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Catalyst: Aggregation-Induced Emission—How Far Have We Come, and Where Are We Going Next?

2020; Elsevier BV; Volume: 6; Issue: 6 Linguagem: Inglês

10.1016/j.chempr.2020.05.018

2451-9308

Kenry Kenry, Ben Zhong Tang, Bin Liu,

Carbon and Quantum Dots Applications

Kenry was a research fellow at the National University of Singapore (NUS), where he obtained his PhD. His research focuses on functional nanomaterials for theranostics and nanomedicine. He is currently a research fellow at the Dana-Farber Cancer Institute and Harvard Medical School.Ben Zhong Tang received his BS and PhD degrees from South China University of Technology and Kyoto University, respectively. He conducted postdoctoral study at the University of Toronto. He joined the Hong Kong University of Science and Technology in 1994 and was promoted to chair professor in 2008. His research interests include macromolecular chemistry, materials science, and theranostics.Bin Liu is currently the Provost’s Chair Professor and head of the NUS Department of Chemical and Biomolecular Engineering. She received her PhD from the same institution and did her postdoctoral work at the University of California, Santa Barbara. Her research focuses on organic functional nanomaterials for energy and biomedical applications. Kenry was a research fellow at the National University of Singapore (NUS), where he obtained his PhD. His research focuses on functional nanomaterials for theranostics and nanomedicine. He is currently a research fellow at the Dana-Farber Cancer Institute and Harvard Medical School. Ben Zhong Tang received his BS and PhD degrees from South China University of Technology and Kyoto University, respectively. He conducted postdoctoral study at the University of Toronto. He joined the Hong Kong University of Science and Technology in 1994 and was promoted to chair professor in 2008. His research interests include macromolecular chemistry, materials science, and theranostics. Bin Liu is currently the Provost’s Chair Professor and head of the NUS Department of Chemical and Biomolecular Engineering. She received her PhD from the same institution and did her postdoctoral work at the University of California, Santa Barbara. Her research focuses on organic functional nanomaterials for energy and biomedical applications. The concept of aggregation-induced emission (AIE) was coined in 2001, when a non-emissive solution of 1-methyl-1,2,3,4,5-pentaphenylsilole turned on its fluorescence after drying on a thin-layer chromatography plate.1Luo J. Xie Z. Lam J.W. Cheng L. Chen H. Qiu C. Kwok H.S. Zhan X. Liu Y. Zhu D. Tang B.Z. Aggregation-induced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole.Chem. Commun. 2001; 2001: 1740-1741Crossref Scopus (5766) Google Scholar This peculiar effect ran counterintuitive to the more established concept of aggregation-caused quenching (ACQ), where the fluorescence of organic fluorogens has been generally known to be quenched in the solid state. In fact, molecular aggregation has long been regarded as detrimental to solid-state light emission. The suppression of aggregate formation to minimize the ACQ effect has, therefore, been the holy grail in the molecular design of luminogens. The emergence of the intriguing AIE effect has rendered molecular aggregation beneficial to light emission, which has flourished since then to guide various important developments in luminogen research. Here, we discuss the progression of AIE research in the last 20 years and highlight several key milestones that have shaped AIE research (Figure 1). In general, these milestones can be categorized into three progressive stages: (1) the emergence of the AIE concept, (2) the identification of potential AIE applications and solidification of the AIE concept, and (3) the expansion of AIE research to more practical technological applications. We will also discuss the challenges in the field and propose viable approaches to overcoming them, as well as provide our perspectives on several exciting potential directions that could mold the AIE research landscape in the years to come. The excavation of the AIE phenomenon has challenged the common belief that luminogen aggregation is harmful to light emission. Instead of fighting against molecular aggregation, AIE enables one to utilize the aggregation process to boost light emission. As opposed to conventional ACQ molecules, luminogens with the AIE characteristic (AIEgens) are weakly emissive or almost non-emissive when they are dissolved as isolated molecular species, but their luminescence is strongly enhanced when they are aggregated.2Mei J. Leung N.L.C. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (5198) Google Scholar With the formulation of more AIEgens over the years, systematic studies have been simultaneously carried out to decipher and understand the operating mechanisms of AIE processes in order to validate and further establish the AIE concept. Given the distinct propeller- or rotor-shaped structures possessed by most AIEgens, the initial works have attributed the occurrence of AIE to the restriction of intramolecular rotations upon aggregate formation,2Mei J. Leung N.L.C. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (5198) Google Scholar where the intramolecular rotations dissipate the exciton energy to quench the molecular fluorescence of AIEgens. Apart from the restriction of intramolecular rotation mechanism, other hypotheses, including conformational planarization, E/Z isomerization, and J-aggregation, have been proposed at this stage as the action mechanisms of the AIE effect, although they have been proved to be valid only for some specific cases. This initial discovery stage, which primarily revolves around understanding the fundamentals of the AIE phenomenon, has laid the groundwork for navigating the next phase of AIE research in exploring the possibilities and potential of AIE applications. As time passes by, more advanced models have been proposed for understanding AIE, and these will be discussed later. Along with the quest to unravel the working mechanisms underlying the AIE effect, concurrent efforts were launched to identify the potential utilities of AIE.2Mei J. Leung N.L.C. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (5198) Google Scholar The first notable event in this exploration phase was the expansion of the AIE concept into polymer science in 2003 with the first report of silole-containing polyacetylenes exhibiting the AIE attribute. This was quickly followed by the expansion of AIE research into the biological sciences in 2004 with the first report of the demonstration of silole nanocrystals as biological labels for immunological assay. The use of silole aggregates with the AIE characteristic has also led to amorphization-driven emission color changes, which have potential for applications in optoelectronics. The formulation of tetraphenylethene (TPE) and its introduction into the AIEgen family in 2006 further widened the scope of AIE study.3Tong H. Hong Y. Dong Y. Häussler M. Lam J.W. Li Z. Guo Z. Guo Z. Tang B.Z. Fluorescent “light-up” bioprobes based on tetraphenylethylene derivatives with aggregation-induced emission characteristics.Chem. Commun. 2006; 2006: 3705-3707Crossref Google Scholar TPE and its derivatives have become arguably the most popular AIEgens because of their ready synthetic accessibility and great structural tunability. In fact, numerous advancements made in the second decade of AIE research have capitalized largely on TPE and its derivatives; in 2010, one study showed that classic ACQ molecules, such as anthracene and pyrene, could be endowed with the AIE characteristic by simply being decorated with single or multiple TPE units. This straightforward approach readily transforms traditional ACQ luminogens into their AIE counterparts and significantly expands the library of AIEgens. As the concept of AIE was gradually validated and the directions of AIE research became more clearly defined, great advancements in luminogen research were made simultaneously. Thanks to the AIE concept, novel intriguing photophysical phenomena—notably crystallization-induced emission (CIE), aggregation-induced phosphorescence, multiphoton-excited AIE, pressurization-enhanced emission or piezo-AIE, CIE-based pure organic room-temperature phosphorescence, and AIE mechanoluminescence and mechanochromism, among many others—have been successfully realized.2Mei J. Leung N.L.C. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (5198) Google Scholar,4Kenry Chen C. Liu B. Enhancing the performance of pure organic room-temperature phosphorescent luminophores.Nat. Commun. 2019; 10: 2111Crossref PubMed Scopus (310) Google Scholar,5Xu S. Duan Y. Liu B. Precise molecular design for high-performance luminogens with aggregation-induced emission.Adv. Mater. 2020; 32: e1903530Crossref PubMed Scopus (219) Google Scholar Driven by these exciting breakthroughs, AIE research progressively garnered more attention and involvement from the international research community toward the end of the first decade, signifying the increasing importance of AIE in multidisciplinary research. The last decade has seen the expansion of AIE research beyond fundamental studies toward more practical real-world applications.2Mei J. Leung N.L.C. