Thermodynamic changes in the photoinduced proton-transfer reaction of the triplet state of safranine-T

Abstract

The enthalpy and volume changes occurring in the triplet excited state proton-transfer reactions of safranine-T (SH⁺) in aqueous solutions at pH 4.8, 8.3, and 10.4 were investigated using time-resolved photoacoustics (TRP). The transient triplet state species were also studied using laser-flash photolysis (LFP). The LFP experiments showed the prompt formation of ³SH⁺ with a triplet quantum yield Φ_T = 0.28 between pH 4.8 and 10.4. At pH 8.3 ³SH⁺ decays directly to the ground state. However, at pH 4.8 and 10.4, ³SH⁺ reacts with protons or hydroxy ions to form the dication ³SH₂²⁺ or the neutral ³S species, with diffusion-controlled rate constants of k_H⁺ = 1.6 × 10¹⁰ M⁻¹ s⁻¹, and k_HO⁻ = 2.6 × 10¹⁰ M⁻¹ s⁻¹, respectively. Under the same experimental conditions, the TRP measurements allowed the accurate determination of the energy content of the rapidly formed triplet state ³SH⁺, i.e.E_T = 175 kJ mol⁻¹. The slow component (0.1–3 µs) of the TRP signal at pH 4.8 and 10.4 was attributed to the formation of the species ³SH₂²⁺ and ³S, respectively. The enthalpy changes associated with the proton-transfer reactions of ³SH⁺, calculated from the values of the heat released as obtained by TRP, were in remarkable agreement with the values estimated from the thermodynamic data of the acid–base equilibria of the triplet states of the dye. The formation of ³SH⁺ was accompanied by a volume expansion of 1.8 cm³ mol⁻¹, which was explained by changes in the hydrogen-bonding interaction of the dye with its solvation sphere. Instead, the volume changes observed upon the formation of ³SH₂²⁺ and ³S accounted for the electrostrictive effect produced by the change in the charge distribution on the dye after the proton-transfer reaction.