Specific solvent effects on the charge separation efficiency in photoinduced electron transfer processes

The photoinduced electron transfer (PET) quenching of singlet excited pyren e and 1,2,5,6-dibenzanthracene (DBA) by 3-cyanopyridine and o-dicyanobenzen e, respectively, was investigated in several protic and aprotic solvents. T he triplet quenching of DBA by p-nitrobenzaldehyde was also studied in the same solvents. In all cases free radical ions yield were measured by laser flash photolysis. In the case of the triplet reaction, forward electron tra nsfer quenching rate constants were also measured. Charge separation effici encies, phi(cs) were determined from the free radical ions yields. The solv ent effect on phi(cs) in singlet state PET may be explained in aprotic solv ents by a model based on the macroscopic properties of the solvent. For alc ohols the efficiencies are lower than the corresponding quantities for apro tic solvents of similar viscosities and dielectric constant. For triplet me diated PET reactions, the forward rate constants are scarcely sensitive to the solvent, while strong specific solvent effects are important in determi ning the back-electron-transfer to ground state. (C) 2000 Elsevier Science S.A. All rights reserved.