Hybrid Mechanism of Nucleation and Cooperative Propagation in a

Jun 25, 2018 - (3) In metals, martensitic transition(4−6) is well studied. ..... values as low as 7.3K and as high as 22.5K. As mentioned before, ...
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Hybrid Mechanism of Nucleation and Cooperative Propagation in a Single-Crystal-to-Single-Crystal Transition of a Molecular Crystal Hyunjoong Chung,† Christian Ruzie,́ ‡ Yves Geerts,‡ and Ying Diao*,† †

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Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana−Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, United States ‡ Laboratoire de Chimie des Polymères, Faculté des Sciences, Université Libre de Bruxelles (ULB), CP206/1, Boulevard du Triomphe, 1050 Brussels, Belgium S Supporting Information *

ABSTRACT: Martensitic transition is a type of solid-phase transition that involves a collective, rapid propagation of crystal structure change. In molecular crystals, such a transition is rarely observed, as most systems exhibit a nucleation and growth mechanism. Thus, the process and mechanism of martensitic transition are underexplored. Here we use in situ microscopy of organic single crystals complemented with interaction energy calculations to provide new insights on martensitic transition. We separate the transition process into three distinct steps where each step corresponds to propagation of structural change in a specific direction. We analyze an initiation stage and two propagation stages from hysteresis and propagation speed during cyclic transitions. We discover the dichotomous role of defects in facilitating the initiation step and hindering the propagation steps of transition. We conclude that the organic martensitic transition shows mixed mechanisms of nucleation and cooperativity. This study presents new experimental evidence of a rare phenomenon that will contribute to expanding the understanding of martensitic transition in molecular crystals.

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Understanding and harnessing cooperativity in molecular crystals has high application potential. Rapid and reversible mechanical motion22,23 in response to external stimuli can be utilized in designing micro actuators and smart materials. Our recent work15 demonstrated application of martensitic transition to organic electronic systems for next-generation multifunctional materials. We observed rapid electronic switching and function memory effect in addition to shape the memory effect possibly driven by the order-to-disorder transition of bulky side chains. Compared to inorganic, metallic counterparts, designing organic compounds exhibiting a martensitic transition has an advantage in the versatility of chemical synthesis and diverse intermolecular interactions. Therefore, a full understanding of the origin and mechanism of cooperativity in organic systems will help push this promising field beyond serendipitous discovery toward rational materials design. In this study, we investigate the mechanism of a single-crystalto-single-crystal (SCSC) cooperative polymorph transition of di-tert-butyl [1]benzothieno[3,2-b][1]1benzothiophene (ditBuBTBT), a high-performance organic semiconductor.24 Using variable temperature polarized optical microscopy (POM), we observe a multistep transition with directional propagation of phase boundary across the crystal. We propose a three-step mechanism of martensitic transition based on in situ POM observations and molecular interaction energy calculations.

artensitic transition is a unique type of solid-state phase transition driven by a collective propagation of molecules. According to Buerger’sclassification1 of solid-state phase transitions,2 it qualifies as a displacive transition: the phases are structurally similar and have a close orientational relationship before and after transition.3 In metals, martensitic transition4−6 is well studied. It is known for its low transition barrier, rapid kinetics, and reversibility. On the other hand, reports of martensitic transitions in organic systems remain rare, likely because of the larger size and less symmetrical shape of organic molecules. Although weak van der Waals and multipole interactions are conducive to abundant polymorphic transitions7 in organic systems, nucleation and growth8−11 is usually observed. Consequently, martensitic transition is rarely seen and usually observed serendipitously. Thus, there is a lack of quantifiable experimental evidence, severely hindering the understanding of the transition mechanism. Nonetheless, cooperativity in molecular crystals during martensitic transition exhibits intriguing behavior that greatly attracts the attention of the research community. Upon transition, the collective movement of molecules is macroscopically manifested as superelasticity,12,13 negative thermal expansion,14 shape memory effect,14−18 or thermosalient effect.19−21 These observations show that cooperativity of molecules can amplify angstrom-scale movement to micro- and millimeter-scale crystal motion. For instance, thermosalient crystals,20,21 or “jumping crystals”, exhibit a burst of movement over distances a thousand times their size due to abrupt phase transition. Some of these examples12,13,15,17,20,21 describe their observations as organic analogues of martensitic transition. © XXXX American Chemical Society

