Chiral Reticular Self-Assembly of Achiral AIEgen into Optically Pure Metal–Organic Frameworks (MOFs) with Dual Mechano-Switchable Circularly Polarized Luminescence

Circularly polarized luminescence (CPL) is attractive in understanding the excited-state chirality and developing advanced materials. Herein, we propose a chiral reticular self-assembly strategy to unite achiral AIEgens, chirality donors, and metal ions to fabricate optically pure AIEgen metal–organic frameworks (MOFs) as efficient CPL materials. We have found that CPL activity of the single-crystal AIEgen MOF was generated by the framework-enabled strong emission from AIEgens and through-space chirality transfer from chirality donors to achiral AIEgens via metal-ion bridges. For the first time, a dual mechano-switched blue and red-shifted CPL activity was achieved via ultrasonication and grinding, which enabled the rotation or stacking change of AIEgen rotors with the intact homochiral framework. This work provided not only an insightful view of the aggregation induced emission (AIE) mechanism, but also an efficient and versatile strategy for the preparation of stimuli-responsive CPL materials.     

Consideration of Molecular Structure in the Excited State to Design New Luminogens with Aggregation‐Induced Emission

Aggregation‐induced emission (AIE) is a photoluminescence phenomenon in which an AIE luminogen (AIEgen) exhibits intense emission in the aggregated or solid state but only weak or no emission in the solution state. Understanding the mechanism of AIE requires consideration of excited state molecular geometry (for example, a π twist). This Minireview examines the history of AIEgens with a focus on the representative AIEgen, tetraphenylethylene (TPE). The mechanisms of solution‐state quenching are reviewed and the crucial role of excited‐state molecular transformations for AIE is discussed. Finally, recent progress in understanding the relationship between excited state molecular transformations and AIE is overviewed for a range of different AIEgens.     

Chiral Reticular Self‐Assembly of Achiral AIEgen into Optically Pure Metal–Organic Frameworks (MOFs) with Dual Mechano‐Switchable Circularly Polarized Luminescence

Circularly polarized luminescence (CPL) is attractive in understanding the excited‐state chirality and developing advanced materials. Herein, we propose a chiral reticular self‐assembly strategy to unite achiral AIEgens, chirality donors, and metal ions to fabricate optically pure AIEgen metal–organic frameworks (MOFs) as efficient CPL materials. We have found that CPL activity of the single‐crystal AIEgen MOF was generated by the framework‐enabled strong emission from AIEgens and through‐space chirality transfer from chirality donors to achiral AIEgens via metal‐ion bridges. For the first time, a dual mechano‐switched blue and red‐shifted CPL activity was achieved via ultrasonication and grinding, which enabled the rotation or stacking change of AIEgen rotors with the intact homochiral framework. This work provided not only an insightful view of the aggregation induced emission (AIE) mechanism, but also an efficient and versatile strategy for the preparation of stimuli‐responsive CPL materials.     

Tetraphenylpyrazine-based AIEgens: facile preparation and tunable light emission

Research on aggregation-induced emission (AIE) has been a hot topic. Due to enthusiastic efforts by many researchers, hundreds of AIE luminogens (AIEgens) have been generated which were mainly based on archetypal silole, tetraphenylethene, distyrylanthracene, triphenylethene, and tetraphenyl-1,4-butadiene, etc. To enlarge the family of AIEgens and to enrich their functions, new AIEgens are in high demand. In this work, we report a new kind of AIEgen based on tetraphenylpyrazine (TPP), which could be readily prepared under mild reaction conditions. Furthermore, we show that the TPP derivatives possess a good thermal stability and their emission could be fine-tuned by varying the substituents on their phenyl rings. It is anticipated that TPP derivatives could serve as a new type of widely utilized AIEgen, based on their facile preparation, good thermo-, photo- and chemostabilities, and efficient emission.     

Recent Development of DNA-modified AIEgen Probes for Biomedical Application

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.     

Bioorthogonal Turn‐On Probe Based on Aggregation‐Induced Emission Characteristics for Cancer Cell Imaging and Ablation

Bioorthogonal turn‐on probes have been widely utilized in visualizing various biological processes. Most of the currently available bioorthogonal turn‐on probes are blue or green emissive fluorophores with azide or tetrazine as functional groups. Herein, we present an alternative strategy of designing bioorthogonal turn‐on probes based on red‐emissive fluorogens with aggregation‐induced emission characteristics (AIEgens). The probe is water soluble and non‐fluorescent due to the dissipation of energy through free molecular motion of the AIEgen, but the fluorescence is immediately turned on upon click reaction with azide‐functionalized glycans on cancer cell surface. The fluorescence turn‐on is ascribed to the restriction of molecular motion of AIEgen, which populates the radiative decay channel. Moreover, the AIEgen can generate reactive oxygen species (ROS) upon visible light (λ=400–700 nm) irradiation, demonstrating its dual role as an imaging and phototherapeutic agent.     

