Achieving ultralow critical micelle concentration of surfactant with an unlocked aggregation-induced emission probe under pressure

The ultralow critical micelle concentration (CMC) for micellar stability is crucial in photovoltaic devices, optoelectronic technologies, and biosensors. While aggregation-induced emission (AIE) probes function as sensitive fluorescence switches for millimolar-level CMC detection, their application in the micromolar regime has been limited by the inherent high fluorescence background. Here, a robust strategy is developed to achieve the ultralow CMC of surfactants by the detection of AIE-active 2-(benzo[d]thiazol-2-yl)-5-(diphenylamino)-phenol (BDTP) system. Under 0.82 GPa, the CMC of typical cetyl trimethyl ammonium bromide probed by BDTP is significantly reduced to 15.0 μM compared to 780.0 μM at 0 GPa. The detection of ultralow CMC is attributed to pronounced fluorescence quenching under pressure. Femtosecond transient absorption spectra reveal a clear picture that pressure-induced fluorescence quenching is associated with the triggered excited-state intramolecular proton transfer process. These results establish a direct structure–property–function relationship and provide a robust approach to leveraging this system for achieving and sensing ultralow CMC.