N 8.14. Found C 59.43, H 4.45, N 8.06.

	inin-10-ium-1-uide (X=S,
	H 3.43, N 4.54.
	3-(4-(dimethylamino)phenyl)-1,1-difluoro-1H-benzo[4,5]thiazolo[3,2-c][1,3,2]oxazabor-

Acknowledgments B.O. and A.M.G. thank the Polish National Science Centre (Grant No. 2017/26/M/ST5/00327). W.B. acknowledges the statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wrocław University of Technology. The authors want to address special and warm gratitude to Dr Robert Zaleśny from Faculty of Chemistry, Wrocław University of Science and Technology for countless valuable discussions which helped to improve manuscript and his kind and precious help with CC2 calculations. Authors are also very graceful to Damian Plazuk from University of Łódź for recording fluorescence quantum yield with the use of the integrating sphere. The calculations were performed at the Wroclaw Center for Networking and Supercomputing.

Supporting Informations

The Supporting Information is available on the Elsevier website. It contains fluorescence decay curves, NMR spectra, Cartesian coordinates, computational results and bond-lenghts analysis.