Received: March 26, 2018 Revised: June 22, 2018 Published: June 25, 2018 A

DOI: 10.1021/acs.cgd.8b00452 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 1. Three step mechanism of martensitic transition in ditBu-BTBT single crystals. (a) Molecular structure of ditBu-BTBT. (b) POM images from in situ POM upon cooling (HT to LT transition) that shows initiation, propagation 1 (P-1), and propagation 2 (P-2) steps. (c) Three neighboring pairs of ditBu-BTBT molecules calculated for interaction energy. The side and top views of crystal are shown, displaying three and one molecular layer, respectively. The a axis is the out-of-plane axis. A schematic of the crystal edges (not drawn to scale) is shown as a reference. (d) Display of the directionality of the proposed 3-step mechanism. The initiation step proceeds through Pair I molecules, the P-1 step through Pair II molecules, and P-2 step through Pair III molecules.

heating/cooling cycles at a constant heating and cooling rate of 2 K min−1. We observed solid−solid polymorphic transition at 356.1 K on average during heating and 341.2 K during cooling. This reversible solid-to-solid transition matched with previous differential scanning calorimetry (DSC) results in single crystals.15 Most of the transitions of the single crystal samples showed the appearance of a distinct phase boundary line that rapidly sweeps across the crystal (Movie S1). We observed clear birefringence color changes between blue and yellow, which was reversible throughout heating and cooling cycles. We note them as lower temperature (LT) and higher temperature (HT) forms, respectively. Although a faint propagation of the phase boundary layer has been observed before in an organometallic crystal25 and in other molecular crystals12,13 upon stress, no other system showed such a clear propagation behavior with a distinct phase boundary that separates the parent and daughter forms. As expected for a martensitic type transition system, ditBu-BTBT only has slight changes in the unit cell between the LT and HT forms. Both forms were solved with single crystal X-ray diffraction at 298 and 370 K, respectively, and the details of the two polymorphic structures are described in our

We decouple the initiation and propagation stages of the transition and investigate the mechanism of each step by analyzing the hysteresis and the propagation speed during cyclic transitions. We also study the role of defects and discover the nucleation and cooperative characteristics of martensitic transition observed in ditBu-BTBT single crystals.

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RESULTS AND DISCUSSION Multistep Transition Mechanism in ditBu-BTBT Single Crystals. Single crystals of ditBu-BTBT (Figure 1a) were fabricated by dropcasting on clean silicon substrates. Microliters of supersaturated solution was left under ambient conditions for more than 10 h for slow evaporation (see Methods for details). This method is capable of preparing single crystals as large as millimeters in size. In this study, we used single crystals ranging 50−400 μm in size with thicknesses of several microns. We employed POM optical microscopy (POM) equipped with a temperature controlled stage to directly observe thermally triggered phase transition in ditBu-BTBT single crystals in situ. We studied 32 single crystal samples subjected to multiple B

