A TIME-RESOLVED ELECTRON-SPIN-RESONANCE AND LASER FLASH SPECTROSCOPY INVESTIGATION OF THE PHOTOLYSIS OF BENZALDEHYDE AND BENZOIN IN HOMOGENEOUS SOLVENTS AND MICELLAR SOLUTIONS
Iv. Khudyakov et al., A TIME-RESOLVED ELECTRON-SPIN-RESONANCE AND LASER FLASH SPECTROSCOPY INVESTIGATION OF THE PHOTOLYSIS OF BENZALDEHYDE AND BENZOIN IN HOMOGENEOUS SOLVENTS AND MICELLAR SOLUTIONS, Journal of physical chemistry, 97(50), 1993, pp. 13234-13242
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.