Substituent Effects on Fluorescent Properties of Imidazo[1,2- a ]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 (λfl = 370.5 nm, Φ = 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[1,2-a]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—190 °C) without much affecting the fluorescent properties of the parent compound (λfl = 374—381 nm, Φ = 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[1,2-a]pyridine (8), respectively) caused marked red shift of their fluorescence (λfl = 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.