DALTONFULL PAPERJ. Chem. Soc., Dalton Trans., 1999, 4217¡V4221 4217This journal is The Royal Society of Chemistry 1999Spectroscopic and electrochemical properties of ruthenium(II)polypyridyl complexesPu-Hui Xie,a Yuan-Jun Hou,a Bao-Wen Zhang,*a Yi Cao,*a Fang Wu,b Wen-Jing Tian b andJia-Cong Shen ba Laboratory of Photochemistry, Institute of Photographic Chemistry, the Chinese Academy ofSciences, Beijing, 100101b Key Lab for Supramolecular Structure and Spectroscopy, Jilin University, ChangChun,130023, People¡¦s Republic of ChinaReceived 20th September 1999, Accepted 11th October 1999Luminescent mixed-ligand ruthenium() complexes of type [Ru(bpy)2L]2 (where bpy = 2,2-bipyridine, L = 3,3-dicarboxy-2,2-bipyridine 1; 4,4-dicarboxy-2,2-bipyridine 2; or 5,5-dicarboxy-2,2-bipyridine 3) were synthesized.Their photophysical, acid¡Vbase and electrochemical properties were investigated.The emission lifetime for 2 was thelongest, and the emission quantum yield was the highest for 2.This reveals that the positions of the carboxylicacids in the 2,2-bipyridine ligand had an important inuence on the photophysical and electrochemical propertiesof the complexes.IntroductionRuthenium() polypyridyl complexes have been employed asecient photosensitizers for the past two decades, due to theirchemical stability, redox properties and excited state reactivity.1For studying the steric eect of CO2H groups on the 2,2-bipyridine ligand in the photoelectric conversion of theruthenium complexes, photosensitizers of cis-[Ru(dcbpy)2-(NCS)2] type were synthesized (dcpy = 4,4-dicarboxy-2,2-bipyridine).This complex was shown to be a most ecientphotosensitizer. Its maximum incident monochromatic photonto-current conversion eciency (IPCE) was greater than 65%.However, such thiocyanate complexes showed very weakluminescence in solution. The emitting state of the above complexhad a luminescence quantum yield of only 0.4% (125 K)and a 50 ns lifetime (298 K).2 So it is dicult to study photophysicalproperties of such complexes by general spectraltechniques.The spectroscopy and photochemistry of complexes[Ru(bpy)2L]2 have been of particular interest (for exampleL = dicarboxy-4,4-bipyridine 3,4) because of their longer emissionlifetimes and higher emission quantum yields. In suchmixed-ligand complexes the electron is largely localized on thatligand which is more easily reduced. If this ligand happens to beprotonatable, the formation of the MLCT excited state causessignicant changes in the acid¡Vbase equilibria of the complex,resulting in large shifts in the pKa of the complex.5 In addition,the photophysical and redox properties of such transition metalcomplexes can provide important information for the natureof the ground and excited states.So bis(2,2-bipyridine)-(3,3-dicarboxy-2,2-bipyridine)ruthenium() chloride 1, bis-(2,2-bipyridine)(4,4-dicarboxy-2,2-bipyridine)ruthenium()chloride 2 as well as bis(2,2-bipyridine)(5,5-dicarboxy-2,2-bipyridine)ruthenium() chloride 3 were synthesized.Detailedstudies on the properties of all the complexes are reported inthis paper. The aim of these experiments was to investigate thesteric eects on the photophysical and electrochemical propertiesof the complexes.ExperimentalSpectral measurementsThe 1H NMR spectra were recorded on a Varian Gemini300 (MHz) spectrometer, element analysis data on an ItalianCarloerba 1160 spectrometer, UV-Vis absorption spectra ona Hitachi U-2001 UV/Vis spectrophotometer and emissionspectra on a Hitachi 850 uorescence spectrophotometer witha computer for data collection and analysis.Emission quantumyields were calculated by comparison with the integrated intensityof the emission spectrum of an absorbance-matched solutionof [Ru(bpy)3]Cl2 (£Xem = 0.042) 6 in water. The emission lifetimeswere measured with a time-resolved spectrouorimeter(Horiba NAES-1100), based on the single-photon-countingmethod.The light source (2 ns, full width half maximum,FWHM) for the excitation was a high pressure hydrogen ashlamp of free-running type. Each sample was bubbled with N2for 30 min before use.Acid/base titration experimentsAcid/base