Unusual photoinduced electron transfer from a zinc porphyrin to a tetrapyridyl free-base porphyrin in a noncovalent multiporphyrin array.
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 (tau=150+/-10 ps) 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.
Ventura
Barbara
B
Istituto per la Sintesi Organica e la Fotoreattività (ISOF), CNR, Via P. Gobetti 101, 40129 Bologna, Italy.
Flamigni
Lucia
L
Beyler
Maryline
M
Heitz
Valérie
V
Sauvage
Jean-Pierre
JP