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

Article abstract of DOI:10.1016/0584-8539(89)80023-6

Pulsed laser excitation (354.7 nm, 10 ns pulse) of a pyridyltritolylporphyrin chromophore covalently linked to a dibenzylviologen, Bz2V²⁺, electron acceptor (porphyrin-viologen, P-V²⁺) in CH3CN leads to intramolecular electron transfer quenching of the porphyrin singlet excited state within the laser pulsewidth to reduce the linked Bz2V²⁺ to Bz2V⁺.Transient Bz2V⁺ 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 CH3CN where Bz2V²⁺ quenches the porphyrin fluorescence, establishing bimolecular excited state electron transfer quenching to yield Bz2V⁺.Confirmation of our assignments of the transient Bz2V⁺ comes from comparison of the spectra with the resonance Raman spectrum of an authentic sample of Bz2V⁺, and of electrochemically reduced P-V²⁺ which has been spectroscopically confirmed to formed P-V⁺.Fluorescence lifetime determinations for P-V²⁺ and P yield a rate constant for intramolecular electron transfer, k_et = 8 x 1E7 s^-1, consistent with the ability to observe electron transfer within the laser pulsewidth.