Water induced electron transfer from hematoporphyrin triplet state to metronidazole

The electron transfer from an optically excited donor (hematoporphyrin) to an acceptor (metronidazole) has been investigated by 532 nm laser flash pho tolysis in water, in organic solvents (alcohols, acetonitrile, formamide, N -methylformamide, N,N-dimethylformamide and dimethylsulfoxide) and in binar y mixtures of these solvents with water. The electron transfer is character ized by the quenching of the donor triplet-triplet transient absorption by metronidazole. The quenching constant has a maximum value of 2.1 x 10(9) M- 1 s(-1) in water. This constant decreases according to an exponential funct ion as the water content of the binary water-cosolvent mixtures decreases. In neat organic solvents, the electron transfer has only been observed in m ethanol and formamide, but at a rate three orders of magnitude lower than t hat in water. The changes in the rate constants and activation parameters o f the electron transfer in going from water to neat solvents are not compat ible with Marcus' theory. The water-induced electron transfer is analyzed i n terms of specific solute-water interactions. (C) 1998 Elsevier Science S. A. All rights reserved.