Effect of Conformational Chirality on Optical Activity Observed in a

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Effect of Conformational Chiralty on Optical Activity Observed in an Achiral, Bent-Core Molecule Yue Shi, Zeyong Sun, Ran Chen, Chenhui Zhu, Richard K. Shoemaker, Ethan Tsai, David M. Walba, Matthew A Glaser, Joseph E. Maclennan, Dong Chen, and Noel A. Clark J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017

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The Journal of Physical Chemistry

Effect of Conformational Chiralty on Optical Activity Observed in an Achiral, Bent-Core Molecule Yue Shi+, †, ‡, Zeyong Sun+, †, §, Ran Chen†, §, Chenhui Zhu‡, Richard K. Shoemaker‖, Ethan Tsai‖, ⊥, David M. Walba‖, Matthew A. Glaser‡, Joseph E. Maclennan‡, Dong Chen*, †, ‡, § and Noel A. Clark*, ‡ †

State Key Laboratory of Fluid Power and Mechatronic Systems, Zhejiang University, Zheda Road No. 38, Hangzhou, 310027, China

‡

Department of Physics and Soft Materials Research Center, University of Colorado, Boulder, CO 80309-0390, USA. §

Institute of Process Equipment, College of Energy Engineering,

Zhejiang University, Zheda Road No.38, Hangzhou, 310027, China ‖

Department of Chemistry and Biochemistry and Soft Materials Research Center, University of Colorado, Boulder, Colorado 80309-0215, USA. ⊥

Department of Chemistry, Metropolitan State University of Denver, Denver, Colorado 80217, USA *E-mail: [email protected] ; [email protected]

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ABSTRACT Smectic layers of achiral, tilted bent-core liquid crystal molecules are chiral but their optical activity is generally small. Here, we study the effect of conformational chirality on optical activity in an achiral, bent-core mesogen, W513. The neat material has a modulated B4 phase, which appears dark under crossed polarizers and shows no observable optical rotation under decrossed polarizers. However, in mixtures of W513 with a rod-like mesogen, 8CB, distinct left- and right-handed chiral domains with opposite optical activity are observed. The optical behavior of the mixtures is consistent with NMR results, which show a splitting of the carbonyl peak of the bent-core molecules into two, indicating a twisted conformation between the two molecular arms of the bent-core molecules as in conventional B4 materials.

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INTRODUCTION The properties of liquid crystal (LC) phases depend strongly on their molecular structures. Rod-like molecules are generally made of a rigid core and one or two flexible alkyl tails and upon cooling these molecules can form nematic, smectic A and smectic C phases 1. In the smectic C phase, the molecules self-assemble into layers with their molecular long axis tilted from the layer normal. The smectic C phase becomes polar when the molecules are chiral and the mirror symmetry is lost 2. In recent years, bent-core liquid crystals have drawn considerable attention 3-6, exhibiting several new phases with distinct molecular packing and phase properties, such as the B1-B8, helical nanofilament (HNF) 7, dark conglomerate (DC)

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and twist-bend

nematic (NTB) phases 9-11. In contrast to rod-like molecules, when bent-core molecules form smectic layers and their molecular axes tilt from the layer normal, there is spontaneous reflection symmetry breaking and the layers become chiral (also called layer chirality), even if the molecules themselves are achiral 12. However, the optical activity of the tilted, achiral, bent-core molecules is generally small and barely observable 13-15. Macroscopic left- and right-handed chiral domains were first observed in the B4 phase (also called the HNF phase) 7, which remains one of the most intriguing bent-core phases 16-24. The B4 phase appears dark under crossed polarizers but in most materials chiral domains with opposite optical rotation are easily distinguished under decrossed polarizers. Previous studies have revealed that left- and right-handed B4 domains are made of left- and right-handed HNFs, which have a typical width of ~30 3



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nm and a pitch of ~200 nm 25. Though the large optical activity typically observed in the B4 phase may be attributed to the microscopic twisted structure of the HNFs rather than the layer chirality, similar optical behavior is observed in the DC phase, which is made of disordered focal conics with no microscopic chiral structure. Conformational chirality (the conformational twisting between the two molecular arms of the bent-core molecules) is expected to be partly responsible for the observed spontaneous reflection symmetry breaking and to contribute to the large optical activity