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (5198) Google Scholar It is also worth highlighting that the second decade of AIE research has witnessed the internationalization of AIE research beyond China and Asia such that it is now being increasingly accepted within the European and American research communities. During this stage, there has been a clear shift toward more sophisticated AIE-based platforms, such as AIE dots, AIE light-up probes, AIE assemblies, AIE metal-organic frameworks, and AIE metallocages.2Mei J. Leung N.L.C. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (5198) Google Scholar,6Feng G. Liu B. Aggregation-induced emission (AIE) dots: emerging theranostic nanolights.Acc. Chem. Res. 2018; 51: 1404-1414Crossref PubMed Scopus (403) Google Scholar,7Kenry Chong K.C. Liu B. Reactivity-based organic theranostic bioprobes.Acc. Chem. Res. 2019; 52: 3051-3063Crossref PubMed Scopus (44) Google Scholar Concurrent with the creation of new AIE structures, mechanistic studies were performed to gain deeper insights into AIE processes, resulting in the introduction of the more general mechanism of restriction of intramolecular motions, which encompasses the restriction of both intramolecular rotations and vibrations.2Mei J. Leung N.L.C. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (5198) Google Scholar In addition, newer mechanisms, notably the suppression of Kasha’s rule and photochemical structural changes,5Xu S. Duan Y. Liu B. Precise molecular design for high-performance luminogens with aggregation-induced emission.Adv. Mater. 2020; 32: e1903530Crossref PubMed Scopus (219) Google Scholar,8Qian H. Cousins M.E. Horak E.H. Wakefield A. Liptak M.D. Aprahamian I. Suppression of Kasha’s rule as a mechanism for fluorescent molecular rotors and aggregation-induced emission.Nat. Chem. 2017; 9: 83-87Crossref PubMed Scopus (274) Google Scholar have further propelled the rational design of AIEgens beyond those with simple twisted π systems, enabling AIEgens with double-bond rotation, plane distortion, and even cluster distortion.5Xu S. Duan Y. Liu B. Precise molecular design for high-performance luminogens with aggregation-induced emission.Adv. Mater. 2020; 32: e1903530Crossref PubMed Scopus (219) Google Scholar Consequently, emerging AIE concepts, such as clusteroluminescence, anion-π+ interaction, through-space conjugation, achirality-helicity transformation, circularly polarized luminescence, and solid-state molecular motion,2Mei J. Leung N.L.C. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (5198) Google Scholar,5Xu S. Duan Y. Liu B. Precise molecular design for high-performance luminogens with aggregation-induced emission.Adv. Mater. 2020; 32: e1903530Crossref PubMed Scopus (219) Google Scholar,9He Z. Ke C. Tang B.Z. Journey of aggregation-induced emission research.ACS Omega. 2018; 3: 3267-3277Crossref PubMed Scopus (191) Google Scholar,10Zhang H. Sun J.Z. Liu J. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Visualizing and monitoring interface structures and dynamics by luminogens with aggregation-induced emission.J. Appl. Phys. 2019; 126: 050901Crossref Scopus (19) Google Scholar have collectively fueled increasing explorations of AIEgen applications in optoelectronic-device fabrication, environmental monitoring, biological-process monitoring, and preclinical diagnosis and therapy.2Mei J. Leung N.L.C. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (5198) Google Scholar,7Kenry Chong K.C. Liu B. Reactivity-based organic theranostic bioprobes.Acc. Chem. Res. 2019; 52: 3051-3063Crossref PubMed Scopus (44) Google Scholar,10Zhang H. Sun J.Z. Liu J. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Visualizing and monitoring interface structures and dynamics by luminogens with aggregation-induced emission.J. Appl. Phys. 2019; 126: 050901Crossref Scopus (19) Google Scholar In the field of optoelectronics, numerous AIEgens with quantum efficiency up to unity and emission spanning the whole visible and near-infrared regions have been formulated for organic light-emitting diodes.2Mei J. Leung N.L.C. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (5198) Google Scholar AIEgens with delayed fluorescence have demonstrated great potential for the construction of white-light-emitting optoelectronics. Similarly, AIEgen-based liquid crystals have been designed to be highly luminescent, enabling the fabrication of light-emitting liquid-crystal displays (LCDs) with simplified structures without the need for backlights, unlike the current LCDs.2Mei J. Leung N.L.C. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (5198) Google Scholar In addition, 3D displays could benefit from the latest availability of AIE-active circularly polarized electroluminescent materials. The recent realization of through-space conjugated AIE systems with blue thermally activated delayed fluorescence is anticipated to further push the boundary of optoelectronics.9He Z. Ke C. Tang B.Z. Journey of aggregation-induced emission research.ACS Omega. 2018; 3: 3267-3277Crossref PubMed Scopus (191) Google Scholar In environmental-monitoring applications, a series of AIE-based chemosensors has been constructed for highly selective and fast detection of harmful metal ions.2Mei J. Leung N.L.C. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (5198) Google Scholar Simultaneously, AIE probes have been developed for detecting environmental pollutants and hazardous gases, as well as environmentally damaging bacteria and toxins in real time with visible signals.2Mei J. Leung N.L.C. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (5198) Google Scholar To facilitate their practical and on-field utilizations, researchers have been gradually integrating AIE probes with portable devices, such as paper strips and smartphones, for real-time sensing. One of the most exciting applications of AIEgens is for in situ monitoring of physical processes and interfaces,10Zhang H. Sun J.Z. Liu J. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Visualizing and monitoring interface structures and dynamics by luminogens with aggregation-induced emission.J. Appl. Phys. 2019; 126: 050901Crossref Scopus (19) Google Scholar and accomplishing this is difficult via traditional methods. AIEgens have been prepared for observing the structures and dynamics of interfaces between different phases, notably gas-solid (e.g., vapor diffusion), liquid-solid (e.g., nanoparticle formation and freezing), solid-solid (e.g., polymer microphase separation), and gas-liquid-solid (e.g., evaporation and interfacial dynamic self-assembly) interfaces.10Zhang H. Sun J.Z. Liu J. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Visualizing and monitoring interface structures and dynamics by luminogens with aggregation-induced emission.J. Appl. Phys. 2019; 126: 050901Crossref Scopus (19) Google Scholar The latest inclusion of prochiral AIEgens with circularly polarized luminescence for interface visualization is expected to further improve the accuracy of the monitoring process.10Zhang H. Sun J.Z. Liu J. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Visualizing and monitoring interface structures and dynamics by luminogens with aggregation-induced emission.J. Appl. Phys. 2019; 126: 050901Crossref Scopus (19) Google Scholar Of all the potential AIE applications unearthed in the second decade of progression, perhaps the most practical advancement was made in the biomedical field, specifically in the monitoring of biological processes and preclinical diagnosis and therapy. The invention of AIE dots and AIE light-up probes in 2012 has greatly accelerated the implementation of AIEgens in the life sciences and nanomedicine.6Feng G. Liu B. Aggregation-induced emission (AIE) dots: emerging theranostic nanolights.Acc. Chem. Res. 2018; 51: 1404-1414Crossref PubMed Scopus (403) Google Scholar,7Kenry Chong K.C. Liu B. Reactivity-based organic theranostic bioprobes.Acc. Chem. Res. 2019; 52: 3051-3063Crossref PubMed Scopus (44) Google Scholar Whereas AIE light-up probes have been successfully used for continuous monitoring of biological processes in vitro and in vivo, AIE dots have been formulated for targeted cellular and subcellular imaging, vascular imaging, disease diagnosis and cancer grading, and image-guided surgery and therapy.6Feng G. Liu B. Aggregation-induced emission (AIE) dots: emerging theranostic nanolights.Acc. Chem. Res. 2018; 51: 1404-1414Crossref PubMed Scopus (403) Google Scholar Recent efforts have been geared toward endowing basic AIEgens with therapeutic functionalities to yield AIE photosensitizers and photothermal agents, which have been used in the development of “one-for-all” AIE theranostic systems.7Kenry Chong K.C. Liu B. Reactivity-based organic theranostic bioprobes.Acc. Chem. Res. 2019; 52: 3051-3063Crossref PubMed Scopus (44) Google Scholar These systems are capable of multimodal imaging and phototherapy and have been effectively