Metal–organic frameworks with aggregation‐induced emission

Aggregation‐induced emission (AIE) materials exhibit remarkable emission in the aggregated or solid state while demonstrating minimal emission in dilute solutions. In contrast to conventional luminescent materials, AIE luminogens (AIEgens) offer several advantages in the aggregate state, including high quantum yield, excellent photostability, and low background signals, making them highly promising for diverse applications. Integrating AIEgens into designable metal–organic frameworks (MOFs) enables tunable and well‐ordered AIE materials, allowing for precise control over photophysical properties and deeper exploration of AIE mechanisms. Numerous AIE MOFs have been constructed and investigated, and several reviews focus on their structure design and applications in sensing and bioimaging. This review highlights the state‐of‐the‐art advancements in AIE MOFs, including mechanisms, design strategies, and applications in chemical sensing, bioimaging, and disease therapy. The challenges associated with practical applications of AIE MOFs are also addressed, with an emphasis on their large‐scale production involved interdisciplinary collaboration. This comprehensive review aims at guiding further development of AIE MOFs and promoting their practical applications in analysis, healthcare, and other luminescence related fields.

observed a fluorescence emission behavior that is contrary to the conventional ACQ effect.Unlike ACQ molecules, these newly discovered molecules exhibit notable emission in the aggregated or solid state, while demonstrating minimal emission in dilute solutions.This unique phenomenon was defined as aggregation-induced emission (AIE).Some investigations of mechanisms have been put forward to elucidate AIE phenomenon, [17] and the restriction of intramolecular motion (RIM) hypothesis, [18][19][20] which encompasses the restriction of intramolecular vibration and intramolecular rotation, [21,22] is widely acknowledged as the most accepted explanation.The inherent property of fluorescence switching in AIE luminogens (AIEgens) provides them with several advantages in the aggregate state, including high quantum yield, [23,24] excellent photostability, [25,26] and low background signals, [27,28] and these characteristics make them highly promising for a wide range of applications in various fields. [29,30]Therefore, the rational design and synthesis of an efficient aggregation system can be used for manipulating AIE properties and extending their scope of applications.
[46] Taking these advantages of MOFs, the integration of AIEgens into MOF frameworks can result in tunable and well-ordered AIE materials.This synergistic combination enables precise control over the photophysical properties and facilitates in-depth investigations into the underlying mechanisms of AIE.On the basis of above considerations, numerous AIE MOFs have been constructed and extensively investigated for their luminescence applications.Several recent reviews have summarized and discussed AIE MOFs, [47][48][49] with a primary focus on their structure design and applications in sensing and bioimaging.To enhance the comprehensive understanding of AIE MOFs, this review highlights the state-of-the-art advancements in AIE MOFs, including their luminescence mechanism, design strategies, and applications in chemical sensing, bioimaging, disease therapy (Figure 1).The challenges associated with the practical applications of AIE MOFs are also addressed, with a particular emphasis on their large-scale production involved interdisciplinary collaboration.We hope this comprehensive review will serve as a guide for further advancements in AIE MOFs and promote their practical applications in various fields, such as analysis, healthcare, and other luminescence-related domains.

Basic luminescence mechanism
The nature of luminescence is the process of releasing energy, such as burning, lightning, and solar fusion.For fluorophore molecules, they absorb photons and release energy to emit characteristic luminescence, and the corresponding absorption and transformation processes, namely Jablonski diagram, are illustrated in Figure 2A.In detail, fluorophore molecules F I G U R E 2 (A) Jablonski diagram: Processes of energy absorption and transformation of luminescent molecules (S 0 , the ground state; S 1 , the singlet excited state; S n , the higher excited state; T 1 , the triplet state; VR, vibrational relaxation; IC, internal conversion; ISC, intersystem crossing).(B) Schematic illustration of the restriction of intramolecular motions mechanisms in AIEgens.Reproduced with permission. [55]Copyright 2020, Wiley-VCH.Reproduced with permission. [14]Copyright 2020, American Chemical Society.
absorb energy from excitation light source, and the ground state (S 0 ) subsequently jumps to the singlet excited state (S 1 ) or the higher excited state (S n , n ≥ 2), contingent upon the excitation wavelength.Then, the unstable excited state radiates energy as luminescence by the following pathways: The excited state S n (n ≥ 2) primarily returns to S 1 state through vibrational relaxation (VR) or internal conversion (IC).The excited state transition from S 1 to S 0 undergoes a radiative decay pathway or a non-radiative decay pathway, resulting in fluorescence or radiant heat, respectively.The excited state S 1 can also transfer energy to the triplet state (T 1 ) through intersystem crossing (ISC) when the energy splitting (∆E ST ) between S 1 and T 1 is small enough or the spin-orbit coupling (SOC) effect is significant.Then, the energy transfers from T 1 to S 0 state through a radiative decay pathway or a non-radiative decay pathway, generating phosphorescence or radiant heat, respectively. [50,51]

AIE mechanism
According to basic luminescence mechanism, [52][53][54] reducing energy losses through non-radiative decay pathways (e.g.vibrational relaxation and internal conversion) can significantly enhance luminescence efficiency, leading to exceptional emission.However, it should be noted that fluorophore molecules are perpetually in motion, making them prefer to undergo non-radiative decay pathways.On the other hand, single fluorophore molecules tend to form aggregates via a spontaneous process at high concentration or in solidstate, leading to luminescence quenching.This phenomenon is defined as ACQ.RIM is a widely employed strategy to address both issues and achieve excellent luminescence emission, and has been studied to understand the AIE behavior and develop AIEgens.RIM mechanisms can be divided into two categories, including restriction of intramolecular rotation (RIR) and restriction of intramolecular vibration (RIV). [21,55]p to now, most of the reported AIEgens are designed based on these mechanisms as illustrated in Figure 2B.

