Organisms in nature are able to change their appearance (such as color) or behavior in response to different environmental stimuli (including light, temperature, force, electricity, etc.).1 For example, once the noctiluca is stimulated by the outside world, it will stimulate the body’s instinct to emit light, which forms the fluorescent sea that we see. Therefore, these complicated stimuli-responsive natural systems provide inspiration for humanity and have given birth to biomimicry. Therefore, the stimuli-responsive behaviors of nature inspired scientists and engineers to create a wide range of smart materials by a mimicking strategy.2 Therefore, stimuli-responsive smart materials are widely used in optoelectronic devices, lighting displays, biological applications, and others.1^,3^,4

Since light stimuli have distinct advantages, such as convenience, speed, high spatiotemporal precision, being environmentally benign, and having a noninvasive manner, the light-responsive system has attracted more interest and been regarded to be a prominent candidate for the development of biotechnology and applications in controlled drug delivery and bioimaging.5 Among them, photoswitchable molecules (also named photochromic molecules) have been extensively studied as a type of classical stimuli-responsive materials. Photoswitchable molecules could switch between two discrete states upon different light irradiation, such as azobenzene (AZO),6^,7 spiropyran (SP),8^,9 diarylethene (DAE),5^,10^,11 dihydropyrenes,12 stilbene,13^,14 alkene-based motors,15^,16 and so on.17^,18 Specifically, trans–cis isomerization of AZO undergoes a light-induced photoisomerization process; colorless closed-ring isomer SP isomerizes to red-color open-ring isomer merocyanine (MC).17 DAE shows reversible photoswitch and significant differences in color from the colorless open-ring form to the strongly colored closed-ring form upon alternating UV light or visible light irradiation.11 In summary, photochromic compounds are important owing to their reversability, controlability, excellent fatigue resistance and thermally irreversible properties, high photoisomerization quantum yields (QYs), and rapid response and can avoid generating waste compared to most chemical-responsive systems.9^,17^,19 That is to say, introducing photoswitchable molecules into combined materials can enhance the photophysical, electronic, mechanistic, or thermal properties of materials, which provide an excellent platform for the design of stimuli-responsive photonic materials.19

To date, a variety of stimuli-responsive, dynamic, and flexible metal–organic framework (MOF) materials have been documented in this review. As the name suggests, a special class of MOFs/porous coordination polymers (PCPs) are those that can reversibly change their framework structure when guest molecules (gas, solvent, and so on) are introduced or the external temperature stimuli change. This results in phenomena such as breathing (pore contraction) or gate opening (pore expansion), where pores open or contract upon adsorption, known as the breathing effect, gating effect, or shape-memory effect.20^,21 Other groups also have reported many temperature-responsive flexible MOFs, such as MIL-53 series materials.22 More importantly, the fascinating characteristics of flexible MOFs have great potential applications in many fields, such as gas storage and separation, sensing, and guest capture and release.23 Remarkably, the breathing effect and accompanying photophysical changes of flexible MOFs provide an excellent platform for the design of stimuli-responsive photonic MOFs.

The research on photonic functional materials involves the processes of absorption, generation, transmission, conversion, detection, storage, and display of photons, which is one of the important material foundations for exploring photonics phenomena and technologies. Because of their material properties, photonic functional materials, including inorganic and organic photonic materials, both have been widely studied. However, there are still areas that need urgent improvement. The preparation of inorganic materials usually requires high temperatures and relies on complex doping processes. Organic materials have poor photostability and are prone to concentration quenching in the solid state. Photonics performance regulation of materials refers to the control of certain internal/external factors to induce materials to produce dynamic and regularly changing photonics properties under light fields. Compared to a single static optical performance, dynamically adjustable optical and photonics performances are more suitable for the development of integrated, functional, and intelligent micro/nano-optoelectronic devices.24 Researchers utilize a large number of materials to study controllable optical properties, explain changes, and expand regulatory methods to achieve the goal of “controlling” light through material media.25^,26

MOFs are an outstanding class of organic–inorganic porous materials and have gained immense attention from researchers over the last decades.27^–29 The integration of high surface area, excellent crystallinity, tunable structure and porosity, diversity of morphology and modifiable chemical functionality endows them with great promise for gas storage and separation, chemical sensing, catalysis, pollutant treatment, drug delivery, and so on.19^,28^,30^–43 Our group is committed to developing advanced photonic functional MOF materials. In the discussion of stimuli-responsive MOFs in this review, we continue the concept of “multiphoton units” (MPUs) proposed in the previous literature (Fig. 1).28 We consider MPUs of photonic functional MOFs, mainly comprising metal ions/clusters, organic ligands, and guests. In this review, we discuss stimuli-responsive MPUs (such as photoswitchable molecules and dyes) that could be incorporated into the framework of MOFs in three different ways: as a guest, as a ligands backbone, or as a ligands side group (Fig. 2).

Due to space limitations, the stimuli-responsive photonic MOFs involved in this review mainly include two types. The first type of particular interest of stimuli-responsive photonic MOFs can be found in a class of MOF materials whose structure and composition may change under external stimuli, including (1) MOFs based on photoswitchable molecular-derived ligands and (2) flexible MOFs. Since photoswitchable molecules as a typical MPU could be integrated inside a MOF matrix, the pore size and pore environment of the photoswitchable MOFs will change under external stimuli, resulting in new functionalities.44 The attractive stimuli-responsive photonic properties of flexible MOFs mainly originate from their flexibility and dynamic behavior. The second type of stimuli-responsive photonic MOFs is mostly focused on the MOF⊃guest materials, constructed by incorporating guest MPU (e.g., photoswitchable molecules and dyes) into the pores of the MOFs. Due to the synergistic effects, combination possibilities, controllable and ordered arrangements of guests and MOFs, MOFs are an optimal platform for controllable design and synthesis of the hybrid crystalline material with novel photonic function. In fact, a series of new photonic functionalities, including polarization-responsive nonlinear optics (NLO) or lasing behaviors have been successfully implemented in MOF⊃guest materials.45^,46 Thanks to the response of photon building unit to external stimuli, the photonic properties of the photonic MOF materials can be regulated by controlling external stimuli.

In this review, stimuli-responsive photonic MOFs are summarized and classified in term of several representative external stimuli. Owing to space limitations, only some recent representative examples were selected to demonstrate the specific stimuli-responsive performance of each type of stimulus, which provides new insights and ideas for designing multifunctional stimuli-responsive photonic MOF materials. In the end, the future challenges of stimuli-responsive photonic MOFs are also briefly discussed.

