A Time-dependent Density Functional Theory (TDDFT) Interpretation of the Optical Spectra of Zinc Phthalocyanine π-cation and π-anion Radicals

The UV–visible and near-IR spectra of the zinc phthalocyanine π-cation and π-anion radicals, [ZnPc(–1)]•+ and [ZnPc(–3)]•–, are investigated by time-dependent density functional theory (TDDFT) calculations using the pure, asymptotically correct, statistical average of (model) orbital potentials (SAOP) functional. The nature and intensity of the main spectral features are highlighted and interpreted on the basis of the ground-state electronic structure of the complexes. Similarities and differences with previous TDDFT/B3LYP results are discussed. TDDFT/SAOP results for the π-anion radical prove to be in excellent agreement with the solution spectra and generally in line with deconvolution analyses of solution absorption and magnetic circular dichroism (MCD) spectra. On the basis of these results a novel interpretation of the Q-band system is proposed. For the π-cation radical TDDFT/SAOP calculations provide a satisfactory description of the UV region of the spectrum. However, they do not reproduce accurately the energy and intensity of the Q band observed at 825 nm. The description of the Q-band region appears to be complicated by the presence of spurious non-Gouterman transitions. Furthermore, the calculations, either in the gas phase or in solution, do not account for the broad absorption near 500 nm that has been suggested to arise from a nondegenerate, z-polarized 2A2g excited state. Theory and experiment can be reconciled if the presence of an axial ligand such as CN– is explicitly considered in the calculations. TDDFT/SAOP results for the axially ligated [ZnPc(–1)(CN)]• species indicate that the 500 nm feature is related to the axial ligation induced symmetry lowering of the π-cation radical and this band is assigned to a z-polarized transition associated with the hole in the 2a1u.