Tia Jacobs

Exceptionally large positive and negative anisotropic thermal expansion of an organic crystalline material.

In general, the relatively modest expansion experienced by most materials on heating is caused by increasing anharmonic vibrational amplitudes of the constituent atoms, ions or molecules. This phenomenon is called positive thermal expansion (PTE) and usually occurs along all three crystallographic axes. In very rare cases, structural peculiarities may give rise either to anomalously large PTE, or to negative thermal expansion (NTE, when lattice dimensions shrink with heating). As NTE and unusually large PTE are extremely uncommon for molecular solids, mechanisms that might give rise to such phenomena are poorly understood. Here we show that the packing arrangement of a simple dumbbell-shaped organic molecule, coupled with its intermolecular interactions, facilitates a cooperative mechanical response of the three-dimensional framework to changes in temperature. A series of detailed structural determinations at 15-K intervals has allowed us to visualize the process at the molecular level. The underlying mechanism is reminiscent of a three-dimensional (3D) folding trellis and results in exceptionally large and reversible uniaxial PTE and biaxial NTE of the crystal. Understanding such mechanisms is highly desirable for the future design of sensitive thermomechanical actuators.

The liquid phase epitaxy approach for the successful construction of ultra-thin and defect-free ZIF-8 membranes: pure and mixed gas transport study

The liquid-phase epitaxy (LPE) method was effectively implemented to deliberately grow/construct ultrathin (0.5-1 μm) continuous and defect-free ZIF-8 membranes. Permeation properties of different gas pair systems (O2-N2, H2-CO2, CO2-CH4, C3H6-C3H8, CH4-n-C4H10) were studied using the time lag technique.

In situ X-ray structural studies of a flexible host responding to incremental gas loading

Crystallographic pressure-lapse snapshots of a porous material responding to gas loading were used to investigate the stepwise uptake of carbon dioxide and acetylene molecules into discrete confined spaces. Based on the data, a qualitative statistical mechanical model was devised that reproduces even subtle features in the experimental gas sorption isotherms.

Anomalous thermal expansion of an organic crystal - Implications for elucidating the mechanism of an enantiotropic phase transformation

Two enantiotropic polymorphs of a dumbbell shaped molecule possess similar packing arrangements, in principle, but one of the polymorphs shows anomalously anisotropic thermal expansion while the other does not.

Single-crystal to Single-crystal Transformations – Guest Removal and Substitution in a Robust Solvent-templated Metallocyclic Compound

The formation, structure, and single-crystal to single-crystal transformations of the dinuclear metallocycle [Ag2L2](PF6)2·2CH3CN (L = 2,3,5,6-tetrafluoro-1,4-bis(2-methylimidazole-1-yl-methyl)benzene) are described. The acetonitrile guest molecules can be removed without loss of single crystallinity to afford a porous material. CO2 sorption isotherms were recorded at various temperatures and the structures and sorption behaviour of the crystals are compared with those of a previously reported analogous system. Upon exposure to acetone vapour, the desolvated crystals undergo a single-crystal to single-crystal transformation to form the acetone solvate.

Templated polar order of a guest in a quasiracemic organic host

A quasiracemic mixture of Dianins compound and its thiol derivative enforces additional anisotropy of the guest-accessible space, thus facilitating a net polar arrangement of guest molecules; guest alignment is rationalized in terms of van der Waals volume considerations.

Solvent diffusion and binding in a 'nonporous' molecular crystal

Crystals of a series of solvated cadmium-based metallocycles undergo single-crystal to single-crystal transformations (SCTSCT) upon desolvation to yield an empty apohost; these transiently porous crystals then allow the diffusion of bulky solvent molecules.

Solid-state structural studies of oxacalix[2]arene[2]naphthalene as a molecular tweezer

A novel oxacalix[2]arene[2]naphthalene host molecule was synthesized and crystallized from a number of solvents. In the solid state the host assumes the 1,3-alternate conformation, allowing it to function as a molecular tweezer by utilizing its naphthalene moieties as pincer arms. An apohost phase was crystallized from DMSO and single-crystal X-ray diffraction analysis reveals that the pincer arms are pinched together rather than interdigitating. Solvate crystals were obtained from dimethyl formamide, dimethyl acetamide, N-methyl pyrrolidone and nitrobenzene and these were also analyzed by means of single-crystal X-ray diffraction. Three of the solvate phases are isoskeletal with a host:guest ratio of 1:1, and with the guest molecules located between the pincer arms. The nitrobenzene solvate has a host:guest ratio of 1:2—one of the guest molecules is complexed by the pincer arms while the second is situated interstitially.

Breaking the trigonal host packing motif of Dianin's compound

Crystallisation of Dianins compound (DC) together with two organic amines (piperazine and piperidine) produces co-crystals of 2:1 DC–piperazine and 1:1 DC–piperidine, respectively. The structures of these adducts contrast sharply with that of the well-known clathrate structure of DC in which organic guests are generally included in ratios of 3:1 or 6:1 DC–guest.

Single-crystal to single-crystal transformations in discrete solvated metallocycles: the role of the metal ion

Crystals of solvated cobalt- and zinc-containing metallocycles undergo single-crystal to single-crystal (SCTSCT) transformations upon desolvation to yield their close-packed forms; this is in stark contrast to the analogous cadmium-based metallocycle, which affords an empty, transiently porous phase upon desolvation.