Recently, stimuli-responsive photonic materials have gained immense attention, mainly owing to the rapid development of smart materials. The so-called stimuli-responsive photonic material refers to the material’s specific perception and response to external stimuli (e.g., temperature, pressure, light, and pH) in the environment, specifically manifested in changes in optical or photonic properties.4^,47 Up till now, various strategies have been developed to build stimuli-responsive photonic materials, including incorporating photoswitchable molecules, modifying side chains, and regulating topological structures.48 However, there are still many difficulties and challenges in designing and constructing stimuli-responsive materials owing to the difficulty of completing switching behavior in confined environments as well as precisely regulating and controlling the molecular structure.

Nowadays, various approaches to construction of photoswitchable molecule-bound polymers have been developed.2 In polymer systems, photoswitchable molecules as modification units can be bound to polymer networks through a noncovalent manner or be directly covalently connected to polymer molecular chains. Combining photochromic units with polymers provides a new strategy for developing functional materials.9^,11^,24^,49 The polymer backbone has a much larger contribution to overall solubility than the photochromic groups. For example, Li et al. developed several examples about the construction of the photoresponsive luminescent polymeric hydrogels as well as their applications in information encryption.50^,51 However, photoswitchable polymer materials usually have complex synthesis routes (method of covalent immobilization), high requirements for reactants, and are not green enough, resulting in limited application scenarios.

Over the past few years, inorganic nanoparticles (NPs), such as gold NPs, lanthanide-doped upconversion NPs (UCNPs), and quantum dots (QDs), have been functionalized and combined for nondestructive optical memory and photoswitching applications.3^,52^–55 For example, Medintz and co-workers employed an immobilization approach to obtain SP-coated CdSe–ZnS core-shell QDs with ultimate control of Förster resonance energy transfer (FRET) efficiency and reversible dual-emitting luminescence.56 Lee et al. reported a UCNP with orthogonal emissions when excited at 808 and 980 nm for controlled two-way photoswitching of SP.57 Significantly, this article reported that UCNPs were dispersed in SP solution to achieve isomerization. However, the future development trend is to obtain solid-state photoswitchable luminescent materials that can produce rapid and significant color changes and excellent luminescence.

In this review, we point out that different stimuli-responsive MPUs allow precise control of the different photonic properties of MOFs. Stimuli-responsive photonic MOFs, against the photoswitch behavior as well as the interaction between photoresponsive units and stimulus sources, can serve as a model platform for research on the photophysics mechanism as well as developing next-generation multifunctional materials. Hence, the fabrication and application of stimuli-responsive photonic MOFs will be the research hotspots for a long time into the future. Next, we will summarize the advantages of stimuli-responsive photonic MOFs.

The immobilization of a stimuli-responsive guest (e.g., photochromic molecules and dyes) in MOFs can isolate the guests from each other, thus suppressing stimuli-responsive guest stack formation and protecting the fluorophores from the aggregation-caused quenching (ACQ) effect.9 As a result, emission QYs are increased. Due to the energy transfer (EnT) process between MOFs and the guest, the fluorescence of the guest can further be enhanced by constructing MOF⊃guest materials, which is particularly important for the development of switchable fluorescent materials.58^,59 Due to the stability and designability of MOF structures, stimuli-responsive MPUs (e.g., photochromic molecules and dyes) could be introduced into the framework of MOFs in three different ways. When stimuli-responsive MPUs are introduced into the ligands of MOFs, the rigidity of MOFs greatly improves the stability of stimuli-responsive MPUs, which will promote the development of multifunctional photonics materials.

Successful switching and communication of the majority of photoswitches under confinement require some degree of conformational freedom. For example, in the solid state, the crowded environment greatly limits the available freedom of switches; in the solution environment, the communication between an optical switch and these loosely restricted systems is weak. When assembling photochromic molecules into the cavity of a MOF or coordination cage, successful switching can be observed when there is an appropriate degree of freedom.12 Benefiting from the unique bistable characteristics and excellent fatigue resistance of molecular switches, rapid photoisomerization can be achieved in limited space environments of MOF ⊃ guests. By reasonably designing photochromic derivative ligands to reduce the spatial hindrance in MOFs, one must improve the photo-responsive performance of stimuli-responsive MOFs. Finally, stimuli-responsive MOFs can be further applied to the development of smart material, molecular machines, etc.

Due to the designability of MOF structures, introducing MPUs into the MOF framework can centralize multiple photonics centers in a multifunctional material, thereby endowing the material with new photonics functions. For example, introducing photochromic molecules into the MOF framework leads to effective intramolecular interactions between different MPUs due to the clear shape and size of the guest. On the other hand, this process enables MOFs to have light-responsive characteristics, which involve structural and photophysical changes. Such systems exhibit broad applications in light-controlled guest release, rewritable molecular inks, photodynamic therapy (PDT), and so on, which is the key to the development of multiresponsive materials and smart devices. In another case, through the inclusion of dyes into the pores of the MOFs, MOF⊃guest materials could demonstrate tunable NLO or lasing performance or even new photonic performance.

To begin with, stimuli-responsive MPUs, such as photoswitchable molecules, can serve as a class of molecular building blocks or organic ligands for constructing MOFs through coordination (Fig. 2). Typically, the light-responsive carboxylic acid linkers or pyridyl linkers are obtained by incorporation of photoswitchable molecules in the backbone of the linkers and then corresponding MOFs were formed through coordination.27^,60 The DAE-based ligands are incorporated into the backbone of MOFs, while the photoisomerization of DAE is only accompanied by small structural changes and tiny decreases in the rigidity of the MOF.59 As to the isomerization of trans–cis AZO and SP-MC, the photoisomerization processes are usually accompanied by large structural changes. For example, upon different external stimuli, this strategy could regulate the window size of MOF through photoisomerization process of trans–cis AZO, and the MOF could be further applied in switchable adsorption separation and contaminant removal.61^–63

There are also numerous photoswitchable side groups, such as AZO, DAE, and SP that can be incorporated into the ligands of MOFs (Fig. 2). For example, Wang et al. reported a photo-responsive AZO-functionalized mechanized UiO-68-azo for on-command cargo release.64 Shustova et al. designed an SP derivative-based linker, 1′,3′,3′-trimethyl-6-nitro-4′,7′-di(pyridin-4-yl)spiro[chromene-2,2′-indoline] (TNDS), which was incorporated into rigid MOF scaffolds by coordinating with Zn- or Zr-metal.63

Given that (1) the isomerization process of photochromic molecules effectively triggers the structural transformation of MOFs, causing a sharp change in the adsorption properties of photoswitchable MOF materials, and (2) the reversible photoisomerization can bring about notable magnetic changes and an increase in the electronic conductance, photoswitchable ligand-based MOFs have shown great promise for gas storage and separation,65^–73 guest adsorption and diffusion,74^–76 proton conductivity,77^,78 drug-controlled release,64 electronic devices,79^,80 and so on81 in recent years. Apart from those applications, photoswitchable ligand-based MOFs also have contributed important progress to the field of photonics.

