TIME-RESOLVED DIFFUSE-REFLECTANCE STUDIES OF BETA-PHENYL KETONES IN THE SOLID-STATE - CONFORMATIONAL AND CHIRAL CONTROL OF TRIPLET LIFETIMES

The triplet states of ketones having a beta-aryl substituent, such as 2 and 3, decay in solution by a charge transfer interaction between th is aryl group and the excited carbonyl. In the solid state, this inter action also provides an efficient mode of deactivation for 3, but not for 2, reflecting the proximity between the pi-ring system in 3, but n ot in 2, where the molecules crystallize (X-ray) in a stretched confor mation. In the case of 3, the triplet lifetime in the solid state is 4 20 ns for the pure R or S enantiomers, but 733 ns for the racemic crys tals, showing an interesting case of chiral discrimination. Powder X-r ay and sold state NMR data suggest that conformational and packing dif ferences between the enantiomers and racemic crystals are responsible for differences in the efficiency of intramolecular deactivation.