Translating microscopic molecular motion into macroscopic body

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Translating microscopic molecular motion into macroscopic body motion: reversible self-reshaping in the solid state transition of an organic crystal Roberto Centore, and Mauro Causà Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00337 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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Crystal Growth & Design

Translating microscopic molecular motion into macroscopic body motion: reversible self-reshaping in the solid state transition of an organic crystal Roberto Centore,*1 Mauro Causà2 1

Department of Chemical Sciences, University of Naples Federico II, Via Cinthia, I-80126 Naples, Italy Department of Chemical, Material and Production Engineering, University of Naples Federico II, Piazzale Tecchio, I80125 Naples, Italy

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ABSTRACT: The amplification of microscopic molecular motions so as to produce a controlled macroscopic body effect is the main challenge in the development of molecular mechanical devices. That amplification requires the coherent and ordered movement of each molecule of a whole macroscopic set, such as that taking place in a single-crystal-to-singlecrystal transition. Actually, single-crystal-to-single-crystal transitions in molecular crystals can produce a variety of mechanical effects potentially useful in the development of smart materials. A challenging issue in these dynamic crystals, propedeutic to many possible applications in devices, is the gaining of a strict control on the mechanical effects associated with the transition. Here we report an example in which the control of the mechanical effects was successfully got. The compound studied undergoes a reversible single-crystal-to-single-crystal transition at 71 °C, from a planar stacked to a herringbone type packing. To this transition, a reversible macroscopic self-reshaping of the crystal is associated. Depending on the morpholgy, the crystal specimen undergoes a reversible longitudinal expansion of about 20 % or a reversible transverse expansion of 20 %, the other two dimensions of the crystal specimen being substantially unchanged. The amount of the macroscopic reshaping effect (20 %) fully matches the relative variation of the sole unit cell parameter that changes during the transition (from 8.139 Å to 9.666 Å) in a sort of scale-invariant process. This represents a striking evidence of controlled translation of sub-nanometer molecular motions up to the macroscopic scale of body motion.

new phase (single-crystal-to-single-crystal transition, hereafter SCSC transition), Fig. 1(b).3

Introduction Self-reshaping of a still macroscopic object requires the ordered and coordinated motion of all of its microscopic individual constituents (atoms/molecules). Solid state transformations between crystal phases seem natural candidates to get this goal. First order phase transitions between crystal phases are classically divided in diffusive/reconstructive and nondiffusive/displacive.1 In many cases of diffusive/reconstructive transitions, Fig. 1(a), a macroscopic single crystal of the initial phase is transformed in a macroscopic specimen of the new phase that basically keeps the shape and dimensions of the parent crystal, but it is no longer a single crystal, being formed instead by a loose mosaic of many small single crystals of the new phase, oriented in a more or less random way with respect to each other. In a topotactic displacive/diffusionless transition, on the other hand, there is a well defined relation between the orientation of the crystal axes before and after the transition.2 If the relation is a one-to-one correspondence, one single crystal of the initial phase is transformed in one single crystal of the

Figure 1. Sketch of a solid-solid phase transition. (a) diffusive/reconstructive transition; (b) displacive, topotactic SCSC transition.

Non topotactic transitions are also nicknamed ”civilian” transitions and SCSC ”military” transitions. This is be-

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cause in an SCSC transition the microscopic movement of individuals (atoms/molecules) is strongly correlated in space and time over macroscopic distances (i. e. over the whole dimensions of the single crystal). As a matter of fact, SCSC transitions are a rare example, if not the only one, in which sub-nanometric displacements of all the molecules of a macroscopic set (i. e. the single crystal) are realized collectively and coherently. This allows, in principle, the amplification of the microscopic molecular motions in such a way as to produce a macroscopic effect, which is the challenging issue in the development of mechanical molecular devices.4

requirements are met by the compound studied in this paper, 4-dimethylaminobenzaldehyde(4-cyanophenylethylidene)hydrazone, hereafter compound GCN, whose chemical diagram is shown in Chart 1.

With respect to the classic nucleation/growth mechanism of first order transitions,5 SCSC transitions seem to have singular basic features. In fact, no new surface is formed after an SCSC transition: the external faces of the parent crystal are kept as external faces of the new crystal. They can only undergo a change in area and dihedral angles at the transition, without losing the monolithic integrity of the single crystal. Macroscopically, this is accomplished, evidently, through a mechanical deformation of the whole parent crystal as a continuous elastic body (i. e. through a single deformation wave running across the crystal during the transition).6-8 In this sense, SCSC transitions seem to have a strong character of mechanical transitions besides that of thermodynamic transitions. The mechanical deformation associated to the SCSC transitions (reshaping) can lead, under particular circumstances,9,10 to the appearance of impressive dynamical phenomena (jumping crystals, thermosalient and photosalient effect11). The study of the mechanical effects in these dynamic solids is a very active branch of crystal engineering today,8,12 in view of the potential applications as high-performance smart materials (macro- or nanoactuators, artificial muscles, etc.). We also remark that these systems, lato sensu, actuate a sudden macroscopic motion as a response to an external stimulus (heat, light, etc.), a feature that had always been considered typical, if not exclusive, of living beings.13

This compound was reported by us some years ago.15 In that paper, the polymorphism of GCN was recognized and very shortly described, but only the crystal structure of the metastable phase (phase II, vide ultra), not involved in the SCSC transition, was studied.

