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

We report on ab-initio calculations of theelectronic structure and optical absorption response of theblack dye sensitizer in gas phase. We show that, despitethe large size of this molecule, the second-order multiconfigurationquasi-degenerate perturbation theory (MCQDPT)can be used to calculate vertical excitationenergies, oscillator strengths and optical absorption spectra.The zeroth-order reference states entering perturbationcalculations are complete active space (CAS) configurationinteraction (CI) wave functions computed for 12 activeelectrons distributed in 12 active orbitals. We found thatthe CI approach is not enough for taking into account thestrong dynamical correlation effects in this system. In fact,the excitation energies of the CAS-CI target states arestrongly renormalized by the MC-QDPT calculations. Inthe calculated absorption spectra, the analysis of the perturbedwavefunctions revealed that the stronger absorptionbands correspond to metal-to-ligand and ligand-to-ligandcharge transfer processes. Comparison with independenttime-dependent extension (TDDFT) calculationsperformed with different functionals shows that correctionsto the long-range behavior of the functional is pivotal toachieve agreement with the MC-QDPT results.