Substituent effects on fluorescent properties of imidazo[1,2-α] pyridine-based compounds

In order to search for novel fluorescent organic compounds, 20 derivatives of imidazo[1,2-a]pyridine (1) were synthesized, and their fluorescent properties were studied. Though the parent compound 1 (lambda(fl) = 370.5 nm, Phi = 0.57 in ethanol) was in the liquid state at ambient temperature, 2-phenylimidazo[1,2-a]pyridine (5), 2-(2-naphthyl)imidazo[1,2-a]pyridine (16), 7-methylimidazo[1,Za]pyridine (3), 7-methyl-2-phenylimidazo[ 1,2-a]pyridine (12), and 7-methyl-2-(2-naphthyl)imidazo[ 1,2-a]pyridine (17) were found to give thermally stable solid compounds (mp 55-19 degrees OC) without much affecting the fluorescent properties of the parent compound (lambda(fl) = 374-381 nm, Phi = 0.50-0.78 in ethanol). Among the 4'-substituted 2-phenyl derivatives, it was found that the introduction of the strong electron-donating amino and dimethylamino groups (2- (4-aminophenyl)imidazo [1,2-a]pyridine (7) and 2- [4-(dimethylamino)phenyl] imidazo[I,Za]pyridine (8), respectively) caused marked red shift of their fluorescence (lambda(fl) = 445 and 446 nm, respectively, in ethanol), thus providing the way for tuning the fluorescence color of the IP derivatives. The observed red shift of the fluorescence of 7 and 8 was ascribed to the contribution of the excited intramolecular charge transfer state.