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

The reaction of Sn(DPP)(OH)₂ (DPP^2- = 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.