Mononuclear Cyclometalated Ruthenium(II) Complexes of 1,2,4,5

Article pubs.acs.org/Organometallics

Mononuclear Cyclometalated Ruthenium(II) Complexes of 1,2,4,5Tetrakis(N-methylbenzimidazolyl)benzene: Synthesis and Electrochemical and Spectroscopic Studies Jiang-Yang Shao,† Jiannian Yao,† and Yu-Wu Zhong*,†,‡ †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *

ABSTRACT: A series of mononuclear cyclometalated ruthenium complexes with 1,2,4,5-tetrakis(N-methylbenzimidazolyl)benzene have been prepared, where two N-methylbenzimidazoles bind to the metal center and others remain intact. Electronic properties of these complexes were investigated by electrochemical and spectroscopic studies and DFT/TDDFT computations. The RuII/III redox potentials of theses complexes can be modulated by attaching substituents of various electronic natures on the noncyclometalating ligand. These complexes display enhanced visible light absorption compared to those without two free-standing Nmethylbenzimidazolyl units as a result of the three-chromophore effect.

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INTRODUCTION Polypyridyl ruthenium complexes are often characterized by broad and intense metal-to-ligand charge-transfer (MLCT) absorptions in the visible region of the electromagnetic spectrum.1 Taking advantage of this feature, these complexes have become the most efficient dyes for sensitized solar cells (DSSCs).2 Since the pioneering work by Grätzel and coworkers in 1991,3 a vast number of polypyridyl dyes have been developed,2 and the highest power conversion efficiency has now reached more than 11%.4 For further improvement of the photovoltaic performance, the development of new dyes is of utmost importance. Cyclometalated ruthenium complexes5 have become promising dyes for DSSCs since the original report from van Koten and co-workers.6 Later studies by Berlinguette,7 Nazeeruddin,8 and others9 have elegantly demonstrated that very impressive results could be achieved with these complexes. The highest occupied molecular orbital (HOMO) level of cyclometalated ruthenium complexes is strongly dependent on the electronic nature of the cyclometalating ligand. On the other hand, the variation of the non-cyclometalating ligands may lead to the adjustment of the lowest unoccupied molecular orbital (LUMO) level of the dye. Appropriate HOMO and LUMO levels are pivotal for efficient dye regeneration and electron injection into the conduction band of a semiconductor, respectively. Working together, the positions of the HOMO and LUMO levels of a cyclometalated ruthenium complex determine its frontier energy gap and absorption spectrum. As a result, the design and synthesis of new complexes with broad © 2012 American Chemical Society

absorption and tunable frontier orbital levels have attracted considerable interest.10 In addition to their applications in DSSCs, cyclometalated ruthenium complexes have been employed as redox sites to construct a number of mixed-valence (MV) systems.11 The presences of covalent Ru−C bonds in these systems induce a stronger metal−metal electronic coupling than non-cyclometalated analogues. We have recently launched a program to study the charge delocalization of 1,4-benzene-bridged cyclometalated bis-ruthenium complexes (Figure 1) and found that these complexes exhibited interesting electronic properties such as redox-noninnocence of the bridge and electrochromism in the near-infrared region.12 Taking into account the fact that auxiliary ligands play a very important role in influencing the charge delocalization of MV systems,13 we designed a potentially new bis-cyclometalating ligand, 1,2,4,5-tetrakis(Nmethylbenzimidazolyl)benzene (Metbib, 1, Figure 1), in the hopes of synthesizing a new bis-ruthenium complex via a double C−H bond (shown in pink) activation process. However, as we found later, the desired bis-ruthenium complexes could not be obtained possibly due to a severe steric hindrance between the adjacent N-methylbenzimidazolyl units on the phenyl ring. Instead, mononuclear cyclometalated ruthenium complexes with various tridentate N∧N∧N ligands were isolated smoothly in acceptable yield. Interestingly, these monoruthenium complexes display more broad absorption than Received: April 10, 2012 Published: May 22, 2012 4302

