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Introduction Luminescent materials are everywhere in our daily lives, from display devices, to biomedical research, personal safety, and solid lighting. They make our life colorful and exciting. Despite the fact that many luminescent materials are very emissive in dilute solutions, their luminescence is weakened or even completely quenched at higher concentrations, partially due to aggregate formation. As common luminescent materials naturally aggregate in the solid state and in aqueous media, the phenomenon of aggregationcaused quenching (ACQ) is a problem in many applications, especially in light-emitting devices and biomedical research. Over the past few decades, tremendous efforts have been made to solve the ACQ problem, with limited success. Due to their large aromatic rings and planar structures, most fluorophores naturally aggregate in the solid state and in aqueous media, but researchers are trying their best to separate them.1 One of the most straightforward solutions to address the ACQ problem was the invention of luminophores with aggregation-induced emission fluorogens (AIEgens). Tang et al. first coined the concept of aggregation-induced emission (AIE) in 2001.2 AIE molecules generally have rotor structures, and one of the most common examples is tetraphenylethylene (TPE [Figure 1]).3 When it is dissolved in a good solvent such as tetrahydrofuran, there is barely

any fluorescence due to the free motions of the molecular rotors, which deactivate the radiative decay channel. Upon aggregate formation caused by the addition of water to the solution, the restriction of the molecular motions lead to bright fluorescence in an aggregate or thin-film state. Detailed studies over the past decade reveal that the mechanism underlying the AIE phenomenon is the restriction of intramolecular motions in the aggregated state.3 The rotor structures also prevent the molecules from approaching each other closely, which results in fluorescence quenching. Although the restriction of intramolecular motion has been used as a primary mechanism to explain the AIE phenomenon, several complementary theories and mechanisms, such as the suppression of Kasha’s rule, have also been proposed.4 Over the past 15 years, we and many other research groups have designed and synthesized a series of AIEgens. Almost all of them show very low luminescence as molecular species, but quantum yields of up to 100% (all absorbed photons emitted as light) have been obtained in the film state.5 Due to their unique features, AIEgens have been employed in a wide range of research applications, including smart materials that are responsive to external stimuli, active materials for energy devices, and tools to monitor biological or physical self-assembly processes. They have proven useful in environmental engineering, such as for water and food quality

Bin Liu, Chemical and Biomolecular Engineering Department, National University of Singapore, Singapore; [email protected] doi:10.1557/mrs.2017.93

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• VOLUME 42 • JUNE 2017Columbia • www.mrs.org/bulletin ©use,