Article pubs.acs.org/crystal
Reversible Thermosalient Effect of N′‑2-Propylidene-4hydroxybenzohydrazide Accompanied by an Immense Negative Compressibility: Structural and Theoretical Arguments Aiming toward the Elucidation of Jumping Phenomenon Ivor Lončarić,†,# Jasminka Popović,‡,# Vito Despoja,§ Sanja Burazer,‡ Ivan Grgičević,∥ Dean Popović,⊥ and Ž eljko Skoko*,§ †
Centro de Física de Materiales CFM/MPC (CSIC-UPV/EHU), P. Manuel de Lardizabal 5, 20018 Donostia-San Sebastián, Spain Laboratory for Synthesis and Crystallography of Functional Materials, Center of Excellence for Advanced Materials and Sensing Devices, Division for Materials Physics, Ruđer Bošković Institute, Bijenička 54, HR-10000 Zagreb, Croatia § Department of Physics, Faculty of Science, University of Zagreb, Bijenička 32, HR-10000 Zagreb, Croatia ∥ Fidelta Ltd., Prilaz baruna Filipovića 29, HR-10000 Zagreb, Croatia ⊥ Institute of Physics, Bijenička 46, HR-10000 Zagreb, Croatia ‡
S Supporting Information *
ABSTRACT: The temperature-induced reversible phase transition of N′-2-propylidene-4-hydroxybenzohydrazide from the polymorphic Form II to Form III, and vice versa, is accompanied by the dramatic change of the macroscopic dimensions of the crystal which resulted in the pronounced mechanical motion (jumping) during the phase transition. Prior to the phase transition, the extremely large uniaxial negative thermal expansion along one crystal axis (b axis) was observed, together with the positive thermal expansions along the other two crystal axes. Form III of N′-2-propylidene-4hydroxybenzohydrazide exhibits the thermal expansion αc = 360 × 10−6 K−1, which is the largest value ever noticed in any organic or metal−organic crystal. From the structural point of view, a thermosalient effect is escorted by the springlike behavior of the zig-zag molecular assemblies along the c axis. First-principles electronic structure calculations show that negative thermal expansion arises from the elastic properties of the crystal which show uniaxial negative compressibilities, NLC. Form III exhibits the negative compressibility along the 001 direction β3 = −28 TPa−1, which is 1 order of magnitude larger than that of any organic compound and, in fact, is comparable to compressibilities of molecular frameworks showing the most pronounced NLC behavior. Elastic properties are also the reason for the reversibility of Form II to Form III transition in contrast to the irreversible Form I to Form II transition. Low energy springlike phonons are easily thermally excited and can assist in the overcoming of the energy barrier between the two phases that precedes thermosalient transition.
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INTRODUCTION Molecular crystals that provide, when thermo- and/or photostimulated, a mechanical response on a macroscopic scale are representing an ideal platform for the design of actuators, artificial muscles, biomimetic kinematic devices, and heat and light sensors.1 The capability of collective molecular response to the external excitation results in various displacements within the crystal lattice. Such deformations of the crystal are manifested, on the macroscopic scale, by the diverse acrobatic forms such as jumping, twisting, curling, bending, and rotating.2 If considered for application as actuators, the main advantage of crystalline materials, compared to the soft materials, lies in the rapidness of the responsemechanical actuations usually take place on the time scale of less than a millisecond.3 The fact that this behavior is visually very attractive and unexpected for the observer is not to be disregarded as well, as it can provide a © XXXX American Chemical Society
direct insight into the strong forces that are accumulated inside the crystal lattice and momentously relaxed in the form of the actuations of the crystals. Thereby, they provide a wonderful natural example on the microscopic level of the transformation of the thermal and/or light energy into mechanical work. Among all the mechanically responsive systems, one class stands out“thermosalient materials”, or more colloquially called “jumping crystals”. So far, the thermosalient effect has been reported for only a dozen of systems; it takes place during the temperature-induced polymorphic phase transitions and is accompanied by the dramatic and anisotropic change of the lattice parameters. Early papers reporting the thermosalient Received: June 6, 2017 Revised: July 11, 2017 Published: July 13, 2017 A
DOI: 10.1021/acs.cgd.7b00785 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
dependence of a free energy of each polymorphic phase. This enables us to rationalize, on a truly microscopic level, their existence in the defined temperature ranges.
