Crystal Structure, Thermal Properties, and Shock-Wave-Induced Nucleation of 1,2-Bis(phenylethynyl)benzene

We report the single crystal structure and thermal properties of 1,2-bis(phenylethynyl)benzene (PEB), revealing that PEB forms a metastable liquid at rt, ca. 35 °C below its melting point. Accelerated nucleation of PEB from its supercooled state was induced with high reproducibility by a shock wave with ca. 15 ns duration and 1.2 GPa peak pressure. By conducting shock wave experiments with varying peak pressures, we observed a correlation between the frequency of accelerated nucleation and shock intensity. The generality of shock-induced nucleation for supercooled liquids was probed with other organic supercooled liquids bearing phenyl rings. However, accelerated nucleation after shock wave impact was only observed for PEB, possibly due to the low rotational energy barrier of the terminal phenyl rings. C crystallization of organic molecules is of great interest for the development of pharmaceuticals, organic electronics, and gas-storage materials. Upon cooling, some molten crystalline materials are capable of forming thermodynamically unstable supercooled liquids. The effect of pressure in the form of compression on crystal nucleation and growth in supercooled liquids has been explored; however, contradictory examples of pressure-promoted or -inhibited crystallization are found in the literature. Other external forces such as ultrasonication, mechanical grinding, and shear are often utilized to assist in crystallization, but the complex mechanisms of crystallization, especially in the presence of such external triggers, still lacks a thorough understanding. Recently, our group demonstrated that a shock wave is capable of inducing structural ordering in an amorphous ionic liquid. A shock wave imparts a transient pressure jump of nanosecond duration, potentially long enough for nuclei to form, and may serve as a useful probe of crystal nucleation in supercooled liquids. While studying the reactivity of aryl-substituted arenediynes, we discovered that 1,2-bis(phenylethylnyl)benzene (PEB) forms a stable supercooled liquid at room temperature. Although PEB has been extensively studied to probe the Bergman cyclization reaction and to investigate its optical properties, to the best of our knowledge, its crystal structure is not reported. Herein, we present the single crystal structure and thermal properties of PEB, along with crystallization of the supercooled liquid. We demonstrate that a shock wave with 1.2 GPa peak pressure and 15 ns duration accelerates nucleation of supercooled PEB (Scheme 1a). Systematic studies of shock wave peak pressures were conducted to correlate pressure effects and frequency of nucleation. The generality of shock-induced nucleation was probed with other supercooled liquids including diphenyl phthalate (DP), benzophenone (BP), salol, and 2biphenylmethanol (PM) (Scheme 1b). However, accelerated nucleation after shock wave impact was only observed for PEB. Several hypotheses are discussed to provide insight into the mechanism of shock-accelerated crystal nucleation of supercooled liquids bearing phenyl rings. PEB was synthesized via a Sonogashira coupling reaction following a literature procedure (Figure S1). Nonplanar molecules with limited conformational flexibility often lead to the formation of amorphous materials, and PEB falls into this category based on simulations with the B3LYP/6-31G(d) basis set (Figure S7). Solution grown, diffraction quality crystals of PEB were attempted from common organic solvents such as cyclohexane, benzene, tetrahydrofuran, and chloroform at different temperatures (see Table S1). In spite of extensive experimentation with typical crystallization conditions, PEB generally remained in solution or oiled-out after solvent evaporation. We were able to successfully obtain single crystals of PEB suitable for X-ray analysis through slow evaporation of a saturated hexanes solution at −20 °C. A single-crystal X-ray analysis revealed that PEB crystallizes in the acentric space group Pca21, with two crystallographically unique molecular conformations (PEB1 and PEB2) in the asymmetric unit (Figure Received: July 27, 2016 Revised: September 27, 2016 Communication pubs.acs.org/crystal © XXXX American Chemical Society A DOI: 10.1021/acs.cgd.6b01119 Cryst. Growth Des. XXXX, XXX, XXX−XXX 1a). The terminal phenyl rings of PEB1 lie twisted from the plane of the central benzene ring at dihedral angles of 28.6° and 74.2°. In PEB2, however, one terminal phenyl ring lies nearly coplanar with the central benzene ring (4.4° twist), while the second terminal phenyl ring is twisted significantly from the plane of the central benzene at 71.0°. The dihedral angle between