Kinetics for acid-dissociation of tetraphenylporphinetetrasulfonate in the ground state measured by laser photolysis relaxation method

Transient absorption spectra (TAS) for tetraphenylporphinetetrasulfonate (tpps(4-)) and its diprotonated form (H(2)tpps(2-); 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 tpps(4-) and H(2)tpps(2-) at pH 2.05 and 7.41 (I , 0.01; 25degreesC), molar absorption coefficients of tpps(4-) and H(2)tpps(2-) in their triplet states were obtained. On the other hand, as the triplet tpps(4-) was deactivated, H(2)tpps(2-) in the triplet state was generated through the protonation of the triplet tpps(4-) in the pH range of 5.0-6.0 (I, 0.01; 25degreesC). This indicated that the triplet tpps(4-) was more basic than the ground state. As H(2)tpps(2-) in the triplet state was deactivated, an excess amount of H(2)tpps(2-) in the ground state was successively generated. The excess H(2)tpps(2-) 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 H(2)tpps(2-) in the ground state was determined as (3.3 +/- 0.4) 10(5) s(-1), independent of pH. From this value and the acid-dissociation constants of H(2)tpps(2-), the overall acid-dissociation kinetics of tpps(4-) was proposed.