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. The existence of conformational chirality is supported by the splitting of

the carbonyl peak of the bent-core molecules observed in nuclear magnetic resonance (NMR), an observation which suggests that in typical B4 materials the conformation of the bent-core molecules is frozen by intermolecular interactions in the smectic phase and that the two molecular arms adopt a twisted conformation with respect to each other 27. In this paper, we further investigate the relationship between conformational chirality and optical activity and study the properties of neat W513, a bent-core mesogen, and its mixtures with 8CB, a rod-shaped mesogen, using depolarized transmission light microscopy (DTLM), freeze-fracture transmission electron microscopy (FFTEM), X-ray diffraction (XRD), and nuclear magnetic resonance (NMR). In the neat material, the B4 phase has in-plane modulation of the layering and is essentially dark showing no observable optical rotation in the microscope. The NMR experiments show that the carbonyl groups on the two molecular arms are identical, which suggests that the two molecular arms are in the same plane and there is no intramolecular twist. However, 4


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in mixtures with 8CB, the B4 phase is more conventional, showing strong optical rotation in distinct left- and right-handed chiral domains. The carbonyl peak in the NMR spectrum splits into two, suggesting that the two molecular arms in this case are twisted with respect to each other and supporting the notion that the observed large optical activity is strongly related to this twist.

EXPERIMENTAL RESULTS AND DISCUSSIONS In the well-known P-n-OPIMB homologous series, the achiral, bent-core molecules possess the classic double Schiff-base structure and the B4 phase generally appears below the B2/B3 phase 7. In contrast to the P-n-OPIMB mesogens, W513 possesses an unsymmetrically substituted bisphenol unit as a rigid central linker, as shown in Figure 1, and exhibits a B4mod phase below the B1 on cooling 28. The B1 phase shows a typical fan-like texture at T=168C, as shown in Figure 2a, and subsequently transitions to the B4mod phase, in which the texture becomes essentially dark under crossed polarizers, as shown in Figures 2b and c. To visualize the microstructure of the B4mod phase directly, the sample is quenched at T=160C and fractured in the bulk. The topology of the fracture plane is replicated by a 2-nm-thick platinum coating shadowed at 45 and reinforced with carbon. The replica is then imaged in a transmission electron microscope (TEM). TEM images of the bulk and of the free surface reveal that the microstructure of the B4mod phase is dominated by saddle-splay curvature of the layers and shows a flame-tip-like texture, as seen in Figures 2d and e.

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This topology is typical of helical nanofilaments, in which smectic layers of tilted bent-core molecules deform to form twisted structures with saddle-splay curvature in order to relieve the internal strain induced by the orthogonal tilt directions of the top and bottom molecular arms 7. The above observations suggest that the B4mod phase has a structure similar to the conventional B4 helical nanofilament phase 7. The presence of saddle-splay curvature, which is driven by the intra-layer structural mismatch of tilted smectic layers, suggests that in the B4mod phase bent-core molecules are tilted from the layer normal. Since smectic layers of tilted bent-core molecules are chiral, the left- and right-handed chiral domains are expected to possess opposite optical activity and become distinguishable under decrossed polarizers, as observed in the P-n-OPIMB materials. However, in contrast to the conventional B4 HNF phase, no left- or right-handed chiral domains can be distinguished in the B4mod phase under decrossed polarizers. To further investigate the B4mod phase, we mixed W513 with different concentrations of 8CB, a rod-like molecule which serves as a solute in the system and lowers the phase transition temperature of the bent-core molecules

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, to further study their phase

properties. As in the previously studied 8CB/P-9-OPIMB mixtures

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, the B1 phase of W513

disappears in the mixture due to freezing point depression and the B4 phase nucleates and grows directly from the isotropic phase, even in the c=10% 8CB/W513 mixture, where c is the concentration of 8CB. On cooling an initially isotropic mixture, the bent-core molecules of W513 first undergo a total phase separation into the B4 phase 6


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with the 8CB remaining isotropic. In contrast to the B4mod phase observed in neat W513, left- and right-handed chiral domains with opposite signs of optical activity are easily seen in 8CB/W513 mixtures under decrossed polarizers, as shown in Figures 3a, b, c and e. Because these domains are the result of the nucleation and chirality-preserving growth

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, each chiral domain is made of HNFs of the same

handedness. FFTEM images confirm that, the microstructure of these chiral domains is dominated by saddle-splay curvature of the layers and that there are distinct helical nanofilaments, visible in Figures 3d and f. Previous studies of the HNFs in the 5CB/P-8-OPIMB mixtures by circular dichroism (CD) suggest that the magnitude of the CD spectra decreases almost linearly as the concentration of 5CB increases

