Substituent Effect on the Photoreduction Kineticsof Benzophenone.

The kinetics of the photoreductionof four benzophenone derivativesby isopropyl alcohol was examined in acetonitrile, namely, tetra-meta-trifluoromethyl-, di-para-trifluoromethyl-,di-para-methoxy benzophenone, and, for comparison,the unsubstituted molecule itself. The basic spectroscopic (absorptionand phosphorescence spectra) and photophysical (quantum yields andexcited state energies) properties were established, and the key kineticparameters were determined by the laser flash photolysis transientabsorption technique. The rate coefficients of both the primary andsecondary photoreduction reaction show remarkable dependence on ringsubstitution. This substantial effect is caused by the considerablechange in the activation energy of the corresponding process. Theexperimental results as well as DFT quantum chemical calculationsclearly indicate that these benzophenone derivatives all react as n–π* excited ketones, and the rate as wellas the activation energy of the reduction steps change parallel withthe reaction enthalpies, the determining factor being the stabilityof the forming aromatic ketyl radicals. The secondary photoreductionof benzophenones by the aliphatic ketyl radical formed in the primarystep occurs via a hydrogen bonded complex. The binding energy of thehydrogen bonded complex between the aliphatic ketyl radical reactantand a solvent molecule is a critical parameter influencing the observablerate of the secondary photoreduction. [ABSTRACT FROM AUTHOR]