Spectroscopic detection of excited-state electron transfer in porphyrin viologen systems

Abstract

Pulsed laser excitation (354.7 nm, 10 ns pulse) of a pyridyltritolylporphyrin chromophore covalently linked to a dibenzylviologen, Bz 2V 2+, electron acceptor (porphyrin—viologen, PV 2+) in CH 3CN leads to intramolecular electron transfer quenching of the porphyrin singlet excited state within the laser pulsewidth to reduce the linked Bz 2V 2+ to Bz 2V +·. Transient Bz 2V +· can be detected directly by resonance Raman spectroscopy. The same transient features are obtained from pulsed laser excitation of a mixture of porphyrin (P) and dibenzylviologen in CH 3CN where Bz 2V 2+ quenches the porphyrin fluorescence, establishing bimolecular excited state electron transfer quenching to yield Bz 2V +·. Confirmation of our assignment of the transient Bz 2V +· comes from comparison of the spectra with the resonance Raman spectrum of an authentic sample of Bz 2V +·, and of electrochemically reduced PV 2+ which has been spectroscopically confirmed to form PV +·. Fluorescence lifetime determinations for PV 2+ and P yield a rate constant for intramolecular electron transfer, k et = 8 × 10 7 s −1, consistent with the ability to observe electron transfer within the laser pulsewidth