Spectral Manifestations of Specific Solvation of 5,10,15,20-Tetrakis-(4-Sulfonatophenyl)-Porphyrin and its Doubly Protonated Form in Aqueous Solutions

The temperature dependence of absorption and fluorescence spectra of 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrin in weakly acidic aqueous solutions was studied in the range 288–333 K. The fraction of molecules in the free-base form in the ground (S0) and lower excited singlet states (S1) was found to increase as the temperature increased because the fraction of molecules in the doubly protonated form decreased. The deprotonation was caused by a shift in the acid–base equilibrium in the macrocycle core due to decreases in pKa(S0) and pKa(S1) with increasing temperature. The difference pKa(S1) – pKa(S0) < 0 for solution temperatures >293 K with pKa(S1) – pKa(S0) > 0 for T < 293 K. The activation energies of deprotonation in the S0- and S1-states for T > 293 K were Ea = 5.0 and 3.4 kJ/mol; for T < 293 K, they increased to 20.3 and 56.2 kJ/mol. These differences were explained by different specific solvation of the tetrapyrrole macrocycle in the ground S0- and lower excited S1-states because of a change in the ratio of two forms of water. So-called form A with disordered H-bonds dominated at higher temperatures. The fractions of forms A and B, which possessed a strongly ordered H-bond system, were comparable as the temperature decreased. Stabilization of the porphyrin free base prevailed at high temperatures; of the doubly protonated form, at low temperatures.