Computational and spectroscopic studies of the imidazole-fused phenanthroline derivatives containing phenyl, naphthyl, and anthryl groups

Abstract Three N,N-bidentate ligands, 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline ( 1 ), 2-(2-naphyl)-1H-imidazo[4,5-f]phenanthroline ( 2 ), and 2-(2-anthryl)-1H-imidazo[4,5-f]phenanthroline ( 3 ) have been synthesized and characterized. Effects of aryl subsistiuents (phenyl, naphthyl, and anthryl) on the photophysical properties of these ligands in solution have been studied. Ligand 1 exhibit the main absorption band at 283 nm with the shoulder bands at 300–350 nm and these bands are assigned as the typical π→π* (imPhen) state (imPhen = 1H-imidazo[4,5-f][1,10]phenanthroline). A similar absorption spectrum was also observed in the case of 2, in which the charge transfer (CT) state should be considered. 3 shows the slightly different absorption properties compared to that of 1 and 2 . The highest-lying absorption band at 257 nm is assigned as a mixed π→π* (imPhen) /π→π* (Anth) (Anth = anthracene) state. Additionally, the characteristic absorption band of anthryl group with three vibronic bands at 352, 367, and 386 nm also observed in the visible region range from 340 to 400 nm. 1 shows the typical ligand-centered 1 π→π* emission, while 2 and 3 emit from the mixed 1 π→π*/CT states. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were employed to rationalize the photophysical properties of these ligands studied. The theoretical data confirm the assignment of the experimental absorption spectra and the nature of the emitting states.