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 this aryl group and the excited carbonyl. In the solid state, this interaction also provides an efficient mode of deactivation for 3, but not for 2, reflecting the proximity between the pi-ring system in 3, but not in 2, where the molecules crystallize (X-ray) in a stretched conformation. In the case of 3, the triplet lifetime in the solid state is 420 ns for the pure R or S enantiomers, but 733 ns for the racemic crystals, showing an interesting case of chiral discrimination. Powder X-ray and sold state NMR data suggest that conformational and packing differences between the enantiomers and racemic crystals are responsible for differences in the efficiency of intramolecular deactivation.