Sequential phase transformation of propeller-like C₃-symmetric liquid crystals from a helical to ordered to disordered hexagonal columnar structure

Abstract

In this paper, we report thermally induced intercolumnar phase transitions of C₃-symmetric liquid crystals (LCs) bearing a triazole-based propeller-like aromatic mesogen. Since the constituting aromatic rings are conjugated through rotatable single bonds, the mesogenic shape is tuneable depending on the degree of conformational motion. Molecule 1 with ninefold octyl peripheries shows a hexagonal columnar liquid crystalline phase transition from ordered mesogenic stacking to disordered mesogenic stacking upon heating. On the other hand, molecule 2 with sixfold octyl peripheries displays a helical hexagonal columnar phase with the P6/mmm space group at ambient temperature as well as the ordered and disordered hexagonal columnar phases at higher temperatures. The intracolumnar helical order can be understood by an interdigitated stacking of the propeller-like mesogens along the columnar axis and the optimized space-filling. Notably, all the intercolumnar phase transformations in this study are revealed as second-order transitions. The thermodynamic nature agrees well with the fact that the conformational motions of the C₃-symmetric aromatic mesogen change abruptly with each columnar transition.

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Introduction

Columnar assemblies formed by aromatic building units have been paid a great deal of attention because they can be employed as a platform material for their application in photovoltaics and field effect transistors.¹ It is well-known that discotic liquid crystals (DLCs) consisting of a flat aromatic disk and flexible chains organize into columnar phases via the orthogonal stacking of aromatic disks.² Even though most columnar structures are derived from the classical two-dimensional (2-D) flat discogens, some molecules consisting of three-dimensional (3-D) aromatic cores such as bowl-like,³ tetrahedral,⁴ octahedral,⁵ star-shaped,⁶ or propeller-like mesogens⁷ are capable of self-assembling into columnar arrays via their unique stereoscopic stackings. Such unconventional 3-D aromatic stacks within a column sometimes induce interesting ferroelectric, charge-carrying, and thermochromic functions.⁸

Depending on the unidirectional interactions and steric requirement, DLCs form helical arrangements within a column. As a result of the intracolumnar helical arrangement, the packing order of aromatic disks can improve, which sometimes increases the charge carrier mobility along the helical column.⁹ As ways to organize helical assemblies, non-covalent interactions,^7b,10e.g., hydrogen-bonding, dipolar forces, and metal complexation, have been employed. In the molecular design of helically organizing DLCs, appropriate functional units should be incorporated with aromatic disks to induce helicity. As a steric element, chiral groups could be attached in the vicinity of a flat aromatic disk in order to promote the helical sense.¹¹

Such a helical columnar structure is possibly derived by self-assembly of 3-D aromatic mesogens. As a strong mesogenic candidate, propeller-like molecules can be taken into consideration. Due to the symmetrically twisted shape, propeller-like molecules are inclined to produce an intracolumnar helical order, which can minimize void. Indeed, some LC examples with fixed propeller shapes were reported to organize into hexagonal columnar phases with helical orders, although they did not possess any chiral element at the periphery.^7c–e Considering the 3-D molecular structure with the aromatic wings and central core, the mesogenic wing parts would be interdigitated to some extent along the columnar axis during the stepwise helical stacking.

In this context, we envisioned that a propeller-like mesogen could be induced by the conformational variation of C₃-symmetric molecules. As shown in Fig. 1a, we designed two LC molecules 1 and 2 with ninefold and sixfold octyl peripheries, respectively. The molecules are based on a C₃-symmetric propeller-like mesogen which consists of three triazole-based aromatic wings and a benzenyl core. Since all the aromatic rings are conjugated through rotatable single bonds, we speculated that the mesogenic shape would be tuneable depending on the degree of the rotational motions of the dendritic wings which would change in response to temperature. Furthermore, we conjectured that the stacking mode of the shape-tuneable mesogen would be closely associated with the number of peripheral chains in terms of the space-filling requirement, and possibly a helical columnar phase could be derived from the C₃-symmetric molecular propellers in cases when the dendritic wings could stack in an interdigitated mode (Fig. 1b).

Fig. 1 (a) Molecular structures of C₃-symmetric LC molecules 1 and 2 with ninefold and sixfold octyl peripheries, respectively. (b) Schematic representation of the assembly of a propeller-like mesogen.

Herein, we report a unique columnar phase sequence, i.e., a helical columnar (Col_hex/h) to an ordered columnar (Col_hex/o) to a disordered columnar (Col_hex/d) phase, as a function of temperature. All the columnar phases display an identical 2-D hexagonal symmetry, but the structural difference occurs in the mesogenic packing order along the columnar axis. To the best of our knowledge, the sequential intercolumnar transition is reported for the first time in this study, although the fragmentary intercolumnar transitions, e.g., helical to ordered,^9c,11c,12 ordered to disordered,¹³ and helical to disordered¹⁴ intercolumnar changes, were reported. In addition to the unique phase phenomenon, another remarkable feature is the thermodynamic nature of the intercolumnar transitions. Both Col_hex/h-to-Col_hex/o and Col_hex/o-to-Col_hex/d transformations turned out to be second-order transitions. In this paper, the unique phase transitional details are addressed.

