Mechanistic studies promote scientific development from phenomena to theories. Aggregation-induced emission(AIE), as an unusual photophysical phenomenon, builds the bridge between molecular science and aggregate mesoscience. With the twenty-year development of AIE, restriction of intramolecular motion(RIM) has been verified as the working mechanism of AIE effect. In this review, these mechanistic works about RIM are summarized from experimental and theoretical perspectives. Thereinto, the experimental studies are introduced from three parts: external rigidification, structural modification and structural characterization. In the theoretical part, calculations on the low-frequency motion of AIEgens have been performed to prove the RIM mechanism. By virtue of the theoretical calculations, some new mechanisms are proposed to supplement the RIM, such as restriction of access to conical intersection, suppression of Kasha transition, restriction of access to dark state, etc. It is foreseeable that the RIM mechanism will unify the photophysical theories for both molecules and aggregates, and inspire more progress in aggregate science.

Aggregation-induced emission(AIE) has emerged as a new concept, giving highly efficient solid-state photoluminescence. Particularly, AIE luminogens(AIEgens) with deep blue emission(400–450 nm) have displayed salient advantages for non-doped organic light-emitting diodes(OLEDs). However, deep blue emitters with Commission Internationale de L’Eclairage(CIE) coordinates less than 0.08 are still rare. In this review, we outline the latest achievements in the molecular guidelines based on the AIE core of tetraphenylbenzene(TPB) for developing efficient deep blue AIEgens. We provide insights into the construction of deep blue emitters with high horizontal orientation by regulating the length of the linear molecule. We also discuss the luminescence mechanisms of these AIEgens-based OLEDs by using the magnetic field effects measurements. Finally, a summary of the challenges and perspectives of deep blue AIEgens for non-doped OLEDs is also presented.

The modern medicine requires precise diagnostic techniques while the fluorescent imaging shows great potential in such applications due to its excellent sensitivity and high resolution. However, conducting fluorescent imaging in deep-tissue is not so easy because most luminogens show short-wavelength excitation, which may undergo severe light scattering by the bio-tissue. The marriage of fluorescent imaging with nonlinear optical(NLO) effect can alleviate such adverse effects by utilizing NIR laser to reduce light scattering. On the other hand, scientists are enthusiastic in pursuing luminescent materials, which can match well with NLO application. Aggregation-induced emission(AIE) materials exhibit huge advantages in such aspect not only because of its high luminescent efficiency in aggregate state but also due to its excellent photo-stability (a key factor to meet laser application because of its ultrahigh energy density). Inspired by this, many interesting and meaningful works have sprung up based on AIE luminogens with NLO effect in recent years, and for such reason, it motivates us to summarize them to give a systematic presentation. Here, we first give a brief introduction of the principle of NLO effect. Secondly, the strategies for improving the NLO effect of AIE materials, such as increasing molecular conjugation, introduction of donor-acceptor effect, induction of centrally asymmetric array of AIE molecules in crystals and introduction of intermolecular interactions are clarified. In the final part, we also present the multiple applications of AIEgens with NLO effect in cell imaging, deep-tissue tumor and brain blood vessel imaging and photodynamic therapy. We believe, with this review, the topic will attract more attention from the scientists in multi-science field to accelerate the development of AIE materials in biomedical applications.

With the development of the global economy, the safety of agricultural production has attracted intense attention. To minimize the risk of harmful ingredients in the whole industry chain, it is very necessary to cover the entire process of safety inspections from planting to production to environmental management. Fluorescence sensing as a promising and powerful screening tool is widely used for the detection of ions, toxic gases, biological molecules and so on. However, traditional fluorescent probes often suffer from aggregation-induced quenching(ACQ) effects, which limits their practical applications. In this regard, aggregation-induced emission luminogens(AIEgens) can perfectly address this notorious issue and have shown great potential in agricultural safety analysis. In this review, we briefly summarize their applications in agricultural safety monitoring, including the fabrication of agricultural film, agricultural sewage treatment and selective detection of harmful residues in agricultural products. The challenge and future development of AIEgens in this field are also discussed and highlighted. Hopefully, this review can inspire more researchers to participate in this fascinating area.

