Mohamed Ali Benmensour

Awatef Ayadi

Huriye Akdas-Kilig

Abdou Boucekkine
email: abdou.boucekkine@univ-rennes1.fr

Jean-Luc Fillaut
email: jean-luc.fillaut@univ-rennes1.fr

Abdelkrim El-Ghayoury
email: abdelkrim.elghayoury@univ-angers.fr

Azobased iminopyridine ligands and their rhenium metal complexes: syntheses, spectroscopic, trans-cis photoisomerization and theoretical studies

Keywords: Syntheses, spectroscopic, trans-cis photoisomerization and theoretical studies Iminopyridine, Azobenzene, UV-Visible spectroscopy, Rhenium complex, X-ray diffraction, DFT

come

Introduction

Increasing interest is currently devoted to the photochemistry of transition metal derivatives incorporating stimuli-responsive functions for their potential applications in photonic memory, photosensing, as well as luminescence imaging [START_REF] Otsuki | Molecular switches for electron and energy transfer processes based on metal complexes[END_REF]. In particular, photochromic metal complexes allow the specific properties of the metal center and the optical response of the photochromic group to be combined. Of course, the photoinduced isomerization of azobenzene and its derivatives is one of the most and well-studied light driven processes. The location of the azobenzene unit in the ligands has been established as an important factor to influence the photochromic properties of azobenzene containing transition metal complexes [START_REF] Pérez-Miqueo | Photoswitchable azobenzene-appended iridium(iii) complexes[END_REF][START_REF] Hasegawa | Recent progress of luminescent metal complexes with photochromic units[END_REF][START_REF] Tylkowski | Power of light -Functional complexes based on azobenzene molecules[END_REF][START_REF] Kume | Metal-based Photoswitches Derived from Photoisomerization[END_REF]. The isomerization is assumed to take place from intraligand excited states localized on the azobenzene unit. Meanwhile, the choice of the metal center also participates to the photochromic properties of these azobenzene containing complexes [START_REF] Nishihara | Combination of redox-and photochemistry of azo-conjugated metal complexes[END_REF]. It would be of great interest to better identify the factors which favor an efficient synergy between the intrinsic properties of the metal complex and that of the photochromic unit.

Our interest recently focused on the possible effects of coordination of iminopyridine azo-based ligands to heavy transition metals on the photochemistry of these ligands. For instance, it has been reported that organoplatinum complexes containing iminopyridine azo-based ligands can undergo trans-cis isomerization of the azo group [START_REF] Moustafa | Switching by Photochemical trans-cis Isomerization of Azobenzene Substituents in Organoplatinum Complexes[END_REF]. In this context, we were interested in the photochemistry of Re(I) polypyridyl complexes having iminopyridine-appended azobenzene ligands. Rhenium(I) tricarbonyl polypyridine complexes generally possess attractive photophysical and photochemical properties such as large Stokes shifts, long emission lifetimes, and high photostability through the coordination of functionalized ligands. In the present paper, we aim to report on the synthesis and full characterization, including X-ray diffraction analysis, of two rhenium metal complexes. The focus of this work is on the trans-cis photoisomerization behavior of the free ligands, L1 and L2, (scheme 1) [START_REF] Ayadi | Azo-based iminopyridine ligands: synthesis, optical properties, theoretical calculations and complexation studies[END_REF] and of the corresponding complexes ReL1 and ReL2. These studies are supported by DFT and TD-DFT computational studies that reveal the importance of relevant barriers on the potential energy surfaces in the S1 excited states, which involve conical intersection, for rationalizing the observed behaviours.

Experimental

General remarks

All manipulations were performed under nitrogen atmosphere using commercial grade solvents. Rhenium(I) pentacarbonyl chloride was of reagent grade quality and used as received. NMR spectra were recorded on a Bruker Avance DRX 300 spectrometer operating at 300 MHz for 1 H and 75 MHz for 13 C. Chemical shifts are expressed in parts per million (ppm) downfield from external TMS. The following abbreviations are used: s, singlet; d, doublet; t, triplet; td, triplet of doublet. UV-visible spectra were recorded at room temperature in quartz cuvettes using Perkin-Elmer spectrophotometer. Mass spectra were collected with Bruker Biflex-III TM. IR spectra were recorded on a Bruker vertex 70.

