Aggregation-induced emission: phenomenon, mechanism and applications Yuning Hong, a Jacky W. Y. Lam a and Ben Zhong Tang* ab Received (in Cambridge, UK) 6th March 2009, Accepted 21st April 2009 First published as an Advance Article on the web 13th May 2009 DOI: 10.1039/b904665h It is textbook knowledge that chromophore aggregation generally quenches light emission. In this feature article, we give an account on how we observed an opposite phenomenon termed aggregation-induced emission (AIE) and identified the restriction of intramolecular rotation as a main cause for the AIE effect. Based on the mechanistic understanding, we developed a series of new fluorescent and phosphorescent AIE systems with emission colours covering the entire visible spectral region and luminescence quantum yields up to unity. We explored high-tech applications of the AIE luminogens as, for example, fluorescence sensors (for explosive, ion, pH, temperature, viscosity, pressure, etc.), biological probes (for protein, DNA, RNA, sugar, phospholipid, etc.), immunoassay markers, PAGE visualization agents, polarized light emitters, monitors for layer-by-layer assembly, reporters for micelle formation, multistimuli-responsive nanomaterials, and active layers in the fabrication of organic light-emitting diodes. Introduction About half a century ago, Fo¨ rster and Kasper discovered that the fluorescence of pyrene was weakened with an increase in its solution concentration. 1 It was soon recognized that this was a general phenomenon for many aromatic compounds. 2,3 This concentration-quenching effect was found to be caused by the formation of sandwich-shaped excimers and exciplexes aided by the collisional interactions between the aromatic molecules in the excited and ground states, which ‘‘are now known to be common to most aromatic hydrocarbons and their derivatives’’, as summarized by Birks in his classic book on Photophysics of Aromatic Molecules in 1970. 2 The ubiquitous concentration–quenching effect has forced researchers to study and utilize fluorophores as isolated single molecules in very dilute solutions. 4 The use of dilute solutions, however, causes many problems. For example, emissions from dilute solutions are often weak, leading to poor sensitivity in fluorescence sensory systems, especially in bioassays of trace amounts of biomolecules. 5 The sensitivity cannot be enhanced by using high fluorophore concentration due to the notorious concentration-quenching effect. The small numbers of the dye molecules in dilute solutions can be quickly photobleached when a harsh laser beam is used as the excitation light source. The development of inorganic quantum dots (QDs) can surmount these disadvantages but poses new problems, such as difficult synthesis, limited variety and high cytotoxicity. 6 Even in dilute solutions, concentration-quenching can still be involved. For example, in a bioassay system, the small fluorophore molecules may accumulate on the surfaces of the biomacromolecules and cluster in the hydrophobic cavities or pockets of the folding structures. 7 This can greatly increase local fluorophore concentration and cause the concentration- quenching problem. The concentration-quenching effect has been a thorny obstacle to the development of biosensor strips for on-site detection because the fluorophore concentration reaches its highest value in the solid state. 4,5 a Department of Chemistry, Nano Science and Technology Program, Bioengineering Graduate Program, The Hong Kong University of Science & Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong, China. E-mail: tangbenz@ust.hk b Department of Polymer Science & Engineering, Institute of Biomedical Macromolecules, Zhejiang University, Hangzhou 310027, China Yuning Hong Yuning Hong received her BSc degree in applied chemistry from Sun Yat-sen University in 2006. She is now a PhD student in the Tang group at HKUST. Her interest is mainly in the development of new AIE molecules and exploration of their biological applications. Jacky W. Y. Lam Jacky W. Y. Lam received his PhD degree from HKUST in 2003 under the supervision of Prof. Tang. In 2003–2007, he did his postdoctoral work on novel polymers with linear and hyperbranched structures and advanced functional properties in the Tang group. He is currently a research assistant professor of chemistry at HKUST. 4332 | Chem. Commun., 2009, 4332–4353 This journal is  c The Royal Society of Chemistry 2009 FEATURE ARTICLE www.rsc.org/chemcomm | ChemComm Published on 13 May 2009. Downloaded by HK University of Science and Technology on 26/08/2015 03:17:33. View Article Online / Journal Homepage / Table of Contents for this issue