Optical absorption spectrum of the N3 solar cell sensitizer by second-order multireference perturbation theory

Here we report second-order n-electron valence state perturbation theory (NEVPT2) excited state calculations on the low-lying transitions of the [Ru(4,4'-COOH-2,2'-bpy)(2)(NCS)(2)] complex, better known as the N3 dye, both in vacuo and water solution. The present results provide unprecedented insights into the crucial role of the solvation effects, previously reported at density functional theory (DFT) and time-dependent DFT (TDDFT) level of theory, to adequately reproduce the optical absorption spectrum. We find that the NEVPT2 results in gas phase, even if slightly blueshifted, already provide a reliable description and assignation of the electronic structure and low-lying excited states of N3, whereas these are achieved in a DFT/TDDFT approach only by means of the solvent, which properly polarizes the ground-state frontier molecular orbitals. Then, inclusion of the solvent at the NEVPT2 level does not substantially modify the ground-state electronic structure as well as the excited states picture found in vacuo, solely yielding the expected spectral blueshift.