STRUCTURAL EFFECTS ON PHOTOINDUCED ELECTRON-TRANSFER IN CAROTENOID-PORPHYRIN-QUINONE TRIADS

meso-Polyarylporphyrins are often used as components of molecules that mimic photosynthetic reaction centers by carrying out photoinduced el ectron-transfer reactions. Studies of these systems have raised questi ons concerning the role of alkyl substituents at the ''beta-pyrrolic'' positions on the porphyrin periphery in limiting pi-pi overlap betwee n the macrocycle and the aryl rings. The degree of overlap affects ele ctronic coupling and, therefore, the rates of electron-transfer reacti ons. There is also evidence that when the Linkages joining porphyrins to electron-acceptor or -donor moieties contain amide bonds, the sense of the amide linkage may strongly affect electron-transfer rate const ants. In this study, three carotenoid-porphyrin-quinone molecular tria ds and various model compounds have been prepared, and electron-transf er has been studied using time-resolved emission and absorption techni ques. The results show that steric hindrance due to methyl groups at t he beta-pyrrolic positions reduces electron-transfer rate constants by a factor of similar to 1/5. In addition, amide-containing donor-accep tor linkages having the nitrogen atom attached to the porphyrin meso-a ryl ring demonstrate electron-transfer rate constants similar to 30 ti mes larger than those for similar linkages with the amide reversed, af ter correction for thermodynamic effects.