DOI: 10.1021/acs.cgd.8b00452 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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likely to proceed first along the Pair I direction, then the Pair II direction, and finally, the Pair III direction. Accordingly, we correlate Pair I to the initiation step, Pair II to the P-1 step, and Pair III to the P-2 step. Combining in situ POM observations with interaction energy calculations, we propose the following molecular picture of the stepwise transition (Figure 1d). The initiation step takes place out of plane through the thickness of the crystal along Pair I prior to P-1 and P-2 steps, most likely in a very rapid fashion. Unfortunately, we cannot directly observe the initiation step, due to the limitation of in situ POM. After the initiation step, transition occurs along the next lowest interaction energy pair, the Pair II direction. This can be correlated to the P-1 step, which we clearly observe under the microscope as the extension of the phase boundary line (Figure 1b). Consequently, the last propagation direction occurs along Pair III as the P-2 step. As the transition proceeds from initiation to P-1 and P-2, we can assume that the total number of molecules that transform cooperatively (in sync) increases. For instance, our observation indicates that P-1 may involve columns of molecules through the thickness of the crystal propagating to form an entire plane of molecules, as the phase boundary line extends to the edge of the crystal. P-2 may involve propagation of entire planes of molecules, as the fully extended phase boundary line propagates, as described in Figure 1d. In one of our crystals which is 6.7 μm thick, a column of molecules propagating during the P-1 step involves approximately 4600 molecules, while a molecular plane of width 120 μm propagating in the P-2 step involves approximately 1.56 × 109 molecules. The two-step propagation process shows that cooperative steps of the transition requires propagation of a significant number of molecules in sync. To the extent of our knowledge, this is the first study that presents a direct observation of a multistep martensitic transition process in molecular crystals. The proposed mechanism presents a new discovery of the details of an organic analogue of martensitic transition. Next we will discuss quantifiable parameters of the martensitic transition, hysteresis, and propagation speed, during initiation and propagation steps, to discern the transition mechanism of each step. Initiation Step and the Role of Defects. From in situ POM observations, we defined initiation to be when the phase boundary line first starts to form. In 16 out of the 32 pristine crystals, we were able to capture the initiation location in the first heating cycle. The initiation behavior in the first heating cycle is important because that is when the sample is in its most pristine state, before occurrence of additional defects induced from consequent thermal cycles. In the rest of the samples the initiation location could not be captured with our frame rate. Interestingly, in 15 out of the 16 samples, the initiation started from a corner of the crystal. In the one sample that did not initiate from the edge, there was a neighboring crystal that was pushing down on the crystal, which seemed to have caused the initiation from that location (Figure S2). This indicates that, without external factors, a pristine crystal favors initiation from a corner of the crystal. We reason that, compared to edge or bulk molecules, the corner molecules are at higher free energy state due to additional interfacial free energy, making them easier to transform first. A single crystal sample over multiple cycles showed a stochastic behavior with varying locations of initiation. In Figure 2a, the initiation occurred in various locations for a single crystal over 6 thermal cycles during heating. It occurred at a corner, a plane that cuts through the middle, or even from two opposite corners.

