The highlight gives an overview of the recent advances in mechanically flexible molecular crystals, with qualitative and quantitative studies performed on different molecular systems. The diverse methods for tuning of...

The distinct mechanical properties of concomitant dimorphs (Form α and Form β) of 1,3-bis(4-nitrophenyl) thiourea−DMSO solvate (TU-DMSO) have been investigated via experimental and computational techniques. This includes analysis of the molecular arrangement for the individual crystalline forms, considering the interaction energies responsible for stabilization of the dimers, comparing their lattice energies, and corroborating the results obtained from nanoindentation experiments. The work gives a plausible explanation for elastic bending of Form α crystals that includes synergistic contributions from electrostatic and dispersion components of intermolecular interactions. In addition to this, it suggests the role of the flexible torsions in thiourea along with expansion−compression in the outer-inner arc as a means to accommodate the strain due to bending. The relatively soft and compliant behavior of the former polymorph is highlighted by almost a 3-fold difference in hardness and elastic modulus values [Form α (115 MPa, 2.7 GPa) and Form β (334 MPa, 7.41 GPa)]. The nanoindentation experiments also support the observance of splitting of Form α crystals on excessive mechanical agitation.

Reversible responses as a result of thermal and mechanical stimuli are investigated for an organic salt polymorph. The dissipation of strain occurs by conformational and rotational changes in molecule leading to exhibition of shape memory effect.

In the current investigation, we report five new co-crystals formed by combinations of triphenylphosphine sulfide (PS) with 1,3,5-triiodo-2,4,6-trifluorobenzene (I3F), triphenylphosphine sulfide (PS) with 1,4-diiodotetrafluorobenzene (IF) which consists of I-centered interactions, primarily consisting of I···S halogen bonding interactions. The distance between the iodine and sulfur atoms in the different crystal structures is much shorter than the sum of the van der Waals radii (shortest I···S distance, 3.163 Å) and is highly directional (most directional C–I···S, 179°) in nature. The three-dimensional deformation density shows that the charge depleted (CD) region on iodine electrostatically interacts with the charge concentrated (CC) region on sulfur. The interaction energy, as obtained from density functional theory calculations, is in the range of −16 to −32 kJ/mol, for the interacting units, at the crystal geometry. The atomic polarizability analysis establishes the mutual polarization of the iodine atom in the presence of the sulfur atom and vice versa. A topological analysis unequivocally establishes the presence of a (3, −1) bond critical point, and the NCI-RDG analysis establishes the attractive nature of the interaction. The elongation of the C–I bond length as a consequence of evident charge transfer from sulfur to iodine for I···S interaction is computed via natural bonding orbitals. Thus, a detailed computational analysis renders insights into the electronic nature of the observed I···S interactions in the solid state.

A solvent‐assisted grinding method has been used to prepare co‐crystals in substituted dihydropyrimidines (DHPM) that constitutes pharmacologically active compounds. These were characterized using FT‐IR, PXRD, and single‐crystal X‐ray diffraction. In order to explore the possibility of formation of halogen (XB) and hydrogen bonding (HB) synthons in the solid state, co‐crystallization attempts of differently substituted DHPM molecules, containing nitro, hydoxy, and chloro substituents, with different co‐formers, such as 1,4‐diiodo tetrafluorobenzene (1,4 DITFB) and 3‐nitrobenzoic acid (3 NBA) were performed. The XB co‐crystals (C2aXB, C2bXB, and C2cXB) prefer the formation of C−I⋅⋅⋅O/C−I⋅⋅⋅S XB synthon, whereas the HB co‐crystal (C2dHB) is stabilized by N−H⋅⋅⋅O H‐bond formation. Hirshfeld surface analysis revealed that the percentage contribution of intermolecular interactions for XB co‐crystals prefer equal contribution of XB synthon along with HB synthon. Furthermore, the interaction energy was analyzed using energy frameworks, which suggests that their stability, a combination of electrostatics and dispersion, is enhanced through XB/HB in comparison to the parent DHPMs.