Photoresponsive Metal-Organic Frameworks as Adjustable Scaffolds in Reticular Chemistry

The easy and remote switching of light makes this stimulus an ideal candidate for a large number of applications, among which the preparation of photoresponsive materials stands out. The interest of several scientists in this area in order to achieve improved functionalities has increase parallel to the growth of the structural complexity of these materials. Thus, metal-organic frameworks (MOFs) turned out to be ideal scaffolds for light-responsive ligands. This review is focused on the integration of photoresponsive organic ligands inside MOF crystalline arrays to prepare enhanced functional materials. Besides the summary of the preparation, properties and applications of these materials, an overview of the future outlook of this research area is provided.

The development of organic ligands having a higher structural complexity is one of the main directions in order to prepare MOFs showing advanced functionalities [31][32][33][34]. Thus, the incorporation of stimuli-responsive scaffolds inside a MOF matrix turned out to be an ideal strategy within reticular chemistry [35][36][37][38], allowing to prepare smart materials [39,40]. The application of an external stimulus induces changes in the organic ligands, leading to modifications of certain properties of the metal-organic material, such as porosity [41,42]. The selected stimulus should be able to promote modifications in the crystalline array without causing damage to its structural integrity.
Among the available stimuli, light is particularly appealing due to: (i) the easy and remote switching control; (ii) the possibility of varying the intensity, wavelength and irradiation time; (iii) the clean and non-destructive operation [43]. Different organic ligands bearing photoactive units have been employed for the preparation of photoresponsive MOFs [44][45][46]. Different strategies can be carried out for this purpose, involving both the design of the photoresponsive ligands and the incorporation of the photoresponsive units by postsynthetic modifications (PSM) [1,2]. The direct assembly of the photoactive ligands directly affords the target material, but the ligand should retain its structural integrity during the MOF formation reaction. In stark contrast, PSM strategies lead to the preparation of materials containing the desired units by using a premade crystalline framework. Thus, preparation of materials containing the desired units by using a premade crystalline framework. Thus, ligands that are not stable to the conditions of formation of the metalorganic material can be used in order to achieve a greater variety of functionalization. However, there are two main limitations: (i) the postsynthetic conditions must not affect the integrity of the crystalline array; (ii) the functionalization with the photoresponsive units may not be homogeneous throughout the material.

Azobenzene-Based MOFs
Azobenzenes are a type of organic compound constituted by two benzene rings connected through an azo functionality. The huge variations in the shape, polarity and size induced by cis/trans photoisomerization ( Figure 2) makes them one of the most extensively studied photoswitchable molecules [54][55][56][57]. Azobenzenes are postulated as convenient scaffolds to induce a photoresponsive behavior in condensed materials through easy isomerization between both geometric isomers [58]. The employment of azobenzenebased compounds as organic ligands in the preparation of MOFs led to different applications such as control over the pore (open/close pore) [59] or capture and release of carbon dioxide [60,61].

Azobenzene-Based MOFs
Azobenzenes are a type of organic compound constituted by two benzene rings connected through an azo functionality. The huge variations in the shape, polarity and size induced by cis/trans photoisomerization ( Figure 2) makes them one of the most extensively studied photoswitchable molecules [54][55][56][57]. Azobenzenes are postulated as convenient scaffolds to induce a photoresponsive behavior in condensed materials through easy isomerization between both geometric isomers [58]. The employment of azobenzene-based compounds as organic ligands in the preparation of MOFs led to different applications such as control over the pore (open/close pore) [59] or capture and release of carbon dioxide [60,61]. Professors Liu, Sun and colleagues prepared azobenzene-functionalized UiO-66 absorbents [62]. First, the reaction of 2-phenyldiazenyl terephthalic acid (1) (Figure 3a), zirconium tetrachloride and acetic acid under solvothermal conditions afforded the crystalline material U-azo having azobenzene ligands connected to Zr6O4(OH)4 clusters through the carboxylate groups. This porous material was suspended in a solution of tetraethylenepentamine (TEPA) in anhydrous methanol and stirred under inert atmosphere for 12 h. After removing the solvent, MOF-based composite materials having up to 10.8 wt% of TEPA were obtained. The TEPA sites embedded in the material allow a tailorable CO2 capture, establishing strong interaction with this guest gas when the trans isomer of the azobenzene is connecting the different metal clusters (Figure 3b, above). Upon visible irradiation of the MOF surface, the trans isomer was photoisomerized to the cis azobenzene, thus positioning the aromatic ring on the TEPA sites (Figure 3b, below), reducing the CO2 adsorption capacity of the composite. Ultraviolet irradiation of the isomerized material led to the trans isomers, showing a remarkable modification of the adsorbed CO2 amount up to 45.6% through cyclic irradiation. Professors Liu, Sun and colleagues prepared azobenzene-functionalized UiO-66 absorbents [62]. First, the reaction of 2-phenyldiazenyl terephthalic acid (1) (Figure 3a), zirconium tetrachloride