ACTIVATION ENTROPY CHANGES OF MACROCYCLE PROTONATION OF THE FREE BASE 21-CH3-OCTAETHYLPORPHYRIN IN THE LOWEST EXCITED SINGLET S1 STATE

UDC 535.37+541.65+543.4

  • Kruk Mikalai Mikalaevich − DSc (Physics and Mathematics), Associate Professor, Head of the Department of Physics. Belarusian State Technological University (13a, Sverdlova str., 220006, Minsk, Republic of Belarus). E-mail: m.kruk@belstu.by

DOI: https://doi.org/10.52065/2520-2669-2022-259-2-150-155

Key words: porphyrin, acid-base equilibria, entropy, enthalpy.

For citation: Kruk M. M. Activation entropy changes of macrocycle protonation of the free base 21-СН3-octaethylporphyrin in the lowest excited singlet S1 state. Proceedings of BSTU, issue 2, Chemical Engineering, Biotechnologies, Geoecology, 2022, no. 2 (259), pp. 150–155 (In Russian). DOI: https://doi.org/10.52065/2520-2669-2022-259-2-150-155.

Abstract

The acid-base equilibria of the 21-CH3,23-H,2,3,7,8,12,13,17,18-octaethylporphyrin in the ground S0 and lowest excited singlet S1 states were studied using the Foerster cycle and spectrofluorometric titration methods. It has been experimentally established that the рKа values in the ground and excited states are equal, while the estimate for the Foerster cycle is ΔрKа = –1,8. The discrepancy is explained by the violation of the activation entropy equality approximation in the lower excited singlet S1 and ground S0 states. It was found that the activation entropy of macrocycle protonation ΔS‡ in the lowest excited singlet S1 state decreases by 49 J mol−1 К−1, and the activation enthalpy ΔН‡ decreases by 14.4 kJ mol−1 correspondingly. This indicates an enthalpy-entropy ΔН‡ -ΔS‡ compensation upon protonation of the macrocycle core of the studied porphyrin. Based on the results obtained, it was proposed that the transition state for protonation in the excited S1 state is characterized by a lower height of the activation barrier and conformational mobility of the macrocycle.

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15.06.2022