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. In

the c=40% 8CB/W513 mixture, two different kinds of boundary are observed: the boundaries between domains with the same chirality are faint, black lines while the boundaries between domains with different chirality are decorated by stubs that penetrate into each domain (Figure 3c). The phenomena are attributed to the twisted boundary condition presented by the homochiral HNFs to guest molecules

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and the

change of boundary twist between domains of HNFs with different handedness. Though the microstructures observed in neat W513 and in 8CB/W513 mixtures are both dominated by saddle-splay curvature, X-ray diffraction from the smectic layers reveals some minor differences. The scattering of the B4mod phase shows the fundamental (q~0.132 Å-1) and the second harmonics of Bragg scattering from the smectic layers and an additional peak (q~0.148 Å-1), which results from an in-plane layer modulation, as shown in Figure 4a. In the c=76% 8CB/W513 mixture, there is 7



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no in-plane layer modulation and the spacing of the smectic layers (d ~ 50 Å) is increased by 2.5 Å, over that observed in neat W513 (d ~ 47.5 Å), as shown in Figure 4b. This behavior is different from the 8CB/P-9-OPIMB system, in which the layer spacing in the neat material (d ~ 48.3 Å) is essentially the same as that in mixtures (d ~ 48.6 Å) 25. The increase of layer spacing in the W513 mixture, therefore, suggests a change of molecular conformation in the mixture that we propose results in the optical activity exhibited by the left- and right-handed chiral domains. To verify this hypothesis and clarify the relationship between molecular conformation and optical activity, we performed NMR measurements to probe the molecular conformation in both the neat material and mixtures. Details of the solid-state NMR experiment were given previously in reference

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and

may be summarized as follows. Solid-state NMR spectra were acquired using a Varian Inova 400 spectrometer equipped with an extended, variable-temperature, 4 mm, cross-polarization, magic-angle-spinning (CPMAS) probe, operating at 100.63 MHz for 13C observation. CPMAS spectra were acquired using a 90º pulse of 3.4 μs and a typical MAS spinning frequency of 11 kHz, with a Hartmann-Hahn spin-locking field of 71.5 kHz for both 1H and 13C. Typical cross-polarization contact times were 4.0 ms, and TPPD 1H decoupling was applied during acquisition. We first recorded the CPMAS NMR spectrum of neat W513 at T=20C and used ACD/Laboratories NMR software (version 2016) to help identify the chemical shift of the carbons, as plotted in Figure 5. Similar to the CPMAS NMR spectrum of P-9-OPIMB 31, the peaks between 10 ppm and 40 ppm are the signals from the tails of 8


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the W513 molecules, the peak at around 67 ppm corresponds to the chemical shift of the carbons in the alkoxyl groups at the end of the bent core, and the peaks between 110 ppm and 170 ppm come from the bent cores. The peak at around 164 ppm corresponds to the chemical shift of the carbons in the two carbonyl groups in the bent core (Figure 1) and the observation of a single peak suggests that the two carbonyl groups of neat W513 are in the same plane and have a similar micro-environment, as indicated in Figure 5a, giving identical chemical shifts. The CPMAS NMR peaks observed in c=10% 8CB/W513 are generally sharper than that in neat W513. This is because the B4 phase of W513 in the mixture grows in from the isotropic and is able to form larger domains, which have fewer inherent inhomogeneities than the small saddles that grow from the B1 phase of neat W513. The long HNFs formed in the mixture give a sharper NMR spectrum than the small saddles. Most importantly, the single peak at around 164 ppm observed in neat W513 splits into two peaks in the mixture, suggesting that there is a conformational change of W513 molecules in the 8CB/W513 mixture so that the two carbonyl groups in the bent core are in different micro-environments, as indicated in Figure 5b. In addition, the peak at around 67 ppm, which corresponds to the chemical shift of the carbons in the alkoxyl groups located at the end of the bent core, also split into two. Similar behaviors are also observed in the HNFs of P-9-OPIMB at room temperature

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, and

the splitting of the chemical shift of the carbons in the alkoxyl groups is also an indication of a conformational change.

