Ligand substitution effect on electronic structure and optical properties of nickel porphyrazines

Self-consistent density functional calculations have been performed on a variety of planar conjugated Ni-centered macrocycles with a basic porphyrazine (tetraazaporphyrin) core and peripheral dithiolene groups (PZ) or fused-benzo groups (PC). Theoretical energy level diagrams, charge distributions, and densities of states have been obtained in order to understand the electronic structure modifications due to peripheral ligand substitution. The substituents role in altering electronic properties and charge distribution of the porphyrazine macrocycles has been used to interpret the observed variations in optical absorption profiles. In the Q-band (similar to 680 nm) region, a single peak is seen for high symmetry (D-4h) macrocycles and a double peak for lower symmetry (D-2h and C-2n) systems. Calculated intensities and band splittings are compared in detail with qualitative molecular orbital models and experiment in the visible and UV regions. Predictions are made for the infrared absorption and semiconducting band gap.