Kinetics and Mechanisms of Photoinduced Reduction of Methylviologen by N-Alkyltetraphenylporphyrinatozinc(II) in Methanol

Article abstract of DOI:10.1246/bcsj.68.1947

N-3-Bromopropyl-5,10,15,20-tetraphenylporphyrinato(1-)chlorozinc(II), , catalyzed the formation of a methylviolagen radical cation (MV) on steady-state irradiation of degassed methanol solutions with visible light (>420 nm) in the presence of triethanolamine (TEOA).The initial rate (V_i) of the formation of MV-radical cation was of first order in the concentration of MV²⁺ and in the intensity of light and was independent of the concentration of TEOA above 0.025 M.Plots of V_i vs. the concentrations of were curved.The fluorescence of ^*, whose lifetimes are τ₁^s = 0.7 ns (48percent) and τ₂^s = 2.0 ns (52percent) in methanol at 25 deg C, was not quenched by MV²⁺.The excited triplet state of (τ₀^t = 3.2 μs in MeOH) was efficiently quenched by MV²⁺ (k_q = (6.0+/-0.6)*10⁷ M^-1 s^-1 at 25 deg C, 1M = 1 mol dm^-3).The thermal back electron-transfer reaction between -radical cation and MV-radical cation was also observed (k_b = (1.2+/-0.2)*10¹⁰ M^-1 s^-1).On the basis of the steady-state irradiation kinetics and ns-laser flash photolysis, the quantum yield of the excited triplet state of can be evaluated to be 0.98+/-0.02. was found to be a more effective photocatalyst than (HPrtpp = N-propyl-5,10,15,20-tetraphenylporphyrin) or (H2tpp = 5,10,15,20-tetraphenylporphyrin) under the present experimental conditions.