previous work15 and registered on the Cambridge Crystallographic Data Center as 1570908 and 1570909. Interestingly, in 10 out of 32 crystal samples, we observed a stepwise formation and propagation of the phase boundary line. An example from in situ POM is shown in Figure 1b. We first observe that the phase boundary line forms across the crystal, initiating from an edge (circled in Figure 1b) and fully extends to the opposite crystal edge. We denote the initiation and the extension of the phase boundary line across the crystal as the initiation step and the propagation step 1 (P-1), respectively (Figure 1b). Afterward, the phase boundary line propagates almost perpendicular to the P-1 direction, until a complete transformation of the crystal to the other polymorph. We denote this as the propagation step 2 (P-2). P-1 occurred very rapidly and was observed in only 10 out of 32 samples (maximum camera frame rate was 40 frames per second). We always observed, however, that the phase boundary line fully extends the entire length of the crystal before starting P-2 (Movie S1). In other words, the P-1 step is a prerequisite for the P-2 step. This behavior will be discussed in detail in later sections. Next, we investigated the possible mechanism underlying the stepwise transition. As discussed in our previous work,15 we pointed to the rotation of the bulky tBu side chains as the driving force for the observed cooperative transition. The bulky side chain rotation will affect the neighboring molecules in a collective fashion. We observed that in all single crystal samples from in situ microscopy, the phase boundary line always forms along a specific direction, verified by the constant angle measured between the phase boundary line and the crystal edge of approximately 80 degrees (Figure S1). In other words, the P-1 step is always occurring along a certain direction. Consequently, the P-2 step is also always occurring nearly perpendicular to the P-1 direction. From that and the stepwise behavior of the transition, we reason that the transition always follows a specific molecular pathway. We hypothesize that the directionality of the transition is dependent on the strengths of molecular interaction between neighboring molecules that are affected by the side chain rotation. The transition will start first in the direction of the lowest interaction energy, as it will experience the lowest free energy barrier for polymorphic transition. In other words, it takes less energy to break or alter the intermolecular bonds that are weak to create a phase boundary line. To evaluate this hypothesis, we calculated the intermolecular interaction energy between neighboring molecular pairs comparing different molecular pathways. In both LT and HT crystal lattices, each ditBu-BTBT molecule is surrounded by two identical molecules out-of-plane and six molecules in-plane. We label the out-of-plane pair as Pair I, and the two unique in-plane pairs as Pair II and Pair III (Figure 1c). We calculated the overall interaction energy of the three pairs of ditBu-BTBT molecules (see Methods for details). The calculated components of interaction energy in both polymorphs are given in Table S1. Similar trends in interaction energies were observed for both LT and HT forms, indicating that there were no significant changes to the interactions after the martensitic transition. For the three different molecular pairs calculated, Pair I had the lowest interaction energy of −7.0 ± 1.0 kJ mol−1. This is reasonable, as the molecular pair only interacts through the side chain and has no overlap of the conjugated core. Pairs II and III both have significantly higher interaction energy, of −28.7 and −38.7 ± 1.0 kJ mol−1, respectively, as both pairs exhibit significant π−π overlap from the herringbone packing. From these results, the transition is C

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Figure 2. Initiation characteristics and role of defects. (a) POM images of a single crystal through six heating cycles showing various initiation locations. (b) Comparison of initial and final cycle hysteresis in 30 crystal samples. All show a distinct decrease from the initial to the final cycle. (c) POM images of two needle-pricked crystals that show that initiation starts from the defects. (d) Comparison of the hysteresis as a function of thermal cycles for pristine and defected crystals. There is a distinct difference in the hysteresis between the two types, and a clear decrease in hysteresis for the pristine as number of cycles increase. POM images of a pristine and a defected crystal is given below.

The reason the initiation locations change from cycle to cycle may be defects (e.g., point defect, dislocations) that are formed during every transition. Martensitic transition involves a displacive movement of the molecules upon transition, which induces stress in the crystal. Thus, an increase in the number of thermal cycles naturally introduce defects into the crystal. The visible damages to the crystal after multiple cycles is shown in Figure S3. Similar to nucleation, the initiation locations in the ditBu-BTBT crystals appear in a stochastic manner. Further similarities between nucleation and the initiation step are discussed later. From frames captured during the initiation step, the hysteresis of a crystal sample was calculated as the difference between the heating and cooling transition temperatures per cycle. A higher hysteresis indicates a larger barrier for initiating the transition since larger superheating or undercooling from the thermodynamic equilibrium transition temperature is required to start the phase transition. A wide range of hysteresis existed from sample to sample in the first cycle, ranging from values as low as 7.3K and as high as 22.5K. As mentioned before, the first-cycle hysteresis provides information on the quality of pristine crystals, since subsequent hysteresis values are affected by new defects introduced from thermal cycling. We compared POM images between crystals with the highest and lowest firstcycle hysteresis and observed a distinct difference in crystal quality: the crystal with the maximum hysteresis had an almost visually defect-free surface, while the one with minimum hysteresis had a highly defective surface (Figure S4). We can infer