Crystallography

X-ray single-crystal diffraction data for both complexes were collected on an Agilent SuperNova diffractometer equipped with Atlas CCD detector and mirror monochromated micro-focus Cu Kα radiation (λ = 1.54184 Å). The two structures were solved by direct methods, expanded and refined on F 2 by full matrix least-squares techniques using SHELX97 programs (G.M. Sheldrick, 1998). S1.

Synthesis of ReL1 and ReL2 metal complexes

Synthesis of Rhenium(I) complex [ReL1(CO)3Cl] (ReL1)

To a solution of L1 (0.01 g, 0.03 mmol) in a solution of toluene and dichloromethane (3:1, v/v) was added [Re(CO)5Cl] (0.016 g, 0.045 mmol). The mixture was refluxed for 6 hours under nitrogen atmosphere. After cooling the resulting mixture to room temperature, the solvent was removed by a rotary evaporator. The dark residue was extracted with dichloromethane and recrystallized from acetone/hexane to yield complex ReL1 as brown crystals with 90% yield (0.017 g, 0.027 mmol); mp 245 °C; 1 H NMR (300 MHz, CDCl3) δ/ppm: 9.14 (d, 1H, J = 4.89 Hz), 8.89 (s, 1H), 8.16 (td, 1H, J = 7.83 Hz, J = 1.39 Hz), 8.06 (m, 1H), 8.02 (d, 2H, J = 8.7 Hz), 7.96 (d, 2H, J = 8.83 Hz), 7.66 (m, 3H), 6.82 (d, 2H, J = 9.04 Hz), 3.17 (s, 6H). 13

Synthesis of Rhenium(I) complex [ReL2(CO)3Cl] (ReL2)

This complex was prepared by following the same procedure as for complex 1. Yield: 88% (0.017 g, 0.027 mmol); mp 205 °C; 1 m/z = 637.0 Da. found: m/z = 637.1; HR-MS (M): calcd for C21H13N5O5ClRe: 637.0163; found: 637.0161.

Results and discussion

Syntheses

The equimolar reaction between L1 or L2 and the [Re(CO)5Cl] precursor at refluxing toluene/dichloromethane mixture, under inert atmosphere, afforded the mononuclear neutral respectively as a brown precipitate for ReL1 and as an orange precipitate for ReL2.

Description of crystal structures

Single crystals of ReL1 and ReL2 metal complexes were obtained by recrystallization from acetone/hexane solutions. Crystal structures are represented in Fig. 1. Both ReL1 and ReL2 complexes crystallize in the triclinic space group P-1 with one independent molecule in the unit cell.

Both ligands L1 and L2 acquired a cis-conformation of the iminopyridyl group, within the complexes.

The rhenium center is surrounded by the bidentate iminopyridyl chelating fragment, three carbonyl groups, and a chlorine atom and its coordination sphere presents the expected, although slightly distorted, octahedral geometry. This minor distortion from the ideal octahedral geometry is caused by the formation of a five-membered chelate ring (the N-Re-N angle) [START_REF] Pino-Cuevas | Rhenium Complexes of Ligands Based on Stilbene -Synthesis, Characterization, Reactivity, and Conformational Analysis[END_REF]. The structures of both complexes highlight the expected facial arrangement of the three carbonyl ligands. For both complexes, the values corresponding to cis angles around the Re(I) ion are very close to the ideal value of 90°. Meanwhile, the N-Re1-N angle shows significantly lower values, 74.33(2)° and 74.69(1)° for ReL1 and ReL2 respectively, as imposed by the small N-N bite distance of a α-iminopyridyl type ligand (average values for ReL1 and ReL2 respectively). The dihedral angle between the iminopyridyl fragment and the adjacent phenyl ring of the azobenzene unit is equal to 32.80°(3) for ReL1 and to 42.67°(3) for ReL2.

