Visualization of Single-Crystal-to-Single-Crystal Phase Transition of

Jun 19, 2018 - Single-crystal-to-single-crystal (SCSC) phase transition is an ideal model to study the structural correlation between the polymorphs i...
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Visualization of Single-Crystal-to-Single-Crystal Phase Transition of Luminescent Molecular Polymorphs Chao Ge, Jie Liu, Xin Ye, Quanxiang Han, Leilei Zhang, Shuangyue Cui, Qing Guo, Guangfeng Liu, Yang Liu,* and Xutang Tao* State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, P. R. China

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S Supporting Information *

ABSTRACT: Single-crystal-to-single-crystal (SCSC) phase transition is an ideal model to study the structural correlation between the polymorphs at the molecular level. In this regard, a transition process with concomitant emission color change is in favor of direct visualization by fluorescence microscope. Here we report an SCSC transition on the luminescent single crystal of large conjugated molecules, which is accompanied by a drastic luminescence color change from red to orange upon heating. The transition process was clearly recorded under both fluorescence and polarized light. Combining with crystallographic analysis, the results indicate that the existence of molecular layers and the oriented motion of the interface between the daughter and the parent phase preserved the integrity of the single crystal, despite of remarkable changes of both conformational and supramolecular structure. Thus, the transition is rationalized to proceed by a nucleation-and-growth mechanism but not a martensitic one. This work on one hand delivers intuitive cognition about the polymorph-dependent optical properties and the mechanism of the phase transition between the polymorphs and, on the other hand, also proved the paramount importance of direct microscopy observation about the actual transition process.

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INTRODUCTION Luminescent organic crystals have attracted great attention in recent years. Besides their application perspectives in the fields of organic lasers, optical waveguides, and light-emitting transistors, etc., the ordered packing of molecules in single crystals definitely provides a direct picture to investigate molecular conformation and intermolecular interactions.1−10 Among the great number of explored luminescent organic single crystals, the phenomenon of polymorphs, in which multiple forms or crystal structures of a compound can exhibit totally distinct physical and/or chemical properties, has drawn special interest. The concurrent existence of polymorphs has long been recognized.11−14 Considering the fact that different polymorphs are composed of molecules with the same molecular structures, they are now regarded as ideal models for disclosing the factors governing structure−property relationships of organic luminescent materials.15−17 Furthermore, the phase transition between polymorphs in response to external stimuli, accompanied by switched fluorescence, has also been widely explored recently. Light, heat, exposure to vapors, and mechanical stress can trigger the transition.18−26 While because organic crystals are generally fragile and easily collapsed under external perturbations, preservation of the integrity of a single crystal during the transition process is not an easy case. Compared with monolithic single crystals, polycrystalline powders are more prone to interact with external stimuli to switch their structures.27−30 Thus, despite of © XXXX American Chemical Society

the abundant reported cases of stimuli-responsive materials based on polymorph transitions, the single-crystal-to-singlecrystal (SCSC) transitions are still rarely discovered.31−33 An important achievement in this field was made on gold(I) complexes. Alternations in aurophilic interactions triggered by vapor exposure,34−36 or even applying a small mechanical force responded for the occurrence of the SCSC.37,38 In virtue of the luminescence color change accompanied by the process of the phase change, the SCSC progression was clearly visualized.37,38 The SCSC phase transitions were also found on pure organic diphenyldibenzofulvene and quinolone/1,8-naphthyridine derivatives, which show polymorphism-dependent emissions.39,40 Generally speaking, the discovery of luminescent polymorphs that can undergo SCSC phase transition is still rare and random. There are scarcely ever studies about the actual SCSC phase transition directly viewed by microscopic and fluorescent techniques in order to infer the mechanism of the transition process. The treatment of phase transitions, which is traditionally limited to metals and alloys, gave rise to many doubts about its applicability of classical dynamical process when meeting to molecular crystals.41,42 This confusion is even more serious for the luminescent organic crystals being comprised of large conjugated and rigid molecules, making Received: May 2, 2018 Revised: June 19, 2018 Published: June 19, 2018 A

DOI: 10.1021/acs.jpcc.8b04164 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. Structures and polymorph-dependent luminescence of TPE-functionalized dicyanopyrazine derivatives. (a) Chemical structures of 1DQCN, 2DQCN and 3DQCN. (b) Fluorescent photographs of the crystalline and amorphous phase of 1DQCN and its mechanochromic behavior. (c) Steady emission spectra and (d) transient lifetime spectra of 1DQCN in crystalline and amorphous states. (e) Fluorescent photographs of the crystalline and amorphous phases of 3DQCN and its mechanochromic behavior. (f) Steady emission spectra and (g) transient lifetime spectra of 3DQCN in crystalline and amorphous state.

