Cyclometalated iridium(iii) complexes of (aryl)ethenyl functionalized 2,2′-bipyridine: synthesis, photophysical properties and trans–cis isomerization behavior

Department of Chemical Sciences, Indian In (IISER) Kolkata, Nadia, Mohanpur, 741246, +91-3473279131; Tel: +91-3473279130 † Electronic supplementary information ( H NMR: L2; Fig. S2: expanded NMR: L spectra L1, L2; Fig. S4: COSY and NOE dependent electronic spectra: L1; Fig. S6: COSY and NOESY NMR: 1; Fig. S8: ex temperature dependent H NMR of 1 and spectra of 2; Fig. S11: ESI-MS of ligands and 2. Fig. S13: C NMR of complexes; angles of 1 and 2. Combined CIF. CCDC 999589. For ESI and crystallographic data DOI: 10.1039/c5ra16214a Cite this: RSC Adv., 2015, 5, 99529


Introduction
Cyclometalated iridium(III) complexes with rich photophysical and electrochemical properties have a variety of applications in the eld of sensors, 1 organic light-emitting diodes, 2 lightemitting electrochemical cells, 3 biological imaging agents, 4 catalysts for water splitting, 5 dye-sensitized solar cells (DSSCs) 6 and organic transformations. 7 The photophysical properties of a wide range of cyclometalated heteroleptic iridium(III) complexes with 4,4 0 -substituted-2,2 0 -bipyridyls as ancillary ligands have also been explored extensively. 1d,8 The presence of two C X N ligands exhibits large ligand-eld stabilization of iridium owing to its high oxidation state. In addition, strong spin orbit coupling constant of iridium (z Ir ¼ 3909 cm À1 ) and long-lived triplet excited state (s $ ms) enable these family of compounds to show signicant photophysical properties. 8a The photoinduced trans-cis isomerization of the (aryl) ethenyl functionality is well studied and it is believed to undergo the isomerization following an excitation to the lowest triplet excited state by suitable sensitizers or to the singlet excited state by absorption of light. 9 trans-Stilbene and its analogues are important chromophores in many organic and organometallic materials for applications in nonlinear optics, light-emitting diodes, organic photovoltaics (OPVs), photochemical molecular devices, optical sensors, and DSSC. 10 Therefore, the study of the photoinduced trans-cis isomerization of the cyclometalated iridium(III) chelated with (aryl) ethenyl system is of fundamental mechanistic interest and also important for their practical applications.
Excluding Pt(II), the 5d transition metal complexes with(aryl) ethenyl substituted bipyridyl systems have not been studied extensively. Baik and Wang have reported a boron containing trans-stilbenoid appended 2,2 0 -bipyridyl ligand that undergoes trans-cis isomerization in the free ligand but complexation with Pt(II) prevent its isomerization. 11 However, the isomerization process of (aryl)ethenyl substituted phenyl pyridines show the isomerization process with  But the process of isomerization did not get enough focus in earlier reports. In this report, our objective is to understand the signicant change in the photoinduced isomerization of the (aryl)ethenyl substituted 2,2 0 -bipyridyl ligands (L1-L2) in the free state, and aer complexation with cationic cyclometalated iridium (1, 2).
It is expected that the generation of low-lying MLCT state and consequently the intramolecular energy transfer may affect the isomerization process of the (aryl)ethenyl functionality in conjugation with the iridium(III) centre. 13 Ligands were judiciously chosen to understand the steric effect on free rotation around a single bond in a ve-membered N-methyl imidazole group, L1 versus a six membered 2-methoxy phenyl group in L2. The 1 H NMR signals of methyl groups are far apart from the aromatic protons that can be easily recognized by photoinduced position changes. The ethenyl moieties photoisomerizes from trans-trans conguration to trans-cis and cis-cis form, when exposed at 366 nm wavelength light. On contrary the complexes 1 and 2 show only trans-cis form upon irradiation with the light of same wavelength. We report the detailed study of the photophysical properties and the photoinduced behavior of the iridium complexes, characterized by spectroscopic techniques and quantum chemical calculation.