titration experiments were performed on samplesdissolved in 1 M NaCl solutions to keep a constant ionicstrength. The ground state pKa0 were determined by spectro-4218 J. Chem. Soc., Dalton Trans., 1999, 4217–4221 Table 1 Absorption and emission data for the ruthenium polypyridine complexes Complex Solvent ëabs max/nm ëem max/nm (RT) ô/ns (RT) ö (RT) 1 [Ru(bpy)2(3,3-dcbpy)] 2 [Ru(bpy)2(4,4-dcbpy)] 3 [Ru(bpy)2(5,5-dcbpy)] water (pH 7) CH3CN EtOH water (pH 7) CH3CN EtOH water (pH 7) CH3CN EtOH 453 453.5 450 459 457 454 450 456 456 615 607 601 628 604 597 650 606 600 109 235 241 471 630 313 ——— 0.012 0.008 0.017 0.031 0.022 0.039 0.0004 0.0005 0.0003 metric titration, and the excited state pKa* by emission titration.The acidities of these solutions were adjusted by addition of dilute HCl and/or NaOH. The pH values were determined using a pH glass electrode. In the strongly acidic regimes, absorption spectra were obtained by titrating with 98.6% H2SO4. Estimated errors were as follows: absorption and emission maxima, ±1 nm; emission lifetime, ±10%; pKa, pKa*, ±0.1. Cyclic voltammetry measurements Cyclic voltammetry measurements were made with HPD-IA equipment. Millimolar solutions of the compounds were prepared in 0.1 M acetonitrile solutions of tetra-n-butylammonium tetra.uoroborate.The acetonitrile was freshly distilled over P2O5, and the solutions were deaerated by purging with N2 over 25 min. All voltammograms were recorded under a N2 atmosphere with a platinum microcylinder working electrode, a platinum wire auxiliary electrode, and a standard calomel reference electrode. Cyclic voltammetry of the ferrocene–ferrocenium redox couple was performed after each experiment to calibrate the pseudo-reference electrode.Preparations 3,3-Dicarboxy-2,2-bipyridine(3,3-dcbpy),7 4,4-dicarboxy- 2,2-bipyridine(4,4-dcbpy),8 5,5-dimethyl-2,2-bipyridine,9 [Ru(bpy)3]Cl2,10 cis-[Ru(bpy)2Cl2],11 and cis-[Ru(4,4-dcbpy)2- Cl2] 1 were prepared by the literature methods. 5,5-Dicarboxy- 2,2-bipyridine(5,5-dcbpy) was prepared in the same manner as 4,4-dcbpy. [Ru(bpy)2(3,3-dcbpy)]Cl2 1. The compounds cis-[Ru(bpy)2- Cl2] (160 mg, 0.33 mmol) and 3,3-dcbpy (101 mg, 0.42 mmol) were re.uxed in water–EtOH (20 ml, 1 : 1 v/v) under N2 in the dark for 8 h.The product was puri.ed on a neutral alumina column using CH3CN as eluent. Calc. for 112H2O: C, 40.68; H, 5.08; N, 8.89. Found: C, 41.20; H, 4.99; N, 8.58%. 1H NMR (D2O): ä 8.53 (d, 4 H), 8.27 (d, 2 H), 8.05 (m, 4 H), 7.78 (d, 2 H), 7.66 (d, 2 H) and 7.40 (m, 8 H). [Ru(bpy)2(4,4-dcbpy)]Cl2 2. This complex was synthesized as described.12 Calc.for 24H2O: C, 48.48; H, 4.00; N, 10.50. Found: C, 48.50; H, 4.01; N, 10.52%.1H NMR (D2O): ä 9.25 (d, 2 H), 8.83 (d, 4 H), 8.30 (d, 2 H), 8.22 (ddd, 4 H), 8.04 (t, 4 H), 7.95 (dd, 2 H) and 7.57 (m, 4 H). [Ru(bpy)2(5,5-dcbpy)]Cl2 3. The compound cis-[Ru(bpy)2Cl2] (160 mg, 0.33 mmol) was mixed with 5,5-dcbpy (101 mg, 0.42 mmol) in 20 mL of EtOH–water, re.uxed for 12 h under N2 and then concentrated. The solid was recrystallized from MeOH– diethyl ether. Calc.for 32H2O: C, 50.26; H, 3.66; N, 10.99. Found: C, 50.29; H, 3.60; N, 10.93%. 1H NMR (D2O): ä 8.64 (d, 2 H), 8.54 (m, 6 H), 8.13 (d, 2 H), 8.06 (m, 4 H), 7.75 (d, 4 H) and 7.42 (m, 4 H). Results and discussion UV-Vis spectra and the ground state pKa 0 of complexes 1, 2, 3 Absorption spectral data for compounds 1, 2, 3 are reported in Table 1. For all the mixed-ligand complexes the low-energy metal-to-ligand charge-transfer (MLCT) band was located anywhere between 450 and 460 nm, being little dependent on the nature of the spectator ligand L and that of the solvent.The UV-Vis absorption spectra for complex 1 in aqueous solution as a function of pH are shown in Fig. 1(a). As the pH decreased the absorbance of the MLCT band decreased and broadened, then a new, low-energy shoulder appeared at ca. 500 nm. The changes of these spectra were completely reversible, and the compound was stable at each pH. There were two isosbestic points at 427 and 496 nm between pH 2 and 7.Below pH 2 the spectra changed little. The absorption changes