and acetic acid under solvothermal conditions afforded the crystalline material U-azo having azobenzene ligands connected to Zr 6 O 4 (OH) 4 clusters through the carboxylate groups. This porous material was suspended in a solution of tetraethylenepentamine (TEPA) in anhydrous methanol and stirred under inert atmosphere for 12 h. After removing the solvent, MOF-based composite materials having up to 10.8 wt% of TEPA were obtained. The TEPA sites embedded in the material allow a tailorable CO 2 capture, establishing strong interaction with this guest gas when the trans isomer of the azobenzene is connecting the different metal clusters (Figure 3b, above). Upon visible irradiation of the MOF surface, the trans isomer was photoisomerized to the cis azobenzene, thus positioning the aromatic ring on the TEPA sites (Figure 3b, below), reducing the CO 2 adsorption capacity of the composite. Ultraviolet irradiation of the isomerized material led to the trans isomers, showing a remarkable modification of the adsorbed CO 2 amount up to 45.6% through cyclic irradiation. Professors Liu, Sun and colleagues prepared azobenzene-functionalized UiO-66 absorbents [62]. First, the reaction of 2-phenyldiazenyl terephthalic acid (1) (Figure 3a), zirconium tetrachloride and acetic acid under solvothermal conditions afforded the crystalline material U-azo having azobenzene ligands connected to Zr6O4(OH)4 clusters through the carboxylate groups. This porous material was suspended in a solution of tetraethylenepentamine (TEPA) in anhydrous methanol and stirred under inert atmosphere for 12 h. After removing the solvent, MOF-based composite materials having up to 10.8 wt% of TEPA were obtained. The TEPA sites embedded in the material allow a tailorable CO2 capture, establishing strong interaction with this guest gas when the trans isomer of the azobenzene is connecting the different metal clusters (Figure 3b, above). Upon visible irradiation of the MOF surface, the trans isomer was photoisomerized to the cis azobenzene, thus positioning the aromatic ring on the TEPA sites (Figure 3b, below), reducing the CO2 adsorption capacity of the composite. Ultraviolet irradiation of the isomerized material led to the trans isomers, showing a remarkable modification of the adsorbed CO2 amount up to 45.6% through cyclic irradiation.  [62]. Color key: metallic grey = azobenzene-functionalized UiO-66 (U-azo); red = nitrogen atoms of the trans isomer; dark blue = nitrogen atoms of the cis isomer; purple = phenyl ring; dark green = CO 2 molecule; white = hydrogen atoms; light blue = TEPA site. Professors Mei and coworkers described the preparation of the thorium-organic framework Th-Azo-MOF through the reaction of the azobenzene-modified photoswitchable ligand 2 ( Figure 4a) and thorium nitrate hexahydrate in the presence of small amounts of nitric acid and water, using N,N-dimethylformamide (DMF) as solvent [63]. This threedimensional material, having an overall formula [Th 6 O 4 (OH) 4 (H 2 O) 6 (2) 6 ], formed huge tetrahedral and octahedral pores (Figure 4b) with an impressive 73.7% of solvent-accessible void volume. The authors monitored the dynamics of this porous material related to cis/trans photoisomerization by UV-vis absorption and 1 H NMR. Irradiation at 365 nm afforded a material having 19.7% of cis azobenzene ligands after 120 min. The reversible isomerization is also possible by using blue light (460 nm) or heating at 60 • C under dark conditions. The irradiation experiment only decreased the amount of cis ligand to 5.51%, while the heating treatment allows the recovery of the pristine material. These actinide MOFs were applied as efficient adsorbents of Rhodamine B, showing an adsorption capacity of 100.01 (±0.68) mg/g in the pristine material. This capacity was slightly reduced to 97.9 (±0.61) mg/g after 1 h of irradiation, showing the potential of these materials in smart photo-induced release applications. nitrogen atoms of the trans isomer; dark blue = nitrogen atoms of the cis isomer; purple = phenyl ring; dark green = CO2 molecule; white = hydrogen atoms; light blue = TEPA site.
Professors Mei and coworkers described the preparation of the thorium-organic framework Th-Azo-MOF through the reaction of the azobenzene-modified photoswitchable ligand 2 ( Figure 4a) and thorium nitrate hexahydrate in the presence of small amounts of nitric acid and water, using N,N-dimethylformamide (DMF) as solvent [63]. This three-dimensional material, having an overall formula [Th6O4(OH)4(H2O)6(2)6], formed huge tetrahedral and octahedral pores (Figure 4b) with an impressive 73.7% of solvent-accessible void volume. The authors monitored the dynamics of this porous material related to cis/trans photoisomerization by UV-vis absorption and 1 H NMR. Irradiation at 365 nm afforded a material having 19.7% of cis azobenzene ligands after 120 min. The reversible isomerization is also possible by using blue light (460 nm) or heating at 60 °C under dark conditions. The irradiation experiment only decreased the amount of cis ligand to 5.51%, while the heating treatment allows the recovery of the pristine material. These actinide MOFs were applied as efficient adsorbents of Rhodamine B, showing an adsorption capacity of 100.01 (±0.68) mg/g in the pristine material. This capacity was slightly reduced to 97.9 (±0.61) mg/g after 1 h of irradiation, showing the potential of these materials in smart photo-induced release applications.  [63]. For clarity, only the structure of one organic ligand per pore is shown. Color key: metallic  [63]. For clarity, only the structure of one organic ligand per pore is shown. Color key: metallic grey = thorium clusters; red = rods representing the azobenzene ligand 2 and nitrogen atoms of the azo functionality; light grey = carbon atoms; white = hydrogen atoms; purple = phenyl ring.

