Ligand substitution effect on optical properties in conducting tetraazaporphyrines

Self-consistent Density Functional calculations have been performed on a variety of planar conjugated Ni-centered macrocycles with a basic tetraazaporphyrinic core and dithiolene groups (PZ) or fused-benzo groups (PC). Theoretical energy diagrams, charge and spin 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 (approx. 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_2v) 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.