ARTICLE pubs.acs.org/JPCA
Formation of a Hybrid Compound Composed of a Saddle-Distorted Tin(IV)-Porphyrin and a Keggin-Type Heteropolyoxometalate To Undergo Intramolecular Photoinduced Electron Transfer Atsutoshi Yokoyama,† Takahiko Kojima,*,‡ Kei Ohkubo,† Motoo Shiro,§ and Shunichi Fukuzumi*,†,|| †
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Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan ‡ Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan § X-ray Research Laboratory, Rigaku Corporation, 3-9-12 Matsubara, Akishima-shi, Tokyo 196-68666, Japan Department of Bioinspired Chemistry, Ewha Womans University, Seoul 120-750, Korea
bS Supporting Information ABSTRACT: Nonplanar Sn(IV)-porphyrin complexes, [Sn(TMPP(Ph)8)Cl2] (1) and [Sn(TMPP(Ph)8)(OMe)2] (2) (TMPP(Ph)8: 5,10,15,20tetrakis(4-methoxyphenyl)-2,3,7,8,12,13,17,18-octaphenylporphyrinato), were prepared and characterized by spectroscopic and electrochemical methods together with X-ray crystallography. Variable-temperature 1H NMR study revealed that the coordination of the methoxo ligand of 2 is weak enough in solution to enhance the axial ligand exchange with a Keggin-type phosphotungstate (R-[PW12O40]3-) due to the steric stress between the axial methoxo ligand and the peripheral phenyl groups of the porphyrin ligand. The formation of a novel 1:1 donor-acceptor complex, [Sn(TMPP(Ph)8)(OMe)(R-[PW12O40])]2- (4) was confirmed by 1H NMR and UV-vis spectral titrations, and also by MALDI-TOF-MS measurements. Electrochemical measurements for the donor-acceptor complex in PhCN revealed that the Sn(IV)-TMPP(Ph)8 moiety acts as an electron donor and the R-[PW12O40]3- moiety acts as an electron acceptor and that the energy level of the electron-transfer (ET) state of the 1:1 complex (1.17 eV) is lower than that of the triplet excited states of the SnTMPP(Ph)8 complex (1.31 eV). Femtosecond and nanosecond laser flash photolysis measurements indicate that intersystem crossing from the singlet excited sate to the triplet excited state occurs followed by intramolecular photoinduced electron transfer from the triplet excited state of the Sn(IV)-TMPP(Ph)8 moiety to the R-[PW12O40]3moiety in the 1:1 complex in benzonitrile.
’ INTRODUCTION In nature, photoinduced electron transfer (PET) is one of the most important events to convert light energy into chemical energy by forming a long-lived charge separated (CS) state as observed in the photosynthetic reaction center.1 To mimic the photosynthetic reaction center, a great number of donor-acceptor (D-A) systems have been prepared to demonstrate photoinduced electron transfer toward the formation of long-lived CS states.2-6 Porphyrins have attracted considerable interest as antenna and electron donor molecules to construct D-A systems because of the synthetic diversity of porphyrin derivatives and their excellent photophysical and electrontransfer properties.6-9 In most D-A systems organic electron acceptors are linked with porphyrins and their metal complexes.2-9 As inorganic electron acceptors, polyoxometalates (POMs) have merit special attention because POMs are quite robust to exhibit a great diversity of sizes, nuclearities, and shapes together with various catalytic reactivities.10-12 Although POMs absorb only UV light, the combination with porphyrins makes it possible to utilize the visible light. Associations between metalloporphyrins and POMs have so far been made by using covalent bond, electrostatic interaction, and coordination bonds.13-19 Among them the most simple but r 2011 American Chemical Society
elegant way to link between metalloporphyrins and POMs with fixed distance and orientation may be the use of coordination of axial ligands to metalloporphyrins, because they may be obtained just by mixing metalloporphyrins and POMs. In this context, we have reported that a nonplanar Mo(V) porphyrin complex can form discrete Mo(V) porphyrin-POM conglomerates, which includes direct coordination bonds between the highly Lewis acidic Mo(V) center of the distorted porphyrin complex and a terminal metal-oxo group of a Keggin-type POM.20 However, the lifetime of the excited state of the paramagnetic Mo(V) porphyrin is too short to undergo any photochemical reaction. There has been no report on formation of coordination complexes of diamagnetic metalloporphyrins with POMs, which can undergo photoinduced electron transfer from the excited state of metalloporphyrins to POMs. We report herein the formation of a 1:1 complex composed of a nonplanar Sn(IV)-porphyrin, [Sn(TMPP(Ph)8)(OMe)2] Received: October 14, 2010 Revised: December 20, 2010 Published: January 26, 2011 986
