Stimuli‐Responsive Metal‐Organic Frameworks with Photoswitchable Azobenzene Side Groups

Light switching for product selectivity control in photocatalysis

Artificial switchable catalysis is a new, rapidly expanding field that offers great potential advantages for both homogeneous and heterogeneous catalytic systems. Light irradiation is widely accepted as the best stimulus to artificial switchable chemical systems. In recent years, tremendous progress has been made in the synthesis and application of photo-switchable catalysts that can control when and where bond formation and dissociation take place in reactant molecules. Photo-switchable catalysis is a niche area in current catalysis, on which systematic analysis and reviews are still lacking in the scientific literature, yet it offers many intriguing and versatile applications, particularly in organic synthesis. This review aims to highlight the recent advances in photo-switchable catalyst systems that can result in two different chemical product outcomes and thus achieve a degree of control over organic synthetic reactions. Furthermore, this review evaluates different approaches that have been employed to achieve dynamic control over both the catalytic function and the selectivity of several different types of synthesis reactions, along with the remaining challenges and potential opportunities. Owing to the great diversity of the types of reactions and conditions adopted, a quantitative comparison of efficiencies between considered systems is not the focus of this review, instead the review showcases how insights from successful adopted strategies can help better harness and channel the power of photoswitchability in this new and promising area of catalysis research.

Photoresponsive Covalent Organic Frameworks: Visible-Light Controlled Conversion of Porous Structures and Its Impacts

Covalent organic frameworks are a novel class of crystalline porous polymers that enable molecular design of extended polygonal skeletons to attain well-defined porous structures. However, construction of a framework that allows remote control of pores remains a challenge. Here we report a strategy that merges covalent, noncovalent, and photo chemistries to design photoresponsive frameworks with reversibly and remotely controllable pores. We developed a topology-guided multicomponent polycondensation system that integrates protruded tetrafluoroazobenzene units as photoresponsive sites on pore walls at predesigned densities, so that a series of crystalline porous frameworks with the same backbone can be constructed to develop a broad spectrum of pores ranging from mesopores to micropores. Distinct from conventional azobenzene-based systems, the tetrafluoroazobenzene frameworks are highly sensitive to visible lights to undergo high-rate isomerization. The photoisomerization exerts profound effects on pore size, shape, number, and environment, as well as molecular uptake and release, rendering the system able to convert and switch pores reversibly and remotely with visible lights. Our results open a way to a novel class of smart porous materials with pore structures and functions that are convertible and manageable with visible lights.

Reticular Synthesis of Flexible Rare-Earth Metal-Organic Frameworks: Control of Structural Dynamics and Sorption Properties Through Ligand Functionalization

An exciting direction in metal-organic frameworks involves the design and synthesis of flexible structures which can reversibly adapt their structure when triggered by external stimuli. Controlling the extent and nature of response in such solids is critical in order to develop custom dynamic materials for advanced applications. Towards this, it is highly important to expand the diversity of existing flexible MOFs, generating novel materials and gain an in-depth understanding of the associated dynamic phenomena, eventually unlocking key structure-property relationships. In the present work, we successfully utilized reticular chemistry for the construction of two novel series of highly crystalline, flexible rare-earth MOFs, RE-thc-MOF-2 and RE-teb-MOF-1. Extensive single-crystal to single-crystal structural analyses coupled with detailed gas and vapor sorption studies, shed light onto the unique responsive behavior. The development of these series is related to the reported RE-thc-MOF-1 solids which were found to display a unique continuous breathing and gas-trapping property. The synthesis of RE-thc-MOF-2 and RE-teb-MOF-1 materials represents an important milestone as they provide important insights into the key factors that control the responsive properties of this fascinating family of flexible materials and demonstrates that it is possible to control their dynamic behavior and the associated gas and vapor sorption properties.

Systematic Investigation into the Photoswitching and Thermal Properties of Arylazopyrazole-based MOF Host-Guest Complexes

A series of arylazopyrazole-loaded metal-organic frameworks were synthesized with the general formula Zn2(BDC)2(DABCO)(AAP)x (BDC = 1,4-benzenedicarboxylate; DABCO = 1,4-diazabicyclo-[2.2.2]octane; AAP = arylazopyrazole guest). The empty framework adopts a large pore tetragonal structure. Upon occlusion of the E-AAP guests, the frameworks contract to form narrow pore tetragonal structures. The extent of framework contraction is dependent on guest shapes and pendant groups and ranges between 1.5 and 5.8%. When irradiated with 365 nm light, the framework expands due to the photoisomerization of E-AAP to Z-AAP. The proportion of Z-isomer at the photostationary state varies between 19 and 57% for the AAP guests studied and appears to be limited by the framework which inhibits further isomerization once fully expanded. Interestingly, confinement within the framework significantly extends the thermal half-life of the Z-AAP isomers to a maximum of approximately 56 years. This finding provides scope for the design of photoresponsive host-guest complexes with high stability of the metastable isomer for long-duration information or energy storage applications.

