Reversible Luminescent Switching of Cu(I)-imidazophenanthroline Complexes in Silica Gel for Visual Acid-Base Vapor Indication

Vapor-responsive luminescent materials are desirable for visual detection and indication, yet reversible switching in Cu(I) complexes remains challenging. Here, a series of ionic and neutral Cu(I)–Imphens–diphosphine complexes were designed and synthesized, and their luminescence response to NH3 and HCl vapors was examined. The complexes were characterized by IR, NMR, X-ray single-crystal diffraction, and ESI-MS. Photophysical studies revealed that absorption and emission are governed by both ligand deprotonation and solvent effects: in dichloromethane, ionic and neutral complexes show markedly different spectral profiles, whereas in DMSO, strong solvation and hydrogen-bond acceptance smooth the spectral differences between ionic and neutral complexes. Notably, when deposited on silica gel TLC plates, complexes 1a and 1b display reversible, visually detectable luminescence switching upon alternating exposure to NH3 and HCl vapors, suggesting their potential as visual indicators. This response is tentatively attributed to reversible protonation/deprotonation, which modulates high-frequency vibrational quenching channels and the extent of π-conjugation, with additional influence from the confined, highly solvated silica gel microenvironment. These preliminary observations provide a useful basis for the further exploration of Cu(I)-based materials for reversible acid–base vapor luminescent indication.