DFT/TDDFT theoretical studies on electronic structures and spectral properties of rhenium(I) phenanthrolineimidazo complexes

The ground state geometries, frontier molecular orbital properties and absorption properties of the four transition metal complexes ReCl(CO)3(phen-imidazo) (1), ReCl(CO)3(5,5′-Br-phen-imidazo) (2), ReCl(CO)3(4,4′-CH3-phen-imidazo) (3) and ReCl(CO)3(4,4′-dimethylformyl-phen-imidazo) (4) were studied theoretically. Density functional theory (DFT) calculations were carried out to optimize the ground state geometries. Upon the introduction of electron-withdrawing groups, the LUMO energy decreases much more than that of the HOMO. The LUMO energy level of 4 has the smallest value (−3.24 eV), while its HOMO energy level (−5.67 eV) is comparable to those of 1, 2 and 3, thus complex 4 has the smallest HOMO–LUMO energy gap. Time-dependent density functional theory (TDDFT) calculations were carried out to predict absorption spectra. The lowest lying absorption bands of the four Re(I) complexes all have HOMO-1 → LUMO transition configurations resulting in MLCT/LLCT transition character. The HOMO-5 → LUMO + 1 and HOMO-2 → LUMO + 2 mixed transitions predominantly contribute to the highest energy absorption bands of 1, 2 and 3, which have MLCT/LLCT/ILCT mixed character. The 259 nm absorption band of 4 is dominated by the HOMO-2 → LUMO + 2 and HOMO-8 → LUMO + 1 transitions giving rise to mixed MLCT/LLCT/ILCT character.