3DQCN is indeed more capable to crystallize, whose crystals were attained by slow evaporation from a methanol/dichloromethane solution. The fluorescence behavior in different solidstate morphologies of 3DQCN has some similarities with that of 2DQCN, but is a sharp contrast to that of 1DQCN. As shown in parts b and e of Figure 1, the green-emissive 1DQCN crystal became orange-emissive in the amorphous state; while the red-emissive 3DQCN crystal became yellow-emissive in the amorphous state. This indicates that the molecule with two TPEs demonstrates crystallization induced blue- shift emission (∼36 nm) while the molecule with one TPE shows opposite crystallization induced red-shift emission (∼75 nm). The morphology-dependent fluorescence property of the transient lifetime also shows the diametrically opposite trend between the two compounds. The blue-shift 1DQCN crystal possesses shorter fluorescent lifetime than that of its amorphous counterpart; while the red-shift 3DQCN crystal has longer lifetime than that of its amorphous counterpart. (Detailed lifetime data are shown in Table S1 of the Supporting Information.) Considering the similarity of the fluorescence between the two compounds in amorphous state, wherein the molecules are relative isolated and free compared with the case in crystals, both of the steady state and the transient fluorescence characteristics suggest a different molecular packing motif in their respective crystals that governs the fluorescence property. The stacking in the 3DQCN crystal should be more severe than that in 1DQCN crystal, but this interaction in-reverse results in less nonradiative decay

the molecular motion mechanism under the solid-state phase transitions to be a terra incognita. Thus, in this work, we systemically studied a SCSC phase transition based on single crystals with high contrast luminescence in different polymorphs by direct microscopy observation. The obvious luminescence revolution in a SCSC transition on one hand facilitates the discrimination of the interface between different polymorphs and on the other hand provides a platform to correlate the molecular conformation and packing motif with the luminescence characteristics and, moreover, to correlate their changes with each other.

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RESULTS AND DISCUSSION In our last work, we have investigated two dicyanopyrazine derivatives 1DQCN and 2DQCN (Figure 1).22 Polymorphs with high contrast luminescence were realized by functionalizing large conjugated and electron-withdrawing dicyanopyrazine unit with two propeller-like tetraphenylethylenes (TPEs), which also demonstrated rapid conversion of fluorescence when amorphization. However, the crystallization was difficult to be implemented due to their large scale and rigid molecular structures, especially for 2DQCN. Thus, we further designed and synthesized 7-(4-(1,2,2-triphenylvinyl)phenyl)dibenzo[f,h]quinoxaline-2,3-dicarbonitrile (3DQCN), a molecule with a planar dicyanopyrazine but only one tetraphenylethylene (Figure 1). Detailed synthesis and characterization information are presented in the Supporting Information. B

DOI: 10.1021/acs.jpcc.8b04164 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. Molecular packing motifs in 1DQCN and 3DQCN crystals. (a) Side view of the columnar stacking array in 1DQCN. (b) Antiparallel molecular pairs bonded by π−π and C−N···π in 1DQCN. (c) Overlap of molecular pairs in 1DQCN. (d) Side view of the layered columnar stacking array in 3DQCN. (e) Antiparallel molecular pairs bonded by π−π and C−N···π in 3DQCN. (f) Overlap of molecular pairs in 3DQCN. Hydrogen atoms are omitted for ease of observation.