Next, we will review related works of light-responsive MOFs in EnT system design, light capture, singlet oxygen (O12) generation, circularly polarized luminescence (CPL), and chemical detection in detail.

In 2013, Yaghi et al. demonstrated the synthesis of a photoswitchable AZO functionalized isoreticular MOF (azo-IRMOF-74-III) [Mg2(C26H16O6N2)] for the controllable release of propidium dye upon an external light stimulus (such as laser on/off) (Fig. 3).82 Under light stimulation, the photoisomerization of photochromic molecules is accompanied by obvious changes in the ultraviolet (UV) absorption spectrum, which provides an excellent platform for the EnT process. For example, Shustova et al. fabricated a rigid scaffold of Zn-MOF using a photoswitchable DAE-based bis(5-pyridyl-2-methyl-3-thienyl)-cyclopentene (BPMTC) ligand and a tetrakis(4-carboxyphenyl)-porphyrin (H4TCPP) ligand (Fig. 4).59 In this case, a previously reported molecular D–A dyad approach was used in which BPMTC (A) was immobilized between H4TCPP (D) layers. Under the irradiation of 365 and 590 nm light, BPMTC can switch between open form and closed form reversibly, leading to changes in its absorption band (colorless to blue), and selectively quench the luminescence of H4TCPP, thus activating the MOF optical switch based on the EnT process. Time-resolved photoluminescence spectroscopy showed that the average emission lifetime of H4TCPP was decreased, which indicated a resonance EnT-based mechanism. In another case, they constructed a series of DAE and SP derivative-based linkers, such as TNDS, 1′,1‴,3′,3′,3‴,3‴-hexamethyl-6,6″-dinitro-4′,4‴-di(pyridin-4-yl)-7′,7‴-bispiro[chromene-2,2′-indoline] HDDB, and BPMTC linkers that were coordinated with Zn- or Zr-metal to obtain rigid MOF materials.63 They also demonstrated the photochromic molecules that were immobilized in the scaffolds of MOFs, which can control the kinetics of the photoisomerization process. When exposed to acid vapor, the skeleton of MOF materials undergoes degradation leading to ligand release, which is accompanied by thermochromic shifts of over 100 nm and drastic emission changes (from purple to green). Following this route, in 2020, Feringa et al. built a visible-light-driven pillared-paddlewheel MOF with porphyrin (PdTCPP) as a light-harvesting unit and bispyridyl-derived molecular motor linker.16 In this MOF system, the photoswitch of the molecular motor was achieved upon irradiation with low-energy green light (530 nm), which indicated an efficient EnT from the porphyrin linker to the motor linker. Considering the unique long-wavelength-driven molecular motor, this MOF system is promising for future applications in photocatalysis, solar cells, and biosensing.

In consideration of the gigantic benefits of light control with high spatial and temporal resolution, catalytic performance can be regulated. The O12 produced by catalytic oxidation process has applications in biological and medical systems (for example, PDT). One example of MOF EnT system containing mixed ligands was reported by Feringa and co-workers.58 They demonstrated a noncovalent strategy to reversibly regulate on and off O12 generation using a DAE photochromic switch and porphyrin photosensitizers. There, the effect is based on the efficient EnT from the porphyrin to the strongly colored closed-ring form of DAE upon irradiation at 420 nm, resulting in the O12 generation conversion to the OFF state, while when the DAE unit is in the colorless open form, the O12 generation state is ON. Zhou and co-workers also developed two classical pillar-layer structures, namely, PC-PCN (photochromic porous coordination network) and SO-PCN (singlet oxygen-generating PCN) with a 1,2-bis(2-methyl-5-(pyridin-4-yl)thiophen-3-yl)cyclopent-1-ene (BPDTE) linker and a Zn-based tetrakis(4-carboxyphenyl)-porphyrin (TCPP) linker (Fig. 5).83 Then they studied the reversible regulation over O12 generation through competition between two EnT pathways upon UV (365 nm) or visible (>450nm) light irradiation in SO-PCN. Furthermore, SO-PCN can efficiently catalyze the photooxidation of 1,5-dihydroxynaphthalene molecules. Subsequently, they continued to synthesize a DAE-based Zr-MOF for PDT.84 By incorporating a porphyrin system (TCPP, dyad) and derivatization of DTE, 1,2-bis(5-(4-carbonxyphenyl)-2-methylthien-3-yl)cyclopent-1-ene (BCDTE) to UiO-66, this Zr-MOF nanoplatform showed enhanced EnT-based PDT efficacy with excellent O12 control.

Overcrowded olefin-based light-driven molecular motors enable large-scale repetitive unidirectional rotations. Although their behavior in solution has been well studied, relatively little research has been done on their behavior in the solid state. In view of this, Feringa and co-workers reported that a novel molecular motor unit embedded in a pillared-paddlewheel crystalline MOF could achieve unhindered, large-amplitude unidirectional rotary motion under UV light irradiation and heating (Fig. 6).15 In particular, Raman spectroscopy allows the in situ monitoring of the isomerization process of the molecular motor in the crystalline moto-MOFs. Impressively, these linker-based “moto-MOFs” have promising potential for controlling dynamic function in crystalline materials.

Chirality is a central theme in nature and has important significance in biology, medicine, and other fields.85 Among them, achieving dynamic control of chirality remains a huge challenge in the field of chemistry. In recent years, Heinke et al. developed several photoswitchable surface-mounted MOF (SURMOF)-based hybrid materials, which meet the developing trends of the controlled release of guest molecules and proton conduction.69^,74^,75^,78 Interestingly, they also reported a chiral and enantioselective enrichment of crystalline nanoporous materials induced by CPL.86 The scaffold of Cu2(F2AzoBDC)2(dabco) SURMOF material attached with a fluorinated AZOs side group (Fig. 7). The AZO side group undergoes a photoisomerization process between trans form and cis form upon green light and violet light irradiation, respectively. Depending on the CPL-handedness, CPL results in chiral photoresolution, causing an optically active material. This work opens new opportunities for photochromic molecular switches for information storage with CPL.

