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Formation of a Hybrid Compound Composed of a Saddle-Distorted Tin(IV)−Porphyrin and a Keggin-Type Heteropolyoxometalate To Undergo Intramolecular Photoinduced Electron Transfer

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journal contribution
posted on 17.02.2011, 00:00 by Atsutoshi Yokoyama, Takahiko Kojima, Kei Ohkubo, Motoo Shiro, Shunichi Fukuzumi
Nonplanar Sn(IV)−porphyrin complexes, [Sn(TMPP(Ph)8)-Cl2] (1) and [Sn(TMPP(Ph)8)(OMe)2] (2) (TMPP(Ph)8: 5,10,15,20-tetrakis(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 (α-[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)(α-[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 α-[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 α-[PW12O40]3− moiety in the 1:1 complex in benzonitrile.