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afforded the desired ligand 1 in good yield (85%).14 Five mononuclear cyclometalated ruthenium complexes (2−6) were then prepared by the reaction of 1 with various Ru(L)Cl3 in the presence of AgOTf and followed by anion exchange with KPF6,15 where L = bis(N-methylbenzidazolyl)pyridine (Mebip),10f,16 4′-p-methoxyphenyl-2,2′:6′,2″-terpyridine (MeOptpy), 4′-tolyl-2,2′:6′,2″-terpyridine (ttpy), 4′-p-trifluoromethylphenyl-2,2′:6′,2″-terpyridine (CF3ptpy), and trimethyl-4,4′,4″tricarboxylate-2,2′:6′,2″-terpyridine (Me3tcbtpy), respectively. These complexes were purified through flash column chromatography on silica gel and obtained as bench-stable compounds. The ligand Mebip is electron-donating, while Me3tcbtpy is electron-deficient. The other three ligands can be smoothly prepared according to a known one-step procedure.17 The attachments of different substituents (MeO, Me, and CF3) on these ligands were selected on purpose to systematically examine the electronic properties of resulting complexes. In an early stage, we speculated that these complexes should have similar 1H NMR signals. However, it turned out that complex 2 displayed very different 1H NMR signals compared to those of 3−6. There are altogether six methyl groups in complex 2, and only three single peaks, at 2.98, 3.84, and 4.40 ppm (1:1:1), are observed for these groups (Figure 2), which indicates that complex 2 has a C2-symmetric structure. A very possible configuration of 2 is with the two methyl groups in the intact N-methylbenzimidazolyl units directed away from the metal center and opposite each other. If any of these methyl groups is directed toward the metal center, an unfavorable steric congestion with the benzene rings of the ancillary Mebip ligand may arise. In stark contrast, four single peaks are found for the four methyl groups of the Metbib ligand of complexes 3−6 (Figure 2; additional peaks of 3 and 6 shown in the figure are due to the methoxy groups). This means that these methyl groups are in different chemical environments. We deduce that the two methyl groups in the intact N-methylbenzimidazolyl units of these complexes are on the same side of the cyclometalating phenyl plane, with one directed away from

Figure 1. Mononuclear complex of Metbib.

conventional mononuclear cyclometalated ruthenium complexes, which would make them potentially useful for DSSCs. We consider that the two intact N-methylbenzimidazolyl units (shown in blue) in these complexes act as two additional chromophores to enhance their light absorption (altogether three chromophores including the metal component itself), as represented by the cartoon in Figure 1. The synthesis, characterization, and spectroscopic, electrochemical, and computational studies of these mononuclear complexes are presented in this paper.

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RESULTS AND DISCUSSION Synthesis and Characterization. Ligand 1 and mononuclear complexes 2−6 were synthesized as outlined in Scheme 1. The condensation of N-methyl-1,2-phenylenediamine with benzene-1,2,4,5-tetracarboxylic acid in polyphosphoric acid Scheme 1. Synthesis of Compounds 1−6

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processes of 2−6 with some possible involvement of the oxidation of the cyclometalating ring.10−13 As a result of the variation of the electronic properties of the ancillary ligands, the RuII/III potentials of these complexes can be modulated from +0.36 to +0.91 V vs Ag/AgCl. The RuII/III potential for 2 (+0.36 V) is less positive than that of a previously reported complex, [(Mebip)Ru(Mebib)](PF6) (+0.43 V),10f where Mebib is bis(N-methylbenzidazolyl)benzene. This reflects that Metbib is more electron-rich than Mebib due to the presences of two additional N-methylbenzidazole units in the former ligand. At more positive potentials, an irreversible oxidation process is evident for 2−6. These peaks are attributable to ligand-based oxidative decomposition.6,7,10 Two chemically reversible cathodic waves at −1.50 and −1.75 V vs Ag/AgCl are observed for complex 2 (Figure S1). They arise from the reductions of the non-cyclometalating (Mebip) and cyclometalating ligand (Metbib), respectively. However, the cathodic waves of complexes 3−6 (except the first reduction wave of 6) are mostly irreversible (Figures S2−S5). Density Functional Theory (DFT) Calculations. DFT calculation of 2 was performed at the B3LYP/Lanl2DZ/631G*/vacuo level (see details in the Experimental Section) to aid in the understanding of its electronic structure. Figure 4