dx.doi.org/10.1021/jp109863d | J. Phys. Chem. A 2011, 115, 986–997
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under air for 1 day at 130 C, the reaction mixture was cooled to room temperature and evaporated to dryness under vacuum. The residue was dissolved in 100 mL of CH2Cl2 and washed with water, and the organic layer was dried over anhydrous sodium sulfate. The green solution was passed through a silica gel column eluted with a CH2Cl2/MeOH (10:1 v/v) mixed solvent, and the first green band was collected.28 After the solvent was removed, the green powder obtained was redissolved into distilled CH2Cl2 and washed with 5% HCl aqueous solution. The organic layer was dried with Na2SO4, and recrystallization from CHCl3/hexane gave green crystals of 1 (120 mg, 52%). Anal. Calcd for C96H68N4O4Cl2Sn 3 CHCl3: C, 70.58; H, 4.21; N, 3.39. Found: C, 70.21; H, 4.37; N, 3.25. 1H NMR (CD3CN, δ (ppm)): 7.47 (d, J = 8.6 Hz, 8H; meso-phenyl o-H), 6.76 (s, 40H; β-phenyl H), 6.30 (d, J = 8.7 Hz, 8H; meso-phenyl m-H), 3.70 (s, 12H; meso-phenyl, p-OMe, H). MALDI-TOF-MS (matrix: CHCA): m/z calcd for C96H68N4ClSn: 1495.78; observed: 1495.28 [M Cl]þ. UV/vis (benzonitrile, λmax (ε, M-1 cm-1)): 480 (2.0 105), 604 (1.4 104), 664 nm (2.2 104). [Sn(TMPP(Ph)8)(OMe)2] (2). 1 (25 mg, 16.5 μmol) and AgPF6 (41 mg, 165 μmol) were dissolved into 50 mL of a CH3CN/MeOH (3:2 v/v) mixed solvent. After 5 mL of distilled triethylamine (Et3N) was added, the mixture was refluxed for overnight and the reaction solution was filtrated to remove white precipitate. After the solvent was removed, the residue was recrystallized from CH3CN/MeOH to give purple crystals of 2 (15 mg, 60%). Anal. Calcd for C98H74N4O6Sn 3 (CH3OH)2: C, 75.71; H, 5.21; N, 3.53. Found: C, 75.41; H, 4.95; N, 3.42. 1H NMR (CD3CN, δ (ppm)): 7.30 (d, J = 8.6 Hz, 8H; meso-phenyl o-H), 6.81-6.70 (m, 40H; β-phenyl H), 6.13 (d, J = 8.8 Hz, 8H; meso-phenyl m-H), 3.66 (s, 12H; meso-phenyl, p-OMe H). UV/vis (benzonitrile, λmax (ε, M-1 cm-1)): 470 (2.0 105), 593 (1.5 104), 652 nm (1.5 104). r-[(n-C4H9)4N]3[PW12O40] (3). R-Na3[PW12O40] was obtained from a commercial source (Wako pure chemicals) and used without further purification. R-Na3[PW12O40] (5.00 g, 1.70 mmol) and [(n-butyl)4N]Br (TBABr) (2.50 g, 7.76 mmol) were dissolved in 20 mL of water and stirred vigorously. White precipitate was collected and washed by a large amount of water. Recrystallization from CH3CN/H2O gave white powder and was filtered and dried under vacuum (4.50 g, 74%). Anal. Calcd for C48H108N3O40PW12: C, 16.00; H, 3.02; N, 1.17. Found: C, 16.00; H, 3.01; N, 1.19. Single Crystal X-ray Crystallography. Single crystals of 1 and 2 were put in liquid paraffin and mounted on a glass fiber with silicon grease. X-ray diffraction data were collected on a Rigaku Mercury CCD diffractometer at 193 ( 1 K. All calculations for structure refinements were carried out on a PC using CrystalStructure29 (Rigaku Corp.) and SHELXL programs.30 Electrochemical Measurements. Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) were obtained on an ALS 630B electrochemical analyzer in deaerated PhCN in the presence of 0.1 M [(n-butyl)4N]PF6 (TBAPF6) as a supporting electrolyte under Ar at room temperature, with use of a glassy carbon electrode as a working electrode, Ag/AgNO3 as a reference electrode, a Pt wire as an auxiliary electrode. The values (vs Ag/AgNO3) were converted to those versus SCE by addition of 0.29 V.31 Femtosecond Laser Flash Photolysis. Femtosecond transient absorption spectroscopy experiments on solutions of 1 and 2 in PhCN were performed using an ultrafast source (Integra-C, Quantronix Corp.), an optical parametric amplifier (TOPAS, Light Conversion Ltd.), and a commercially available optical detection system (Helios, Ultrafast Systems LLC). The sources for the pump and probe pulses were derived from the fundamental output of Integra-C (780 nm, 2 mJ per pulse and fwhm = 130 fs) at a repetition
Figure 1. (a) Nonplanar Sn(IV)-porphyrins, [Sn(TMPP(Ph)8)Cl2] (1) and [Sn(TMPP(Ph)8)(OMe)2] (2), and (b) a Keggin-type POM, R-[PW12O40]3-, used in this study.