at 453 nm as a function of pH between 0.6 and 10.5 for complex 1 showed two in.ection points (pKa1 0 = 0.2 and pKa2 0 = 2.2) in Fig. 1(b) (because the protonation steps were close together, it was not possible to draw good sigmoidal curves to determine pKa; the acidity at which the rate of change in absorbance is greatest was chosen to calculate the pKa for each protonation step 13). Thus in Fig. 1(b) the spectrum at pH 4.7 represented that of the deprotonated form; at pH 1.9, the monoprotonated form; at pH 0, the diprotonated form.The observations of two isosbestic points and two in.ection points in the titration plot demonstrated that two di.erent protonation steps occurred for 1, eqns. (1) and (2), Fig. 1 (a) Absorption spectra of complex 1 in aqueous solution for various values of pH. From 1 to 7: 6.16; 4.7; 4.15; 3.14; 2.6; 2.3; 2.11. (b) Spectrophotometric titration of 1 showing absorbance change at 453 nm for various values of pH.J.Chem. Soc., Dalton Trans., 1999, 4217–4221 4219 [Ru(bpy)2(3,3-dcbpy)H2]2 pKa1 0 = 0.2 [Ru(bpy)2(3,3-dcbpy)H] H (1) [Ru(bpy)2(3,3-dcbpy)H] pKa2 0 = 2.2 [Ru(bpy)2(3,3-dcbpy)] H (2) which could be used to describe the acid/base behavior of this complex. Protonation of complex 2 also led to a large perturbation of the absorption spectra. Upon lowering of pH the MLCT band located at 459 nm showed a small decrease in intensity.Two isosbestic points at 422 and 478 nm were also found at pH between 1 and 6. Below pH 1 the spectra shifted slightly o. the isosbestic but otherwise changed little. The absorption changes at 459 nm as a function of pH between 1 and 11 yielded two pKa values for the ground state as 1.7 and 2.9. Over the pH range 13.0–5.5 the absorption spectra of complex 3 were pH independent. Below pH 5.5 lowering of pH led to signi.cant spectral changes.Fig. 2(a) presents absorption spectra of the fully deprotonated and protonated forms of the complex. The MLCT absorption maximum located at 450 nm was red shifted, and a shoulder appeared at 507 nm with slight decrease in intensity with decreasing pH. Fig. 2(b) shows such a .t for analysis at 450 nm with a single pKa2 at 3.0 (pKa1 was below 0.8). The stronger acidity of complex 1 as compared to its 4,4- dcbpy and 5,5-dcbpy analogs could be explained in terms of intramolecular hydrogen-bonding interaction between the two carboxy acid substituents in 3,3 positions.The hydrogen bonding interaction could occur between one of the oxygen atoms bonded to the carbonyl carbon atom in the 3 position on the bipyridine ring with the hydrogen atom bonded to the carboxy group in the 3 position. The distance of the 3,3 positions might fall within the hydrogen-bonding domain, and be more favorable than the 4,4 and 5,5 positions. The hydrogenbonding interaction might withdraw the electron from the carboxy groups, and would decrease the electron density on the Fig. 2 (a) Absorption spectra of complex 3 in aqueous solution for the following values of pH: 1, 5.48; 2, 3.39; 3, 2.75; 4, 2.11; 5,1.72; 6, 1.09; 7, 0.83. (b) Spectrophotometric titration of 3 showing the absorbance change at 450 nm for various values of pH. carboxy group and increase the acidity of 1. So the pKa values of 1 were accordingly smaller than those of the other two.However, the hydrogen bonding between the CO2H groups in the C3 and C3 positions had less in.uence on the 1H NMR spectra than the OH groups in the same positions,14 as two CO2H substituents of the ligand formed an intramolecular O–H O hydrogen bonding in a twisted manner, while two OH groups form a six- or seven-membered ring which was more stable. Complex 3 was the least acidic among the compounds as seen from the above analysis. A possible reason for this was the increase in electron density on the 5,5-dcbpy positions. Therefore, there was a substantial decrease in the ground state pKa2 upon going from 5,5- to 4,4- and 3,3-based dyes.Emission properties Fig. 3(a) shows the emission spectra for complex 1 under various pH conditions. The luminescence intensity was almost pH independent above pH 7. The emission intensities of 1 