Results and discussion

The design and synthesis of the C₃-symmetric molecules utilized click chemistry.¹⁵ To prepare the molecules (1 and 2) through a click reaction, ethynyl and azido precursors were synthesized by following the procedures described previously.¹⁶ The target molecules were obtained by a click reaction of the obtained azido and alkyne precursors using CuSO₄·5H₂O and sodium ascorbate as catalysts (Scheme S1†). The molecules were characterized by ¹H and ¹³C-NMR spectroscopy techniques, elemental analysis, and gel permeation chromatography (GPC). All the experimental data agree well with the molecular structures (Fig. S1 and S2†).

The thermal behavior was investigated by differential scanning calorimetry (DSC). In Fig. 2, molecule 1 with ninefold octyl chains shows a reversible endothermic peak in the heating and cooling scans. Since the degree of supercooling (ΔT = 3.4 °C) and the corresponding enthalpy change (7.1 kJ mol⁻¹) are small, the endothermic peak is considered to be a LC-to-liquid transition. Similar to 1, 2 with sixfold octyl peripheries exhibits an endothermic peak at 175.1 °C, which indicates the formation of an LC phase. However, a signal like a glass transition is observed at 66.5 °C. Considering the nature of the molecular components such as the aromatic and flexible octyl groups, the transition may be attributed to the conformational change of the aromatic mesogen.

Fig. 2 DSC thermograms of C₃-symmetric LC molecules 1 and 2. Col_hex/o, ordered hexagonal columnar; Col_hex/d, disordered hexagonal columnar; Col_hex/h, helical hexagonal columnar; iso, isotropic liquid phase. The values in parentheses are the enthalpy change (kJ mol⁻¹) of each transition.

To characterize their LC phases, we examined the optical textures of birefringent phases using polarized optical microscopy (POM). From the POM observations, both molecules 1 and 2 display a fan-like texture over the entire temperature range from 25 °C to the isotropization temperatures (Fig. S3†). This POM texture is typically observed from the columnar LC phases of conventional DLCs.¹⁷ Interestingly, the fan-like texture in the LC of 2 does not change all the way down to 30 °C, and the cover glass of the sandwich-type sample cell of 2 does not move at all when pushing it. These observations indicate that the transition at 66.5 °C is a second-order glass transition.

To demonstrate the microstructures of the birefringent phases, temperature-variable small- and wide angle X-ray scattering (SAXS and WAXS) experiments were performed. Fig. 3 represents the SAXS and WAXS data of 1. In the SAXS data, 1 reveals an identical reflection pattern over the entire temperature range of 25 °C–180 °C (Fig. 3a). Four reflections with a q-spacing ratio of 1 : √3 : √4 : √7 are observed, and they can be indexed to the (100), (110), (200), and (210) planes of a 2-D hexagonal lattice (Table 1). In contrast, the WAXS data alters upon heating. In the WAXS pattern at 30 °C, two broad reflections are observed (Fig. 3b and S4†). One reflection near 1.45 Å⁻¹ corresponds to the mean distance of molten octyl chains, and the other reflection near 1.78 Å⁻¹ is derived from the average packing distance (3.5 Å) between the aromatic segments. As the temperature elevates, the aromatic stacking reflection becomes gradually weaker, and finally disappears. Consequently, the gradual intercolumnar transition involves the loss of the long-range stacking correlation of the aromatic cores. The lower and higher temperature LC phases are assigned as Col_hex/o and Col_hex/d phases, respectively. Although the transition temperature of the gradual intercolumnar transformation is difficult to pinpoint, it may be determined through a Gaussian peak separation of the WAXS data. In the peak separation, the two peak positions are set near 1.45 and 1.79 Å⁻¹, respectively. Up to 140 °C, the peak separation yields two noticeable peaks. But, the peak at 1.79 Å⁻¹ cannot be assigned above 150 °C, which is approximately determined to be the transition temperature (Fig. S5†).

Fig. 3 (a) SAXS data of the bulk sample of 1 at 30 °C and 160 °C and (b) WAXS data of the bulk sample of 1 at various temperatures.

