Synthesis of π-Expanded Azole-Fused Imidazo[1,2-a]pyridine Derivatives and their Photophysical Properties

A Highly efficient copper-catalyzed one-pot sequential approach has been developed for the synthesis of azole-fused imidazo[1,2-a]pyridines with 2-(2-bromophenyl)imidazo[1,2-a]pyridine-3-carbaldehydes as substrates. The one-pot approach involved a sequential imidazole/benzimidazole formation followed by a copper-catalyzed intramolecular Ullmann type C–N coupling. The method tolerated a variety of functional groups and offered the desired products in good to excellent (50–85 %) yields. The photophysical properties of the compounds synthesized were evaluated by UV/Vis and fluorescence spectroscopy in CH₃CN. The π-expanded azole-fused imidazo[1,2-a]pyridines displayed high fluorescence emission with large Stokes shifts and moderate to good quantum yields. A pronounced positive solvatochromism and aggregation caused quenching (ACQ) was observed for 2,3-bis(4-methoxyphenyl)-12-methylimidazo[1,2-a]pyrido[2',1':2,3]imidazo[4,5-c]quinoline.