Evidence of Biaxial Order in the Cybotactic Nematic Phase of Bent

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Evidence of Biaxial Order in the Cybotactic Nematic Phase of BentCore Mesogens Francesco Vita,† Tatum Tauscher,‡ Frank Speetjens,‡ Edward T. Samulski,§ Eric Scharrer,‡ and Oriano Francescangeli*,† †

Dipartimento SIMAU and CNISM, Università Politecnica delle Marche, via Brecce Bianche, 60131 Ancona, Italy Department of Chemistry, University of Puget Sound, 1500 N. Warner, Tacoma, Washington 98416,United States § Department of Chemistry, University of North Carolina, CB#3290, Chapel Hill, North Carolina 27599, United States ‡

S Supporting Information *

T

signature of long-range biaxiality);9 this was recently confirmed by direct observation of cybotactic groups in cryo-transmission electron microscope (cryo-TEM) images.10 However, the clusters’ biaxial order could not be inferred directly from XRD measurements. Indeed, local biaxiality has usually been assumed based on the following: (i) by analogy with the Sm phases of bent-core mesogens (banana phases), which exhibit well-known biaxial and polar properties;6a (ii) from the results of molecular simulations performed on BCNs;9a,11 (iii) indirectly, via several experimental observations (e.g., dynamic light scattering,12 field-induced biaxiality,13 ferroelectric response,9a,14 and extraordinary field sensitivity15), whose explanation entails the existence of biaxial, and possibly polar, clusters; and (iv) above all, from a detailed analysis of NMR spectra.8b,16 Here we report a comparative study of the WA XRD pattern of a series of ODBP BCNs, some of them possessing lateral methyl groups (Chart 1 and Supporting Information). The splitting of the WA diffuse crescents that we observe in trimethylated compounds 4 and 5 represents the first unequivocal XRD evidence of biaxial order in the fluid cybotactic N phase of bent-core mesogens. The parent compound, 1, is a short alkyl tail homologue of the dodecyloxy-tailed ODBP mesogen first reported to exhibit a Nb phase;7 both BCNs show a four-spot SA pattern indicative of cybotactic order.9b,e The synthesis, phase behavior, and structural characterization of methylated derivatives 2−5 have been described elsewhere:17 while all of them exhibit an enantiotropic cybotactic N phase, trimethylated compounds 4− 5 show much lower N onset temperatures and a propensity for supercooling the cybotactic order down to RT in a metastable, highly viscous state. XRD measurements were performed on samples placed in capillaries at the BM16 beamline of the European Synchrotron Radiation Facility (ESRF), France. A horizontal magnetic field B (B = 1 T) was applied orthogonal to the beam direction to align the n molecular director, i.e., the average orientation of the long molecular axes. Parts a−d and e−h of Figure 1 show typical WA patterns from samples 2 and 4, respectively, taken on cooling from the isotropic (I) melt under a B field (see Supporting Information for patterns of other BCNs). In the SA

hermotropic biaxial nematicsmelts exhibiting three distinct directions of molecular orientational order1 have attracted considerable interest as promising candidates for a new generation of ultra-fast switching displays. However, despite decades of intensive investigation, the realization of such an unusual fluid phase is still a hotly debated topic in liquid crystal science. After Freiser’s theoretical prediction in 1970,2 it took 10 years before Yu and Saupe reported the first experimental demonstration of a biaxial nematic (Nb) phase in a lyotropic system consisting of amphiphilic aggregates (micelles) in solution.3 Subsequently, biaxial order was reported for a number of low molecular weight thermotropic nematics characterized by large molecular shape biaxiality.4 However, these latter claims were primarily based on optical measurements and, contrary to Yu and Saupe’s system, could not be confirmed by nuclear magnetic resonance spectroscopy (NMR).1a Among the papers published at that time, one by Wedler et al. received little attention:5 it reported a room temperature (RT) glassy Nb phase in mixtures of low molecular weight β-naphthylester derivatives, signified by the splitting of the wide-angle (WA) X-ray diffraction (XRD) maxima and confirmed by conoscopic measurements. However, these signatures of biaxiality disappeared at higher temperatures, indicating an apparent uniaxial fluid nematic (Nu) phase. More recently, the development of bent-core nematics (BCNs) suggested new strategies for realizing biaxial order:1b−d,6 Two papers published in 2004, one based on small-angle (SA) XRD measurements,7a the other one on NMR spectroscopy,7b reported experimental evidence of biaxiality in BCNs based on the 2,5-bis(p-hydroxyphenyl)-1,3,4-oxadiazole (ODBP) mesogenic core. These results soon fueled an intense scientific debate: some authors cited the difficulty of an undisputable verification of biaxiality and proposed alternative interpretations of the experimental data; others cast doubts on the true nature of the observed biaxial orderlocal vs macroscopic, spontaneous vs induced (e.g., by surfaces or external fields).1c,6b,8 In spite of these contentious and opposing views, a general consensus has finally emerged: the unique properties of BCNs result from the presence of nanometric size clusters of molecules (cybotactic groups) characterized by shortrange smectic (Sm) positional order and biaxial orientational order.8−10 Clear evidence of tilted Sm−C-like order within clusters is provided by the four-spot SA XRD pattern typical of BCNs (the same pattern that was initially interpreted as a © XXXX American Chemical Society