planes is 3.258 Å (the other one is 3.287 Å), indicating strong π−π stacking interactions (Figure 2d). In addition to the π−π interactions, the columnar stacking is further stabilized by weak C−N···π interactions with a distance of 3.734 Å (Figure 2e). Obviously, the intermolecular π−π interactions get to be a major difference in determining the crystalline molecular stacking between the two materials. This difference is considered to play an important role in the morphologydependent fluorescence. To deeply understand the microscopic structure to help explain the contrast between their fluorescence behaviors, we also calculated the optimized geometry of the two molecular structures by density functional theory and compared them with the respective single molecular configuration in crystals (Figure S1 in the Supporting Information). The optimized 1DQCN shows two symmetrical arms with dihedral angles of 34.96° and 36.29°, and the dihedral angle in the optimized 3DQCN connecting TPE and the fused plane is 36.29°. (The dihedral angles in TPE were not counted considering its similar severe twisted configuration.) In crystals the dihedral angles in 1DQCN are not symmetrical and are ranging from 30.88° to 39.70°, and in 3DQCN is 33.17°. These data manifest two issues: one is the similarity of the fluorescence between the two materials in amorphous state stemming from the similarity in the isolated molecular configuration; the other is 1DQCN taking a relative distorted configuration to fit into the crystalline lattice compared to that in the amorphous state, while 3DQCN taking a more coplanar conformation in crystals than in its isolated state. Thus, combing the above analyzed molecular packing motif in crystals, we can conclude that 1DQCN packs loosely in crystals (no π−π interactions and only weak C−N···π interactions) and adopts a more twisted conformation; once subjected to external force, the crystalline lattice is easy to destroy and is accompanied by conformation planarization. This leads to the occurrence of mechanochromic luminescence in 1DQCN and also a crystallization induced blue-shifted emission. For 3DQCN, the large overlapped and

pathways in the 3DQCN crystal. This inference can be evidenced by another contrast between the two kinds of crystals. The green-emissive 1DQCN crystal turns into orangeemissive amorphous powders by mechanical grinding. However, grinding on the red-emissive 3DQCN crystal makes neither an amorphization nor a change on its emission. The different mechanochromic luminescence behavior also implies a strong intermolecular force and close packing in the 3DQCN crystal. The moderate crystallization capability of both 1DQCN and 3DQCN enables us to directly unveil the detailed molecular arrangements in crystals through conducting single crystal Xray diffraction (SCXRD) analyses. As depicted in Figure 2, SCXRD analysis of 1DQCN displays a staggered and antiparallel packing mode between the two adjacent molecules, in which the stacked molecules alternately take the opposite orientation. Because each molecule possesses two bulky propeller-like TPEs, most parts of the molecules have no overlap between each other, except for the dicyanopyrazine units. The dicyanopyrazine cores overlapped well, forming a columnar stacked array along the a axis, with a shortest distances of 4.461 Å (the other face-to-face distance in the dicyanopyrazine columnar is 5.792 Å) (Figure 2a), indicating there is almost no π−π interactions. (A general standard π−π distance is considered to be less than 3.5 Å.) The main crystallization force is the weak C−N···π interactions (3.605 Å) between the cyano group and the phenyl ring of the adjacent molecules (Figure 2b). In 3DQCN, SCXRD analysis also reveals an antiparallel packing mode between the two adjacent molecules (Figure 2d). Because of the fused coplanar dibenzoquinoxaline-2,3-dicarbonitrile planes and only one bulky TPE in each molecule, the face-to-face overlapped fraction is large in the formed columnar stacked array along the a axis. As compared in Figure 2, parts c and f, the overlap ratio of the π−π stacking in 3DQCN is 48%, apparently larger than that of 25% in 1DQCN. The shortest face-to-face distance between the adjacent dibenzoquinoxaline-2,3-dicarbonitrile C

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Figure 3. SCSC phase transition from 3DQCNred to 3DQCNorange upon heating. (a) Photographs of 3DQCN crystals before and after phase transition under day light, UV light and polarized light (scale bar, 100 μm). (b) Transition process from 3DQCNred to 3DQCNorange observed under UV light (scale bar, 50 μm). (c) Transition process from 3DQCNred to 3DQCNorange observed under polarized light (scale bar, 50 μm). The arrows indicate the initial points of the phase transition.

