Synthesis, photoluminescence and computational study of rhenium(I) diimine complexes with [1,3,4]oxadiazole substituted 2,2′-bipyridine ligands

Four bipyridine ligands substituted by the [1,3,4]oxadiazole moieties at different positions, 4,4′- bis(5-phenyl-[1,3,4]oxadiazol-2-yl)-2,2′-bipyridine (L1), 4,4′-bis[5-(4-tert-butyl-phenyl)-[1,3,4] oxadiazol-2-yl]-2,2′-bipyridine (L2), 5,5′-bis(5-phenyl-[1,3,4]oxadiazol-2-yl)-2,2′-bipyridine (L3), and 5,5′-bis[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-2,2′-bipyridine (L4), and their corresponding rhenium(I) Re(CO)3Cl(L) complexes (1–4) were synthesized and characterized by the elemental analysis, IR, 1H NMR and UV–vis spectroscopy. The complex 2 is also determined by the crystal structure analysis. In the UV–vis spectra complexes 1, 2 and 4 show intense spin-allowed intraligand (π→π*) transitions and weak metal to ligand charge-transfer dπ (Re)→π* (diimine) (MLCT) bands in THF solution. Complexes 1, 2 and 3 show efficient photoluminescent emission assigned to dπ (Re)→π* (diimine) MLCT phosphorescence at ambient temperature, and the different substitution position of [1,3,4]oxadiazole moieties in bipyridine ligands has a great effect on both absorption and emission properties of complexes. Among complexes 1, 2 and 3, the 4,4′-disubstituted complex 1 exhibits the strongest phosphorescence emission both in aerated THF solution (a quantum yield of 1.1%) and solid state. Meanwhile, the ground-state electronic structure and absorption spectra of 1–4 are investigated theoretically by DFT and TD-DFT methods.