An Intelligent AIEgen with Nonmonotonic Multi-Responses to Multi-Stimuli

Intelligent stimulus-response (S/R) systems are the basis of natural process and machine control, and have been intensively explored in biomimetic design, analytical chemistry and biological applications. However, nonmonotonic multi-S/R systems are still rarely studied so far. Now, we propose a rational design strategy to achieve such a unique S/R system by integrating opposite luminescence behaviors in one molecule. When solvent polarity increases, many heterocycles often become more emissive due to the suppression of the proximity effect. However, molecules with donor-acceptor (D-A) structures tend to be less emissive because of the twisted intramolecular charge transfer. Meanwhile, protonation on D/A moieties will weaken/strengthen the D-A interaction to result in blue/red-shifted emissions. By combining a protonatable heterocyclic acceptor and a protonatable donor together in one molecule, we can easily achieve nonmonotonic brightness responses to polarity stimuli and nonmonotonic color responses to pH stimuli. In this work, a simple molecule, namely ASQ is chosen as the model compound to verify the design strategy feasibility. It successfully shows two opposite trends of responses to polarity and pH stimuli, and aggregation-induced emission (AIE) with a nonmonotonic AIE curve. Moreover, the acidified ASQ solution is also a pure organic up-conversion and white-light-emitting system. A new mechanistic viewpoint is established to explain its unique anti-Stokes emission. Besides, ASQ shows multivalent functionalities including albumin protein sensing, ratiometric pH sensing, and amine gas sensing, etc. Therefore, ASQ is proved to be a fundamentally important and versatile functional “intelligent” AIE luminogen with nonmonotonic multi-responses to multi-stimuli.