CuO-CuAl2O4 and D-glucose catalyzed synthesis of a family of excited state intramolecular proton transfer imidazo[1,2-a]pyridine analogues and their optical properties

A simple and efficient route to luminescent imidazo[1,2-a]pyridines from 2-aminopyridines, phenylacetylene and arylaldehydes was achieved by an one-pot, tandem process starting with an A3-coupling followed by 5-exo-dig cyclization. Copper(II)oxide/copper aluminate composite (5 wt% CuO-CuAl2O4 composite) and D-glucose catalyzed optimal conditions for the reaction were established after examining various reaction parameters such as solvent, catalyst and temperature. The synthesis was much compatible with various functionalities. 2-Aryl-imidazo[1,2-a]pyridines not capable of excited state intramolecular proton transfer emission displayed moderate fluorescence quantum yields in the blue-green region due to intramolecular charge transfer process. In addition, they exhibited peculiar solvatochromic behaviour in chlorinated solvents like chloroform and dichloromethane. Products containing 2-(2'-hydroxyphenyl) substituent underwent excited state intramolecular proton transfer in non polar and polar-aprotic solvents. Although such type of emission in solvents was very weak, the Stokes shifts values were high (14,638-11,448 cm(-1)) with strong solid-state emission in green to yellow region. (C) 2015 Elsevier Ltd. All rights reserved.