Photocycloaddition reaction is also one of the important strategies for constructing dynamic responsive luminescent materials. In particular, the photoisomerization process that occurs in MOF materials is based on stilbene derivative ligands.14^,87^,88 For instance, Lang et al. reported photoreactive olefinic species are integrated into a MOF, [Zn(bdc)(3-F-spy)] (1,3-F-spy=4-(3-fluorostyryl)pyridine; H2bdc=1,4-benzenedicaboxylic acid). This MOF 1 goes through [2+2] photodimerization reactions upon illumination by 365 nm UV light (Fig. 8).13 On this basis, other photopolymerization-driven polyvinyl alcohol composite film and stilbene-based Zn2(sdc)2 (sdc =4,4′-stilbenedicarboxylate) MOF film have also been reported by Lang and Gu and others.89^–91

As aforementioned, stimuli-responsive fluorescent MOFs have broad application prospects in security protection fields, such as information encryption and anticounterfeiting displays, mainly due to their ability to successfully prevent information or data from being stolen, mimicked, or forged.92^,93 Among them, the biggest advantage of light-responsive MOFs is that their chemical and structural changes can be regulated through light stimulation, providing an alternative way for constructing MOFs with switchable optical absorption and fluorescence properties. Following this path, Qian et al. reported the first two-photon responsive MOF for three-dimensional (3D) two-photon patterning and fluorescence imaging (Fig. 9).94 MOF “ZJU-56-0.20” was synthesized by a photoactive zwitterionic pyridinium linker 2,5-bis(isophtalic-5-yl)-1-methylpyridinium hydroxide (H4L1·OH) and a 5,5′-(pyridine-2,5-diyl)-diisophthalic acid (H4L2) linker via a multivariate (MTV) strategy. Under UV light (365 nm), the fluorescence emission intensity of ZJU-56-0.20 gradually increases, accompanied by an obvious redshift of the emission band from 450 to 550 nm. Furthermore, they found that the same photochemical change could be induced with a 710 nm laser, but only created two-photon excited fluorescence (TPEF) without photochemical transformation using 900 nm laser as excitation wavelength. To take advantage of the TPEF, the 3D two-photon patterning was developed in which the pattern can be written and read reversibly with the altering 710 and 900 nm laser irradiation. It is worth noting that this work opens the door for two-photon responsive MOFs in 3D reconstruction and data storage.

In addition to the dynamic responsive luminescence of light-responsive ligands, dynamic responsive luminescence can also be achieved based on the changes of metal clusters. In another case, Milichko et al. demonstrated an all-optical MOF switch (HKUST-1, [Cu3(BTC)2(H2O)3]n) [1,3,5-benzenetricarboxylic acid (BTC)].95 There are reversible hydration and dehydration processes in the structure of HKUST-1, accompanied by color changes from cyan to navy. Owing to dehydration and the concomitant shrinking of the structure-forming [Cu2C4O8] cages, a reversible blueshift of the absorption band was found upon ultrafast near-infrared (NIR) laser pulses radiated. In summary, the light-induced switching can reversibly turn off/on the visible light passing through the MOF single crystal, and the initial PL intensity can be controlled remotely via NIR laser pulses.

Since MOF has permanent porosity, uniform cavities, and tunable pore sizes, while the photoswitch process requires some degree of freedom, the photoswitchable guests confined in the pores of MOFs can control their photophysical properties, which can be converted into photoswitch integration and meet the requirements of optical or photonic communications. On the other hand, the derivation and polymer of photochromic molecules are often complex and expensive. Comparatively speaking, several photoswitches and dyes are commercially available, so the preparation of light-responsive MOF⊃guest compounds does not have the aforementioned synthesis limitations, which can greatly overcome synthesis challenges.96^,97

Four factors need to be considered: (1) the photoisomerization of photoswitchable compounds can effectively impart structural transitions and switching of adsorption properties to a MOF host–guest composite; (2) taking SP as an example, the MC state has a positively charged indole group and a negatively charged phenolic group, which can be used as an important binding site for anions and cations; (3) the reversible photoisomerization of SP-MC could generate different mobile acidic protons in response to the light; and (4) the reversible photoisomerization can result in significant magnetic changes. Photoswitchable MOFs⊃guests have been extensively applied in gas storage and separation,6^,61^,98^–100 sustainable water desalination,87^,101^,102 proton conductivity,103 magnetic materials,104 and photoelectronic devices105^,106 in recent years.107 However, research about photoswitchable host–guest MOFs in the field of optics and photonics is still in its infancy, so we only discuss several photoswitchable host–guest MOFs for optical anticounterfeiting and other applications to help readers understand the mechanism of this stimuli-responsive behavior.

Because obtaining photoswitchable host-guest MOFs systems is a challenging task, Zhu et al. encapsulated nitrobenzospiropyran (BSP) derivatives into pores of JUC-120 by a microwave-assisted crystallization inclusion route to obtain the BSP/JUC-120 films, which exhibit high reversibility and thermal stability of photochromism properties.108 In another example, Benedict et al. confirmed that DTE is aligned within the pores of DMOF-1 through linear dichroism, and DMOF-1@DTE shows photochromic properties upon light irradiation.109 In 2014, Klajn and co-workers described two families of porous aromatic framework (PAF) covalent incorporating the SP molecular switch. Upon desolvation of this material, the color of PhotoPAF-3.6% powder was observed to change from yellow to deep blue (λmax≈550nm), suggesting the conformational freedom of SP-MC isomerization.110 In a recent paper, Ruschewitz et al. confirmed the successful incorporation of SP into the pores of different prototypical MOFs, including MOF-5, MIL-68(In), and MIL-68(Ga).111 Surprisingly, strong photochromic behaviors were all observed in various pore environments.

As a special example, anthracene (ANT) is a special kind of photoswitchable compound that could undergo the process of photoisomerization and convert to nonfluorescent photodimer under UV irradiation. Interestingly, this photodimer can be decomposed by photochemical irradiation in the range of 250 to 290 nm or thermal decomposition at 250°C to 340°C. In 2019, Ameloot et al. first loaded ANT in the spherical cavity of a ZIF-8 framework to obtain an ANT@ZIF-8 photochromic system through a post-synthetic sublimation route [Figs. 10(a) and 10(b)].112 The mechanism behind the photoswitching of ANT@ZIF-8 was systematically investigated. Upon UV light irradiation for 30 min, two yellow excimer ANT molecules from the pairs’ photodimerization formed a nonluminescent photodimer; the other two ANT molecules displayed purple monomer emission, which resulted in reversible yellow-to-purple photoswitching of the fluorescence emission. The reversible solid-state photoswitching and thermoswitching accessibility in ANT@ZIF-8 were further applied in photopatternable, erasable, and rewritable paper. In the subsequent work, Xu and co-workers reported Eu3+@HPU-14@ZIF-8@ANT and Tb3+@HPU-14@ZIF-8@ANT composites for the anticounterfeiting application by the same photodimerization reaction.113