Planar AIEgens with Enhanced Solid‐State Luminescence and ROS Generation for Multidrug‐Resistant Bacteria Treatment

Planar luminogens have encountered difficulties in overcoming intrinsic aggregation‐caused emission quenching by intermolecular π‐π stacking interactions. Although excited‐state double‐bond reorganization (ESDBR) can guide us on designing planar aggregation‐induced emission (AIE) luminogens (AIEgens), its mechanism has yet been elucidated. Major challenges in the field include methods to efficiently restrict ESDBR and enhance AIE performance without using bulky substituents (e.g., tetraphenylethylene and triphenylamine). In this study, we rationally developed fluoro‐substituent AIEgens with stronger intermolecular H‐bonding interaction for restricted molecular motions and increased crystal density, leading to decreased nonradiative decay rate by one order of magnitude. The adjusted ESDBR properties also show a corresponding response to variation in viscosity. Furthermore, their aggregation‐induced reactive oxygen species (ROS) generations have been discovered. The application of such planar AIEgen in treating multidrug‐resistant bacteria has been demonstrated in a mouse model. The relationship between ROS generation and distinct E/Z‐configurational stacking behaviors have been further understood, providing a design principle for synthesizing planar AIEgen‐based photosensitizers.     

AIEgens in cell-based multiplex fluorescence imaging

Fluorescence imaging is an important branch of bioimaging. It is non-invasive and provides superior spatial and temporal resolution during the real-time monitoring of biological samples of interest. Although the spatial resolution limit of optical microscopes is about 200 nm, due to the diffraction limit, with the application of super-resolution fluorescence microscopy technologies this limit has been pushed below 30 nm. This makes it feasible to visualize biological structures in subcellular levels and to monitor subcellular biological processes in real time. However, due to the complexity of the biological structure and components within cells, simultaneous staining and monitoring multiple intracellular components with different coloured fluorophores is often needed during multiplex imaging, to better understand biological processes. Aggregation-induced emission luminogens(AIEgen) and AIEgen based nanoparticles(NPs) have presented many advantages in fluorescence imaging, with strong potential for biological science and nano-medicine. Herein this review, we focus on the advantages of AIEgen and AIEgen NP in cell-based fluorescence imaging, and the latest advances of AIEgens in cell-based multiplex imaging are summarized and discussed. The future perspectives are proposed.     

Tumor‐Exocytosed Exosome/Aggregation‐Induced Emission Luminogen Hybrid Nanovesicles Facilitate Efficient Tumor Penetration and Photodynamic Therapy

The development of novel photosensitizing agents with aggregation‐induced emission (AIE) properties has fueled significant advances in the field of photodynamic therapy (PDT). An electroporation method was used to prepare tumor‐exocytosed exosome/AIE luminogen (AIEgen) hybrid nanovesicles (DES) that could facilitate efficient tumor penetration. Dexamethasone was then used to normalize vascular function within the tumor microenvironment (TME) to reduce local hypoxia, thereby significantly enhancing the PDT efficacy of DES nanovesicles, and allowing them to effectively inhibit tumor growth. The hybridization of AIEgen and biological tumor‐exocytosed exosomes was achieved for the first time, and combined with PDT approaches by normalizing the intratumoral vasculature as a means of reducing local tissue hypoxia. This work highlights a new approach to the design of AIEgen‐based PDT systems and underscores the potential clinical value of AIEgens.     

Calixarene-Based Supramolecular AIE Dots with Highly Inhibited Nonradiative Decay and Intersystem Crossing for Ultrasensitive Fluorescence Image-Guided Cancer Surgery

Host–guest complexation between calix[5]arene and aggregation-induced emission luminogen (AIEgen) can significantly turn off both the energy dissipation pathways of intersystem crossing and thermal deactivation, enabling the absorbed excitation energy to mostly focus on fluorescence emission. The co-assembly of calix[5]arene amphiphiles and AIEgens affords highly emissive supramolecular AIE nanodots thanks to their interaction severely restricting the intramolecular motion of AIEgens, which also show negligible generation of cytotoxic reactive oxygen species. In vivo studies with a peritoneal carcinomatosis-bearing mouse model indicate that such supramolecular AIE dots have rather low in vivo side toxicity and can serve as a superior fluorescent bioprobe for ultrasensitive fluorescence image-guided cancer surgery.     

