Synthesis, Structural Characterization, and Luminescence Switching

Apr 27, 2018 - Apart from incorporating π-delocalization in the ligand frame, diarylethenes can also be considered as model trans/cis photoisomerizat...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Synthesis, Structural Characterization, and Luminescence Switching of Diarylethene-Conjugated Ru(II)-Terpyridine Complexes by trans− cis Photoisomerization: Experimental and DFT/TD-DFT Investigation Poulami Pal, Shruti Mukherjee, Dinesh Maity, and Sujoy Baitalik* Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India S Supporting Information *

ABSTRACT: We synthesized and thoroughly characterized a new family of diarylethene-conjugated mononuclear Ru(II)terpyridine complexes and investigated in detail their photophysical, electrochemical, and spectroelectrochemical behaviors. Interestingly, the compounds show moderately strong room-temperature luminescence predominantly from their 3 MLCT state with luminescence lifetime varying between 8.43 and 22.82 ns. Because of the presence of diarylethene unit, all the monometallic complexes underwent trans-to-cis photoisomerization upon interaction with UV light with substantial changes in their absorption and luminescence spectra. Reverting back from the cis to the trans form is also made possible upon treatment with visible light or by heat. Trans-to-cis isomerization leads to almost complete quenching of luminescence, while backward cis-to-trans isomerization gives rise to restoration of the original luminescence for all the complexes. Thus, “on−off” and “off-on” emission switching was made possible upon successive interaction of the complexes with UV and visible light. Computational investigation involving density functional theory (DFT) and time-dependent DFT methods was done for proper assignment of the experimental absorption and emission spectral bands in the complexes. Finally, experimentally observed trend on the absorption and emission spectral behaviors of the complexes upon photoisomerization was also compared with the calculated results.

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INTRODUCTION Smart molecular systems capable of reversibly changing their physicochemical properties upon treatment with different external stimuli are receiving great attention in recent years with regard to their probable practical applications such as for the design of molecular switches and memory devices.1−6 Among the various external stimuli, light is the most suitable and convenient, because it is a clean energy source and required wavelength of light can easily be used for exciting a particular molecular fragment within very short time scale. To this end, photoisomerization studies of some organic molecules such as azobenzenes, diarylethenes, and spiropyrans have been extensively investigated.7−13 But reports on similar photoisomerization studies with metal complexes are relatively few in the literature compared to pure organic compounds.14−17 Coordination complexes based on transition metals have gained increasing attention in recent years as materials in photovoltaic, optoelectronic, photonic devices as well as in sensors and in different photocatalytic processes.18−22 The versatility of the complexes is related to their widely tunable optoelectronic properties, which can be incorporated upon variation of the metal as well as the ligand. Among the various coordination complexes, those derived from Ru(II) metal and in combination with polypyridine ligands contributed significantly © XXXX American Chemical Society

due to their outstanding photophysical and redox properties.23−26 Both bipyridine- and terpyridine-type ligands have widely been utilized to synthesize a large variety of Ru(II) complexes. But the octahedral complexes based on bisterpyridine-type ligands are superior over tris-bipyridine type with regard to the construction of achiral multinuclear architectures.27−33 However, this class of complexes often suffers from their low intrinsic quantum yields because of closeness of energy between the radiative metal-to-ligand charge transfer (MLCT) states and nonradiative metal-centered (MC) states arising out of their distorted octahedral geometries.34 Nonetheless, it had already been shown that their room-temperature emission properties can be improved by adopting different strategies. In general, improved luminescence properties can be accomplished by synthetic routes, that is, by insertion of electron-donating or -accepting substituent35−39 as well as polyaromatic chromophores40−43 to control the electronic coupling of π−π* excited state and simultaneously present MLCT states. The use of cyclometalated ligands can give rise to destabilization of the nonemitting 3MC state.44−47 Alternatively, incorporation of different heterocyclic rings can Received: December 8, 2017

A

DOI: 10.1021/acs.inorgchem.7b03096 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