Within the crystal structure of ReL1 the Re-N distances (Re-N4 = 2.206(5) Å, Re-N5 = 2.177(5) Å) are similar to those found in complexes of the {ReX(CO)3} fragment with the iminopyridyl based coordinating group (Table S2 and Figure S1 in supplementary information) [START_REF] Shavaleev | Syntheses and structures of mononuclear {Re(CO)3Cl(NN)} 'complex ligands' with a pendant imino-pyridine binding site, and preparation of some heterodinuclear Re(I)-lanthanide(III) complexes[END_REF][START_REF] Alvarez | Carbonyl complexes of manganese, rhenium and molybdenum with 2-pyridylimino acid ligands[END_REF][START_REF] Pramanik | Re(I) carbonyl complexes of N-[(2-pyridyl)methyliden]-α (or β)-aminonaphthalene: Synthesis, structure, electrochemistry and DFT analysis[END_REF]. All three Re-CO bond lengths are very close, and the Re-C-O angles present minor deviations from linear structure, values ranging from (177.50(9)° to 177.78(8)°).

In the crystal structure of ReL2 complex, the ligands surrounding the Re(I) ion are also involved in hydrogen bonding interactions type. Thus, two of the three CO ligands establish short C-H 5) Å) with hydrogen atoms from pyridine linking neighbor molecules into layers parallel with the ac crystallographic plane (Fig. S2).

•••O contacts (H11•••O1 2.600(6) Å; H15•••O2 2.550(

UV-Visible absorption spectroscopy

The UV-visible absorption spectra (Fig. 3 and Fig. 4) of the two ligands L1 and L2 and their corresponding metal complexes ReL1 and ReL2 were recorded in dichloromethane solution (~ C = 5.9.10 -5 M) at room temperature. As previously reported [START_REF] Ayadi | Azo-based iminopyridine ligands: synthesis, optical properties, theoretical calculations and complexation studies[END_REF], ligand L1 exhibits two strong absorption bands at  = 266 nm and 438 nm that are assigned to the * and n* ( 1 ILCT) transitions together with a slight intramolecular charge transfer (ICT) from the donating dimethylamino donor to the

A C C E P T E D M A N U S C R I P T
accepting iminopyridine unit. The spectrum of ReL1 complex shows the same characteristics as ligand L1 with a bathochromic shift and a broadening of the visible absorption band. The bathochromic shift (ca. 40 nm) indicates an increase of the electron acceptor character of the iminopyridyl fragment upon complexation with rhenium (I) which acts as a strong Lewis acid while the broadening of the band can be assigned to the presence of of a metal to ligand charge transfer Re(d  ligand(*) 1 MLCT transition [START_REF] Dominey | (2-PP) [2-PP = N-(2-pyridinylmethylene)phenylamine] and ClRe(CO)3(2-PC) [2-PC = N-(2pyridinylmethylene)cyclohexylamine[END_REF]. We assume that, in ReL1, the two absorption bands corresponding to the 1 ILCT and 1 MLCT are superimposed. These features are confirmed by means of the TD-DFT computations which have been carried out (vide infra).

In the case of ligand L2 [START_REF] Ayadi | Azo-based iminopyridine ligands: synthesis, optical properties, theoretical calculations and complexation studies[END_REF] a strong absorption band around 380 nm assigned to -* and n-* transitions is observed. In the case of ReL2, because of the presence of a nitro electron withdrawing group in L2, the absorption band in the visible region is blue-shifted, by comparison with L2, while a broad absorption band, of lower intensity ( = 4500 L.mol -1 .cm -1 ) is observed in the visible region around max = 452 nm which is characteristic of a metal to ligand charge transfer (Re(d  ligand(*) 1 MLCT transition [START_REF] Dominey | (2-PP) [2-PP = N-(2-pyridinylmethylene)phenylamine] and ClRe(CO)3(2-PC) [2-PC = N-(2pyridinylmethylene)cyclohexylamine[END_REF]. Note that no significant emission behavior has been observed for the two rhenium complexes.