Functional dynamics in framework materials

Dynamic crystalline materials have emerged as a unique category of condensed phase matter that combines crystalline lattice with components that display dynamic behavior in the solid state. This has involved a range of materials incorporating dynamic functional units in the form of stimuli-responsive molecular switches and machines, among others. In particular, it has been possible by relying on framework materials, such as porous molecular frameworks and other hybrid organic-inorganic systems that demonstrated potential for serving as scaffolds for dynamic molecular functions. As functional dynamics increase the level of complexity, the associated phenomena are often overlooked and need to be explored. In this perspective, we discuss a selection of recent developments of dynamic solid-state materials across material classes, outlining opportunities and fundamental and methodological challenges for their advancement toward innovative functionality and applications.

Photo-Modulating CO2 Uptake of Hypercross-linked Polymers Upcycled from Polystyrene Waste

Incorporating photo-switches into skeletal structures of microporous materials or as guest molecules yield photo-responsive materials for low-energy CO₂ capture but at the expense of lower CO₂ uptake. Here, we overcome this limitation by exploiting trans-cis photoisomerization of azobenzene loaded into the micropores of hypercross-linked polymers (HCPs) derived from waste polystyrene. Azobenzene in HCP pores reduced CO₂ uptake by 19 %, reaching 37.7 cm³  g^-1 , but this loss in CO₂ uptake was not only recovered by trans-cis photoisomerization of azobenzene, but also increased by 22 %, reaching 56.9 cm³  g^-1 , when compared to as-prepared HCPs. Computational simulations show that this increase in CO₂ uptake is due to photo-controlled increments in 10-20 Å micropore volume, i. e., adsorption sites and a photo-reversible positive dipole moment. Irradiating these HCPs with visual-range light reverted CO₂ uptake to 33 cm³  g^-1 . This shows that it is feasible to recycle waste polystyrene into advanced materials for low-energy carbon capture.

Recent Progress in Photoresponsive Biomaterials

Photoresponsive biomaterials have garnered increasing attention recently due to their ability to dynamically regulate biological interactions and cellular behaviors in response to light. This review provides an overview of recent advances in the design, synthesis, and applications of photoresponsive biomaterials, including photochromic molecules, photocleavable linkers, and photoreactive polymers. We highlight the various approaches used to control the photoresponsive behavior of these materials, including modulation of light intensity, wavelength, and duration. Additionally, we discuss the applications of photoresponsive biomaterials in various fields, including drug delivery, tissue engineering, biosensing, and optical storage. A selection of significant cutting-edge articles collected in recent years has been discussed based on the structural pattern and light-responsive performance, focusing mainly on the photoactivity of azobenzene, hydrazone, diarylethenes, and spiropyrans, and the design of smart materials as the most targeted and desirable application. Overall, this review highlights the potential of photoresponsive biomaterials to enable spatiotemporal control of biological processes and opens up exciting opportunities for developing advanced biomaterials with enhanced functionality.

Design and Construction of an Azo-Functionalized POP for Reversibly Stimuli-Responsive CO2 Adsorption

A porous azo-functionalized organic polymer (JJU-2) was designed and prepared via oxidative coupling polymerization promoted by FeCl3. JJU-2 exhibited reversibly stimuli-responsive CO2 adsorption properties as a result of the trans/cis isomerization of the polymer’s azo-functionalized skeleton. Under UV irradiation and heat treatment, this porous material displayed various porous structures and CO2 adsorption properties. The initial Brunauer-Emmett-Teller (BET) surface area of JJU-1 is 888 m2 g−1. After UV irradiation, the BET surface area decreases to 864 m2 g−1, along with the decrease of micropores around 0.50 nm and 1.27 nm during the trans-to-cis isomerization process. In addition, CO2 sorption isotherms demonstrate an 8%t decrease, and the calculated Qst of CO2 has decreased from 29.0 kJ mol−1 to 26.5 kJ mol−1 due to the trans to cis conversion of the azobenzene side group. It is noteworthy that JJU-2′s CO2 uptakes are nearly constant over three cycles of alternating external stimuli. Therefore, this azo-functionalized porous material was a potential carbon capture material that was responsive to stimuli.