RATIONAL DESIGNS OF AIE MOFS
MOFs, assembled by metal nodes and organic ligands, are emerging materials, featuring designable structures, high porosity, and large specific surface area.According to these advantages, the incorporation of AIEgens into MOFs can be achieved by using AIE ligands or accommodating AIE composites.The detailed construction strategies of AIE MOFs are shown as below.

Direct strategy: MOFs with AIE ligands
The direct synthesis strategy of AIE MOFs relies on AIE ligands, and a majority of AIE MOFs are synthesized using this approach.Several organic molecules containing AIEgens and either carboxyl or pyridine functional groups have been utilized to fabricate AIE MOFs with exceptional luminescence according to the aforementioned mechanisms.Figure 3  were obtained, [56] and showed fluorescence lifetimes consistent with those of closely packed molecular aggregates.Zn 2 (TCPE)(H 2 O) 2 ⋅4DEF exhibited distinct responses when exposed to different analytes.Exposure to ethylenediamine caused a blue-shift of 10 nm (from 467 to 457 nm) nm, while exposure to cyclohexanone and acetaldehyde resulted in red-shifts of 6 and 10 nm, respectively.Afterward, the extended H ). [57] Several MOFs (namely NUS-13) were obtained by regulating the ratios of H 2 L 2 and H 2 L 1 , exhibiting the linear relationship between emission intensity and H 2 L 1 ratio.Luminescence detection investigations revealed that NUS-13-100% showed a high linearity response with good sensitivity for detecting trace toxic benzene vapor, facilitating the development of molecular machine-based dynamic luminescent materials.

Post-modification strategy: MOFs with AIE composites
Leveraging the modifiability of MOFs, post-modification strategy provides a simple approach to incorporate AIE composites, yielding AIE MOFs.Inspired by the large porosity of MOFs, AIE molecules can be integrated as guests within MOFs.Zhao and coworkers reported TMPyPE@Zn-MOF through in situ encapsulation of TMPyPE within Zn-MOF. [58]TMPyPE@MOF showed an absolute quantum yield of 78.5%, and demonstrated the capability to detect NFZ and NFT in the water phase with low detection limits of 0.110 ppm and 0.134 ppm, respectively, exhibiting the potential application in environmental pollution monitoring.In addition to AIE molecules, nanomaterials with AIE properties can also be incorporated into MOFs.Zhang and coworkers constructed AIE-type AuNCs using glutathione and incorporated them into ZIF-8 to form an AIE MOF. [59]he AuNCs/ZIF-8 composites exhibited AIE effects and could be utilized for the selective and sensitive detection of glucose.In terms of practical application, creating large-area, optically high-quality thin films of efficiently performing AIE chromophores is useful but challenging.Howard and coworkers demonstrated the successful achievement of using a surface-anchored MOF thin film coating as a host substrate for printing TPE-based AIE chromophores. [60]Inkjet printing onto the SURMOF substrate enabled easy creation of AIE chromophore patterns with feature sizes as small as 70 μm, and the corresponding photoluminescent quantum yield could reach 50% by adjusting the loading amount of AIE chromophores.This work offers exciting prospects for utilizing AIE chromophores in the production of patterned phosphor thin films for applications in displays or lighting.

APPLICATIONS OF AIE MOFS
Combination the characteristics of MOFs and AIE molecules, many AIE MOFs have been constructed and exhibit fascinating luminescent properties, as well as the promising applications in chemical detection, bioimaging, disease therapy, and so on.

Chemical detection
Taking the advantages of excellent luminescence of AIEgens and unique structures in MOFs, AIE MOFs can form strong interactions (e.g.hydrogen bond and coordination bond) with guest molecules to influence their emissions, enabling the luminescence detection of analytes.The porous structures with unique pores and coordinated sites can selectively enrich analytes to increase their concentration, resulting in high selectivity and sensitivity.Thus, AIE MOFs are considered as the promising candidates for luminescence detection.The recent progresses of AIE MOFs for chemical detection have been summarized and discussed in the following section.

