2-LASER, 2-COLOR PHOTOCHEMISTRY FROM UPPER TRIPLET-STATES OF 2-BROMONAPHTHALENE AND 9-BROMOPHENANTHRENE IN BENZENE

The photochemistry of 2-bromonaphthalene (BrN) and 2-bromophenanthrene (BrP) in benzene was examined using two-laser, two-color techniques. These experiments center on the photochemistry of the strongly absorbi ng triplet states of these molecules. Triplet photobleaching quantum y ields in benzene are 0.12 and 0.15 for BrN and BrP, respectively. It i s believed that these yields coincide with C-Br bond cleavage to yield in-cage radical pairs. Their magnitude reflects excited-state partiti oning between C-Br bond cleavage and return to the lowest triplet surf ace. Bromine atoms were detected taking advantage of pi-complex format ion in benzene with lambda(max) similar to 540 nm. Less than half of t he bleaching events result in the formation of detectable bromine atom s. Geminate radical recombination of the radical pair to regenerate st arting material must compete with cage escape, which accounts for the remaining bleaching events.