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-nitrobenzyl monoesters and diesters of mesoporphyrin existing preferentially in folded conformations. This process was compared with the intermolecular electron transfer between mesoporphyrin dimethyl ester and the 2-, 3-and 4-nitrobenzyl acetates. In both cases, electron transfer occurs from the porphyrin first excited singlet state, and the rate constant is lower in the ortho isomer, although intermolecular electron transfer is less isomer selective. The selectivity in the intermolecular case can be related to the isomer redox potentials, but in the dyads an ortho effect must also be taken into account; this effect produces a deviation from coplanarity between the nitro group and the phenyl ring. Electron paramagnetic resonance (EPR) in a low temperature matrix permits the charge transfer product to be detected. The process does not lead to the production of long-lived species, as shown by optoacoustic calorimetry. The efficiency of photoinduced electron transfer strongly depends on the ability of the solvent to stabilize the radical ion pair; this process occurs in CH₃OH, CH₃CN and CH₂Cl₂, but is inhibited in toluene and benzene. In the last two solvents, singlet oxygen is formed instead via energy transfer from the triplet state, with the same quantum yield as for the nitrobenzyl-free porphyrin.