Unusual Anisotropic Thermal Expansions with Reversible Axial Switching and Record-Wide Thermal Hysteresis in Single-Component Purely Organic Molecular Crystals

Single-component purely organic molecular
crystals exhibiting negative
thermal expansion (NTE) are extremely rare. Systematic studies on
bromophenyl-substituted dihydro-imidazole, a novel single-component
organic lightweight material, have resulted in dimorphs (centrosymmetric
and noncentrosymmetric) at room temperature and two more crystal forms
at low temperature (LT) upon reversible isosymmetric single-crystal-to-single-crystal
transformation. While the noncentrosymmetric forms with a notable
second harmonic generation activity exhibit colossal NTE and positive
thermal expansion (PTE) effects with axial switching, the centrosymmetric
LT phase experiences a moderate NTE but a colossal PTE. The interplay
of the hydrogen bonds and weak interactions leading to the scissor
jack-like molecular motion resulted in the unusual anisotropic thermal
expansions over a large temperature range with record-wide (∼40
to ∼110 K) thermal hysteresis. These environmentally benign
thermoresponsive materials experiencing wide thermal hysteresis, minimal
volumetric thermal expansions, and reversible axial (biaxial and uniaxial)
switching may find important technological applications in nonexpansive
organic electronics and thermomechanical devices.