that the more defective the crystal is, the smaller the hysteresis and the lower the initiation barrier. In other words, it is easier for martensitic transition to initiate on a defective crystal. This resembles the role of defects in nucleation.11,26,27 For instance, Mnyukh’s experimental observations with p-dichlorobenzene single crystals27 showed that crystal defects serve as nucleation sites and that less defective crystals require larger overheating or undercooling, in agreement with what we observed in ditBuBTBT single crystals. Another observation to support the idea that defects facilitate the initiation step was that all 32 crystal samples showed a decrease in hysteresis between the first and the last cycle (Figure 2b). By naturally introducing defects from thermal cycling, a detailed investigation of the role of defects in martensitic transition was possible. The first and last cycle hystereses showed Gaussian distributions and a 7° decrease of the mean from the first to the last cycle (Figure S5). It was evident that hysteresis decreased as more defects were introduced at later cycles. For instance, a crystal that displayed a large difference between the first and the last cycle hysteresis, 17.7K and 7.1K, respectively, showed significant damage after cyclic transitions than in the initial state (Figure S6a). In comparison, for a sample that had a small difference between the first and last cycle hysteresis, 19.1K and 13.5K, the crystal did not exhibit visible defects (Figure S6b). In line with the previous observation, this trend indicates that more defects result in a decrease in transition temperature hysteresis and the initiation free energy barrier to the transition. We can speculate that defects, which D

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Figure 3. Propagation characteristics and role of defects. (a) Comparison of the P-1 and P-2 propagation speeds observed from in situ microscopy. (b) Comparison of the initial and final cycle P-2 propagation speed in selected crystal samples. These samples were selected for their similar range of speed values. Only the propagations occurring along the same direction were compared. All show a significant decrease from the initial to the final cycle. (c) POM images of two different pristine crystals with naturally occurring defects that appeared in later cycles (d) POM image of a artificially mechanically damaged crystal, where the defects were induced by syringe needle. It shows a drastic decrease in propagation speed as the phase boundary line travels through the defects.

can be macroscopic cracks as well as molecular level dislocations, point defects, and vacancies, help trigger martensitic transition, similar to the case of nucleation. To verify the role of macroscopic defects on the initiation step, we introduced new defects to the crystal by pricking it with a syringe needle. Defects were introduced to four different crystal samples, and two are shown in Figure 2c with the defects circled. Consistent with our hypothesis that defects initiate transition, the initiation steps occurred in the proximity of the defects in all four defected crystals. In one crystal, several initiating phase boundary lines originated from the defect in the middle (Figure 2c top). Also, we were able to observe a clear three-step mechanism with much slower P-1 and P-2 speeds (Figure S7, Movie S2). This observation again validates the directional characteristic of each step of transition. The defects also played a role in slowing down the propagation speed overall, allowing us to directly observe the process in multiple frames. We will discuss the effect of defects on the propagation step later. We also compared the hysteresis between the needle-pricked and pristine crystals over multiple cycles. When comparing the first cycle hysteresis, the pricked crystals showed distinctively smaller values with an average of 5.6 ± 1.7 K while the average of the pristine crystals was 16.4 ± 3.3 K (Figure S8). The much smaller hysteresis of the pricked crystals prove that the introduced defects are helping initiation and therefore lowering the initiation free energy barrier. The hysteresis trend for multiple cycles also prove this point. As shown in Figure 2d, the pricked crystals did not show a decrease in their hysteresis up to 8 cycles, whereas the pristine crystals (four crystals selected for better visualization) show a clear decreasing trend. This indicates that the pricked crystals, which have a relatively higher defect density than the pristine crystals, have a low

