Ab Initio Study of the Emissive Charge-Transfer States of Solvated Chromophore-Functionalized Silsesquioxanes

Recent experimental advances in the ability to tune the optical properties of silsesquioxanes by functionalizing them with photoactive ligands have made these compounds attractive candidates for building blocks of photovoltaic materials. We employ state-of-the-art ab initio methodologies to determine the nature of the excited charge-transfer (CT) states that give rise to a large red-shift between absorption and emission in these molecules, in comparison to the corresponding red-shift in the individual ligand. The calculations are based on time-dependent density functional theory and employ the recently developed Baer-Neuhauser-Livshits range-separated hybrid (RSH) functional. Solvent effects are accounted for via a combination of charge-constrained density functional theory and the polarizable continuum model. We find that the experimentally observed red-shift is consistent with identifying the emissive state as a ligand-to-ligand, rather than a ligand-to-silsesquioxane, CT state. We also find that the enhanced red-shift cannot be explained without accounting for solvation effects, and we demonstrate the importance of using a RSH functional to obtain reliable predictions regarding the emissive state.