Figure 2. 1H NMR spectra of 2−6.

the metal center and another toward the metal. This is reasonable because the ancillary ligands around the metal in 3− 6 (2,2′:6′,2″-terpyridine) are less sterically demanding than that of 2 (Mebip). Electrochemical Studies. The electronic properties of 2− 6 were first studied by cyclic voltammetry (CV) studies (Figures S1−S5 in the Supporting Information). Corresponding data are summarized in Table 1. All of these complexes Table 1. Electrochemical Data of 2−6 and Related Complexes complex

E1/2a anodic

E1/2a cathodic

2, [(Mebip)Ru(Metbib)](PF6) 3, [(MeOptpy)Ru(Metbib)](PF6) 4, [(ttpy)Ru(Metbib)](PF6) 5, [(CF3ptpy)Ru(Metbib)](PF6) 6, [(Me3tcbtpy)Ru(Metbib)](PF6) [Ru(Mebip)(dpb)](PF6)

+0.36 +0.66, +1.73b +0.68, +1.64b +0.73, +1.83b +0.91, +1.66b +0.43, +1.49b

−1.50, −1.75 −1.45b −1.49b −1.41b −1.02, −1.28b −1.56

Figure 4. Isodensity plots of selected frontier orbitals for 2+ along with the energy level label. All orbitals have been computed at the B3LYP/ Lanl2DZ/6-31G*/vacuo level with an isovalue of 0.02.

shows the isodensity plots of selected frontier orbitals for 2 along with the energy level label. The closely spaced LUMO and LUMO+1 are associated with the non-cyclometalating ligand (Mebip). The LUMO+2 has a dominating contribution from the cyclometalating ligand (Metbib). This is in agreement with the previous electrochemical assignment that the reduction of Mebip occurs at a less negative potential than Metbib. The HOMO of 2 is of metal character. The HOMO−1 level displays an electronic configuration that is commonly observed in the HOMOs of many cyclometalated ruthenium complexes.10−13 Namely, this orbital is associated with both the metal center and the cyclometalating phenyl ring. The HOMO−2 of 2 is of metal character as well. The two freestanding N-methylbenzidazole units in Metbib are responsible for a relatively low-lying HOMO−3 level. In order to probe the electronic structures of complexes 3−6, we carried out DFT computations on a slightly simplified structure, [(tpy)Ru(Metbib)]+, where tpy is 2,2′:6′,2″-terpyridine. A set of molecular orbitals with very similar ordering and electronic configurations to those of 2 are found (Figure 5). However, [(tpy)Ru(Metbib)]+ is predicted to have lower lying

a

The potential is reported as the E1/2 value vs Ag/AgCl unless otherwise noted. bEp, irreversible.

exhibit a chemically reversible redox couple at relatively low potentials (Figure 3). These waves are ascribed to the RuII/III

Figure 3. Cyclic voltammograms of 2−6 in acetonitrile containing 0.1 M nBu4NClO4 as the supporting electrolyte at a scan rate of 100 mV/s. The working electrode was glassy carbon, and the counter electrode was a platinum wire. The reference electrode was Ag/AgCl in saturated aqueous NaCl solution. 4304

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Figure 5. Isodensity plots of selected frontier orbitals for [(tpy)Ru(Metbib)]+ along with the energy level label. All orbitals have been computed at the B3LYP/Lanl2DZ/6-31G*/vacuo level with an isovalue of 0.02.

