Giant Thermosalient Effect in a Molecular Single Crystal: Dynamic Transformations and Mechanistic Insights

Abstract

The exploration of mechanical motion in molecular crystals under external stimuli is of great interest because of its potential applications in diverse fields, such as electronics, actuation, or sensing. Understanding the underlying processes, including phase transitions and structural changes, is crucial for exploiting the dynamic nature of these crystals. Here, we present a novel organic compound, PT-BTD, consisting of five interconnected aromatic units and two peripheral alkyl chains, which forms crystals that undergo a drastic anisotropic expansion (33% in the length of one of its dimensions) upon thermal stimulation, resulting in a pronounced deformation of their crystal shape. Remarkably, the transformation occurs while maintaining the single-crystal nature, which has allowed us to follow the crystal-to-crystal transformation by single-crystal analysis of the initial and expanded polymorphs, providing valuable insights into the underlying mechanisms of this unique thermosalient behavior. At the molecular level, this transformation is associated with subtle, coordinated conformational changes, including slight rotations of the five interconnected aromatic units in its structure and increased dynamism in one of its peripheral alkyl chains as the temperature rises, leading to the displacement of the molecules. In situ polarized optical microscopy reveals that this transformation occurs as a rapidly advancing front, indicative of a martensitic phase transition. The results of this study highlight the crucial role of a soft and flexible structural configuration combined with a highly compact but loosely bound supramolecular structure in the design of thermoelastic materials.

Figure 1

(a) Chemical structure of PT-BTD. (b) Evolution of the crystallization of PT-BTD visualized under a polarizing optical microscope by diffusion of MeOH vapors into a THF solution of the compound, showing the transformation of polymorph α into β.

Figure 2

DSC first heating (red) and cooling (blue) cycles of polymorphs (a) α and (b) β. DSC cycles were recorded at 10 °C/min. Endo up. (c) Schematic representation of the thermal polymorphic transformations.

Figure 3

Comparison between the Raman spectra of all of the studied phases collected at 785 nm excitation.

Figure 4

Microphotographs of a single crystal of PT-BTD (polymorph β) before (a), during (b), and after (c) thermal expansion showing its transformation to polymorph δ captured using an optical microscope equipped with a Linkam hot stage. The scale units are microns.

Figure 5

(a) Different torsion angles shown by the independent molecules of PT-BTD in polymorphs α, β, and δ, showing variation in the representative angles. (b) Dihedral potential energy hypersurface E = E(ϕ₁,ϕ₂) for an isolated PT-BTD molecule calculated at the ωB97XD/6-31G** level of theory.

Figure 6

Lamellar arrangement of polymorphs α, β, and δ of PT-BTD viewed along b-, a-, and b-axes, respectively.

Figure 7

Schematic representation of the overall crystal parameters changes upon the transition between polymorphs β and δ.

Figure 8

Comparison of the centroids’ positions for the six-membered rings of the central BTD unit (a, c) and the peripheral chains (b, d) in polymorphs β and δ, viewed along the c- and a-axes, respectively. The elements of the two different molecules in the asymmetric unit in polymorph β are depicted in different colors. The red/blue arrows illustrate the expansion/shrinkage of the b/c-axis when heating/cooling the crystals polymorphs β and δ, respectively.

Figure 9

Comparison of the centroids’ positions for the six-membered rings of the central BTD unit (a, c) and the peripheral chains (b, d) in polymorphs β and δ viewed along the a- and b-axes, respectively. The elements of the two different molecules in the asymmetric unit in polymorph β are illustrated in different colors. The red/blue arrows illustrate the shrinkage/expansion of the c/a-axis when heating/cooling the crystals.

Figure 10

Illustration of how the interaction involving the aromatic section of the molecules changes when transitioning from (a) polymorphs β and (b) δ. The alkyl chains have been removed for clarity.

Figure 11

(a) Illustration of the interactions giving rise to the formation of the layers: CH–π and N–S interactions connecting the dimers (in blue) and methoxy–methoxy contacts and OCH₃–π interactions (in red) between adjacent dimers. (b) Network of N–S interactions (blue) and OCH₃–π interactions (black) that held together the layers in polymorph α (alkyl chains have been omitted for the sake of clarity).