Tumor-Exocytosed Exosome/Aggregation-Induced Emission Luminogen Hybrid Nanovesicles Facilitate Efficient Tumor Penetration and Photodynamic Therapy

The development of novel photosensitizing agents with aggregation-induced emission (AIE) properties has fueled significant advances in the field of photodynamic therapy (PDT). An electroporation method was used to prepare tumor-exocytosed exosome/AIE luminogen (AIEgen) hybrid nanovesicles (DES) that could facilitate efficient tumor penetration. Dexamethasone was then used to normalize vascular function within the tumor microenvironment (TME) to reduce local hypoxia, thereby significantly enhancing the PDT efficacy of DES nanovesicles, and allowing them to effectively inhibit tumor growth. The hybridization of AIEgen and biological tumor-exocytosed exosomes was achieved for the first time, and combined with PDT approaches by normalizing the intratumoral vasculature as a means of reducing local tissue hypoxia. This work highlights a new approach to the design of AIEgen-based PDT systems and underscores the potential clinical value of AIEgens.     

Calixarene‐Based Supramolecular AIE Dots with Highly Inhibited Nonradiative Decay and Intersystem Crossing for Ultrasensitive Fluorescence Image‐Guided Cancer Surgery

Host–guest complexation between calix[5]arene and aggregation‐induced emission luminogen (AIEgen) can significantly turn off both the energy dissipation pathways of intersystem crossing and thermal deactivation, enabling the absorbed excitation energy to mostly focus on fluorescence emission. The co‐assembly of calix[5]arene amphiphiles and AIEgens affords highly emissive supramolecular AIE nanodots thanks to their interaction severely restricting the intramolecular motion of AIEgens, which also show negligible generation of cytotoxic reactive oxygen species. In vivo studies with a peritoneal carcinomatosis‐bearing mouse model indicate that such supramolecular AIE dots have rather low in vivo side toxicity and can serve as a superior fluorescent bioprobe for ultrasensitive fluorescence image‐guided cancer surgery.     

Activation of apoptosis by rationally constructing NIR amphiphilic AIEgens: surmounting the shackle of mitochondrial membrane potential for amplified tumor ablation

In recent years, the use of aggregation-induced emission luminogens (AIEgens) for biological imaging and phototherapy has become an area of intense research. However, most traditional AIEgens suffer from undesired aggregation in aqueous media with “always on” fluorescence, or their targeting effects cannot be maintained accurately in live cells with the microenvironment changes. These drawbacks seriously impede their application in the fields of bio-imaging and antitumor therapy, which require a high signal-to-noise ratio. Herein, we propose a molecular design strategy to tune the dispersity of AIEgens in both lipophilic and hydrophilic systems to obtain the novel near-infrared (NIR, ∼737 nm) amphiphilic AIE photosensitizer (named TPA-S-TPP) with two positive charges as well as a triplet lifetime of 11.43 μs. The synergistic effects of lipophilicity, electrostatic interaction, and structure-anchoring enable the wider dynamic range of AIEgen TPA-S-TPP for mitochondrial targeting with tolerance to the changes of mitochondrial membrane potential (ΔΨ_m). Intriguingly, TPA-S-TPP was difficult for normal cells to be taken up, indicative of low inherent toxicity for normal cells and tissues. Deeper insight into the changes of mitochondrial membrane potential and cleaved caspase 3 levels further revealed the mechanism of tumor cell apoptosis activated by AIEgen TPA-S-TPP under light irradiation. With its advantages of low dark toxicity and good biocompatibility, acting as an efficient theranostic agent, TPA-S-TPP was successfully applied to kill cancer cells and to efficiently inhibit tumor growth in mice. This study will provide a new avenue for researchers to design more ideal amphiphilic AIE photosensitizers with NIR fluorescence.

In this contribution, based on a “step-by-step” molecular design strategy, a novel NIR amphiphilic AIEgen TPA-S-TPP with a triplet lifetime of 11.43 μs and surmounting the shackle of MMP was successfully fabricated for amplified tumor ablation.     

Donor-Acceptor Typed AIE Luminogens with Near-infrared Emission for Super-resolution Imaging

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.     

