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

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.

Cs2Cd(C2H)2(C2): A Crystalline Acetylide with Bridging C2 Units.

Cs2Cd(C2H)2(C2) was synthesized by heating known Cs2Cd(C2H)4 either in a dry argon atmosphere at 200 °C or under ammonothermal conditions (130 °C, ∼ 100 bar). The crystal structure of the resulting dark orange-brown microcrystalline material was solved and refined from synchrotron powder diffraction data (Cmcm, Z = 4). Cs2Cd(C2H)2(C2) is composed of Cd2+ cations tetrahedrally coordinated end-on by four acetylide groups. Two of them are terminating C2H- groups, whereas the other two positions are occupied by bridging C22- anions. Thus, a polymeric ∞1[Cd(C2H)2(C2)2/22-] chain-like anion results and these chains are separated by Cs+ cations. So obviously Cs2Cd(C2H)2(C2) is formed from Cs2Cd(C2H)4 by a condensation reaction of two of its four C2H- groups under the release of one acetylene (C2H2) molecule. This reaction mechanism is supported by DSC/TGA measurements, and the crystal structure of Cs2Cd(C2H)2(C2) is further supported by IR spectroscopic investigations.

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

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.

AI SeC2 H (AI =K, Rb, Cs): Crystalline Compounds with the Elusive - Se-C≡C-H Anion.

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 AI 2 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.