Quinoxaline-based azamacrocycles: synthesis, AIE behavior and acidochromism

Abstract

The development of luminescent molecular materials has advanced rapidly in recent decades, primarily driven by the synthesis of novel emissive compounds and a deeper understanding of excited-state mechanisms. Herein, we report a streamlined synthetic approach to light-emitting diazapolyoxa- and polyazamacrocycles N₂C_nO_xQ and N_yC_nQ (n = 3–10; x = 2, 3; y = 2–5), incorporating a 2,3-diphenylquinoxaline residue (DPQ). This synthetic strategy based on macrocyclization through Pd-catalyzed amination reaction yields the target macrocycles in good or high yields (46–92%), enabling precise control over their structural parameters. A key role of the PhPF-tBu ligand belonging to the JosiPhos series in this macrocyclization was elucidated through DFT computation. This macrocyclization reaction eliminates the need for complex protecting-deprotecting procedures of secondary amine groups, offering a convenient and scalable method for the preparation of target compounds. Moreover, it boasts a potentially broad substrate scope, making it promising for structure-properties studies within photophysics, sensor development, and material synthesis. Photophysical properties of representative macrocycles were investigated, employing spectroscopic techniques and DFT computation. It was demonstrated that DPQ-containing macrocycles display aggregation-induced emission in a DCM–hexane solvent mixture despite the presence of flexible tethers within their structures. Single-crystal X-ray diffraction analysis of a representative compound N₂C₈O₃Q allowed us to gain deeper insight into its molecular structure and AIE behaviour. The emissive aggregates of the N₂C₁₀O₃Q macrocycle were immobilized on filter paper yielding AIE-exhibiting test strips for measuring acidity in vapors and in aqueous media.