There are many open problems, however, with SCSC transitions in molecular crystals. For instance, it is not clear if (and how) symmetry restraints hold for SCSC transitions: Is it possible an SCSC transition between a centrosymmetric and a non centrosymmetric polar phase?14 How similar must two packings be in order that an SCSC transition be possible between them? What is the role of specific intermolecular interactions in triggering and coherently propagating the molecular displacements that actuate the transition? What is the relation between the packings of the phases involved in the transition and the dynamic phenomena recorded at macroscopic level? In order to address these issues, very interesting cases are represented by molecular compounds with many crystal phases, and transitions between the different phases both SCSC and not, and for which the crystal structure has been determined for all the phases involved. These

Chart 1. Chemical diagram of the compound studied.

Results and discussion Thermal and morphological analysis of the transitions GCN is a trimorphic compound, as it can be deduced from the DSC curves reported in Fig. 2. In the first heating run of the bulk sample, two broad endothermic signals are present at 76.7 °C and 118 °C, before the melting endotherm at 174 °C. On cooling from the liquid phase, after crystallization at 166 °C, only one sharp exothermic signal is observed at 62 °C. The corresponding transition is reversible, as it can be clearly deduced from the second heating run in which it is observed at 71 °C (∆H = 2.95 kJ/mol).

Figure 2. DSC thermograms of compound GCN. Scanning rate 10 K/min. (a): heating run of an as prepared bulk sample; (b) first cooling from the melt; (c) heating run of the sample crystallized from the melt. The inset shows curve (a) between 60 °C and 130 °C with enhancement.

These observations correspond to the trimorphism early assessed for the compound, and summarized in the Chart 2 below. Chart 2. Sketch of the phase transformations of GCN.

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Crystal Growth & Design Figure 3. Snapshots at the polarizing microscope of three different single crystals of GCN taken before, (a)/(c)/(e), and after, (b)/(d)/(f), the SCSC transition I to III. The single crystals of photographs (a)/(b) and (e)/(f) are observed under crossed polarizers.

The polymorphism of phase I is enantiotropic. Concerning phase II, the heat-of-transition rule16 would also suggest an enantiotropic relation with phase III because of the endothermic signal in the DSC curve, but we have not observed the reversed transition III→II on cooling, perhaps because of kinetic factors, nor we have observed any interconversion between phases I and II. The reversible transition I to III is SCSC while the irreversible transition II to III is not. We have also noted that the transition I to III is significantly affected by the thermal hystory of the sample: the transition temperature is higher in the as prepared bulk sample or in virgin single crystals, as compared with bulk samples crystallized from the melt or with single crystals no longer virgin, i. e. that have already undergone the transition to phase III (see SI). By performing crystallization experiments in ethanol solutions, phases I and II are concomitantly obtained, in the form of larger flat plates (phase II) and smaller squares or elongated prisms (phase I). In Fig. 3 are reported snapshots of three different single crystals of GCN taken before and after the SCSC transition I → III.

The macroscopic increase in the size of the crystals upon the transition I →III is evident. The single crystals of Figs. 3(a)/(b) and 3(c)/(d) undergo an increase in the transverse length by about 20 %, the longitudinal size being unchanged, while in the specimen of Figs. 3(e)/(f) the same relative increase is observed longitudinally, the transverse size being unchanged (see also Fig. S1 of SI for accurate measurements of the size of crystals). The possibility of growing single crystals with different, selective and orthogonal features of macroscopic deformation (i. e. longitudinal or transverse) is quite remarkable. The full reversibility of the macroscopic crystal reshaping (see Fig. S1), that we have verified over several cycles of transitions I↔III without observing any mechanical damage of the crystal (provided it is free from fissures or cracks), is particularly appealing for potential applications of this phenomenon in the development of actuators. In fact, in several cases reported in literature, dramatic mechanical effects observed in dynamic crystals are related with irreversible phase transitions,17-19 and so they can be exploited only once. Examples of reversible mechanical effects related with a (reversible) phase transition are comparatively fewer.4,20 The observation at the polarizing microscope of the two transitions, with video-recording (movies 1-5), evidences differences at a qualitative and quantitative level. The SCSC transition I→III appears simply as an elastic deformation of the crystal as a whole. Of course, we use the word ”elastic” just to convey the information that on cooling the crystal from phase III to phase I there is a complete recovery of the initial shape and size (see also SI). Depending on the morphology, size (mainly the thickness)12 and presence of fissures or cracks, the deformation of the single crystal is accomplished in a larger interval of time (movie 1), or almost instantly (movies 2 and 3). We have measured time intervals ranging from 0.2 s to 2 s for crystals of maximum dimension between 0.5 mm and 0.6 mm. Under crossed polarizers, the crystals extinguish uniformly before and after the transition, see also Fig. 3(a)/(b) and Fig. 3(e)/(f). In some cases there is no clear evidence of a phase front of differential birefringence advancing across the crystal during the transition (movies 1 and 3), this suggesting small changes in the orientation of the optical principal axes (vide ultra). In other cases, (movie 4) a faint phase front is observed and it is a quite regular straight line, Fig. 4, as it has been reported in few other cases of SCSC transitions.7,21

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II→III is not SCSC), when it is cooled down, the mechanical effects of the transition to phase I distance the single blocks of the mosaic from each other, with consequent fragmentation of the sample.