Diarylethene-Based MOFs
Diarylethenes are a type of light-responsive organic compound constituted by aromatic rings connected through a carbon-carbon double bond [64]. These molecules easily exchange the open ring and closed ring forms by the application of a photochemical stimulus ( Figure 5). Along with their diverse structural functionalization, diarylethenes are suitable candidates for the development of photoresponsive metal-organic porous materials due to their exceptional photoreactivity and thermal stability [65][66][67]. grey = thorium clusters; red = rods representing the azobenzene ligand 2 and n azo functionality; light grey = carbon atoms; white = hydrogen atoms; purple =

Diarylethene-Based MOFs
Diarylethenes are a type of light-responsive organic compound c matic rings connected through a carbon-carbon double bond [64]. Thes exchange the open ring and closed ring forms by the application of a ph ulus ( Figure 5). Along with their diverse structural functionalization suitable candidates for the development of photoresponsive metal-org rials due to their exceptional photoreactivity and thermal stability [65-  (Figure 6b) [68]. The react linker 3, nip 2− and cadmium nitrate using a mixture of DMF and meth forded colorless prismatic crystals of the target MOF. The single crystal that this material crystallizes in the P42/n space group of the tetragona Cd 2+ adopts a pentagonal bipyramidal geometry. The pentagonal plane nip 2− carboxylates and a nitrogen atom from one of the bipyridyl m rylethene ligand 3, while the axial positions are coordinated to other the ligand 3 and to a water molecule. The solid structure showed fou connected by four diarylethene ligands forming a square-shaped macr where the square grids composed of cadmium and ligand 3 are connec the c-axis (Figure 6d) affording a nanotubular structure. These metallo cally interlocked with four adjacent nanotubular structures.  (Figure 6b) [68]. The reaction of the organic linker 3, nip 2− and cadmium nitrate using a mixture of DMF and methanol as solvent afforded colorless prismatic crystals of the target MOF. The single crystal structure revealed that this material crystallizes in the P4 2 /n space group of the tetragonal system, in which Cd 2+ adopts a pentagonal bipyramidal geometry. The pentagonal plane is occupied by two nip 2− carboxylates and a nitrogen atom from one of the bipyridyl moieties of the diarylethene ligand 3, while the axial positions are coordinated to other bipyridyl group of the ligand 3 and to a water molecule. The solid structure showed four cadmium atoms connected by four diarylethene ligands forming a square-shaped macrocycle (Figure 6c), where the square grids composed of cadmium and ligand 3 are connected by nip 2− along the c-axis ( Figure 6d) affording a nanotubular structure. These metallogrids are mechanically interlocked with four adjacent nanotubular structures.
The carbon atoms in the thienyl groups of the diarylethene ligands are placed 3.52 Å spaced out from each other showing an antiparallel conformation, which is adequate for carrying out a photocyclization reaction [69]. Thus, the irradiation of a suspension of DTE MOF in DMF/MeOH at 305-315 nm instantly turned the colorless crystals of the metal-organic porous material into a homogeneous dark blue solution. The photostationary state was reached after 2 h, leading to a mixture having 80% of closed ring diarylethene linkers. This photocyclization reaction led to the breaking of the coordination bonds between carboxylate and pyridyl groups and cadmium atoms, degrading the crystalline array. Further attempts to obtain a metal-organic framework using the closed form of 3 as starting material were unsuccessful, suggesting that the geometrically demanding closed ring form could not be fitted to a coordination geometry in order to make the target MOF. Interestingly, the reversible isomerization can photochemically regenerate the initial DTE MOF by exposing a solution with the obtained fragments to visible light. This smart photochemically breakable and recoverable metal-organic material paves the way to the development of enhanced applications in greatly effective switchable release of guest compounds encapsulated within a MOF matrix.  showing the connection between one side of two different macrocycle-shaped diarylethene-containing metallogrids [68]. I terpenetration is omitted for clarity in the solid structure. Color key: grey = cadmium atoms; blac = carbon atoms; red = oxygen atoms and carbon atoms of the diarylethene motif; white = hydroge atoms; yellow = sulfur atoms; green = fluor atoms; purple = nitrogen atoms.
The carbon atoms in the thienyl groups of the diarylethene ligands are placed 3.52 spaced out from each other showing an antiparallel conformation, which is adequate fo carrying out a photocyclization reaction [69]. Thus, the irradiation of a suspension of DT MOF in DMF/MeOH at 305-315 nm instantly turned the colorless crystals of the meta organic porous material into a homogeneous dark blue solution. The photostationary sta was reached after 2 h, leading to a mixture having 80% of closed ring diarylethene linker  [68]. Interpenetration is omitted for clarity in the solid structure. Color key: grey = cadmium atoms; black = carbon atoms; red = oxygen atoms and carbon atoms of the diarylethene motif; white = hydrogen atoms; yellow = sulfur atoms; green = fluor atoms; purple = nitrogen atoms.
Professors Zheng, Luo and coworkers prepared the zinc-organic framework ECUT-30 combining two photoresponsive organic ligands through the solvothermal reaction of zinc nitrate, the dipyridine-functionalized diarylethene 3 ( Figure 6a) and the azobenzene-based ligand 4 ( Figure 7a) bearing two carboxylate groups [70]. The solid structure of the synthesized metal-organic porous material ( Figure 7b) revealed that the MOF crystallizes in the monoclinic C2/c space group, with a five-coordinated pyramidal zinc(II) site having attached four azobenzene oxygen atoms and one pyridinic nitrogen atom of the diarylethene ligand 3. The zinc paddlewheel clusters are connected by four carboxylate groups of azobenzene ligands 4, creating a four-fold interpenetration. This material was employed in the capture of C 2 H 2 , C 2 H 4 and CO 2 , obtaining a fine-tuning adsorption selectivity towards different guests by the application of a photochemical stimulus.
cally breakable and recoverable metal-organic material paves the way to the developmen of enhanced applications in greatly effective switchable release of guest compounds en capsulated within a MOF matrix.
