SPECTRAL MANIFESTATIONS OF SPECIFIC SOLVATION OF THE 5,10,15,20-TETRAKIS-(4-SULFONATOPHENYL)-PORPHYRIN AND ITS DOUBLY PROTONATED FORM IN WATER SOLUTIONS

Abstract

The temperature dependence of the electronic absorption and fluorescence spectra of 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrin in weakly acidic aqueous solutions was studied in the temperature range of 288–333 K. It was found that the fraction of molecules in the form of a free base in the ground (S₀) and lower excited singlet (S₁) states increases with the temperature increase, due to a decrease in the fraction of molecules in the doubly protonated form. Deprotonation is caused by a shift in the acid-base equilibrium in the macrocycle core due to a decrease in рK_а(S₀) and рK_а(S₁) with the temperature increase. At solution temperatures >293 K, the difference рK_а(S₁)–рK_а(S₀) < 0, and at Т < 293 K the value рK_а(S₁)–рK_а(S₀) >0. It was found that activation energy of deprotonation in the S₀ and S₁ states is E_a = 5.0 and 3.4 kJ/mol at T > 293 K; it increases to 20.3 and 56.2 kJ/mol at T < 293 K. These differences are explained by different specific solvation of the tetrapyrrole macrocycle in the ground S₀ and lower excited S₁ states due to a change in the proportion of two forms of water. At higher temperatures so-called form A with disordered hydrogen bonding dominates, and when temperature decreases the amounts of the form A and form B, possessing strongly ordered system of hydrogen bonding, are comparable. As a result the porphyrin free base stabilization prevails at high temperatures, whereas the doubly protonated form dominates at lower temperatures.