PhCH2PPh3Br) were synthesized following reported literature procedures.67,79 [(tpy-PhCH3)RuCl3] was prepared by refluxing RuCl3·3H2O and tpy-PhCH3 (1:1 molar ratio) in EtOH. The terpyridine ligands incorporating diarylethene were synthesized by modification of the literature procedures adopted by Schmehl80 and Magennis,81 and the synthetic details along with the relevant characterization data were given in Supporting Information (Figures S1−S8). Synthesis of the Metal Complexes. A general procedure as described below was adopted for the synthesis of the complexes. [(tpy-PhCH3)Ru(tpy-pvp-H)](BF4)2·H2O (1). [Ru(tpy-PhCH3)Cl3] (0.13 g, 0.25 mmol) and AgBF4 (0.17 g, 0.87 mmol) were successively added to 40 mL of acetone and stirred under refluxing condition for ∼3 h. The white solid of AgCl was quickly filtered out, and the filtrate was treated with ∼40 mL of EtOH, and the remaining Me2CO was rotary evaporated. Tpy-pvp-H (0.11 g, 0.26 mmol) was then added in its finely powdered form to the solution and refluxed for 6 h with continuous stirring under argon protection. The red compound that deposited upon cooling was collected by filtration and thoroughly washed with chloroform and dried. Acetonitrile solution of the resulting compound was subjected to column chromatography (silica gel) and eluted with MeCN−PhCH3 (10:1, v/v) mixture. The desired compound was collected upon evaporation to a small volume. Recrystallization of the complex from MeCN−MeOH (1:1, v/v) mixture was done for further purification (0.16 g, yield: 61%). Anal. Calcd for C51H40N6B2F8ORu: C, 59.78; H, 3.92; N, 8.17. Found: C, 59.67; H, 3.99; N, 8.05%. 1H NMR (300 MHz, deuterated dimethyl sulfoxide (DMSO-d6)): δ 9.45 (s, 2H, 2H3′), 9.41 (s, 2H, 2H3′), 9.07 (t, 4H, J = 7.0 Hz, H6), 8.47 (d, 2H, J = 7.9 Hz, 2H7), 8.33 (d, 2H, J = 7.7 Hz, 2H7), 8.04−7.97 (m, 6H, 4H4 + 2H8), 7.70 (d, J = 7.7 Hz, 2H, 2H8), 7.57−7.52 (m, 8H, 4H3 + 2H11 + 2H12), 7.46 (nr, 1H, H13′), 7.42 (d, 1H, J = 7.2 Hz, H9), 7.34 (d, 1H, J = 7.2 Hz, H10), 7.26 (t, 4H, J = 6.2 Hz, H5), 2.49 (s, 3H, CH3). Electrospray ionization mass spectrometry (ESI-MS; positive, MeCN) m/z = 417.81 (100%) [(tpy-PhCH3)Ru(tpy-pvp-H)]2+ and m/z = 922.52 (5%) [(tpy-PhCH3)Ru(tpy-pvp-H)(BF4)]+. [(tpy-PhCH3)Ru(tpy-pvp-Me)](BF4)2·H2O (2). Yield: 0.16 g (63%). Anal. Calcd for C52H42N6B2F8ORu: C, 59.96; H, 4.06; N, 8.18. Found: C, 59.79; H,4.18; N, 8.28%. 1H NMR (300 MHz, DMSO-d6): δ 9.48 (s, 2H, 2H3′), 9.43 (s, 2H, 2H3′), 9.09 (t, 4H, J = 6.7 Hz, H6), 8.47 (d, 2H, J = 7.4 Hz, 2H7), 8.35 (d, 2H, J = 7.1 Hz, 2H7), 8.05 (t, 4H, J = 7.1 Hz, H4), 7.97 (d, 2H, J = 7.5 Hz, 2H8), 7.61−7.53 (m, 10H, 4H3 + 2H8 + 2H11 + 2H12), 7.48 (nr, 1H, H9), 7.35 (d, 1H, J = 9.0 Hz, H10), 7.29−7.24 (m, 4H, H5), 2.34 (s, 3H, CH3), 2.49 (s, 3H, CH3). ESI-MS (positive, MeCN) m/z = 425.10 (100%) [(tpyPhCH3)Ru(tpy-pvp-CH3)]2+ and m/z = 937.24 (12%) [(tpy-PhCH3)Ru(tpy-pvp-Me)(BF4)]+. [(tpy-PhCH3)Ru(tpy-pvp-Cl)](BF4)2·2H2O (3). Yield: 0.17 g (65%). Anal. Calcd for C51H41N6B2F8ClO2Ru: C, 56.71; H, 3.83; N, 7.78. Found: C, 56.60; H, 3.99; N, 7.62%. 1H NMR (300 MHz, DMSO-d6): δ 9.43 (s, 2H, 2H3′), 9.38 (s, 2H, 2H3′), 9.04 (t, 4H, J = 6.6 Hz, H6), 8.45 (d, 2H, J = 8.0 Hz, 2H7), 8.31 (d, 2H, J = 8.1 Hz, 2H7), 8.04− 7.96 (m, 6H, 4H4 + 2H8), 7.72 (d, 2H, J = 8.5 Hz, 2H8), 7.57−7.46 (m, 10H, 4H3 + 1H9 + 1H10 + 2H11 +2H12), 7.26 (t, 4H, J = 6.0, H5), 2.53 (s, 3H, CH3). ESI-MS (positive, MeCN) m/z = 435.01 (100%) [(tpy-PhCH3)Ru(tpy-pvp-Cl)]2+. [(tpy-PhCH3)Ru(tpy-pvp-NO2)](BF4)2·H2O (4). Yield: 0.16 g (60%). Anal. Calcd for C51H39N7B2F8O3Ru: C, 57.11; H, 3.66; N, 9.14. Found: C, 57.01; H, 3.78; N, 9.01%. 1H NMR (300 MHz, DMSO-d6): δ 9.50 (s, 2H, 2H3′), 9.43 (s, 2H, 2H3′), 9.09 (t, 4H, J = 6.2 Hz, H6), 8.52 (d, 2H, J = 7.5 Hz, 2H7), 8.35 (d, 2H, J = 7.4 Hz, 2H7), 8.30 (d, 2H, J = 7.6 Hz, 2H8), 8.07−8.05 (m, 6H, 4H4 + 2H8), 7.96 (d, 2H, J = 7.5 Hz, H12), 7.71 (nr, 2H, 1H9 + 1H10), 7.58−7.53(m, 6H, 4H3 + 2H11), 7.29−7.27 (nr, 4H, H5), 2.49 (s, 3H, CH3). ESI-MS (positive, MeCN) m/z = 440.59 (100%) [(tpy-PhCH3)Ru(tpy-pvp-NO2)]2+ and 968.21(10%) [(tpy-PhCH3)Ru(tpy-pvp-NO2)(BF4)]+. [(tpy-PhCH3)Ru(tpy-pvp-Ph)](BF4)2·3H2O (5). Yield: 0.17 g (62%). Anal. Calcd for C57H48N6B2F8O3Ru: C, 60.06; H, 4.24; N, 7.37. Found: C, 59.82; H, 4.67; N, 7.05%. 1H NMR (300 MHz, DMSO-d6): δ 9.47 (s, 2H, 2H3′), 9.41 (s, 2H, 2H3′), 9.07 (t, 4H, J = 7.5 Hz, H6),