(TMPP(Ph)8: 5,10,15,20-tetrakis(4-methoxyphenyl)-2,3,7,8,12,13, 17,18-octaphenylporphyrinato), and a Keggin-type POM, R-[PW12O40]3- (Figure 1), by the axial ligand exchange reaction between the MeO- ligand and R-[PW12O40]3-. A diamagnetic and six-coordinate Sn(IV) can be used as a metal center of a porphyrin complex, because it allows axial coordination of POMs and Sn(IV)porphyrins have been known to act as electron donors upon photoexcitation.21-25 As the Sn(IV) ion is a hard Lewis acid, relatively strong interaction can be expected to form a complex with the POM through the coordination of the terminal oxygen of the POM as a hard Lewis base even in a polar solvent. Femtosecond and nanosecond laser flash photolysis measurements of the 1:1 complex have been performed to examine the photoinduced electron-transfer dynamics. This study provides a convenient way to use POMs as electron acceptors in photoinduced electron-transfer reactions of the POM with metalloporphyrins.
’ EXPERIMENTAL SECTION General Information. Chemicals were purchased from commercial sources and used without further purification, unless otherwise noted. Benzonitrile (PhCN) was distilled from P2O5 and stored over 4 Å molecular sieves. Dichloromethane (CH2Cl2) and acetonitrile (CH3CN) were distilled from CaH2 under N2 just prior to use. All other solvents were special grade and were used as received from commercial sources without further purification. Column chromatography was performed on a silica gel (Wako-gel C-200 (60-200 mesh)) or activated alumina (ca. 200 mesh), both of which were obtained from Wako pure chemicals. Toluene was distilled over sodium benzophenone ketyl and used for the synthesis of H2TMPP(Ph)8. UV-vis spectroscopy was carried out on JASCO Ubset-50 UV/vis and JASCO V-570 spectrometers at room temperature. All 1 H NMR measurements were performed on a JEOL EX270 spectrometer, and the chemical shifts were determined by using the residual solvent peak as reference. Fluorescence spectra were measured on a JASCO FP-6500 spectrofluorometer at room temperature. Phosphorescence of an Ar saturated solution of 1 and 2 in 2-methyltetrahydrofuran in a quartz tube (3 mm in diameter) in liquid nitrogen (77 K) was measured using a SPEX Fluorolog τ3 fluorescence spectrophotometer by excitation at 600 nm. MALDI-TOF-MS spectra were recorded on a Kratos Compact MALDI I (Shimadzu) using Rcyano-4-hydroxycinnamic acid (CHCA) as a matrix. H2TMPP (5,10, 15,20-tetrakis(4-methoxyphenyl)porphyrin)26 and H2TMPP(Ph)827 were prepared according to the literature. [Sn(TMPP(Ph)8)Cl2] (1). H2TMPP(Ph)8 (200 mg, 150 μmol) and SnIICl2 (140 mg, 750 μmol) were dissolved in 80 mL of pyridine and heated at 130 C for 6 h under Ar. After continuous heating 987
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Figure 2. (a) Top view and (b) side view of the crystal structure of 1. (c) Top view and (d) side view of the crystal structure of 2. Colors: gray, carbon; blue, nitrogen; red, oxygen; pink, tin; green, chlorine.
rate of 1 kHz. A75% level of the fundamental output of the laser light was introduced into TOPAS, which has optical frequency mixers, resulting in a tunable range from 285 to 1660 nm, while the rest of the output was used for white-light generation. Prior to generating the probe continuum, a variable neutral density filter was inserted in the path to generate a stable continuum, and then the laser pulse was fed to a delay line that provides an experimental time window of 3.2 ns with a maximum step resolution of 7 fs. In our experiments, a wavelength at 430 nm of TOPAS output, which is the fourth harmonic of the signal or idler pulses, was chosen as the pump beam. Since this TOPAS output consists of not only the desirable wavelength but also unnecessary wavelengths, the latter were deviated using a wedge prism with wedge angle of 188. The desirable beam was irradiated at the sample cell with a spot size of 1 mm diameter where it was merged with the white probe pulse in a close angle (
