The geometries, electronic structures, polarizabilities, and hyperpolarizabilities of organic dye sensitizer 3-Phenoxyphthalonitrile were studied based on Hartree–Fock (HF) and density functional theory (DFT) using the hybrid functional B3LYP. Ultraviolet–visible (UV–vis) spectrum was investigated by time dependent DFT (TD-DFT). Features of the electronic absorption spectrum in the visible and near-UV regions were assigned based on TD-DFT calculations. The absorption bands are assigned to π→π* transitions. Calculated results suggest that the three excited states with the lowest excited energies in 3-Phenoxyphthalonitrile is due to photoinduced electron transfer processes. The interfacial electron transfer between semiconductor TiO₂ electrode and dye sensitizer 3-Phenoxyphthalonitrile is due to an electron injection process from excited dye to the semiconductor's conduction band. The role of phenoxy group in 3-Phenoxyphthalonitrile in geometries, electronic structures, and spectral properties were analyzed.