Table 1 Characterization of the X-ray data of 1^a

	d cal/Å	d exp/Å	(hkl)	Lattice parameters
a d cal, calculated lattice spacing; dexp, experimental lattice spacing.
30 °C Colhex/o	28.2	28.2	(100)	a = 32.5 Å, 3.5 Å, (π–π stacking)
	16.3	16.3	(110)
	14.1	14.1	(200)
	10.7	10.7	(210)
140 °C Colhex/o	27.3	27.3	(100)	a = 31.5 Å, 3.6 Å, (π–π stacking)
	15.8	15.8	(110)
	13.7	13.6	(200)
	10.4	10.3	(210)
160 °C Colhex/d	27.0	27.0	(100)	a = 31.1 Å
	15.6	15.6	(110)
	13.5	13.5	(200)
	10.2	10.2	(210)

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It should be noted that no peak is found at the Col_hex/o-to-Col_hex/d transition in the DSC data (Fig. 2). Therefore, this intercolumnar transition is a second-order transition rather than a first-order one. On the other hand, in rigid fused aromatic ring-based discotic LCs, similar intercolumnar transitions were reported to be first-order transitions previously.^{7a,12a–d,13} An endothermic peak appeared in the DSC data. On the basis of our previous results, we assume that the thermodynamic nature of such an intercolumnar transition is strongly dependent upon the mesogenic structure. For the propeller-like mesogens in this study, the packing correlation becomes diminished by the rotations of the bonds connecting aromatic rings, which results in the second-order transition. In contrast, for the fused aromatic mesogens like triphenylene or porphyrazine, a certain amount of energy has to be supplied to weaken the stronger interdisc π–π interactions, which results in the first-order transition. Consequently, the degree of the interdisc interactions associated with the aromatic structure should be taken into account to evaluate the thermodynamic properties.

Molecule 2 with the sixfold octyl peripheries displays a more unique phase sequence with temperature. In addition to the Col_hex/o and Col_hex/d LC phases, a 3-D helical columnar structure is formed below the T_g of 66.5 °C. Fig. 4 represents the bulk SAXS and WAXS data of 2. Similar to the intercolumnar transition of 1, the Col_hex/o and Col_hex/d phases of 2 show the 2-D hexagonal reflections at 100 °C and 160 °C, respectively. Additionally, the WAXS reflection corresponding to the aromatic packing disappears upon entering into the Col_hex/d phase. However, a drastic change in the SAXS pattern occurs below the T_g. The SAXS data at 30 °C exhibits a complicated reflection pattern including the 2-D hexagonal peaks, and concomitantly the WAXS reflection becomes even intensified.

Fig. 4 (a) SAXS data of the bulk sample of 2 at 30 °C, 100 °C and 160 °C and (b) WAXS data of the bulk sample of 2 at various temperatures.

For the accurate structural analysis of the phase below the T_g, grazing incidence X-ray scattering (GIXS) experiments were performed with the surface-oriented sample. The aligned sample was prepared by casting the chloroform solutions (∼3 wt%) on 3-aminopropyltriethoxysilane (APS)-coated Si-wafers, and was slowly cooled to room temperature (RT) from the isotropic transition temperature.¹⁸Fig. 5a represents the spotty GIXS image of 2 at 30 °C. The hexagonal reflections corresponding to the (100), (110), and (200) planes are found at the equator, which indicates that columns stand on the substrate. On the other hand, two intense reflections are revealed along the meridian. The reflection in the wider angle corresponds to the stacking order of aromatic segments. The stacking distance is 3.4 Å. In particular, one that appears at q = 1.84 Å⁻¹ strongly suggests an intracolumnar order which is a helical stacking between propeller-like mesogens. Furthermore, two off-axis reflections are observed in the small angle region. Based on the X-ray reflections, the meridian component can be divided into twenty equal distances. From the GIXS pattern, the helical parameter (φ, a mutual rotational angle between neighboring propellers) is calculated to be 18° (Fig. 5b).^7a,9b In the wide angle region of the GIXS data, two symmetric reflections at q = 1.61 Å⁻¹ are found (Fig. 5a). Since this helical phase forms below the T_g, the conformation of the aromatic mesogens is considered to be fixed. Thus, these reflections are attributed to the tilted aromatic wings with respect to the central benzenyl group. From the azimuthal plot of the reflections, the fixed tilt angle is measured to be 27° (Fig. 5c and d).^7a,9b The tilted conformation of the aromatic wings may lead to an effective space-filling.

Fig. 5 (a) GIXS data of 2 at 30 °C, (b) the helical arrangement of the Col_hex/h phase, (c) the azimuthal plot of the reflection at q = 1.61 Å⁻¹ (green region in the GIXS data), (d) the molecular model with the fixed tilt angle of 27° in the Col_hex/h phase, and (e) the 3-D arrangement of the helical units. In (a), the reflections in the small angle region are magnified below. In (b), (d), and (e), the octyl peripheries are omitted for clarity.

As shown in the magnified small angle region (the below part in Fig. 5a), the off-meridional reflections on the L = 2 and 3 layers can be assigned to the (102) and (103) planes, which demonstrates the correlation between intercolumnar helical units. Consequently, the assembled structure below the T_g can be interpreted as a 3-D helical columnar structure (Col_hex/h) with the P6/mmm space group (Fig. 5e). The lattice parameters are estimated to be a = 32.8 Å and c = 69.5 Å. The complicated pattern of the bulk SAXS data at 30 °C in Fig. 4a also agrees well with the Col_hex/h structure (Table 2).

