Ian M. Walton

The role of atropisomers on the photo-reactivity and fatigue of diarylethene-based metal–organic frameworks

Photo-responsive metal–organic frameworks (MOFs) are one example of light controlled smart materials for use in advanced sensors, data storage, actuators and molecular switches. Herein we show the design, synthesis and characterization of a photo-responsive linker that is subsequently reacted to yield MOF single crystals. The photo-responsive properties of the resulting UBMOF-2 arise from the photo-induced cyclization of the diarylethene moiety designed into the linker. Computational modeling to assess the relative energies of linker atropisomers reveals a large energetic barrier preventing facile interconversion between key species. The role of this barrier on the observed photo-induced fatigue provides useful insight into the development of advanced photo-responsive nanoporous materials.

A versatile environmental control cell for in situ guest exchange single-crystal diffraction

In situ single-crystal diffraction experiments provide researchers with the opportunity to study the response of crystalline systems, including metal–organic frameworks and other nanoporous materials, to changing local microenvironments. This paper reports a new environmental control cell that is remarkably easy to use, completely reusable, and capable of delivering static or dynamic vacuum, liquids or gases to a single-crystal sample. Furthermore the device is nearly identical in size to standard single-crystal mounts so a full unrestricted range of motion is expected for most commercial goniometers. In situ single-crystal X-ray diffraction experiments performed under dynamic gas-flow conditions revealed the cell was capable of stabilizing a novel metastable intermediate in the dehydration reaction of a previously reported metal–organic framework.

Polymorphism and the influence of crystal structure on the luminescence of the opto-electronic material 4,4′-bis(9-carbazolyl)biphenyl

Three polymorphs of 4,4′-bis(9-carbazolyl)biphenyl were prepared and characterized by X-ray diffraction and luminescence spectroscopy. Electronic structure calculations were performed to examine the influence of the molecular geometry on the HOMO and LUMO energy levels and calculated electronic transitions.

On the design of atropisomer-separable photochromic diarylethene-based metal–organic framework linkers

The relative stability and accessibility of atropisomers plays a prominent role in the efficacy of diarylethene-based photochromic materials. Herein, DFT methods, using the ωB97XD functional and a 6-31G(d) basis, are employed to determine the local energetic minima and maxima which describe the rotation of a thiophene group in derivatives of 9,10-bis(2-methyl-5-pheylthiophen-3-yl)phenanthrene-2,7-dicarboxylic acid, a photochromic linker molecule which exhibits atropisomer-formation-related fatigue when incorporated into a metal–organic framework. Results of these potential energy surface mapping calculations as well as their applications to atropisomer separability are discussed.

Structural response to desolvation in a pyridyl-phenanthrene diarylethene-based metal–organic framework

A phenanthrene-based diarylethene linker with linear pyridyl connectivity, 4,4′-(9,10-bis(2,5-dimethylthiophen-3-yl)phenanthrene-2,7-diyl)dipyridine linker (TPDPy) was prepared and subsequently used to synthesize an air-stable metal–organic framework, UBMOF-3 (Zn3(BDC)3(TPDPy)1(DMF)1.5, BDC = 1,4-benzenedicarboxylate, DMF = N,N-dimethylformamide). Upon irradiation with ultraviolet light, this photo-responsive framework, composed of terephthalate, TPDPy, and zinc pinwheels, exhibits strong linear dichroism consistent with the crystal structure. Activation (desolvation) of the crystal leads to a significant change in the crystal structure that improves the ability to crystallographically resolve the photochromic linker.

Photoactive and Physical Properties of an Azobenzene-Containing Coordination Framework*

A new three-dimensional coordination framework, [Zn4(tbazip)3(bpe)2(OH)2]·bpe·{solvent} (where bpe = 1,2-di(4-pyridyl)ethene) containing the novel photoactive ligand tbazip (tbazip = 5-((4-tert-butyl)phenylazo)isophthalic acid) has been synthesised and crystallographically characterised. The photoactivity of discrete tbazip was investigated and compared with its photoactivity while incorporated within the framework. The effect of isomerisation of the incorporated azobenzene on the chemical and physical properties of the framework were investigated using UV-vis and Raman spectroscopies. The framework is porous only to hydrogen gas at 77 K, but displayed an appreciable uptake for CO2 at 195 K.

Solvent exchange in a metal–organic framework single crystal monitored by dynamic in situ X-ray diffraction

Understanding the processes by which porous solid-state materials adsorb and release guest molecules would represent a significant step towards developing rational design principles for functional porous materials. To elucidate the process of liquid exchange in these materials, dynamic in situ X-ray diffraction techniques have been developed which utilize liquid-phase chemical stimuli. Using these time-resolved diffraction techniques, the ethanol solvation process in a flexible metal-organic framework [Co(AIP)(bpy)0.5(H2O)]·2H2O was examined. The measurements provide important insight into the nature of the chemical transformation in this system including the presence of a previously unreported neat ethanol solvate structure.

Photochromism in a New Light: Temperature-Dependent in situ Photocrystallography

Organic photochromic molecules including diarylethenes are of particular interest for their potential applications in fields of high density optical data storage and light-activated switches, among many others. However, one of the limitations in diarylethene-based systems has been the low photoconversion observed in neat single crystals which is often less than 20%. The low conversion is typically believed to be the result of screening effects in which the photoisomerized molecules at the surface absorb incident light preventing full isomerization of the crystal. To assess the effect of screening on a model diarylethene system, photocrystallographic experiments on microcrystals of the compound were performed using synchrotron radiation at the Advanced Photon Source at Argonne National Labs. During the course of the study, we discovered that the photoconversion of the diarylethene crystals exhibits highly unusual temperature dependent behavior which is incongruent with current computational models of diarylethene photochemistry. Herein we report the first temperature-dependent ‘constant irradiation’ in situ photocrystallography experiments performed on a photochromic system. Through the application of this technique, combined with spectroscopic analysis, we demonstrate that the steady-state population arising from the photo-cyclization reaction shows a temperature dependence which has been heretofore unobserved. Possible explanations for this anomalous behavior and its role in the photochemical reactivity of this and other diarylethene systems will be presented.

Recent Progress in the Synthesis and Characterization of Diarylethene-based MOFs

Metal-organic Frameworks (MOFs) remain an extremely active area of research given the wide variety of potential applications and the enormous diversity of structures that can be created from their constituent building blocks. While MOFs are typically employed as passive materials, next-generation materials will exhibit structural and/or electronic changes in response to applied external stimuli including light, charge, and pH. Herein we present recent results in which advanced photochromic diarylethenes are combined with MOFs through covalent and non-covalent methods to create photo-responsive permanently porous crystalline materials. This presentation will describe the design, synthesis, and characterization of next-generation photo-switchable diarylethene based ligands which are subsequently used to photo-responsive MOFs. These UBMOF crystals are, by design, isostructural with previously reported non-photoresponsive frameworks which enables a systematic comparison of their physical and chemical properties. While the photoswitching of the isolated ligand in solution is fully reversible, the cycloreversion reaction is suppressed in the UBMOF single crystalline phase. Spectroscopic evidence for thermally induced cycloreversion will be presented, as well as a detailed analysis addressing the limits of X-ray diffraction techniques applied to these systems.