ANALYSIS OF THE ACID-BASE EQUILIBRIA IN THE LOWEST EXCITED SINGLET S1 STATE OF PORPHYRINS USING FÖRSTER CYCLE

UDC 535.37+541.65+543.4

  • Sokhibova Anarkhan Muhfizullokyzy – student. Belarusian State Technological University (13a, Sverdlova st., 220006, Minsk, Republic of Belarus). E-mail: anarxan.soxibova@bk.ru

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

DOI: https://doi.org/10.52065/2520-2669-2021-242-2-25-30

Key words: porphyrin, acid-base equilibria, peripheral substitution, excited state.

For citation: Sokhibova A. M., Kruk M. M. Analysis of the acid-base equilibria in the lowest excited singlet S1 state of porphyrins using Förster cycle. Proceedings of BSTU, issue 3, Physics and Mathematics. Informatics, 2021, no. 1 (242), pp. 25–30 (In Russian). >DOI: https://doi.org/10.52065/2520-2669-2021-242-2-25-30.

Abstract

Using the Förster cycle the value of the basicity changes upon the population of the lowest singlet excited S1 state ΔрKа has been estimated for unsubstituted porphine molecules and for six 5,10,15,20-aryl-substituted derivatives, which differing in number and position of phenyl and mesityl groups. The analysis of the basicity changes has been carried out for equilibria between free base and monoprotonated form ΔрKа3, and between monoprotonated and doubly protonated forms ΔрKа4. It was found that value and direction of changes of ΔрKа3 and ΔрKа4 depend substantially on the architecture of peripheral substitution of macrocycle. Porphine molecule in its lowest excited S1 state is a stronger acid compared to the ground state: ΔрKа3 = –1.1, and ΔрКа4 = –0.46. For 5,10,15,20-aryl-substituted derivatives increase in the basicity in S1 state was found according to increase in the number of phenyl substituents: value ΔрKа3 increases linearly from –0.39 for 5,10,15,20-tetramesitylporphyrin, up to 1.1 for 5,10,15,20-tetraphenylporphyrin. The changes are likely to be due to differences in: а) macrocycle conformation flexibility and b) dihedral angle between the planes of the aryl fragments and the macrocycle mean plane. Attachment of the second proton increases the basicity; the value ΔрKа4 is positive for all the derivatives and barely depends on the substitution architecture.

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References

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07.12.2020