Time dependent density functional theory study of the absorption spectrum of the [Ru(4,4′-COO^--2,2′-bpy)₂(X) ₂]^4- (X = NCS, Cl) dyes in water solution

We present a combined experimental and computational density functional theory (DFT)/time-dependent DFT study of the geometry, electronic structure and optical absorption spectrum of the solar-cell sensitizers cis-[Ru(4,4′-COO^--2,2′-bpy)₂(X) ₂]^4- (X = NCS, Cl) in water solution. The experimentally observed red-shift of the spectrum upon substitution of NCS-by Cl-is well reproduced by our theoretical model and appears to be related to stabilization of the t_2g-π* HOMOs in the NCS^- complex. The π* LUMOs of the Cl complex are destabilized by a smaller extent with respect to the HOMOs, due to the increased π-back bonding interactions with the bipyridine ligands.