Porphyrins Basicity in the Lowest Excited Singlet S1 State: the Role of Peripheral Substitution and Macroheterocycle Structure

Abstract

The influence of peripheral substitution and the nature of the macroheterocyclic heteroatoms on the basicity of hydrophilic 5,10,15,20-tetraarylporphyrins in the lowest excited singlet S₁ state has been studied using absorption and fluorescence spectroscopies. It has been established that in the proper porphyrins, the basicity of the lowest excited singlet S₁ state decreases compared to the ground S₀ state, and the difference in the basicity constants рK_a depends on the nature of electronic communication between the macrocycle and peripheral substituents. In case of the inductive effect of substituents, the difference in the basicity constants рK_a is small, while mesomeric effects lead to significant differences in рK_a values. It has been shown that the cooperative nature of protonation, leading to almost simultaneous addition of two protons, is preserved in both cases. It has been established that the replacement of the pyrrole fragment with the thiophene one in the macroheterocycle leads to a decrease in cooperativity both in the ground S₀ and in the lowest excited singlet S₁ states. It has been found that in S₁ state the basicity constants characterizing the addition of the second and the third protons by thia-substituted porphyrin differ significantly (рK_a2 - рK_a3 = 2), while in the ground state they are close. The activation entropy change ΔΔS‡ upon protonation of porphyrins in the lowest excited S₁ state compared to that in the ground S0 state has been determined.