In this Review, we showcase the upsurge in the development and fundamental photophysical studies of more than 100 metal-organic frameworks (MOFs) as versatile stimuli-responsive platforms. The goal is to provide a comprehensive analysis of the field of photoresponsive MOFs while delving into the underlying photophysical properties of various classes of photochromic molecules including diarylethene, azobenzene, and spiropyran as well as naphthalenediimide and viologen derivatives integrated inside a MOF matrix as part of a framework backbone, as a ligand side group, or as a guest. In particular, the geometrical constraints, photoisomerization rates, and electronic structures of photochromic molecules integrated inside a rigid MOF scaffold are discussed. Thus, this Review reflects on the challenges and opportunities of using photoswitchable MOFs in next-generation multifunctional stimuli-responsive materials while highlighting their use in optoelectronics, erasable inks, or as the next generation of sensing devices.

Photophysics Modulation in Photoswitchable Metal–Organic Frameworks

In this review, the dependence of the photophysical response of chromophores in the confined environments associated with crystalline scaffolds, such as metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and molecular cages, has been carefully evaluated. Tunability of the framework aperture, cavity microenvironment, and scaffold topology significantly affects emission profiles, quantum yields, or fluorescence lifetimes of confined chromophores. In addition to the role of the host and its effect on the guest, the methods for integration of a chromophore (e.g., as a framework backbone, capping linker, ligand side group, or guest) are discussed. The overall potential of chromophore-integrated frameworks for a wide-range of applications, including artificial biomimetic systems, white-light emitting diodes, photoresponsive devices, and fluorescent sensors with unparalleled spatial resolution are highlighted throughout the review.

Confinement-guided photophysics in MOFs, COFs, and cages

Metal–organic fireworks: MOFs as integrated structural scaffolds for pyrotechnic materials

A novel metal-organic framework (MOF), formulated as [Cd2(TTVTC)Cl2(H2O)3]·2H2O (1), was synthesized from a tetracarboxylate ligand ([TTVTC]2-) functionalized with the thiazolothiazole extended viologen (TTV2+) fluorophore. The MOF features three-dimensional (10,3)-d frameworks with 6-fold interpenetration. The MOF exhibits reversible photochromism, due to photoinduced electron transfer from carboxylate to TTV2+. The photoactivity benefits from the electron donor-acceptor contacts enabled by mutual interpenetration of the frameworks. This is the first demonstration of photochromism in TTV2+ derivatives. In addition, the fluorescence arising from the TTV2+ fluorophore can be reversibly modulated during the photochromic process. The work demonstrates the great potential of extended viologen based ligands in the construction of MOFs with dual photomodulable optical properties, which could find future applications in photoelectronics.

Interpenetration-Enabled Photochromism and Fluorescence Photomodulation in a Metal-Organic Framework with the Thiazolothiazole Extended Viologen Fluorophore.

New fluorinated alkaline earth metal–organic frameworks were successfully synthesized by milling of metal hydroxides M(OH)2 with tetrafluoroterephthalic acid H2pBDC-F4. Both calcium- and strontium-tetrafluoroterephthalates are tetrahydrated, while the barium tetrafluoroterephthalate is free of coordinating water molecules. The two isomorphic structures Ca(pBDC-F4)·4H2O and Sr(pBDC-F4)·4H2O were solved from the powder diffraction data by ab initio structure determination and subsequent Rietveld refinement. The products were thoroughly characterized by elemental analysis, thermal analysis, magic-angle spinning NMR, Fourier transform infrared spectroscopy, scanning electron microscopy imaging, and Brunauer–Emmett–Teller measurements. Our findings suggest that the mechanochemical synthesis route is a promising approach for the preparation of new fluorinated alkaline earth metal–organic frameworks.

Mechanochemical Synthesis, Characterization, and Structure Determination of New Alkaline Earth Metal-Tetrafluoroterephthalate Frameworks: Ca(pBDC-F4)·4H2O, Sr(pBDC-F4)·4H2O, and Ba(pBDC-F4)

Fifteen new photochromic hybrid materials were synthesized by gas phase loading of fluorinated azobenzenes, namely ortho-tetrafluoroazobenzene (tF-AZB), 4H,4H'-octafluoroazobenzene (oF-AZB), and perfluoroazobenzene (pF-AZB), into the pores of the well-known metal-organic frameworks MOF-5, MIL-53(Al), MIL-53(Ga), MIL-68(Ga), and MIL-68(In). Their composition was analysed by elemental (CHNS) and DSC/TGA. For pF-AZB0.34 @MIL-53(Al), a structural model based on high-resolution synchrotron powder diffraction data was developed and the host-guest and guest-guest interactions were elucidated from this model. These interactions of O-H⋅⋅⋅F and π⋅⋅⋅π type were confirmed by significant shifts of the O-H frequencies in loaded and unloaded MOFs of the MIL-53 and MIL-68 series. Most remarkably, all of the synthesized F-AZB@MOF systems can be switched with visible light, and some of them show almost quantitative (>95 %) photo-isomerization between its E and Z forms with no significant fatigue after repeated switching cycles.

Metal-Organic Frameworks as Hosts for Fluorinated Azobenzenes: A Path towards Quantitative Photoswitching with Visible Light.