LUMO and HOMO levels than those of 2. This correlates well with the previous electrochemical results, which show that complexes 3−6 have less negative ligand-based reduction waves but more positive RuII/III potentials than those of 2. Spectroscopic Studies and Time-Dependent DFT (TDDFT) Calculations. The absorption spectra of 2−6 were recorded to further probe their electronic properties (Figure 6a and c), which consist of the broad MLCT transitions in the visible region and the intraligand transitions in the ultraviolet (UV) region. In order to assist the detailed interpretations of the MLCT envelopes of these complexes, TDDFT computations were performed for 2 and [(tpy)Ru(Metbib)]+ on the basis of the above DFT-optimized structures. Predicted main low-energy excitations are shown in Figure 6b and d, and corresponding parameters are summarized in Table 2. In comparison, complex 2 with the Metbib ligand displays a much broader MLCT envelope than a previously reported complex,10f [(Mebip)Ru(Mebib)](PF6), without two freestanding N-methylbenzimidazolyl motifs (Figure 6a). Interestingly, this difference results in a significant change in the colors of these two complexes in acetonitrile. A solution of complex 2 is violet, while [(Mebip)Ru(Mebib)](PF6) is pink. TDDFT results suggest that the MLCT transitions of 2 are largely associated with the S6, S7, and S9 excitations (shown in blue in Table 2), which are dominated by the HOMO → LUMO+2, HOMO−1 → LUMO+2, and HOMO−2 → LUMO transitions. However, we note that the HOMO−3 level (shown in red in Table 2), which is associated with the two free-standing N-methylbenzimidazolyl units, is also predicted to contribute to the visible absorptions (S 9 , S 10 , and S 11 excitations) by ligand-to-ligand charge-transfer (LLCT) transitions. We consider that this is partially responsible for the enhancement of the light absorption of 2 compared to [(Mebip)Ru(Mebib)](PF6). Complexes 3−6 show absorptions in a similar region to 2, with a plateau between 500 and 600 nm. The colors of solutions of these complexes are violet as well, except 6, which is brownish in acetonitrile. TDDFT computations of [(tpy)Ru(Metbib)]+ show two major excitations in the visible region (S6 and S7), and they can be interpreted as the MLCT transitions with HOMO → LUMO+2 and HOMO−1 → LUMO+2 character. Similarly, the LLCT transitions from the HOMO−3 level of [(tpy)Ru(Metbib)]+ make an appreciable contribution to its absorptions in the visible region. Finally, it

Figure 6. (a) UV/vis spectra of 2 and [(Mebip)Ru(Mebib)]+ in acetonitrile. The inset shows the colors of these two complexes at a concentration of 5 × 10−5 M in acetonitrile. (b) TDDFT-predicted excitations of 2. (c) UV/vis spectra of 3−6 in acetonitrile. (d) TDDFT-predicted excitation of [(tpy)Ru(Metbib)]+.

should be noted that these complexes are only very weakly emissive or virtually nonemissive. For instance, complex 2 was found to emit a very weak light at 800 nm when excited at 560 nm (the quantum yield is less than 0.01%; see Figure S6 in the Supporting Information). No distinct emission spectra could be recorded for other complexes using the spectrofluorimeter at hand.

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CONCLUSION To conclude, a series of mononuclear cyclometalated ruthenium complexes with Metbib were successfully prepared. The RuII/III redox potentials of these complexes, which are correlated to their HOMO levels, could be modulated by attaching substituents of different electronic nature on the ancillary ligand. In general, these complexes display enhanced visible light absorption compared to those without two freestanding N-methylbenzimidazolyl units. The reasons are twofold. First, these N-methylbenzimidazolyl units act as electron-donating groups to decrease the RuII/III redox potentials and consequently the frontier energy gaps of these complexes. Second, the LLCT transitions from the Nmethylbenzimidazolyl units-associated occupied orbitals (HOMO−3) make appreciable contributions to the absorp4305

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Table 2. Calculated Excitation Energy (E), Oscillator Strength ( f), Dominant Contributing Transitions and Associated Percent Contribution, and Assignment of Complexes 2 and [(tpy)Ru(Metbib)]+a

a Computed at the TDDFT/B3LYP/LANL2DZ/6-31G*/vacuum level of theory. bThe actual percent contribution = (configuration coefficient)2 × 2 × 100%.

