Picosecond Fluorescence Studies on Intramolecular Photochemical Electron Transfer in Porphyrins Linked to Quinones at Two Different Fixed Distances
Fluorescence lifetimes were determined for porphyrins linked to quinones at two different fixed distances by bicyclo[2.2.2]octyl spacers. Electron-transfer rate constants for the one- and two-spacer homologues were calculated from the lifetimes. The incremental effect of the second spacer is to decrease the electron-transfer rate by at least a factor of 500-1600 depending on solvent. Lifetime measurements in solvents of different polarities indicate that the electron-transfer rate decreases with increasing solvent polarity. Experiments at low temperature (77 K) revealed fast nearly temperature-independent electron transfer characterized by nonexponential decays in the fluorescence profiles of the one-spacer compound, in contrast to essentially monoexponential decays at room temperature. Low temperatures appear to freeze out the rotational motion of the chromophores and the observed fluorescence decays may be explained as electron transfer from an ensemble of rotational conformations.