Photoisomerization studies

It is well known that azobenzenes experience a trans-cis isomerization by irradiation at a light wavelength close to their absorption maxima [START_REF] Bandara | Photoisomerization in different classes of azobenzene[END_REF]. L1, L2 (that can be considered as being push-pull azobenzenes (ppAB) types [START_REF] Dominey | (2-PP) [2-PP = N-(2-pyridinylmethylene)phenylamine] and ClRe(CO)3(2-PC) [2-PC = N-(2pyridinylmethylene)cyclohexylamine[END_REF]) and their corresponding rhenium complexes were systematically studied in order to establish if trans-cis isomerization occurs in these systems. Fig. 2 exhibits the UV-visible absorption spectra of L1 in dichloromethane, showing drastic modifications of the UV-visible absorption spectra upon irradiation at 450 nm revealing the photoisomerization of the azobenzene chromophore in L1. Upon irradiation the intensity of the absorbance at 440 nm, attributed to the * transition of the trans-azobenzene group, decreased gradually, reaching a relatively stable value at a maximum wavelength of 460 nm. At the same time, the intensity of a new signal in the 300-400 nm range, which we assign to the * transition of the cis isomer, increased, leading to two isobestic points at 380 and 530 nm for L1. These variations are consistent with the trans-cis isomerization of the azobenzene group in L1, as described for similar compounds [START_REF] Mohamed | Azopyridinefunctionalized benzoxazine with Zn(ClO4)2 form high-performance polybenzoxazine stabilized through metal-ligand coordination[END_REF], and at the photostationary state, the intensity of the main absorption band collapsed by a factor approximately 3. Indeed, the TD-DFT simulations of the UV-visible spectra of the trans and cis isomers of L1 (Fig. S3), which compare very well with the observed ones, confirms the occurrence of the transcis isomerization under irradiation.

A C C E P T E D

M A N U S C R I P T
In the case of ligand L2 we did not observe any photoisomerization, whatever is the solvent (dichloromethane or acetonitrile).

For complexes ReL1 and ReL2 we did not observe any change of the absorption spectra in acetonitrile or in dichloromethane under irradiation or in the dark. The trans-to-cis photoisomerization property is completely switched off for the complex ReL1, compared to the ligand L1. Clearly, the photoisomerization dynamics is strongly influenced by the conjugation pathway and modified upon coordination of a metal to the azobenzene containing conjugated ligand, as previously demonstrated by Fillaut and coworkers for a series of Ruthenium(II) tris(bipyridine) complexes [START_REF] Amar | Photoisomerization in Aminoazobenzene-Substituted Ruthenium(II) Tris(bipyridine) Complexes: Influence of the Conjugation Pathway[END_REF]. More recently Freixa and coworkers reported Iridium(III) cyclometalated complexes, in which the use of an aliphatic spacer unit between the azobenzene and the coordinating unit of the ligand allowed the electronic communication to be disrupted and the observation of photochromic properties for these coordination complexes [START_REF] Pérez-Miqueo | Photoswitchable azobenzene-appended iridium(iii) complexes[END_REF].

Simulated UV-Visible absorption spectra and theoretical calculations

All calculations were performed at the DFT level of theory, employing the G09 suite of programs [17].

Based on the results of several previous studies on similar systems, the B3LYP [START_REF] Lee | Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density[END_REF][START_REF] Becke | Density-functional thermochemistry. III. The role of exact exchange[END_REF][START_REF] Stephens | Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields[END_REF] and CAM-B3LYP functionals [START_REF] Yanai | A new hybrid exchange-correlation functional using the Coulombattenuating method (CAM-B3LYP)[END_REF][START_REF] Tozer | Improving virtual Kohn-Sham orbitals and eigenvalues: Application to excitation energies and static polarizabilities[END_REF] have been chosen to perform the computations. The 6-31G(d) and LanL2DZ basis set [START_REF] Hay | Ab initio effective core potentials for molecular calculations. Potentials for K to Au including the outermost core orbitals[END_REF] augmented with polarization function on all atoms, except hydrogen ones, have been used for the ligands and their corresponding complexes respectively. Solvent effects were taken into account using the polarisable continuum model (PCM) [START_REF] Barone | A new definition of cavities for the computation of solvation free energies by the polarizable continuum model[END_REF]. All ground state geometries were optimized at these levels of theory and checked to be true minima on the potential energy surface, the computed frequencies of the normal modes of vibration being all real. Then, time-dependent density functional theory (TD-DFT) [START_REF] Casida | Recent Advances in Density Functional Methods[END_REF] computations were performed, using the optimized ground state geometries, to obtain excitation energies and simulate the UV-visible spectra of the considered species.

The simulated UV-visible spectra of the considered ligands and complexes agree very well with the observed ones, when using the CAM-B3LYP functional, so that our discussion will be based on the CAM-B3LYP results only. This reliability of the CAM-B3LYP functional for the UV-visible spectra simulation has already been observed in the case of the ligands [START_REF] Ayadi | Azo-based iminopyridine ligands: synthesis, optical properties, theoretical calculations and complexation studies[END_REF].

