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

N-3-Bromopropyl-5,10,15,20-tetraphenylporphyrinato(1-)chlorozinc(II), [Zn(BrPrtpp)Cl], catalyzed the formation of a methylviologen 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 (Vi) of the formation of MV+• 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 Vi vs. the concentrations of [Zn(BrPrtpp)Cl] were curved. The fluorescence of [Zn(BrPrtpp)Cl]*, whose lifetimes are τ1s = 0.7 ns (48%) and τ2s = 2.0ns (52%) in methanol at 25 °C, was not quenched by MV2+. The excited triplet state of [Zn(BrPrtpp)Cl] (τ0t = 3.2 μs in MeOH) was efficiently quenched by MV2+ (kq = (6.0 ± 0.6) × 107 M−1 s−1 at 25 °C, 1 M = 1 mol dm−3). The thermal back electron-transfer reaction between [Zn(BrPrtpp)Cl]+• and MV+• was also observed (kb = (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 [Zn(BrPrtpp)Cl] can be evaluated to be 0.98 ± 0.02. [Zn(BrPrtpp)Cl] was found to be a more effective photocatalyst than [Zn(Prtpp)Cl] (HPrtpp = N-propyl-5,10,15,20-tetraphenylporphyrin) or [Zn(tpp)] (H2tpp = 5,10,15,20-tetraphenylporphyrin) under the present experimental conditions.