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

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 (SH+)-S-3 with a triplet quantum yield Phi(T)=0.28 between pH 4.8 and 10.4. At pH 8.3(3)SH(+) decays directly to the ground state. However, at pH 4.8 and 10.4, (SH+)-S-3 reacts with protons or hydroxy ions to form the dication (SH22+)-S-3 or the neutral S-3 species, with diffusion-controlled rate constants of k(H)= 1.6 X 10(10) M-1 s(-1), and k(HO) = 2.6 X 10(10) M-1 s(-1), respectively. Under the same experimental conditions, the TRP measuren-tents allowed the accurate determination of the energy content of the rapidly formed triplet state (SH+)-S-3, i.e. E-T = 175 kJ mol(-1). The slow component (0.1-3 mus) of the TRP signal at pH 4.8 and 10.4 was attributed to the formation of the species (SH22+)-S-3 and S-3, respectively. The enthalpy changes associated with the proton-transfer reactions of (SH+)-S-3, 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 (SH+)-S-3 was accompanied by a volume expansion of 1.8 cm(3) 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 (SH22+)-S-3 and S-3 accounted for the electrostrictive effect produced by the charge in the charge distribution on the dye after the proton-transfer reaction.