Reversible Regulation of Polar Gas Molecules by Azobenzene-Based Photoswitchable Metal-Organic Framework Thin Films

The development of tunable molecule separation membranes requires materials with remote controllability and ultra-high separation capability. In this paper, a novel photoswitchable metal organic framework (MOF) thin film (Cu₂(AzoBPDC)₂) was prepared by liquid phase epitaxial layer-by-layer assembly to realize the reversible remote-controlled switching. The azobenzene side groups in the Cu₂(AzoBPDC)₂ thin film showed excellent reversible photoswitching performance under UV (365 nm) and Vis (450 nm) irradiation, achieving the remote-controlled mode of the diffusion flux of polar gas molecules in the MOF thin film.

Recent Advances in Responsive Membrane Functionalization Approaches and Applications

In recent years, significant advances have been made in the field of functionalized membranes. With the functionalization using various materials, such as polymers and enzymes, membranes can exhibit property changes in response to an environmental stimulation, such as heat, light, ionic strength, or pH. The resulting responsive nature allows for an increased breadth of membrane uses, due to the developed functionalization properties, such as smart-gating filtration for size-selective water contaminant removal, self-cleaning antifouling surfaces, increased scalability options, and highly sensitive molecular detection. In this review, new advances in both fabrication and applications of functionalized membranes are reported and summarized, including temperature-responsive, pH-responsive, light-responsive, enzyme-functionalized, and two-dimensional material-functionalized membranes. Specific emphasis was given to the most recent technological improvements, current limitations, advances in characterization techniques, and future directions for the field of functionalized membranes.

Unraveling the timescale of the structural photo-response within oriented metal-organic framework films

Fundamental knowledge on the intrinsic timescale of structural transformations in photo-switchable metal-organic framework films is crucial to tune their switching performance and to facilitate their applicability as stimuli-responsive materials. In this work, for the first time, an integrated approach to study and quantify the temporal evolution of structural transformations is demonstrated on an epitaxially oriented DMOF-1-on-MOF film system comprising azobenzene in the DMOF-1 pores (DMOF-1/AB). We employed time-resolved Grazing Incidence Wide-Angle X-Ray Scattering measurements to track the structural response of the DMOF-1/AB film upon altering the length of the azobenzene molecule by photo-isomerization (trans-to-cis, 343 nm; cis-to-trans, 450 nm). Within seconds, the DMOF-1/AB response occurred fully reversible and over several switching cycles by cooperative photo-switching of the oriented DMOF-1/AB crystallites as confirmed further by infrared measurements. Our work thereby suggests a new avenue to elucidate the timescales and photo-switching characteristics in structurally responsive MOF film systems.

Rational Design of Smart Metal-Organic Frameworks for Light-Modulated Gas Transport

Smart metal-organic frameworks (MOFs) are constructed by introducing stimuli-responsive functional groups into MOF platforms. Through membrane systems containing smart MOFs, external field-modulated gas transport can be achieved, which finds potential applications in chemical engineering. In this work, we design a series of Mg-MOF-74-III-based frameworks functionalized by arylazopyrazole groups. Methyleneamine chains with various lengths are attached to the photoresponsive azopyrazole moiety. Molecular dynamics simulations show that CO₂ diffusion can be remarkably changed by controlling the cis-to-trans isomerization of the functional unit due to the tunable adsorbate-adsorbent and adsorbate-adsorbate interactions of the two states. With the optimal length of the functional chain, the spatial hindrance and adsorbate-adsorbent interaction exhibit a synergetic effect to maximize the stimuli-responsive kinetic separation of N₂ over CO₂. This work provides a promising strategy for elevating smart MOFs' potential in gas separation.

Conjugated microporous polymer membranes for light-gated ion transport

Inspired by the light-gated ion channels in cell membranes that play important roles in many biological activities, herein, we developed an artificial light-gated ion channel membrane out of conjugated microporous polymers. Through bottom-up design of the monomer molecular structure and by the electropolymerization method, the membrane pore size and thickness were precisely controlled on the molecular level. The obtained membrane exhibited uniform pore size and highly sensitive light-switchable response. The photoisomerization of the polymer chain resulted in a reversible "on and off" light control over the pore size and subsequently led to light-gated ion transport across the membrane for a series of ions including hydrogen, potassium, sodium, lithium, calcium, magnesium, and aluminum ions.

Cooperative light-induced breathing of soft porous crystals via azobenzene buckling

Although light is a prominent stimulus for smart materials, the application of photoswitches as light-responsive triggers for phase transitions of porous materials remains poorly explored. Here we incorporate an azobenzene photoswitch in the backbone of a metal-organic framework producing light-induced structural contraction of the porous network in parallel to gas adsorption. Light-stimulation enables non-invasive spatiotemporal control over the mechanical properties of the framework, which ultimately leads to pore contraction and subsequent guest release via negative gas adsorption. The complex mechanism of light-gated breathing is established by a series of in situ diffraction and spectroscopic experiments, supported by quantum mechanical and molecular dynamic simulations. Unexpectedly, this study identifies a novel light-induced deformation mechanism of constrained azobenzene photoswitches relevant to the future design of light-responsive materials.

