Aggregation-Induced Emission: A New Research Frontier.

Abstract

Development of luminescent materials is of critical importance and has far-reaching practical implications. A great number of luminophores have been developed and found to be highly luminescent in dilute solutions. Their emissions, however, are weakened or even completely quenched upon aggregate formation. Although various approaches have been taken to minimize the aggregation-caused quenching effect, only limited success has been realized. The development of a luminogenic system in which the aggregation plays a constructive rather than destructive role in the light emission process will bring forth a revolution both conceptually and technically. Molecular rotors have long existed in nature, but they could never have become so popular and valuable without Ben Zhong Tang. Fifteen years ago, Tang coined the concept of “Aggregation-induced Emission (AIE)”, which stands for an intriguing process in which non-emissive isolated mole­cules are induced to emit strongly by aggregate formation. This discovery is of great scientific value as a new theorem needs to be established to understand such an abnormal phenomenon, while the new concept of AIE changes the way people think of the roles of aggregation in the light emission processes of luminogenic molecules. The area of AIE research is advancing rapidly, as evidenced by the fact that hundreds of laboratories over fifty countries are now performing AIE studies, and the exponential increase in the numbers of publications and citations (e.g., 6600 in 2013, 11 100 in 2014 and over 24 000 in 2015) on this theme. AIE has been ranked the 2nd hot research fronts in the fields of chemistry and materials science by Thomson Reuters in 2015. In recognizing the increasing importance of AIE, a number of international conferences have been held in recent years and attracted thousands of scientists from different disciplines to participate. A community of AIE has formed, which is expanding exponentially over the years! The AIE field has grown tremendously over the past fifteen years, showing increasing impact on energy, optoelectronics, environment, and life sciences. We are very delighted that some of the best regarded names in the research community have decided to join our effort, leading to the publication of a collection of exciting AIE research including five review articles, one concept paper, three communications, and nine full papers. The research findings reported in this Special Issue cover cutting-edge advances in molecular design and working mechanism as well as nanotechnology platforms for sensing, imaging, optoelectronics, theranostics, etc. These articles also reveal the evolution of AIE materials over the years from molecules to polymers to supramolecules to organic-inorganic hybrids, and to unconventional nonconjugated nanoclusters. Along with the advancement of materials, the working mechanism is also broadened from the initial restriction of intramolecular rotation (RIR) to the restriction of intra­molecular motion (RIM) by taking into consideration of the restriction of intramolecular vibration (RIV). These AIE materials have been made into highly emissive films, which show efficient fluorescence and circularly polarized luminescence for applications in light-emitting devices. They are also formulated into organic nanoparticles for sensing of small molecules (such as ions and explosives), and big poly­mers (such as DNA and β-amyloids) and visualization of important chemical and biological processes (such as organic-inorganic hybridization and stem cell homing). Of particular interest is the exploration of the utility of multi-functional AIE mole­cules in the area of theranostics, which represents a new research direction of the AIE materials for biomedical applications. The development of photosensitizers with simultaneously high luminescence efficiencies and singlet-oxygen yields in the aggregate state is very difficult to achieve with traditional luminophores but can be readily accomplished with AIE luminogens. The unique examples of single AIE molecules in multi-modality imaging and image-guided therapy and surgery also distinguish them from existing fluoro­phores. Altogether, we hope these works will stimulate enthusiasm and effort in the creation of innovative materials and functions that can address the key challenges faced in the areas of energy, environmental and biomedical research, such as improvements in energy saving and environmental protection as well as innovations in diagnostics and therapy. In closing this Editorial, we would like to sincerely thank the editorial team of Small, especially Drs. José Oliveira and Jipei Yuan, who have contributed and helped us to identify excellent papers for this Special Issue. It is our great honor to work with them to present the excellence in AIE to the scientific research community. We would also like to express gratitude to our colleagues who have chosen to participate in this Special Issue and present their cutting-edge research findings on AIE, from fundamentals to applications. We hope the work presented in this issue will inspire researchers from across a wide range of scientific disciplines to continue developing AIE platforms that will make an impact on our everyday life. Bin Liu received a BS degree from Nanjing University and Ph.D. degree from the National University of Singapore (NUS) before her postdoctoral training at the University of California, Santa Barbara. She joined the Chemical and Biomolecular Engineering Department of NUS in late 2005, and was named Dean's Chair Professor in 2014. Her current research focuses on organic nanomaterials for biomedical and energy applications.