Time-resolved fluorescence monitoring of aromatic radicals in photoinitiated processes.

Photolytic initiation of free radical reactions is important to many areas of technology; time-resolved monitoring of submicromolar concentrations of radicals produced during the course of these reactions is needed to provide information about the rate of initiation and its competition with radical recombination. In this work, time-resolved laser-induced fluorescence is evaluated for monitoring of diphenylketyl radicals produced by photoreduction of the triplet state of benzophenone. Fluorescence from the doublet-doublet transition of the radical is excited with a continuous wave laser and provides a sensitive method to detect these intermediates at nanomolar concentrations and to study their kinetics in solution on time scales from a few microseconds to hundreds of milliseconds. The ketyl radical fluorescence measurements of radical initiation reactions allowed the H atom abstraction rate constant by triplet benzophenone from both 2-propanol and benzhydrol to be determined, where k(H) = (2.1 ± 0.1) × 10(6) M(-)(1) s(-)(1) for 2-propanol and k(H) = (4.4 ± 0.1) × 10(6) M(-)(1) s(-)(1) for benzhydrol. The diphenylketyl radical recombination rate constant was also determined by time-resolved fluorescence monitoring of the decay of the radical population and found to be k(r) = (1.9 ± 0.2) × 10(8) M(-)(1) s(-)(1). Formation kinetics could be measured on a microsecond time scale from radical populations as low as 45 nM; decay kinetics could be followed on a millisecond time scale from 20 nM radical concentrations.