Table 2 Characterization of the X-ray data of 2^a

	d cal/Å	d exp/Å	hkl	Lattice parameters
a d cal, calculated lattice spacing; dexp, experimental lattice spacing.
30 °C Colhex/h	28.4	28.4	(100)	a = 32.8 Å, c = 69.5 Å, 3.4 Å, (π–π stacking)
	26.3	26.4	(101)
	22.1	22.2	(102)
	17.9	17.8	(103)
	16.4	16.4	(110)
	14.2	14.3	(200)
	13.9	13.9	(005)
	13.1	13.1	(202)
	12.5	12.4	(105)
	12.1	12.1	(203)
	11.0	11.0	(204)
60 °C Colhex/h	28.4	28.4	(100)	a = 32.8 Å, c = 69.5 Å, 3.4 Å, (π–π stacking)
	22.1	22.3	(102)
	17.9	17.8	(103)
	16.4	16.4	(110)
	14.2	14.3	(200)
	13.9	13.8	(005)
	12.5	12.5	(105)
70 °C Colhex/o	28.3	28.3	(100)	a = 32.7 Å, 3.5 Å, (π–π stacking)
	16.4	16.4	(110)
	14.2	14.2	(200)
100 °C Colhex/o	27.7	27.7	(100)	a = 31.9 Å, 3.5 Å, (π–π stacking)
	16.0	16.0	(110)
	13.8	13.8	(200)
150 °C Colhex/o	27.0	27.0	(100)	a = 31.2 Å, 3.5 Å, (π–π stacking)
	15.6	15.6	(110)
	13.3	13.4	(200)
160 °C Colhex/d	26.7	26.7	(100)	a = 30.9 Å
	15.5	15.4	(110)
	13.4	13.5	(200)

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According to our current work and previous research studies, complex factors such as mesogenic geometry,^3–7 degree of branching,^{7b,h,8c,9a,19} peripheral chain length,^{12a,b,13,14e,20} and the size of the aromatic core^{6d,e,7h,14c,d} may be involved in the intracolumnar molecular packing. These factors work cooperatively, and thereby it is difficult to say individually the effect of each factor. Nevertheless, some factors may be considered independently to elucidate the packing tendency in intracolumnar assemblies. Especially, for the helical columnar formation, the molecular shape including mesogenic and peripheral chain parts is crucial, although other molecular parameters like a stereogenic chiral group¹¹ is also taken into consideration. In the intracolumnar helical structure, mesogens should stack on top of each other with a certain mutual rotation angle. At a given peripheral chain, symmetric star-shaped or propeller-like mesogens may be more advantageous than flat aromatic cores. In terms of helical spacing-filling, the former with a radially undulated molecular shape can pack tightly via an interdigitated stacking on top of each other, which is more likely to form a helical stacking than the latter with the planar shape.

Likewise, the density of peripheral chains in DLCs should be symmetric and unequally distributed in the design of the helical columnar assembly. The chain density distribution is affected by the degree of chain branching and chain length relative to the aromatic core. This is exemplified in molecules 1 and 2 with the identical aromatic core. The Col_hex/h structure is not observed in 1 with ninefold octyl peripheries. Thus, the number of octyl peripheries significantly influences the helical space-filling in a column, and guides the stacking mode of the propeller-like aromatic mesogen.

Like the stacking model proposed in Fig. 1b, the aromatic mesogen of 2 is assumed to interdigitate to some extent on top of each other along the columnar axis. This assumption can be clarified by investigating the number (N) of molecules in a columnar slice of the RT structure, Col_hex/o and Col_hex/h for 1 and 2, respectively. The N value can be calculated by the volume (V_cs) of a columnar slice/the molecular volume (V_mol). By dividing the molecular weight (M.W.) by the measured density (ρ) at RT, the V_mol values for 1 and 2 are obtained to be 3220.5 ų and 2518.9 ų, respectively. On the basis of the X-ray data, the V_cs values can be estimated by the columnar cross-sectional area (S = a × d₍₁₀₀₎) × the thickness (h) of a columnar slice. Although the V_mol of 2 is approximately 22% smaller than that of 1, their V_cs values are almost identical. Using the obtained V_cs and V_mol, the N values at RT are calculated to be 1.00 and 1.26 for 1 and 2, respectively (Table 3). Similar to conventional flat discogens, a single molecule of 1 constitutes the columnar slice. In contrast, the columnar slice of 2 is composed of more than one molecule. The non-unit N value indicates an interdigitated stacking of the aromatic mesogens. The dendritic wings of the molecular propeller primarily interdigitate in the columnar stacking. This stacking mode may enable the efficient filling of columnar space. In terms of space-filling, the interdigitation occurs more effectively in the columnar assembly of 2 with the sixfold octyl chains. Since its dendritic wings are less bulky than those of 1, more empty space will be generated if only a molecule covers the columnar slice.

