Understanding the interactions between the single components of hybrid systems is essential to drive the development of advanced functional materials. A prerequisite for this is the systematic variation of the building blocks of such compounds. Focusing on spiropyran@metal–organic framework (MOF) composite materials with noncovalently attached spiropyran dyes, both the host scaffold and the dye molecules can be systematically tuned. In this work, a broad substitution pattern was applied to systematically elucidate the characteristics of the resulting hybrid materials as a function of the supplemental substitution on spiropyran. The newly developed 12 composites exhibit substitution and host-dependent optical characteristics, which are particularly affected by the substitution of the 6′-position on the chromene ring. Through the favorable combination of the MOF host’s polarity and an adequate strength of the spiropyran’s indolinedonor–chromeneacceptor pair, reversible conversion between photoisomers is efficiently accomplished, especially for nitro-substituted spiropyrans inside MIL-68­(In).

In functional materials, the understanding of the interactions between the individual components is essential for further development. One example for such materials are guest@host systems, with a metal−organic framework (MOF) as the host matrix. These porous compounds consist of metal nodes and organic linker molecules and are therefore inorganic−organic hybrid materials. Combining MOFs with photoswitchable dyes such as spiropyrans (SPs) resulted in new functional materials with fascinating properties. To obtain such materials with technologically relevant properties, knowledge of the individual adjusting screws to modulate, for example, the optical response is obligatory to further push material's design. Within this work, we systematically studied the optical properties of a nitro-substituted SP inside an MIL-68(In) series as a function of linker substitution. In particular, 1,3,3-trimethylindolino-6′-nitrobenzopyrylospiran ( SP-Nitro) was non-covalently attached into the pores of MIL-68(In) with the terephthalate linker molecule being functionalized either with a Br-, NH 2 -, or a NO 2group. Three different hybrid systems were obtained, exhibiting both photochromic and solvatochromic response. The observed optical characteristics were found to strongly depend on the substitution pattern of the linker molecule. Based on the resulting structural and dielectric properties of the substituted terephthalic acid molecules calculated at B3LYP and MP2 levels, the observed UV/vis reflectance spectra were directly correlated with the out-of-plane rotation of the carboxyl group in the vicinity of the respective substituent.

Ice XIX represents the latest discovery of ice polymorphs and exists in the medium pressure range near 1–2 GPa. Ice XIX is a partially hydrogen-ordered phase, by contrast to its disordered mother phase ice VI, which shares the same oxygen-atom network with ice XIX. Ice XIX differs in terms of the ordering of the hydrogen-atom sublattice, and hence the space group, from its hydrogen-ordered sibling ice XV, which also features the same type of oxygen network. Together, ice VI, XV, and XIX form the only known trio of ice polymorphs, where polymorphic transformations from order to order, order to disorder, and disorder to order are possible, which also compete with each other depending on the thermodynamic path taken and the cooling/heating rates employed. These transitions in the H-sublattice have barely been investigated, so we study here the unique triangular relation in the ice VI/XV/XIX trio based on calorimetry experiments. We reveal the following key features for H-sublattice transitions: (i) upon cooling ice VI, domains of ice XV and XIX develop simultaneously, where pure ice XV forms at ≤0.85 GPa and pure ice XIX forms at ≥1.60 GPa, (ii) ice XIX transforms into ice XV via a transient disordered state, (iii) ice XV recooled at ambient pressure features a complex domain structure, possibly containing an unknown H-ordered polymorph, (iv) recooled ice XV partly transforms back into ice XIX at 1.80 GPa, and (v) partial deuteration slows down domain reordering strongly. These findings not only are of interest in understanding possible hydrogen-ordering and -disordering processes in the interior of icy moons and planets but, more importantly, also provide a challenging benchmark for our understanding and parameterizing many-body interactions in H-bonded networks.

Exploring synthesis methods fosters new ways to design materials in faster and cheaper ways. In this fundamental study, we present mechanochemical loading of photoactive dye molecules into porous metal-organic frameworks (= MOFs) as an alternative and promising strategy to form hybrid switch@-MOF systems. Six different spiropyrans were inserted into the two MOFs MOF-5 and MIL-68(In) in varying compositions. By this, the concentration-dependent enclosure characteristics were determined. This fast and simple synthesis strategy has never been reported so far, and thus provides completely new possibilities for the formation of two-or even multi-component systems.