It is promising to incorporate photochromic molecules into MOF cavities for reversible optical data storage and information anticounterfeiting. In 2019, Li et al. loaded the open-form OF-DAE into the channel of the Eu-MOF ZJU-88 to obtain ZJU-88⊃OF-DAE material.114 DAE molecules undergo reversible open/closed ring structure transitions under alternating UV and visible light irradiation, corresponding to OF-DAE and CF-DAE, respectively. As to DAE⊃ZJU-88 composite, the red emission was gradually quenched via the FRET between CF-DAE and Eu3+ as OF-DAE transforms to CF-DAE upon 300 nm UV irradiation. Considering these unique advantages of this remote light-induced reversible on–off switching of the fluorescence emission, the DAE⊃ZJU-88 composite was developed as luminescent QR code. In addition, chiral porous crystalline materials have an ordered assembly structure, which is of great significance in the construction and performance improvement of CPL materials.85 Recently, by designing “turn-on” DAE derivative (DAEC) and UCNPs-loaded chiral MOFs, Duan and co-workers realized multiple light responsive CPL solid-state switches, and the amplification of CPL was achieved through the EnT process.115 The light-responsive CPL-active L-MOF⊃DAEC was prepared by loading the DAEC into the chiral Ln-MOFs. Due to the reversible photoisomerization process of DAEC molecules, the downshifting (DS) CPL of L-MOF⊃DAEC reversibly switched from on to off under different light irradiation. Because of the effective EnT from the UCNPs to the ring-closed (DAECc) components, another L-UCMOF⊃DAEC composite with tunable UC-CPL was achieved upon NIR and visible light irradiation. In addition, compared to the DS-CPL of L-MOF⊃DAEC (|glum|=0.014 to 0.020 at 546 nm), the UC-CPL of L-UCMOF⊃DAEC achieved an enhanced |glum| of 0.078 at 546 nm.

Developing photo-stimuli-responsive luminescent materials that can be applied in security fields such as information encryption is currently an urgent challenge needing to be solved. Although various photoisomerizable molecules have been developed, they generally operated in solution because they were hampered in the solid state; the isomerization process requires some degree of conformational freedom. In addition, most of these materials exhibit the single “turn-on” or “turn-off” switching of fluorescence emission, which means that the stimuli-responsive luminescence is hard to be monitored when in the switch-off state, preventing the accurate quantitative readout of storage information. It is hoped that the designability of MOFs makes them become one of the solid-state light-responsive materials to address the above requirements [Fig. 11(a)]. It is precisely because the pore confinement of porous MOF crystals can suppress the nonradiative decay pathway of photochromic molecules, which avoids the ACQ effect caused by self-stacking; it effectively enhances the fluorescence behavior of photochromic molecules. Apart from that, the suitable distance between the MOF material (donor) and the photochromic molecules (acceptor) could effectively promote the EnT process as well as improve the fluorescence intensity and fluorescent QY of photochromic molecules. Furthermore, unlike the single and static fluorescent behavior of the conventional MOFs system, the MOF⊃photochromic molecules composite is a dynamic dual-emissive platform that exhibits a time-dependent fluorescent color-tunable property and the dynamic tunable ratio of PL intensity, which is very important for dynamic information anticounterfeiting applications and high-security multiplexed encoding.

Hence, as proof of concept, Cui et al. reported a photo-stimuli-responsive dual-emitting ZJU-128⊃SP composite, which was constructed by assembling SP molecule into ZJU-128, thus adopting a host–guest space-confined strategy.116 The UV-visible absorbance of SP-MC can be adjusted througth the photoisomerization process of SP between ring-closed and ring-open forms. The significant FRET process between the blue-emitting ZJU-128 donor and the red-emitting SP acceptor can be adjusted dynamically by changing the stimulus source (UV/visible light) and stimulus time. Thereby, the fluorescence output and emission wavelength can be controlled dynamically [Figs. 11(b) and 11(c)]. Anticounterfeiting is a permanent issue owing to its abuse in banknotes, diplomas, product labels, and so on. Furthermore, due to the unusual dynamic properties of the color and fluorescence intensity of ZJU-128⊃SP composites, a dual-emitting ZJU-128⊃SP polydimethylsiloxane (PDMS) film has been constructed, which provides the method to reversibly regulate the output signal of advanced dynamic information anticounterfeiting compared with typical on–off switching information encryption materials [Fig. 11(d)]. On the other hand, the emissions intensity ratio between H4TCPP ligand and SP (I447/I650) in dual-emitting ZJU-128⊃SP provides a self-calibrated signal, which could eliminate the external interference and realization of the precise quantitative assessment. More interestingly, the dynamic dual-emitting feature and quantitative analysis of the intensity ratio of blue to red of ZJU-128⊃SP architectures offer a promising strategy for multiplexed coding (for example, 3D/4D coding) and multilevel encryption. This work supplied an inspiration for the preparation of information encryption materials and advanced anticounterfeiting materials.

The detection and monitoring of gases around the local environments, such as sulfur dioxide (SO2), oxygen (O2), hydrogen (H2), hydrogen sulfide (H2S), ammonia (NH3), and volatile organic solvents (VOCs), are important due to the harmfulness and toxicity of some of them.117 Gas molecules will affect the electron migration during the MOF luminescence process, or produce specific recognition with groups on the ligand, resulting in “turn on” or “turn off” type fluorescence responses.

As mentioned earlier, the flexible MOFs can exhibit dynamic structural changes under external stimuli (such as gas), providing an excellent platform for constructing stimuli-responsive photonic materials. For instance, Kitagawa’s team introduced fluorescent reporter distyrylbenzene (DSB) into nanochannels of flexible PCP to realize a luminescence detection of gas molecules [Fig. 12(a)].118 The synthesized PCP-DSB material could selectively adsorb CO2 over other atmospheric gases, such as N2, O2, and Ar. Intriguingly, under the pressure of CO2 gas, the conformation of DSB molecules varied with the change of the pore shape and pore size of the host framework, which brings about the change of the fluorescence peak position and intensity. The PCP-DSB material exhibits various fluorescence responses to various gases that have similar physicochemical properties, such as CO2 and acetylene. NLO switches in the solid state are badly needed in the present, especially based on noncentrosymmetric framework materials. Gascon et al. also reported that the crystal structure of the flexible NH2-MIL-53(Al) material changes after CO2 absorption [from NH2-MIL-53(Al)vnp to NH2-MIL-53(Al)lp], resulting in the disturbance of polar ordering for strong electronic groups-NH2; then the NLO response switches from on to off [Fig. 12(b)]. The NLO response intensity of MOF is relatively obvious and can be used as a solid-state NLO switch.119 This work provides guidance for the preparation of ordered arrangement in MOF structures and the development of NLO switches induced by external stimuli (such as gas and light).