Table 3 Characterization of the columnar phases of 1 and 2 at RT^a

	M.W. (g mol^−1)	ρ (g cm^−3)	V mol (Å^3)	S (Å^2)	h (Å)	V cs (Å^3)	N (Vcs/Vmol)
a ρ, density measured at RT; Vmol, molecular volume; S, columnar cross-sectional area; h, thickness of a columnar cross-section from WAXS data; Vcs, volume of a columnar slice; N, number of molecules in a columnar slice.
Colhex/o (1)	1661.5	0.857	3220.5	916.4	3.5	3207.4	1.00
Colhex/h (2)	1276.8	0.842	2518.9	933.4	3.4	3173.6	1.26

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It is worth noting that in our recent work, an analogous C₃-symmetric molecule with slightly flexible benzylic units formed a lamellar structure rather than a columnar one by adopting a Y-shaped conformer.²¹ Through the conformational variation, the analogue LC could fill the space efficiently without vacancy. However, 2 with the fully conjugated aromatic mesogen cannot adopt such a Y-shaped conformer, but it primarily forms the C₃-symmetric conformer due to its limited conformation. Consequently, interdigitation stacking is the way to meet the space-filling requirement by keeping the C₃-symmetric conformer.

Upon heating above the T_g, the Col_hex/h phase of 2 is transformed into a Col_hex/o phase. In the GIXS data, the (005) reflection at the meridian and the diagonal reflections disappear, while the two-dimensional hexagonal reflections at the equator and the aromatic stacking reflection at the meridian still remain. This GIXS pattern is consistent with the bulk X-ray data (Fig. 6a). In addition, the off-meridional reflections at q = 1.61 Å⁻¹ disappear in the Col_hex/o phase, as shown in the azimuthal plot (Fig. 6b). This means that the torsional angle of the triazolyl group with respect to the central benzenyl group is not fixed, and the bond begins to rotate upon transitioning into the Col_hex/o phase (Fig. 6c). In the Col_hex/o phase, the degree of interdigitation reduces, and thereby the average shape of the aromatic mesogen may become a two-dimensional disk.

Fig. 6 (a) GIXS data of 2 at 150 °C and (b) the azimuthal plot of the reflection at q = 1.61 Å⁻¹, and (c) the schematic conformational state in the Col_hex/o phase. Although the bonds connecting the central benzenyl and triazolyl rings are rotatable, the degree of the rotations is somewhat restricted state in the Col_hex/o phase.

The conformational motions increase the mesogenic fluctuation along the columnar axis. This can be confirmed by examining the full width at half maximum (FWHM) of the peak near 1.80 Å⁻¹. Like the way to determine the Col_hex/o-to-Col_hex/d transition temperature of 1, the peak corresponding to the aromatic stacking is able to be isolated by the Gaussian peak separation of the WAXS data. The peak isolation of 2 can be performed only up to 150 °C. In the Col_hex/d phase, the peak cannot be extracted from the peak separation. In Fig. 7, the FWHM increases suddenly near 70 °C. Interestingly, this transitional temperature is consistent with the T_g in the DSC data. This result indicates that the long-range correlation between aromatic mesogens somewhat decreases by the conformational rotations of the aromatic mesogen.

Fig. 7 FWHM plot of the aromatic stacking peak (near 1.80 Å⁻¹) of 2 as a function of temperature.

Upon further heating to the Col_hex/d phase, aromatic mesogens become distributed more randomly within a column. In the WAXS data, no peak is observed near 1.80 Å⁻¹ (Fig. 4b). The long-range correlation of the aromatic cores disappears because the conformational motions become even greater in the Col_hex/d phase. It is worth noting that the sequential phase transitions from the Col_hex/h to Col_hex/o to Col_hex/d phase are second-order transitions as identified by the DSC data. Therefore, by considering that the change in the conformational motions does not require a considerable energy, the conformational argument for the sequential intercolumnar transitions is reasonable.

Thermally-induced intercolumnar phase transitions are led by the variation in the degree of mesogenic stacking order which is influenced by peripheral chain motions. At an intercolumnar transition, mesogenic ordering should be altered. According to previous publications, molecular examples showing only the fragmentary intercolumnar transitions (e.g., Col_hex/o-to-Col_hex/d, Col_hex/h-to-Col_hex/o) mostly consist of fused aromatic cores such as hexa-peri-hexabenzocoronenes,^9c phthalocyanine,^11c triphenylene,^12d,13a and porphyrazine.^13b In comparison to molecule 2 showing the multiple intercolumnar transitions in this study, these fused aromatic mesogens are considered to interact more strongly with each other due to their large, planar and rigid mesogenic structures, assuming that an identical peripheral chain group is attached. Strong intermesogenic interactions may cause the discogen stacking order to be less sensitive to the variation of flexible chain motions. Accordingly, DLCs with the large fused aromatic mesogens may show only a single intercolumnar transformation if any. In contrast, the C₃-symmetric mesogen of 2 is composed of rotatable aromatic segments. Therefore, the mesogenic stacking order may be altered readily in response to the conformational motions of peripheral chains.

