Low-lying electronic excitations and optical absorption spectra of the black dye sensitizer: a first-principles study

We report on ab-initio calculations of the electronic structure and optical absorption response of the black dye sensitizer in gas phase. We show that, despite the large size of this molecule, the second-order multiconfiguration quasi-degenerate perturbation theory (MC-QDPT) can be used to calculate vertical excitation energies, oscillator strengths and optical absorption spectra. The zeroth-order reference states entering perturbation calculations are complete active space (CAS) configuration interaction (CI) wave functions computed for 12 active electrons distributed in 12 active orbitals. We found that the CI approach is not enough for taking into account the strong dynamical correlation effects in this system. In fact, the excitation energies of the CAS-CI target states are strongly renormalized by the MC-QDPT calculations. In the calculated absorption spectra, the analysis of the perturbed wavefunctions revealed that the stronger absorption bands correspond to metal-to-ligand and ligand-to-ligand charge transfer processes. Comparison with independent time-dependent extension (TDDFT) calculations performed with different functionals shows that corrections to the long-range behavior of the functional is pivotal to achieve agreement with the MC-QDPT results.