First, we observe that the band shifts observed for the UV-visible spectra upon complexation are well reproduced by the computations, namely the bathochromic one in the case of the complexation of ligand L1 and the hypsochromic one in the case of ligand L2 (Fig. 3 and Fig. 4).

A C C E P T E D M A N U S C R I P T

The results of the TD-DFT computations which permit the assignment of the UV-visible absorption bands of the ligands are given in Table 1. a H = HOMO; L =LUMO The assignments of the observed UV-visible bands of the free L1 and L2 ligands have been described in reference [START_REF] Ayadi | Azo-based iminopyridine ligands: synthesis, optical properties, theoretical calculations and complexation studies[END_REF]. To consider the photoisomerization issue, the properties of the free L1 and L2 ligands are compared to those of their corresponding complexes, reminding that ligand L1 only undergoes the trans to cis isomerization.The first excited states of both L1 and L2 derive from transitions involving the diazo group, thus likely to trigger the trans-cis photoisomerization of the azobenzene group. Indeed, HOMO-2 of L1 and HOMO-1 of L2, exhibit a diazo lone pair character, whereas the reached MOs upon excitation, namely LUMO and LUMO+1 for L1, LUMO and LUMO+2 for L2 are  type MOs (Fig. 5), so that the computed excitations at 431 nm for L1 and 468 nm for L2 exhibit mainly a n-* character. However, the oscillator strength of the electronic transition is ca. 0.0050 for L1 and ca. 0.0001 for L2 which is very low, fifty times smaller than that for L1. This is certainly a factor contributing to the fact that ligand L2 does not exhibit a photoisomerization, contrarily to ligand L1. It is worth noting that in our previous TD-DFT study of the ligands [START_REF] Ayadi | Azo-based iminopyridine ligands: synthesis, optical properties, theoretical calculations and complexation studies[END_REF] we were interested in the UV-visible spectra only so that the electronic transitions of low oscillator strength were not considered. These latter transitions, i.e. HOMO-2 to LUMO for L1 and HOMO-1 to LUMO for L2 are of importance (vide supra) for the isomerization processes, explaining why they are considered now.

The assignment of the UV-visible absorption bands of the ReL1 and ReL2 complexes has also been done on the basis of the TD-DFT results given in Table 2 and Table 3 whereas the frontier MO diagrams of these species are displayed on Fig. 6.

For the ReL1 complex, the computed excitation at 431 nm exhibits an n-π* character, the involved occupied molecular orbital HOMO-4 is localized on the diazo function (Table 2). This excitation is similar to that computed for the ligand L1, but with a lower oscillator strength. However, it appears that this complex does not isomerize under irradiation contrarily to ligand L1. Thus, for ReL2, we observe the same situation as for the ligand L2 alone; the computed excitation at 467 nm exhibits an n-π* character with a very low oscillator strength of ca. 0.0007, the involved occupied HOMO-4 being localized on the azo function (Table 2). Similarly to the L2 ligand, the relevant excited state will not be easily reached and then the photoisomerization should hardly be possible.

Another interesting difference appears between the UV-visible spectra of ReL1 and ReL2, when considering the most intense absorption band computed at 419 nm for ReL1 and at 354 nm for ReL2.

Contrarily to ReL1, the absorption band of ReL2 exhibits a significant MLCT character, since the weight of the metal in the involved occupied MOs of the corresponding transitions (Table 4) namely HOMO

A C C E P T E D M A N U S C R I P T
of ReL1, is ca. 0.15% whereas for the HOMO-2 of ReL2, this weight is ca. 26%. In the case of ReL1 the two excitations at 395 and 374 nm with a smaller oscillator strength exhibit also a MLCT character, the metal weights in the corresponding occupied MOs being equal to ca. 50% and 48% respectively. The low intensity bands computed at 402 nm and 380 nm of ReL2 exhibit also an important MLCT character. Finally, it is interesting to note that whereas the HOMO of ReL2 has an important metallic character (ca. 49%) it is not the case for ReL1, mainly due to the strong donating character of the NMe2 group of the L1 ligand which contributes strongly to push the ligand HOMO to higher energies than the metallic MO.