varied mildly in dilute acidic solutions. The intensity decreased rapidly in medium acidic solutions and was almost unchanged in strong acid (below pH 2). The luminescence peak at 615 nm in solutions of pH 6.17 and at 635 nm in pH 1.9 were assigned to the deprotonated and monoprotonated forms of 1 in the excited state respectively, suggesting that proton transfer occurred within the emission lifetime of the excited state.The emission intensity of the monoprotonated form was very weak (ca. 7%) as compared to that of the deprotonated one. As for complex 2, at pH above 5.38, the emission maxima and intensities were independent of pH.When the pH decreased from 5.38 to 4.25 a sudden decrease in intensity occurred. Then between pH 4.25 and 2.99 the emission intensities decreased slowly; at pH below 2.99 the emission maxima and intensities were again almost independent of pH. As the pH decreased from 5.38 to 2.72 the luminescence peak of 2 at 628 nm red shifted to 665 nm, which was assigned to the excited Fig. 3 (a) pH Dependence of emission for complex 1.From 1 to 9, the values of pH are 10.47; 6.16; 4.70; 4.15; 3.14; 2.60; 2.30; 2.11; 1.90 respectively. (b) Titration curve for relative emission intensity with pH for 1 (at 615 nm).4220 J. Chem. Soc., Dalton Trans., 1999, 4217–4221 states of the deprotonated and monoprotonated species respectively. The e.ect of variations in pH on the emission spectra vs. intensity of complex 3 are presented in Fig. 4. The fully ionized form, present at pH = 5.0, had its emission maximum at 650 nm.The emission was rather weak and short-lived as compared to that of 2. Lowering of pH led to a decrease in intensity as well as a red-shift to ca. 700 nm at pH 1.7. A summary of the emission quantum yields and lifetimes of complexes 1, 2, 3 at di.erent pH is given in Table 2. The data suggested a profound in.uence of the positions of the carboxy groups in the 2,2-bipyridine ligand on the photophysical properties of the complexes. The di.erences in the emission lifetimes as well as emission quantum yields between these basic and acidic forms in the dicarboxybipyridine chelates were rather signi.cant.The emission lifetimes between the deprotonated and protonated forms were consistent with the emission intensity ratios measured in the steady-state luminescence measurements. As shown above, complex 1 emitted at shorter wavelength and had a shorter emission lifetime and smaller quantum yield than 2. This behavior was opposite to the trend predicted by the energy gap law (3) 15 where C is the slope in cm and Eem the energy in cm1 at the emission maximum.According to eqn. (3), knr of 1 should be smaller than that of 2 and therefore Knr . eCEem (3) the emission quantum yield of 1 should be higher than that of 2. This may be attributed to a thermal population of a highlying ligand .eld (3LF) state.16 In general, photoexcitation of ruthenium polypyridyl complexes generates a singlet metal-to-ligand charge-transfer state which undergoes e.cient intersystem crossing (Fisc � 1) to a manifold of closely spaced triplet states (3MLCT).17 These states can then decay back to the ground state through both radiative (kr) and non-radiative (knr) mechanisms, in addition to a thermal population of high-lying ligand .eld (3LF) states.This 3LF state undergoes fast non-radiative decay to the ground state or ligand dissociation.18 The energy of the LF excited state depends on the .eld strength, which depends on the s-donor and p-acceptor properties of the ligand, the steric crowding around the metal and the bite angle of the polydentate Fig. 4 pH Dependence of emission for complex 3. From 1 to 5, the values of pH are 5.48, 3.39, 2.75, 2.11, 1.72 respectively. Table 2 The emission quantum yields and lifetimes at di.erent pH for ruthenium complexes (.ex = 454 nm, .em = 610 nm for 1, .ex = 460 nm, .em = 630 nm for 2, .ex = 450 nm, .em 650 nm for 3) Compound pH t/ns 103 fem 1 2 3 13.38 0.6 13.38 0.6 13.38 0.6 341 77 492 190 —— 12 3 30 8 0.54 0.30 ligands.19 For 1 the bite angle cannot be optimized because of the steric hindrance on the 