Solvent Dependence Of Photoinduced Intramolecular Electron Transfer

This work is a study of the solvent and temperature dependence of photoinduced intramolecular electron transfer (ET). The molecule containing a porphyrin and a quinone linked by a phenylalanine amino acid residue was synthesized. Excitation of this molecule (PPAQ) causes electrons to be transferred from the porphyrin to the quinone entity. The ET rate constants were measured by comparing the porphyrin fluorescence lifetime with or without the possibility of ET.;The ET rate constants of PPAQ were measured for several solvents and compared with data of other similar porphyrin-quinone linked molecules. A temperature dependence of ET rate constants study was also carried out. Finally, ET rate constants were measured in some solvents with added MeI{dollar}\sb2{dollar}. All the experimental data were analyzed and compared with the experimental results obtained in an earlier study. Analysis of the results of this investigation reveals three solvents effects.;Solvents with heavy atoms were found to have the strongest effect on the ET reaction. Although the reason for enhanced ET in these solvents is still an open question, experimental data indicates that both the nuclear and electronic factors are increased in these solvents. One possible explanation is that there may be a new ET route in the intramolecular electron transfer process, namely from the singlet excited molecule state to the triplet radical-ion pair state; another explanation is that in the heavy atom containing solvents there may be a weak charge transfer complex formed, which increases the electronic coupling between the donor and the acceptor, and decreases the Gibbs activation energy for the reaction.;Aromatic groups in the solvent can lead to another solvent effect. The small energy gap between the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals increases the electronic coupling of donor and acceptor, which thus enhances the rate of electron transfer.;The third solvent effect is that of polarity, which is smaller and hence shielded by the other two effects. Polarity affects the electronic coupling term by changing the molecular conformations; it also affects the Gibbs activation energy of reaction by changing the reorganization energy and the Gibbs energy of reaction