Aggregation-Induced Emission: Past, Present, and Future

ConspectusFor centuries, the reductionist view that "the whole equals the sum of its parts" has guided scientific study, particularly materials design. Nature, however, often defies this logic: an aggregate (whole) can display emergent properties that are totally absent in its individual parts. Aggregation-induced emission (AIE) exemplifies this "anomaly": nonluminescent molecules become emissive upon aggregation, achieving a qualitative "0-to-1" leap that challenges the reductionist tenet and provides a unique lens through which to view the emergence of new properties.Since it was proposed as a concept in 2001, AIE has been mechanistically understood as arising from the restriction of molecular motion (RMM) in the excited state. In dilute solutions, molecular rotors and vibrators dissipate exciton energy through active motions, leading to nonradiative decay. Upon aggregation, these motions are physically restricted by molecular packing and noncovalent interactions, impeding nonradiative channels and opening radiative pathways. This mechanistic understanding has motivated extensive research into AIE and expanded the field into a diverse platform of aggregation-enabled luminescent systems, including clusteroluminescence (CL), room-temperature phosphorescence (RTP), and circularly polarized luminescence (CPL)─all absent in the isolated molecular constituents and emerging through aggregation.With accumulated knowledge in AIE, the attention has broadened toward the exploration of aggregation-generated function (AGF). From this perspective, molecular motions─previously viewed as energy "wasted" that reduced emission─can be harnessed to convert excited-state energy into heat through rotations and vibrations. By channeling the same exciton energy that underlies luminescence into nonradiative decay pathways, we can engineer aggregates to exhibit emergent photothermal (PT), photoacoustic (PA), and photocatalytic (PC) activities. These functions open new application avenues, including solar energy conversion, high-resolution deep-tissue imaging, and "intelligent" actuation.From the serendipitous encounter with AIE to the systematic study of AGF, advances in the field have shifted scientific attention from isolated molecules to complex aggregates. With the elucidation of principles governing emergent properties, it is becoming clear that a paradigm shift is needed─from molecularism to aggregatism or from molecular science to aggregate science (AS). Guided by emergentism, AS studies how molecules, through noncovalent interactions and hierarchical organization, give rise to macroscopic functions absent in their individual constituents. Notably, the materials we use and the life we see around us are all aggregates. This aggregate-level perspective enables the development of new systems with complex functionalities (e.g., advanced multimodal theranostics) and deepens our understanding of life─an archetypal multiary system in which the aggregation of nonliving biomolecular constituents yields a living organism.In this Account, we detail the intellectual trajectory from AIE to AGF and finally to AS. We distill the guiding principles and outline future directions, including transitions from unary to multiary systems, static structures to dynamic processes, and descriptive aggregate science to prescriptive aggregate engineering. A deeper understanding of AS will enable new scientific discoveries and technological innovations, inviting us to imagine a future designed not merely with matter but with the sophisticated organizational logic that endows it with "life-like" functions.