Excited-state intramolecular proton transfer in 2 '-(2 '-hydroxyphenyl)imidazo[1,2-a]pyridines

摘要

The key observation made by Acuna revealed the unusual optical properties of previously unknown 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridines (HPIPs). Although structurally similar to 2'-(2'-hydroxyphenyl)benzoxazoles and 2-(2'-hydroxyphenyl)benzimidazoles, 2-(2'-hydroxyphenyeimidazo[1,2-a]pyridines exhibit strikingly different optical properties, especially in the solid state. The luminescence color of HPIPs depends on the polymorphic form and can be tailored from green to red. The inability to assume the neutral keto tautomer is their other distinct characteristic, setting them apart from analogous azoles and their benzoanalogs. For these azoles and related luminescent compounds, the dual emission in polar solvents is usually observed, dominated by excited-state intramolecular proton transfer (ESIPT) fluorescence. However, for 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridines, clear ESIPT emission exists only in non-polar solvents and it is very weak, while in polar and especially protic solvents, strong emission from the Franck-Condon state is exclusively observed. The solvent-related effect is attributed to the tendency to form strong intramolecular hydrogen bonds in the solid state and in non-polar solvents, while hydrogen bond formation is much weaker in polar, and especially, in protic solvents. The present study focuses on HPIPs and demonstrates that ESIPT process is a promising mechanism for packing-directed luminescence control and offers a novel design concept for tunable organic luminescent solids. This review presents not only optical properties in various solutions, but also in the solid state, and it is supported by a synthetic overview, computational studies, and the description of pi-expanded derivatives. (C) 2016 Published by Elsevier B.V.