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

Molecular and electronic structures of transoid and cisoid forms of oxodi(fluoroboron(III)) complex of phthalocyanine [O(FB)(2)Pc] 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 pi-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.