Ju Mei,†,‡,∥ Nelson L. C. Leung,†,‡,∥ Ryan T. K. Kwok,†,‡ Jacky W. Y. Lam,†,‡ and Ben Zhong Tang*,†,‡,§ †HKUST-Shenzhen Research Institute, Hi-Tech Park, Nanshan, Shenzhen 518057, China ‡Department of Chemistry, HKUST Jockey Club Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, State Key Laboratory of Molecular Neuroscience, Division of Life Science, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China

Aggregation-Induced Emission: Together We Shine, United We Soar!

Luminogenic materials with aggregation-induced emission (AIE) attributes have attracted much interest since the debut of the AIE concept in 2001. In this critical review, recent progress in the area of AIE research is summarized. Typical examples of AIE systems are discussed, from which their structure–property relationships are derived. Through mechanistic decipherment of the photophysical processes, structural design strategies for generating new AIE luminogens are developed. Technological, especially optoelectronic and biological, applications of the AIE systems are exemplified to illustrate how the novel AIE effect can be utilized for high-tech innovations (183 references).

Aggregation-induced emission

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

The Halogen Bond

"United we stand, divided we fall."--Aesop. Aggregation-induced emission (AIE) refers to a photophysical phenomenon shown by a group of luminogenic materials that are non-emissive when they are dissolved in good solvents as molecules but become highly luminescent when they are clustered in poor solvents or solid state as aggregates. In this Review we summarize the recent progresses made in the area of AIE research. We conduct mechanistic analyses of the AIE processes, unify the restriction of intramolecular motions (RIM) as the main cause for the AIE effects, and derive RIM-based molecular engineering strategies for the design of new AIE luminogens (AIEgens). Typical examples of the newly developed AIEgens and their high-tech applications as optoelectronic materials, chemical sensors and biomedical probes are presented and discussed.

Aggregation-Induced Emission: The Whole Is More Brilliant than the Parts

Blue organic light-emitting diodes that harness thermally activated delayed fluorescence are realized with an external quantum efficiency of 19.5% and reduced roll-off at high luminance.

Efficient blue organic light-emitting diodes employing thermally activated delayed fluorescence

Phosphorescence is among the many functional features that, in practice, divide pure organic compounds from organometallics and inorganics. Considered to be practically non-phosphorescent, purely organic compounds (metal-free) are very rarely explored as emitters in phosphor applications, despite the emerging demand in this field. To defy this paradigm, we describe novel design principles to create purely organic materials demonstrating phosphorescence that can be turned on by incorporating halogen bonding into their crystals. By designing chromophores to contain triplet-producing aromatic aldehydes and triplet-promoting bromine, crystal-state halogen bonding can be made to direct the heavy atom effect to produce surprisingly efficient solid-state phosphorescence. When this chromophore is diluted into the crystal of a bi-halogenated, non-carbonyl analogue, ambient phosphorescent quantum yields reach 55%. Here, using this design, a series of pure organic phosphors are colour-tuned to emit blue, green, yellow and orange. From this initial discovery, a directed heavy atom design principle is demonstrated that will allow for the development of bright and practical purely organic phosphors.

Activating efficient phosphorescence from purely organic materials by crystal design

Potassium is an important cation in biology, and quantitative detection of the extracellular potassium level is important. However, selective detection of extracellular physiological potassium is a challenging task due to the presence of sodium in a much higher concentration. In this contribution, we describe the development of practical polydiacetylene (PDA) liposome-based microarrays to selectively detect potassium even in the presence of sodium. We utilize the fact that the G-rich ssDNA can fold into a G-quadruplex via intramolecular hydrogen bonding by wrapping around a potassium ion exclusively. We rationally design the PDA liposome in such a way that the G-rich ssDNA probes are presented densely at the liposome surface and form bulky quadruplexes upon binding with K+. The resulting bulky quadruplexes are sterically hindered and repulse each other and impose mechanical stress on the PDA backbone, resulting in the conformational change of the ene-yne backbone of the PDA. As a result, polydiacetylene liposomes turn into the emissive red phase from the nonfluorescent blue phase.

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Polydiacetylene Liposome Arrays for Selective Potassium Detection

We investigated the effectiveness of p-dopants to generate holes in a hole transporting material by comparing the absorption in visible-near-infrared and infrared regions and current density-voltage characteristics. CuI, MoO3, and ReO3 having different work functions were doped in a hole transporting organic material, 4,4′,4″-tris(N-(2-naphthyl)-N-phenylamino)-triphenylamine (2TNATA). Formation of charge transfer (CT) complexes increases linearly with increasing doping concentration for all the dopants. Dopants with higher work function (ReO3>MoO3>CuI) are more effective in the formation of CT complexes and in the generation of the charges in the doped films.

