In situ Growth Transition Towards Helical Crystals of Achiral Molecules

The synthesis of chiral crystals such as helical crystals is important due to their chirality‐dependent applications in various fields. Since chiral crystals of achiral molecules are getting more attention, it is important to establish synthetic strategies for helical crystals of achiral molecules. Quite surprisingly, there are numerous examples of helical crystals of achiral molecules, although they are mostly obtained without specific synthetic strategies. Herein, it is proposed that Eshelby twist can be applied to noncovalent molecular crystals by introducing screw dislocations and demonstrates a successful synthesis of helical tube crystals of achiral titanyl phthalocyanine by vapor‐phase crystal growth. The dislocations form where interfaces are created via growth transition from wire growth to tube growth upon growth temperature change. The screw dislocation is confirmed by Burgers vector analysis using transmission electron microscopy. The screw dislocation induces crystal twisting along the growth direction to dissipate the strain applied around the dislocation. The diffraction peak broadening of the (200) and (002) planes confirms the nonuniform strain, i.e., twisting force. This work offers a new approach for the synthesis of helical crystals from achiral molecules.


Introduction
][4][5][6][7][8][9][10][11] Especially, helical morphology is getting much attention due to its outstanding properties, such as high power conversion efficiency in photovoltaics, [12] high separation efficiency in chiral separation, [6] and flexible polarization rotators with better adaptability. [13]Interestingly, the formation of helical crystals of achiral molecules is quite frequently observed such as hippuric acid, [14] oligoaniline derivatives, [15] and benzamide. [16]The twisting mechanisms of these crystals were revealed and offered the possibility of applying them to other molecules.However, strategic methods for the synthesis of helical crystals of achiral molecules are deficient and still demand to be developed.[25] Our strategy is to introduce a screw dislocation through a crystallization transition.That is, when there are two crystallization pathways available for a molecule, leading to two morphologies, then asymmetric crystals can be produced by inducing a symmetry breakdown via an in situ transition from one crystallization pathway to another.For this strategy, we propose vapor-phase crystallization, since it allows a smooth crystallization transition as the reaction temperature conditions change. [26]As a target crystal for the proof of concept, a titanyl phthalocyanine (TiOPc) molecule was selected, as it is an achiral molecule known to form wire and rectangular tube morphologies upon growth temperature change. [27,28]Our key hypothesis is that the in situ growth transition from wire crystals to rectangular tube crystal introduces screw dislocation defects, resulting in helical crystals (Figure 1).
To demonstrate this, we studied the vapor-phase growth of TiOPc crystals at two different growth temperatures as well as sequential growth at two growth temperatures.Wire crystals and rectangular tube crystals were selectively formed at growth temperatures of 230 and 280 °C, respectively, while a high yield of TiOPc helical crystals was obtained upon sequential growth at temperatures of 230 °C and then 280 °C.Transmission electron microscopy (TEM) study with Burgers vector analysis confirmed that the twisting of the TiOPc crystal originated from the screw dislocation line defect.To our knowledge, this is the first example of the synthesis of helical molecular crystals of achiral molecules through a strategic introduction of screw dislocations.

