Theoretical studies on the structures and spectroscopic properties of rhenium(I) acetylide diimine complexes

Electronic structures and spectroscopic properties of Re(bpy)(CO)3(CCR) (bpy = 2,2′-bipyridine; R = H (1), CH2OH (2), and C6H5 (3)) were studied by the ab initio and DFT methods. The ground- and excited-state structures were optimized by the DFT-B3LYP and CIS methods, respectively. The absorption and emission spectra in the dichloromethane solution were calculated by the TD-DFT method at PBE1PBE level associated with the PCM model. The calculated results indicate that the variation of the substituents on the acetylide ligand slightly changes the structures in the ground and excited states, but leads to a great difference in the electronic structures. The HOMOs of 1-3, which are of d(Re), π(CC), and p(CO) character (For 3, there are considerable compositions of the phenyl group in HOMO), are sensitive to the change of the substituents on the acetylide ligand. And the energy levels of HOMOs increase obviously with the introduction of the electron-donating groups, but those of the bpy-based LUMOs with the π*(bpy) character vary slightly. The lowest-lying absorptions at 443, 457, 495 nm for 1, 2, and 3, respectively, which arise from the HOMO → LUMO excitation configuration were attributed to the singlet MLCT/ LLCT transitions, while the lowest-lying emissions at 626 (1), 648 (2), and 668 nm (3) were categorized as the triplet 3MLCT/3LLCT transitions. Both the lowest-lying absorptions and emissions for 1-3 are red-shifted in the order 1 < 2 < 3 when the electron-donating groups are introduced into the acetylide ligand. By comparison of the results obtained by using the different functionals embedded in the TD-DFT method, the calculated results indicate that the PBE1PBE functional is appropriate for the Re(I) acetylide complexes to get the satisfactory results.