Transient absorption spectra (TAS) for tetraphenylporphinetetrasulfonate (tpps4−) and its diprotonated form (H2tpps2−; protonated at the two N-atoms of the porphyrin-ring) after laser photolysis (532 nm, pulse width 6 ns) showed the generation of their respective triplet states in aqueous solutions. From the analysis of TAS for tpps4− and H2tpps2− at pH 2.05 and 7.41 (I, 0.01; 25 °C), molar absorption coefficients of tpps4− and H2tpps2− in their triplet states were obtained. On the other hand, as the triplet tpps4− was deactivated, H2tpps2− in the triplet state was generated through the protonation of the triplet tpps4− in the pH range of 5.0–6.0 (I, 0.01; 25 °C). This indicated
that the triplet tpps4− was more basic than the ground state. As H2tpps2− in the triplet state was deactivated, an excess amount of H2tpps2− in the ground state was successively generated. The excess H2tpps2− faded to attain the acid-dissociation equilibrium in the ground state again. The total decay curves were successfully fitted to triple-exponential decay functions, and the rate constant for acid-dissociation of H2tpps2− in the ground state was determined as (3.3 ± 0.4) × 105 s−1, independent of pH. From this value and the acid-dissociation constants of H2tpps2−, the overall acid-dissociation kinetics of tpps4− was proposed.