Conclusions

In this study, we prepared two C₃-symmetric molecules 1 and 2 with ninefold and sixfold octyl peripheries, respectively, using click chemistry. The structural analysis proved that 1 underwent a second-order phase transition from the Col_hex/o to the Col_hex/d LC phases as the temperature increased. On the other hand, 2 with less bulky dendritic wings displayed a helical hexagonal columnar phase (Col_hex/h) at RT as well as the subsequent Col_hex/o to Col_hex/d phases upon heating (Fig. 8). In particular, the unique sequential intercolumnar phase transformation of 2 can be elucidated by the stepwise conformational variation of the C₃-symmetric mesogen and the optimized longitudinal space-filling. Therefore, an interdigitated packing of the propeller-like mesogens can be proposed as the plausible model for the helical formation. The C₃-symmetric molecular system provides a versatile approach to fine-tune the aromatic packing structure within the 1-D aromatic column, and thereby the assembling concept will be useful in the design of 1-D electro-active materials with morphology-dependent tuneable mobility.

Fig. 8 Sequential intercolumnar transformations of 2 as a function of temperature.

Acknowledgements

The present research was conducted by the research fund of Dankook University in 2014. We acknowledge the Pohang Accelerator Laboratory (Beamline 9A), Korea.

Notes and references

1. (a) J. Nelson, Science, 2001, 293, 1059–1060; (b) C. W. Tang, Appl. Phys. Lett., 1986, 48, 183–185; (c) L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, E. Moons, R. H. Friend and J. D. MacKenzie, Science, 2001, 293, 1119–1122; (d) J. P. Schmidtke, R. H. Friend, M. Kastler and K. Müllen, J. Chem. Phys., 2006, 124, 174704; (e) H. E. Katz, A. J. Lovinger, J. Johnson, C. Kloc, T. Siegrist, W. Li, Y.-Y. Lin and A. Dodabalapur, Nature, 2000, 404, 478–481; (f) W. Pisula, A. Menon, M. Stepputat, I. Lieberwirth, U. Kolb, A. Tracz, H. Sirringhaus, T. Pakula and K. Müllen, Adv. Mater., 2005, 17, 684–689; (g) M. Katsuhara, I. Aoyagi, H. Nakajima, T. Mori, T. Kambayashi, M. Ofuji, Y. Takanishi, K. Ishikawa, H. Takezoe and H. Hosono, Synth. Met., 2005, 149, 219–223; (h) A. Kraft, ChemPhysChem, 2001, 2, 163–165.
2. (a) I. Paraschiv, K. de Lange, M. Giesbers, B. van Langen, F. C. Grozema, R. D. Abellon, L. D. A. Siebbeles, E. J. R. Sudhölter, H. Zuilhof and A. T. M. Marcelis, J. Mater. Chem., 2008, 18, 5475–5481; (b) S. Sergeyev, W. Pisula and Y. H. Geerts, Chem. Soc. Rev., 2007, 36, 1902–1929; (c) R. J. Bushby and O. R. Lozman, Curr. Opin. Colloid Interface Sci., 2002, 7, 343–354; (d) S. Kumar, Chem. Soc. Rev., 2006, 35, 83–109.
3. (a) K. Isoda, T. Yasuda and T. Kato, Chem.–Asian J., 2009, 4, 1619–1625; (b) Y. Matsuo, A. Muramatsu, Y. Kamikawa, T. kato and E. Nakamura, J. Am. Chem. Soc., 2006, 128, 9586–9587; (c) T. Hatano and T. Kato, Chem. Commun., 2006, 1277–1279; (d) K. Sato, Y. Itoh and T. Aida, J. Am. Chem. Soc., 2011, 133, 13767–13769; (e) B. Xu and T. M. Swager, J. Am. Chem. Soc., 1993, 115, 1159–1160.
4. (a) X. H. Cheng, S. Diele and C. Tschierske, Angew. Chem., Int. Ed., 2000, 39, 592–595; (b) A. Pegenau, T. Hegmann, C. Tschierske and S. Diele, Chem.–Eur. J., 1999, 5, 1643–1660; (c) A. Pegenau, P. Göring and C. Tschierske, Chem. Commun., 1996, 2563–2564.