Anthracene Modified by Aldehyde Groups Exhibiting Aggregation‐Induced Emission Properties

In order to get an easy way to achieve the transformation from aggregation‐caused quenching luminophores (ACQphores) to aggregation‐induced emission luminogens (AIEgens), we took aldehyde groups as the modifying group to decorate anthracene. The fluorescence performances of 9‐anthraldehyde (AnA) and 9,10‐anthracenedicarboxaldehyde (AnDA) in solution and aggregated state were studied. We found out that the aldehyde group can transform anthracene with aggregation‐caused quenching properties to AIEgen. The single‐crystal structures analysis of AnA and AnDA showed that their structure characteristics are responsible for the AIE properties of AnA and AnDA. On one hand, the aldehyde group can cause steric effects to lower intermolecular π‐π packing style in aggregated state. On the other hand, intermolecular H‐bonding interactions can restrict the intramolecular rotation and suppress internal charge transfer. These results may supply a new simple method for the transformation from ACQphores to AIEgens on the point of the molecular design.     

Non-Doped Blue AIEgen-Based OLED with EQE Approaching 10.3 %

Aggregation-induced emission (AIE) luminogens (AIEgens) are attractive for the construction of non-doped blue organic light-emitting diodes (OLEDs) owning to their high emission efficiency in the film state. However, the large internal inversion rate (k_IC^(Tn)) between high-lying triplet levels (T_n) and T_n-1 causes a huge loss of triplet excitons, resulting in dissatisfied device performance of these AIEgens-based non-doped OLEDs. Herein, we designed and synthesized a blue luminogen of DPDPB-AC by fusing an AIEgen of TPB-AC and a DMPPP, which feature hot exciton and triplet-triplet annihilation (TTA) up-conversion process, respectively. DPDPB-AC successfully inherits the AIE feature and excellent horizontal dipole orientation of TPB-AC. Furthermore, it owes smaller k_IC^(Tn) than TPB-AC. When DPDPB-AC was applied in OLED as non-doped emitting layer, an outstanding external quantum efficiency of 10.3 % and an exceptional brightness of 69311 cd m⁻² were achieved. The transient electroluminescent measurements and steady-state dynamic analysis confirm that both TTA and hot exciton processes contribute to such excellent device performance. This work provides a new insight into the design of efficient organic fluorophores by managing high-lying triplet excitons.     

Interplay of Anion-π+ and π+-π+ Interactions in Novel Pyrido[2,1-a]isoquinolinium-Based AIEgens - Substituent- and Counterion-Dependent Fluorescence Modulation and Applications in Live Cell Mitochondrial Imaging**

Recently, the concept of anion-π⁺ interactions has witnessed unique applications in the field of AIEgen development. In this contribution, we disclose a consolidated study of a library of N-doped ionic AIEgens accessed through silver-mediated cyclization of pyridino-alkynes. A thorough photophysical, computational and crystallographic study has been conducted to rationalize the observed substituent- and counterion-dependent fluorescence properties of these luminogens. We further elucidate the prominent role of anion-π⁺ interactions, π⁺-π⁺ interactions and other non-covalent interactions, in inhibiting the undesired ACQ effect. Finally, we have also demonstrated the application of selected AIEgens for imaging of mitochondria in live cells.     

AIEgen with Fluorescence–Phosphorescence Dual Mechanoluminescence at Room Temperature

We report the first example of an AIEgen (DPP-BO) with fluorescence–phosphorescence dual emission under mechanical stimulation. By carefully analyzing the crystal structure of DPP-BO, the efficient intermolecular and intramolecular interactions should account for its unique mechanoluminescence (ML) properties, especially the abnormal phosphorescence, as further confirmed by controlled experiments and theoretical calculations for the presence of ISC transitions. These results provide important information for understanding the complex ML process, possibly opening up a new way to study the inherent mechanism of ML by broadening the application of AIEgens.     

AIEgen with Fluorescence–Phosphorescence Dual Mechanoluminescence at Room Temperature

We report the first example of an AIEgen (DPP‐BO) with fluorescence–phosphorescence dual emission under mechanical stimulation. By carefully analyzing the crystal structure of DPP‐BO, the efficient intermolecular and intramolecular interactions should account for its unique mechanoluminescence (ML) properties, especially the abnormal phosphorescence, as further confirmed by controlled experiments and theoretical calculations for the presence of ISC transitions. These results provide important information for understanding the complex ML process, possibly opening up a new way to study the inherent mechanism of ML by broadening the application of AIEgens.     

Structural Controls of Tetraphenylbenzene-based AIEgens for Non-doped Deep Blue Organic Light-emitting Diodes

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.