In this section, selected reports about the pressure-responsive behaviors of photonic MOFs will be discussed in detail. Piezofluorochromic or mechanofluorochromic materials have attracted increasing attention for their practical applications, such as anticounterfeiting, mechanical sensing, data storage, and smart devices. The pressure-responsive photonic MOFs exhibit dynamically adjustable photophysical properties upon external pressure stimuli, such as grinding, shearing, scratching, stretching, cleaving, or compressing.1^,49^,120

Owing to the unique luminescence merits of aggregation-induced emission (AIE) luminogens in the stacked state, plentiful AIEgens are pressure-responsive, which serve as an essential class of solid-state piezofluorochromic materials. In particular, AIE building blocks derived from AIEgens are an important means of AIE-MOFs construction strategy, which have interesting pressure-responsive properties.1 In 2015, Zhou et al. introduced the first piezofluorochromic MOF, PCN-128, which is constructed by eight connected Zr6 clusters and H4ETTC (4′,4‴,4‴′′,4‴′‴-(ethene-1,1,2,2-tetrayl)tetrakis-(([1,1′-biphenyl]-4-carboxylic acid))) ligand (Fig. 13).121 When compressed, the white PCN-128W powder changed to a yellow PCN-128Y powder. The blue emitting (470 nm) PCN-128W converts to green emitting (538 nm) PCN-128Y under a UV 365 nm environment. Further, after treatment with trifluoroacetate (TFA), the compressed PCN-128Y can be recovered to PCN-128W. Further structural analysis indicates that the reversibility is mainly owed to the coordination of TFA groups, which restricted the movement of Zr6 clusters, thereby preventing further deformation of the ETTC connector. In summary, studies on its transformation mechanism indicate that PCN-128W can be regarded as a pressure-responsive microscissor lift.

To further enrich pressure-responsive materials with tunable and controllable piezofluorochromic performance, another mixed-linker MOF, LIFM-66W was observed by Su and Pan et al.122 Later, a series of H4ETTC-based MOFs (LIFM-66, LIMF-114, LIMF-223, PCN-94, PCN-128) were prepared and then they compared the pressure-induced one/two/three-photon multiphoton excited fluorescence (MPEF) and fluorochromic performance [Fig. 14(a)].123 Further analysis found that the MPEF of the MOFs is associated with the MOF’s structural characteristics, as well as the pressure-induced deformations process. In other words, the flexibility and deformability of the compressed MOFs greatly affect their emission intensity and color. Such AIE-MOFs featuring different pressure-dependent MPEF behavior within the same material could be a very attractive material. Su and co-workers reported a flexible two-fold interpenetrating LIFM-114 material with multiple external stimuli responses to solvents, heat, and pressure [Fig. 14(b)].124 A reversible switch between the blue and green emission bands in LIFM-114 was achieved due to the obvious breathing contraction of the interpenetrated framework of LIFM-114 and related conformation change inside the ETTC linkers under different external stimuli.

Apart from the previously reported work about pressure-responsive AIE-MOFs,125 Zou et al. showed the green-emitting Tb(BTC)(H2O)6 with pressure-responsive performance (Fig. 15).126 When the pressure below 2.5 GPa, Tb(BTC)(H2O)6 behaved obvious PL enhancement. It is worth noting that when the pressure is completely released and maintained for more than half a year, the QY of the Tb(BTC)(H2O)6 material increases significantly from 50.6% to 90.4%. Further research suggests that the mechanism of this pressure-responsive MOF can be attributed to the enhanced hydrogen bonds lock causing enhanced intersystem crossing to effectively drive ligand-to-metal energy transfer. In addition, the same group found that a non-emission MOF-2 and Y(BTC)(H2O)6 also exhibit pressure-responsive photoluminescence.127^,128 Moggach et al. also reported pressure-induced photoluminescence of Hf-peb MOF due to the linker rotation.129

The asymmetry between the direction of vibration and propagation is called the polarization phenomenon. The polarization of light can provide the possibility for the analysis and processing of scattered light signals. Polarized light has been extensively applied in optical modulation devices, radar remote sensing, high-resolution display, and other fields.45^,130 However, microcavity laser materials with high polarization emission (>99%) have rarely been reported. Among them, high-order multiphoton pumped polarized microlasers have important application prospects in biophotonics, such as bioinformatics imaging and PDT.130^–133

The A–π–D push–pull type molecules (A = acceptor and D = donor) possess a strong intramolecular charge transfer effect, such as dyes. It is essential that the centrosymmetric multipolar geometry of A–π–D type molecules possesses donor groups and acceptor groups, resulting in an enhanced polarizability, which is a prerequisite for strong NLO/multiphoton absorption (MPA) response.131^,134 On the basis of their previous work about bio-MOF-1⊃DMASM (DMASM, 4-[p-(dimethylamino)styryl]-1-methylpyridine), size-matched inclusion of dyes periodically aligned in one-dimensional (1D) pores of MOFs, which will improve the pore confinement efficiency of MOFs. Therefore, Qian et al. reported an anionic MOF crystal H2[Zn3O(CPQC)3] (ZJU-68) with subnanoscale 1D channels, which was designed using zinc ion and a novel multidentate organic bridging ligand 7-(4-carboxylic acid phenyl)-3-carboxylic acid-quinoline (H2CPQC).135 The in situ assembly method successfully controlled the chromophore ion DMASM with unidirectional transition dipole moment to assemble into channels of ZJU-68, thus resulting in the host–guest hybrid single crystal ZJU-68⊃DMASM (Fig. 16). Due to the highly oriented and periodic arrangement of DMASM in the 1D ordered channels of ZJU-68, the assembly concentration of DMASM could be further increased to enhance the QY from 0.45% of DMASM solution to 24.28% of ZJU-68⊃DMASM. Particularly, a low lasing threshold of the 3PP lasing has been successfully achieved upon excitation at 1380 nm. Both the 3PP WGMs optical-feedback mechanisms and especially F–P lasing in ZJU-68⊃DMASM microcrystals show a high degree of polarization (DOP, over 99.9%). These results provide a new approach and theoretical basis for the design and construction of novel multiphoton pumped polarized microcavity laser materials and devices.