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The splitting of the NMR peaks of the carbons in the carbonyl groups and the carbons in the alkoxyl groups from single peak in neat material into two peaks in mixtures is accompanied by the change of the phase sequences. In neat W513, the B4mod phase appears below a two-dimensionally-ordered B1 phase, while in the mixtures the B4 phase forms directly from the isotropic. The double peaks observed in the NMR spectra of the mixtures are similar to those observed in the HNFs of conventional bent-core molecules, suggesting a twist conformation of the two molecular arms 32

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. These results are consistent with the optical observations. In the neat material, the

B4mod phase is dark under crossed polarizers and no left- and right-handed chiral domains can be distinguished when the polarizers are decrossed. This implies that in neat W513 there is no twist of the bent core and the optical activity of left- and right-handed chiral domains due to the layer chirality is too small to be observed 14. In the mixtures, the planes of the two molecular arms of the bent-core molecules adopt a twisted conformation and the resulting chiral structure on the molecular scale strongly increases the optical activity of the left- and right-handed chiral domains. 8CB is a conformationally labile molecule, with a number of low-energy, thermally accessible conformations involving one or more gauche “bends” in the alkyl chains (see Table II in reference 33, which is an MD simulation study of 8CB SmA), and there are also several distinct equal-energy twisted conformations of the biphenyl core (see Figure 14 in reference 33). Although 8CB is chemically achiral, the vast majority of molecular conformations are chiral (lack mirror reflection symmetry). However, distinct chiral conformations related by mirror reflection occur at equal probability in 10


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the fluid phases of 8CB. Note also that the energy barriers between distinct rotational isomers are on the order of a few kBT at room temperature (see Figure 14 in reference 33). Interconversion between distinct molecular conformations is rapid in the fluid phases of 8CB (~10-100 ps). It’s seems highly unlikely that there is sufficient local chirality in the isotropic phase of 8CB to significantly influence the chiral nucleation and growth of HNFs. Especially, the HNFs that form in 8CB/W513 mixtures are nearly pure W513, with most 8CB expelled from the HNFs when they form. This also suggests that the conformational behavior of 8CB is largely irrelevant to the molecular organization of the HNFs. A likely scenario of the effect of 8CB is that this is a purely kinetic phenomenon: in pure W513, the B4mod phase nucleates and grows from the B1 phase, whereas in the 8CB/W513 mixtures the HNF phase nucleates and grows from the isotropic phase. In this scenario, the B4mod phase may be a metastable state, which is more kinetically accessible than the HNF phase on cooling from the B1 phase.

SUMMARY In neat W513, a bent-core liquid crystal, a B4mod phase appears below the B1 phase and is characterized by saddle-splay curvature of the smectic layers. In thin cells the material presumably forms domains corresponding to left- and right-handed helical nanofilaments. In contrast to conventional B4 phase materials, however, these domains have no observable optical activity. NMR measurements confirm that in neat

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material the two carbonyl groups are identical and that there is no twist conformation in the molecule. In mixtures with 8CB, the B4 phase appears directly from the isotropic on cooling. These domains show strong optical rotation and distinct left- and right-handed chiral domains are observed under decrossed polarizers. The optical observations are consistent with the NMR results that the two molecular arms of the bent core adopt a twisted conformation, suggesting the strong effect of conformational chirality on optical activity.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]（D.C.） *E-mail: [email protected]（N.A.C）

ORCID Zeyong Sun: 0000-0002-1339-5561 Dong Chen: 0000-0002-8904-9307

Author Contributions +These authors contributed equally

Note The authors declare no competing financial interest.

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ACKNOWLEDGEMENTS

D.C. acknowledges the Youth Funds of the State Key Laboratory of Fluid Power and Mechatronic Systems (Zhejiang University) and the “Thousand Talents Program” for Distinguished Young Scholars. This work was supported by the Soft Materials Research Center under NSF MRSEC Grants DMR-0820579 and DMR-1420736.

