Dinabandhu Das

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.

Synthon evolution and unit cell evolution during crystallisation. A study of symmetry-independent molecules (Z′ > 1) in crystals of some hydroxy compounds

A kinetically favoured crystal, with many molecules in the asymmetric unit, may be a fossil relic of the crystal nucleus of a more stable polymorph.

Reversible single-crystal to single-crystal polymorphic phase transformation of an organic crystal

Two polymorphs of a substituted dihydroanthracene derivative were prepared by crystallization from different solvents. Each polymorph is reversibly interconvertible to the other by means of a single-crystal to single-crystal phase transformation.

Polymorphism of a hexa-host: isolation of four different single-crystal phases by melt crystallization.

A hexa-host compound yields at least four polymorphic forms from its melt phase. All four phases have been characterized using single-crystal X-ray diffraction; two of the phases were obtained by means of thermal stress, and the different forms exhibit conformational polymorphism.

Effects of the substituent on the formation of dimers and catemers in phenylpyruvic acids

The formation of dimers and catemers in a number of phenylpyruvic acids is strongly influenced by the nature of the substituent group. In all cases, the acid exists as the enol tautomer in the solid state.

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.

Concomitant formation of two different solvates of a hexa-host from a binary mixture of solvents

Crystallization of hexakis(4-cyanophenyloxy)benzene from a mixture of two different solvents produces two different solvates concomitantly, which were characterized by X-ray diffraction, thermal analysis and NMR spectroscopy.

A rare syn–anti catemer in 4‐nitro­phenyl­propiolic acid

The crystal structure of 4-nitrophenylpropiolic acid, C 9 H 5 NO 4 , is described. The carboxylic acid functional group adopts the rare syn-anti conformation and the molecules are arranged in a catemeric chain.

Uniaxial negative thermal expansion induced by moiety twisting in an organic crystal

Variable temperature single-crystal X-ray diffraction reveals uniaxial negative thermal expansion (NTE) of 1,4-bis(1′-hydroxycyclopentyl)-1,3-butadiyne crystal. The length of the unit cell axis a decreases with increasing temperature, whereas the b and c axes increase. The mechanism of the negative thermal expansion in the organic crystal has been derived by analysis of the crystal structures at different temperatures. Steric adjustment of the molecules in the crystal structure requires twisting of the terminal cyclopentyl ring, which drives the contraction of one of the principal axes.