Efficient solid-state photoswitching of methoxyazobenzene in a metal-organic framework for thermal energy storage

Efficient photoswitching in the solid-state remains rare, yet is highly desirable for the design of functional solid materials. In particular, for molecular solar thermal energy storage materials high conversion to the metastable isomer is crucial to achieve high energy density. Herein, we report that 4-methoxyazobenzene (MOAB) can be occluded into the pores of a metal-organic framework Zn2(BDC)2(DABCO), where BDC = 1,4-benzenedicarboxylate and DABCO = 1,4-diazabicyclo[2.2.2]octane. The occluded MOAB guest molecules show near-quantitative E → Z photoisomerization under irradiation with 365 nm light. The energy stored within the metastable Z-MOAB molecules can be retrieved as heat during thermally-driven relaxation to the ground-state E-isomer. The energy density of the composite is 101 J g-1 and the half-life of the Z-isomer is 6 days when stored in the dark at ambient temperature.

Dye adsorption of aluminium- and zirconium-based metal organic frameworks with azobenzene dicarboxylate linkers

The high efficiency of metal-organic-frameworks (MOFs) such as the ZIF, MIL and UiO type species in dye adsorption is well established. Recently, an emerging class of photoresponsive azobenzene-based MOFs has found suitable application in gas adsorption. However, there is a dearth of research on their use in the adsorption of dyes and other water pollutants. In this research, two microporous photoresponsive azobenzene dicarboxylate MOFs of Al³⁺ (Al-AZB) and Zr⁴⁺ (Zr-AZB) were synthesized for the adsorption of congo red (CR) dye. The surface and textural properties of the synthesized MOFs were characterized by FTIR, PXRD, SEM, TGA, BET and pore analysis. Both MOFs were crystalline, thermally stable up to 300 °C and stable in aqueous medium at room temperature. The Al-AZB displayed a higher surface area (2718 m²/g) than the Zr-AZB (1098 m²/g), which significantly impacted the higher adsorption of CR. Besides, pore volumes of 0.86 cm³/g and 0.35 cm³/g were obtained for Al-AZB and Zr-AZB, respectively. The maximum adsorption capacity of Al-AZB and Zr-AZB was 456.6 mg/g and 128.9 mg/g, respectively, with the former superior to other potent adsorbents. The pseudo-second-order and Langmuir models were well correlated with the dye uptake on the MOFs. Thermodynamics revealed random and endothermic sorption of CR dominated by chemisorption, while efficient regeneration and reuse of both MOFs were achieved using dimethylformamide as eluent. The results proved the potency of the synthesized photoresponsive MOFs, as highly efficient and reusable materials for dye adsorption.

Coordination flexibility aided CO2-specific gating in an Iron Isonicotinate MOF

Coordination flexibility assisted porosity has been introduced into an Iron-isonicotinate metal organic framework (MOF), (Fe(4-PyC) 2 .(OH). The framework showed CO 2 -specific gate opening behavior, which gets tuned as a function of temperature and pressure. The MOF's physisorptive porosity towards CO 2 , CH 4 , and N 2 was investigated; it adsorbed only CO 2 via a gate opening phenomenon. The isonicotinate, representing a borderline soft base, is bound to the hard Fe 3+ centre through monodentate carboxylate and pyridyl nitrogen. This moderately weak binding enables isonicotinate to spin like a spindle under the CO 2 pressure opening the gate for a sharp increase in CO 2 uptake at 333 mmHg (At 298K, the CO 2 uptake increases from 0.70 to 1.57 mmol/g). We investigated the MOF's potential for CO 2 /N 2 and CO 2 /CH 4 gas separation aided by this gating. IAST model reveals that the CO 2 /N 2 selectivity jumps from 325 to 3131 when the gate opens, while the CO 2 /CH 4 selectivity increases three times. Interestingly, this Fe-isonicotinate MOF did not follow the trend set by our earlier reported Hard-Soft Gate Control (established for isostructural M 2+ -isonicotinate MOFs (M = Mg, Mn)). However, we account for this discrepancy using the different oxidation state of metals confirmed by X-ray photoelectron spectroscopy and magnetism.