Results and Discussion
The crystals were synthesized using a physical vapor transport (PVT) method that yields products of high chemical purity DOI: 10.1002/sstr.202300134 The synthesis of chiral crystals such as helical crystals is important due to their chirality-dependent applications in various fields.Since chiral crystals of achiral molecules are getting more attention, it is important to establish synthetic strategies for helical crystals of achiral molecules.Quite surprisingly, there are numerous examples of helical crystals of achiral molecules, although they are mostly obtained without specific synthetic strategies.Herein, it is proposed that Eshelby twist can be applied to noncovalent molecular crystals by introducing screw dislocations and demonstrates a successful synthesis of helical tube crystals of achiral titanyl phthalocyanine by vapor-phase crystal growth.The dislocations form where interfaces are created via growth transition from wire growth to tube growth upon growth temperature change.The screw dislocation is confirmed by Burgers vector analysis using transmission electron microscopy.The screw dislocation induces crystal twisting along the growth direction to dissipate the strain applied around the dislocation.The diffraction peak broadening of the ( 200) and (002) planes confirms the nonuniform strain, i.e., twisting force.This work offers a new approach for the synthesis of helical crystals from achiral molecules.and allows easy control of growth temperature. [29]About 10 mg of TiOPc (>99%, Sigma Aldrich) was loaded in a ceramic boat placed in a protective quartz tube at the center of a horizontal tube furnace (Figure 1).A piece of SiO 2 substrate was attached upside-down inside the quartz tube to collect products.The growth temperature was controlled by adjusting the position of the substrate in the furnace.For the growth transition, growth proceeded at 230 °C for 10 min and then the protective quartz tube was pushed 1 cm into the furnace where the growth temperature was 280 °C.Detailed experimental procedures are provided in Supporting Information.At 230 °C, wire crystals grew in the four corners of a square base, indicating the preferred growth mode as wires (Figure 2a).Prolonged growth for up to 20 min only lengthened the wires.
Meanwhile, at 280 °C, straight rectangular tube crystals were obtained, indicating the preferred growth mode as tubes at this temperature (Figure 2b).According to our hypothesis, these initial results indicate that TiOPc can grow into helical crystals through induction of a screw dislocation via a growth transition from a wire growth mode to rectangular tube growth mode by controlling the growth conditions.Indeed, a high yield of TiOPc helical crystals was obtained from sequential growth at 230 °C and then 280 °C (Figure 2c).Therefore, the in situ growth transition from wire to tube growth via a growth temperature change is a salient mechanism for fabricating helical crystals.The helicity was observed under an optical microscope (OM) as periodic bright and dark regions (Figure 3a).
Apparent helical tube crystal morphologies were also observed in scanning electron microscopy (SEM) images (Figure 3b,c).The lengths of the helical crystals ranged from tens to hundreds of micrometers, while the thickness ranged from hundreds of nanometers to a few micrometers.The crystals exhibited a mixture of right-and left-handed twists.The ratio of left-and  right-handed crystals measured from 471 crystals prepared in 10 turned out to be 47.7% and 52.3%, respectively, showing approximately equal probability by measuring (Table S1, Supporting Information).
The dependence of twist rate (twist in radians per unit area, α) and cross-sectional size (thickness of the crystal, h) obeys a power function α ∝ 1 h n .Other twisting mechanisms such as heterometry and strain between intergrowing crystallites follow the power function with the exponent n close to 1, but Eshelby twist arises the function with n = 2. [17,19,21] The relationship between the twist rate (α) and the inversed square of cross-sectional size (1/h 2 ) was analyzed by a scatterplot of 104 helical crystals (Figure S1, Supporting Information).It shows a linear function that reveals that the twisting mechanism is Eshelby twist.To examine other potential twist mechanisms than Eshelby twist, of which twist rate is not affected by the length of wires, the effect of the length of wires and twist rate depending on the growth time at 230 °C was examined.The wire became longer as the growth time at 230 °C was increased, as shown in Figure S2, Supporting Information.Nevertheless, there is no effect on the twist rate (Figure S2d, Supporting Information).
To investigate the structural relationship between the straight crystals and the helical crystals, a powder X-ray diffraction (PXRD) study was performed with manually collected straight and helical crystals.The results show that both types of crystals exhibit a strong and sharp (200) and ( 002) diffraction peak at 2θ = 5.618°, d = 12.674 Å.This finding suggests that the helical crystals have the same crystal structure basis as the straight crystals (Figure 3d). [28]This sameness is exactly what is expected under our hypothesized scenario of the formation of helical crystals through twisting of straight crystals.This interpretation is further supported by the PXRD peak shape.It is known that a uniform strain, such as a stretch, induces a diffraction peak shift, while a nonuniform strain, such as bending or twisting, causes a diffraction peak to broaden. [30,31]The full-width at half-maximum (FWHM, 0.0486) the overlapping ( 200) and (002) peaks of helical crystals was 59% broader than that (0.0305) of straight crystals (Figure 3d inset), implying that the twisting force was employed to straight crystals to induce helicity.It should be noted that the (200) and (002) peaks are shown overlapped because the peak positions of ( 200) and (002) are inherently indistinguishable.C-phase TiOPc straight crystals grew along the [010] direction along intersections of ( 200) and (002) planes (Figure 3e). [28]More detailed information about the molecular packing of the straight crystal is described in Figure S3, Supporting Information.The broadening of ( 200) and (002) diffraction peaks thus indicates that two sides of the straight crystal were twisted, confirming that twisting of straight crystals was the key mechanism in the observed helical crystal formation.Furthermore, selected-area electron diffraction (SAED) patterns of helical and straight crystals exhibited identical patterns with a similar d-spacing (Figure S4, Supporting Information).These XRD and SAED results support the interpretation of helical crystal formation from the twisting of straight crystals.
The prime suspect for the twisting of crystals is screw dislocation, whose presence is known to dissipate extra strain around the screw dislocation by twisting the structure. [23,32]In our case, it seems that a screw dislocation was introduced upon the transition between successive crystal growth modes, i.e., wire crystal growth followed by tube crystal growth.In the transition from wire crystal growth to tube crystal growth, the space between wires was filled in with planar surfaces, providing an interface where a screw dislocation was introduced (Figure S5, Supporting Information).The introduction of this screw dislocation was confirmed by TEM analysis.A dark-field TEM image of a helical tube crystal clearly shows a dark gray line, interpreted as a dislocation line, along the [010] length direction at the center of the crystal (Figure 3f ).A dislocation is classified as either edge dislocation or screw dislocation depending on the geometric relationship between the dislocation line vector and Burgers vector (b): perpendicular for edge dislocation, parallel for screw dislocation. [25,33]Therefore, Burgers vector analysis was performed on the basis of g • b contrast in TEM images (see Supporting Information for details).The dislocation was imaged with various g reflections and became invisible at g • b = 0 and visible at g • b 6 ¼ 0. The dislocation line is visible at g = (110) in Figure 3f, implying that g • b 6 ¼ 0, while it is invisible at g = (200) in Figure S6b, Supporting Information, i.e., g • b = 0. Therefore, the direction of Burgers vector is determined to be along the [010] direction and turns out to be parallel to the dislocation line direction [010].This parallel relationship confirms the classification of the dislocation as a screw dislocation.In contrast, the straight tube crystal showed no dislocation line (Figure S7, Supporting Information), verifying that the screw dislocation was essential for twisting.

