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

• Published in: Journal of applied spectroscopy Vol. 91; no. 3; pp. 489 - 495
• Main Authors: Shakel, A. Yu, Melnik, A. D., Kruk, M. M.
• Format: Journal Article
• Language: English
• Published: New York Springer US 01.07.2024; Springer Nature B.V
• Subjects: Activation energy; Analytical Chemistry; Atomic/Molecular Structure and Spectra; Basicity; Entropy of activation; Excitation spectra; Physics; Physics and Astronomy; Porphyrins; Protonation; Protons; Substitutes; macroheterocycles; basicity; fluorescence; acid-base equilibria
• ISSN: 0021-9037; 1573-8647
• DOI: 10.1007/s10812-024-01745-5

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 S 1 state was studied using absorption and fluorescence spectroscopies. The basicity of the porphyrins themselves in the lowest excited singlet S 1 state was found to decrease as compared to the ground S 0 state. The difference in basicity constants p K 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 p K a was small, while mesomeric effects led to significant differences in the p K 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 S 0 and the lowest excited singlet S 1 states. The basicity constants characterizing the addition of the second and third protons by thia-substituted porphyrin differed significantly in the S 1 state (p K a 2 – p K a 1 = 2), while they were close in the ground state. The activation entropy change ΔΔ S ‡ upon protonation of the porphyrins in the lowest excited S 1 state was compared to that in the ground S 0 state.