MOF materials with NLO performance have broad application prospects in photoelectric conversion, optical switches, optical information processing, temperature sensors, biological imaging, and detection of living tissues. A single NLO response is difficult to meet the development of multifunctional integrated materials or devices. The development of new materials with multiple NLO response integration, especially the design of materials with controllable NLO performance is a topic with some research urgency. However, there is still a lack of suitable material systems and construction strategies, making it difficult to integrate multiple NLO properties into the same micro/nanomaterial and achieve performance control. Switching between different strong signals [second-harmonic generation (SHG), two-photon fluorescence (TPF), and so on] in a single medium could avoid signal cross-contamination. Qian et al. selected 1-ethyl-4-[4-(p-dimethylaminophenyl)-1,3-butadienyl]pyridinium perchlorate (LDS-722) dye encapsulated into ZJU-68 by an in situ self-assembly strategy.136 The polarization direction and transition dipole moment are highly arranged along the ID channel of ZJU-68, resulting in the second- and third-order NLO effects. The polarization ratios of TPF and SHG are both as high as 99%. The above NLO performance is excitation wavelength-dependent. Consequently, the signal output of TPF/laser, SHG, and THG can be controlled by adjusting the excitation wavelength. Switching between two-photon-pumped (TPP) lasing and SHG signals is scientifically significant but difficult to achieve. Therefore, Qian et al. proposed the concept of NLO building units (NBUs); two types of D-A type NBUs were reasonably arranged in MOF single crystals to achieve polarization-regulated multiple NLO performances. Specifically, they introduced a dipolar donor (D)–acceptor (A) DSM dye to a 1D channel of MOF to prepare the ZJU-24-Eu⊃DSM host–guest material (Fig. 17).137 ZJU-24-Eu was precisely fabricated using an octupolar electron D–A structural 4′,4‴,4‴″-nitrilotris[1,10-biphenyl]-4-carboxylic acid (H3NBB) ligand. By changing the polarization direction of NIR femtosecond lasers and the angle between the long axis of the crystal (θEx), hybrid single crystals exhibit SHG signals that are excitation polarization-dependent, with maximum SHG signals at θEx=90deg. The spatial arrangement of two-photon dye DSM and the smooth surface of regular MOF crystals enable ZJU-24-Eu⊃DSM single crystals to behave with the strongest TPP laser when emitted at θEx=0deg. Therefore, the alternating occurrence of SHG and TPP lasers can be controlled by orthogonally changing the excitation polarization direction from the angle θ=0deg to 90 deg, with a DOP of 81.2% and 96.1%, respectively.

Constructing heterostructures is an effective method for integrating and spatially separating multiple optically active media in a single structure with component gradients. The design and fabrication of controllable MOF heterostructures through epitaxial growth strategy enable them to obtain the multiwavelength and multicolor emission. Qian et al. used in situ assembly of cationic linear dye molecules DMASM, (E)-1-methyl-4-(2-(1-methyl-1H-pyrrol-2-yl)vinyl)pyridinium (MMPVP) and 4-((1E,3E)-4-(4-(dimethylamino) phenyl)buta-1,3-dienyl)-1-methylpyridinium (DPBDM) in the channels of ZJU-68 to obtain homoepitaxy ZJU-68⊃DPBDM+DMASM+MMPVP microcrystals. Surprisingly, this hybrid MOF heterostructure achieved dynamically tunable multicolor single-mode highly polarized lasing.138 Following this method, a composition-graded DABP@DASE@ZJU-68 heterostructure was fabricated through the same strategy with the inclusion of two dye molecules E-4(4-dimethylaminostyryl)-1-ethylpyridinium (DASE) and 4-((1E,3E)-4-(4(dimethylamino) phenyl) buta-1,3-dien-1-yl)-1-methylpyridinium (DABP) into ZJU-68 channels.139 On the basis of this excited position-dependent two-photon polarized lasing in NIR region, an extension work is carried out. Similarly, LDS-798@LDS-722@ZJU-67 heterostructure (LDS-798, 4-[4-[4-(dimethylamino)phenyl]-1,3-butadienyl]-1-ethyl quinolinium perchlorate) was constructed and used for three-level photonic encoding based on the switchable dual-wavelength NIR lasing.140 It increases the complexity and security of photonic tags, making the MOF-based heterostructure promising for advanced anticounterfeiting and information encryption in the future.

In addition to a strategy to construct MOF heterostructures, Qian et al. also designed a novel sandwich-like MOF-based mixed-matrix membrane to realize NLO switching.141 A new anionic MOF material H2[Mn3O(C17H9NO4)3] (ZJU-67) with 1D channels was synthesized by manganese ion and H2CPQC ligands. DASE@ZJU-67 and spirooxazine were utilized as laser emission unit and absorption unit, respectively, which were hierarchically encapsulated in PDMS (Fig. 18). Due to the strong space confinement and high-concentration gain media, DASE@ZJU-67 exhibits anisotropic two-photon lasing in the red band with E’th of 1.95mJ/cm2 and a high degree of linear polarization of ∼99.9% (λEx=1064nm). Owing to the rapid photochemical reaction of photochromic molecules, the molecular structure changes reversibly under light stimuli, which results in the sharp red absorption band variations, ultimately causing a decrease in emission or quenching of the upconversion laser.

Similar to the MOF⊃dye host–guest system developed by Qian’s group, Boom et al. used a one-step solvothermal approach to construct oriented surface-bound MOFs (sMOFs), mainly through the control of morphology as well as the dimensions of the crystal;142 they then successfully encapsulated resorufin sodium salt into the 1D channels of the sMOFs (Fig. 19). Due to the aligned arrangement of dyes in the crystal, hybrid MOF crystals exhibit red-polarized emission with a high polarization degree. In their next work, the same group demonstrated the size-selective encapsulation of dyes in differently sized channels of MOFs.143 The AdDB ligand-based MOF has two types of continuous channels, which is suitable for inclusion of sodium resorufin (SR) and sodium fluorescein (SF) dyes. Interestingly, the SR/SF@MOF crystals are dichroic under polarized light, and the yellow emission from SF (donor) at 541 nm is gradually decreased and accompanied by a red emission of around 608 nm of SR (acceptor) that increases. Further analysis indicates that a significant FRET process was achieved when the SR concentration was increased. The FRET mechanism has been verified by fluorescence lifetime and photobleaching of the acceptor SF.