We carried out all the reactions in dark condition to afford the trans-conguration of the (aryl)ethenyl framework. Under the dark condition (t ¼ 0 min), AB system in 1 H NMR in DMSOd 6 located at d (ppm) 7.57; 7.44 ( 3 J H-H ¼ 16.04 Hz) for L1 (Fig. 1), and 7.80; 7.18 ( 3 J H-H ¼ 16.04 Hz) for L2 (Fig. S1 †) conrm the presence of trans-trans (t-t) conguration of (aryl)ethenyl groups. However, both the ligands undergo facile olenic transcis (t-c) and cis-cis (c-c) isomerization at 366 nm light exposure. Under photoirradiation, the time dependent 1 H NMR spectra ( Fig. 1) indicate that the isomerization process for ligand L1 reaches to the photostationary state aer $90 min (L2 $ 100 min) at ambient temperature. The generation of new associated peaks close to the existing peaks at d (ppm) 7.56; 7.46 (ABq, 2H, J AB ¼ 16.04 Hz) of L1, and 7.73; 7.26 (ABq, 2H, J AB ¼ 16.7 Hz) of L2 (Fig. S1 †) authenticate the presence of trans-cis isomers. Moreover, the growth of a completely new set of peaks at d (ppm) 6.79-6.73 (L1) and 6.88-6.69 (L2) with J AB ¼ 12 Hz describe the presence of cis-cis isomers. The characteristic methyl proton peak initially at d 3.82 ppm of L1 (Fig. S2 †) and at 3.91 ppm of L2 (Fig. S1 †) gradually diminished with the growth of new peaks at d 3.668, 3.662 ppm and 3.89, 3.78 ppm, respectively. 1 H NMR data further revealed that the trans-cis isomers contribute $20% to the total isomerization process along with $8% cis-cis isomers of L1 and $22% trans-cis isomers along with $7% cis-cis isomers of L2 at the photostationary state. The rotational barrier for L1 is found to be $8 kcal mol À1 (ref. 15) but it was difficult to calculate for L2 due to the overlap of proton signals (Fig. S3 †). However, the spectrum predicts that the rotational barrier in case of L2 will be in the same order. Moreover, the proton couplings of L1 and L2 have been conrmed from the correlation diagram of 1 H-1 H COSY, 1 H-1 H NOESY NMR in DMSO-d 6 (500 MHz) (Fig. S4 †).
The structural elucidation of ligands L1 and L2 support the trans-trans disposition of the (aryl)ethenyl functionality at t ¼ 0 min (Fig. 2). Both the ligands L1 and L2 possess a crystallographically imposed inversion center and the (aryl)ethenyl moieties show some deviation from planarity. The two bipyridyl rings are in the same plane with a dihedral angle between the bipyridyl and the N-methyl imidazolyl (L1, 19.83 ) or 2-methoxy phenyl ring (L2, 13.57 ). The crystallographic parameters are tabulated in Table 1 and bond parameters in Table S1. † The ligands L1 and L2 dissolved in dry dichloromethane show very intense bands in absorption spectra (Fig. S5 †) at l < 400 nm (3 z 8-10 Â 10 5 M À1 cm À1 ) due to spin-allowed 1 p-p* transitions and intense emission at 410 and 404 nm ( Fig. 3) with quantum yield of 0.28 and 0.22 for L1 and L2, respectively. The energy dissipation of light by the olen bonds due to facile trans-cis and cis-cis isomerisation causes about 70% decrease in quantum yield of L1 and 44% of L2 (Table 2).

Synthesis of complexes 1 and 2 (trans, trans)
The syntheses of complexes 1-2 have been done by reuxing [Ir(ppy) 2 Cl] 2 (ppy ¼ 2-phenylpyridine) with ligands L1-L2 in DCM : MeCN (1 : 1) solution at 70 C for 3 h (Scheme 1) under dark condition to avoid the trans-cis isomerization during the synthesis. To check the purity of the complexes, HPLC experiment (equipped with a reversed-phase column), 1 H and 13 C NMR spectroscopy were done. Both the complexes show single peak in the HPLC (Fig. S6A †) and the NMR spectra were shown      Table 3. The time dependent 1 H NMR, 1 H-1 H COSY and 1 H-1 H NOESY experiments of the complexes provide strong evidence of the presence of only trans-cis conguration along with the transtrans conguration of ethenyl groups, once exposed to light ( Fig. S7-S9 †). The cis-cis conguration at both the sides of the bipyridyl ring has never formed. 1 H NMR data further revealed that the trans-cis isomers contribute $30% to the total isomers for 1 and around 50% for 2 at the photostationary state. We have tried to calculate the rotational barrier from variable temperature 1 H NMR spectrum, but could not succeed due to the complicated nature of the same. The overlap of the aromatic proton signals from phenylpyridines is responsible for the complication (Fig. S11 †).

