Conformational Control of the Photochemistry and Photophysics of Diphenylacetone

We report the direct observation of the lowest triplet states of 1,3-diphenylacetone (DBK) and two methylated derivatives by direct detection time-resolved electron spin resonance (TRESR) at 15 K in methylcyclohexane glass. The spectral features are broad with multiple peaks in the A M 2 = 2 region, which we assign to a multitude of conformations frozen into the glass. Successive experiments on a single sample provide evidence that certain of these conformations can be photochemically 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 parameters, triplet sublevel populating rates, and rates of triplet-state a-cleavage appear to be more sensitive functions of conformation than would be expected for nominally aliphatic ketones. We conclude that a significant portion of the unpaired electron density of the triplet state of DBK (and its derivatives) is delocalized into the phenyl rings and propose that this delocalization is responsible for the previously observed change in rate of triplet-state a-cleavage with substitution in the phenyl rings. 1