INTRAMOLECULAR AND INTERMOLECULAR PHOTOINDUCED ELECTRON-TRANSFER IN ISOMERIC MESOPORPHYRIN NITROBENZYL ESTERS - STRUCTURE AND SOLVENT EFFECTS

Photoinduced intramolecular electron transfer was studied in a series of flexible dyads consisting of the isomeric ortho-, meta- and para-ni trobenzyl monoesters and diesters of mesoporphyrin existing preferenti ally in folded conformations. This process was compared with the inter molecular electron transfer between mesoporphyrin dimethyl ester and t he 2-, 3- and 4-nitrobenzyl acetates. In both cases, electron transfer occurs from the porphyrin first excited singlet state, and the rate c onstant is lower in the ortho isomer, although intermolecular electron transfer is less isomer selective. The selectivity in the intermolecu lar case can be related to the isomer redox potentials, but in the dya ds an ortho effect must also be taken into account; this effect produc es a deviation from coplanarity between the nitro group and the phenyl ring. Electron paramagnetic resonance (EPR) in a low temperature matr ix permits the charge transfer product to be detected. The process doe s not lead to the production of long-lived species, as shown by optoac oustic calorimetry. The efficiency of photoinduced electron transfer s trongly depends on the ability of the solvent to stabilize the radical ion pair; this process occurs in CH3OH, CH3CN and CH2Cl2, but is inhi bited in toluene and benzene. In the last two solvents, singlet oxygen is formed instead via energy transfer from the triplet state, with th e same quantum yield as for the nitrobenzyl-free porphyrin.