Professors Zheng, Luo and coworkers prepared the zinc-organic framework ECUT 30 combining two photoresponsive organic ligands through the solvothermal reaction o zinc nitrate, the dipyridine-functionalized diarylethene 3 (Figure 6a) and the azobenzene based ligand 4 ( Figure 7a) bearing two carboxylate groups [70]. The solid structure of the synthesized metal-organic porous material (Figure 7b) revealed that the MOF crystallizes in the monoclinic C2/c space group, with a five-coordinated pyramidal zinc(II) site having attached four azobenzene oxygen atoms and one pyridinic nitrogen atom of the dia rylethene ligand 3. The zinc paddlewheel clusters are connected by four carboxylate groups of azobenzene ligands 4, creating a four-fold interpenetration. This material was employed in the capture of C2H2, C2H4 and CO2, obtaining a fine-tuning adsorption selec tivity towards different guests by the application of a photochemical stimulus.  [70]. Interpenetration is omitted for clarity in the solid structure. Color key: grey = zinc atoms; black = carbon atoms of the azobenzene ligand 4; red = oxygen atoms and carbon atoms of the diarylethene ligand 3; white = hydrogen atoms; yellow = sulfur atoms; green = fluor atoms; purple = nitrogen atoms.

Spiropyran-Based MOFs
Spiropyrans are a type of organic compounds in which a pyran ring is bonded to a second ring, usually a heterocyclic compound, in a spiro way [71]. These molecules are widely known for their photochromic properties, reversibly changing to merocyanines by photochemical irradiation (Figure 8). The employment of spiropyrans ligands in the construction of metal-organic frameworks also led to photoresponsive crystalline porous materials that can be employed in different applications, such as photophysics modulation [72,73] and light-dependent gas adsorption [74].
second ring, usually a heterocyclic compound, in a spiro way [71]. widely known for their photochromic properties, reversibly changing photochemical irradiation (Figure 8). The employment of spiropyran struction of metal-organic frameworks also led to photoresponsive cr terials that can be employed in different applications, such as photo [72,73] and light-dependent gas adsorption [74]. Professor Shustova and collaborators reported the control over c ics using spiropyran ligand-based zinc-organic frameworks [75]. W searchers carried out a synthetic approach in which the spiropyran lig pyridyl moieties (Figure 9a) is attached to zinc(II) paddlewheel clus DBTD MOF, constructed with a tetracarboxylate organic linker 7 (Figu ing different 2D layers of this premade MOF matrix to afford a thre ture (Figure 9c). Professor Shustova and collaborators reported the control over cycloreversion kinetics using spiropyran ligand-based zinc-organic frameworks [75]. With this aim, the researchers carried out a synthetic approach in which the spiropyran ligand (5 or 6) bearing pyridyl moieties ( Figure 9a) is attached to zinc(II) paddlewheel clusters of the premade DBTD MOF, constructed with a tetracarboxylate organic linker 7 (Figure 9b), thus connecting different 2D layers of this premade MOF matrix to afford a three-dimensional structure (Figure 9c).
The authors studied the photochromic cycloreversion kinetics of ligand 5 both in solution and within the crystalline array of Spiro−1 MOF, demonstrating that the material mimics the solution behavior. Thus, a MOF matrix affords a photoswitchable isomerization in the solid state providing an arrangement having enough void space in comparison to the crowded packing observed for the free ligands. However, Spiro−2 MOF, in which the organic ligand 6 bearing two photoswitchable spiropyran groups was used, exhibited a limited photoisomerization. These results can be explained by the confined environment that provides a high steric hindrance to the spiropyran linkers.
Additionally, Spiro−1 MOF, in which spiropyran 5 connects different layers, was employed to map the acidic degradation of the material by using photoluminescence spectroscopy and powder X-ray diffraction (PXRD). This work showed the fine-tuning of the photoisomerization rate of spiropyrans inside MOFs as a function of the structure of the framework, and, also, provided a toolbox for the development of a new class of light-responsive markers.
The same research group also studied the tailoring and tunability of the optoelectronic properties of mono-and heterometallic actinide-containing spiropyran-based metal-organic frameworks [76]. This work demonstrated that the electronic properties of these materials can be tuned by different approaches: (i) incorporation of photoswitches; (ii) integration of a secondary metal; (iii) inclusion of a guest inside the MOF pores. This research group went a step further and developed the first photochromic field-effect transistor (FET) based on a zirconium-organic framework backbone encapsulating 7,7,8,8-tetracyanoquinodimethane, along with its incorporation into a two-light-emitting diode (LED) failsafe circuit. The drain current of the FET system could be regulated by the application of different gate voltages, and, also, ultraviolet irradiation that induces the conversion of the spiropyrans ligands to the merocyanine linkers.

DASA-Based MOFs
Donor-acceptor Stenhouse adducts are a type of heterocyclic photoswitchable organic compound that experiences reversible triene cyclisation through the application of a photochemical stimulus (Figure 10), efficiently exchanging between open ring and closed ring forms [77].
metal-organic frameworks [76]. This work demonstrated that the electronic properties of these materials can be tuned by different approaches: (i) incorporation of photoswitches; (ii) integration of a secondary metal; (iii) inclusion of a guest inside the MOF pores. This research group went a step further and developed the first photochromic field-effect transistor (FET) based on a zirconium-organic framework backbone encapsulating 7,7,8,8-tetracyanoquinodimethane, along with its incorporation into a two-light-emitting diode (LED) failsafe circuit. The drain current of the FET system could be regulated by the application of different gate voltages, and, also, ultraviolet irradiation that induces the conversion of the spiropyrans ligands to the merocyanine linkers.
