Structures, frontier molecular orbitals and UV–vis spectra of RuX(PPh3)(NHCPh2)L (X = Tp and Cp*; L = Cl and N3): A DFT study

• RuX(PPh3)(NHCPh2)L (X = Tp and Cp*; L = Cl and N3) may be excited by sunlight.
• The strongest UV–vis absorption feature is assigned to the HOMO − 2 → LUMO transition.
• The HOMO − 2 → LUMO is assigned to MLCT or MLCT/LLCT depending on co-ligands.
• HOMO–LUMO energy gap can be reduced by a π electron-rich ligand such as Cp* or N3.
• The red shifts of UV–vis absorption features are predicted on the RuCp* complexes.

Geometries of RuX(PPh3)(NHCPh2)(L) (X = hydridotris(pyrazolyl)borate (Tp) and pentamethylcyclopentadiene (Cp*); L = Cl and N3) are predicted from density functional theory (DFT) calculations at pure and hybrid functionals with DZVP2/DZVP mixed basis sets. The calculated values for the geometries are in very good agreement with experimental values; moreover, we have found that the pure functional (BP86) shows the better results in geometry calculations than hybrid functionals (B3LYP and M06). The frontier molecular orbitals (FMOs) and electronic transitions have been investigated as well. Our calculations show that a π electron-rich co-ligand (N3) increases the energies of occupied orbitals and reduces the energy gap of the HOMO–LUMO (ΔEL–H) in these complexes. The simulated UV–vis spectra of these complexes in methanol have been studied by using time-dependent density functional theory (TD-DFT). The conductor-like polarizable continuum model (CPCM) was employed to account for the solvent effect. A number of adsorption peaks are predicted in the visible region (400–800 nm) with non-zero oscillator strength. The strongest adsorption feature is associated to a transition from HOMO − 2 to LUMO which is assigned to metal-to-ligand charge transfer (MLCT) or metal/ligand-to-ligand charge transfer (MLCT/LLCT) depending on co-ligands. Compared to the Tp ligand, the red shift occurs obviously on RuCp* based complexes due to the π electron-donor and electron-rich characters of the Cp* ligand. According to our results, these complexes are good candidates for dye-sensitized solar cells (DSSCs) owing to their absorption intensities and wide absorption bands in the visible region.

These complexes can be excited in visible region and red shifts of UV–vis absorption features are predicted. A HOMO – 2 → LUMO transition results in the strongest absorption and the HOMO–LUMO energy gap can be reduced by a π electron-rich ligand. Figure optionsDownload as PowerPoint slide