applied for image-guided cancer therapy and bacterial ablation. Moreover, utilizing anion-π+ interactions to prepare inherently charged AIE bioprobes as well as solid-state molecular motion to formulate AIEgens for photoacoustic imaging and photothermal therapy has further underlined the wide applicability of AIEgens in the biomedical field. Altogether, it is evident that the second decade of AIE research has been fruitful, and the various advancements made in this period have paved the way for the maturation of AIE-based technologies. Representing one of the very few successful strategies to overcome the ACQ problem with far-reaching practical implications, the AIE effect has generated tremendous interest over the last few years. This is evident from the many breakthroughs in functional materials and potential applications of AIE. However, AIE research also faces challenges, including (1) short wavelength absorption due to the rotor-shaped structures, (2) broad emission due to the presence of multiple emission species, (3) molecular-packing-dependent brightness, (4) sub-optimal emission and photothermal effect due to random solid-state molecular motion, and (5) difficulty in making turn-on sensing strips because of the always-on feature of AIEgens in the solid state.5Xu S. Duan Y. Liu B. Precise molecular design for high-performance luminogens with aggregation-induced emission.Adv. Mater. 2020; 32: e1903530Crossref PubMed Scopus (219) Google Scholar These challenges might not necessarily be problems, but they need to be dealt with judiciously. Rational strategies can be taken to (1) enable long-wavelength excitations through the design of donor-acceptor structures or utilization of non-linear optics effects (e.g., multiphoton absorption and upconversion processes), (2) narrow emission bandwidths by taking advantage of Förster resonance energy transfer or photonic crystals, (3) enhance luminescence brightness through nanocrystallization, (4) optimize emission and photothermal effect by regulating solid-state movement, and (5) turn on solid-state emission by blocking the initially incorporated quenching processes, such as photoinduced electron transfer and twisted intramolecular charge transfer.5Xu S. Duan Y. Liu B. Precise molecular design for high-performance luminogens with aggregation-induced emission.Adv. Mater. 2020; 32: e1903530Crossref PubMed Scopus (219) Google Scholar Apart from addressing the challenges discussed above, it is also worth pursuing other potential directions. These include but are not limited to (1) the rational structural design (e.g., modeling-guided and machine-learning- or artificial-intelligence-assisted designs) and synthesis of new multifaceted AIEgens (e.g., non-conjugated and natural AIE systems, stimuli-responsive AIEgens, and AIEgens with room-temperature phosphorescence and triboluminescence), (2) the comprehensive elucidation of their fundamental aspects (e.g., action mechanisms driving new AIEgen-based structures, their structure-property relationships, the effects of molecular packing and polymorphism, and bioactivity), and (3) the identification of their niche and dominant applications in energy, environmental, and biomedical fields, which are superior to the existing systems. With these vast possibilities, the next decade of AIE research is anticipated to witness more exciting discoveries and provide solutions to the currently unmet scientific and technological needs in the areas outlined above and beyond. Audaces fortuna iuvat: let’s embrace the great opportunities and explore new territories in the area of AIE research. The authors would like to thank the International Collaboration Program of the National Natural Science Foundation of China (51620105009), Singapore National Research Foundation (NRF) Competitive Research Program (R279-000-483-281), NRF Investigatorship (R279-000-444-281), and National University of Singapore (R279-000-482-133) for financial support. Reaction: The Impact of the AIE ConceptGu et al.ChemJune 11, 2020In BriefIn the past 20 years, aggregation-induced emission (AIE) research has developed rapidly and obtained vast achievements in the energy, environmental, and biomedical fields, which is attributed to the fact that the simple but useful AIE concept provides a totally new design rationale for the exploration of advanced functional materials. Gu and Zhang further emphasize the significance of the AIE concept in the development of AIEgens as probes and optical agents for life science and biomedical applications. Full-Text PDF Open Archive