Photodynamics of the Paterno-Buchi cycloaddition of stilbene to quinone. Unusual modulation of electron-transfer kinetics by solvent and added salt

Oxetanes are produced in the Paterno-Buchi cycloaddition of stilbene (S) to quinone (Q) via an efficient photoinduced electron transfer. Kinetics anal ysis of the time-resolved absorption spectra over three distinctive (ps, ns , mu s) timescales establishes the coupling (k(C)) of the initially;formed ion-radical pair (3)[S+., Q(-.)] to the 1,4-biradical SQ as the critical st ep toward oxetane formation. The (rather slow) rate constant Of k(C) less t han or equal to 10(7) s(-1) in acetonitrile must compete with other faster decay pathways of the ion pair involving ionic separation, ion exchange (wi th added salt) and back electron transfer. As such, solvent polarity and do nicity as well as added salts play an unusually prominent role in modulatin g the ion-pair microdynamics. Donor-acceptor complexation of the photoexcit ed-quinone with the solvent and cis-->trans isomerization of(Z)-stilbene mu st also be considered in the overall photodynamics of electron transfer.