2,2-bipyridine which leads to weakening of the ligand .eld strength.Thus, the energy of the LF excited state of 1 decreases. Furthermore, the MLCT energy of 1 is slightly higher than that of 2. Therefore, knr increases exponentially with decreasing energy gap between MLCT and LF excited states. In addition, the formation of a hydrogen bond can lead to signi.cant enhancement of non-radiative decay rates.An increase in knr may be the reason that the quantum yield of 1 is smaller than that of 2. However, for 3 the longest emitting wavelength and the shortest emission lifetime as well as the smallest emission quantum yield were due to the increase in non-radiative decay rates. Excited-state emission spectral data on the ruthenium polypyridine complexes examined are also given in Table 1.For all of the mixed-ligand complexes the emission MLCT band was located anywhere between 610 and 700 nm, being much dependent on the positions of the carboxy groups, on the spectator ligand L and the nature of the solvent. In CH3CN each complex emitted at shorter wavelength than in water; the solvent stabilization of the 3MLCT was ca. 214, 633 and 1117 cm1 for the 3,3, 4,4 and 5,5 Ru complexes respectively. These results showed that solvatochromic shifts in emission can be used to alter the emission energies of [Ru(bpy)2L]2 to produce a solvent-dependent switching.Excited state pKa* The titration curve of the luminescence intensity at 615 nm for complex 1 in Fig. 3(b) over the pH range of 2–11 shows one in.ection point at 3.8. The in.ection point represents the equilibrium between the deprotonated form and monoprotonated form and indicates that the pH for the other equilibrium (between monoprotonated and diprotonated form) was lower than 1.9.The results illustrate that the excited complex could be protonated without electronic deactivation. As an example, when the deprotonated form of 1 was excited at pH 3.14 the emission of the monoprotonated form was observed predominantly. The pKa in the excited state (pKa2*) corresponding to the equilibrium between the deprotonated and monoprotonated form was evaluated in two empirical ways based on Forster’s cycle, using the emission and absorption spectra and their titration curves.20 First, in the titration method, the pKa2* was given by eqn.(4) where the pH is taken at the in.ection point of pH = pKa2* log(tH/t) (4) 1 in the luminescence curve, tH (102 ns) and t (341 ns) are the excited state lifetimes of the monoprotonated and the deprotonated species respectively. Then, the pKa2* of 1 was determined to be 3.3. Secondly the Forster treatment results in eqn. (5), which pKa* = pKa 0 (0.625/T) (.B .BH) (5) describes the relationship between ground and excited state pKa based on pure 0–0 transitions in wavenumbers (cm1) for the deprotonated (.B) and monoprotonated (.BH) forms respectively.At room temperature the ground state pKa2 0 of 1 was 2.2, the emission energy maxima for the deprotonated and monoprotonated species were .B = 1.63 × 104, .BH = 1.58 = 104 cm1, and the predicted excited state pKa2* was 3.2 which agrees well with the value measured by emission titration. From the titration of the emission intensity at 645 nm for complex 2 over the pH range of 2–11 one in.ection point was derived at pH 4.5.According to eqn. (4), and the lifetimes of the deprotonated (t = 471 ns) and monoprotonated form (t = 243 ns) of 2, the pKa2* of 2 was 4.2. Using .B = 1.538 × 104, .BH = 1.47 × 104 cm1, the resulting value of pKa2* was 4.3 according to eqn. (5).J. Chem. Soc., Dalton Trans., 1999, 4217.4221 4221 Table 3 Data for the ground state and excited state acid.base equilibria in ruthenium polypyridyl complexes pKa MLCT (nm) Emission ¥ëmax/nm Compound 1 2 pKa2* deprot.prot. pH > 7.0 pH < 0 123 0.2 1.7 2.2 2.9 3.0 3.3 4.3 3.5 453 459 450 520 500 510 615 628 650 635 665 700 Fitting of the total emission intensity variation as a function of pH for complex 3 yielded an in.ection point (pKa* value) at 3.5. Comparison of similar data for the other two complexes showed that the shift in the pKa upon formation of the MLCT excited