This porous material exhibited a bistable behavior, which the closed ring form, obtained upon white light irradiation, turned out to be stable for up to 30 days. After a heating treatment, the material underwent the regeneration of the open ring form of the DASA ligands. Furthermore, the stability of the system was tested under multiple photoswitchingheating cycles, observing a partial deterioration of the crystalline matrix. Despite the short-term durability of this system, its bistability could serve for applications in which an extended permanence of both bistable forms is a requirement, such as the development of non-volatile memory (NVM) materials. This porous material exhibited a bistable behavior, which the closed ring form, obtained upon white light irradiation, turned out to be stable for up to 30 days. After a heating treatment, the material underwent the regeneration of the open ring form of the DASA ligands. Furthermore, the stability of the system was tested under multiple photoswitching-heating cycles, observing a partial deterioration of the crystalline matrix. Despite the short-term durability of this system, its bistability could serve for applications in which an extended permanence of both bistable forms is a requirement, such as the development of non-volatile memory (NVM) materials.

Molecular Motor-Based MOFs
A synthetic molecular motor is a molecular machine-type compound that experiences a rotational motion upon activation [79,80].
One of the topics of the Nobel Prize in Chemistry in 2016 was the development of molecular motors envisioned by Professor Feringa [81]. The Nobel Prize laureate researcher has substantial expertise in the preparation of light-driven unidirectional rotary molecular motors [82,83]. In 1999, his research group reported the preparation and operation of the overcrowded alkene-based molecular motor 12 [84], in which a unidirectional rotary motion was accomplished by a four-step isomerization cycle (Figure 12). The development of enhanced molecular motors makes this type of molecular machine a tunable scaffold in several fields of research, including biomolecular technology and smart materials [51].
The well-defined spatial organization provided by the MOF matrix led to the precise positioning of the molecular motor linkers, overcoming the Brownian motion that precludes cooperativity in solution [51].
Professors Browne, Wezenberg, Feringa and coworkers reported the first preparation of a zin-organic framework containing overcrowded alkene-type molecular motors as linkers [85]. The molecular motor-based linkers 13 bearing pyridyl units at the ends could experience a four-step isomerization cycle by the application of light and thermal stimuli (Figure 13a). The researchers carried out a solvent-assisted linker exchange (SALE) of the previously reported BrYO-MOF constituted by 1,4-dibromo-2,3,5,6-tetrakis(4-carboxyphenyl)benzene (TPCB) and dipyridyl-naphthalenediimide (DPNI) linkers (Figure 13bd) [86]. This postsynthetic functionalization of a MOF backbone involved the exchange of the pyridine-based pillars DPNI with the corresponding overcrowded alkene-type motor linker 13 by immersing crystals of BrYO-MOF in a solution of 13 in DMF at 60 °C. Yellow crystals of moto-MOF1 were obtained by this methodology using the organic linker 13a (Figure 13a) after 24 h of heating. Moto-MOF2-E and moto-MOF2-Z required an additional 72 h to completely replaced the DPNI pillars with the molecular motor ligands 13b (Figure 13a). The successful ligand exchange was confirmed by Raman spectroscopy, not observing bands attributable to DPNI. The development of enhanced molecular motors makes this type of molecular machine a tunable scaffold in several fields of research, including biomolecular technology and smart materials [51].
The well-defined spatial organization provided by the MOF matrix led to the precise positioning of the molecular motor linkers, overcoming the Brownian motion that precludes cooperativity in solution [51].
Professors Browne, Wezenberg, Feringa and coworkers reported the first preparation of a zin-organic framework containing overcrowded alkene-type molecular motors as linkers [85]. The molecular motor-based linkers 13 bearing pyridyl units at the ends could experience a four-step isomerization cycle by the application of light and thermal stimuli (Figure 13a). The researchers carried out a solvent-assisted linker exchange (SALE) of the previously reported BrYO-MOF constituted by 1,4-dibromo-2,3,5,6-tetrakis(4-carboxyphenyl)benzene (TPCB) and dipyridyl-naphthalenediimide (DPNI) linkers (Figure 13b-d) [86]. This postsynthetic functionalization of a MOF backbone involved the exchange of the pyridine-based pillars DPNI with the corresponding overcrowded alkene-type motor linker 13 by immersing crystals of BrYO-MOF in a solution of 13 in DMF at 60 • C. Yellow crystals of moto-MOF1 were obtained by this methodology using the organic linker 13a (Figure 13a) after 24 h of heating. Moto-MOF2-E and moto-MOF2-Z required an additional 72 h to completely replaced the DPNI pillars with the molecular motor ligands 13b (Figure 13a). The successful ligand exchange was confirmed by Raman spectroscopy, not observing bands attributable to DPNI.
The authors studied the photochemical and thermal isomerization of the molecular motor 13 in solution and integrated in the moto-MOFs matrix by using Raman spectroscopy. These results revealed that the overcrowded alkene motor struts could perform an unimpeded 360 • unidirectional rotary motion in the solid state, having a similar dynamic behavior to that observed in solution. These finding are remarkably promising in the research area of molecular machinery, paving the way for the development of applications where a switchable unidirectional motion is a requirement, such as photoswitchable microfluidic pumps and photodriven mass transport.