initiation barrier from the beginning that does not change in later cycles. On the other hand, the pristine crystals have a higher initiation barrier that decreases as more defects form in later cycles. This point is also proven by the fact that in the pricked crystals, the initiation locations consistently emanated from the defects (Figure S9), whereas in pristine crystals, the locations varied as more defects formed in later cycles (Figure 2a). All of these results indicate that the initiation step of martensitic transition resembles nucleation. We note that although nucleation in martensitic transition of metal alloys have been well studied,4,28,29 it has not been shown experimentally in organic systems that exhibit martensitic transition. Propagation Steps and Role of Defects. Next, we investigate the mechanism of propagation by quantifying the propagation speeds via in situ POM. Our observation shows reversible propagation of the phase boundary line over multiple cycles. P-1 propagation speed was much faster than P-2, so that in most cases, P-1 speed was not detectable with our frame rate. For the cases when we detected the speed of P-1, we compared it to the measured P-2 speeds (Figure 3a). The distribution of the P-1 and P-2 speeds are also given in Figure S10. P-1 speeds reached a maximum of up to 6787 μm/s while maximum P-2 speed was 807 μm/s. P-1 speeds measured were usually orders of magnitude higher than those of P-2. We can infer that this is due to the fact that P-1 may involve columns of molecules in sync during propagation, while P-2 involves whole planes of molecules (Figure 1c). As discussed previously, the number of molecules involved in a plane of cooperative molecules (∼109 molecules) is more than 5 orders of magnitude higher than a column of molecules (∼4600 molecules). The fact that P-1 is much faster and that it occurred prior to P-2 indicates that the free energy barrier for P-1 propagation is smaller than that of P-2. E

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1.75 μm s−1 while the speed afterward was 18.6 μm s−1. Natural or artificial, defects had the same effect in slowing down the overall martensitic transition process. Moreover, we again observed that the delay is predominantly caused by the cracks in the path of the P-1 step. Interestingly, in the needlepricked crystal, P-1 propagation eventually extends to the end by overcoming the defect, indicating that the defect did not completely hinder the P-1 propagation as was in the case of Figure 3c (Movie S3). These observations indicate that, depending on the nature of cracks, they can completely block or slow down the propagation process. The former case is shown in Figure 3c when the defects prevent the full extension of the phase boundary line during the P-1 step. This forces the propagation to continue to the P-2 step with a partial plane that does not extend through the entire crystal. The latter case is when the crack slows down the P-1 step as shown in Figure 3d and Movie S3. It is also observed in the significant decrease in P-2 speed with the increase in number of cycles (Figure 3b). In both cases, the role of defects validate the cooperative nature of propagation.

We note that the measured P-1 speeds are several orders of magnitude smaller than the propagation speeds expected from reported martensitic transition systems, which are on the order of the speed of sound4 (i.e., 4000 m/s in polycrystalline ice30). But we note that, we were not able to obtain the first cycle P-1 speeds, as most of the P-1 speeds measured were from later thermal cycles. This indicates that they were measured after a significant decrease in speed. Almost all first cycle P-1 speeds were not detected from our experiments due to the limitation of the camera frame rate. With a much slower propagation speed, the P-2 speed was detected in 30 out of 32 pristine crystals and 4 out of 4 needlepricked crystals over multiple thermal cycles. Because of its common occurrence, P-2 speed was a crucial parameter that we analyzed in detail, similar to hysteresis. We note that the local speeds of the phase boundary line also varied, as shown in Figure S11, but the average speed was used in all samples. Similar to how we investigated the effect of defects on the initiation barrier via hysteresis, we can study the effect of defects on propagation speed over multiple cycles to obtain information on the propagation free energy barrier and the cooperative nature of propagation. A drastic decrease in speed was observed from the initial to the final cycle in 42 out of 45 samples studied, which are displayed in Figure 3b. Note that the number of samples here do not correlate to number of crystals, as oftentimes multiple P-2 speed data was obtained per crystal (details in methods). The distributions of the initial and final cycle P-2 speeds are also given in Figure S12. The decrease in speed was significant, as seen from Table S2, where more than half of the total samples showed an order of magnitude or more reduction in speed. From that we can conclude that the defects are significantly slowing down P-2 propagation. Cooperative movement requires entire layers of molecules to propagate across the length of the crystal, so that crystal defects that are in the path of propagation will disturb the concerted movement and cause the process to slow down. As more defects form over more cycles, the speed continues to slow down since more defects have to be overcome during propagation. Therefore, this observation validates the cooperative nature of propagation. We directly observed that macroscopic defects slow down the propagation speed. When a crack formed near the forming phase boundary line, it prevented the propagation in the P-1 step, forcing the process to continue to the P-2 step (Figure 3c). The significant amount of time that the crystals take in the proximity of the cracks indicates that the propagation energy barrier is increased. For both crystals shown in Figure 3c, the crack prevented the full extension of the phase boundary line during the P-1 step and caused a significant delay. The observation suggests that defects can obstruct the completion of the P-1 step and consequently delay the P-2 step, validating the cooperative nature of the P-1 and P-2 steps. These observations also provide further evidence of the sequential nature of the stepwise mechanism, specifically between the P-1 and P-2 step. To confirm the role of defects in the propagation steps, we investigated the P-2 speed in the needle-pricked crystals. As expected, the defects induced by syringe needles had the same effect as the cracks and significantly slowed down the P-2 speed (Figure 3d). The transition time was 14.3 s as the phase boundary line passed through the defects, and only 2.9 s for the same propagation distance without visible damage. The propagation speed through the defects was calculated to be

