Strong and Anomalous Thermal Expansion Precedes the Thermosalient Effect in Dynamic Molecular Crystals

Abstract

The ability of thermosalient solids, organic analogues of inorganic martensites, to move by rapid mechanical reconfiguration or ballistic event remains visually appealing and potentially useful, yet mechanistically elusive phenomenon. Here, with a material that undergoes both thermosalient and non-thermosalient phase transitions, we demonstrate that the thermosalient effect is preceded by anomalous thermal expansion of the unit cell. The crystal explosion occurs as sudden release of the latent strain accumulated during the anisotropic, exceedingly strong expansion of the unit cell with αa = 225.9 × 10(-6) K(-1), αb = 238.8 × 10(-6) K(-1) and αc = -290.0 × 10(-6) K(-1), the latter being the largest negative thermal expansivity observed for an organic compound thus far. The results point out to the occurence of the thermosalient effect in phase transitions as means to identify new molecular materials with strong positive and/or negative thermal expansion which prior to this work could only be discovered serendipitously.

Figure 1. Chemical structures and phase transitions of IMACET and its deuterated analogues.

Figure 2. Thermal behavior and thermosalient effect in crystals of protiated and deuterated variants of IMACET recorded by variable-temperature optical microscopy.

(a) IMACET-H₆ on cooling by liquid nitrogen. (b) IMACET on heating. (c) IMACET-D₂ on cooling. (d) IMACET-D₂ on heating. (e) IMACET-D₆ on cooling. (f) IMACET-D₆ on heating (the background of this set of images appears darker due to the use of polarizing filter). (g) IMACET and IMACET-D₂ on heating. (h) Form I and form III of IMACET-D₂ on heating.

Figure 3. Thermal (DSC) profile of crystals of IMACET, IMACET-D₂ and IMACET-D₆ above room temperature showing the phase transitions between forms I–III.

The two consecutive thermal cycles are shown in different colors.

Figure 4

Face indices (a–c) and snapshots of the thermosalient effect in crystals of the three isotopic variants of IMACET (d–g). The images of protiated IMACET (denoted H₆ for clarity; green circles), IMACET-D₂ (D₂, yellow circles) and IMACET-D₆ (D₆, purple circles) in panels (d–g) were extracted from high-speed video recordings (recording rate: (2-5)·10³ s⁻¹).

Figure 5. Correlation among surface tension, crystal habit and modes of parting of freely standing IMACET crystals during the thermosalient transition.

(a) Wulff-Gibbs polyhedra calculated for forms I, II and III. (b) Face indices of the crystal before the transition (left), and of two typical fragments obtained after the phase transition (center, right). (c) SEM images of the typical fragments, schematically shown in panel b, that are obtained by splintering of the crystals in the course of the thermosalient transition (the first and the second image show different views of the same fragment).

Figure 6. Correlation between the mode of splitting, crystal habit and thermal expansion during the thermosalient effect of affixed (constrained) IMACET crystals.

(a) Schematic of a typical disintegration of a crystal glued on the (01) face. (b) SEM images of a typical crystal glued with its (01) face before (far left panel) and after the phase transition (the other panels). The circled areas are zoomed for clarity. (c) Schematic showing typical disintegration of a crystal that was glued with its (100) face onto the basis (d) SEM images before (the two panels on the far left) and after (the other panels) the transition of a typical crystal glued onto the surface with its (100) face. The dashed red arrow shows top view of the parent crystal before the transition. (e) Correlation between the mode of splitting of the crystal and the thermal expansion that causes build-up of stress.

Figure 7. Thermal expansion and mechanism of the TS effect in IMACET.

(a) Thermal variation of the unit cell axes (the standard deviations are shown as the thickness of the connecting lines). (b) Thermal expansion coefficients and plot of the expansivity indicatrices along the principal axes calculated as α_l  = (1/l) (δl/δt)_P with PASCal. The principal axes X₁, X₂ and X₃ coincide with the crystallographic axes c, a and b, respectively. Red and blue colors represent positive and negative thermal expansion. (c) Change of the unit cell parameters across the TS phase transition (the standard deviations are smaller than the symbols). (d–f) Schematic representation of the intermolecular hydrogen bond O1—H1···O2 related to the negative thermal expansion along the c axis. (g,j) Variations of the distance between the donor (D) and the acceptor (A) in the hydrogen bonds O1—H1···O2 and N1—H1A···O2 that account for anisotropy in the thermal expansion. (h,i) Cartoon showing gradual decrease in the O1···O2 distance on heating that results in supramolecular jack-like distortion. This deformation causes concomitant positive thermal expansion along the b axis and negative thermal expansion along the c axis. (k,l) Cartoon showing the increase in the distance of the hydrogen bond N1···O2 that acts as hinge-screw. The elongation of this bond on heating accounts for the positive thermal expansion along the a axis.