state was the least pronounced (.pKa = 0.5 for 3, as compared to ¡Ã1.1 for the other two).Table 3 summarizes the absorption and emission properties measured in this work for each dcbpy complex in the deprotonated and protonated forms. The higher apparent pKa* values of the compounds as compared to each ground-state pKa value indicated that the excitedstate deprotonated forms of each compound were slightly more basic than the ground-state analogue. The higher pKa2* value of each complex suggested that the ligand electron density is signi.cantly higher in the excited state than in the ground state, due to the charge transfer transition from metal to ligand.In the excited state the electron was located on the dcbpy ligand; such redistribution of charge creates more electron density on the carboxy groups, thus causing them to be more basic. Electrochemical data Redox potentials for the complexes in CH3CN were obtained by cyclic voltammetry. Results are listed in Table 4.For these complexes the oxidation corresponds to removal of an electron from the d orbital of RuII to give RuIII. The reduction corresponds to reduction of the ligand containing the carboxy acid groups. In mixed-ligand complexes like these the electron transition upon optical absorption would occur between the metal center and the ligand which is most easily reducible. The electron-withdrawing character of the carboxy group would shift the reduction potential of the ligand positively relative to that of the unsubstituted bipyridine ligand.From 1 to 3 the oxidation potentials decreased with increasing pKa. Data presented in the above tables on the complexes with carboxy groups reveal several interesting e.ects on the Ru¡æL luminescence. Along the ligand series 3,3, 4,4, 5,5 one can note that: (a) the absorption maxima of deprotonated and monoprotonated forms and the emission maxima of deprotonated forms all blue shift with increasing excited-state lifetime Table 4 Ground and excited state energies and cyclic voltammetry data for model compounds in CH3CNa Oxidations c Reductions Compound E0,0 b/eV E1/2/V E1/2*/V E1/2/V E1/2*/V 123 2.18 2.25 2.26 1.29 1.27 1.26 0.89 0.98 1.00 1.53 1.50 1.44 0.65 0.75 0.82 a All potentials for 103 M compounds with 0.1 M tetra-n-butylammonium tetra.uoroborate as supporting electrolyte.Volts vs. SCE, error in potentials ¡¾0.002 V, T = 25 ¡¾ 1 C, scan rate = 100 mV s1.b The values of E0,0 were obtained from the 298 K emission spectra, it should be noted that E0,0 cannot be rigorously determined, because of the approximations required in the spectral modelling of the lower frequency modes and the lack of spectral resolution. c Mean of cathodic and anodic half-wave potential values. and emission intensity; (b) the pKa2* value for the Ru¡æL CT emission is larger than each pKa 0 value; (c) the emission properties are sensitive to ¡°.ne tuning¡± of the excited-state energies.Conclusion A series of mixed-ligand polypyridyl ruthenium(..) complexes [Ru(bpy)2(3,3-dcbpy)]2, [Ru(bpy)2(4,4-dcbpy)]2, [Ru(bpy)2- (5,5-dcbpy)]2 were designed and synthesized. The ground state pKa and excited state pKa* of [Ru(bpy)2L]2 type complexes were determined. Complex 3 had the shortest emission lifetime and smallest quantum yield. The strongest acidity of 1 could be attributed to intramolecular hydrogen bonding interaction. Compared to the ground state pKa, the higher value for pKa* revealed that the excited state species was a slightly stronger base than its ground state analogue.The positions of the carboxy groups in the 2,2-bipyridine ligand had an e.ect on the photophysical properties of the complexes. Acknowledgements The authors wish to thank the NNSFC for .nancial support (No. 29672034 and 29733100). References 1 M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Muller, P. Liska, N. Vlachopoulos and M. Gratzel, J. Am. Chem. Soc., 1993, 115, 6382. 2 S. Ferrere and B. A. Gregg, J. Am. Chem. Soc., 1998, 120, 843. 3 P. J. Giordano, C. R. Bock, M. S. Wrighton, L. V. Interrante and R. F. X. Williams, J. Am. Chem. 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