CONFORMATIONAL CONTROL OF THE PHOTOCHEMISTRY AND PHOTOPHYSICS OF DIPHENYLACETONE

We report the direct observation of the lowest triplet states of 1,3-d iphenylacetone (DBK) and two methylated derivatives by direct detectio n time-resolved electron spin resonance (TRESR) at 15 K in methylcyclo hexane glass. The spectral features are broad with multiple peaks in t he Delta M(2) = 2 region, which we assign to a multitude of conformati ons frozen into the glass. Successive experiments on a single sample p rovide evidence that certain of these conformations can be photochemic ally removed. DBK and 1-p-tolyl-3-phenylacetone (p-MeDBK) give single- exponential fluorescence lifetimes of 2.7 ns. 1,3-di-p-tolylacetone p, p'-diMeDBK) gives a multiexponential fluorescence decay. ZFS parameter s, triplet sublevel populating rates, and rates of triplet-state alpha -cleavage appear to be more sensitive functions of conformation than w ould be expected for nominally aliphatic ketones. We conclude that a s ignificant portion of the unpaired electron density of the triplet sta te of DBK (and its derivatives) is delocalized into the phenyl rings a nd propose that this delocalization is responsible for the previously observed change in rate of triplet-state alpha-cleavage with substitut ion in the phenyl rings.