DFT and TD DFT study of oxodiboron(III) phthalocyanine and peculiarities of spectral behavior and acid–base properties of its tert-butyl substituted derivative in solutions

Abstract

Molecular and electronic structures of transoid and cisoid forms of oxodi(fluoroboron(III)) complex of phthalocyanine [O(FB)₂Pc] in the ground and excited states were studied using DFT and TD DFT methods. The peculiarities of the electrochemical and spectral properties of the tetra-tert-butyl substituted complex existing in the transoid form in organic solvents and in acid medium were considered in comparison with 1:1 complexes formed by aluminum subgroup metals and boron(III) subphthalocyanine. Coordination of two boron atoms in the coordination cavity of the phthalocyanine macrocycle leads to its tetragonal distortion both in the cisoid and especially in the transoid forms and stabilizes the frontier π-molecular orbitals, especially the lowest unoccupied MO. This results in the bathochromic shift of the long-wave Q band and strongly facilitates the reduction of macrocycle occurring for the transoid form at −0.19 V versus Ag/AgCl in dimethylformamide (DMF). Spectrophotometric titration in CH₂Cl₂-CF₃COOH medium showed that the basicity of the meso-nitrogens is decreased upon coordination of two boron atoms. Slow deborylation can occur either in a strongly acid medium (96% H₂SO₄) or in the presence of water in DMF. The absence of fluorescence and ability to generate singlet oxygen is explained by quenching of the excited state due to conformation flexibility of the macrocycle and inversion of the coordination center in the transoid form.

Keywords:

• Phthalocyanine
• subphthalocyanine
• diboron complex
• spectral and acid–base properties

Acknowledgment

The authors express their gratitude to Dr. Yu.A. Zhabanov and D.A. Lazovskiy for help with quantum calculations.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by Russian Foundation for Basic Research (joint Russian-Czech project Nr. 20-53-26004).