X-ray crystallography
It is noteworthy that the complex 1 gets crystallized with the same trans-trans disposition ( Fig. 6) of ethenyl groups before and aer irradiation. Several attempts to grow good quality crystal were done but the quality of the crystal remains very poor in all the cases. Incidentally, aer photoirradiation signicant rotation around the single bond is observed in case of complex 2 (apart from the trans-cis isomerization). The X-ray    crystallographic structures of complex 2 obtained from slow diffusion of dichloromethane solution (with added NH 4 PF 6 ) before and aer the irradiation (2 and 2 nal ) (UV-light of 366 nm) (Fig. 7), always provides different disposition of ethenyl groups. The 2 nal structure is the most stable structure and it is also evident from the DFT study (see later). For all the cases, the cis-[Ir(C X N) 2 (N X N)] complexes were in slightly distorted octahedral geometry in which Ir-C(ppy), Ir-N(ppy) and Ir-(N X N) bond distances are comparable to the previously reported complexes. 16 The light exposed complexes are thermally switchable to the initial state once heated at 70 C in DCM : MeCN (1 : 1) solution under dark. The crystallographic parameters are tabulated in Table 1 and bond parameters in Table S1. † Photophysical studies of complexes 1 and 2 (pure trans, trans form) The photophysical studies of the complexes were done with the complexes with trans, trans conguration.
The absorption spectra of the cyclometalated iridium(III) complexes usually witness metal-to-ligand charge transfer (MLCT), where an electron is promoted from a metal d orbital to a vacant p* orbital on one of the ligands, and ligand-centered (LC) transitions in which an electron is promoted from p orbital on one of the coordinated ligands. Absorption spectra (Fig. 8, Table 2) of the complexes 1 and 2 were recorded in dry DCM and dominated by intense high energy bands around 300 nm with 3 z 5 Â 10 5 M À1 cm À1 , assigned to spin allowed ligand-centered 1 IL transitions ( 1 p / p*) from both the ppy and bpy ligands. 17 The moderately high energy absorption shoulders with high extinction coefficients around 382-390 nm have been assigned to an admixture of spin-allowed metal-toligand charge transfer ( 1 MLCT), dp-p*(bpy) and ligand-toligand charge transfer ( 1 LLCT), p(bpy)-p*(bpy) processes (Table 2). 18 The low intensity band (3 z 8 Â 10 3 M À1 cm À1 ) around 470 nm is due to the spin-forbidden transitions owing to 3 MLCT and 3 LLCT/ 3 LC transitions. The emission spectra of 1 and 2 (10 mM, dry DCM) showed l max at 406 nm (Fig. 8) and 408 nm for 1 and 2 (Fig. S12 †) respectively, when excited at 350 nm.
The excitation at 410 nm shows the 3 MLCT/ 3 LC based emission at 556 nm (620 nm, vibronic progression) and 561 nm for the complexes 1 and 2 (Fig. 8), respectively. The emission spectrum of the complex 1 show l max at 556 nm with vibronic progression at 620 nm, but such feature is absent in the case of 2.