Advances and opportunities in the exciting world of azobenzenes

Azobenzenes are archetypal molecules that have a central role in fundamental and applied research. Over the course of almost two centuries, the area of azobenzenes has witnessed great achievements; azobenzenes have evolved from simple dyes to 'little engines' and have become ubiquitous in many aspects of our lives, ranging from textiles, cosmetics, food and medicine to energy and photonics. Despite their long history, azobenzenes continue to arouse academic interest, while being intensively produced for industrial purposes, owing to their rich chemistry, versatile and straightforward design, robust photoswitching process and biodegradability. The development of azobenzenes has stimulated the production of new coloured and light-responsive materials with various applications, and their use continues to expand towards new high-tech applications. In this Review, we highlight the latest achievements in the synthesis of red-light-responsive azobenzenes and the emerging application areas of photopharmacology, photoswitchable adhesives and biodegradable materials for drug delivery. We show how the synthetic versatility and adaptive properties of azobenzenes continue to inspire new research directions, with limits imposed only by one's imagination.

Rational Construction of a Responsive Azo-Functionalized Porous Organic Framework for CO2 Sorption

An azo-functionalized porous organic framework (denoted as JJU-1) was synthesized via FeCl3-promoted oxidative coupling polymerization. By virtue of a porous skeleton and a light/heat responsive azo functional group, this task-specific JJU-1 displays a reversible stimuli-responsive adsorption property triggered by UV irradiation and heat treatment. The initial Brunauer-Emmet-Teller (BET) surface area of this porous material is 467 m2 g-1. The CO2 sorption isotherms exhibit a slight decrease after UV irradiation because of the trans to cis conversion of the azo functional skeleton. It is worth mentioning that the responsive CO2 adsorption performance can be recycled for three cycles via alternating external stimuli, confirming the excellently reversible switchability of trans-to-cis isomerization and controllable CO2 adsorption.

An Azobenzene-Modified Photoresponsive Thorium-Organic Framework: Monitoring and Quantitative Analysis of Reversible trans-cis Photoisomerization

Monitoring and quantification of the photoresponsive behavior of metal-organic frameworks that respond to a light stimulus are crucial to establish a clear structure-activity relationship related to light regulation. Herein, we report the first azobenzene-modified photoresponsive thorium-organic framework (Th-Azo-MOF) with the formula [Th₆O₄(OH)₄(H₂O)₆L₆] (H₂L = (E)-2'-p-tolyldiazenyl-1,1':4',4'-terphenyl-4,4″-dicarboxylic acid), in which the utilization of a thorium cluster as a metal node leads to one of the largest pore sizes among all the azobenzene-containing metal-organic frameworks (MOFs). The phototriggered transformation of the trans isomer to the cis isomer is monitored and characterized quantitatively by comprehensive analyses of NMR and UV spectroscopy, which reveals that the maximum isomerization ratio of ^cisTh-Azo-MOF in the solid state is 19.7% after irradiation for 120 min, and this isomerization is reversible and can be repeated several times without apparent performance changes. Moreover, the isomerization-related difference in the adsorption of the Rhodamine B guest is also illustrated and a possible photoregulated mechanism is proposed. This work will shed light on new explorations for constructing functionalized actinide porous materials by the elegant combination of actinide nodes with tailored organic ligands and furthermore will provide a comprehensive understanding of photoisomerization processes in MOF solids and insight into the mechanism on photoregulated cargo adsorption and release by photoactive MOFs.

Beyond structural motifs: the frontier of actinide-containing metal-organic frameworks

In this perspective, we feature recent advances in the field of actinide-containing metal-organic frameworks (An-MOFs) with a main focus on their electronic, catalytic, photophysical, and sorption properties. This discussion deviates from a strictly crystallographic analysis of An-MOFs, reported in several reviews, or synthesis of novel structural motifs, and instead delves into the remarkable potential of An-MOFs for evolving the nuclear waste administration sector. Currently, the An-MOF field is dominated by thorium- and uranium-containing structures, with only a few reports on transuranic frameworks. However, some of the reported properties in the field of An-MOFs foreshadow potential implementation of these materials and are the main focus of this report. Thus, this perspective intends to provide a glimpse into the challenges, triumphs, and future directions of An-MOFs in sectors ranging from the traditional realm of gas sorption and separation to recently emerging areas such as electronics and photophysics.

Photoswitchable Metal-Organic Framework Thin Films: From Spectroscopy to Remote-Controllable Membrane Separation and Switchable Conduction

The preparation of functional materials from photoswitchable molecules where the molecular changes multiply to macroscopic effects presents a great challenge in material science. An attractive approach is the incorporation of the photoswitches in nanoporous, crystalline metal-organic frameworks, MOFs, often showing remote-controllable chemical and physical properties. Because of the short light-penetration depth, thin MOF films are particularly interesting, allowing the entire illumination of the material. In the present progress report, we review and discuss the status of photoswitchable MOF films. These films may serve as model systems for quantifying the isomer switching yield by infrared and UV-vis spectroscopy as well as for uptake experiments exploring the switching effects on the host-guest interaction, especially on guest adsorption and diffusion. In addition, the straightforward device integration facilitates various experiments. In this way, unique features were demonstrated, such as photoswitchable membrane separation with continuously tunable selectivity, light-switchable proton conductivity of the guests in the pores, and remote-controllable electronic conduction.