Synthesis of [(Mebip)Ru(Metbib)](PF6) (2). To 10 mL of dry acetone were added Ru(Mebip)Cl310f (54.8 mg, 0.1 mmol) and AgOTf (100 mg, 0.39 mmol). The mixture was refluxed for 2 h before cooling to room temperature. The resulting AgCl precipitate was removed by filtration. The filtrate was concentrated to dryness. To the residue were added Metbib (1) (59.9 mg, 0.1 mmol), 15 mL of DMF, and 15 mL of t-BuOH. The mixture was then refluxed for 24 h before cooling to room temperature. The solvent was removed under reduced pressure. The residue was dissolved in a proper amount of methanol. After adding an excess of aqueous KPF6, the resulting precipitate was collected by filtering and washing with water and Et2O. The obtained solid was subjected to flash column chromatography on silica gel (eluent: CH2Cl2/CH3CN, 15:1) to give 28 mg of complex 2 in a yield of 24%. MALDI-TOF (m/z): 1038.1 for [M − PF6]+. 1H NMR (400 MHz, CD3CN): δ 2.98 (s, 6H), 3.84 (s, 6H), 4.40 (s, 6H), 6.03 (d, J = 8.3 Hz, 2H), 6.28 (d, J = 8.2 Hz, 2H), 6.81 (t, J = 7.6 Hz, 2H), 6.88 (t, J = 7.8 Hz, 2H), 7.00 (t, J = 7.5 Hz, 2H), 7.07 (d, J = 8.1 Hz, 2H), 7.21 (t, J = 7.7 Hz, 2H), 7.34 (t, J = 7.4 Hz, 2H), 7.40 (m, 4H), 7.58 (d, J = 8.0 Hz, 2H), 7.73 (d, J = 7.9 Hz, 2H), 8.01 (s, 1H), 8.42 (t, J = 8.1 Hz, 1H), 8.86 (d, J = 7.2 Hz, 2H). Anal. Calcd for C59H46F6N13PRu·2H2O: C, 58.13; H, 4.13; N, 14.94. Found: C, 57.78; H, 3.98; N, 14.72. Synthesis of [(MeOptpy)Ru(Metbib)](PF6) (3). According to a synthetic procedure similar to that for 2, complex 3 was prepared from Ru(MeOptpy)Cl3 (27.7 mg, 0.051 mmol) and 1 (30.0 mg, 0.05 mmol) in a yield of 49%. MALDI-TOF: 1038.2 for [M − PF6]+. 1H NMR (400 MHz, CD3CN): δ 3.53 (s, 3H), 3.67 (s, 3H), 3.91 (s, 3H), 3.93 (s, 3H), 4.25 (s, 3H), 5.86 (d, J = 8.7 Hz, 1H), 6.69 (t, J = 8.0 Hz, 1H), 7.12 (t, J = 7.6 Hz, 1H), 7.15−7.35 (m, 6H), 7.35−7.50 (m, 6H), 7.56 (m, 3H), 7.65 (m, 2H), 7.85 (d, J = 8.1 Hz, 1H), 7.94 (m, 2H), 8.08 (d, J = 8.7 Hz, 2H), 8.21 (t, J = 8.0 Hz, 1H), 8.36 (d, J = 7.9 Hz, 1H), 8.52 (s, 1H), 8.55 (s, 1H), 8.84 (d, J = 8.1 Hz, 1H), 8.87 (s, 1H). Anal. Calcd for C60H46F6N11OPRu·4H2O: C, 57.41; H, 4.34; N, 12.28. Found: C, 57.59; H, 4.04; N, 12.33. Synthesis of [(ttpy)Ru(Metbib)](PF6) (4). According to a synthetic procedure similar to that for 2, complex 4 was prepared from Ru(ttpy)Cl3 (26 mg, 0.049 mmol) and 1 (30.7 mg, 0.051 mmol) in a yield of 43%. MALDI-TOF: 1022.2 for [M − PF6]+. 1H NMR (400 MHz, CD3CN): δ 2.47 (s, 3H), 3.47 (s, 3H), 3.59 (s, 3H), 3.93 (s, 3H), 4.25 (s, 3H), 5.84 (d, J = 8.7 Hz, 1H), 6.63 (t, J = 7.9 Hz, 1H), 7.06 (t, J = 7.6 Hz, 1H), 7.13 (t, J = 6.7 Hz, 1H), 7.25−7.35 (m, 4H), 7.35−7.48 (m, 7H), 7.56 (m, 3H), 7.80 (m, 3H), 7.97 (d, J = 8.0 Hz, 2H), 8.07 (m, 2H), 8.31 (d, J = 8.7 Hz, 2H), 8.50 (s, 1H), 8.60 (s, 1H), 8.77 (d, J = 8.1 Hz, 1H), 8.79 (s, 1H). Anal. Calcd for C60H46F6N11PRu·Et2O·2H2O: C, 60.18; H, 4.73; N, 12.06. Found: C, 60.16; H, 4.33; N, 12.24. Synthesis of [(CF3ptpy)Ru(Metbib)](PF6) (5). According to a synthetic procedure similar to that for 2, complex 5 was prepared from Ru(CF3ptpy)Cl3 (31 mg, 0.052 mmol) and 1 (31 mg, 0.052 mmol) in

