Static and dynamic paramagnetism of the undistorted triplet state of benzophenone

Abstract

Existing discrepancies in reports of the EPR fine structure of triplet traps in uv-excited benzophenone are clarified by studies of the population kinetics and the EPR of triplet benzophenone-h10 in crystals of deuterobenzophenone. Benzophenone-h10 provides a triplet trap of depth ≈30 cm−1 below the triplet exciton band of the deuterobenzophenone host and phosphoresces copiously at liquid helium temperatures. In deuterated crystals containing 1 mole% benzopheone-h10, only the phosphorescence of ordinary benzophenone is detectible below 4.2°K; the quantum yield is 42%. The ordinary benzophenone traps are shown to have the same conformation as the triplet molecules formed by direct T1←S0 absorption and the molecules responsible for the excitonic phosphorescence of ordinary benzophenone crystals. The traps earlier reported by Sharnoff are shown to have nearly this conformation. The triplet states of the traps reported by Chan, Schmidt, Veeman, and van der Waals and by Winscom and Maki are shown not to have this conformation. The kinetics of depopulation of triplet benzophenone are shown to be sensitive to the conformation of the emitting state. The decay rates from the spin sublevels of undistorted triplet benzophenone-h10 molecules are kz = 768 sec−1, ky = 70 sec−1, and kx = 60 sec−1. Population heavily favors (85%) the z sublevel. At liquid helium temperatures the zero-field EPR transitions have the frequencies 1241 MHz (x ⇄ y), 3930 MHz (y ⇄ z), and 5171 MHz (z ⇄ x). The z sublevel is highest in energy.