The light-driven molecular motor linker 13a was employed in a SALE functionalization protocol in the premade PdTCPP-MOF, constituted by meso-α,β-di(4-pyridil)glycol pillars (DPG) and the photosensitizer porphyrin-based struts 14 connected through zinc paddlewheel clusters (Figure 14a-c) [87]. The newly prepared molecular motor-type moto-MOF3 (Figure 14d) also exhibited an unhindered rotary motion of the overcrowded alkene within the crystalline array. Interestingly, an efficient energy transfer between the DPG ligands and the molecular motor-based pillars 13a was determined, allowing the photoisomerization to take place using green light as irradiation source. showing a portion of the lattice framework [86]; (e) simulated stick schematic representation of the moto-MOF1 containing the overcrowded alkene-based molecular rotor 13a as organic linker [85].
tion protocol in the premade PdTCPP-MOF, constituted by meso-α,β-di(4-pyridil)g pillars (DPG) and the photosensitizer porphyrin-based struts 14 connected through paddlewheel clusters (Figure 14a-c) [87]. The newly prepared molecular motor moto-MOF3 (Figure 14d) also exhibited an unhindered rotary motion of the overcrow alkene within the crystalline array. Interestingly, an efficient energy transfer betwee DPG ligands and the molecular motor-based pillars 13a was determined, allowin photoisomerization to take place using green light as irradiation source.

Interlocked Fumaramide-Based MOFs
Mechanically interlocked molecules (MIMs) are a type of compound in which at least two components are intertwined with each other [88,89]. Along with molecular motors, the use of MIMs for the development of molecular machines was the topic of the Nobel Prize in Chemistry in 2016 [90][91][92]. Rotaxanes, a type of MIMs having at least a linear component threaded into a cyclic one, stand out for the greatest variety of motions, being possible to exert control over the dynamics of the counterparts [93,94].
Light has been shown to be an effective stimulus in rotaxanes bearing photoresponsive scaffolds [95][96][97][98]. Thus, interlocked fumaramides are postulated as a useful tool to obtain molecular machines operating through photoisomerization to their equivalent intertwined maleamides ( Figure 15) [99][100][101][102]. component threaded into a cyclic one, stand out for the greatest variety of motion possible to exert control over the dynamics of the counterparts [93,94].
As an eldest member of the MOF family having benzylic amide macrocyc organic ligands [111,112], Professor Berna and coworkers reported the preparatio UMUMOF-(E)-3 having the interlocked fumaramide (E)-15 (Figure 16a) as an org and [113]. The reaction of (E)-15 and copper nitrate in the presence of nitric aci solvothermal conditions afforded blue prismatic crystals of a MOF having an ove mula [Cu2((E)-15)2(H2O)2(DMF)2]. This porous material crystallized in the mo P21/n space group, forming a two-dimensional net of non-interpenetrated rhomb grids, in which the copper(II) paddlewheels are coordinated to (E)-15 ligands at th and to water molecules at axial positions (Figure 16b). The stacking of these rhomb metallogrids stablished well-ordered channels along the a-axis. Interestingly, the has enough free volume to undergo the fumaramide/maleamide isomerizatio upon irradiation at 312 nm of a suspension of UMUMOF-(E)-3 crystals in d methane for 8 h, a new material having 20% of interlocked maleamides (Z)-15 (Fig  was obtained, showing a higher pore size compared to that of the pristine mate The incorporation of rotaxanes inside a MOF matrix has turned out to be a suitable strategy to exploit the properties of this type of MIMs in the solid state [103][104][105][106], including the study of the component's dynamics [107][108][109][110]. As an eldest member of the MOF family having benzylic amide macrocycle-based organic ligands [111,112], Professor Berna and coworkers reported the preparation of the UMUMOF-(E)-3 having the interlocked fumaramide (E)-15 (Figure 16a) as an organic ligand [113]. The reaction of (E)-15 and copper nitrate in the presence of nitric acid under solvothermal conditions afforded blue prismatic crystals of a MOF having an overall formula [Cu 2 ((E)-15) 2 (H 2 O) 2 (DMF) 2 ]. This porous material crystallized in the monoclinic P2 1 /n space group, forming a two-dimensional net of non-interpenetrated rhombohedral grids, in which the copper(II) paddlewheels are coordinated to (E)-15 ligands at the vertex and to water molecules at axial positions (Figure 16b). The stacking of these rhombohedral metallogrids stablished well-ordered channels along the a-axis. Interestingly, the material has enough free volume to undergo the fumaramide/maleamide isomerization. Thus, upon irradiation at 312 nm of a suspension of UMUMOF-(E)-3 crystals in dichloromethane for 8 h, a new material having 20% of interlocked maleamides (Z)-15 (Figure 16a) was obtained, showing a higher pore size compared to that of the pristine material. The authors also prepared the metal-organic crystalline material UMUMOF-(Z)-3 using the interlocked maleamide (Z)-15 as struts, displaying even more porosity.
The researchers envisioned the employment of these materials as molecular dispensers as the result of a breathing-like process of the pores induced by a photochemical stimulus. In order to test this idea, p-benzoquinone was selected as a model cargo, accomplishing its loading inside UMUMOF-(E)-3 through the suspension of this material in a 1.2 M solution of quinone in chloroform for 8 h. The same isomerization conditions abovementioned, which provided 20% of maleamides struts within the crystalline array, were carried out, achieving the complete release of the cargo as a consequence of this photoirradiation. In contrast, a similar experiment with UMUMOF-(Z)-3 did not require light activation to reach similar results in a shorter time. Conveniently, the photoisomerized material can be regenerated by a thermal treatment, being possible to reuse the UMUMOF-(E)-3 in the operation cycle involving quinone loading, photorelease and thermal recovery. This cyclic reusability of the material showed a retaining of the cargo capacity close to 80% after three iterations.