The solid-state emission was done with thin lm of dichloromethane solution on glass. The complex 1 exhibits bright orange emission with an intense, signicantly red shied band at 568 nm with lower energy vibronic progression at 628 nm (room temperature; l max : 556 nm and 620 nm). However, in case of the complex 2, signicant blue-shied emission at 544 nm with vibronic progression at 606 nm (room temperature; l max : 561 nm) is observed. In both the cases, at low temperature alteration of the l ex from 410 nm to 480 nm provides well resolved spectra (Fig. 8). At 77 K, the PLQY is difficult to measure with acceptable accuracy but the observed lifetime can provide quantitative information. The main emission peak of both the complexes exhibit biexponential decay pattern with a long excited state lifetime of 38.49 mS and 24.21 mS in the solid state. These observations conrm that the emissive center owns a triplet character and the long excited state lifetime also support that the emission   originates from a 3 MLCT/ 3 LC excited state. The signicant red or blue shi in the l max of the complexes attributes to the changes in the intermolecular p-p interactions by aggregation in the solid state. Strong spin-orbit coupling from the iridium(III) center facilitates intersystem crossing to energetically similar triplet states and enables the formation of an emissive mixed (triplet) excited state. This vibronic structure indeed supports considerable contribution of 3 LC character to the emission. To conrm the contribution of 3 LC character, we recorded the emission in the solid state (thin lm on glass, Fig. 8). To understand the nature of emission more precisely, theoretical studies have also been done.

Photoirradiation at 366 nm
The emission intensity diminished slowly upon irradiation with ultraviolet light (l ¼ 366 nm). Comparison of the l em at around 406 nm of the complexes with the ligands (Table 2) indicates the ligand-based character of the emission at the said wavelength. It was noted that the complexes 1 and 2 reached at the photo stationary states aer 48 and 42 minutes, respectively at ambient temperature. This signies that the energy is dissipated through non-radiative processes. As discussed, the excitation at 410 nm before irradiation shows the 3 MLCT/ 3 LC based emission at 556 nm (620 nm, vibronic progression) and 561 nm for the complexes 1 and 2 (Fig. 9) and the emission of the complex 1 show l max at 556 nm with vibronic progression at 620 nm (2, l em ¼ 561 nm). The emission spectra aer photoirradiation showed similar pattern with reduced emission intensity (complex 1: 546 nm, 624 nm; complex 2: 568 nm) (Fig. 9). The mixture of isomers obtained could not be separated by column chromatography.
To separate the isomers of complexes 1 and 2 aer irradiation, we tried HPLC equipped with a reversed-phase column. The mobile phase was a gradient of H 2 O and acetonitrile (50%  for 10minutes + gradient 50% to 100% (v/v) of acetonitrile for 20 minutes), containing 0.1% (v/v) of HCl. The retention time of the two peaks of complex 1 were t ¼ 9.53 min and t ¼ 10.01 min, whereas for complex 2, only one peak at t ¼ 10.39 min is observed (Fig. S6(B) †). 20 Theoretical studies The ground state optimization of the trans-trans, trans-cis and cis-cis isomers of ligands L1 and L2 by performing density functional theory (DFT) at the B3LYP/(6-31G**) level were done and the optimized structures with energy differences are shown in Fig. 10. The optimized geometries of trans-cis and cis-cis isomers provide quite clear insight of the energy differences between the isomers. The time-dependent density functional (TDDFT) calculations were done only with the trans-trans isomer.
The molecular and electronic structures of the complexes 1 and2 were investigated by performing combined density functional theory (DFT) and time-dependent density functional (TDDFT) calculations at the B3LYP/(6-31G**)+LANL2DZ level. The calculations correctly reproduce the near-octahedral coordination of the metal centre observed in the X-ray structures and predicted geometric parameters are in good agreement with the X-ray structural data. Interestingly, the complex 2 always optimized in the 2 nal geometry. It implies that the stable geometry of the complex is depicted in the 2 nal state. Energies of the frontier molecular orbitals were determined and plots of the HOMO (HOMO to HOMOÀ3 for L1, 1, 2 and HOMO to HOMOÀ4 for L2) and LUMO (LUMO to LUMO+1 for L1, L2, 1 and LUMO to LUMO+2 for 2) orbitals for the ligands and the non-substituted [Ir(ppy) 2 (arylethenylbpy)] + cation are depicted in Fig. 11.
The calculations help us to correlate the spectral data and assignment for the transitions accordingly. The lowest energy transition of the ligand L1 has oscillator frequency (f) 1.6388 (>1) of L1, whereas it is slightly less than 1 in case of L2 (f ¼ 0.9779) ( Table 2). It occurs especially on highly symmetric molecules.
