Polarity profiling of porous architectures: solvatochromic dye encapsulation in metal-organic frameworks.

Metal-organic frameworks (MOFs) have gathered significant interest due to their tunable porosity leading to diverse potential applications. In this study, we investigate the incorporation of the fluorosolvatochromic dye 2-butyl-5,6-dimethoxyisoindoline-1,3-dione ([double bond, length as m-dash]Phth) into various MOF structures as a means to assess the polarity of these porous materials. As a purely inorganic compound, zeolite Y was tested for comparison. The fluorosolvatochromic behavior of Phth, which manifests as changes in its emission spectra in response to solvent polarity, provides a sensitive probe for characterizing the local environment within the MOF pores. Through systematic variation of the MOF frameworks, we demonstrate the feasibility of using (fluoro-)solvatochromic dyes as probes for assessing the polarity gradients within MOF structures. Additionally, the fluorosolvatochromic response was studied as a function of loading amount. Our findings not only offer insights into the interplay between MOF architecture and guest molecule interactions but also present a promising approach for the rational design and classification of porous materials based on their polarity properties.

Structural Properties of Metal-Organic Frameworks at Elevated Thermal Conditions via a Combined Density Functional Tight Binding Molecular Dynamics (DFTB MD) Approach.

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.

First ternary tungsten tellurate(IV) WTe2O7 with unique crystal structure type.

At multianvil high-pressure/high-temperature conditions of 10 GPa and 1273 K, the first ternary tungsten tellurate WTe2O7 is formed, starting from a stoichiometric mixture of WO3 and TeO2. The compound crystallizes triclinic in a hitherto unknown crystal structure type with the space group P1̄; (no. 2), and was refined from single-crystal X-ray diffractometer data: a = 538.3(1), b = 687.5(1), c = 802.3(1) pm, α = 72.4(1)°, ß = 85.7(1)°, γ = 68.1(1)°, wR2 = 0.0323, GooF = 1.048, 3157 F2 values, and 106 variables. The main motifs of the crystal structure are pairs of edge-linked [WO6]6- octahedra and fourfold oxygen-coordinated Te4+ atoms. The oxidation state of W6+ and Te4+ was further verified by measuring the characteristic binding energy values for the W 4f and the Te 3d core levels via X-ray photoelectron spectroscopy (XPS). In addition, DFT calculations of the structure, the associated electron localisation functions (ELF) and vibrational spectra have been carried out. The theoretical data clearly demonstrates the impact of the residual electron density located at the Te4+ ions, which can be directly interpreted as the presence of lone electron pairs within the solid structure.

Fundamental Study of the Optical and Vibrational Properties of Fx-AZB@MOF systems as Functions of Dye Substitution and the Loading Amount.

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.

Cationic Cycloheptatrienyl Cyclopentadienyl Manganese Sandwich Complexes: Tromancenium Explored with High-Power LED Photosynthesis.

In this contribution, we revisit the neglected and forgotten cationic, air-stable, 18-valence electron, heteroleptic sandwich complex (cycloheptatrienyl)(cyclopentadienyl)manganese, which was reported independently by Fischer and by Pauson about 50 years ago. Using advanced high-power LED photochemical synthesis, an expedient rapid access to the parent complex and to functionalized derivatives with alkyl, carboxymethyl, bromo, and amino substituents was developed. A thorough study of these "tromancenium" salts by a range of spectroscopic techniques (1H/13C/55Mn-NMR, IR, UV-vis, HRMS, XRD, XPS, EPR), cyclic voltammetry (CV), and quantum chemical calculations (DFT) shows that these manganese sandwich complexes are unique metallocenes with quite different chemical and physical properties in comparison to those of isoelectronic cobaltocenium salts or (cycloheptatrienyl)(cyclopentadienyl) sandwich complexes of the early transition metals. Electrochemically, all tromancenium ions undergo a chemically partially reversible oxidation and a chemically irreversible reduction at half-wave or peak potentials that respond to the substituents at the Cp deck. As exemplarily shown for the parent tromancenium ion, the product generated during the irreversible reduction process reverts at least partially to the starting material upon reoxidation. Quantum-chemical calculations of the parent tromancenium salt indicate that metal-ligand bonding is distinctly weaker for the cycloheptatrienyl ligand in comparison to that of the cyclopentadienyl ligand. Both the HOMO and the LUMO are metal and cycloheptatrienyl-ligand centered, indicating that chemical reactions will occur either metal-based or at the seven-membered ring, but not on the cyclopentadienyl ligand.