Highly Cooperative Photoswitching in Dihydropyrene Dimers

We present a strategy to achieve highly cooperative photoswitching, where the initial switching event greatly facilitates subsequent switching of the neighboring unit. By linking donor/acceptor substituted dihydropyrenes via suitable π-conjugated bridges, the quantum yield of the second photochemical ring-opening process could be enhanced by more than two orders of magnitude as compared to the first ring-opening. As a result, the intermediate mixed switching state is not detected during photoisomerization although it is formed during the thermal back reaction. Comparing the switching behavior of various dimers, both experimentally and computationally, helped to unravel the crucial role of the bridging moiety connecting both photochromic units. The presented dihydropyrene dimer serves as model system for longer cooperative switching chains, which, in principle, should enable efficient and directional transfer of information along a molecularly defined path. Moreover, our concept allows to enhance the photosensitivity in oligomeric and polymeric systems and materials thereof.

Water Adsorption in Soft and Heterogeneous Nanopores

ConspectusLiquids under confinement differ in behavior from their bulk counterparts and can acquire properties that are specific to the confined phase and linked to the nature and structure of the host matrix. While confined liquid water is not a new topic of research, the past few years have seen a series of intriguing novel features for water inside nanoscale pores. These unusual properties arise from the very specific nature of nanoporous materials, termed "soft porous crystals"; they combine large-scale flexibility with a heterogeneous internal surface. This creates a rich diversity of behavior for the adsorbed water, and the combination of different experimental characterization techniques along with computational chemistry at various scales is necessary to understand the phenomena observed and their microscopic origins. The range of systems of interest span the whole chemical range, from the inorganic (zeolites, imogolites) to the organic (microporous carbons, graphene, and its derivatives), and even encompass the hybrid organic-inorganic systems (such as metal-organic frameworks).The combination of large scale flexibility with the strong physisorption (or even chemisorption) of water can lead to unusual properties (belonging to the "metamaterials" category) and to novel phenomena. One striking example is the recent elucidation of the mechanism of negative hydration expansion in ZrW₂O₈, by which adsorption of ∼10 wt % water in the inorganic nonporous framework leads to large shrinkage of its volume. Another eye-catching case is the occurrence of multiple water adsorption-driven structural transitions in the MIL-53 family of materials: the specific interactions between water guest molecules and the host framework create behavior that has not been observed with any other adsorbate. Both are counterintuitive phenomena that have been elucidated by a combination of experimental in situ techniques and molecular simulation.Another important direction of research is the shift in the systems and phenomena studied, from physical adsorption toward studies of reactivity, hydrothermal stability, and the effect of confinement on aqueous phases more complex than pure water. There have been examples of water adsorption in highly flexible metal-organic frameworks being able to compete with the materials' coordination bonds, thereby limiting its hydrothermal stability, while tweaking the functional groups of the same framework can lead to increased stability while retaining the flexibility of the material. However, this additional complexity and tunability in the macroscopic behavior can occur from changes in the confined fluid rather than the material. Very recent studies have shown that aqueous solutions of high concentration (such as LiCl up to 20 mol L^-1) confined in flexible nanoporous materials can have specific properties different from pure water and not entirely explained by osmotic effects. There, the strong ordering of the confined electrolyte competes with the structural flexibility of the framework to create an entirely new behavior for the {host, guest} system.

Configurational Selection in Azobenzene-Based Supramolecular Systems Through Dual-Stimuli Processes

Azobenzene is one of the most studied light-controlled molecular switches and it has been incorporated in a large variety of supramolecular systems to control their structural and functional properties. Given the peculiar isomeric distribution at the photoexcited state (PSS), azobenzene derivatives have been used as photoactive framework to build metastable supramolecular systems that are out of the thermodynamic equilibrium. This could be achieved exploiting the peculiar E/Z photoisomerization process that can lead to isomeric ratios that are unreachable in thermal equilibrium conditions. The challenge in the field is to find molecular architectures that, under given external circumstances, lead to a given isomeric ratio in a reversible and predictable manner, ensuring an ultimate control of the configurational distribution and system composition. By reviewing early and recent works in the field, this review aims at describing photoswitchable systems that, containing an azobenzene dye, display a controlled configurational equilibrium by means of a molecular recognition event. Specifically, examples include programmed photoactive molecular architectures binding cations, anions and H-bonded neutral guests. In these systems the non-covalent molecular recognition adds onto the thermal and light stimuli, equipping the supramolecular architecture with an additional external trigger to select the desired configuration composition.

