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

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 was studied using absorption and fluorescence spectroscopies. The basicity of the porphyrins themselves in the lowest excited singlet S₁ state was found to decrease as compared to the ground S₀ state. The difference in basicity constants pK_a depended on the nature of electronic communication between the macrocycle and peripheral substituents. In the case of an inductive effect of the substituents, the difference in the basicity constants pK_a was small, while mesomeric effects led to significant differences in the pK_a values. The cooperative nature of protonation, leading to almost simultaneous addition of two protons, was shown to be preserved in both cases. Replacement of a pyrrole with a thiophene in the macroheterocycle led to a decrease in cooperativity in both the ground S₀ and the lowest excited singlet S₁ states. The basicity constants characterizing the addition of the second and third protons by thia-substituted porphyrin differed significantly in the S₁ state (pK_a2 – pK_a1 = 2), while they were close in the ground state. The activation entropy change ΔΔS^‡ upon protonation of the porphyrins in the lowest excited S₁ state was compared to that in the ground S₀ state.