tions in the visible region. This light absorption enhancement will make them potentially useful for DSSCs. However, we note that big dihedral angles are present between the free-standing N-methylbenzimidazolyl units and the cyclometalating benzene plane (calculated to be 44−49°). To further enhance the light absorption capability by means of the three-chromophore effect,18 groups with small steric hindrance and efficient conjugation are desired, which will be the focus of our future investigations.

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EXPERIMENTAL SECTION

Spectroscopic Measurement. All optical ultraviolet−visible (UV/vis) absorption spectra were obtained using a TU-1810DSPC spectrometer from Beijing Purkinje General Instrument Co. Ltd. at room temperature in acetonitrile, with a conventional 1.0 cm quartz cell. Emission spectra were recorded using a F-380 spectrofluorimeter from Tianjin Gangdong Sci & Tech Development Co. Ltd., with an R928F red-sensitive photomultiplier tube. Electrochemical Measurement. All cyclic voltammetry measurements were taken using a CHI620D potentiostat with a onecompartment electrochemical cell under an atmosphere of nitrogen. All measurements were carried out in 0.1 M Bu4NClO4/acetonitrile at a scan rate of 100 mV/s. The working electrode was glassy carbon with a diameter of 0.3 mm. The electrode was polished prior to use with 0.05 μm alumina and rinsed thoroughly with water and acetone. A large-area platinum wire coil was used as the counter electrode. All potentials are referenced to a Ag/AgCl electrode in saturated aqueous NaCl without regard for the liquid junction potential. Computational Methods. DFT and TDDFT calculations are carried out using the B3LYP exchange correlation functional19 and implemented in the Gaussian 03 program package.20 The electronic structures of complexes were determined using a general basis set with the Los Alamos effective core potential LanL2DZ basis set for ruthenium and 6-31G* for other atoms under vacuum.21 Synthesis of Metbib (1). A mixture of 1,2,4,5-benzenetetracarboxylic acid (0.52 g, 2.05 mmol) and N-methyl-1,2-phenylenediamine (1.74 g, 8.94 mmol) in 10 mL of polyphosphoric acid was stirred at 210 °C for 8 h. After being cooled to room temperature, the reaction mixture was poured into 50 mL of water and neutralized by 5 M aqueous NaOH. The resulting precipitate was collected by filtering and washing with water. The obtained solid was subjected to flash column chromatography on silica gel (eluent: CH2Cl2/methanol, 10:1) to give 1.05 g of 1 as a pale yellow solid in a yield of 85%. 1H NMR (400 MHz, CDCl3): δ 3.33 (s, 12H), 7.20 (d, J = 6.6 Hz, 4H), 7.27 (m, 8H), 7.77 (d, J = 6.6 Hz, 4H), 8.45 (s, 2H). ESI-MS: 599 for [M + H]+. ESI-HRMS (m/z): calcd for C38H31N8 599.2672, found 599.2655. 4306