Enlightening Materials with Photoswitches

Incorporating molecular photoswitches into various materials provides unique opportunities for controlling their properties and functions with high spatiotemporal resolution using remote optical stimuli. The great and largely still untapped potential of these photoresponsive systems has not yet been fully exploited due to the fundamental challenges in harnessing geometrical and electronic changes on the molecular level to modulate macroscopic and bulk material properties. Herein, progress made during the past decade in the field of photoswitchable materials is highlighted. After pointing to some general design principles, materials with an increasing order of the integrated photoswitchable units are discussed, spanning the range from amorphous settings over surfaces/interfaces and supramolecular ensembles, to liquid crystalline and crystalline phases. Finally, some potential future directions are pointed out in the conclusion. In view of the exciting recent achievements in the field, the future emergence and further development of light-driven and optically programmable (inter)active materials and systems are eagerly anticipated.

Thin Films of Homochiral Metal–Organic Frameworks for Chiroptical Spectroscopy and Enantiomer Separation

Chiral nanoporous solids are a fascinating class of materials, allowing efficient enantiomer separation. Here, we review the status, applications, and potential of thin films of homochiral metal–organic frameworks (MOFs). Combining the advantages of MOFs, whose well-defined, crystalline structures can be rationally tuned, with the benefits of thin films enables new opportunities for the characterization of the enantioselectivity, e.g., via chiroptical spectroscopy and straightforward molecular uptake quantifications. By incorporating photoresponsive molecules in the chiral MOF films, the enantioselectivity of the material can be dynamically remote-controlled. The most promising application of MOF films is their use as membranes, where the enantioselective separation of chiral molecules is demonstrated and parameters for further improvements are discussed.

Atomistic Insight Into the Host-Guest Interaction of a Photoresponsive Metal-Organic Framework

Photoresponsive functional materials have gained increasing attention due to their externally tunable properties. Molecular switches embedded in these materials enable the control of phenomena at the atomic level by light. Metal-organic frameworks (MOFs) provide a versatile platform to immobilize these photoresponsive units within defined molecular environments to optimize the intended functionality. For the application of these photoresponsive MOFs (pho-MOFs), it is crucial to understand the influence of the switching state on the host-guest interaction. Therefore, we present a detailed insight into the impact of molecular switching on the intermolecular interactions. By performing atomistic simulations, we revealed that due to different interactions of the guest molecules with the two isomeric states of an azobenzene-functionalized MOF, both the adsorption sites and the orientation of the molecules within the pores are modulated. By shedding light on the host-guest interaction, our study highlights the unique potential of pho-MOFs to tailor molecular interaction by light.

Design of Collective Motions from Synthetic Molecular Switches, Rotors, and Motors

Precise control over molecular movement is of fundamental and practical importance in physics, biology, and chemistry. At nanoscale, the peculiar functioning principles and the synthesis of individual molecular actuators and machines has been the subject of intense investigations and debates over the past 60 years. In this review, we focus on the design of collective motions that are achieved by integrating, in space and time, several or many of these individual mechanical units together. In particular, we provide an in-depth look at the intermolecular couplings used to physically connect a number of artificial mechanically active molecular units such as photochromic molecular switches, nanomachines based on mechanical bonds, molecular rotors, and light-powered rotary motors. We highlight the various functioning principles that can lead to their collective motion at various length scales. We also emphasize how their synchronized, or desynchronized, mechanical behavior can lead to emerging functional properties and to their implementation into new active devices and materials.

Light-Switchable One-Dimensional Photonic Crystals Based on MOFs with Photomodulatable Refractive Index

Photonic crystals are solids with regular structures having periodicities comparable to the wavelength of light. Here, we showcase the photomodulation of the refractive index of a crystalline material and present a quasi-one-dimensional photonic crystal with remote-controllable optical properties. The photonic material is composed of layers of TiO₂ and films of a nanoporous metal-organic framework (MOF) with azobenzene side groups. While the rigid MOF lattice is unaffected, the optical density is reversibly modified by the light-induced trans-cis-azobenzene isomerization. Spectroscopic ellipsometry and precise DFT calculations show the optical-density change results from the different orbital localizations of the azobenzene isomers and their tremendously different oscillator strengths. The photomodulation of the MOF refractive index controls the optical properties of the quasi-one-dimensional photonic crystal with Bragg reflexes reversibly shifted by more than 4 nm. This study may path the way to photoswitchable photonic materials applied in advanced, tunable optical components and lens coatings and in light-based information processing.

