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Articolo in rivista, 2010, ENG, 10.1002/chem.201000375

Unusual Photoinduced Electron Transfer from a Zinc Porphyrin to a Tetrapyridyl Free-Base Porphyrin in a Noncovalent Multiporphyrin Array

B. Ventura, L. Flamigni, M. Beyler, V. Heitz, J.-P. Sauvage

a. Istituto per la Sintesi Organica e la Fotoreattività (ISOF), CNR, Via P. Gobetti 101, 40129 Bologna (Italy) b. Laboratoire de Chimie Organo-Minérale, Institut de Chimie, Université de Strasbourg, CNRS/UMR 7177, 4, rue Blaise Pascal, 67070 Strasbourg-Cedex (France)

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.

Chemistry (Weinh., Print) 16 (29), pp. 8748–8756