Understanding isomer distribution of tF-AZB inside DMOF-1 as a function of host–guest and guest–guest interaction.

The performance of different density functional tight binding (DFTB) methods for the description of six increasingly complex metal−organic framework (MOF) compounds have been assessed. In particular the self-consistent charge density functional tight binding (SCC DFTB) approach utilizing the 3ob and matsci parameter sets have been considered for a set of four Zn-based and two Al-based MOF systems. Moreover, the extended tight binding for geometries, frequencies, and noncovalent interactions (GFN2-xTB) approach has been considered as well. In addition to the application of energy minimizations of the respective unit cells, molecular dynamics (MD) simulations at constant temperature and pressure conditions (298.15 K, 1.013 bar) have been carried out to assess the performance of the different DFTB methods at nonzero thermal conditions. In order to obtain the XRD patterns from the MD simulations, a flexible workflow to obtain time-averaged XRD patterns from (in this study 5000) individual snapshots taken at regular intervals over the simulation trajectory has been applied. In addition, the comparison of pair-distribution functions (PDFs) directly accessible from the simulation data shows very good agreement with experimental reference data obtained via measurements employing synchrotron radiation in case of MOF-5. The comparison of the lattice constants and the associated X-ray diffraction (XRD) patterns with the experimental reference data demonstrate, that the SCC DFTB approach provides a highly efficient and accurate description of the target systems.

Controlling the switching efficiency of photoactive hybrid systems is an obligatory key prerequisite for systematically improving the design of functional materials. By modulating the degree of fluorination and the amount being embedded into porous hosts, the E / Z ratios of fluorinated azobenzenes were adjusted as both functions of substitution and the degree of loading. Octafluoroazobenzene (F8-AZB) and perfluoroazobenzene (F10-AZB) were inserted into porous DMOF-1. Especially for perfluoroazobenzene (F10-AZB), an immense stabilization of the E isomer was observed. In complementary molecular dynamics simulations performed at the DFTB (density functional tight binding) level, an in-depth characterization of the interactions of the different photoisomers and the host structure was carried out. On the basis of the resulting structural and energetic data, the experimentally observed increase in the amount of the Z conformer for F8-AZB can be explained, while the stabilization of E -F10-AZB can be directly related to a fundamentally different interaction motif compared to its tetra- and octafluorinated counterparts.

Three novel hybrid systems containing thioindigo (TI) as a guest molecule and MIL-68(In), , and MOF-5 as host matrices were synthesized and studied with respect to their optical properties. Surprisingly, TI@metal−organic framework (MOF) systems do not exhibit photochromic response as a result of strong host−guest interactions, in which the E isomer is trapped in a parallel interaction motif between TI and the phenyl rings of the linker molecules. This binding site is assumed to be the origin of the occurring bright red solid-state fluorescence of TI inside the different MOF hosts. It was found that the corresponding decay times are significantly longer for TI@MOF than for pure TI. The emission wavelength of pure solid TI is also longer compared to its TI@MOF counterpart. These findings demonstrate that insertion into the MOF prevents aggregation between the TI molecules, which is beneficial to enhance the photoluminescence quantum yield. In addition to the experimental measurements, a series of density functional tight binding molecular dynamics (DFTB MD) simulations of E-and Z-thioindigo embedded in MOF-5 and MIL-68(Ga) have been performed, providing detailed insight into the host−guest interaction at the molecular level.

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.