Fluorescence imaging based on luminogens with aggregation-induced emission(AIE) effect has drawn great attention in recent two decades, due to their superior advantages to overcome the technical difficulties. Thus, the AIE-active bioprobes with targeted ability at the subcellular level have been widely investigated to visualize the subcellular structures and monitor the biological processes. Considering the very rapid developments and the significance of selective imaging of subcellular structures, we summarize the recent two-year achievements about the AIEgens for targeted imaging of subcellular organelles including nuclei, membranes, lipid droplets(LDs), endoplasmic reticulum(ER), lysosomes, mitochondria and cytoplasm. The designed protocols and advantages of AIEgens, their mechanisms for targeted staining at organelles and the imaging performance are discussed. These AIE bioprobes exhibit great potentials for early diagnosis and therapeutics of diseases that related to subcellular organelles. Finally, the perspectives about AIEgens for these applications are also discussed.

A variety of DNA-based probes are utilized for the detections of multiple analytes and DNA nanotechnology has been thriving for recent decades and achieving numerous nanostructures, mainly focusing on DNA morphology modulation and multifunctional systems engineered into to the complicated works. Among the numerous detections, fluorescence method is a non-invasive, highly selective and sensitive means for varieties of applications, but their emissions are often compromised by the aggregation-caused quenching(ACQ) effect, which weakens their applications. The aggregation induced emission luminogens(AIEgens) are created with non emissive or weakly emissive in a low concentration but emit strong fluorescence in a high concentration with aggregated states. Herein, numerous functionalized AIEgens have been emerged and used for detection and imaging and DNA-modified AIEgen probes are introduced. In this vein, here we report the progress on DNA-modified AIEgen probes in recent years and highlight their conjugation strategies including covalent bonding, electrostatic interaction and their applications of biosensing. Moreover, multiple DNA strands are needed to introduce into the DNA-modified AIEgen probes for more purposes. At the end, some challenges are mentioned to discuss the new trend of DNA-modified AIEgen probes.

Bioimaging, as a powerful and helpful tool, which allows people to investigate deeply within living organisms, has contributed a lot for both clinical theranostics and scientific research. Pure organic room temperature phosphorescence(RTP) materials with the unique features of ultralong luminescence lifetime and large Stokes shift, can efficiently avoid biological autofluorescence and scattered light through a time-resolved imaging modality, and thus are attracting increasing attention. This review classifies pure organic RTP materials into three categories, including small molecule RTP materials, polymer RTP materials and supramolecular RTP materials, and summarizes the recent advances of pure organic RTP materials for bioimaging applications.

With the rapid development of materials science, photosensitive materials have been widely used in the field of immunogenic cell death(ICD), which was on account of the reactive oxygen species(ROS) generation by photosensitizer under light irradiation inducing cellular oxidative stress during the dying of cells. Considerable researches related to photodynamic therapy(PDT) induced ICD were conducted and exhibited brilliant performance in cancer immunotherapy. Herein, a variety of different strategies for PDT induced ICD have been summarized and discussed to provide researchers more inspiration for cancer immunotherapy.

Polymer science entails the structural study at multi-levels from nano- to micro- and mesoscale, which is highly important to transfer or even amplify the molecular information to macroscopic materials. Multiple polymer structural transitions from lower-order to higher-order superstructures are normally involved to achieve selective, efficient and sophisticated functions. Therefore, in-situ visualization of these processes is highly important, not only for fundamental understanding the structural evolution, but also for the optimization of the process flow during the materials processing. Fluorescence imaging based on aggregation-induced emission(AIE) provides an ideal tool that offers a simple, accurate, and easy-readable method to fulfill the above requirements. Owing to the twisted propeller-like structure of AIE luminogens(AIEgens), they show high fluorescence sensitivity to the surrounding microenvironment(e.g., viscosity, rigidity, and polarity) through intramolecular motions. In this short review, we summarize the recent applications of AIEgens to serve as “built-in” sensors to analyze the process of polymerization, microphase separation, glass/vitrification transition, polymer solvation, crystallization, etc. The perspective on the future application of AIE technology in polymer engineering, especially fiber materials, is also discussed.

