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.

These 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 as PowerPoint slide