Dissociation of highly excited triplet benzophenone into phenyl radicals and carbon monoxide: determination of the reaction quantum yield and the heat of reaction by time-resolved thermal lensing technique

Abstract

The photodissociation path and quantum yield of benzophenone in the highly excited triplet (T n ) state were determined in benzene solution at room temperature using two-photon absorption (TPA) and time-resolved thermal lensing (TRTL) techniques. The TRTL signals measured were composed of fast and slow components: the fast one rose with a time constant less than the instrumental response time (≈ 100 ns), while the slower rose with a time constant of about 3 μs. The fractions of the fast and slow TRTL signals increased and decreased, respectively, with the laser power. The TRTL signals were quantitatively analyzed on the basis of the probe-light tracing in the nonuniform refractive index medium. The efficiency of thermal lensing (TRTL signal intensity/heat available to thermal lens) depends on the spatial profile of thermal distribution. Therefore, the TRTL signal due to TPA is more intense than that due to single photon absorption (SPA): TPA was about four times more sensitive in the TRTL signal than SPA under the experimental conditions employed. The TRTL signals were simulated using a kinetic model including the S 1←S 0 and T n ←T 1 absorptions of benzophenone and the subsequent dissociation of the T n molecule. The reaction quantum yield (Φ r) and the heat of reaction (Δ H) in benzene solution were determined to be 0.39 and 117 kcal mol −1, respectively, based on the experiment and simulation. The dissociation path open for benzophenone in the T n state was thus confirmed to be Ph 2CO→2Ph+CO.