Organic mechanochromic materials(also known as piezochromic materials), whose color or emission changes under mechanical force, have attracted great interest owing to their potential applications in pressure sensors, rewritable materials, optical storage, and security ink. Organic mechanochromic materials with aggregation-induced emission(AIE) features have better development prospects and research value owing to their excellent optical properties. To date, mechanochromism has mostly been realized by means of mechanical grinding. Nevertheless, the magnitude of the grinding force is usually uncontrollable and its direction is anisotropic, making it awkward to study the mechanism of mechanochromic materials. On the contrary, hydrostatic pressure, whose magnitude and direction are controllable, is a more valid and governable method to investigate the mechanism of mechanochromic materials, which can help us to construct a meaningful structure-property relationship and understand the latent origin of the mechanochromism. Furthermore, it is conducive to developing other mechanochromic material systems with desired chemical and physical properties. In this review, we focus on the recent progress in the mechanism of organic mechanochromic materials with AIE features under hydrostatic pressure. Four types of mechanisms are included: intermolecular interaction change, intramolecular conformation change, transformation from locally excited state to intramolecular charge-transfer state, and intra- and inter-molecular effects induced by hydrostatic pressure, respectively.

Fluorescent probes used for cell imaging are powerful tools in cell-based assays and research. In this study, we exhibited a water-soluble aggregation-induced emission fluorogen(AIEgen), BSPO-TPE, specifically stained cytoplasm in live cells and had an excellent photostability when compared to that of two widely used commercial fluorescent dyes. The long cytoplasm retention time of BSPO-TPE demonstrated its suitability as a live cell cytoplasm tracker.

Although hydrophobic interface regions adjacent to water droplets play a vital role in microemulsion-based studies, their widespread applications have not been explicitly evoked owing to their small spaces. Herein, we designed and synthesized a novel double-tailed anionic surfactant(TPE-di-C8SS) by linking the propeller-shaped tetraphenylethylene(TPE) with two octyl chains and an anionic sulfonate headgroup through a methoxy-butyl spacer. The extra spacer and steric hindrance between rigid TPE groups can create the large cavities in hydrophobic interface regions, which we call the hydrophobic interface cages(HICs). The potentials and advantages of HICs in the easily-prepared TPE-di-C8SS microemulsion have been implemented by comparing the extraction efficiency towards cationic rhodamine B with Aerosol OT(AOT) microemulsion. The results provided solid evidence that HICs rather than water droplets contributed to a higher extraction efficiency. This work not only proposes a concept of HICs but also provides a new perspective of their utilization in microemulsion-based applications.

Fluoran salicylaldehyde hydrazone metal complex(FSHMC) is a kind of recently reported photo-responsive system, which has the advantages of simple synthesis, multiple colors as well as distinct color change before and after UV light irradiation. However, the emission property of FSHMC is relatively unitary. In solid state, especially, only fluorescence quench is induced after UV light irradiation, which limits their applications. In this work, a typical aggregation-induced emission(AIE) moiety of tetraphenylethene(TPE) was introduced to the design of FSHMC. The obtained FSHMC, 2-Zn, exhibited reversible color and fluorescence changes upon UV light irradiation. Due to the AIE feature of compound 2, 2-Zn exhibited different emission changes upon UV light irradiation in THF and in solid matrix, because of the fluorescence resonance energy transfer(FRET) process from TPE moiety to rhodamine B moiety.

The development of fluorescent nanocrystals based on organic small molecules is of great importance in bioimaging due to the merits of easy modification, high brightness and excellent photostability, however suffering from the emission-detrimental aggregation-caused quenching(ACQ) effect. Herein, we successfully designed and synthesized an AIE-active di(N, N-dimethylaniline)-dibenzofulvene(named as NFTPE), which exhibits the crystallization-induced emission enhancement(CIEE) effect. Interestingly, two types of yellow- and orange-emissive crystals for NFTPE were obtained, exhibiting aggregation microenvironment-dependent emission tuning in the solid state. Single-crystal analysis and density functional theory(DFT) calculations reveal that different aggregation microenvironments result in the distinct molecular conformation for various emission. Excitingly, the crystallization of NFTPE in an aqueous solution under the assistance of amphiphilic PEG polymer matrices could be monitored in situ by the fluorescence changes, facilitating the preparation of NFTPE nanocrystals(NFTPE-NCs) by adjusting the aggregation microenvironment. The obtained NFTPE-NCs exhibit the superior performance in cell imaging in respect to high brightness, photostability, and biocompatibility, thus demonstrating the potential in bioimaging applications.

