Structures and properties of metal-free and copper tetrakis(thiadiazole) porphyrazine and metal-free tetrakis(selenodiazole) porphyrazine based on density functional theory calculations

Density functional theory (DFT) and time-dependent DFT calculations were carried out to comparatively describe the molecular structures, atomic charges, molecular orbital, UV-vis spectra, and infrared (IR) spectra of metal-free and copper tetrakis(thiadiazole) porphyrazine and metal-free tetrakis(selenodiazole) porphyrazine. The calculated structures, electronic absorption, and IR spectra of TTDPzH₂ (1), TSeDPzH₂ (2), and Cu(TTDPz) (3) [(TTDPz)^2- = dianion of tetrakis(thiadiazole) porphyrazine; (TSeDPz)^2- = dianion of tetrakis (selenodiazole) porphyrazine] are in good consistency with the experimental results. In comparison with metal-free phthalocyanine, the presence of soft atoms such as S or Se together with the N atoms in the peripheral heterocyclic rings of tetraazaporphyrin has significant effect on the charge density distribution of C_β atoms, while the charges of other atoms change very little. Media effects have little influence on the charge distribution of TTDPzH₂ (1) and TTDPzCu (3) with the largest charge difference of only 0.03e. Comparison between the orbital energies of TTDPzH₂ (1) in vacuum system, dichloromethane, and chlorobenzene, and of TTDPzCu (3) in vacuum system and chlorobenzene indicates that the absolute orbital energies are sensitive to the solvent but the HOMO-LUMO energy gap is not significantly altered. According to our calculation results, the natures of the main transitions in electronic absorption spectra of the three compounds have been assigned. With the assistance of animated pictures produced on the basis of the normal coordinates, the vibration modes in the IR spectra of all the three compounds were also identified.