Volatile organic compounds
Volatile organic compounds (VOCs) encompass a range of volatile pollutants, such as toluene, benzene, xylene, ethylbenzene, ketones, aldehydes, styrene, and chlorinated hydrocarbons.These pollutants pose a significant threat to human health when inhaled.Prolonged exposure to VOCs at parts per million (ppm) levels can lead to damage to (E) On-off luminescent detection.Reproduced with permission. [64]opyright 2020, American Chemical Society.
[63] Therefore, the development of effective VOC detection methods is a pressing research concern.The luminescent detection of guest molecules can be realized through the enhancement of in-situ RIM induced by guest locking (Figure 4A).Lang and co-workers demonstrated the effectiveness of a rigid Cu(I)-MOF ([Cu 4 I 4 (Py 3 P) 2 ] n , Py 3 P = tris(2-pyridyl)phosphine) with hexagonal channels in accommodating guest molecules. [64]he luminescence intensity of the Cu(I)-MOF significantly increased upon the introduction of chlorinated hydrocarbons (Figure 4B).Single crystal X-ray diffraction revealed that the short distance between CH 2 Cl 2 /CHCl 3 and Cu(I)-MOF formed weak interactions, which can lock the molecular vibrations to enhance luminescence (Figure 4C).For other gases, the long distance or high disorder of the guest molecules formed too weak interactions that cannot lock the molecular vibrations.DFT calculations were used to evaluate the influence of CH 2 Cl 2 , CHCl 3 , and CCl 4 on Cu(I)-MOF.The results showed that the pore space of Cu(I)-MOF can be fully occupied by three molecules of CH 2 Cl 2 or CHCl 3 , leading to enhanced luminescent properties.However, the inclusion of three CCl 4 molecules was found to be unfavorable, resulting in insufficient vibration locking and diminished luminescent properties.The guest-lock induced emission enhancement enables the specific detection of chlorinated hydrocarbons by the Cu(I)-MOF, with a response time of less than 0.6 s (Figure 4D).Additionally, the on-off luminescent detection of CH 2 Cl 2 was achieved (Figure 4E).This study introduces a novel approach, the guest-lock-induced light-up route, for ultrafast luminescent detection of guest molecules.
Zhu and co-workers demonstrated a tetraphenylethylenefunctionalized UiO-66 (UiO-66-TBPE), exhibiting the characteristic blue emission of TBPE ligands. [65]UiO-66-TBPE proved to be an effective luminescence sensor for fast and selective detection of p-xylene and styrene vapor with the LOD of 1.295 and 0.906 ppm, respectively.The underlying mechanism responsible for the distinct luminescence enhancement and quenching effects induced by p-xylene and styrene was elucidated through a combination of advanced characterization techniques and calculations.The mechanism of UiO-66-TBPE for detecting p-xylene and styrene vapor was established.The luminescence enhancement of UiO-66-TBPE on p-xylene was mainly due to the strong interaction between p-xylene and the incomplete inhibition of TBPE's benzene, as well as the π−π stacking between their benzene rings.PXRD and luminescence spectrum suggested that the luminescence quenching of UiO-66-TBPE by styrene were not caused by the framework collapse and FRET.DFT calculation results revealed that UiO-66-TBPE possessed a higher excited LUMO energy level than styrene, enabling charge transfer pathway upon 365 nm excitation.Thus, the quenching was likely caused by PET from UiO-66-TBPE to styrene.To further enhance its practical performance, UiO-66-TBPE integrated with the polymer polyacrylate (PA) created a flexible hybrid membrane that demonstrated fast detection capabilities for styrene vapor within 30 s.
The AIE mechanism can be used for the detection of energetic compounds.Wang and co-workers introduced three TABD-MOFs with different outer-shell electron configurations of their metal nodes, resulting in both highly emissive and non-emissive luminescence. [68]Typically, heterocyclic energetic compounds containing C═N and/or N═N bonds can weaken the coordination between metal ions and carboxylate groups in MOFs, leading to partial replacement of linkers.AIE MOFs thereby incorporating carboxylate groups can be employed for energetic compounds detection, where AIE molecules can be released and reassembled to form emissive aggregates.However, the addition of NTO (5nitro-2,4-dihydro-3H-1,2,4-triazole-3-one) compromises the integrity of TABD-MOF due to competitive coordination substitution.As a result, when a trace amount of TABD-MOFs is deposited on a paper strip, free TABD-COOH molecules are released into the solution.Upon evaporation of the THF solvent, the dissociated TABD-COOH molecules can reaggregate, enabling detection.Utilizing a disposable paper strip with a small amount of TABD-MOFs allowed for naked-eye detection of highly dangerous five-memberedring energetic heterocyclic compounds (5MR-EHCs) within seconds, demonstrating unprecedented detection sensitivity (6.5 ng on a 1 cm 2 testing strip).This novel AIE-MOF method offers universality, high sensitivity, ease of visualization, and holds potential for the development of turn-on chemo/biosensing probes.
Luo and co-workers developed a luminescent sensor array consisting of eight AIE MOFs. [69]Through luminescence investigations, they were able to differentiate nine energetic compounds, which could be classified into three categories: nitroaromatics, nitramines, and nitrogen-rich heterocycles (Figure 5A).The detection range of the array spanned from 300 μM to 10 nM, and it exhibited excellent selectivity against various interferences (Figure 5B).The highly nitrated NACs (TNT, TNP, LTNR) efficiently quenched electron-rich AMx.DFT calculations demonstrated that NACs (TNT, TNP, LTNR) exhibited lower LUMO compared to AIE linkers (Figure 5C).The absorbances of TNP and LTNR (400-500 nm) overlapped with the emission spectrum of AMx.These results suggested the involvement of PET and RET mechanisms may contribute to the luminescence quenching.In contrast to NACs homologues, four NRHCs (NTO, DHT, DABT, DNBT) with diverse structures and functional groups exhibited different quenching mechanisms.DABT, due to high LUMO and no spectrum overlap, inefficiently quenched AMx luminescence.These findings explained poor EDNA quenching and the possible coexistence of PET and RET in FOX-7 quenching.The dynamic RIM effect of AMx and improved RIM in AM8 mitigated quenching.The luminescence response mechanisms depend on AMx topology and energetic compound properties.In comparison to sensors based on single AIE MOFs, the luminescent array constructed using multicomponent AIE MOFs provides a simpler approach to construct more complex sensing systems for the detection of similar substances.