close face-to-face stacking and strong π−π interactions in its crystals ensure its higher lattice energy. Thus, external force cannot destroy the crystalline unit cell. And the strong intermolecular π−π interactions, plus the relative coplanar conformation in crystals, should be also responsible for the crystallization induced red-shifted emission. These findings demonstrate the immediate relationship between the packing structure and the optical performances, which provides property modulation via morphology control. However, with robust crystal packing and no mechanochromic luminescence ability of 3DQCN, its crystals were found to be phase-transformable under thermal treatment. Most surprisingly, the phase transition occurring on the 3DQCN single crystal can go through an SCSC type of change without collapse of the single crystal integrity. The process of the SCSC transition can be visualized clearly under polarized light. Thanks to the morphology-dependent fluorescence property, the process can be witnessed by the accompanied photoluminescence color change. As shown in Figure 3a, a needlelike 3DQCN single crystal was subjected to a phase transition when it was heated from room temperature to 270 °C. After phase transition, the shape and integrity of the single crystal were preserved; however, both the color under day light and polarized light and the fluorescence changed obviously. The color of the crystal shallowed and its original red luminescence became orange under UV light. We defined the red luminescent 3DQCN crystals as a 3DQCNred form, and the orange luminescent 3DQCN crystals after phase transformation as a 3DQCNorange form. Figure 3b shows the fluorescence images of the transition process. The new phase of 3DQCNorange occurred from one end of the crystal, with reduced emission intensity compared to that of the 3DQCNred form. An interface between the two phases can be observed clearly. Along with the proceeding of the phase transition the interface traverses toward the other tip of the crystal, and the 3DQCNorange domain gradually expands, ending with the entire crystal being the 3DQCNorange form in less than 3 min.

A more detailed transition process was recorded by polarized light microscopy. The birefringence difference shows clear phase interface and interface motion trajectory. As shown in Figure 3c, the location of the birthplace of the daughter phase was not one end of the crystal, but was from the middle section. And two starting points arose simultaneously on one crystal. The interface plane is regular, and keeping smooth in the whole process, indicating that the interphase should be a crystallographic plane in the daughter phase. While the orientations of the two daughter phase domains are different. The daughter phase individually spreads by migration of its well-defined fronts in a parallel heading direction. Finally, the two daughter phase domains join together to complete the phase transition in 7 min. In the subsequent X-ray single crystal diffraction analysis, this crystal, after phase transition, was seemed to be a twin crystal but not a single crystal. This demonstrates the orientation of the daughter phase is defined at the nucleation stage, and there should be orientational correlation between the daughter and the parent phase. One may note that the duration of the phase transition varied a lot for different single crystals. In order to verify the reproducibility of the phase transitions, we present detailed information on the phase transition on 20 single crystals in Table S3, including fluorescent photographs before and after phase transition, propagation length, initial temperature of the transition, duration time, and the corresponding velocity. We can see the propagation rate range from 0.16 to 3.08 μm/s. This propagation speed is much lower than the hundreds of μm/s of martensitic transition in a newly reported system.31 The dispersion in transition rate is found to be dependent on the crystalline quality, the crystal size and even the heating rate. For crystals with high degree of crystal perfection, heating in a slow rate (e.g., < 10 °C/h) cannot trigger the phase transition. That is why 3DQCNred crystals are stable in ambient environment. Even an ongoing transition process will be halted when heating is removed; and the phase transition resumes by heating anew (Movie S1 and S2 in the Supporting D

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Figure 4. Thermal property of 3DQCN and the luminescence property before and after phase transition. (a) DSC curves of 3DQCNred under a rate of 20 °C min−1 in two heating cycles. The black dashed frame corresponds to the phase transition from 3DQCNred to 3DQCNorange. The blue dashed frame represents where polychromatic wave was found. (b) Steady emission spectrum and (c) transient lifetime spectrum of 3DQCNred and 3DQCNorange..