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CONCLUSIONS In conclusion, we propose a hybrid mechanism of nucleation followed by two-step cooperative propagation during a singlecrystal-to-single-crystal transition of ditBu-BTBT. Such a mechanism is proposed on the basis of analyzing the transition kinetics, transition temperature hysteresis, and their relationship with macroscopic defects induced either by cyclic transitions or by mechanical force. The mechanism is further substantiated with calculated pairwise interaction energies of each transition step. Specifically, we discovered a stepwise mechanism consisting of initiation, P-1, and P-2 along three different directions, taking place in a sequence from the step with lowest pairwise interaction energy (initiation) to the one with the highest interaction energy (P-2). We determined that the initiation process resembles nucleation in that more defects help lower the initiation energy barrier. For the subsequent propagation steps, P-1 and P-2, defects act as barriers for phase transition, slowing down the phase transition and thus validating the cooperative nature of propagation. The study provides new experimental evidence of a rare organic martensitic transition containing mixed mechanisms of nucleation and cooperative propagation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b00452. Methods section with associated figures and tables (PDF) Direct observation of the three-step mechanism during the second cooling cycle of a crystal sample. Note that in the P-1 step, the phase boundary line completely extends to the ends of the crystal before the P-2 step starts. (AVI) Direct observation of the three-step mechanism in a defected crystal. The in situ POM video is from the sixth heating cycle of a crystal with defects induced by a syringe needle. A clear directionality exists from the P-1 step and the P-2 step. (AVI) Direct observation of the transition from the P-1 to the P-2 step in a defected crystal. The P-2 step can only F