5. (a) S. T. Trzaska, H.-F. Hsu and T. M. Swager, J. Am. Chem. Soc., 1999, 121, 4518–4519; (b) H. Zheng and T. M. Swager, J. Am. Chem. Soc., 1994, 116, 761–762.
6. (a) M. Lehmann, R. I. Gearba, M. H. J. Koch and D. A. Ivanov, Chem. Mater., 2004, 16, 374–376; (b) M. Lehmann and M. Jahr, Chem. Mater., 2008, 20, 5453–5456; (c) M. Lehmann, M. Jahr and J. Gutmann, J. Mater. Chem., 2008, 18, 2995–3003; (d) M. Lehmann, M. Jahr, F. C. Grozema, R. D. Abellon, L. D. A. Siebbeles and M. Müller, Adv. Mater., 2008, 20, 4414–4418; (e) M. Lehmann, M. Jahr, B. Donnion, R. Graf, S. Gemming and I. Popov, Chem.–Eur. J., 2008, 14, 3562–3576.
7. (a) M. Peterca, M. R. Imam, C.-H. Ahn, V. S. K. Balagurusamy, D. A. Wilson, B. M. Rosen and V. Percec, J. Am. Chem. Soc., 2011, 133, 2311–2328; (b) J. Barberá, L. Puig, P. Romero, J. L. Serrano and T. Sierra, J. Am. Chem. Soc., 2006, 128, 4487–4492; (c) Q. Xiao, T. Sakurai, T. Fukino, K. Akaike, Y. Honsho, A. Saeki, S. Seki, K. Kato, M. Takata and T. Aida, J. Am. Chem. Soc., 2013, 134, 18268–18271; (d) T. Metzroth, A. Hoffmann, R. Mrtín-Rapún, M. M. J. Smulders, K. Pieterse, A. R. A. Palmans, J. A. J. M. Vekemans, E. W. Meijer, H. W. Spiess and J. Gauss, Chem. Sci., 2011, 2, 69–76; (e) J. J. van Gorp, J. A. J. M. Vekemans and E. W. Meijer, J. Am. Chem. Soc., 2002, 124, 14759–14769; (f) M. L. Bushey, T.-Q. Nguyen and C. Nuckolls, J. Am. Chem. Soc., 2003, 125, 8264–8269; (g) M. L. Bushey, A. Hwang, P. W. Stephens and C. Nuckolls, J. Am. Chem. Soc., 2001, 123, 8157–8158; (h) E. Beltrán, J. L. Serrano, T. Sierra and R. Giménez, J. Mater. Chem., 2012, 22, 7797–7805.
8. (a) D. Miyajima, F. Araoka, H. Takezoe, J. Kim, K. Kato, M. Takata and T. Aida, Science, 2012, 336, 209–213; (b) T. Yasuda, T. Shimizu, F. Liu, G. Ungar and T. Kato, J. Am. Chem. Soc., 2011, 133, 13437–13444; (c) J. Kim, S. Cho and B.-K. Cho, Chem.–Eur. J., 2014, 20, 12734–12739.
9. (a) J. Wu, M. D. Watson and K. Müllen, Angew. Chem., Int. Ed., 2003, 42, 5329–5333; (b) W. Pisula, Ž. Tomović, M. D. Watson, K. Müllen, J. Kussmann, C. Ochsenfeld, T. Metzroth and J. Gauss, J. Phys. Chem. B, 2007, 111, 7481–7487; (c) J. Wu, M. D. Watson, L. Zhang, Z. Wang and K. Müllen, J. Am. Chem. Soc., 2004, 126, 177–186.
10. (a) F. Vera, J. L. Serrano and T. Sierra, Chem. Soc. Rev., 2009, 38, 781–796; (b) E. Beltrán, E. Cavero, J. Barberá, J. L. Serrano, A. Elduque and R. Giménez, Chem.–Eur. J., 2009, 15, 9017–9023.
11. (a) Ž. Tomović, J. van Dongen, S. J. George, H. Xu, W. Pisula, P. Leclére, M. M. J. Smulders, S. D. Feyter, E. W. Meijer and A. P. H. J. Schenning, J. Am. Chem. Soc., 2007, 129, 16190–16196; (b) S. Ishihara, Y. Furuki and S. Takeoka, Polym. Adv. Technol., 2008, 19, 1097–1104; (c) C. F. van Nostrum, A. W. Bosman, G. H. Gelinck, P. G. Schouten, J. M. Warman, A. P. M. Kentgens, M. A. C. Devillers, A. Meijerink, S. J. Picken, U. Sohling, A.-J. Schouten and R. J. M. Nolte, Chem.–Eur. J., 1995, 1, 171–182.
