Fluorescent pH sensor constructed from a heteroatom-containing luminogen with tunable AIE and ICT characteristics

A heteroatom-containing organic fluorophore 1-(4-pyridinyl)-1-phenyl-2-(9-carbazolyl)ethene (CP₃E) was designed and synthesized. CP₃E exhibits the effect of intramolecular charge transfer (ICT) caused by the donor–acceptor interaction between its carbazole and pyridine units. Whereas it emits faintly in solution, it becomes a strong emitter in the aggregated state, demonstrating the phenomenon of aggregation-induced emission (AIE). Its emission can be reversibly switched between blue and dark states by repeated protonation and deprotonation. Such behaviour enables it to work as a fluorescent pH sensor in both solution and the solid state and as a chemosensor for detecting acidic and basic organic vapors. Analyses by NMR spectroscopy, single-crystal X-ray diffraction and computational calculations suggest that the change in electron affinity of the pyridinyl unit and molecular conformation of CP₃E upon protonation and deprotonation is responsible for such sensing processes.