Diverse Mechanical Properties of 1,3-Bis(4-nitrophenyl) thiourea–DMSO Dimorphic Solvates

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.