Research Progress of Aggregation-Caused Quenching (ACQ) to Aggregation-Induced Emission (AIE) Transformation Based on Organic Small Molecules

Most of the traditional organic fluorescent molecules have notorious aggregation-induced quenching (ACQ) effect, which limits their application in many fields. Therefore, how to achieve the transition from ACQ to aggregation-induced emission (AIE) while maintaining the functional properties of traditional conjugated molecules is of great significance. In recent years, many research groups have done a lot of breakthrough works in this field. According to the construction method and structural characteristics of AIE luminogen (AIEgen), the ACQ to AIE transformation strategy can be divided into two types. Based on the restriction of intramolecular motion (RIM) mechanism, the first one is to decorate ACQphore with typical AIEgen (e.g. tetraphenylethene unit). As the second one, some functional units (e.g. triphenylamine and cyanoethylene moiety) are introduced to realize the construction of AIE molecules from zero starting point as the RIM mechanism. In the latter, the construction of other AIEgen systems involving isomerism and excited-state intramolecular proton transfer (ESIPT) mechanisms has also been prominent recently. Thus, according to these classifications, the new progress on the construction strategy of ACQ-to-AIE transformation in the past five years is summarized from the four aspects of AIEgens' structure, performance, mechanism and application.