Dynamical and Environmental Effects on the Optical Properties of an Heteroleptic Ru(II)-Polypyridine Complex: A Multilevel Approach Combining Accurate Ground and Excited State QM-Derived Force Fields, MD and TD-DFT.

An integrated multilevel approach is here built by combining classical molecular dynamic (MD) simulations, time-dependent density functional theory (TD-DFT) calculations, and solvation dynamics linear response (LR) analysis, and successively employed to investigate the optical properties and solvation structure of a prototypical heteroleptic Ru(II)-polypyridyl complex, widely employed in dye sensitized solar cells. The MD simulations are performed with an accurately parametrized intramolecular force field (FF), specifically derived from the quantum chemical (DFT) description of the molecule, both for its singlet and triplet ground states. Solvent effects, in ethanol (EtOH) and dimethyl sulfoxide (DMSO), are taken into account at different levels of approximation, going from a totally implicit description (polarizable continuum) to an hybrid explicit/implicit scheme. Our results show that the developed FFs were able to accurately describe and preserve the octahedral coordination of the Ru(II) center along the MD trajectories, yielding an accurate picture of the solute dynamics. Noticeably, the dynamical effects and the inclusion of an explicit microsolvation shell were found to be crucial to get a good agreement with the experimental absorption spectrum in EtOH, in both shape and positions of the main bands. The significant experimental blue-shift of the two low-energy bands in DMSO, that is not reproduced by the simulated thermal-averaged spectra, is, instead, attributed to deprotonation phenomena of the carboxylic groups, induced by the strong nucleophilic character of the solvent. Finally, analysis of the solvent response shows that the structural changes in the first solvation shell, following the metal-ligand to ligand charge transfer excitation, cause, in the protic medium the breakdown of the linear response approximation, which, on the contrary, holds for DMSO.