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

Abstract

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.