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

C. D. BORSARELLI; S.G. BERTOLOTTI; C.M. PREVITALI

Photochemical and Photobiological Sciences

Royal Society of Chemistry

Año: 2002 vol. 1 p. 574 - 580

1474-905X

Enthalpy and volume changes occurring in the triplet excited state proton-transfer reactions of safranine-T (SH+) in aqueous solutions were investigated using time-resolved photoacoustics (TRP). The transient triplet state species formed at pH between 4 and 11 were also studied using laser-flash photolysis (LFP). The LFP experiments showed the prompt formation of the 3SH+ with a triplet quantum yield FT= 0.28. At pH 8.3 the 3SH+ decays directly to the ground state. However, at pH’s 4.8 and 10.4, the 3SH+ reacts with proton or hydroxyl ions forming the dication 3SH22+ or the neutral 3S species, with diffusional controlled rate constants of kH+= 1.6x1010 M-1s-1, and kHO-= 2.6x1010 M-1s-1, respectively. Under the same experimental conditions, the TRP measurements allowed the accurate determination of the energy content of the prompt triplet state 3SH+, i.e. ET = 175 kJmol-1. The slow component (0.1-3 ms) of the TRP signal at pH’s 4.8 and 10.4 was attributed to the formation of the 3SH22+ and 3S species, respectively. The enthalpy changes associated with the proton-transfer reactions of 3SH+, calculated with the heat released values 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, 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 the 3SH22+ and 3S accounted the electrostrive effect produced by the change on the charge distribution after the proton-transfer reaction.