Biomarkers
Biomarkers, including amino acids, metal ions, and proteins, are specific molecules that can be measured or detected in samples obtained from within or outside of an organism. [70,71]Through the measurement and analysis of these biomarkers, valuable biological information and diagnostic evidence can be obtained.In the field of medicine, biomarkers are extensively employed for early disease detection, prognosis evaluation, treatment monitoring, and various other applications.They play an important role in enhancing the accuracy of disease diagnosis and the effectiveness of personalized treatment.
Yin and co-workers reported an AIE MOF (Eu-MOF) constructed by AIE ligand of tetrakis(4-carboxyphenyl)pyrazine, emitting strong dual emissions form ligand and Eu 3+ ions. [72]hrough ratiometric luminescence, Eu-MOF demonstrated selective detection of arginine within the concentration range of 0-160 μM.This was achieved with a lowest LOD of 15 nM, which outperformed other luminescent detection methods.Structural analysis reveals that arginine molecules of appropriate size could penetrate the channels of Eu-MOF, with the nitrogen atoms on the ligand forming multiple hydrogen bonds with the guanidyl groups of arginine.These hydrogenbonding interactions, in combination with the presence of arginine, restricted the motion of the benzene ring in the ligand.The conjugation length was extended, leading to improved blue emission.Meanwhile, these interactions do not disrupt the antenna effect from the ligand to Eu 3+ ions.The resulting red emission from the antenna effect remained stable and served as a reference for ratiometric luminescence sensing.Notably, the color shift from red to blue, occurring with increasing arginine content, could be easily discerned by the naked eye.This work offers a convenient and promising application for arginine detection (Figure 6).
Point-of-care (POC) biochemical sensors have been widely utilized in clinical diagnosis and environmental monitoring, but they often show poor sensitivity.To improve the detection sensitivity, Tang, Jiang, and co-workers presented the rational design and modification of metal-AIEgen frameworks (MAFs), exhibiting a near 99% quantum yield. [73]eanwhile, MAFs could be used for ultrasensitive and user-friendly POC diagnostics on the basis of hybridization detection system (HDS) and lateral flow immunoassay (LFIA) For environmental monitoring, this luminescent system could detect 10 nM Cr 3+ by naked eyes through color changes.On the other hand, the sensor could be utilized for The theoretical calculation of sensing mechanism.Reproduced with permission. [69]Copyright 2021, Elsevier.
detecting alpha-fetoprotein (AFP), a serum marker used for diagnosing hepatocellular carcinoma (HCC) and monitoring therapy.This study presents a simple method to acquire novel materials for POC sensors with high sensitivity, offering the promising practical applications.

Other detection
Stimuli-responsive luminescent materials possess remarkable properties that enable the generation of photofluorescence emission transfer in response to external stimuli such as light, pH value, temperature, pressure, and other analytes.These materials exhibit great potential for various detection applications.Shi and co-workers reported two novel porous MOFs based on tetraphenylethylene (TPE), namely Sr-ETTB and Co-ETTB. [74]By modifying the outer shell electron configurations of Sr 2+ and Co 2+ ions, the luminescence intensity of the corresponding MOFs could be adjusted from strong emission to complete non-emission.Sr-ETTB MOF exhibited strong blue luminescence and demonstrated reversible variations in response to changes in temperature and pressure.
The sensing ranges for temperature and pressure were 80-400 K and 1 atm to 12.38 GPa, respectively.This luminescence behavior was directly linked to the reversible deformation of the crystal structure.On the other hand, non-emissive Co-ETTB showed a turn-on luminescent enhancement when stimulated by the analyte histidine.The enhanced emission was attributed to the release of AIE ligand through competitive coordination substitution, leading to in the manifestation F I G U R E 6 AIE MOF with dual emission for arginine detection.Reproduced with permission. [72]Copyright 2019, American Chemical Society.
of AIE behavior.This provides a simple approach for design stimuli-responsive luminescence materials.
The inhibition of acetylcholinesterase activity by organophosphorus pesticides (OPs) can lead to the development of neurological disorders.Therefore, it is crucial to develop a rapid and sensitive monitoring strategy for OPs.Liu and co-workers developed AuNCs@ZIF-8 with AIE effect, [75] resulting in strong emission with a fluorescence lifetime of 6.83 μs and a quantum yield of 4.63%.A dualsignal biosensor was developed by combining fluorescence and colorimetric signals, utilizing the enzymolysis product of acetylcholinesterase and choline oxidase on AuNCs@ZIF-8.The enzymolysis product could initially decompose ZIF-8, thereby reducing the constraint on AuNCs and resulting in a decrease in fluorescence signal.Subsequently, the released AuNCs functioned as peroxidase mimics, leading to the oxidation of 3,3′,5,5′-tetramethylbenzidine and the generation of a visible blue color.Consequently, a fluorescence-colorimetric dual-signal biosensor was established by harnessing the inhibition of acetylcholinesterase activity by OPs.Furthermore, colorimetric paper strips were developed to facilitate visual semiquantitative detection, while a smartphone app was designed to improve the precision of visualization results and enable real-time monitoring of pesticide contamination.
As a short summary of AIE MOFs for luminescent detection.AIE MOFs with excellent luminescence have found extensive applications in highly sensitive and selective detection, even being distinguishable to the naked eyes and smartphones.To enable quantitative and portable analysis of analytes, the integration of AIE MOFs with electronic devices should be pursued to bring them closer to practical applications.The fabrication techniques of AIE MOF-based electronic sensors must demonstrate high compatibility with complementary metal-oxide-semiconductor (CMOS) processes to facilitate large-scale manufacturing.Several studies have reported the use of screen printing methods for the development of MOF-based sensors.However, there is a need to explore novel printing techniques such as ink-jet printing and aerosol printing, as well as AIE MOFs inks, to achieve large-scale fabrication of sensors based on AIE MOFs.Additionally, systematic investigations into the mechanical properties of AIE MOFs are crucial to enhance the service life of AIE MOF-based sensors.