effect claim that it has been observed serendipitously, usually by the visual observation of the crystals’ behavior on the hot stage.4−13 The main obstacle for the comprehensive elucidation of the phenomenon lies in the fact that integrity of the crystals is rarely preserved during the thermosalient phase transition, hence making the full structural characterization difficult. The first true systematic study of this phenomenon3 was carried out in 2010 by Skoko et al., and after that, there has been a significant number of publications aiming at the revelation of this effect,1,14−26 proving a rising interest in this impressive, visually very exciting, yet exclusive, process which embodies an example par excellence of the conversion of heat energy into the useful mechanical work. Centore et al. published, indeed, a fascinating solid-state polymorphism of the title compound, N′-2-propylidene-4hydroxybenzohydrazide, having three polymorphic forms I, II, and III.21 Phase transitions from I to II and from II to III are topotactic, i.e., single-crystal-to-single-crystal transitions. The authors reported the thermosalient bursting of the single crystals of Form I during the heating process, upon the irreversible transition from I → II (at 147 °C). This phase transition is accompanied by the strong compression of the polar axis. In this system, the polar axis is parallel to the c direction and it compresses by approximately 15%. The study of Centore et al. also reports the reversible transition from III ↔ II (at 80 °C during the cooling and 88 °C during the heating) which does not involve any burst or jumping of the single crystals. During this phase transition, the polar axis expands by approximately 14%. The authors propose that the mechanical response observed during the first phase transition is caused by the compression of the polar axis, while the second phase transition, lacking the c axis compression, therefore does not exhibit any crystal motion.21 Just very recently, another study was published by Centore et al., once again claiming that N′-2-propylidene-4-hydroxybenzohydrazide is characterized by only one thermosalient transition.20 Our study, however, contradicts those findings and it will show that the reversible phase transition III ↔ II is, as well, accompanied by the mechanical motion, which means that thermosalient behavior appears in the case of the expansion of the polar axis, as well. Finally, we believe that revelation of thermosalient behavior during the phase transition II to III is extremely important since it has more potential to be exploited in the actuator-related research being reversible, unlike the onetime bursting effect occurring during the heating of Form I to II. This study has been carried out by the means of the in situ high temperature X-ray powder diffraction study (HT-XRPD) and the first-principles electronic structure calculations which are, to the best of our knowledge, the first attempt to elucidate phase transitions in any thermosalient system by the state-ofthe-art theoretical arguments. Large unit cells of the crystal usually prohibit the use of high-level quantum chemistry calculations; however, with the recent progress in development of density functional theory (DFT) with the van der Waals (vdW) interactions, the good accuracy and predictability in modeling of molecular crystals has become accessible.27−34 Of several vdW implementations in DFT, we choose the nonemipirical vdW-DF-cx functional which proved to be capable of accurately describing the structure and properties of a wide range of systems35−37 and specifically, for molecular systems.38,39 Using such a theoretical framework, we perform phonon calculations which provide insight into the temperature
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EXPERIMENTAL SECTION
Synthesis. Synthetic preparation of the title compound was slightly modified from the one reported by Centore et al.21 The compound has been prepared by refluxing 2.5 g of 4-hydroxybenzohydrazide in 20 mL of acetone for 18 h. Subsequently, half of the solution volume was removed in vacuum and the suspension was filtered to obtain 2.3 g of desired product as a white crystalline solid. The solid was dried in a vacuum oven at 50 °C for 2 h. Analytical data of title product correspond to the data reported by Centore et al.21 Methods and Calculations. X-ray Powder Diffraction (XRPD). Temperature-induced structural changes have been followed by the in situ HT XRPD using a Philips PW 1710 diffractometer equipped with a high temperature chamber. XRPD data have been collected in the 2θ range 10−50°. Data were collected in the temperature range 0−200 °C. Crystal structures were refined by the Rietveld method in HighScoreXpert Plus (Version 4.5, March 2016). The thermal expansion coefficients were calculated from the refined unit-cell parameters obtained from variable temperature diffraction data. The axial thermal expansion coefficients along the principal axes were calculated by using the software PASCal.40 Thermal Analysis. Differential Scanning Calorimetry (DSC) was carried out on a Mettler Toledo DSC 822e instrument in a dynamic helium atmosphere (flow rate 50 mL/min) in the temperature range between −100 and 200 °C. Three heating runs and two cooling runs were measured (Supporting Information Figure S1). DFT Calculations. For all calculations, we have used a plane-wave basis set code Quantum Espresso41 with GBRV pseudopotentials42 and vdW-DF-cx35−37 exchange-correlation functional. The choice of the functional is justified by good agreement between calculated and experimentally determined lattice constants of all three forms. The agreement is much better compared to ones obtained with the commonly used PBE43 functional, or with the original vdw-DF functional (Supporting Information Table S1). The plane-wave basis set cutoff is 680 eV for phonon calculations and 820 eV for stress calculations. The first Brillouin zone is sampled by a 3 × 3 × 3 Monkhorst−Pack k-point mesh. In each calculation, atoms were relaxed until the change in the total energy was