Since most of the transitions are forbidden, the f-strengths are concentrated on one main peak. 21 The spectral assignments are consistent with the ndings of TDDFT calculations (Table 2, Fig. 11). The transition in the ligands are mostly p-p* based. 22 The lower energy transitions of the complexes are MLCT and LC based, and mostly p (ppy, bpy) and p*bpy are involved in both 1 and 2. To gain insight into the photophysical properties of 1 and 2, we optimized the geometry of the lowest-energy triplet excited state (T 1 ) by using the spin-unrestricted UB3LYP approach. 23 The excitation to T 1 involves electron promotion from the orbital with electron density residing on the ppy fragment to the ancillary bpy ligand. Both the complexes have a similar spindensity distribution, mainly localized on the C X N ligands together with a small contribution (about 0.13 e) from iridium. Therefore, T 1 can be described as a cyclometalating-ligandcentered triplet state, with a small metal contribution (Table 4). From a simple visual analysis of the positions of the major transitions depicted in Fig. 12, it can be seen that the energies of the calculated transitions are in good agreement with the experimentally recorded spectra. The predominant ligand centered nature of the emitting T 1 state explains the well resolved vibronic structure of the luminescence spectra of the complexes (Fig. 9).
Summarily, the ligands L1 and L2 isomerize only in the presence of UV-light. The well-established mechanism for the trans-cis isomerization of stilbenes describes that the formation of a singlet excited state causes promotion of an electron from HOMO to LUMO which repopulates the ground state primarily Fig. 10 The calculated energy difference between trans-trans, transcis and cis-cis geometrical forms of the ligands L1 and L2. Fig. 11 Schematic representation showing the isovalue contours and energy differences between HOMO and LUMO calculated for the ligands L1, L2 and complexes 1, 2. in solution by isomerization to the cis-form. 9 At singlet excited state, the trans-form can twist around the central C]C bond orienting the planes perpendicular to both aryl rings to afford a mixture of trans-and cis-isomers. The isomerization process in the (aryl)ethenyl-functionalized bipyridyls possibly occurs exactly in the similar way. The calculation shows that for the free ligand the main transition is from HOMOÀ1 to LUMO+1 ($82 kcal mol À1 , HOMO-LUMO $ 81.58 kcal mol À1 ) in L1 and to LUMO in L2 ($85 kcal mol À1 , HOMO-LUMO $ 94.75 kcal mol À1 ). The rotational barrier for L1 has been calculated around 8 kcal mol À1 (for L2, 1, and 2, it could not be calculated exactly, but of the same order) which is much lower than the excitation energy. The metal chelation, in contrast, populate the 1 MLCT and 3 LC, 3 MLCT state by means of internal conversion and intersystem crossing, compared to the free ligand as evident from the TDDFT calculation (Table 2, Fig. S13 †). The energy released in the T 1 / S o transition, is much less ($50 kcal mol À1 ) compared to the ligand but denitely higher than the rotational barrier. Thus, trans-cis isomer formation from transtrans variety is witnessed in the complexes. On a separate note, due to the bound character of the MLCT state, perpendicular intermediate formation is difficult. The difficulty in the formation of the transition state might be the reason for the restriction of isomerization of the chelated ligands in the complexes to only trans-cis isomerized state. The isomerization process is very slow in the ligands in presence of visible light. The presence of the low-lying MLCT states enables the photoisomerization process to occur fast in the presence of visible light, in the chelated (aryl)ethenyl-functionalized bipyridyls.

Synthetic materials and methods
The starting materials IrCl 3 $3H 2 O, 2-phenylpyridine, 4,4 0dimethyl-2,2 0 -bipyridine, and required aldehydes were purchased from Sigma-Aldrich and used without further purication. All the solvents were dried by usual methods prior to use. The cyclometalated iridium(III) chloro bridged dimer [Ir(ppy) 2 Cl] 2 was prepared according to the literature methods. 24

Physical measurements
IR spectra were obtained on a Perkin-Elmer Spectrum RXI spectrophotometer with samples prepared as KBr pellets. Elemental analyses were performed on a Perkin-Elmer 2400 series II CHN series. Electronic spectra were recorded on a U-4100, HITACHI spectrometer. 1 H NMR spectra were obtained on a Bruker Avance III-500 NMR spectrometer using TMS as the internal standard. HPLC was done with Waters 600 HPLC system with CHIRALCEL OD-H column. Electrochemical measurements were made using a PAR model 273 potentiostat. A platinum disk working electrode, a platinum wire auxiliary electrode and an aqueous saturated calomel reference electrode (SCE) were used in a three electrode conguration. Electrochemical measurements were made under nitrogen atmosphere. Mass spectra were recorded on a Q-Tof-Micromass spectrometer by positive-ion mode electrospray ionization. Fluorescence spectra were taken on a HORIBA JOBINYVON spectrouorimeter. Quantum yields were determined in CH 2 Cl 2 with quinine sulfate in 0.1 M H 2 SO 4 solution as a reference (F ¼ 0.577), and calculated with the following equation: F, I, A, h are quantum yield, integral emission intensity, absorbance and refractive index of the solvents respectively, in which the sample or reference was dissolved.