Similarly, D-A type derived ligands can be used as building blocks and NLO/MPA photonic units of MOFs, such as triphenylamine-derived linkers, dye-derived linkers, and others, that have been reported.144^–146 On the basis of the previous work, D-A type ligands-based MOFs that exhibit polarized NLO/MPA performance have also been developed. For instance, a seven-fold interpenetrated MOF (1) is transformed into an eight-fold interpenetrated MOF (2) by a single crystal-to-single-crystal approach.147 The transformation of these two MOFs is accompanied by significant improvements of the SHG performance (ca. 125 times) and TPF performance (ca. 14 times). In 2019, Ji et al. obtained a rhodamine B (Rh B) dye-coordinated MOF with a high thermal and chemical stability. This dye-coordinated MOF microplate could achieve a low-threshold multiphoton-pumped microlaser without an external cavity (Fig. 20).148 Particularly, the lasing thresholds are in the range from 0.34 to 0.12μJcm−2 upon various optical pumping conditions, mainly owing to the exciton–polariton lasing mechanism as well as the large MPA cross sections of the three dye-coordinated MOF microplates. Subsequently, they continued to demonstrate the stack of 2D nanosheets of rhodamine B molecular network for constructing a MOF with a large polariton–polariton scattering strength (g).149 In 2022, Liu et al. employed a mixed ligand strategy to synthesize a series of MPA frameworks in combination of AIE-linker H4TCPE and four pillared D–π–A ligands [1,4-bis(4-pyridylethenyl)benzene (L1), 1,4-bis(4-pyridylethenyl)-naphthalene (L2), 9,10-bis(4-pyridylethenyl)anthracene (L3, An2Py), and 4,9-bis(4-pyridylethenyl)pyrene (L4)].150 Complex 1 and isostructural complexes 2 to 5 exhibited efficient multiphoton excited photoluminescence (MEPL) performance and THG performance. Complex 1 shows the polarization dependence of 2PPL, 3PPL, and THG when excited at 720 and 1217 nm (Fig. 21).

Meldrum’s group synthesized a linear organic chromophore Np-P4VB and assembled it with H2TDC and Zn2+ to form a MOF (TDC-MOF-8).151 TDC and Zn2+ are assembled into 2D ZnTDC layers, which are connected by Np-P4VB molecules to form a 3D framework (Fig. 22). Therefore, Np-P4VB molecules are aligned along the same axis in the framework. Due to this special arrangement, the light emission of this MOF is highly polarized, with an anisotropic value between 0.7 and 0.8.

Apart from D-A type MOFs, Ln-MOFs also possess polarization characteristics. For example, Yan and Zhao et al. reported three isostructural Ln-MOFs, Tb-/Eu-BTC, and Eu@Tb-BTC 1D microrods. As a result, the Ln-BTC MOFs exhibit both linearly and circularly polarized emission.152 Similarly, Zhai et al. further reported an easy method to design and prepare Eu-BTC with color-tunable polarization-dependent emission for high-capacity photonic barcodes.153

Many studies have shown the unique advantages and broad applications of stimuli-responsive photonic MOFs in the past few years. In this review, we mainly discussed the representative results of stimuli-responsive photonic MOFs upon different types of external stimuli, including light, force, gas, and polarization. We emphasized the basic work principles, to provide a better understanding of the stimuli-responsive process and then gave guidance for the design and fabrication of desirable stimuli-responsive photonic MOF materials.

So far, although some crucial progress has been achieved in dynamic responsive MOFs, the development of stimuli-responsive photonic MOFs is still in its infancy, far behind its gas storage and separation applications. Therefore, there is an urgent need to further develop smart dynamic responsive photonic MOFs and control their responsive behavior as needed, which will greatly improve and promote optoelectronic applications.

First, it is necessary to design and fabricate photoswitchable molecule-based ligands for the construction of stimuli-responsive photonic MOFs. Meanwhile, currently reported stimuli-responsive MPU-based ligands are mainly carboxyl- or pyridyl-based. Therefore, exploring chelating groups of MPUs-based ligands will enrich the coordination patterns and luminescent properties of stimuli-responsive MOFs. On the other hand, there are still numerous switchable moieties that have not been converted to ligands of MOFs.

Second, multivariate components and multifunction could be integrated into the dynamically responsive photonic MOF materials. The tunable and designable structure of stimuli-responsive MOFs as well as the synergistic effect between different MPUs could affect their photonic performance. Therefore, it is necessary to systematically consider the relationship between corresponding photonic properties and material preparation strategies, structural modulation as well as synergistic effects, which will promote the rapid development of photonic functional MOFs. For example, traditional photoswitchable moieties (such as DAE) have intrinsic shortcomings—weak emission and lower QY. Hence, by rational design of a FRET system between MOFs and a photoswitchable moiety-based guest, tunable structure and luminescence and even photonic function will be achieved, which could reveal the structure-activity relationship of stimuli-responsive behaviors and expand its application fields.

Third, applications of photoswitchable MOFs still need to be explored in the future. For example, much more attention should be paid to the development of stimuli-responsive MOF-based photonic biosensors for intracellular sensing, which will achieve meaningful advancements in biomedical diagnostics, therapeutics, and so on. In addition, developing alternative cost-effective and high-quality luminescent materials with good encryption capabilities for advanced information storage (such as 3D/4D coding) and security protection applications remains an urgent challenge.

Fourth, we reviewed the fluorescence properties and applications of photochromism MOFs, although the NLO properties, upconversion, and lasing in stimuli-responsive MOFs are rarely discussed. However, according to reports, AZO and SP dyes or derivatives can also exhibit NLO and lasing properties. Therefore, the introduction of photoswitchable molecules into MOF channels and the orderly arrangement of photoswitchable chromophores in MOFs are expected to achieve the NLO, upconversion luminescence, and laser performance in dynamically responsive MOFs. Note that this is also one of the important topics currently being studied by our group. It is undoubtedly one of the important development trends in the future to apply them to photonic biological imaging and PDT.

He-Qi Zheng received his master’s degree from Fujian Agriculture and Forestry University in 2020. He is currently a PhD candidate in the School of Materials Science and Engineering at Zhejiang University under the supervision of Prof. Yuanjing Cui and Prof. Guodong Qian. His research interests focus on the stimuli-responsive emission and aggregation-induced emission of photonic functional metal−organic frameworks.

Lin Zhang received her bachelor’s degree in materials science and engineering from Zhejiang University in 2018. She is currently a PhD student under the supervision of Prof. Guodong Qian in the School of Materials Science and Engineering, Zhejiang University. Her research interests mainly focus on photonic functional porous crystalline materials.

Yuanjing Cui received his BS and PhD degrees in materials science and engineering from Zhejiang University in 1998 and 2006, respectively. Currently, he is a full professor in the School of Materials Science and Engineering at Zhejiang University. His research interest focuses on organic–inorganic hybrid photonic materials.

Guodong Qian received his bachelor’s and master’s degrees in materials science from Zhejiang University in China, in 1988 and 1992, respectively. He joined the Materials Department of Zhejiang University after obtaining his PhD from Zhejiang University in 1997. He was promoted to associate professor, full professor, and Cheung Kong professor in 1999, 2002, and 2011, respectively. His current research interests include hybrid organic–inorganic photonic functional materials and multifunctional porous materials.