Electronic and optical response of functionalized Ru(II) complexes: joint theoretical and experimental study

New photovoltaic and photocatalysis applications have been recently proposed based on the hybrid Ru(II)-bipyridine-complex/semiconductor quantum dot systems. In order to attach the Ru(II) complex to the surface of a semiconductor, a linking bridge -- a carboxyl group -- needs to be added to one or two of the 2,2'-bipyridine (bpy) ligands. Such changes in the ligand structure affect electronic and optical properties and, consequently, the charge transfer reactivity of Ru(II)-systems. In this study, we analyze the effects brought by functionalization of bipyridine ligands with the methyl, carboxyl, and carboxilate groups on the electronic structure and optical response of the [Ru(bpy){sub 3}]{sup 2+} complex. First principle calculations based on density functional theory (DFT) and time dependent DFT (TDDFT) are used to simulate the ground and excited-state properties, respectively, of functionalized Ru-complexes in the gas phase and acetonitrile solution. In addition, an effective Frenkel exciton model is used to explain the optical activity and splitting patterns of the low-energy excited states in all molecules. All theoretical results nicely complement and allow for detailed interpretation of experimental absorption spectra of Ru-complexes that have been done in parallel with our theoretical investigations. We found that the carboxyl group breaks the degeneracy of two low-energy optically bright excited states and red-shifts the absorption spectrum, while leaves ionization and affinity energies of complexes almost unchanged. Experimental studies show that deprotonation of the carboxyl group in the Ru-complexes results in a slight blue shift and decrease of oscillator strengths of the low energy absorption peaks. Comparison of experimental and theoretical linear response spectra of deprotonated complexes demonstrate strong agreement if the theoretical calculations are performed with the addition of a dielectric continuum model. A polar solvent is found to play an important role in calculations of optical spectra: it stabilizes the energy of states localized on the carboxyl or carboxylate groups. Thus, the excited-state structure of the functionalized Ru-complexes, specifically in the case of the deprotonated functions, can be accurately modeled by TDDFT with the addition of a dielectric continuum in simulations