Bioimaging
AIE MOFs possess outstanding luminescence properties, including strong resistance to photo-bleaching, robust luminosity, and absence of random blinking even in complex biological environments.These unique characteristics allow them highly suitable for bioimaging applications.Thus, many AIE MOFs have been employed for high-resolution imaging in cancer cells, exhibiting their potential for advanced biomedical applications.Small guest molecules with AIE properties can be utilized for bioimaging.Chen and workers have showcased a photosensitizer-conjugated AIE-Au cluster.The assembly of glutathione-protected Au clusters into aggregates through a cationic polymer resulted in a 5.2-fold enhancement in X-ray-excited luminescence.The AIE-Au cluster exhibited remarkable capabilities in low-dose X-ray-induced tumor imaging and photodynamic therapy, with minimal adverse effects.To further enhance the stability of AIE molecules, porous MOFs can serve as suitable platforms for encapsulating these molecules.Herein, Zhao and co-workers reported AIE-COF@ZIF-8, resulting in composites of a AIE-based material. [76]It not only enhanced the stability of AIE-based 2D COF nanosheets but also retained a strong luminescence emission suitable for bioimaging (Figure 7).This approach effectively addresses the limitations of inefficient cellular uptake and the ACQ effect observed in conventional luminescent nanosheets, which tend to aggregate into large particles and result in poor physiological stability.This study presents the first instance of delivering hydrophobic 2D organic nanosheets into viable cancer cells through encapsulation within biocompatible MOFs.This advancement is expected F I G U R E 7 Schematic diagram of the challenge in bioimaging and AIE-COF@ZIF-8 for bioimaging.Reproduced with permission. [76]Copyright 2019, American Chemical Society.
to enhance the development of ultrathin 2D nanomaterials for diverse biological applications.
Scintillators, a class of functional materials known for their capacity to convert high-energy X-ray radiation into visible light, have gained increasing attention due to their potential applications in security inspection, radiation monitoring, X-ray astronomy, and medical radiography.The growing demand for X-ray imaging of 3D irregular objects has sparked considerable interest in the development of largearea flexible X-ray imaging membranes based on scintillating materials.Liu and presented a flexible composite scintillating membrane with superior imaging performance, achieved by embedding an AIE MOF scintillator (Y-PCN-94) into a polymer matrix (PDMS) (Figure 8A,B). [77]Y-PCN-94 exhibited a strong AIE effect under both ultraviolet (UV) light (Figure 8C) and X-ray irradiation (Figure 8D), marking the first observation of the AIE effect in the MOF system under an ionizing radiation field.Furthermore, Y-PCN-94@PDMS showed superior flexibility (Figure 8E), and demonstrated promising radioluminescence properties, including a low X-ray detection limit of 1.6 μGy s −1 and high imaging resolution of 14.3 lp mm −1 , attributed to the combination of the AIE effect and strong X-ray stopping power.This work showcases the potential of AIE MOFs as an alternative approach to achieve high-performance X-ray scintillators, offering promising applications in high-resolution bioimaging of cancer cells.
According to these results, AIE MOFs showed excellent performance in bioimaging of cancer cells, but it still faces several challenges as below.Firstly, the stability of AIE MOFs within the biological environment remains a key concern.The complexity of the biological environment and the acidity/basicity within the organism may lead to collapse of AIE MOFs, thereby affecting their luminescence properties and biocompatibility.Secondly, the biotoxicity of AIE MOFs needs to be carefully considered.While some studies indicate that AIE MOFs exhibit minimal toxicity to cells at low concentrations, assessing their biosafety is still necessary for long-term biological applications and in vivo bioimag-ing.Additionally, the metabolism of AIE MOFs within the biological system is an important consideration.Understanding the metabolic pathways of AIE MOFs within the body and whether they accumulate in specific organs or tissues is crucial for further application in bioimaging.In conclusion, although AIE MOFs have tremendous potential in the field of bioimaging, challenges related to stability, biosafety, and metabolism need to be addressed.

Disease therapy
AIE MOFs can serve as photosensitizers (PSs) to generate reactive oxygen species (ROS) through localized activation, leading to cell apoptosis or destroying tumor tissues.Furthermore, the porous structure of MOFs allows them to effectively load drugs.When exposed to the acidic microenvironment around tumor cells, these MOFs will decompose to release the drugs, offering a certain level of targeting.These exceptional characteristics of AIE MOFs contribute to minimizing damage to normal cells, making them a promising material for the development of photodynamic therapy and antibacterial therapy.

