This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Certain chemical substances, broadly called luminogens, have an inherent ability to emit light. Most fluorescent molecules emit light in solution but lose their emission in the solid state, where the molecules aggregate.
Aggregation-induced emission (AIE) luminogens, or AIEgens, show the opposite behavior: They emit weakly in dilute solution but become strongly luminescent upon aggregation or in the solid state. This unusual property has made AIEgens attractive for a wide range of applications, including organic light-emitting diodes (OLEDs), microbial sensors, bioimaging technologies and photodynamic therapy. What are the fundamental factors that govern the emission behavior of AIEgens?
Associate Professor Gen-ichi Konishi of the Department of Chemical Science and Engineering at Institute of Science Tokyo, Japan, and his colleagues have proposed that AIE behavior is associated with large structural changes involving substantial reorganization of π-electron systems, including processes accompanied by substantial weakening or cleavage of π bonds in double bonds or aromatic systems. In solution, such structural changes can proceed along reaction pathways that efficiently deactivate the excited state without light emission, whereas in the solid state, these processes are suppressed, allowing the molecules to emit light. These processes can be understood in terms of excited-state potential energy surfaces, particularly through reaction pathways leading to conical intersections (CIs), where excited molecules can rapidly return to the ground state without emitting light.
However, accurately calculating and analyzing such complex potential energy surfaces is computationally demanding. Consequently, it has been difficult to identify simple molecular parameters that can be used to predict AIE behavior, and many AIEgens have historically been discovered through empirical trial and error rather than rational molecular design. Against this backdrop, a research team led by Konishi succeeded in predicting AIE behavior using a small number of descriptors derived from quantum chemical calculations.
The team further discovered that the introduction of donor and acceptor groups and, importantly, their substitution positions are key structural factors that determine the excited-state potential energy surface and induce AIE behavior. Their work, published in Advanced Science, combined quantum chemical calculations with organic synthesis and experimental spectroscopy to establish a computation-guided framework for understanding and exploring AIE materials. The team focused on low-lying conical intersections, which are critical regions of a molecule's excited-state potential energy surface.
Acting as "energy funnels," CIs allow excited molecules to rapidly return to the ground state without emitting light. Because the accessibility of these intersections strongly affects nonradiative excited-state deactivation, the researchers hypothesized that relatively simple structural modifications could control the excited-state potential energy surface and thereby determine whether a molecule exhibits AIE. To test this hypothesis, the researchers selected bridged stilbenes, a well-studied family of photoresponsive molecules, as a model system.
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