Information). In contrast, larger heating rate (e.g., > 20 °C/ min) can accelerate the phase transition process. To better understand the thermodynamic relations between 3DQCNred and 3DQCNorange, differential scanning calorimetry (DSC) measurements were conducted on the crystals of the two forms. As shown in Figure 4a, upon heating of 3DQCNred crystals, the DSC curve showed broad exothermic peaks from 245 to 270 °C, which are confirmed to be corresponding to the phase transition from 3DQCNred to 3DQCNorange after checking the samples left in the crucible. The broad DSC peaks are caused by the broad size and/or crystalline quality distribution of the crystalline samples, which coincides the aforementioned observations on the transformation rate and duration disparity for different crystals under hot stage microscopy. The enthalpy change ΔH of the phase transition obtained from the DSC data is −7.08 kJ mol−1. The entropy change ΔS at the transition temperature, according to the equation ΔS = ΔH/T, is −13.38 J mol−1 K−1. According to the Boltzmann formula for the statistical entropy S = kln N, where k is the Boltzmann constant being of 1.381 × 10−23 J K−1 and N is the number of microstates in the system, we could estimate N for 3DQCNorange to be 0.2 when assuming N = 1 for 3DQCNred in our system. This indicates in theory the orange phase is thermodynamically more favored than the red phase. According to our former study about an oriented SCSC phase transition initiated by mechanical stimulation, such a thermal characteristic, being without characteristic transition temperature, normally indicates transition from a metastable monotropic phase to the stable phase, but not enantiotropic phase transition.43 Beyond the broad phase transition peaks, another small exothermic peak at about 240 °C appears ahead. Hot stage microscopy observations reveals obvious polychromatic wave rolling over the crystal at the same temperature range (Figure S2 in Supporting Information). This phenomenon was observed before the phase transition on

almost every crystal, but at present we can conclude no further inference for this state. Upon cooling, or second heating on 3DQCNorange crystalline sample, there is no heat flow peak on the DSC curve. This demonstrates that the phase transition from 3DQCNred to 3DQCNorange is indeed monotropic, and 3DQCNorange form is thermodynamically more favored than 3DQCNred. The specific fluorescence properties of 3DQCNorange form were measured and compared with those of its parent 3DQCNred form. As shown in Figure 4b, 3DQCNorange exhibited an emission maximum at 586 nm, leading to a blue shift compared to 3DQCNred. This emission wavelength is still longer than that of 3DQCN in amorphous state (562 nm). However, the emission lifetime of 3DQCNorange, as shown in Figure 4c, is shorter than that for both 3DQCNred and 3DQCN in the amorphous state. (Detailed lifetime data are shown in Table S1 of the Supporting Information. The absorption and excitation spectra of 3DQCN red and 3DQCNorange are shown in Figure S3.) The blue-shift emission and shorter lifetime of 3DQCNorange indicate a probable weaker π−π stacking but more severe nonradiative decay than in 3DQCNred. The crystalline sample after phase transition (3DQCNorange) has enough quality for SCXRD analysis, which on one hand confirms the SCSC phase transition, and more importantly enables us to obtain concrete information about the changes during the SCSC transition in molecular level on the same crystal. SCXRD analysis reveals the crystal structure of 3DQCNorange being of triclinic space group P1̅, which is different from that of its parent form 3DQCNred (Orthorhombic space group Pbca, Table S2 in the Supporting Information). The structural symmetry of the resultant phase decreases after the SCSC transition. As depicted in Figure 5 and described vide supra, 3DQCNred takes an antiparallel packing mode between every two adjacent molecules, which E

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Figure 5. Crystal structure changes of the SCSC phase transition. (a) Orientation relationship of crystal packing in 3DQCNred and 3DQCNorange upon SCSC transition defined by SCXRD face indexing. (b) Side view of the layered columnar stacking array in 3DQCNred. (c) Antiparallel molecular pairs bonded by π−π and C−N···π in 3DQCNred. (d) Overlap of molecular stacking in 3DQCNred and (h) 3DQCNorange. The fused dibenzoquinoxaline-2,3-dicarbonitrile planes were drawn in space-filling mode. (e) Intralayer C−H···N interactions in 3DQCNred and (i) 3DQCNorange. (f) Side view of the layered columnar stacking array in 3DQCNorange. (g) Paired-antiparallel molecular stacking bonded by π−π, C− N···π, C−H···π, and C−H···N in 3DQCNorange. The dihedral angle between TPE and the fused plane was labled. Partial hydrogen atoms are omitted for ease of observation.