Photodynamic therapy
Investigations of photodynamic therapy (PDT) have revealed that the binding process between glutathione (GSH) and the copper node in AIE MOFs could be enhanced.This controlled activation of PDT minimizes the loss of ROS in both cancer cells and zebrafish, resulting in improved treatment outcomes.Building upon these findings, Liu and co-workers demonstrated that Cu-MOF (MOF-199, Figure 9A) with AIE activity can serve as stimuli-responsive materials to minimize ROS loss and enhance PDT efficacy. [78]To achieve this, they incorporated the AIE-based activatable PSs of 2-(4-(diphenylamino)phenyl)anthracene-9,10-dione (TPAAQ) into MOF-199, forming TPAAQ@MOF-199 nanoparticles Reproduced with permission. [77]Copyright 2023, American Chemical Society.
(Figure 9B).These nanoparticles exhibited efficient internalization by tumor cells.Upon photo-activation, the endogenous GSH broke down the MOF-199 structure through coordination competition, releasing the loaded AIE-based PSs to generate ROS.In vitro and in vivo tests demonstrated that TPAAQ@MOF-199 nanoparticles effectively ablated tumor cells and exhibited minimal phototoxicity to normal cells, making them suitable for antitumor PDT (Figure 9D).This approach can be applied to PSs that possess both AIE and ACQ properties, enabling the development of activatable imaging-guided PDT.Furthermore, the regulation of the tumor microenvironment by alleviating tumor hypoxia and reducing GSH enables targeted drug delivery and controlled release of a trapped chemodrug.Liu and co-workers loaded a platinum (IV)-diazido complex (Pt (IV)) into MOF-199 as Pt (IV)@MOF-199, which was further modified by AIE PSs of TBD conjugated to polyethylene glycol, namely TBD-Pt (IV)@MOF-199 (Figure 9C). [79]Upon internalization by tumor cells, TBD-Pt (IV)@MOF-199 could deplete intracellular GSH and degrade into fragments to release Pt (IV), which could generate O 2 to relieve tumor hypoxia and produced a Pt (II)-based chemodrug within tumor cells under light irradiation.Moreover, TBD could efficiently generate ROS and exhibited bright luminescence emission, enabling synergistic image-guided phototherapy and chemotherapy (Figure 9E).This proposed formulation of AIE-based composite photomaterials holds great promise for synergistic antitumor therapy with optimal efficacy.PSs for PDT has also gained significant attention because of their potential to minimize nonspecific phototoxic damage, leading to improved treatment outcomes.Liu and co-workers presented a simple and universal strategy for producing PSs by combining MIL-100(Fe) with various types of AIE PSs. [80]The photosensitization capability of PSs enclosed within MIL-100(Fe) was significantly enhanced due to their isolation from O 2 .The reaction between iron(III) in MIL-100(Fe) generated H 2 O 2 , which then triggers the collapse of the MIL-100(Fe) framework.This collapse enabled the enclosed AIE PSs to be exposed to O 2 , resulting in the activation of photosensitization, while H 2 O 2 decomposition could further generate O 2 to alleviate tumor hypoxia and enhance the effectiveness of PDT.Moreover, Liu and co-workers also utilized ZIF-8 as a platform for encapsulating AIE PSs, where the porous structure of ZIF-8 enhanced oxygen transport and boosts 1 O 2 production inside tumors during light irradiation.As-prepared PS@ZIF-8-PMMA-S-S-mPEG showed the ability to self-assemble through intratumoral reduction of the disulfide bond.This self-assembly leads to optimal tumor retention and high intratumoral ROS generation efficiency, thereby optimizing the efficacy of PDT.Therefore, they offer a universal approach to construct PSs@MOFs for efficient PDT. [81]

Antibacterial therapy
The combination of drug and AIE also can be used for antimicrobial treatment.Liu and co-workers developed a novel approach by developing nanocomposites of AIE MOF for in vivo bacterial metabolic labelling and image-guided antibacterial therapy (Figure 10). [82]They loaded 3-Azidod-alanine (d-AzAla), a molecule used to label bacterial metabolism, into MIL-100(Fe) to create composites called d-AzAla@MIL-100.These composites enabled the targeted release of d-AzAla at the site of bacterial infection by the decomposition of MIL-100 (Fe).After the infected bacteria F I G U R E 9 (A) Crystalline structure of MOF-199.Synthetic schemes of (B) TPAAQ@MOF-199 NPs and (C) TBD-Pt (IV)@MOF-199 NPs.Illustration of tumor treatment using (D) TPAAQ@MOF-199 NPs and (E) TBD-Pt (IV)@MOF-199 NPs.Reproduced with permission. [78]Copyright 2019, American Chemical Society.Reproduced with permission. [79]Copyright 2020, Wiley-VCH.
absorbed the released d-AzAla, the azide groups could be attached to the bacterial wall.Next, ultra-small nanoparticles based on AIE with dibenzocyclooctyne (DBCO) groups were used to track methicillin-resistant staphylococcus aureus (MRSA) in infected skin tissues.Under light irradiation, these nanoparticles efficiently eradicated the bacteria and reduced acute inflammation.
Wang and co-workers also demonstrated that the excitation of localized surface plasmon resonance (LSPR) in gold nanostars (AuNSs) significantly enhances the antibacterial activity of Zn-MOF nanosheets. [83]The AuNSs/Zn-MOF nanosheets exhibited a 2.5-fold increase in ROS generation for bacterial inactivation under light irradiation.Mechanistic investigations attributed this enhancement to a plasmoninduced "dual excited synergistic effect".On one hand, the photosensitive Zn-MOF nanosheets can be excited to gen-erate ROS with antibacterial properties.Moreover, LSPR excitation results in the generation of abundant plasmonic hot electrons on the surface of AuNSs.These electrons are transferred from AuNSs to Zn-MOF through energy matching.Consequently, Zn-MOF nanosheets create an electron-rich environment that activates the adsorbed O 2 molecules and promotes their decomposition into ROS for bacterial inactivation.This study emphasizes the effectiveness of LSPR excitation in enhancing the antibacterial activity of MOFs and provides a novel strategy for efficient antibacterial therapy.
On the basis of these studies, AIE MOFs, as emerging PSs and drugs carriers, displayed high performances in disease therapy.In order to realize the treatment efficacy and minimize side effects of AIE MOFs in disease therapy, there are still several challenges to be further addressed.Firstly, it is crucial to further enhance the photosensitivity of AIE F I G U R E 1 0 Schematic illustration of the proposed strategy for antibacterial therapy using d-AzAla@MIL-100(Fe) NPs.Reproduced with permission. [82]Copyright 2018, Wiley-VCH.
MOFs and their targeting capacity to achieve a high concentration of ROS in the targeted region, thereby resulting in an excellent treatment effect.Secondly, understanding the metabolic pathways of AIE MOFs within the body is essential to reduce the residual presence of metal ions and organic ligands, thereby avoiding potential secondary toxicity.Overall, AIE MOFs have significant potential for disease treatment, but further research and development are required to realize their widespread clinical applications.