Received: May 31, 2014 Revised: July 14, 2014

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dx.doi.org/10.1021/cm5019822 | Chem. Mater. XXXX, XXX, XXX−XXX

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Chart 1. Structures and Phase Behavior of BCNsa and Schematic Drawing of Molecular Packing within a Cybotactic Clusterb

hand, the diffuse crescents centered on the equatorial axis (i.e., perpendicular to n) in the WA region reflect the mesogens’ lateral ordering. In this regard, a clear difference is observed between BCNs 1−3 and BCNs 4−5: in fact, contrary to the former, the trimethylated compounds show radially asymmetric WA crescents, whose maxima are shifted toward larger q values. The different intensity profiles of 4 and 5 are apparent if one looks at the q-scans obtained by azimuthal integration of narrow (5°) intensity slices orthogonal to n:18 apart from the low hump in the SA region which comes from the four-spot feature, BCN 2 exhibits a single symmetric WA peak irrespective of the temperature (Figure 1i); conversely, BCN 4 shows an asymmetric maximum clearly consisting of two distinct diffraction peaks (Figure 1j). The latter effect is more evident at lower temperature but is still visible at T = 115 °C, i.e., very close to the clearing point. The visual impression is confirmed by a quantitative analysis of the intensity profiles: while the WA peak of BCNs 1−3 (at any temperature) can be fitted very well by a single Voigt line shape (indicated as p1 in Figure 2a), the maxima of BCNs 4−5 require the superposition of two Voigt functions (p1 and p2 in Figure 2b−d; see Supporting Information for further examples). The values of the d-spacing obtained from the peak positions in q-space are summarized in Table S1 of Supporting Information at a few selected temperatures for each sample, together with the corresponding correlation lengths ξ indicating the extent of the short-range positional order. Following Leadbetter et al.,19 the latter were calculated from ξ = 9.92/Δq, where Δq is the peak full width at half-maximum (fwhm). BCNs 1−3 exhibit dspacings, i.e., average transverse intermolecular distances, in the range 4.6−4.3 Å, typical for BCNs; these slightly decreasing as the temperature is lowered. Transverse correlation lengths are very short (∼13 Å), as expected in nematics whose liquid-like positional order only reflects nearest-neighbor correlations. On the other hand, BCNs 4 and 5 are characterized by two d-spacings, corresponding to two distinct intermolecular lateral distances: d1 ≈ 4.9 Å and d2 ≈ 3.8 Å (see Chart 1), the former value being only slightly larger than the d-spacings of BCNs 1− 3. For both 4 and 5 the difference d1−d2 slightly decreases with increasing temperature, but it is still ≥1.0 Å on approaching the clearing point. As generally recognized in the literature,1c such a split of the WA pattern is a clear signature of biaxial order. In fact, d2 is typical of the face-to-face distance between stacked πsystems, while d1 is closer to the width of a planar aromatic ring (e.g., compare with values reported for other systems, such as biaxial nematic glasses,5 biaxial Sm-A phases,20 and lowtemperature Nb phase of shape-persistent V-shaped mesogens21). Interestingly, the correlation lengths are significantly different along the two lateral directions, being slightly larger than 2d1 in the plane of the core, and between ∼4d2 and ∼6d2 (depending on the sample) in the orthogonal direction. These values are the same order of magnitude as the transverse size of cybotactic clusters estimated from the width of the SA diffraction spots.17 To correctly interpret these data, one should consider that positions and widths of the WA features provide direct information on transverse positional order, whereas biaxiality relates to orientational order. However, positional and orientational order are not independent of each other, as close packing of nonlinear mesogens dictates that changes in the latter affects the former. In particular, the widths of the WA peaks are related to the short-range positional order in the two transverse directions; they are not directly connected to the spatial extent

a

Transition temperatures (°C) measured on heating with isotropization enthalpies (kJ/mol). Abbreviations: crystal (Cr), nematic (N), isotropic (I). bk indicates the normal to the smectic plane (green), n, m, and l are the three orthogonal molecular directors; d1 and d2 are the intermolecular distances in the plane normal to n (purple).

