A TIME-RESOLVED ELECTRON-SPIN-RESONANCE AND LASER FLASH SPECTROSCOPY INVESTIGATION OF THE PHOTOLYSIS OF BENZALDEHYDE AND BENZOIN IN HOMOGENEOUS SOLVENTS AND MICELLAR SOLUTIONS

Both photochemical alpha-cleavage of triplet benzoin (BZ) and hydrogen abstraction by triplet benzaldehyde (BA) from ground-state benzaldehy de produce geminate radical pairs of identical chemical structure. A s earch for ''memory effects'' in the chemically identical geminate radi cal 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 i n homogeneous organic solvents and in sodium dodecyl sulfate (SDS) mic ellar solutions leads to chemically induced dynamic electron polarizat ion (CIDEP) of benzoyl and alpha-hydroxybenzyl radicals consisting of a strong emission (E) due to the triplet mechanism (TM) in the generat ion of electron polarization. Photolysis of BA in hydrogen-donating or ganic solvents results in a E/A (or E/A) CIDEP pattern of alpha-hydro xybenzyl radicals due to the radical pair mechanism (RPM). In solvents which are poor hydrogen donors (benzene, acetonitrile) and/or at rela tively high concentrations of BA, the photoreduction of triplet BA by ground-state BA generates benzoyl and alpha-hydroxybenzyl radicals, wh ich manifest an E/A CIDEP spectrum assigned to RPM. Photoreduction of BA by KCNS in aqueous acetonitrile results in absorptive (A) CIDEP of alpha-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 alpha-hydroxybenzyl and the (CNS)2.- radic al). Computer simulation allows the estimation of the g factor of this inorganic polarized radical to be in the range 2.015 < g < 2.03. Unde r conditions of low occupancy number of BA, the photolysis of BA in SD S micellar solution displayed CIDEP spectra assigned to a spin-correla ted geminate radical pair (SCRP) consisting of alpha-hydroxybenzyl and alkyl radicals of SDS. At a higher occupancy number, a different SCRP spectrum is observed and is assigned to a polarized alpha-hydroxybenz yl and benzoyl mdical pair. Computer simulation confirms all of the pr oposed assignments. Measurements of the decay kinetics of the alpha-hy droxybenzyl radical, observed by time-resolved absorption spectroscopy , show that the application of an external magnetic field of 0.30 T le ads 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 alpha-hydroxybenzyl radicals produced from BZ. These resu lts imply that radicals formed by alpha-cleavage of triplet BZ escape from micelles faster than the chemically identical geminate pair gener ated by the photoreduction of triplet BA by ground-state BA.