Smart nanoporous metal-organic frameworks by embedding photochromic molecules - state of the art and future perspectives.

Smart, molecularly structured materials with remote-controllable properties and functionalities attract particular attention and may enable advanced applications. In this respect, the embedment of stimuli-responsive molecules, such as azobenzenes, spiropyrans or diarylethenes, in metal-organic frameworks (MOFs) is a very fascinating approach, resulting in easily accessible photoswitchable, nanoporous hybrid materials. It is an attractive alternative to the incorporation of the smart moieties in the MOF scaffold, which usually demands complex synthetic efforts. Here, the opportunities, properties and perspectives of the embedment of photochromic molecules in MOF pores are reviewed. In addition to presenting a straightforward route to prepare smart materials with, e.g., photoswitchable adsorption properties that can be used for remote-controllable membrane separation, the photoswitch@MOF compounds also represent unique model systems to investigate the dye as well as the MOF properties and their interactions with each other. For instance, the MOF pores possess a polarity similar to a solvent, so that the optical properties of the resulting materials may be influenced by a careful choice of the respective host material.

Metal-organic frameworks as hosts for photochromic guest molecules.

Several metal-organic framework compounds (MOF-5, MIL-68(Ga), MIL-68(In), MIL-53(Al)) were loaded with azobenzene (AZB), as confirmed by XRPD measurements and elemental analysis. By IR spectroscopy, it was shown that the light-induced trans/cis isomerization of AZB in these hybrid host-guest compounds is improved compared to that of solid AZB. A population of the excited cis state up to 30% has been obtained for AZB0.66@MIL-68(In). However, no light-induced trans/cis isomerization was observed for AZB0.5@MIL-53(Al). Structural models obtained from high-resolution synchrotron powder diffraction data show that AZB molecules are densely packed within the channels of MIL-53(Al) so that no trans/cis isomerization can occur. A different situation was observed for AZB in the larger channels of MIL-68(Ga). Thus, this investigation shows the influence of the host material on the switching behavior of the embedded AZB molecules.

Ultra-slow Li ion dynamics in Li(2)C(2)--on the similarities of results from (7)Li spin-alignment echo NMR and impedance spectroscopy.

Li diffusion and transport parameters of binary lithium carbide Li(2)C(2) were complementarily investigated by (7)Li (nuclear magnetic resonance) NMR and impedance spectroscopy. Long-range Li diffusion parameters were measured by using mixing-time-dependent and temperature-variable stimulated echo NMR spectroscopy. The method is sensitive to ultra-slow Li hopping processes which were probed from an atomic-scale point of view. Two-time phase correlation functions S(2) obtained can be parameterized by stretched exponentials only. The corresponding echo decay rates τ(-1), which were recorded at a resonance frequency of e.g. 155.5 MHz, show Arrhenius behaviour revealing an activation energy of 0.80(2) eV. This value is in very good agreement with that deduced from dc conductivity measurements (0.79(2) eV) probing Li transport processes on a macroscopic length scale. The comparison of impedance data with the measured NMR echo decay functions showed that both methods reflect diffusion processes being characterized by very similar motional correlation functions.

Ternary alkali metal transition metal acetylides A2MC2 (A = Na, K; M = Pd, Pt).

Ternary transition metal acetylides A2MC2 (A = Na, K; M = Pd, Pt) can be synthesised by reaction of the respective alkali metal acetylide A2C2 with palladium or platinum in an inert atmosphere at about 350 degrees C. The crystal structures are characterised by (infinity)1[M(C2)(2/2)2-] chains, which are separated by the alkali metals (P3m1, Z = 1). The refinement of neutron powder diffraction data gave C-C = 1.263(3) A for Na2PdC2 (Na2PtC2: 1.289(4) A), which is distinctively longer than the expected value for a C-C triple bond (1.20 A). On the basis of band-structure calculations this can be attributed to a strong back-bonding from the metal into the anti-bonding orbitals of the C2 unit. This was further confirmed by Raman spectroscopic investigations, which showed that the wavenumbers of the C-C stretching vibrations in Na2PdC2 and Na2PtC2 are about 100 cm(-1) smaller than in acetylene. 13C MAS-NMR spectra demonstrated that the acetylenic C2 units in the title compounds are very different from those in acetylene. Electrical conductivity measurements and band-structure calculations showed that the black title compounds are semiconductors with a small indirect band gap (approximately 0.2 eV).

Structural phase transitions in CaC2.

Pure CaC2, free of CaO impurities, was obtained by the reaction of elemental calcium with graphite at 1,070 K. By means of laboratory X-ray and synchrotron powder diffraction experiments, the phase diagram was investigated in the temperature range from 10 K to 823 K; this confirmed the literature data that reported the partial coexistence of up to four modifications. Aside from a cubic high-temperature modification CaC2 IV (Fm3m, Z = 4) and the well-known tetragonal modification CaC2 I (I4/mmm, Z = 2), a low-temperature modification CaC2 II (C2/c, Z =4) that crystallizes in the ThC2 structure type and a metastable modification CaC2 III (C2/m, Z = 4) that crystallizes in a new structure type were found. It was shown that phase transition temperatures as well as the relative amounts of the various CaC2 modifications depend upon the size of the crystallites, the thermal treatment. and the purity of the sample, as a comparison with technical CaC2 confirmed.

Novel Ternary Alkali Metal Silver Acetylides M(I)AgC(2) (M(I)=Li, Na, K, Rb, Cs).

Three different packings of (1)(infinity)[Ag(C(2))(2/2)(-)] chains (represented by rods in the picture) have been found in the crystal structures of the first ternary alkali metal silver acetylides, which were obtained by the reaction of M(I)C(2)H (M(I)=Li-Cs) with AgI in liquid ammonia and subsequent heating of the remaining residue to 120 degrees C.