Modulating the Optical Characteristics of Spiropyran@Metal-Organic Framework Composites as a Function of Spiropyran Substitution.

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).

Band-Gap Tuning in All-Inorganic CsPbxSn1-xBr3 Perovskites.

We investigate all-inorganic perovskite CsPbxSn1-xBr3 thin films to determine the variations in the band gap and electronic structure associated with the Pb/Sn ratio. We observe that the band gap can be tuned between 1.86 eV (x = 0) and 2.37 eV (x = 1). Intriguingly, this change is nonlinear in x, with a bowing parameter of 0.9 eV; furthermore, a slight band gap narrowing is found for low Pb content (minimum x ∼ 0.3). The wide tunability of the band gap makes CsPbxSn1-xBr3 a promising material, e.g., for a wide-gap subcell in tandem applications or for color-tunable light-emitting diodes. Employing photoelectron spectroscopy, we show that the valence band varies with the Pb/Sn ratio, while the conduction band is barely affected.

Structural, dynamical, and photochemical properties of ortho-tetrafluoroazobenzene inside a flexible MOF under visible light irradiation.

Considering porous materials as host matrices is an elegant way to enable photoswitching of non-covalently attached organic dyes even in the solid state. By focusing on the resulting optical properties as a function of loading degree and synthesis procedure, the occurring host-guest and guest-guest interactions can be determined and further exploited. In the course of this study, the photochromic behavior of ortho-tetrafluoroazobenzene (tF-AZB) inside flexible DMOF-1 was investigated from these points of view. It was found that depending on the loading degree and temperature, tF-AZB shows varying E/Z ratios and switching efficiency. For systems with low loading, reversible visible light induced isomerization was observed over ten switching cycles: Upon violet light exposure, formation of 100% E isomer was generated, while green light irradiation resulted in ∼60% Z-tF-AZB. Complementary molecular dynamics simulations at DFTB (density functional tight binding)-level revealed changing binding sites for Z-tF-AZB inside DMOF-1. For the E isomer, only low oscillations have been found, which in turn display a rare T-stacking interaction. Although the interaction strengths of the E and Z isomers with DMOF-1 are in the same range, the different mobility of both isomers due to varying binding sites explains the preference of the E isomer even upon green light exposure.

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.

Tracking the light-induced isomerization processes and the photostability of spiropyrans embedded in the pores of crystalline nanoporous MOFs via IR spectroscopy.

The embedment of photochromic dyes into porous host matrices has attracted increasing interest in recent years. Especially the class of spiropyrans has been considered because of its outstanding photochromic and solvatochromic response. We herein present a comprehensive infrared spectroscopic characterization of the photoresponse and photostability of a nitro-substituted spiropyran "SP-Nitro" (namely 1,3,3-trimethylindolino-6'-nitrobenzopyrylospiran) non-covalently attached to different crystalline nanoporous MOF (metal-organic framework) host lattices. The TTC mesomeric form of SP-Nitro has been found to be preferably generated upon UV light exposure inside the different MOF hosts. Additionally, the excited isomer was found to be stable for prolonged irradiation times of 1-1.5 h.

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.

Solution-Like Behavior of Photoswitchable Spiropyrans Embedded in Metal-Organic Frameworks.

1,3,3-Trimethylindolino-6'-nitrobenzopyrylospiran (SP-1) as an example of a photoswitchable spiropyran was loaded into the pores of different prototypical metal-organic frameworks, namely MOF-5, MIL-68(In), and MIL-68(Ga), by a vapor-phase process. The successful incorporation in the pores of the MOF was proven by X-ray powder diffraction, and the amount of the embedded photoswitchable guest was determined by X-ray photoelectron spectroscopy and elemental analysis. In contrast to the sterically hindered crystalline state, SP-1 embedded in solid MOF hosts shows photoswitching under irradiation with UV light from the spiropyran to its merocyanine form with a nearly complete photoisomerization. Switching can be reversed by heat treatment. These switching properties were confirmed by means of UV/vis and IR spectroscopy. Remarkably, the embedded guest molecules show photoswitching and absorption properties similar to those in the dissolved state, so that MOFs might be considered as "solid solvents" for photoswitchable spiropyrans. In contrast to that, embedment of SP-1 in the smaller pores of MIL-53(Al) was not successful. SP-1 is mainly adsorbed on the surfaces of the MIL-53(Al) particles, which also leads to photoswitching properties.