Other applications
Besides their applications in sensing, bioimaging, and disease therapy, AIE MOFs also exhibit versatility in other fields.They demonstrate promise in anti-counterfeiting, utilizing their unique luminescent properties for secure materials.AIE MOFs also contribute to efficient LED technology, enabling high-performance lighting.Moreover, their potential in laser switch allows for precise control of laser output.These expanding applications highlight the wide-ranging capabilities of AIE MOFs, demonstrating them as highly promising materials for innovation.
Counterfeit products, such as pharmaceuticals and currency, pose significant risks to both public health and the economy, and require the use of anti-counterfeit tags capable of authenticating items throughout the entire supply chain and during their utilization.Pan and co-workers demonstrated two AIE MOFs (LIFM-102 and LIFM-103), [84] exhibiting two-photon excited luminescence performance and high quantum yields of 64.9% and 79.4%, respectively.Benefiting from their excellent luminescence performance, an information encryption device with pixel arrays was fabricated using LIFM-102 and a modulated inorganic phosphor BaMgAl 10 O 17 :Eu (Figure 11).Information was extracted from pixel dots coated with samples using one or two-photon F I G U R E 1 1 Schematic diagram and photographs of encryption device.Reproduced with permission. [84]Copyright 2022, Wiley-VCH.excitation and a specified scan direction.The information encryption device emitted different signals when excited by specific wavelengths, indicating the presence or absence of information.Thus, by applying either phosphor or LIFM-102 to various pixels, the concealed information can be diversified, thereby achieving anti-counterfeiting measures.
F I G U R E 1 2 (A) Structure of tpbe-Cd.(B) Illustration of the tpbe-Cd microlaser measurement.Reproduced with permission. [87]Copyright 2019, American Chemical Society.
By modifying these guest molecules, circular dichroism, circularly polarized luminescence, white-light emission, and room-temperature phosphorescence were realized, respectively.This work provides a valuable strategy for adjusting luminescence in crystalline porous materials.AIEgens can also function as guest molecules for incorporation into MOFs to modulate luminescence.Su and co-workers synthesized two cationic dyes, namely 2,3,4,5tetrakis(4-methoxyphenyl)oxazol-3-ium (MOTPO) and 1-methyl-4-(4-(1,2,2-triphenylvinyl)phenyl)pyridin-1-ium (TPEPy), that exhibit bright blue and yellowish green emissions, respectively. [86]These dyes were then integrated into the pore of the anionic framework ZJU-28.The combination of AIE characteristics and confinement effects in the hybrid material leads to a stronger emission signal than that of the individual components.By optimizing the ratio between the two guests, a quantum efficiency of 64.9% was achieved, falling in the highest reported values for WLE MOF composites to date.The resulting WLED exhibited demonstrated an external quantum efficiency of 12.67%, and an excellent color-rendering index value of 86, as well as an moderate correlated color temperature (CCT) of 4076 K.This work offers novel method for the development of excellent WLEDs.
Miniaturized lasers have attracted considerable interest owing to their promising applications, such as highthroughput sensing and on-chip optical communication.To enable the realization of highly versatile integrated photonic elements, micro-/nanolasers must exhibit wavelength variability.Zhao and co-workers demonstrated a laser switch that was controlled by steric hindrance in a MOF-based system. [87]The Cd-MOF ([Cd 3 (tpbe) 2 Cl 5 (DMAC) 6 ]) was synthesized using AIE luminescent linkers (Figure 12A).The well-faceted MOF microwires, combined with luminescent materials, functioned as conventional Fabry-Pérot microlasers.The presence of guest molecules caused steric hindrance around the AIE linkers, which impeded their rotation and resulted in a red-shifted gain behavior.The gain region could be easily switched by modifying the steric hindrance through the desorption/adsorption of guest molecules.Then, a gas flowing system was constructed using Cd-MOF microwires, enabling the reversible switching of dual-wavelength lasing (Figure 12B).This work offers a new perspective on the stimulated emission of MOFs and provide valuable insights for the development of miniaturized lasers with desired performance.
Upon these results, AIE MOFs have demonstrated tremendous potential in interdisciplinary fields such as anti-counterfeiting, LED technology, and laser switch.However, they also face certain challenges.In terms of anticounterfeiting, it is necessary to further enhance the stability and fabrication processes of AIE MOFs to ensure their long-term effectiveness in practical applications.For LED technology, efforts are needed to overcome limitations in the luminescent efficiency and color purity of AIE MOFs to meet the high-performance lighting requirements in various domains.In the field of laser switch, in-depth research on the luminescence properties and energy transfer mechanisms of AIE MOFs is essential to achieve precise control and modulation of laser output.With continuous technological advancements and a deeper understanding of AIE MOFs, it is believed that these challenges will be effectively addressed, leading to further innovation and application prospects of AIE MOFs in the interdisciplinary fields, such as anti-counterfeiting, LED technology, and laser modulation.