Photosensitizers that can target and accumulate in mitochondria are expected to achieve good therapeutic effects in photodynamic therapy, as mitochondria are the energy generation factory in cells. Herein, we designed and synthesized a novel mitochondrion-targeting photosensitizer TPC-Py with aggregation-induced emission characteristics for image-guided photodynamic therapy. TPC-Py possessed an efficient production of ¹O₂, with a quantum yield of 11.65%, upon mild white light irradiation (6 mW/cm²). TPC-Py exhibited good biocompatibility under dark condition, but showed remarkable cytotoxicity towards human cervical carcinoma(HeLa) cells with a half maximal inhibitory concentration(IC₅₀) of 3.2 µmol/L when exposed to white light irradiation(14.4 J/cm²). In addition, the Stokes shift of TPC-Py was as high as 150 nm, so that it could prevent self-absorption and increase the signal-to-noise ratio of fluorescence imaging. The excellent performance of TPC-Py makes it a promising candidate in imaging-guided clinical PDT for cancer in the near future.

Aggregation-induced emission(AIE) luminogens(AIEgens) with high brightness in aggregates exhibit great potentials in biological imaging, but these AIEgens are seldom applied in super-resolution biological imaging, especially in the imaging by using the structural illumination microscope(SIM). Based on this consideration, we synthesized the donor-acceptor typed AIEgen of DTPA-BTN, which not only owns high brightness in the near-infrared(NIR) emission region from 600 nm to 1000 nm(photoluminescence quantum yield, PLQYs=11.35%), but also displays excellent photo-stability. In addition, AIE nanoparticles based on 4,7-ditriphenylamine-[1,2,5]-thiadiazolo[3,4-c]pyridine(DTPA-BTN) were also prepared with highly emissive features and excellent biocompatibility. Finally, the developed DTPA-BTN-based AIE nanoparticles were applied in the super-resolution cellular imaging via SIM, where much smaller full width at half-maximum values and high signal to noise ratios were obtained, indicating the superior imaging resolution. The results here imply that highly emissive AIEgens or AIE nanoparticles can be promising imaging agents for super-resolution imaging via SIM.

With the antibiotic abuse and the resulting increased antibiotic resistance, the bacterial infection has posed a serious threat to human health. Photodynamic therapy is an effective tool for treating localized and superficial infections. It is a promising approach for the treatment of superbugs and with minimal risk of induced antibiotic resistance. Herein, an isoquinolinium-based photosensitizer, LIQ-TPE, with aggregation-induced emission properties is designed and synthesized. It is with high ¹O₂ generation efficiency and shows efficient antibacterial performance towards both Gram-negative and Gram-positive bacteria, and even methicillin-resistant Staphylococcus aureus(MRSA). LIQ-TPE thus shows great promise as an effective antimicrobial agent to combat the menace of multi-drug resistant bacteria.

Luminogens with aggregation-induced emission(AIE) characteristics(or AIEgens) have been widely used in various applications due to their excellent luminescent properties in molecular aggregates and the solid state. A deep understanding of the AIE mechanism is critical for the rational development of AIEgens. In this work, the “state-crossing from a locally excited to an electron transfer state”(SLEET) model is employed to rationalize the AIE phenomenon of two (bi)piperidylanthracenes. According to the SLEET model, an electron transfer(ET) state is formed along with the rotation of the piperidyl group in the excited state of (bi)piperidylan-thracene monomers, leading to fluorescence quenching. In contrast, a bright state exists in the crystal and molecular aggregates of these compounds, as the intermolecular interactions restrict the formation of the dark ET state. This mechanistic understanding could inspire the deployment of the SLEET model in the rational designs of various functional AIEgens.

There are numerous numbers of hypoxia-selective luminescent probes based on oxygen quenching of phosphorescence. We show a unique design for luminescent probes to detect hyperoxia utilizing hybrid networks consisting of aggregation-induced emission(AIE)-active dyes and disulfide linkers. At the initial state, emission from the AIE-active dyes is inducible by suppressing energy-consumable intramolecular motions in the hybrid matrices, while the decrease in intensity was detected by releasing molecular motions corresponded to bond scission at the disulfide linkers. Particularly, it was shown that this process selectively proceeds in hypoxia. As a result, positive luminescent signals were obtained in hyperoxia.

