Category: Papers

Papers

Squeezing conductivity from a molecular crystal

Most molecular materials don’t conduct electricity well because the electrons in them can’t travel around easily. Materials that do conduct have pathways for their electrons, formed by overlapping atomic orbitals–the space where electrons usually stay–between molecules. In order to increase the conductivity of a molecular crystal, Nickel 5,6-dihydro-1,4-dithiin-2,3-dithiolate, we used high pressure to squeeze the molecules together.

This work was an international collaboration with some really talented scientists in the RIKEN institute, Japan, Kumamoto University, Japan, and Diamond Light Source synchrotron, UK.

Hengbo and Reizo at RIKEN made the molecular crystals and measured their conductivity under high pressure. They did this by attaching tiny wires to the crystal inside a Diamond Anvil Cell (DAC)–a device that generates pressure larger than found at the bottom of the ocean. They found that, whilst normally the crystal didn’t conduct electricity at all, when it was squeezed its conductivity increased dramatically.

In order to understand why, we used X-ray diffraction to work out the crystal structure–where the atoms are and how they are bonded together. Aided by Chloe and Mark, we fired intense X-ray beams produced at Diamond Light Source through our crystal in the DAC and measure how they scattered. From the scattering, we could show for the first time that the molecules got closer and closer together under pressure.

The final piece in the puzzle came from Takao at Kumamoto, who used theoretical calculations based on the X-ray crystal structures to work out the changes in orbital interactions. And the calculations showed that the orbitals between the molecules indeed overlapped more and more with pressure, explaining why the crystals’ conductivity got higher.

A link to the paper can be found here. It’s Open Access, meaning anyone can read it for free!

Thanks and well done to Hengbo, Takao, Chloe, Reizo and Mark!

Papers

Pre-equilibrium species in MOF crystallization

We’re very pleased to announce our paper on the crystallization of ZIF-8 has just been accepted! It’s been a challenging piece of work, not least because it all began when we made the surprising observation that crystallization got SLOWER when we increased the concentration of our reactants…

There is an increasingly large amount of interest in metal-organic frameworks (MOFs) for a variety of applications, from gas sensing and separations to electronics and catalysis. Their exciting properties arise from their modular architectures, which self-assemble from different combinations of metal-based and organic building units. However, the exact mechanisms by which they crystallize remain poorly understood, thus limiting any realisation of real “structure by design”. We report important new insight into MOF formation, gained using in situ X-ray diffraction, pH and turbidity measurements to uncover for the first time the evolution of metastable intermediate species in the canonical zeolitic imidazolate framework system, ZIF-8. We reveal that the intermediate species exist in a dynamic pre-equilibrium prior to network assembly and, depending on the reactant concentrations and the progress of reaction, the pre-equilibrium can be made to favour under- or over-coordinated species, thus accelerating or inhibiting crystallization, respectively. We thereby find that concentration can be effectively used as a synthetic handle to control particle size, with great implications for industrial scale-up and gas sorption applications. This finding enables us to rationalise the apparent contradictions between previous studies and, importantly, opens up new opportunities for the control of crystallization of network solids more generally, from the design of local structure to assembly of particles with precise dimensions.

The paper is published with Angewandte Chemie, International Edition and can be found here. A previous version can also be downloaded for free on ChemRxiv.

Many thanks to all co-authors, Diamond for beamtime, SCG Innovation for funding and everyone else that helped out along the way!

Papers

Compositional inhomogeneity in mixed-metal ZIF-8 analogues

Adam’s work on mixed-metal ZIF-8 analogues is out in Chemical Communications!

We study the structural and thermomechanical effects of cation substitution in the compositional family of metal–organic frameworks Zn1−xCdx(mIm)2 (HmIm = 2-methylimidazole). We find complete miscibility for all compositions x, with evidence of inhomogeneous distributions of Cd and Zn that in turn affect framework aperture characteristics. Using variable-temperature X-ray powder diffraction measurements, we show that Cd substitution drives a threefold reduction in the magnitude of thermal expansion behaviour. We interpret this effect in terms of an increased density of negative thermal expansion modes in the more flexible Cd-rich frameworks.

The paper can be found here and the preprint is available for free on ChemRxiv.

Congratulations Adam!

Papers

Thermodynamics and Kinetics of MOF formation

We’re excited to announce that we’ve got a new paper out– what a start to 2018!

It’s a review of research over the last 20 years on the fundamental factors affecting why certain metal-organic frameworks (MOFs) form the structures that they do. It’s entitled “Thermodynamic and Kinetic Effects in the Crystallization of Metal–Organic Frameworks” and it’s published in the journal Accounts of Chemical Research.

The paper describes how our understanding has grown over the years from simple qualitative observations in the 90’s to detailed, quantitative results provided by computational models and powerful in situ experiments today. The authors, my former supervisor Tony Cheetham, Gregor Kieslich at Munich and Hamish delve into how we’re able to understand not just the structures, but the enthalpic, entropic and kinetic factors that shape each material too.

Take a look at the paper here.