give rise to less distorted planar ground-state configuration.48−52 The key factor for the improvement of their photophysical properties with respect to their probable practical application is an extension of the electron delocalization and the minimization of nonradiative relaxation channels.53−58 With regard to our ongoing research interest for designing luminescent terpyridine-based Ru(II) complexes,59−68 we report herein a new family of heteroleptic Ru(II) complexes of composition [(tpy-PhCH3)Ru(tpy-pvp-X)](BF4)2 (Chart 1) Chart 1

by incorporating a photoisomerizable diarylethene group within the ligand frame (tpy-pvp-X). Apart from incorporating πdelocalization in the ligand frame, diarylethenes can also be considered as model trans/cis photoisomerization systems.69−73Some reports are available on photoisomerization studies of azo-bridged complexes based on terpyridine ligands,4,14−17,74−78 but to our knowledge, no such studies have hitherto been performed on luminescent Ru(II) complexes containing coupled terpyridine-diarylethene motifs. It is expected that conjunction of linear bis-tridentate Ru(II) complex and diarylethene group could lead to the construction of potential molecular-switching systems. This paper deals with synthesis, structural characterization, photophysical, electrochemical, and detailed photoisomerization studies of a new series of heteroleptic Ru(II) complexes of composition [(tpyPhCH3)Ru(tpy-pvp-X)](BF4)2. For fine-tuning of the optical properties as well as the rate of photoisomerization process, the electronic nature of tpy-pvp-X was varied with X = H, Me, Cl, NO2, and Ph group. Finally, computation works were also performed on both trans and cis forms of the complexes by employing density functional theory (DFT) and time-dependent (TD) DFT methods for getting some insight about the electronic structure of the complexes and also for appropriate interpretation of experimentally observed absorption and emission bands.