form columnar stacked array along the a axis. SCXRD face indexing reveals the a axis is along the long axis of the 3DQCNred crystal. After SCSC transition the long axis of the crystal becomes b axis of the 3DQCNorange form, as shown in Figure 5a. Columnar stacked array of 3DQCN molecules is still found to be along the long axis, now being of b axis of the 3DQCNorange crystal, but in a different stacking motif. Parts b− i of Figure 5 show the comparisons between the two. An important distinction is that the antiparallel packing mode within the columnar stacked array of 3DQCNred becomes a paired-antiparallel packing mode between every two adjacent molecular pairs in 3DQCNorange. In the columnar stacked array of 3DQCNred crystal there is only one type of antiparallel molecular pairs bonding through strong π−π interactions (3.258 Å) between the well-overlapped fused dibenzoquinoxaline-2,3-dicarbonitrile planes. In 3DQCNorange crystal, there become two types of molecular pairs, one of which is bonded by π−π interactions with the same distance as in 3DQCNred of 3.258 Å; and in the other one the π−π interaction between the two adjacent molecules is with a little longer distance of 3.330 Å. The paired-antiparallel stacking results a staggered and displaced overlap over the fused dibenzoquinoxaline-2,3dicarbonitrile planes, as compared in parts d and h of Figure 5. The overlap ratios of the π−π stacking in the two kinds of antiparallel pairs are 42% and 36%, respectively, distinctly smaller than the overlap ratio of 48% in 3DQCNred. These

characteristics (longer distance and displaced overlap) indicate a relative weak π−π interaction in 3DQCNorange than in 3DQCNred, on the other hand, facilitate the formation of other intermolecular interactions such as C−N···π, C−H···π, and C− H···N interactions as both intralayer and interlayer connections (Figure 5, parts c, e, g, and i). The multiple interactions are believed to be responsible for the thermodynamic stabilization of 3DQCNorange than 3DQCNred to some extent. Furthermore, an even more concrete change occurred in the SCSC transition on the molecular conformation. The 3DQCN molecules in 3DQCNred adopt a more twisted conformation with the dihedral angle between TPE and the fused plane of 33.17° as described vide supra. In 3DQCNorange there are two different kinds of molecular conformations for the molecules, one with a corresponding dihedral angle of 31.86° and the other being of 14.46°. As shown in Figure 5g, the two kinds of conformational molecules comprise respectively the two types of antiparallel molecular pairs in 3DQCNorange. Thus, we consider that the SCSC transition is a synergistic effect of both conformational changes in molecular level and supramolecular structure adjustments in the packing mode. It becomes more “molecular disordered” during the exothermic transition occurred at high temperature. 3DQCNorange crystal, which is composed of molecules with more coplanar conformations, but possesses blue-shifted emission and reduced lifetime. This indicates that the intermolecular stacking, especially π−π interactions, play F

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Figure 6. Schematic diagram of the nucleation-and-growth mechanism of the SCSC transition. (a) Nucleation of 3DQCNorange phase from microcavity in 3DQCNred crystal. (b) Growth of 3DQCNorange phase by molecular relocation at contact interface. (c) Completion of the SCSC transition process by forming an intact 3DQCNorange crystal.

daughter phases. According to Mnyukh’s and Herbstein’s analysis, the layer stacking was regarded as a prerequisite for some oriented phase transition.28,44 Here although being with of layered crystal structure, the other characteristics: (i) the comparable intra- and interlayer bonding, (ii) the considerable structure change inside the layer after the transition, and (iii) the distinct unit cell with each other (Table S2), render the situation here different with their analysis. We indeed observed that the orientation of the molecular layers is almost unchanged upon transition and the interface remained parallel to itself during the transition process. The retentive molecular layers and interface motion are believed critical to avoid singlecrystallinity deterioration and thus to preserve an SCSC transition. That was the true case for the transition on most of the crystals in our experiments; however, the fact that a real crystal is never homogeneously perfect causes the nucleation to be always heterogeneous and to initiate at random points in the crystal. For some crystals with low quality, we could only obtain textured polycrystals but not a single crystal of the 3DQCNorange form.