In this work, a fluorescent probe(TPEBe-I) was developed for adenosine triphosphate(ATP) detection based on the synergetic effect of aggregation-induced emission and counterion displacement. TPEBe-I gave weak emission in aqueous solution due to the heavy-atom effect of counter iodide ion. However, upon the addition of ATP, the new aggregate complex(TPEBe-ATP) was formed between the cationic unit of TPEBe-I and ATP through electrostatic interactions, which not only restricted the intramolecular motion of luminogen but also eliminated the quenching effect of iodide ion. As a result, the fluorescent light-up detection for ATP was successfully achieved. Moreover, TPEBe-I exhibited high selectivity towards ATP and showed a wide linear detection region towards the logarithm of ATP concentration(5—600 µmol/L) with a detection limit of 1.0 µmol/L, enabling TPEBe-I as a promising probe for ATP quantitative analysis.

Near-infrared(NIR) lights are powerful tools to conduct deep-tissue imaging since NIR-I wavelengths hold less photon absorption and NIR-II wavelengths serve low photon scattering in the biological tissues compared with visible lights. Two-photon fluorescence lifetime microscopy(2PFLM) can utilize NIR-II excitation and NIR-I emission at the same time with the assistance of a well-designed fluorescent agent. Aggregation induced emission(AIE) dyes are famous for unique optical properties and could serve a large two-photon absorption(2PA) cross-section as aggregated dots. Herein, we report two-photon fluorescence lifetime microscopic imaging with NIR-II excitation and NIR-I emission using a novel deep-red AIE dye. The AIE-gens held a 2PA cross-section as large as 1.61×10⁴ GM at 1040 nm. Prepared AIE dots had a two-photon fluorescence peak at 790 nm and a stable lifetime of 2.2 ns under the excitation of 1040 nm femtosecond laser. The brain vessels of a living mouse were vividly reconstructed with the two-photon fluorescence lifetime information obtained by our home-made 2PFLM system. Abundant vessels as small as 3.17 µm were still observed with a nice signal-background ratio at the depth of 750 µm. Our work will inspire more insight into the improvement of the working wavelength of fluorescent agents and traditional 2PFLM.

Nonconventional luminophores without large conjugated structures are attracting increasing attention for their unique aggregation-induced emission(AIE) properties and promising applications in optoelectronic and biomedical areas. The emission mechanism, however, remains elusive, which makes rational molecular design difficult. Recently, we proposed the clustering-triggered emission(CTE) mechanism to illustrate the emission. The clustering of electron-rich nonconventional chromophores with π and/or n electrons and consequent electron cloud overlap is crucial to the luminescence. Herein, based on the CTE mechanism, nonaromatic polymers containing multitype heteroatoms(i.e., O, N, and S) and involving amide(CONH) and sulfide(-S-) groups were designed and synthesized through facile thiol-ene click chemistry. The resulting polymers demonstrated typical concentration-enhanced emission, AIE phenomenon, and excitation-dependent emission. Notably, compared with polysulfides, these polymers exhibited much higher solid-state emission efficiencies, because of the incorporation of amide units, which contributed to the formation of emissive clusters with highly rigidified conformations through effective hydrogen bonding. Furthermore, distinct persistent cryogenic phosphorescence or even room temperature phosphorescence(RTP) was noticed. These photophysical behaviors can well be rationalized in terms of the CTE mechanism, indicating the feasibility of rational molecular design and luminescence regulation.

In order to solve the problem of the synthesis and structural characterization of 15- and 16-vertex closo-carboranes, Xie et al. introduce the method of using silyl groups to both cage carbons, stabilizing the corresponding nido-carborane dianions and promoting the capitation reaction with HBBr₂·SMe₂. This work demonstrates the exist of carboranes with more than 14 vertices and may open the door for further studying supercarborane chemistry.

Electrochromic(EC) materials with a dark-to-transmissive switch have potential applications in optical communications, infrared wavelength detectors for spacecraft, and infrared camouflage coatings. Recently, Yu et al. proposed an innovative low-voltage dark-to-transparent switchable electrochrome based on a donor-acceptor two-dimensional covalent organic framework(COF) material. The compound can be facilely processed in a large-area thin film, which allows for the wide applications in smart displays, windows, and clothing. This work has been published online in Nature Communications on November 2nd, 2020.