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EXPERIMENTAL SECTION

Materials. RuCl3·xH2O, AgBF4, and different para-substituted benzaldehydes were purchased from Sigma. Other analytical reagentgrade chemicals as well as solvents were procured from local vendors. The precursors that were used for the synthesis of the ligands such as 4′-(p-methylphenyl)-2,2′:6′,2″-terpyridine (tpy-PhCH3), 4′-(p-bromomethyl phenyl)-2,2′:6′,2″-terpyridine (tpy-PhCH 2Br), and 4′(2,2′:6′,2″-terpyridyl-4)-benzyltriphenyl phosphonium bromide (tpyB

DOI: 10.1021/acs.inorgchem.7b03096 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Perspective view of 22+ with 50% probability ellipsoid plots. 8.48 (d, 2H, J = 8.2 Hz, 2H7), 8.33 (d, 2H, J = 8.1 Hz, 2H7), 8.04− 7.99 (m, 6H, 4H4 + 2H8), 7.82−7.72 (m, 6H, 1H12 + 2H13 + 2H14 + 1H15), 7.62−7.52 (m, 9H, 4H3 + 2H8+ 2H11 + 1H12), 7.47 (d, 1H, J = 7.5 Hz, H9), 7.40 (d, 1H, J = 7.3 Hz, H10), 7.26 (t, 4H, J = 6.3 Hz, H5), 2.49 (s, 3H, CH3). ESI-MS (positive, MeCN) m/z = 455.85 (100%) [(tpy-PhCH3)Ru(tpy-pvp-Ph)]2+. Instruments and Physical Methods. Descriptions of different instruments and detailed procedures for various physical measurements were given in the Supporting Information. Computational details were also described in the Supporting Information.

resonances) and within the chemical shift range of 7.34−7.71 ppm are clearly due to the ethylenic double bond. Mass Spectra. ESI mass spectra of complexes were acquired in MeCN and presented in Figures S13−S16 (Supporting Information). Figure S15 (Supporting Information) shows that the isotopic distribution pattern of the peak at m/z = 435.01 (3) and 455.85 (5), separated by 0.5 Da, match nicely to the calculated pattern for [(tpy-PhCH3)Ru(tpy-pvpCl)]2+ and [(tpy-PhCH3)Ru(tpy-pvp-Ph)]2+ species, respectively. Thus, the most abundant peak in all the complexes indicates dipositive cation, [(tpy-PhCH3)Ru(tpy-pvp-X)]2+. The weak peak at m/z = 922.52 for 1 (Figure S13, Supporting Information), m/z = 937.24 for 2 (Figure S14, Supporting Information), and m/z = 968.21 for 4 (Figure S16, Supporting Information) indicates the presence of the monopositive ion [(tpy-PhCH3)Ru(tpy-pvp-X)(BF4)]+. Description of the Crystal Structure of [(tpy-PhCH3)Ru(tpypvp-Me)](ClO4)2 (2). Perspective view of 22+ together with its atom numbering is provided in Figure 1, and representative bond distances and bond angles are summarized in Table S2 (Supporting Information). Crystal structure indicates distorted octahedral geometry around Ru(II) center with two terpyridine ligands coordinated in meridional fashion. 22+ crystallizes in monoclinic system with C2/c space group. Crystal structure also indicates the trans orientation of two substituted aryl groups around the ethylenic double bond. The bite angles are within the range of 78.80°(2)−102.10°(2), whereas the trans angle are within the range of 158.0°(2)−178.80°(2). Ru(II)−N distances in 22+ vary between 1.940(5) and 2.064(6) Å. The bond distance between Ru(II) and N atom of central pyridine moiety of tpy unit is found to be substantially shorter compared with the two terminal pyridine rings due to substantial overlap between t2g orbital of Ru(II) and π* orbitals of the said pyridine moiety. The relatively large Ru−N distances associated with the terminal pyridine units are due to the strain developed on the terpyridine moiety of the ligand upon transformation from their transoid to the cisoid conformation in the resulting complex. DFT and TD-DFT Calculations. Geometry optimizations of both trans and cis forms of the complexes were performed in MeCN by Gaussian 09 program82 employing DFT method with Becke’s three-parameter hybrid functional and Lee− Yang−Parr’s gradient-corrected correlation functional (B3LYP) level of theory.83,84 Either 6-31G(d) or 6-31G* basis set was employed for the C, H, and O. By contrast, SDD basis set was employed for heavier Ru atom. The optimized structures of the complexes are shown in Figure S17 (Supporting Information), and metrical parameters are summarized in Tables S2−S6 (Supporting Information). Geometrical parameters indicate

