Sonochemical multicomponent synthesis of 2-(2´-hydroxyphenyl)imidazo[1,2-a]pyridine analogs with an intramolecular hydrogen-bonded eight-membered ring exhibiting an excited-state intramolecular proton transfer fluorescent process.

Multicomponent reactions are excellent tools to generate complex structures with broad chemical diversity and fluorescent properties. The sonochemical multicomponent one-pot synthesis of novel organic luminescent heterocycles, such as 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridine (2-HPIP) analogs, is described. The Groebke–Blackburn–Bienaymé reaction (GBBR) affords good to excellent yields (75–95%). Notably, the compound 4b forms an intramolecular hydrogen-bonded eight-membered ring (8-MR) in the crystalline state and is luminescent in this phase; in solution, this compound exhibits dual band emission with a very large Stokes shift of 18 382 cm⁻¹ attributed to an excited-state intramolecular proton transfer (ESIPT) process. Time-dependent density functional theory (TD-DFT) and additional calculations are performed to evaluate the HOMO–LUMO distributions and theoretical absorption spectra, being in agreement with the experimental data. Additionally, the TD-DFT calculations in solution (chloroform) show the ESIPT process and 8-MR formation become thermodynamically favorable in 4b. This work represents the first synthesis of prompts feasibility ESIPT heterocyclic luminescent materials via an eight-membered ring (8-MR) with an N···HO connection.