Synthesis, structural characterization and photoluminescence properties of rhenium(I) complexes based on bipyridine derivatives with carbazole moieties

Abstract

Three N,N-bidentate ligands, 5,5′-dibromo-2,2′-bipyridine (L1) and two carbazole containing ligands of 5-bromo-5′-carbazolyl-2,2′-bipyridine (L2), 5,5′-dicarbazolyl-2,2′-bipyridine (L3), and their corresponding rhenium Re(CO)₃Cl(L) complexes (ReL1–ReL3) have been successfully synthesized and characterized by elemental analysis, ¹H NMR and IR spectra. Their photophysical properties and thermal analysis, along with the X-ray crystal structure analysis of L3 and complexes ReL1 and ReL3 are also described. In CH₂Cl₂ solution at room temperature, all complexes display intense absorption bands at ca. 220–350 nm, which can be assigned to spin-allowed intraligand (π→π*) transitions, and the low energy broad bands in the 360–480 nm region are attributed to the metal to ligand charge-transfer dπ(Re)→π* (diimine) (MLCT). The introduction of carbazole moieties improves the MLCT absorption and molar extinction coefficient of these complexes. Upon excitation at the peak maxima, all complexes show strong emissions around 620 nm, which are assigned to dπ(Re)→π* (diimine) MLCT phosphorescence. The photoluminescence lifetime decay of Re(I) complexes were measured and the quantum efficiencies of the rhenium(I) complexes were calculated by using air-equilibrated [Ru(bpy)₃]²⁺·2Cl⁻ aqueous solution as standard (Φ_std = 0.028). The complexes with appended carbazole moieties exhibit enhanced luminescence performances relative to ReL1.