Considering in more details the photoisomerization issue, the ligand and the complexes behaviors have been investigated. The photoisomerization of azobenzene in solution has already been studied experimentally and by means of theoretical calculations [START_REF] Chong | A New Trans-to-Cis Photoisomerization Mechanism of Azobenzene on the S1(n,π*) Surface[END_REF][START_REF] Chang | Photoisomerization Dynamics of Azobenzene in Solution with S1 Excitation: A Femtosecond Fluorescence Anisotropy Study[END_REF][START_REF] Quick | Photoisomerization Dynamics and Pathways of trans-and cis-Azobenzene in Solution from Broadband Femtosecond Spectroscopies and Calculations[END_REF]. It has been found theoretically that the isomerization from the trans conformation to the cis one after the n-π* vertical excitation, occurs on the S1 potential energy surface (PES), in three steps. The first two steps correspond to a relaxation to a local minimum, then to an intermediate state through an energy barrier. The third step corresponds to a S1 → S0 relaxation via a conical intersection (CI) S1 / S0 with an energy barrier of 12 kJ / mol, towards the trans to cis isomerization [START_REF] Chong | A New Trans-to-Cis Photoisomerization Mechanism of Azobenzene on the S1(n,π*) Surface[END_REF].

Note that in the singlet ground state (S0) the ligands are more stable in a trans conformation. The energy barrier between the two trans and cis forms is high in the ground state (S0) up to about 50 kcal/mol. Interestingly, we found that the trans structure undergoes a distortion in the S1 state leading to a non-planar structure of the phenyl azo moiety. Therefore, we determined for the two ligands the minima on the S1 PES, then we identified for each ligand intermediate (I-cis and I-trans) and transition states on this S1 PES using the QST2 method, in order to estimate the energy barriers between the cis forms and the TSs. Moreover, as expected [START_REF] Chong | A New Trans-to-Cis Photoisomerization Mechanism of Azobenzene on the S1(n,π*) Surface[END_REF][START_REF] Chang | Photoisomerization Dynamics of Azobenzene in Solution with S1 Excitation: A Femtosecond Fluorescence Anisotropy Study[END_REF][START_REF] Quick | Photoisomerization Dynamics and Pathways of trans-and cis-Azobenzene in Solution from Broadband Femtosecond Spectroscopies and Calculations[END_REF] we observed the occurrence of a conical intersection, namely that the minimum energy structure on the S1 PES corresponds to a TS on the ground state S0 PES.

According to Fig. 7, we note that the intermediate I-cis (S1) state of L2, although lower in energy than the TS(S1) is higher in energy than the min(S1)=TS(S0) so that the relaxation of this intermediate via the S1/S0 CI leads to the trans conformation, whereas for L1 (Fig. 7), the intermediate I-cis (S1) state is lower in energy than the min(S1)=TS(S0); then, the relaxation of this intermediate leading to a cis conformation is facilitated. There is also another difference between L1 and L2, which is the cis to trans backward barrier on S1; as shown in Fig. 7 and Fig. 8, the barrier in the case of L2 is ca. 5.8 kcal/mol which is lower than that for L1 (23. kcal/mol) so that the latter ligand cannot undergo the reverse cis-trans conversion. Thus, either on the S1 PES or via the conical intersection, the trans-cis isomerization appears to be highly favoured for the L1 ligand contrarily to the L2 one, thus explaining the observed facts.

We have also explored the PES for the two ReL1 and ReL2 complexes; these complexes are more stable in the singlet ground state (S0) in a trans conformation, whereas in the S1 PES this conformation is distorted as already observed for the ligands. In the same way as for the ligands, we have located for each complex the transition state (TS), and then the energy barrier (cis-TS).

We found that these complexes behave as ligand L2. Indeed, the backward cis to trans barrier is low, ca. 4 kcal/mol for ReL1 and is ca. 6 kcal/mol for ReL2, thus of the same order of magnitude as L2 (5.8 kcal/mol). We think that this low energy barrier can explain the nonphotoisomerization for the two complexes, since it is likely to allow the return from cisconformation to trans-conformation.

We have also investigated theoretically the isomerization process in the triplet state reminding that in our case, the experiments which have been carried out do not involve this excited state.

The obtained computational results show the same trend as observed for the ligands, namely that the triplet state as well, should allow the trans-cis isomerization for the L1 ligand but not for L2 and the complexes.