Synthesis of Functionalized Azobiphenyl- and Azoterphenyl- Ditopic Linkers: Modular Building Blocks for Photoresponsive Smart Materials

Modular synthesis of structurally diverse functionalized azobiphenyls and azoterphenyls for the realization of optically switchable materials has been described. The corresponding synthesis of azobiphenyls and azoterphenyls by stepwise Mills/Suzuki-Miyaura cross-coupling reaction, proceeds with high yields and provides facile access to a library of functionalized building blocks. The synthetic methods described herein allow combining several distinct functional groups within a single unit, each intended for a specific task, such as 1) the -N=N- azobenzene core as a photoswitchable moiety, 2) aryls and heteroaryls, functionalized with carboxylic acids or pyridine at its peripheries, as coordinating moieties and 3) varying substitution, size and length of the backbone for adaptability to specific applications. These specifically designed azobiphenyls and azoterphenyls provide modular bricks, potentially useful for the assembly of a variety of polymers, molecular containers and coordination networks, offering a high degree of molecular functionality. Once integrated into materials, the azobenzene system, as a side group on the organic linker backbone, can be exploited for remotely controlling the structural, mechanical or physical properties, thus being applicable for a broad variety of 'smart' applications.

Electrochemical and spectroscopic properties of a cobalt framework with (3,7)-c topology

A Co(ii) framework containing a 7-c Co dimer forms a (3,7)-c binodal net incorporating redox-active triarylamine and light-active azobenzene moieties.

Unexpected Nonresponsive Behavior of a Flexible Metal-Organic Framework under Conformational Changes of a Photoresponsive Guest Molecule

In this article, we describe the synthesis, characterization, and optical properties of a photochromic-guest-incorporated metal-organic framework (MOF). The photochromic guest molecule, 2-phenylazopyridine (PAP), was introduced into a pre-synthesized porous crystalline host MOF, [Zn₂(1,4-bdc)₂(dabco)] _n (1). The successful embedment of PAP has been confirmed by elemental analysis, powder X-ray diffraction measurements, IR spectroscopy, etc. The number of PAP molecules per unit cell of host was 1.0, as evidenced by elemental and thermogravimetric analyses of the host-guest composite, 1⊃PAP. The 1⊃PAP composite did not adsorb N₂, revealed by the adsorption isotherm of 1⊃PAP, which indicates the pore blockage by the close contact of the host framework with the guest PAP in the trans form. The light-induced trans/cis isomerization with partial reversibility of the guest molecule (PAP) in this hybrid host-guest compound (1⊃PAP) has been investigated by detailed IR spectroscopy and UV-vis spectroscopy. The structural transformation from tetragonal in 1 to orthorhombic in 1⊃PAP exhibits dynamic nature of the framework upon inclusion of guest in the framework, which remarkably becomes nonresponsive with the photoirradiation of guest PAP, retaining its orthorhombic structure in the photoirradiated complex, 1⊃PAP(UV).

Preparation of carbon-based AuAg alloy nanoparticles by using the heterometallic [Au4Ag4] cluster for efficient oxidative coupling of anilines

We herein report the preparation of unique heteroatom-doped and carbon-based AuAg alloy nanoparticles (NPs) via the pyrolysis of a structurally defined octanuclear heterometallic Au(i)-Ag(i) cluster [Au4Ag4(Dppy)4(Tab)4(MeCN)4](PF6)8 (2, Dppy = diphenylphosphine-2-pyridine and Tab = 4-(trimethylammonio)benzenethiolate). This cluster-precursor approach exerts a fine control over the spatial arrangement, size and uniformity of the AuAg alloy NPs as well as the doped heteroatoms (P, N, F and S). The optimized material prepared at 450 °C efficiently catalyzes the oxidative coupling of anilines to yield azobenzenes under mild conditions.

Thermal cis-to-trans Isomerization of Azobenzene Side Groups in Metal-Organic Frameworks investigated by Localized Surface Plasmon Resonance Spectroscopy

The energy barrier for cis-to-trans isomerization is among the key parameters for photoswitchable molecules such as azobenzene. Recently, we introduced a well-defined model system based on thin films of crystalline, nanoporous metal-organic frameworks, MOFs. The system enables the precise investigation of the thermal cis-to-trans relaxation of virtually isolated azobenzene pendant groups by means of infrared spectroscopy in vacuum. Here, this approach is extended by using localized surface plasmon resonance spectroscopy. This simple and relatively inexpensive setup enables the investigation of the thermal cis-to-trans isomerization in different environments, here in argon gas or in liquid butanediol. The energy barrier for the cis-to-trans-relaxation in argon, 1.17±0.20eV, is identical to the barrier in vacuum, while the energy barrier in liquid butanediol is slightly larger, 1.26±0.15eV.