Figure 4. Consecutive snapshots of a single crystal of GCN taken during the SCSC transition III→I, on cooling. The wavefront of differential birefringence is indicated by the red arrow. It advances from the right to the left while the crystal is udergoing a longitudinal contraction. The temperature of snapshot (a) is higher than (b).

The presence of such a wavefront is generally believed as a proof of the nucleation/growth mechanism.21-24 However, a very intriguing feature is that the movement of the phase front during the transition III→I, from right to left in Fig. 4, is exactly reversed in the reversed transition I→III of the same crystal (from left to right, see also movie 4). We already made this observation in another example of SCSC transition.7 So, it seems that there is a complete reversibility in the thermodynamic, mechanical and optical features of the SCSC transition. This cannot keep us from thinking to a reversibility also in the microscopic movements of molecules during the direct and reversed transition. The non topotactic transition II to III, on the other hand, is accomplished more slowly, in about 40 s for a crystal of approximately 0.4 mm as maximum dimension (movie 5); the phase front of differential birefringence is clearly evident and it irregularly spreads out while advancing along the crystal; the final shape and dimensions of the specimen are unchanged, but its mosaic structure (vide supra) is quite evident under crossed polarizers, Fig. 5.

Analysis of the crystal packing The X-ray structure of phase II was reported in ref. 15. Now, we have succesfully determined the crystal structures of the other two phases I and III involved in the SCSC transition. The molecular structure and conformation is basically unchanged in the three phases (see Fig. S4 of SI). Some relevant features of the crystal packing of phase I (triclinic, P-1, Z=2) are shown in Fig. 6. Phase I has a layered stacking structure. Molecules are arranged in infinite planar layers, Fig. 6(a), parallel to the lattice planes ( 2 1 1 ), with a short interplanar stacking distance, d 211 = 3.315 Å, Fig. 6(b). Within the layers, molecules are held by weak H bonding interactions involving the cyano acceptor and aromatic C-H donors of centrosymmetrically related molecules (the full list of weak H bonding parameters is given in Table S3 of SI).24 In particular, the cyano group acts as bifurcated acceptor and two patterns can be recognized, Fig. 6(a). In one case, the donor is a C-H group ortho to the CN of another molecule, and ring patterns R22 (10 ) are formed (C-H···N: 0.950 Å, 2.586 Å, 3.421(3) Å, 147°). In the other case, the donor is a C-H group in meta to the dimethylamino group, and ring patterns R22 (26) are formed (C-H···N: 0.950 Å, 2.558 Å, 3.492(4) Å, 167°). The two patterns are highlighted with red and blue lines in Fig. 6. In particular, the red pattern connects molecules longitudinally, i. e. in the direction of the long molecular axis, while the blue pattern connects the molecules laterally. It is important to note that the four molecules involved in the two patterns are almost coplanar, as it is evident from Fig. 6(b).

Figure 5. Snapshots at the polarizing microscope of a single crystals of GCN taken before, (a), and after, (b), the irreversible transition II to III. The single crystal is observed under crossed polarizers.

Moreover, since the sample of phase III obtained from phase II has no longer a monolithic nature (the transition

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Crystal Growth & Design

Figure 6. Partial crystal packing of phase I of GCN, with indication of some weak H bonding interactions. (a): face view of a cluster of four molecules of a planar layer; (b) edge view of three stacked clusters, along the direction of the red pattern.

The cluster of Fig. 6(a) with four molecules held by weak H bonding interactions is retained in phase III (triclinic, P-1, Z=4), Fig. 7(a) (see Table S3 of SI). An important difference, however, is that the four molecules of the cluster are no longer coplanar, Fig. 7(b). In particular, the two molecules involved in the blue pattern are above and below the plane of the two molecules involved in the red one. So, the infinite planar layers of Fig. 6 are replaced, in phase III, by corrugated, stair-like layers, Fig. 7(b). This, coupled with the fact that the two independent molecules A and B have their average planes making a dihedral angle of 37.4(3)°, produces a typical herringbone pattern for phase III, Fig. 7(c). So, the transition I→III can be considered as a transition from a layer stacked to a herringbone-type packing; this is also confirmed by the analysis of the Hirshfeld fingerprint plots of the two phases, reported in the SI.

red pattern; (c) edge view of three clusters formed by A and B independent molecules.

The enantiotropic nature of the transition I→III is correctly predicted by theoretical calculations. In Fig. 8 is reported the computed difference ∆G between the Gibbs free energy of phase III and phase I as a function of the temperature. The transition temperature, for which ∆G=0, is predicted at 367 K (94 °C). For T0 and so phase I is thermodynamically stable as compared with III, while for T>367 K it is ∆G