Influence of heteroatom substitution in benzene rings on structural features and spectral properties of subphthalocyanine dyes

Abstract

Peculiarities of the molecular geometry, electronic structure and spectral properties of heterocyclic subphthalocyanines analogues containing electron-deficient heterocycles have been considered using DFT and TD DFT methods. Subporphyrazine with annulated 1,2,5-selenadiazole rings was prepared and characterised for the first time and molecular parameters of the 1,2,5-thiadiazole analogue have been determined by single crystal X-ray diffraction. The theoretical results are compared with the experimental data and the effect of heteroatom substitution in benzene rings of subphthalocyanines on spectral-luminescence properties was revealed. Fusion of pyrazine, 1,2,5-thiadiazole or 1,2,5-selenadiazole rings instead benzene fragments leads to considerable stabilization of the frontier π-molecular orbitals of subporphyrazine macrocycle. The HOMO-LUMO gap is increased when pyrazine or 1,2,5-thiadiazole rings are fused, but becomes narrower in the presence of 1,2,5-selenadiazole rings. The intense Q band observed in the visible region (530–575 nm) originates from the lowest ππ* transition (2a2→1e*) and its energy follows the changes in HOMO-LUMO gap. Two transitions (2a1→1e* and 1a1→1e*) contribute predominantly to closely lying excited states (B1, B2) responsible for the intense Soret band in the UV-region (305–310 nm) of subphthalocyanine and its pyrazine and 1,2,5-thiadiazole analogues. For the Se-analogue the energy of the 2a1→1e* transition is decreased and B1 band is shifted bathochromically to 342 nm. Fusion of π-electron-deficient 1,2,5-chalcogenadiazole rings leads to appearance of the less intense band at 390 (S) or 408 nm (Se) due to electronic transition from HOMO 2a2 to 2e* orbital localized predominantly on peripheral heterocyclic fragments (CT band). The heavy atom effect of chalcogens decreases the fluorescence quantum yield in heterocyclic subphthalocyanines analogues, but promotes the intersystem crossing and strongly enhances the singlet oxygen quantum yield for 1,2,5-selenadiazole fused subporphyrazine (ΦΔ = 0.98 in CH2Cl2).