A Time-Resolved Electron Spin Resonance and Laser Flash Spectroscopy Investigation of the Photolysis of Benzaldehyde and Benzoin in Homogenous Solvents and Micellar Solutions

Article abstract of DOI:10.1021/j100152a030

Both photochemical α-cleavage of triplet benzoin (BZ) and hydrogen abstraction by triplet benzaldehyde (BA) from ground-state benzaldehyde produce geminate radical pairs of identical chemical structure.A search for "memory effects" in the chemically identical geminate radical pairs generated from different photochemical pathways was examined using the techniques of time-resolved electron spin resonance (TRESR) and time-resolved optical absorption spectroscopy.Photolysis of BZ in homogeneous organic solvents and in sodium dodecyl sulfate (SDS) micellar solutions leads to chemically induced dynamic electron polarization (CIDEP) of benzoyl and α-hydroxybenzyl radicals consisting of a strong emission (E) due to the triplet mechanism (TM) in the generation of electron polarization.Photolysis of BA in hydrogen-donating organic solvents results in a E/A (or E^*/A) CIDEP pattern of α-hydroxybenzyl radicals due to the radical pair mechanism (RPM).In solvents which are poor hydrogen donors (benzene, acetonitrile) and/or at relatively high concentrations of BA, the photoreduction of triplet BA by ground-state BA generates benzoyl and α-hydroxybenzyl radicals, which manifest an E/A CIDEP spectrum assigned to RPM.Photoreduction of BA by KCNS in aqueous acetonitrile results in absorptive (A) CIDEP of α-hydroxybenzyl radicals, assigned to a rare case of RPM for which the g factor difference overwhelms the hyperfine interactions of the pertinent radical pair (the α-hydroxybenzyl and the (CNS)2^.- radical).Computer simulation allows the estimation of the g factor of this inorganic polarized radical to be in the range 2.015 < g < 2.03.Under conditions of low occupancy number of BA, the photolysis of BA in SDS micellar solution displayed CIDEP spectra assigned to a spin-correlated geminate radical pair (SCRP) consisting of α-hydroxybenzyl and alkyl radicals of SDS.At a higher occupancy number, a different SCRP spectrum is observed and is assigned to a polarized α-hydroxybenzyl and benzoyl radical pair.Computer simulation confirms all of the proposed assignments.Measurements of the decay kinetics of the α-hydroxybenzyl radical, observed by time-resolved absorption spectroscopy, show that the application of an external magnetic field of 0.30 T leads to a decrease in the rate of micellized geminate recombination in the photoreduction of micellized BA and to an increase in the rate of radical escape.No significant magnetic field effect was found on the decay of the α-hydroxybenzyl radicals produced from BZ.These results imply that radicals formed by α-cleavage of triplet BZ escape from micelles faster than the chemically identical geminate pair generated by the photoreduction of triplet BA by ground-state BA.