A Novel Ru(II) Polypyridine Black Dye Investigated by Resonance Raman Spectroscopy and TDDFT Calculations

The optical properties of a new (bipyridine)2Ru(4H-imidazole) complex presenting a remarkable broad absorption in the visible range are investigated. The strong overlap of the absorption with the solar radiation spectrum renders the studied complex promising as a black absorber and hence as a starting structure for applications in the field of dye-sensitized solar cells. The correlations between structural and electronic features for the unprotonated and protonated forms are studied by means of UV–vis absorption and resonance Raman (RR) spectroscopy modeled with the help of time-dependent density functional theory (TDDFT) calculations. The absorption spectra show two bands in the visible region, which TDDFT assigns to a metal-to-ligand charge-transfer (MLCT) state and to a superposition of three excited states with MLCT and intraligand charge-transfer character, respectively. Additionally, the analysis of the molecular orbitals and RR spectra in resonance with the first MLCT band shows that the effects of protonation favor a charge-transfer photoexcitation to the 4H-imidazole ligand. The RR spectra simulated for several excitation wavelengths covering the visible region are in excellent agreement with experimental data. In particular, it is noteworthy that the calculations are able to reproduce the wavelength dependence of the RR spectra provided that corrected excitation energies are employed. Interference effects between the electronic states contributing to the RR scattering are small for the investigated complex.