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Effectiveness of p-dopants in an organic hole transporting material

Immunofluorescence labeling is a very important analytical technique for probing the structure of living cells. Such labeling is conventionally carried out by using fluorescent organic dyes. However, some drawbacks including a limited range of light emission and particularly poor photo-stability have limited the usage of organic dyes. After the introduction of semiconducting quantum dots many research groups have investigated inorganic quantum dots for immunofluorescence labeling due to their high quantum yield, high molar extinction coefficients, broad absorption with narrow light emission, and good photo physical and chemical stability. Despite the promising properties of semiconducting quantum dots for immunofluorescence labeling, cyto-toxicity is a critical problem in any live-cell or animal experiments. Alternative choices are dye-loaded latex particles and dye-doped silica colloids having improved photostability compared to conventional dye molecules. However, dye-loaded beads also have a critical limit of brightness due to self-quenching when high density of dyes present at the nanoparticle surface. In this context, developing highly emissive, biocompatible, and chemically readily modifiable luminescent materials is strongly desired. Recently, a few papers have been published reporting conjugated organic small molecules that showed higher quantum yield in the solid state than in solution. In other words, self-quenching does not exist in these unique molecules. It has opened a new possibility to develop non-toxic organic nanoparticles with enhanced emission for various applications including immunofluorescence labeling. The reason for the lower quantumyield in solution of someof thesemolecules is believed that the conjugated backbone of the molecules is significantly twisted by steric hindrance and the radiative decay pathway of the resulting twisted chromophores is generally suppressive. Therefore, planarization of the twisted conjugated backbone upon aggregation in the solid state accordingly enhances the emissive property of the molecules. On the other hand, another class ofmolecules that has a larger quantumyield in the solid state has a different reason for the enhanced emissive property. Thesemolecules in solution have a lowquantumyield because they dissipate energy for molecular rotation. Therefore, they have even 20%higher quantumyield in the solid state because the molecular rotation is hindered by adjacent molecules in the solid state. We have developed highly emissive organic nanoparticles by using colloidal self-assembly of a hydroxyphenyl-benzoxazole (HBO) derivative and diacetylene monomers. Various heterocyclic molecules including HBO have been investigated by many research groups due to their chemical and thermal stability as well as their high electron mobility. HBO has a high extinction coefficient and excellent stability in the UV range and has shown potential as a UV stabilizer. HBO is also known to undergo excited state intramolecular proton transfer (ESIPT) upon photo-excitation as illustrated in Figure 1A. We synthesized 1,4-di(3-(benzoxazol-2-yl)-4-hydroxyphenyl)2,5-dihexyloxybenzene (DBO) as shown in Figure 1B. The absorption and photoluminescence spectra of DBO are shown in Figure 2C. Dilute solution of DBO in tetrahydrofuran (THF) has absorption lmax at 335 nm assigned to syn-enol and emits at 518 nm, showing a large Stokes shift due to ESIPT. A nanoparticle dispersion ofDBOwas prepared by adding THF solution of DBO to water to induce aggregate formation. The prepared nanoparticles in themixed solution ofTHF:H2O (1:9 v/v) showa broad aggregationbandabove 400nmdue toMie scattering (Fig. 2C). Interestingly their fluorescence quantum yield (FF) was 10%,which ismore than 3 times larger than the quantumyield of the THF solution (3%). The enhanced fluorescence emission intensity in the aggregates is believed to originate from a more planar structure induced by J-type aggregation through [*] Prof. J. Kim, Dr. H.-J. Kim, T.-H. Kim Department of Materials Science and Engineering University of Michigan Ann Arbor, MI 48109 (USA) E-mail: jinsang@umich.edu

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Highly Emissive Self‐assembled Organic Nanoparticles having Dual Color Capacity for Targeted Immunofluorescence Labeling

We present a newly devised technique, the dynamic layer-by-layer (LbL) deposition method, that is designed to take advantage of the LbL deposition method and fluidic devices. Polyelectrolyte solutions are sequentially injected through the fluidic LbL deposition device to quickly build well-defined multilayer films on a selected region with a linear increase in the material deposited. Multilayer film fabrication by this new method on a specific region was proven to be fast and effective. The effects on film quality of the processing parameters such as concentration of polyelectrolytes, flow rate, and contact time were investigated. A half-tethered self-standing film on a substrate was fabricated to demonstrate the effectiveness and the region-selective deposition capability of the devised dynamic LbL deposition method.

Hyong-Jun Kim

Papers

Activating efficient phosphorescence from purely organic materials by crystal design

Polydiacetylene Liposome Arrays for Selective Potassium Detection

Effectiveness of p-dopants in an organic hole transporting material

Highly Emissive Self‐assembled Organic Nanoparticles having Dual Color Capacity for Targeted Immunofluorescence Labeling

Dynamic sequential layer-by-layer deposition method for fast and region-selective multilayer thin film fabrication.