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Unusual anisotropic thermal expansions with reversible axial switching and record-wide thermal hysteresis in single-component purely organic molecular crystals
Published in American Chemical Society
Volume: 124
Issue: 50
Pages: 27413 - 27421
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. © 2020 American Chemical Society
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