Assessing and applying DFT approaches for geometries and UV–Vis absorption spectra of tetragonal iron(II) complexes

The performance of density functional theory (DFT)-based calculations combined with different basis set was evaluated on potential iron (II) photosensitizer. BP86-D3(BJ), PBE-D3(BJ), TPSS-D3(BJ), OPBE-D3(BJ), B3LYP-D3(BJ), PBE0-D3(BJ) and TPSSh-D3(BJ) functionals were assessed against X-ray data and the PBE, B3LYP, O3LYP, PBE0, CAM-B3LYP and ωB97X functionals for UV–Vis absorption spectrum. For obtaining the geometries, PBE is a good choice, regardless the basis set size, while OPBE does well only when larger basis set are employed. Regarding the excited states energies, the referred protocol III, \({E}_{\text{EE},\text{gas}}^{\text{TD}-\text{DFT}}\left({R}_{\text{solv}}^{\text{GS}}\right)-{E}_{\text{GS},\text{gas}}\left({R}_{\text{solv}}^{\text{GS}}\right)\) $E_{\text{EE},\text{gas}}^{\text{TD}-\text{DFT}}\left(R_{\text{solv}}^{\text{GS}}\right)-E_{\text{GS},\text{gas}}\left(R_{\text{solv}}^{\text{GS}}\right)$ , was found to meet the “Pauling Point” criteria, a situation wherein the cancelation of significant errors yields a physically balanced model, allowing an affordable and reliable way to understand their excited state’s electronic structure. Specifically, TD-DFT calculations using def2-TZVP basis set combined with B3LYP or CAM-B3LYP functionals can be recommended to treat the excited states of the explored iron (II) complex. The study additionally looks at the molecular and electronic structures of a series of similar complexes, collecting understanding to help tune their properties and of related systems as photosensitizers.