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

Abstract

Oxetanes are produced in the Paterno–Büchi cycloaddition of stilbene (S) to quinone (Q) via an efficient photoinduced electron transfer. Kinetics analysis of the time-resolved absorption spectra over three distinctive (ps, ns, µs) 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 step toward oxetane formation. The (rather slow) rate constant of k_C ≤ 10⁷ s^–1 in acetonitrile must compete with other faster decay pathways of the ion pair involving ionic separation, ion exchange (with added salt) and back electron transfer. As such, solvent polarity and donicity as well as added salts play an unusually prominent role in modulating the ion-pair microdynamics. Donor–acceptor complexation of the photoexcited quinone with the solvent and cis→trans isomerization of (Z )-stilbene must also be considered in the overall photodynamics of electron transfer.

References

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