Protonation effects on the resonance Raman properties of a novel (terpyridine)Ru(4H-imidazole) complex: an experimental and theoretical case study

The optically active states in a novel (terpyridine)Ru(4H-imidazole) complex displaying an unusually broad and red-shifted absorption in the visible range are investigated experimentally and theoretically. Since this property renders the complex promising for an application as sensitizer in dye-sensitized solar cells, a detailed knowledge on the correlation between features in the absorption spectrum and structural elements is indispensable in order to develop strategies for spectroscopy/theory-guided design of such molecular components. To this aim, time-dependent density functional theory calculations, including solvent effects, are employed to analyze the experimental UV-vis absorption and resonance Raman (RR) spectra of the unprotonated and protonated forms of the complex. This provides a detailed photophysical picture for a complex belonging to a novel class of Ru-polypyridine black absorbers, which can be tuned by external pH stimuli. The complex presents two absorption maxima in the visible region, which are assigned by the calculations to metal-to-ligand charge transfer (MLCT) and intra-ligand states, respectively. RR simulations are performed in resonance with both bands and are found to correctly reproduce the observed effects of protonation. Finally, the examination of the molecular orbitals and of the RR spectra for the MLCT state shows that protonation favors a charge transfer excitation to the 4H-imidazole ligand.