Formation of a long-lived photoinduced electron-transfer state in an electron acceptor-donor-acceptor porphyrin triad connected by coordination bonds

Abstract

The reaction of Sn(DPP)(OH)2 (DPP2- = 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin dianion) with H 2F16DPPCOOH (2,3,7,8,12,13,17,18-octakis(3,5- difluorophenyl)-5-(4-carboxyphenyl)-10,15,20-triphenylporphyrin) afforded a porphyrin triad, Sn(DPP)(H2F16DPPCOO)2 (1), in which the Sn(DPP) unit is linked with the two H2F 16DPPCOO- units by strong coordination bonds. The H 2F16DPPCOO- unit of Sn(DPP)(H2F 16DPPCOO)2 was diprotonated by the reaction with trifluoroacetic acid (CF3COOH) to afford a robust electron acceptor-donor-acceptor porphyrin triad, Sn(DPP){(H4F 16DPPCOO)(CF3COO)2}2 (2), in which the Sn(DPP) unit and the H4F16DPP2+COO - (H4F16DPPCOO+) unit act as an electron donor and an acceptor, respectively. The photodynamics of 1 was examined by femtosecond laser flash photolysis measurements in PhCN to reveal that the energy transfer occurs from the singlet excited state of the Sn(DPP) unit to the H2F16DPPCOO- unit to generate the singlet excited state of H2F16DPPCOO-. In contrast to the case of 1, the transient absorption spectra of 2 that contains the diprotonated form (H4F16DPPCOO+), observed by femtosecond laser flash photolysis, clearly indicated the occurrence of fast electron transfer from the singlet excited state of the Sn(DPP) unit to the H4F16DPPCOO+ unit. The resulting singlet electron-transfer (ET) state composed of Sn(DPP)•+ and H 4F16DPPCOO• decays to the ground state with the rate constant of 1.4 × 1010 s-1 in competition with generation of the triplet ET state, which was also detected by the nanosecond transient absorption spectroscopy. The lifetime of the triplet ET state (50 μs) was much longer than that of the singlet ET state (71 ps) due to the spin-forbidden character of the back electron-transfer process. © 2010 American Chemical Society.