Conclusion
In conclusion, we demonstrated the formation of helical molecular crystals from achiral molecules based on a strategic introduction of screw dislocation by PVT method.The growth transition of TiOPc crystals was successfully controlled by changing the growth temperature, with the TiOPc helical tube crystals generated by a change in growth mode from wire to tube growth.PXRD analysis revealed a broadening of the overlapping (200) and (002) diffraction peaks in the helical tube compared to the straight tube, confirming twisting as the key mechanism for the growth of helical crystals.The screw dislocation was the origin of the twisting that releases the strain energy caused by the dislocation.This phenomenon was clearly observed in dark-field TEM, and the screw character of the dislocation was confirmed by Burgers vector analysis.

Figure 1 .
Figure 1.Schematic of the PVT system used to grow TiOPc helical tube crystals.The in situ growth transition from wire to tube growth introduces screw dislocations in the crystal, resulting in helical tubes.

Figure 2 .
Figure 2. Diagrams and time-dependent SEM images of TiOPc crystals under various growth conditions.a) Wire growth at 230 °C.b) Tube growth at 280 °C.c) Wire !tube growth transition induced by changing the growth temperature (230 °C !280 °C) after 10 min of growth time.The insets show magnified images of each crystal morphology.

Figure 3 .
Figure 3. Analysis of crystal morphology and structure for TiOPc helical tube crystals.a) OM and b) SEM images of TiOPc helical crystals on an SiO 2 substrate.c) Magnified SEM image of (b).d) PXRD patterns of TiOPc helical and straight tubes.The inset details the peak in the red dashed box.e) Graphical representation of twisting deformation and crystal's hkl planes.f ) Dark-field TEM image of the TiOPc helical tube using (110) reflection under the two-beam condition.