Aggregate Materials beyond AIEgens

Abstract

ConspectusAggregate materials focus on the materials that are constituted by more than a single entity. It could be dimers, trimers, or multimers formed by the group of same molecules or mixtures formed by different molecules. Although materials have different forms such as liquid, hydrogel, colloidal suspension, and powder, aggregates undisputedly are among the most commonly existing forms, which involve many aspects of human daily life such as the clothes we wear, the drugs we take, the food we eat, and the tools and devices we use. Sciences related to aggregate materials are also rich. From molecules to aggregates, different aspects of science such as crystallography, interfacial science, supramolecular science, and engineering science may be involved. Therefore, research on aggregate materials and related mechanisms that dominate the properties of aggregate materials is significant.However, it is noteworthy that reductionism is the dominant research philosophy for the design of traditional materials, and it claims that a molecule is the smallest particle of a substance that retains all of the properties of the substance. Consequently, scientists have expended a lot of effort to develop new molecules and to study the relationships between molecular structure and properties without paying attention to aggregate tuning. Actually, more and more research supports the fact that aggregate control such as the tuning of the packing mode, self-assembly, and morphology could work as a more versatile and effective strategy to endow materials with rich properties and functions. A prototype of the aggregate research is the aggregation-induced emission (AIE) phenomenon and materials. AIE research mainly focuses on the luminescence change accompanied by the aggregation process. After more than 20 years of flourishing development, AIE research has made great advances in new material development, mechanism studies, and high-technology applications. More importantly, AIE research opens the window to aggregate science and demonstrates that new behaviors and functions that are absent in single molecular species can be generated in molecular aggregates.In this Account, we highlight our efforts toward aggregate materials beyond AIE luminogens (AIEgens), with a special focus on new properties in addition to luminesce endowed by aggregation. We first summarize the chirality that is induced by the aggregation process of prochiral molecules. Then we present the aggregation resulting from the transformation between hydrophilicity and hydrophobicity. After that, the aggregation-induced generation of reactive oxygen species (ROS) is described and the regulation of the aggregation state for tuning the ROS generation efficiency is discussed. We also discuss how to achieve the switch between radiative decay and nonradiative decay through the regulation of the aggregation extent to afford luminescence and photothermy (photothermal effect), respectively. Additionally, different strategies to accelerate the solid-state molecular motion are presented and could be used to produce efficient photothermal materials. In particular, the concept of intramolecular motion-induced photothermy has been proposed, which worked as a reliable strategy to generate photothermal materials. At the end of the Account, a brief summary of this area and outlooks in this field are presented.