This new approach to incorporate photoswitchable rotaxane-based components into MOFs paves the way for the development of new functional rotaxane-based materials operating as molecular machines in the solid state. authors also prepared the metal-organic crystalline material UMUMOF-(Z)-3 using the interlocked maleamide (Z)-15 as struts, displaying even more porosity.  [113]. Color key: grey = copper atoms; purple = carbon atoms of the benzylic amide macrocycle and nitrogen atoms; red = carbon atoms of the fumaramide and oxygen atoms; green = p-benzoquinone molecules; purple toroid = benzylic amide macrocycle; red rod = fumaramide thread; blue rod = maleamide thread.
The researchers envisioned the employment of these materials as molecular dispensers as the result of a breathing-like process of the pores induced by a photochemical stimulus. In order to test this idea, p-benzoquinone was selected as a model cargo, accomplishing its loading inside UMUMOF-(E)-3 through the suspension of this material in a 1.2 M solution of quinone in chloroform for 8 h. The same isomerization conditions abovementioned, which provided 20% of maleamides struts within the crystalline array, were carried out, achieving the complete release of the cargo as a consequence of this photoirradiation. In contrast, a similar experiment with UMUMOF-(Z)-3 did not require light activation to reach similar results in a shorter time. Conveniently, the photoisomerized material  [113]. Color key: grey = copper atoms; purple = carbon atoms of the benzylic amide macrocycle and nitrogen atoms; red = carbon atoms of the fumaramide and oxygen atoms; green = p-benzoquinone molecules; purple toroid = benzylic amide macrocycle; red rod = fumaramide thread; blue rod = maleamide thread.
The incorporation of photoactive guests, such as styrylpyridinium derivatives, in the cavities of cucurbituril with a macrocycle-photoactive unit binding ratio of 1:2 has led to a cucurbituril-promoted photodimerization upon visible light irradiation ( Figure 17) [129][130][131]. Professors Mei, Feng, Shi and colleagues reported the preparation of the uraniumorganic framework U-CB [8]-MPyVB by using cucurbit [8]uril-based pseudorotaxanes having a pair of carboxylic acid-functionalized styrylpyridinium linear components [53]. The synthesis of U-CB[8]-MPyVB proceeds via a one-pot solvothermal reaction through the assembly of uranium nitrate oxide, (E)-4-[2-(methylpyridine-4-yl) vinylbenzoic acid (16) and cucurbit [8]uril. Thus, yellow crystals of the target MOFs were obtained, in which two nonparallel styrylpyridinium guests 16 are surrounded by the cucurbituril macrocycle (Figure 18a,b). The carboxylic acid group placed at the end of each styrene-based derivative is coordinated to one different uranium cluster, preventing a dethreading process to take place. Two identical interlocked motifs, showing different conformations are established, differing in the photoactivities as a consequence of the different uranyl coordination patterns. In the photoinert motif, in which the distances between C=C bond of styrylpyridinium guests is 4.50 Å, the photodimerization is impeded. By contrast, in the photoactive motif, this distance is above 4.20 Å, fitting with Schmidt's topochemical criteria [132], thereby allowing the photodimerization process. Professors Mei, Feng, Shi and colleagues reported the preparation of the uraniumorganic framework U-CB [8]-MPyVB by using cucurbit [8]uril-based pseudorotaxanes having a pair of carboxylic acid-functionalized styrylpyridinium linear components [53]. The synthesis of U-CB[8]-MPyVB proceeds via a one-pot solvothermal reaction through the assembly of uranium nitrate oxide, (E)-4-[2-(methylpyridine-4-yl) vinylbenzoic acid (16) and cucurbit [8]uril. Thus, yellow crystals of the target MOFs were obtained, in which two nonparallel styrylpyridinium guests 16 are surrounded by the cucurbituril macrocycle (Figure 18a,b). The carboxylic acid group placed at the end of each styrenebased derivative is coordinated to one different uranium cluster, preventing a dethreading process to take place. Two identical interlocked motifs, showing different conformations are established, differing in the photoactivities as a consequence of the different uranyl coordination patterns. In the photoinert motif, in which the distances between C=C bond of styrylpyridinium guests is 4.50 Å, the photodimerization is impeded. By contrast, in the photoactive motif, this distance is above 4.20 Å, fitting with Schmidt's topochemical criteria [132], thereby allowing the photodimerization process.
Upon 365 nm irradiation of the U-CB[8]-MPyVB MOF, a single-crystal-to-single-crystal regioselective [2 + 2] photodimerization reaction of the styrene motifs in the solid state is accomplished. This reaction leads to the conversion of the interlocked styrylpyridinium linear components within the photoactive motifs into interlocked cyclobutanes (Figure 18c). Thus, a photomechanical bending of the metal-organic crystalline material is induced due to the macroscopic deformation of the MOF matrix (Figure 18b,c). Very likely, this development would allow substantial advances in the preparation of photoactuator devices with special relevance in microrobotics and optomechanics (c) stick representation of the crystal structure of the interlocked photodimerization product within the MOF matrix (left) and cartoon representation of the deformation in the crystalline array as a consequence of the photoirradiation (right) involving quinone loading, photorelease and thermal treatment [53]. Color key: grey = uranium atoms; purple = carbon atoms of the cucurbituril macrocycle and nitrogen atoms; red = carbon atoms of the styrylpyridinium-based compounds and oxygen atoms; blue = carbon atoms of the cyclobutane-based compounds; purple toroid = cucurbit [8]uril macrocycle; red rods = styrylpyridinium-based compounds; crossed blue rods = cyclobutane-based compounds.
