Abstract
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.
