Unusual photoinduced electron transfer from a zinc porphyrin to a tetrapyridyl free-base porphyrin in a noncovalent multiporphyrin array

摘要

Excitation of the peripheral Zn porphyrin units in a noncovalent five-porphyrin array, formed by gable-like zinc(II) bisporphyrins and a central free-base meso-tetrakis(4-pyridyl)-porphyrin in a 2:1 ratio, (ZnP _2)/_2·(TPyP), does not lead to a quantitative sensitization of the luminescence of the free-base porphyrin acceptor, even though there is an effective energy transfer. Time resolution of the luminescence evidences a quenching of TPyP upon sensitization by the peripheral ZnP_2. The time evolution of the TPyP fluorescence in the complex can be described by a bi-exponential fitting with a major component of 180 ps and a minor one of 5 ns, compared to an isolated TPyP lifetime of 9.4 ns. The two quenched lifetimes are shown to be correlated to the presence of 2:1 and 1:1 complexes, respectively. No quenching of TPyP fluorescence occurs in (ZnP _2)_2·(TPyP) at 77 K in a rigid solvent for which only an energy-transfer process (τ=150±10ps) from peripheral ZnP _2 to the central TPyP is observed. An unusual HOMO-HOMO electron-transfer reaction from ZnP_2 to the excited TPyP units, responsible for the observed phenomena, is detected. The resulting charge-separated state, (ZnP_2)~+_2-(TPyP) ~- is found to recombine to the ground state with a lifetime of 11 ns.