Molecules, which reversibly transform between two structural configurations upon excitation with electromagnetic radiation, are attractive candidates for the design of smart materials e. g. memory devices. A possible approach for the development of such smart materials is the construction of hybrid systems that contain these photochromic molecules as well as a porous host matrix, which enables their switching process in the solid state. We herein present the first light-responsive materials consisting of the photoswitchable spirooxazine 1,3,3-trimethyl indolino-naphthospirooxazine (SP-O) and crystalline porous MOF (met-al-organic framework) hosts, namely MOF-5, MIL-68(In), MIL-68 (Ga), and MIL-53(Al), which were combined to form hybrid SP-O@MOF composites. These systems show a reversible photochromic response and host-dependent absorption maxima of the incorporated guest molecules. Most remarkably, SP-O is extremely photostable upon repetitive and prolonged UV light exposure especially inside MOF-5, making these composite materials attractive candidates for potential applications in data storage devices.

https://hal.univ-lorraine.fr/hal-02947411/document

Novel Photoactive Spirooxazine Based Switch@MOF Composite Materials

The incorporation of photochromic dyes into porous metal–organic frameworks (MOFs) is an attractive way to transfer the photochromic properties of the dye to a solid crystalline material. In this work, the well-known P-type chromophore 1,2-bis[2-methylbenzo[b]thiophen-3-yl]-3,3,4,4,5,5-hexafluoro-1-cyclopentene (DTE) is embedded in three different MOFs, namely MOF-5, MIL-68(In), and MIL-68(Ga). The successful filling of the MOF pores with the DTE guest was proven by X-ray powder diffraction, while the amount of the embedded guest molecules was investigated by X-ray photoelectron spectroscopy (XPS), liquid-state NMR and thermal analysis (DSC/TGA). The measurements reveal an unexpectedly low filling of the MOF pores with the DTE guest (e.g. in MOF-5 only every fifth MOF pore is filled with a guest molecule) as well as an inhomogeneous loading throughout the material. Reflection spectra clearly show the transitions of the colourless open-ring and the coloured closed-ring forms of the DTE guest upon UV (λ = 365 nm), blue (λ = 405 nm) and green (λ = 535 nm) light exposure, where the latter is usually suppressed in crystalline DTE. Remarkably, no fatigue after ten switching cycles was observed and a high thermal stability of the coloured closed-ring form (at 50 °C for 1 h) was achieved.

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High fatigue resistance of a photochromic dithienylethene embedded into the pores of a metal-organic framework (MOF).

Two new anhydrous salts of tetrafluoroterephthalic acid (H2tF-BDC), namely K2tF-BDC (1) and Rb2tF-BDC (2), were synthesized and structurally characterized. The crystal structure of 1 (I2/m, Z = 2) was solved and refined from X-ray single crystal data. K+ (CN = 8) is coordinated by six oxygen atoms of five different tF-BDC2– linkers and two fluorine atoms of two of these linkers, which connect the KO6F2 polyhedra to a 3D network. The crystal structure of 2 (P, Z = 1) was refined from X-ray powder diffraction data. It crystallizes in a structure type already known from Tl2tF-BDC. Rb+ (CN = 8) is also coordinated by six oxygen and two fluorine atoms. But six tF-BDC2– linkers are involved in the coordination. Again, a 3D network is formed. Both compounds can be obtained as single-phase samples. They are not porous and stable in air. The thermal stability was investigated by DTA/TG measurements. The decomposition starts at approx. 200 °C (1) and 230 °C (2). Their good solubility in water makes them interesting starting materials for the synthesis of new MOFs with the perfluorinated linker tF-BDC2–.

Two Anhydrous Salts of Tetrafluoroterephthalic Acid (H2tF-­BDC): K2tF-BDC and Rb2tF-BDC †‡

By reaction of alkali metal acetylides, AI C2 H (AI =K, Rb, Cs), with elemental selenium in liquid ammonia highly crystalline powders of AI SeC2 H were obtained. The structure analysis based on the resulting synchrotron powder diffraction data revealed that all compounds crystallize in an orthorhombic unit cell (Cmc21 , Z=4) exhibiting the elusive - SeC2 H anion, which is unprecedented in a crystalline compound up to now. Elemental analysis and IR spectroscopic data confirm this finding. Upon heating, AI SeC2 H compounds release acetylene based on DSC/TGA experiments resulting in powders with the proposed composition AI2 Se2 (C2 ). The resulting powders were indexed with small cubic unit cells, but a reasonable structural model could not be developed up to now. Upon exposure of AI SeC2 H compounds to water elemental selenium is formed again.

Melanie Werker

Papers

Metal-Organic Frameworks as Hosts for Fluorinated Azobenzenes: A Path towards Quantitative Photoswitching with Visible Light.

Novel Photoactive Spirooxazine Based Switch@MOF Composite Materials

High fatigue resistance of a photochromic dithienylethene embedded into the pores of a metal-organic framework (MOF).

Two Anhydrous Salts of Tetrafluoroterephthalic Acid (H2tF-BDC): K2tF-BDC and Rb2tF-BDC †‡

AISeC2H (AI=K, Rb, Cs): Crystalline Compounds with the Elusive −Se‐C≡C‐H Anion

Melanie Werker

Papers

Metal-Organic Frameworks as Hosts for Fluorinated Azobenzenes: A Path towards Quantitative Photoswitching with Visible Light.

Novel Photoactive Spirooxazine Based Switch@MOF Composite Materials

High fatigue resistance of a photochromic dithienylethene embedded into the pores of a metal-organic framework (MOF).

Two Anhydrous Salts of Tetrafluoroterephthalic Acid (H2tF-­BDC): K2tF-BDC and Rb2tF-BDC †‡

AISeC2H (AI=K, Rb, Cs): Crystalline Compounds with the Elusive −Se‐C≡C‐H Anion

Two Anhydrous Salts of Tetrafluoroterephthalic Acid (H2tF-BDC): K2tF-BDC and Rb2tF-BDC †‡