12. (a) V. Percec, H.-J. Sun, P. Leowanawat, M. Peterca, R. Graf, H. W. Spiess, X. Zeng, G. Ungar and P. A. Heiney, J. Am. Chem. Soc., 2013, 135, 4129–4148; (b) V. Percec, S. D. Hudson, M. Peterca, P. Leowanawat, E. Aqad, R. Graf, H. W. Spiess, X. Zeng, G. Ungar and P. A. Heiney, J. Am. Chem. Soc., 2011, 133, 18479–18494; (c) V. Percec, M. Peterca, T. Tadjiev, X. Zeng, G. Ungar, P. Leowanawat, E. Aqad, M. R. Imam, B. M. Rosen, U. Akbey, R. Graf, S. Sekharan, D. Sebastiani, H. W. Spiess, P. A. Heiney and S. D. Hudson, J. Am. Chem. Soc., 2011, 133, 12197–12219; (d) E. Fontes and P. A. Heiney, Phys. Rev. Lett., 1988, 61, 1202–1205; (e) V. Percec, E. Aqad, M. Peterca, J. G. Rudick, L. Lemon, J. C. Ronda, B. B. De, P. A. Heiney and E. W. Meijer, J. Am. Chem. Soc., 2006, 128, 16365–16372; (f) D. Adam, P. Schuhmacher, J. Simmerer, L. Häussling, K. Siemensmeyer, K. H. Etzbach, H. Ringsdorf and D. Haarer, Nature, 1994, 371, 141–143.
13. (a) B.-K. Cho and S.-H. Kim, Soft Matter, 2014, 10, 553–559; (b) K. Ohta, S. Azumane, W. Kawahara, N. Kobayashi and I. Yamamoto, J. Mater. Chem., 1999, 9, 2313–2320.
14. (a) V. Percec, M. Goldde, M. Peterca, A. Rapp, I. Schnell, H. W. Spiess, T. K. Bera, Y. Miura, V. S. K. Balagurusamy, E. Aqad and P. A. Heiney, Chem.–Eur. J., 2006, 12, 6298–6314; (b) V. Percec, E. Aqad, M. Peterca, M. R. Imam, M. Glodde, T. K. Bera, Y. Miura, V. S. K. Balagurusamy, P. C. Ewbank, F. Würthner and P. A. Heiney, Chem.–Eur. J., 2007, 12, 3330–3345; (c) V. Percec, M. R. Imam, M. Peterca, D. A. Wilson and P. A. Heiney, J. Am. Chem. Soc., 2009, 131, 1294–1304; (d) V. Percec, M. R. Imam, M. Peterca, D. A. Wilson, R. Graf, H. W. Spiess, V. S. K. Balagurusamy and P. A. Heiney, J. Am. Chem. Soc., 2009, 131, 7662–7677; (e) H. C. Holst, T. Pakula and H. Meier, Tetrahedron, 2004, 60, 6765–6775.
15. S. Choi and B.-K. Cho, Soft Matter, 2013, 9, 4241–4248.
16. (a) M.-H. Ryu, J.-W. Choi and B.-K. Cho, J. Mater. Chem., 2010, 20, 1806–1810; (b) K. Han and B.-K. Cho, Soft Matter, 2014, 10, 7588–7594.
17. (a) M. Yoshio, T. Mukai, H. ohno and T. Kato, J. Am. Chem. Soc., 2004, 126, 994–995; (b) V. Percec, C. M. Mitchell, W.-D. Cho, S. Uchida, M. Glodde, G. Ungar, X. Zeng, Y. Liu, V. S. K. Balagurusamy and P. A. Heiney, J. Am. Chem. Soc., 2004, 126, 6078–6094.
18. M. Yoshio, T. Kagata, K. Hoshino, T. Mukai, H. Ohno and T. Kato, J. Am. Chem. Soc., 2006, 128, 5570–5577.
19. (a) E. Beltrán, J. L. Serrano, T. Sierra and R. Giménez, Org. Lett., 2010, 12, 1404–1407; (b) J. Barberá, L. Puig, J. L. Serrano and T. Sierra, Chem. Mater., 2004, 16, 3308–3317.
20. (a) K. Katoh, S. Vehara, S. Kohmoto and K. Kishikawa, Chem. Lett., 2006, 35, 322–323; (b) J. Barberá, J. Jiménez, A. Laguna, L. Orion, S. Pérez and J. L. Serrano, Chem. Mater., 2006, 18, 5437–5445; (c) M. Lehmann, G. Kestemont, R. G. Aspe, C. Buess-Herman, M. H. J. Koch, M. G. Debije, J. Piris, M. P. de Haas, J. M. Warman, M. D. Watson, V. Lemaur, J. Cornil, Y. H. Greets, R. Gearba and D. A. Ivanov, Chem.–Eur. J., 2005, 11, 3349–3362.
21. S. Park, M.-H. Ryu, T. J. Shin and B.-K. Cho, Soft Matter, 2014, 10, 5804–5809.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, synthesis schemes, ¹H and ¹³C-NMR data, GPC data, optical textures, GIXS data of 1, and Gaussian peak separation of the WAXS data of 1. See DOI: 10.1039/c4sm02004a

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