Development of Imidazo[1,2- a]pyridine Derivatives with an Intramolecular Hydrogen-Bonded Seven-Membered Ring Exhibiting Bright ESIPT Luminescence in the Solid State.

Imidazo[1,2- a]pyridine derivatives with different hydroxyaryl units (1-3), which could potentially form an intramolecular hydrogen-bonded seven-membered ring in either a planar or a twisted conformation, were newly developed, and the effect of conformation and steric repulsion on the excited-state intramolecular proton transfer (ESIPT) luminescence was evaluated. Among them, 1 and 2 formed an intramolecular hydrogen-bonded seven-membered ring in the crystalline state and exhibited efficient ESIPT luminescence in the solid state (quantum yield up to 0.45).