more crucial role than the molecular conformation in determining the fluorescent properties in aggregates. Considering the complexities of the phase transitions of molecular crystals, it is intractable to reproduce all the characteristics occurred in them. Mnyukh has proposed a nucleation-and-growth theory to describe the transition mechanism of the first-order transitions, however, the models he and the researchers after him studied were basically simple compounds such as p-dichlorobenzene and ammonium chloride.28,44 Here, based on the observation of the phase transition process and structural analysis, we consider that our SCSC transition also proceeds by a nucleation-and-growth mechanism, although the molecules it involves possess large scale and long conjugation. Figure 6 depicted the scheme of the proposed nucleation and growth mechanism during the SCSC transition. The nucleation of this temperature-induced phase transition starts from the defects with optimum size (e.g., microcavities), where the formation of the 3DQCNorange lowered the activation energy (Figure 6a). Thus, the initial point of the transition can be at the end of the crystal or any location of its body depending on the lowest activation energy. After nucleation initiated by heating, the transition proceeds by molecular relocation at the contact interface, just like the way crystal growth from the melt (Figure 6b). That is why the transition in this stage still needs continuous heating to keep its way, but not a spontaneous move after initiation, like the case in other systems.28 The interface separating the two phases should be a crystallographic plane in the daughter phase, where the molecules are detached from the parent phase and rearrange to the daughter phase, making the interface to advance. After the interface traversing the whole crystal, the phase transition completes (Figure 6c). So this transition is totally distinguished from those diffusionless transitions such as “martensitic transition” that occurs without the long-range diffusion, regarding its interface, its slow and variable velocity, and the monotropic characteristic, which are in contrast with the martensitic transition.45,46 Another important aspect is about the orientation relationship between parent and

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CONCLUSIONS In conclusion, we first disclose the structural features that govern the effect of crystallization on the luminescence behaviors of TPE-functionalized dicyanopyrazine derivatives. Different π−π stacking modes over the planar acceptors are responsible for the crystallization induced blue-shifted emission of 1DQCN and the crystallization induced redshifted emission of 3DQCN, and also for their mechanochromic abilities. Polymorphs with high contrast luminescence can be realized between amorphous−crystalline and different crystalline phases. Then we focused on the SCSC phase transition between 3DQCNred and 3DQCNorange. In virtue of the remarkable photoluminescence change during the transition, the transition process can be clearly visualized in detail. Single crystal structure analysis before and after the SCSC transition provided molecular-level understanding of the process. Despite of remarkable structural changes, the oriented G

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motion of molecular layers and interface preserved the integrity of the single crystal. A clear picture about how evolution in molecular layer stacking mode induces the SCSC process and affects the emission property of the crystal was drawn, which revealed that the transition proceeds by a nucleation-and-growth mechanism, but not displacive martensitic transition. The results on one hand deliver intuitive and in-depth cognition about the polymorph-dependent optical properties and the mechanism of the phase transition between the polymorphs. Because people still know little about the transition mechanism especially for the organic polymorphs being composed of large conjugated molecules, an SCSC type transition exposing changes in crystal structure is of unique significance. On the other hand, this work also proved the paramount importance of direct microscopy observation about the actual transition process.

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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.jpcc.8b04164. General information and methods for common measurements, single molecular conformations, absorption and excitation spectra, detailed emission lifetime information, single-crystal analysis data, detailed phase transition information, synthesis, 1H NMR and 13C NMR spectra, high resolution mass spectra, and movie information (PDF) X-ray crystal structure of 1DQCN (CCDC-1825690) (CIF) X-ray crystal structure of 3DQCNorange (CCDC1829280)(CIF) X-ray crystal structure of 3DQCN red (CCDC1827755)(CIF) checkCIF/PLATON report for 1DQCN (PDF) checkCIF/PLATON report for 3DQCNorange (PDF) checkCIF/PLATON report for 3DQCNred (PDF) Movie 1, phase transition upon heating removed (AVI) Movie 2, phase transition upon heating recovered (AVI)

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Article

AUTHOR INFORMATION

Corresponding Authors

*(Y.L.) E-mail: [email protected]. *(X.T.) E-mail: [email protected]. ORCID

Guangfeng Liu: 0000-0001-5029-3794 Xutang Tao: 0000-0001-5957-2271 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the National Natural Science Foundation of China (Grant Nos. 21772115, 51321091, 51227002, and 51272129), National Key Research and Development Program of China (Grant No. 2016YFB1102201), and the Program of Introducing Talents of Disciplines to Universities in China (111program No. b06015). Y.L. is grateful for the support from Qilu Young Scholars and Tang Scholars. H

DOI: 10.1021/acs.jpcc.8b04164 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.8b04164 J. Phys. Chem. C XXXX, XXX, XXX−XXX