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RESULTS AND DISCUSSION Synthesis and Characterization. The terpyridine ligands were obtained in appreciable yields by reacting 4′-(2,2′:6′,2″terpyridyl-4)-benzyltriphenylphosphonium bromide (tpyPhCH2PPh3Br) with different 4-substituted benzaldehydes in dichloromethane under argon atmosphere at ∼0−5 °C. All the ligands were purified by column chromatography and recrystallization techniques. For the synthesis of the metal complexes, [(tpy-PhCH3)Ru(Me2CO)3]3+, obtained upon treating [(tpy-PhCH3)RuCl3] and AgBF4 in appropriate molar ratio, was used, as it functions as a superior precursor compared to [(tpy-PhCH3)RuCl3] with regard to reaction time. Column chromatography and recrystallization techniques were again employed for purification of the complexes. Characterization of the ligands and their metal complexes were performed by using NMR and mass spectroscopic techniques along with their elemental (C, H, and N) analyses. All the characterization data have been provided in the Experimental Section. Single-crystal X-ray structure of complex 2 was also determined in the solid state (Table S1, Supporting Information). NMR Spectra. 1H and {1H−1H} homonuclear correlated spectroscopy (COSY) NMR spectra of 1−5 were acquired in DMSO-d6 and presented in Figure S9 and Figures S10−S12 (Supporting Information), respectively. In spite of complexity in the spectra, all the peaks were tentatively assigned by their COSY spectra as well as by comparing the data with related complexes.59−68 The singlet that appears at ∼2.5 ppm (not shown in Figure S9, Supporting Information) and counts for three protons in each case, except 2 can be assigned as the protons of −CH3 group of tpy-PhCH3 moiety. For 2, two singlets are observed at 2.34 and 2.49 ppm due to two types of −CH3 group within the complex. Two closely situated singlets that appeared at the most downfield region are assignable to the H3′ proton associated with two tpy motifs situated at the opposite site of the Ru(II) center. Interestingly, H9 and H10 protons that appeared as a pair of doublets (although not clearly visible in all cases due to overlapping with other proton C

DOI: 10.1021/acs.inorgchem.7b03096 Inorg. Chem. XXXX, XXX, XXX−XXX

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terpyridine moiety of tpy-pvp-X. Very intense bands in the UV region are primarily due to π−π* transitions within tpyPhCH3 as well as tpy-pvp-X. Calculated lowest-energy band in cis form was blue-shifted, and the extent of shift depends on the electronic behavior of the substituent X. Natural transition orbital (NTO) analysis plots as well as electron density difference map (EDDM) are also useful for appropriate assignment of the bands. NTO plot, which is primarily based on calculated transition density matrices, indicates that the lowest-energy band is primarily composed of both MLCT and ILCT transitions (Figures S29−S33, Supporting Information). EDDM plots (Figure S34, Supporting Information) again support the conclusion drawn from NTO plots, and mixed MLCT and ILCT nature for the said band gets verified. Unrestricted Kohn−Sham (UKS) computations directly on the triplet state of both the trans and cis form of the complexes were performed to get some insight about their luminescence behaviors. Very little variation of the geometrical parameters is observed among the ground and excited states (Tables S2−S6, Supporting Information). The calculated luminescence maximum of the complexes was found to vary between 612 and 635 nm (Table S17, Supporting Information). Experimental Absorption Spectral Behaviors. Absorption spectra of 1−5 acquired in MeCN are displayed in Figure 2, and related data are presented in Table 1. The spectral