Upon 365 nm irradiation of the U-CB[8]-MPyVB MOF, a single-crystal-to-singlecrystal regioselective [2 + 2] photodimerization reaction of the styrene motifs in the solid state is accomplished. This reaction leads to the conversion of the interlocked styrylpyridinium linear components within the photoactive motifs into interlocked cyclobutanes (Figure 18c). Thus, a photomechanical bending of the metal-organic crystalline material is (c) stick representation of the crystal structure of the interlocked photodimerization product within the MOF matrix (left) and cartoon representation of the deformation in the crystalline array as a consequence of the photoirradiation (right) involving quinone loading, photorelease and thermal treatment [53]. Color key: grey = uranium atoms; purple = carbon atoms of the cucurbituril macrocycle and nitrogen atoms; red = carbon atoms of the styrylpyridinium-based compounds and oxygen atoms; blue = carbon atoms of the cyclobutane-based compounds; purple toroid = cucurbit [8]uril macrocycle; red rods = styrylpyridinium-based compounds; crossed blue rods = cyclobutane-based compounds.

General Remarks
The photoresponsive metal-organic frameworks highlighted in this review are highly important because of their advantageous properties and numerous potential practical applications (Table 1). This section addresses a general discussion of these MOFs, remarking some current applications and postulating some potential implementations. Due to the intrinsic porosity that characterizes MOFs, most of them can be applied in selective adsorption or controlled cargo release (Table 1, entries 1-4 and 8) [62,63,68,70,113]. The incorporation of photoactive units in the organic struts can lead to a tailored pore size, affording suitable materials for such applications. One of the biggest challenges in this type of application is to improve the reusability of MOFs. This reusability is affected by several factors, including the partial degradation of the framework by the applied light stimulus and the persistence of part of the host molecule within the crystalline material after the release cycle. In this way, the regenerable DTE MOF, showing a reversible crystalline assembly [68], paves the way to the development of enhanced adsorption/desorption implementations, avoiding the abovementioned issues. However, this material affords an immediate release of the cargo, while other materials, such as UMUMOF-(E)-3 [113], allow control of the release rate, affording a dosage of the cargo. This interlocked fumaramidebased material is limited by the photostationary state, not being able to convert most of the trans isomer to the cis one. Future research should be focused on the development of enhanced photoactive MOFs combining fast and slow release of host molecules at will.
The morphological changes induced by the application of light in these photoresponsive materials which induce different adsorption capacities or release rates may lead to consider its use in the protection of unstable reactants within the MOF matrix and its switchable release during the reaction course. In order to accomplish this interesting application, the selected wavelength should not produce any effect on the rest of reactants present in the reaction medium.
These MOFs also have potential applications in the research areas of Pharmacy and Medicine, highlighting those related to advanced drug delivery [133][134][135][136][137]. The use of photoresponsive MOFs to encapsulate drugs would permit the release at a specific time or a target place by applying a light input. The main trend of this research should be focused on the use of non-cytotoxic materials, organic struts and metal nodes. Furthermore, to accomplish biomedical applications, the selected wavelength must not damage cells or tissues. Thus, the development of photoactive metal-organic materials operating by the application of visible light turns out to be of special relevance. One potential strategy to achieve this goal could be the one followed by Professor Feringa in a molecular motor-based MOF (Table 1, entry 7), in which the incorporation of a photosensitizer strut leads to the effective phoroisomerization using green light as irradiation source through an efficient energy transfer between this photosensitizer and the photoisomerizable linker [87].
Most of the currently implemented applications of these photoresponsive MOFs are at the research laboratory level. The spiropyran-based MOFs synthesized by Professor Shustova have been successfully used for the development of FET and LED devices (Table 1, entry 5) [76]. This technological application makes evident the importance of the incorporation of photocromic MOF scaffolds placed at a FET device in order to orthogonally control the drain current.

Summary and Outlook
The incorporation of photoresponsive molecules within MOFs has led to the preparation of materials which change their properties in response to light irradiation. In order to incorporate this photoswitchable ligands, the well-stablished synthetic protocols for obtaining this type of material have been employed, such as mixed-linker solvothermal conditions and SALE methodology. The vast design possibility forecasts materials in which future prospects are almost limitless. Additionally, the precise modulation of the photochemical stimulus, being able to modify time, wavelength and irradiation power, leads to forecasting a promising future for this field of research.
Although the use of "conventional" ligands, including azobenzenes, diarylethenes, spiropyrans and DASA, has allowed the development of a wide variety of applications, this research will still provide many results potentially allowing enhanced gas separations and preparation of new optoelectronic devices, among other functions. The combination of reticular chemistry and molecular machinery, by incorporating light-responsive molecular motors and rotaxanes inside a MOF matrix, further expands the range of options for the development of photoresponsive materials showing improved functionalities.
Despite the heartening reported results so far, this is not a fairy tale and there are still some difficulties to overcome. In addition to the issue with the photostationary equilibrium that is obtained when some of the photoresponsive ligands are used, the photochemical stimulus should fall equally on all the layers of the material. Another issue is the scale-up production of the material in order to apply these light-responsive MOFs in the industry, sometimes limited by the yield of the organic strut. Regarding industrial applications, materials having a high-performance iterability must be developed to make them cost-effective.
Photoresponsive metal-organic frameworks have a bright outlook as adjustable scaffolds to prepare smart materials, which makes this area of study really attractive to a wide range of scientists from different research fields. This interdisciplinarity would benefit the progress of these interesting porous materials.