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 ph enothiazine in nitrogen-saturated acetonitrile and cyclohexane was inv estigated. The lowest excited triplet state (T(l)) of phenothiazine wa s efficiently generated following 308-nm excitation in both acetonitri le and cyclohexane. In cyclohexane, generation of the phenothiazine ne utral radical via homolytic N-H bond cleavage from S1 accompanied inte rsystem crossing. In acetonitrile, photoionization via the singlet man ifold yielding the cation radical competes with intersystem crossing a nd 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 triple t 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 eff icient in acetonitrile. Enhanced neutral radical production in cyclohe xane and enhanced production of the neutral and cation radicals in ace tonitrile were observed concurrently with T(l) depletion. In cyclohexa ne, 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 che mical efficiencies were 0.53 and 0.45, respectively. Thus, bond cleava ge and ionization from T(n) are competitive.