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 field of chemical sensing is becoming ever more dependent upon novel materials. Polymers, crystals, glasses, particles, and nanostructures have made a profound impact and have endowed modern sensory systems with superior performance. Electronic polymers have emerged as one of the most important classes of transduction materials; they readily transform a chemical signal into an easily measured electrical or optical event. Although our group reviewed this field in 2000,1 the high levels of activity and the impact of these methods now justify a subsequent review as part of this special issue. In this review we restrict our discussions to purely fluorescence-based methods using conjugated polymers (CPs). We further confine our detailed coverage to articles published since our previous review and will only detail earlier research in this Introduction to illustrate fundamental concepts and terminology that underpin the recent literature.

Chemical sensors based on amplifying fluorescent conjugated polymers.

Aggregation-induced emission: phenomenon, mechanism and applications.

"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

Tuning the Properties of Conjugated Polyelectrolytes through Surfactant Complexation

We report the observation of a “superquenching” for water-soluble poly-l-lysine derivatives containing an appended cationic cyanine dye on each repeat unit. The formally nonconjugated cyanine dye chromophores strongly associate in a “J” aggregate structure characterized by a sharp red-shifted absorption (compared to the monomer) and a similarly sharp red-shifted fluorescence. Superquenching is manifested by very large “Stern−Volmer” constants for fluorescence quenching by oppositely charged electron acceptors or energy transfer dyes; substantial quenching is observed at levels of quencher corresponding to one to four molecules per polymer chain. The quenching observed for these polymers is equivalent or greater to that previously observed for conjugated polyelectrolytes. We have been able to exploit the superquenching of the “J” aggregate polymer fluorescence in a competitive bioassay.

Superquenching and Its Applications in J-Aggregated Cyanine Polymers

Repeated bilayers of a polycation and a polyanion were assembled by adsorption from their aqueous solutions. The polycations used were PEI (poly(ethylenimine)), PAH (poly(allylamine hydrochloride)), and generation 3.0 dendrimer (poly(propylenimine)). The polyanion was PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt]). Second harmonic generation (SHG) and UV−vis spectroscopy were used to study the structure and formation of the superlattices. The linear increase in absorbance versus the number of bilayers suggested that a uniform and consistent amount of polymer adsorbed during each deposition cycle. The observation of a second harmonic (SH) signal from the film showed that the conjugated chromophores in the PAZO polymer were aligned with a net dipole. However, the SHG signal did not continue to increase as the number of bilayers was increased. On the basis of thorough studies of the UV and SH properties for each bilayer for 10 total bilayers, we propose that t...

Reversal of Interfacial Dipole Orientation in Polyelectrolyte Superlattices Due to Polycationic Layers

Driven by weak intermolecular interactions between the complementary H-bonds, melamine derivatives and barbituric acids spontaneously self-assemble into the supramolecular ribbon shown. Surprisingly, the supramolecular ribbons stack into a polar, asymmetric (head-to-tail) multilayer structure during film formation. The structure−property relationship of these thick films was investigated using X-ray diffraction and several optical techniques.

and Duncan McBranch

Papers

Tuning the Properties of Conjugated Polyelectrolytes through Surfactant Complexation

Superquenching and Its Applications in J-Aggregated Cyanine Polymers

Reversal of Interfacial Dipole Orientation in Polyelectrolyte Superlattices Due to Polycationic Layers

Spontaneously self-assembled polar asymmetric multilayers formed by complementary H-bonds