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a yield of 16%. MALDI-TOF: 1076.1 for [M − PF6]+. 1H NMR (400 MHz, CD3CN): δ 3.53 (s, 3H), 3.68 (s, 3H), 3.92 (s, 3H), 4.27 (s, 3H), 5.83 (d, J = 8.7 Hz, 1H), 6.69 (t, J = 7.9 Hz, 1H), 7.13 (t, J = 7.6 Hz, 1H), 7.24 (t, J = 7.8 Hz, 1H), 7.29 (m, 4H), 7.37 (m, 2H), 7.46 (m, 3H), 7.54 (m, 3H), 7.66 (m, 2H), 7.87 (d, J = 8.2 Hz, 1H), 7.95 (m, 2H), 8.00 (d, J = 8.2 Hz, 2H), 8.25 (m, 3H), 8.38 (d, J = 7.9 Hz, 1H), 8.53 (s, 1H), 8.62 (s, 1H), 8.85 (d, J = 8.0 Hz, 1H), 8.93 (s, 1H). Anal. Calcd for C60H43F9N11PRu·6H2O: C, 54.22; H, 4.17; N, 11.59. Found: C, 53.99; H, 4.18; N, 12.10. Synthesis of [(Me3tcbtpy)Ru(Metbib)](PF6) (6). According to a synthetic procedure similar to that for 2, complex 6 was prepared from Ru(Me3tcbtpy)Cl3 (30.1 mg, 0.049 mmol) and 1 (29.6 mg, 0.049 mmol) in a yield of 56%. MALDI-TOF: 1106 for [M − PF6]+. 1H NMR (400 MHz, CD3CN): δ 3.42 (s, 3H), 3.63 (s, 3H), 3.88 (s, 3H), 3.90 (s, 3H), 4.06 (s, 3H), 4.10 (s, 3H), 4.43 (s, 3H), 5.61 (d, J = 8.7 Hz, 1H), 6.69 (t, J = 7.9 Hz, 1H), 7.20−7.35 (m, 4H), 7.35−7.45 (m, 4H), 7.48 (d, J = 7.8 Hz, 1H), 7.60 (m, 4H), 7.74 (d, J = 5.8 Hz, 1H), 7.78 (m, 1H), 7.88 (d, J = 8.2 Hz, 1H), 7.96 (m, 1H), 8.16 (d, J = 5.8 Hz, 1H), 8.52 (s, 1H), 8.80 (s, 1H), 8.95 (s, 1H), 9.24 (s, 1H), 9.28 (s, 1H). Anal. Calcd for C59H46F6N11O6PRu·2H2O: C, 55.06; H, 3.92; N, 11.97. Found: C, 54.78; H, 4.17; N, 11.61.

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ASSOCIATED CONTENT

S Supporting Information *

CV profiles and 1H NMR spectra for 2−6, emission spectrum of 2, and DFT-optimized Cartesian coordinates for 2 and [(tpy)Ru(Metbib)]+. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (No. 21002104), the National Basic Research 973 Program of China (No. 2011CB932301), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry of China, and Institute of Chemistry, Chinese Academy of Sciences (“100 Talent” Program) for funding support.

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