2-LASER PHOTOCHEMISTRY OF PHENOTHIAZINE IN SOLUTION - IONIZATION AND BOND-CLEAVAGE FROM UPPER STATES

The one (308 nm)- and two (308 plus 480 nm)-laser photochemistry of phenothiazine in nitrogen-saturated acetonitrile and cyclohexane was investigated. The lowest excited triplet state (T(l)) of phenothiazine was efficiently generated following 308-nm excitation in both acetonitrile and cyclohexane. In cyclohexane, generation of the phenothiazine neutral radical via homolytic N-H bond cleavage from S1 accompanied intersystem crossing. In acetonitrile, photoionization via the singlet manifold yielding the cation radical competes with intersystem crossing and homolytic bond cleavage. An additional route to the neutral radical in acetonitrile is deprotonation of the cation radical. Generation of upper triplet states (T(n)) by 480-nm excitation of T(l) resulted in permanent bleaching of the T-T absorption in both solvents. The triplet bleaching quantum yield was 0.120 in cyclohexane, but only 0.004 in acetonitrile, indicating that relaxation from T(n) to T(l) is more efficient in acetonitrile. Enhanced neutral radical production in cyclohexane and enhanced production of the neutral and cation radicals in acetonitrile were observed concurrently with T(l) depletion. In cyclohexane, the chemical efficiency of two-laser radical production was 0.90, indicating that the majority of T(n) states that do not relax to T(l) undergo bond cleavage. In acetonitrile, neutral and cation radical chemical efficiencies were 0.53 and 0.45, respectively. Thus, bond cleavage and ionization from T(n) are competitive.