Tracking a Paternò-Büchi reaction in real time using transient electronic and vibrational spectroscopies

A detailed mechanistic investigation of the early stages of the Paternò-Büchi reaction following 267 nm excitation of benzaldehyde in cyclohexene has been completed using ultrafast, broadband transient UV-visible and IR absorption spectroscopies. Absorption due to electronically excited triplet state benzaldehyde decays on a 80 ps time scale via reaction with cyclohexene. The growth and subsequent decay of the biradical intermediate produced following C-O bond formation is followed by transient vibrational spectroscopy. The biradical decays by ring closure to an oxetane or by dissociating, reforming the ground state reactants. Detailed kinetic analysis allowed derivation of quantum yields and rate constants for these competing biradical decay processes, ϕ(oxetane) = 0.53, ϕ(diss) = 0.47, koxetane = 0.27 ± 0.09 ns(-1) and k(diss) = 0.24 ± 0.09 ns(-1). This study provides a striking illustration of the ways in which contemporary ultrafast transient absorption spectroscopy methods can be used to dissect the mechanism and kinetics of a classic photoreaction.