X-ray crystallography
Single crystal of L1 and L2 were crystallized from saturated DMSO solution and by slow evaporation of DCM solution, respectively. 2 and 2 nal were obtained by slow evaporation of DCM solution containing ammonium hexauorophosphate and slow evaporation of chloroform solution of complex 1 yielded good quality crystals. Crystal data of the complexes were collected on a Bruker SMART APEXII CCD area-detector diffractometer using graphite monochromated Mo Ka radiation (l ¼ 0.71073 A). For L1 and L2, date were collected on a CrysAlis Pro, Super Nova, Eos four-circle diffractometer using mirror monochromated Mo Ka radiation (l ¼ 0.71073 A). For crystals L1 and L2, X-ray data reduction was carried out using the CrysAlis PRO, Agilent Technologies, Version 1.171.36.24 program and for the complexes, X-ray data reduction was carried out using the Bruker SAINT program. The structures were solved by direct methods using the SHELXS-97 program and renement using SHELXL-97 program. Selected crystal data and data collection parameters for all the complexes are given in Table 3. X-ray structure solution and renement were done using the SHELXL-97 program package. 27 Four nal cycles of renement converged with discrepancy indices R[F 2 > 2s(F 2 )] ¼ 0.0628 and wR(F 2 ) ¼ 0.1457 for L1, R[F 2 > 2s(F 2 )] ¼ 0.0539 and wR(F 2 ) ¼ 0.1484 for L2, R[F 2 > 2s(F 2 )] ¼ 0.0678 and wR F 2 ¼ 0.1646 for 1, R[F 2 > 2s(F 2 )] ¼ 0.0701 and wR(F 2 ) ¼ 0.1898 for 2, and R[F 2 > 2s(F 2 )] ¼ 0.0679 and wR(F 2 ) ¼ 0.1428 for 2 nal .

Computational method
The ground-state geometry of the ligands and the complexes has been optimized in the absence of the counter-ion at the Density Functional Theory (DFT) level, using the X-ray structures. The basis set for the description of the electrons of nonmetallic atoms is B3LYP/6-31G, while for iridium the B3LYP/LANL2DZ basis set has been used. The characterization of the nature of the lowest-lying singlet and triplet excited states involved in absorption and emission properties, respectively, relies on time-dependent density functional theory (TD-DFT) calculations performed on the basis of the ground-state geometry, using the same basis set. All calculations were performed with the Gaussian09 package. 28

Conclusions
We have synthesized two (aryl)ethenyl functionalized 2,2 0bipyridine and their iridium(III) complexes. We report the dynamic photophysical properties of these cyclometalated Ir(III) complexes with detail analysis of spectroscopic and spectrophotometric characterizations. In addition, quantum chemical calculation rationalizes the experimental observations of the anomalous photoinduced behavior of complexes 1 and 2. To the best of our knowledge, this is the rst report of detail study for photophysical properties of these cyclometalated iridium(III) complexes to investigate the mechanism of photoinduced trans-cis isomerism. The trans-cis photoisomerization mechanism in the ligands involve singlet excited state, whereas the iridium(III) complexes of the same ligands involves low lying 3 MLCT state. This is expected from the above observations that a careful choice of the substituted (aryl)ethenyl appended bipyridyl ligands in the cyclometalated Ir(III) complexes could prevent the isomerization process completely. The selectivity of photoinduced isomers might have crucial importance in development of cyclometalated iridium(III) complexes for applications in photochromic systems.