Figure 1. Color-coded XRD diffraction patterns of BCN 2 (a−d) and 4 (e−h) recorded at selected temperatures while cooling the samples under a horizontal magnetic field and the corresponding equatorial intensity profiles for BCN 2 (i) and 4 (j).18 Patterns d and f−h are in the supercooled N phase.

region of the XRD pattern all of the samples exhibit the characteristic four spots of cybotactic BCNs. On the other B



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Figure 2. Fit of the equatorial intensity profiles with two (a) or three (b−d) Voigt lineshapes: (a) BCN 2 at 140 °C; (b) BCN 4 at 115 °C; (c) BCN 4 at 85 °C; and (d) BCN 4 at 25 °C.

clearing point, with no sign of a biaxial−uniaxial N phase transition. Finally, we emphasize that the observed biaxial order is essentially spontaneous, as it was observed in bulk samples, without using electric fields or aligning surfaces. In fact, it is expected that the applied magnetic field only affects the ordering of the long molecular axes, having a negligible effect on the transversal ordering of the mesogens. In conclusion, we provide the first direct XRD evidence of spontaneous biaxial order in low molecular weight thermotropic BCNs via a splitting of the WA crescents. Although our data point to a link between biaxiality and the ability of BCNs to be supercooled, the splitting was observed over the whole cybotactic N range. In the absence of a monodomain biaxial sample, we could not determine whether biaxial order extends beyond the size of cybotactic clusters on a macroscopic length scale. However, the fact that the WA pattern of BCNs 4−5 is so different from that of other cybotactic BCNs, such as 1−3 clearly makes our trimethylated compounds prime candidates for true, i.e., spontaneous and macroscopic, biaxiality. We are pursuing further investigations by complementary experimental techniques on this class of biaxial nematics.

of orientational correlations. The situation is analogous to the scattering from aligned uniaxial nematics, where the fwhm of the WA crescents provides the nearest-neighbor positional correlation length, which is much shorter than the macroscopic range of orientational order.19a In our case, evaluating the range of orientational order would require a monodomain specimen with the transverse molecular directors macroscopically aligned. In principle this could be achieved by simultaneous application of mutually orthogonal electric and magnetic fields. The unambiguous evidence of biaxial order provided by our XRD data naturally raises the question of why similar evidence has never been observed before in BCNs, even though cybotactic clusters are generally assumed to be biaxial. The most obvious answer would be that, given the very diffuse nature of the WA scattering in N phases, small differences of the mean lateral distances are not detected unless they are large enough to resolve the WA feature into two distinct maxima. In line with the arguments of Tschierske and Photinos,1c the difference between the two lateral distances in bent-core mesogens (unlike for board-like molecules) is expected to be very small as the molecular biaxiality would result only from the nonlinear connection of two rod-like units; accordingly, they conclude, “...XRD is most probably blind to biaxiality in the case of bent-core mesogens.” In this context, our results provide the first example of BCNs whose biaxial order can be accessed by XRD. As the molecular shape biaxiality of 4 and 5 does not differ significantly from that of 1−3 (and of most of the BCNs reported so far in the literature), we must infer that the WA splitting observed in our trimethylated compounds reflects a higher degree of local biaxial order compared to the other BCNs. This means an enhanced orientational correlation in the transverse molecular packing, possibly promoted by stronger anisotropic interactions between nearest-neighbor mesogens. Indeed, there seems to be a clear relationship between the split of the WA crescents and the ability of the mesogens to be supercooled into an RT N phase. This is not surprising, as Freiser originally emphasized that retarding crystallization might be a prerequisite for accessing the low temperature Nb phases.1 Moreover, biaxiality is naturally promoted by the quenching of rotational diffusion, so that it is often observed in supercooled systems on approaching the glass transition,5,21 in particular in the case of biaxial N polymers.22 In our compounds, this behavior is certainly determined by the presence of the additional methyl groups in the outer part of the molecular structure, but it remains to be determined the extent to which the enhanced orientational correlation underlying the observed effect is due to steric hindrance effects (e.g., peculiar conformations induced by the outer substituents). Furthermore, it is noteworthy that, contrary to previous reports, we observe evidence of biaxial order even close to the

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ASSOCIATED CONTENT

S Supporting Information *

Molecular structures and phase behavior on heating and cooling; XRD experimental details; XRD patterns and fitted profiles for BCNs 1, 3, and 5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge F. Fauth, C. Ferrero, J. Gorini, E. Di Cola, T. Narayanan (ESRF), and I. F. Placentino (Università Politecnica delle Marche) for their support in XRD measurements. E.T.S. acknowledges support from the Boshamer Professorship. E.S. wishes to thank NSF (DMR-1005923) for financial support of this research.

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REFERENCES

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Evidence of Biaxial Order in the Cybotactic Nematic Phase of Bent

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