EXCITED-STATE CALORIMETRY STUDIES OF TRIPLET BENZOPHENONE USING TIME-RESOLVED PHOTOTHERMAL BEAM DEFLECTION SPECTROSCOPY

Photothermal beam deflection spectroscopy is found to be a useful method for determining the kinetics and energetics of photochemical and photophysical processes. The accumulation of heat from the reaction and decay of excited states is observed with a step-response function on time scales from nanoseconds to milliseconds. Using this method, we study hydrogen abstraction from ethanol by triplet benzophenone and determine the rate constant of this reaction to be 1.43 (±0.08) X 106 M-1 s-1. From the relative amplitudes of the photothermal transients, we derive the enthalpy of the photoproducts and estimate the C-H bond dissociation energy of benzhydrol and the O-H bond energy of the ketyl radical to be 87 (±2) and 95 (±2) kcal/mol, respectively. We also investigate the intensity dependence of heat released from triplet benzophenone to determine the influence of excited-state absorption on the calorimetric results. The data are fit to a four-level, excited-state absorption model, and the triplet excited-state absorption cross section of benzophenone at 322 nm is found to be 2.7 (±0.5) X 10-17 cm2. © 1990, American Chemical Society. All rights reserved.