substantial distortion from idealized octahedral geometry in each case. The calculated Ru−N bond distances vary between 2.006 and 2.113 Å, and for the methyl derivative whose singlecrystal X-ray structure is available, the correlation between calculated and experimental metrical parameters was found to be good (Table S2, Supporting Information). Upon photoisomerization from trans to cis form, coordination environment around the Ru(II) center remains almost unaltered (Tables S2−S6, Supporting Information). The most affected part upon isomerization is the diarylethylene unit in the complex as expected. The calculated results indicate that the magnitude of dihedral angle between the aryl rings across the double bond in all complexes is ∼179.9°, while the corresponding angle in the cis form is ∼7.7°. It would of interest to compare the relative strain in the corresponding ligands. To this end, we optimized the geometry of both cis and trans forms of free ligands. Interestingly, the dihedral angle between the aryl groups across the double bond in the free ligands is almost identical to that of the metal complexes. Moreover, the difference of energy among the two forms of the metal complexes [varying between 24.42 kJ/mol (3) and 25.00 kJ/mol (4)] is almost the same as those of the free ligands [lying between 23.49 kJ/mol (tpy-pvp-Cl) and 24.55 kJ/mol (tpy-pvp-NO2)]. Thus, it appears that the amount of strain incorporated across the diaryethylene unit upon photoisomerization is almost same for both metal complexes as well as the ligands. Frontier molecular orbital (FMO) sketch (Figures S18−S22, Supporting Information) as well as their compositions show that, with few exceptions, highest occupied molecular orbitals (HOMOs) are primarily located on Ru(II) and to some extent on the vinylphenyl and tpy-PhCH3 moiety (Tables S7−S11, Supporting Information), while the lowest unoccupied molecular orbitals (LUMOs) are based on the terpyridines. For nitro derivative, lower-energy LUMOs are mainly localized on nitrobenzene part of tpy-pvp-NO2. On going from trans to their cis forms, only small differences are observed in the compositions of MOs. The distribution of electronic charge density in the complex backbone can be seen by plotting electrostatic surface potential (ESP) over the total electron density (Figure S23, Supporting Information). The electronrich portion is indicated by green color, while electron-deficient area is indicated by blue color in the ESP plots. Trans-to-cis isomerization does not alter the electronic charge density around the complex backbone to a great extent as expected. To obtain calculated UV−vis spectra, TD-DFT computations were also performed in MeCN. Calculated data along the band assignment were provided in Tables S12−S16 (Supporting Information). The FMOs that participate in the lowestenergy band were displayed in Figures S24−S28 (Supporting Information). The calculated band that obtained between 477 nm (for 3 and 4) and 491 nm (for 5) in their trans forms is a combination of metal-to-ligand (Ru(II) → terpyridine) charge transfer (MLCT) and intraligand (phenylvinyl → terpyridine) charge transfer (ILCT) transitions (Figures S24−S28, Supporting Information). In case of nitro derivative, the lowest-energy band consists of combined charge transfer from Ru(II) and phenylvinyl part to the nitrobenzyl unit of tpy-pvp-NO2. Thus, taking into consideration the participation of the relevant FMOs, the calculated lowest-energy band can be assigned as admixture of both MLCT and ILCT. The next higher band obtained between 353 nm (for 1) and 372 nm (for 5) is primarily due to ILCT transition from phenylvinyl to

Figure 2. UV−Vis absorption spectra of 1−5 in acetonitrile.

signature of the complexes is more or less similar containing several very intense peaks in the UV and one moderately intense band in the visible region. Outcomes of TD-DFT calculations and literature data of related complexes suggest the band that appears at ∼500 nm can be assigned as admixture of 1 [RuII(d)6] → 1[RuII(d)5tpy-pvp-X(π*)1] MLCT as well as phenylvinyl → terpyridine ILCT transitions. The next higherenergy band that appears between 354 and 364 nm, depending upon the electronic nature of the substituent (X) on tpy-pvp-X, is primarily ILCT in nature, although some π−π* character is also involved therein. The intense bands that appeared in the UV region primarily consist of π−π* transitions of the coordinated tpy-PhCH3 and tpy-pvp-X ligands. The correlation between calculated and experimental data was found to be reasonably good. The MLCT band is red-shifted in the complexes compared to the parent [Ru(tpy)2]2+ (474 nm)25 probably because of extended delocalization induced by diarylethenes moiety at the 4′-position of the terpyridine unit of tpy-pvp-X. In addition, the ILCT band is also red-shifted with enhanced electron acceptor capability of X. Experimental Luminescence Spectral Behaviors. Steady-state emission spectra of 1−5 were acquired in MeCN D

DOI: 10.1021/acs.inorgchem.7b03096 Inorg. Chem. XXXX, XXX, XXX−XXX

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2.12 1.04 10.81 0.51 598 628, 681 (sh)

In MeCN. bMeOH/EtOH (1/4, v/v) glass. ck1 is the summation of kr and knr at 77 K. dReference 25. eReference 25.

