THE pKa OF DIPROTONATED SEMIMETHYLENE BLUE, IN 5% ETHANOL AND 50% ACETONITRILE AQUEOUS SOLUTIONS

Abstract— Semimethylene blue was generated by reductive quenching of triplet methylene blue, ³MBH²⁺, with diphenylamine at pH 0.62–3.4. A Q-switched ruby laser flash-photolysis-kinetic spectro-photometric apparatus was used to characterize the absorption spectrum of semimethylene blue from 350 to 900 nm and a number of physical constants at 25°C with μ= 0.4 M and Cl^? as the anion. The specific rate of quenching of ³MBH²⁺ by DPA is 2.8 × 10⁹M^?1 s^?1 in 5% EtOH-95% water and 1.2×10⁹M^?1 s^?1 in 50 v/v% aq. CH₃CN. Corresponding efficiencies of net electron transfer are, respectively, 0.15 and 0.62. Spectral characteristics in 5% EtOH are, for MBH²₂±, λ_max= 375 nm, ε₃₇₅= 9000 M^?1 cm^?1; λ_max= 880 nm, ε₈₈₀= 12700 M^?1 cm^?1; for MBH±, λ_max= 410 nm, ε₄₁₀= 9800 M^?1 cm^?1, λ_max= 880 nm, ε₈₈₀= 33000 M^?1 cm^?1; for MBH± in 50 v/v% AN, λ_max= 400 nm, ε₄₀₀= 11000 M^?1 cm^?1 and λ_max= 880 nm,ε₈₈₀= 39000 M^?1 cm^?1. The pK_a of MBH²₂ε calculated from the pH dependence of the absorption spectrum is 1.86 × 0.04 in 5% EtOH and 1.15 in 50 v/v% AN. Rate constants, k_decay, for reaction DPAH±⁺ with MBH²₂ε and MBH± in 5% EtOH are, respectively, 3.9 × 10⁹ and 9.5 × 10⁹M^?1 s^?1. The value of pK_a of MBH²₂ε calculated from the dependence of k_decay on pH is 1.75 in 5% EtOH.