Conclusions

In this paper, we describe the synthesis, full characterization and the theoretical studies of two new rhenium complexes ReL1 and ReL2, L1 and L2 being azo-based iminopyridine ligands. The UV-visible spectrophotometric data show a red shift for complex ReL1 indicating an increase of the electron acceptor behavior of the iminopyridine fragment in ligand L1 upon complexation with Re (I) and a blue shift for complex ReL2 with appearance of new broad absorption band in the visible region indicating the occurrence of a MLCT transition. The observed bands were fully assigned using the TD-DFT results.

The distinct behaviors of the ligands and complexes regarding the trans-cis photoisomerization process have been rationalized, using DFT and TD-DFT computations. Several parameters have been considered for that, among them the relevant barriers on the potential energy surfaces in the S1 excited states, which involve conical intersection.

  All non-H atoms were refined anisotropically and the H atoms were included in the calculation without refinement. Multiscan empirical absorption was corrected using the CrysAlisPro program (CrysAlisPro, Agilent Technologies, V1.171.37.35g, 2014). Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC 1837472 for complex ReL1 and CCDC 1837473 complex ReL2. Details about data collection and solution refinement are given in Table

  rhenium(I) metal complexes ReL1 and ReL2 (scheme 1), described as [ReL1(CO)3Cl] and [ReL2(CO)3Cl]

8

 8 Fig.8, the barrier in the case of L2 is ca. 5.8 kcal/mol which is lower than that for L1(23.8

Scheme 1 .Fig. 1 .

 11 Scheme 1. Synthetic route of rhenium complexes ReL1 and ReL2.

Fig. 2 .

 2 Fig. 2. Changes in the UV-Visible absorption spectra upon irradiation a 450 nm of ligand L1 in CH2Cl2.

Fig. 3 .Fig. 4 . 9 Fig. 5 .

 3495 Fig. 3. Experimental (C = 2.5.10 -5 M) and simulated UV-Visible absorption spectra of ligand L1 and ReL1 complex (in dichloromethane solution at room temperature)

Fig. 6 .

 6 Fig. 6. Frontier MO diagrams of complexes ReL1 and ReL2 with main transitions (CAM-B3LYP level).

Table 1 .

 1 TD-DFT results (absorption wavelengths, oscillator strengths) for the ligands Compounds  in nm (ΔE in eV)

	f	Main transitions a

a H = HOMO; L =LUMO

Table 2 .

 2 TD-DFT results for the complexes

					U S C R I P T
	Compounds	 (nm)		f	N	Main transitions
		431		0.0030 A H-4→L(67%) H-4→L+1(21%)
	ReL1	419 395	1.4566 0.0805 M	H→L(52%) H→L+1(35%) H-1→L(85%)
	374 467 402 380 354 C C E P T E D ReL2	0.0823 0.0007 0.0114 0.1444 1.2107	H-2→L(88%) H-4→L(68%) H-4→L+2(21%) H→L+1(59%) H→L(36%) H-1→L+1(57%) H-1→L(37%) H-2→L(69%)
	A

Table 3 .

 3 Rhenium, metal group and azo percentage weights in frontier molecular orbitals

	MO	compounds	Rhenium weight %	Metal group a weight %	Azo group weight %
		ReL1		0.49		2.74	1.14
	LUMO+2 LUMO+1 LUMO HOMO	ReL2 ReL1 ReL2 ReL1 ReL2 ReL1 ReL2		0.83 1.29 2.24 2.18 0.5 0.15 49.14	2.15 3.67 5.9 5.50 1.26 0.28 91.67 U S C R I P T 22.93 37.78 2.70 5.92 23.20 8.60 0.19
	HOMO-2 HOMO-1	ReL2 ReL1 ReL2 ReL1	47.24 47.93 26.29 M A 49.75 N	89.36 91.71 31.94 94.02	0.33 6.92 0.14 0.09
	HOMO-3 HOMO-4 a Metal group = Re(CO)3Cl ReL1 ReL2 ReL1 ReL2 C C E P T E D	44.00 44.45 1.91 0.17		54.32 77.07 02.01 0.53	6.05 4.19 73.23 76.47
	A

Acknowledgements.

We acknowledge the HPC resources of CINES and of IDRIS under the allocations 2017-[x2015080649] and 2018-[x2016080649] made by GENCI (Grand Equipement National de Calcul Intensif).