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(16) Yao, Z.-S.; Mito, M.; Kamachi, T.; Shiota, Y.; Yoshizawa, K.; Azuma, N.; Miyazaki, Y.; Takahashi, K.; Zhang, K.; Nakanishi, T.; Kang, S.; Kanegawa, S.; Sato, O. Molecular motor-driven abrupt anisotropic shape change in a single crystal of a Ni complex. Nat. Chem. 2014, 6 (12), 1079−1083. (17) Karothu, D. P.; Weston, J.; Desta, I. T.; Naumov, P. ShapeMemory and Self-Healing Effects in Mechanosalient Molecular Crystals. J. Am. Chem. Soc. 2016, 138 (40), 13298−13306. (18) Su, S.-Q.; Kamachi, T.; Yao, Z.-S.; Huang, Y.-G.; Shiota, Y.; Yoshizawa, K.; Azuma, N.; Miyazaki, Y.; Nakano, M.; Maruta, G.; Takeda, S.; Kang, S.; Kanegawa, S.; Sato, O. Assembling an alkyl rotor to access abrupt and reversible crystalline deformation of a cobalt(II) complex. Nat. Commun. 2015, 6, 8810. (19) Liu, G.; Liu, J.; Ye, X.; Nie, L.; Gu, P.; Tao, X.; Zhang, Q. SelfHealing Behavior in a Thermo-Mechanically Responsive Cocrystal during a Reversible Phase Transition. Angew. Chem., Int. Ed. 2017, 56 (1), 198−202. (20) Skoko, Ž .; Zamir, S.; Naumov, P.; Bernstein, J. The Thermosalient Phenomenon. “Jumping Crystals” and Crystal Chemistry of the Anticholinergic Agent Oxitropium Bromide. J. Am. Chem. Soc. 2010, 132 (40), 14191−14202. (21) Sahoo, S. C.; Panda, M. K.; Nath, N. K.; Naumov, P. Biomimetic Crystalline Actuators: Structure-Kinematic Aspects of the Self-Actuation and Motility of Thermosalient Crystals. J. Am. Chem. Soc. 2013, 135 (33), 12241−12251. (22) Sun, Y.; Lei, Y.; Dong, H.; Zhen, Y.; Hu, W. Solvatomechanical Bending of Organic Charge Transfer Cocrystal. J. Am. Chem. Soc. 2018, 140 (20), 6186−6189. (23) Commins, P.; Desta, I. T.; Karothu, D. P.; Panda, M. K.; Naumov, P. Crystals on the move: mechanical effects in dynamic solids. Chem. Commun. 2016, 52 (97), 13941−13954. (24) Schweicher, G.; Lemaur, V.; Niebel, C.; Ruzie, C.; Diao, Y.; Goto, O.; Lee, W. Y.; Kim, Y.; Arlin, J. B.; Karpinska, J.; Kennedy, A. R.; Parkin, S. R.; Olivier, Y.; Mannsfeld, S. C. B.; Cornil, J.; Geerts, Y. H.; Bao, Z. N. Bulky End-Capped [1]Benzothieno[3,2-b]benzothiophenes: Reaching High-Mobility Organic Semiconductors by Fine Tuning of the Crystalline Solid-State Order. Adv. Mater. 2015, 27 (19), 3066−3072. (25) Panda, M. K.; Runcevski, T.; Sahoo, S. C.; Belik, A. A.; Nath, N. K.; Dinnebier, R. E.; Naumov, P. Colossal positive and negative thermal expansion and thermosalient effect in a pentamorphic organometallic martensite. Nat. Commun. 2014, 5, 1 DOI: 10.1038/ ncomms5811. (26) Jiang, Q.; Shtukenberg, A. G.; Ward, M. D.; Hu, C. NonTopotactic Phase Transformations in Single Crystals of β-Glycine. Cryst. Growth Des. 2015, 15 (6), 2568−2573. (27) Mnyukh, Y. Mechanism and kinetics of phase transitions and other reactions in solids. Am. J. Condens. Mater. Phys. 2013, 3 (4), 89−103. (28) Olson, G. B.; Cohen, M. A mechanism for the strain-induced nucleation of martensitic transformations. J. Less-Common Met. 1972, 28 (1), 107−118. (29) Cech, R. E.; Turnbull, D. Heterogeneous Nucleation of the Martensite Transformation. JOM 1956, 8 (2), 124−132. (30) Ruocco, G.; Sette, F.; Bergmann, U.; Krisch, M.; Masciovecchlo, C.; Mazzacurati, V.; Signorelli, G.; Verbeni, R. Equivalence of the sound velocity in water and ice at mesoscopic wavelengths. Nature 1996, 379, 521.

begin after a full extension of the phase boundary line during the P-1 step. The video is obtained from the fourth heating cycle of a defected crystal. (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yves Geerts: 0000-0002-2660-5767 Ying Diao: 0000-0002-8984-0051 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the startup funds of University of Illinois. This work was conducted in part in G.L. Clark X-ray facility at the University of Illinois at Urbana− Champaign. C.R. and Y.G. thanks the financial help of the Walloon Region (WCS Project No. 1117306) and from the Belgian National Fund for Scientific Research (FNRS-Project No. T.0058.14).

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DOI: 10.1021/acs.cgd.8b00452 Cryst. Growth Des. XXXX, XXX, XXX−XXX