the two terminal phenyl rings is 87.9° for PEB1 and 73.0° for PEB2, confirming that PEB crystallizes in a nonplanar geometry. The two PEB rotamers pack in an alternating PEB1 | PEB2 pattern along the a-axis (Figure 1b), and neighboring central benzene rings engage in π-facial interactions (3.32 Å separation). Conformers PEB1 and PEB2 pack into alternating columns that extend along the b-axis (Figure 1c), sustained by edge-toface CH···π interactions (ca. 3.5 Å separation). Although we were able to obtain a single crystal of PEB, we sought to further investigate its solid-state structure to test whether it adopts planar conformations, or whether twisted conformations are typical. One approach to facilitate the crystallization of PEB involves forming stabilized electrostatic interactions via cocrystallization with perfluorinated analogues. In fact, cocrystals comprising octafluoronaphthalene (OFN) and PEB were readily obtained using various conditions (see SI). A single-crystal X-ray analysis revealed that OFN and PEB crystallize in a 3:2 molar ratio in the space group P1̅. One terminal phenyl ring of PEB lies nearly coplanar with the central benzene ring of PEB (6.9° twist), while the second terminal phenyl ring is twisted significantly from the plane of the central benzene (52.8° twist). The coplanar rings engage in phenyl−perflorophenyl interactions with one crystallographically uniqueOFN (centroid−centroid distance of 3.60 Å (central benzene−OFN) and 3.73 Å (phenyl−OFN)), while the twisted phenyl ring engages in phenyl−perflorophenyl interactions with the second crystallographically unique OFN (centroid−centroid distance: 3.71 Å) (Figure 1d). The dihedral angle between the two terminal phenyl rings of PEB is 55.3°, further demonstrating propensity for PEB to crystallize in nonplanar geometries. While crystalline PEB melts at 55 °C as seen in the DSC trace (Figure S2), subsequent cooling of the molten PEB results in a supercooled liquid phase that is stable down to ca. 0 °C. The supercooled liquid phase persists during the second heating cycle without showing any crystallization peaks, indicating that it forms a supercooled liquid with good thermal stability. Using the Hoffman equation, the Gibbs free energy difference (ΔG) between supercooled and crystalline PEB at 25 °C was estimated as −1.84 kJ/mol (Figure S2), which is comparable to the ΔG between tolfenamic acid polymorphs and is over 1 order of magnitude smaller than the ΔG between cisand transazobenzene. The relatively small ΔG of PEB suggests that PEB has a relatively large critical radius of nucleation (r*), the minimum nucleus size that can grow spontaneously. Thus, the large r* of PEB suppresses nucleation, which potentially explains the formation of supercooled PEB. Since external forces often trigger the crystallization of amorphous materials and crystal nucleation in liquids is known to take place within nanoseconds, we investigated the possibility of using a laser-induced shock wave with gigapascal peak pressure and nanosecond duration to induce the nucleation of supercooled PEB. The shock waves were generated by impingement of a high-energy Nd:YAG pulsed laser on a 400nm-thick aluminum energy-absorbing layer (see SI). The transferred laser power drives rapid expansion of the aluminum producing a high-amplitude stress wave, i.e., a shock wave, which propagates through the specimen and impacts the supercooled liquid. The peak pressure of a shock wave was controlled by systematic variation of the laser fluence (Figure S5). Interestingly, immediately after shock wave exposure, a nucleation site Scheme 1. (a) Depiction of Shock-Wave-Induced Crystallization of Supercooled PEB. (b) Chemical Structures of Supercooled Liquids Used in This Study and Their Corresponding Melting Points (mp) as Determined by Differential Scanning Calorimetry (DSC) Figure 1. Single-crystal X-ray structures involving PEB: (a) PEB1 and PEB2 rotamers, (b) alternating packing along a-axis, (c) column packing, and (d) cocrystal with OFN highlighting phenyl−perfluorophenyl interactions. Crystal Growth & Design Communication DOI: 10.1021/acs.cgd.6b01119 Cryst. Growth Des. XXXX, XXX, XXX−XXX B within the PEB liquid was observed with optical imaging (Figure 2), whereas no nucleation site was observed after 1 week for liquid PEB without impact. The powder X-ray diffraction (PXRD) patterns of the solidified PEB after shock wave impact are consistent with the predicted pattern from the single crystal X-ray data (Figure 3). No chemical change in the PEB sample was observed by NMR, UV−vis, or IR following shock wave impact (Figure S6). These findings suggest that the nucleation rate of supercooled PEB is significantly increased upon shock wave impact. Using the relationship = ∂