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.

Hydrogenation of hexagonal Ce2Ni7 was investigated by synchrotron X-ray and neutron powder diffraction. In contrast to the recently investigated lanthanum analogue, which remains hexagonal (La2Ni7D6.5: space group P63/mmc), the cerium compound becomes orthorhombic (Ce2Ni7D approximately 4: space group Pmcn). As in the structurally related CeNi3D2.8, deuterium occupies CeNi2 slabs only, while the bulk of the CeNi5 slabs remains empty. A significant amount of deuterium is bonded in tetrahedral NiD4 units similar to those in nickel-based complex metal hydrides. These findings provide further evidence for directional bonding effects in hydrides that are traditionally considered as "interstitial". Ce2Ni7D approximately 4 displays various orthorhombic lattice distortions, delta = (b/ radical3 - a)/a. Hydrogen pressures of approximately 30 bar stabilize a phase having a negative distortion (delta < 0). Upon a decrease in the pressure, this phase transforms via a two-phase region into another phase having a positive distortion (delta > 0). Both phases are nearly isostructural and have the same space group symmetry and nearly the same composition. This situation is typical for a so-called anti-isostructural phase transition in which delta is considered to be an order parameter. Neither magnetic nor structural transitions have been detected down to 1.5 K.

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