A DFT study on structures, frontier molecular orbitals and UV–vis spectra of [M(L)(N3)(C7H5N)(PPh3)] (M= Ru and Fe; L= Tp and Cp)

•The Fe-based complexes have shorter metal-ligand bond distances than Ru-based complexes.•ΔEL-H can be reduced by a low electronegativity metal atom, such as Fe.•These Ru- and Fe-based complexes can be activated by sunlight.•Fe and Cp result in red shift for UV–vis spectra.•The UV absorption can be enhanced by solvent, such as CH3OH.

In order to investigate whether Ru can be replaced by inexpensive Fe in photo-sensitizers for dye-sensitized solar cells (DSSCs), the differences in spin states, structures, frontier molecular orbitals and UV–vis spectroscopic properties of Ru- and Fe-based complexes are investigated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT). These Ru- and Fe-based complexes are expected to be in singlet state (low-spin) because of low related energy. The calculated structural parameters for the [RuTp]-based complex are in very good agreement with experimental values; moreover, the geometries of the [RuCp]-, [FeTp]- and [FeCp]-based complexes have been predicted as well. The metal-ligand bond distances for the Fe-based complexes are predicted to be slightly shorter than these of the Ru-based complexes due to the smaller spatial extent of 3d wave functions of the Fe atom. These Ru- and Fe-based complexes display the HOMO with metal d-orbital and π(N3) orbital characters and the LUMO with metal d-orbital and π*(C7H5N) orbital characters. The HOMO-LUMO energy gap (ΔEL-H) may be reduced by a low electronegativity of central metal atom, such as Fe. A low electronegativity metal atom (such as Fe) as well as an electron-rich ligand (such as Cp) may result in red shift for UV–vis spectra. Besides, the UV–vis absorption-enhanced by solvent, such as CH3OH, has been predicted. Our results show that Ru can be replaced by inexpensive Fe in photo-sensitizers for DSSCs. In addition, these Ru- and Fe-based complexes are good candidates for photo-sensitizers due to their absorption intensities and rich absorption bands in visible region.

Graphical abstractThese complexes can be excited in visible region, red shifts of UV–vis absorption features and UV–vis absorption-enhanced by solvent are predicted. A HOMO-2→LUMO transition results in the strongest absorption and the HOMO-LUMO energy gap can be reduced by a low electronegativity metal atom.Figure optionsDownload full-size imageDownload as PowerPoint slide