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) (DPP2- = 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin dianion) with H2F16DPPCOOH (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 H2F16DPPCOO- units by strong coordination bonds. The H2F16DPPC00- unit of Sn(DPP)(H,F16DPPCOO)2 was diprotonated by the reaction with trifluoroacetic acid (CF3C001-1) to afford a robust electron acceptor donor acceptor porphyrin triad, Sn(DPP)I(H4F16DPPCOO)(CF3COO)212 (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 femtosecord 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 H2F16DPPC00- 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 (H4FI6DPPC00+), 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 H4F16,DPPCOO+ unit. The resulting singlet electron-transfer (ET) state composed of Sn(1)1313r and Sn(DPP)(H2F16DPPCOO)(2) decays to the ground state with the rate constant of 1.4 x 1010 s 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 its) was much longer than that of the singlet ET state (71 ps) due to the spin-forbidden character of the back electron-transfer process.