Dual Stress and Thermally Driven Mechanical Properties of the Same Organic Crystal: 2,6-Dichlorobenzylidene-4-fluoro-3-nitroaniline

An
elastic organic crystal, 2,6-dichlorobenzylidine-4-fluoro-3-nitroaniline
(DFNA), which also shows thermosalient behavior, is studied. The presence
of these two distinct properties in the same crystal is unusual and
unprecedented because they follow respectively from isotropy and anisotropy
in the crystal packing. Therefore, while both properties lead from
the crystal structure, the mechanisms for bending and thermosalience
are quite independent of one another. Crystals of the low-temperature
(α) form of the title compound are bent easily without any signs
of fracture with the application of deforming stress, and this bending
is within the elastic limit. The crystal structure of the α-form
was determined (P21/c, Z = 4, a = 3.927(7) Å, b = 21.98(4) Å, c = 15.32(3) Å).
There is an irreversible phase transition at 138 °C of this form
to the high-temperature β-form followed by melting at 140 °C.
Variable-temperature X-ray powder diffraction was used to investigate
the structural changes across the phase transition and, along with
an FTIR study, establishes the structure of the β-form. A possible
rationale for strain build-up is given. Thermosalient behavior arises
from anisotropic changes in the three unit cell parameters across
the phase transition, notably an increase in the b axis parameter from 21.98 to 22.30 Å. A rationale is provided
for the existence of both elasticity and thermosalience in the same
crystal. FTIR studies across the phase transition reveal important
mechanistic insights: (i) increased π···π
repulsions along [100] lead to expansion along the a axis; (ii) change in alignment of C–Cl and NO2 groups result from density changes; and (iii) competition between
short-range repulsive (π···π) interactions
and long-range attractive dipolar interactions (C–Cl and NO2) could lie at the origin of the existence of two distinctive
properties.