CONCLUSION AND PERSPECTIVES
Over the past two decades, both AIE phenomenon and MOFs continuously attract numerous attention due to their outstanding luminescence properties and structural characteristics, respectively.Taking these advantages, the integration of AIE properties into MOFs has emerged as a promising approach for the development of luminescent materials.This review highlights the state-of-the-art advancements of AIE MOFs, focusing on their mechanisms, design strategies, and applications in chemical sensing, bioimaging, disease therapy, and so on.Despite the rapid progresses and excellent results that have been achieved in the field of AIE MOFs, the corresponding structure-function relationships and directional design are currently explained primarily through speculations and assumptions rather than well-established theories.The structure-function relationship of AIE MOFs lacks a deep understanding and requires further development in some aspects, including directional synthesis, in situ characterization, and theoretical calculations.In the context of directional synthesis, parameters such as temperature, solvent, and reaction time play a crucial role in regulating structures at the molecular level, resulting in AIE MOFs with varying topologies, pore sizes, and functional groups.To facilitate analysis, the establishment of a corresponding database is necessary, and artificial intelligence algorithms can be employed for its analysis.Based on the obtained results, the luminescent performances of these AIE MOFs can provide semiempirical structure-function relationships, making directional synthesis a primary concern.In the case of in situ characterization, further development is needed for X-ray single crystal diffraction, scanning/transmission electron microscopy, and luminescence/infrared/Raman spectroscopy to reveal guesthost interactions and energy transfer processes in AIE MOFs, thereby obtaining experimental mechanisms.These investigations can further support the aforementioned semiempirical structure-function relationships.Additionally, theoretical calculations should be conducted to determine energy levels, energy transfer processes, and especially the intermediate states of energy transfer that cannot be obtained through experiments.Theoretical calculation results can provide indepth explanations of the structure-function relationships, offering effective guidance for designing AIE MOFs.
Moreover, the majority of studies on AIE MOFs are still in the proof-of-concept stage, and practical applications are yet to be fully realized.In order to achieve practical applications of AIE MOFs, it is crucial to address several challenges that require collaborative efforts from different research fields.Further explorations should be carried out in the following aspects to gain a more indepth understanding and enable practical applications of AIE MOFs.For practical applications of AIE MOFs, large-scale production is the most important factor.In 2023, BASF achieved the first commercial-scale production of MOFs for carbon capture.However, challenges persist in industrial MOFs synthesis, particularly in conditions, purity, and cost.Achieving industrial-scale production requires addressing synthesis challenges, though laboratory-scale synthesis is relatively straightforward.Controlling and repeating synthesis reactions is vital for stable products.Optimizing reaction conditions, including temperature, pressure, and solvent selection, enhances MOFs quality and yield.Safety aspects must be prioritized.MOFs purity is critical for performance and applications.Controlling and removing impurities during synthesis, along with developing purification techniques, ensures high-purity products meeting application standards.Expensive metal ions and organic linkers limit cost-effectiveness in MOFs synthesis.To achieve sustainable development and commercial applications, cost reduction is crucial.This involves optimizing synthesis methods, reducing raw material usage, improving yield and purity, and developing cost-effective routes.Overcoming synthesis challenges requires interdisciplinary collaboration in chemistry, engineering, and materials science.Continuous research and innovation enable industrial-scale AIE MOFs production, expanding applications in various fields.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflicts of interest.

F I G U R E 1
Schematic illustration of topic in this review.

F I G U R E 4
(A) Model of light-up luminescent detection by a guest-lock process.(B) Emission spectra of Cu(I)-MOF treated by guest molecules.(C) The structure of CH 2 Cl 2 @Cu(I)-MOF.(D) The response curves of Cu(I)-MOF to CH 2 Cl 2 .

F
I G U R E 5 (A) Schematic diagram of multicomponent AIE MOFs sensor array for energetic compounds detection.(B) The sensitivity and selectivity of AIE MOFs.(C)
This work was supported by the National Natural Science Foundation of China (Grants No. 22121005, 21931004, and 22271159) and the Ministry of Education of China (grant ref: B12015).
THBDBA) is a unique AIEgen based on both RIR and RIV mechanisms.For AIE MOFs, the designable features of MOFs can be used to regulate their luminescence, and the structures of crystalline MOFs can be simply resolved and analyzed to facilitate in-depth investigations into the underlying mechanisms of AIE.