629 640 [Ru(tpy)2]2+ d [Ru(tpyPhCH3)2]2+ e

5

4

3

(τ(T))−1 = (k1 + k 2exp[ −ΔE/RT ])/(1 + exp[−ΔE /RT ] (1)

where k1 corresponds to the temperature-independent rate constant, which is the summation of radiative (kr) and nonradiative (knr) rate constants from the 3MLCT state to the ground state at low temperature (77 K). K2 corresponds to temperature-dependent rate constant, which takes into account the rate of population of the 3MC state from 3MLCT state, and ΔE corresponds to the activation energy required for this process. Nonlinear fit of the experimental data to eq 1 gives rise to the values of k1, k2, and ΔE {Figure 4c,d and Figure S37c,d, Supporting Information}. The estimated ΔE values were found to vary between 1863 ± 36 and 3391 ± 49 cm−1, depending upon the nature of X. The values of k1 (varying between 1.61 × 104 and 2.22 × 104 s−1) that were obtained from 77 K emission data were utilized for fitting eq 1. Calculated k2 value ranges between 8.56 × 1010 and 1.32 × 1013 s−1. Substantial increase in the value of ΔE is noticed in all complexes compared to the

a

0.04 ≤0.05 ≤0.03

90.9

2.18, 0.33 0.47 664

670

663

662 2

1

at room temperature (RT) as well as in EtOH−MeOH (4:1, v/ v) glass at 77 K. The spectra are shown in Figure 3a and Figure S35 (Supporting Information), while relevant data are provided in Table 1. One moderately intense luminescent band in the range between 661 (1) and 670 nm (4) at RT and between 653 (1) and 662 nm (5) at 77 K was observed in each case upon excitation at their MLCT maxima. Luminescence lifetime acquired at RT were found to vary between 8.43 ns (2) and 22.82 ns (4) (Figure 3b), while at 77 K the lifetimes increased significantly and vary between 44.96 μs (2) and 62.04 μs (4) (Figure S36, Supporting Information). On the basis of the luminescence data of the related Ru(II)terpyridine complexes, it can be concluded that the origin of emission in the present complexes is predominantly 3MLCT in nature.25−28 Blue shift of the emission maximum and substantial increase in intensity and lifetime in frozen glass is also indicative of typical 3MLCT emitters. The energy (E00) of the 3MLCT state was also evaluated from their emission maximum at 77 K (varying between 1.87 and 1.90 eV). Thus, the most important characteristic of the present complexes is that they are luminescent at RT with moderately long lifetimes compared to nonluminescent [Ru(tpy)2]2+ (τ = 0.25 ns) parent complex.34 In addition, improved emission characteristics of the complexes were acquired without too much lowering of their excited-state energy. It may be mentioned that reasonably good correlation between the calculated (based on UKS) and experimentally observed emission maximum was observed for the trans form of complexes (Table S17, Supporting Information). The luminescence band is also red-shifted in 1−5 compared to the parent [Ru(tpy)2]2+. The red shift, although to a very small extent, was found to depend upon the electron-accepting nature of X in tpy-pvp-X. For better assessment of deactivation dynamics, emission spectra and lifetimes of the complexes were acquired by varying temperature (from 265 to 323 K), and the experimental findings are shown in Figure 4 and Figure S37 (Supporting Information). Irrespective of their electronic nature, luminescence intensity, quantum yield, and luminescence lifetime gradually decrease upon increasing temperature for all complexes. Nonlinear regression analysis of temperature versus lifetime data by using eq 1 led to an idea about the involvement of different deactivation channels in the excited-state decay process.23,85

47.16 662

1.61 0.72 8.86 0.55 62.04 661

146.98, 4.38 46.41, 6.99 7.79, 0.23

2.26, 0.36 0.39

0.53

2.20 1.38 8.14 0.37 45.46 653

2.22 1.16 10.68 0.48 44.96 654

66.64, 11.86 58.11, 9.40 2.86, 0.49 0.43

2.06 1.16 9.09 0.44 48.42 653 65.33, 11.0 2.74, 0.46 0.42

1.53 (9.3%), 9.08 (90.7%) 1.50 (11.3%), 8.43 (88.7%) 1.72 (12.5%), 10.63 (87.5%) 0.68 (56.5%), 22.82 (43.5%) 2.15 (11.3%), 14.30 (88.7%) 0.25