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(26) Sekine, T.; Niori, T.; Sone, M.; Watanabe, J.; Choi, S. W.; Takanishi, Y.; Takezoe, H. Origin of Helix in Achiral Banana-Shaped Molecular Systems. Jpn. J. Appl. Phys. 1997, 36, 6455-6463. (27) Walba, D. M.; Eshdat, L.; Eva Körblova, A.; Shoemaker, R. K. On the Nature of the B4 Banana Phase: Crystal or Not a Crystal? Cryst. Growth Des. 2005, 5, 2091-2099. (28) Tsai, E.; Richardson, J. M.; Korblova, E.; Nakata, M.; Chen, D.; Shen, Y.; Shao, R.; Clark, N. A.; Walba, D. M. A Modulated Helical Nanofilament Phase. Angew. Chem., Int. Ed. 2013, 52, 5254. (29) Otani, T.; Araoka, F.; Ishikawa, K.; Takezoe, H. Enhanced Optical Activity by Achiral Rod-Like Molecules Nanosegregated in the B4 Structure of Achiral Bent-Core Molecules. J. Am. Chem. Soc. 2009, 131, 12368-12372. (30) Chen, D.; Tuchband, M. R.; Horanyi, B.; Korblova, E.; Walba, D. M.; Glaser, M. A.; Maclennan, J. E.; Clark, N. A. Diastereomeric Liquid Crystal Domains at the Mesoscale. Nat. Commun. 2015, 6, 7763. (31) Chen, D.; Zhu, C.; Shoemaker, R. K.; Korblova, E.; Walba, D. M.; Glaser, M. A.; Maclennan, J. E.; Clark, N. A. Pretransitional Orientational Ordering of a Calamitic Liquid Crystal by Helical Nanofilaments of a Bent-Core Mesogen. Langmuir 2010, 26, 15541. (32) Kurosu, H.; Kawasaki, M.; Mitsuyo Hirose, A.; Yamada, M.; Kang, S.; Thisayukta, J.; Sone, M.; Hideo Takezoe, A.; Watanabe, J. Solid-State 13C NMR Study of Chiral Twisted Conformation Attributable to Chirality in Smectic Phases of Achiral Banana-Shaped Molecules. J. Phys. Chem. A 2004, 108, 4674-4678. (33) Lansac, Y.; Glaser, M. A.; Clark, N. A. Microscopic structure and dynamics of a partial bilayer smectic liquid crystal. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2001, 64, 051703.

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Figure 1.Chemical structure of the achiral, bent-core mesogen W513 and its phase

diagram. Each arm of the bent core includes a carbonyl group. In neat W513, the isotropic melt of bent-core molecules first transitions to the B1 phase and then to the B4mod phase on cooling. However, in a c=40% 8CB/W513 mixture, the B4 phase appears directly below the isotropic phase.

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Figure 2.Depolarized transmission light microscope and transmission electron

microscope images of neat W513. (a)-(c) DTLM images showing the B1-B4mod phase transition at around T=168C: (a) fan-like texture of the B1 phase; (b) and (c) the B4mod phase is essentially dark between crossed polarizer and analyzer. (d) FFTEM image of the B4mod phase quenched at T=160C and then fractured in the bulk. (e) TEM image of the B4mod phase quenched at T=155C and imaged at the air/liquid crystal interface. The microstructure of the B4mod phase is dominated by smectic layers with saddle-splay curvature and shows a flame-tip-like texture. The scale bars in (a), (b) and (c) are all 100 µm and the cell thickness is about 4 μm.

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Figure 3.DTLM and FFTEM images of 8CB/W513 mixtures in the B4 phase. (a) and

(b) Optical texture of a c=10% 8CB/W513 mixture, showing left- and right-handed chiral domains between decrossed polarizers. (c) Optical texture of a c=40% 8CB/W513 mixture. There are distinct boundaries between the left- and right-handed chiral domains. (d) FFTEM image of a c=40% 8CB/W513 mixture quenched at T=60C (where 8CB is isotropic) and fractured in the bulk. Helical nanofilaments are observed in the mixture. The dark, faint lines are residuals of HNFs left on the replica. (e) DTLM and (f) FFTEM images of a c=76% 8CB/W513 mixture. The sample is quenched at T=37C (where 8CB is nematic) and fractured in the bulk. 18


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Figure 4.X-ray diffraction of neat W513 and a c=76% 8CB/W513 mixture. (a) In neat

W513, in addition to the fundamental (q~0.132 Å-1) and second harmonics of Bragg scattering from the smectic layers, there is an additional peak (q~0.148 Å-1), corresponding to an in-plane modulation. (b) In a c=76% 8CB/W513 mixture, the layer spacing is increased by ~2.5 Å and there is no layer modulation.

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Figure 5.Analysis of molecular conformation in neat W513 and a c=10% 8CB/W513

mixture. (a) In neat material, the two molecular arms of the bent core are in the same plane and the two carbonyl groups have a similar micro-environment. (b) In mixtures, 20


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the two molecular arms are twisted with respect to each other and the two carbonyl groups are in different micro-environments. (c) NMR spectrum of neat W513 and a c=10% 8CB/W513 mixture at T=20C. In neat W513, there is only one peak corresponding to the chemical shift of the carbons in the two carbonyl groups, suggesting that the chemical environments of the carbonyl groups on the two molecular arms are similar. In the c=10% 8CB/W513 mixture, this single peak splits into two, indicating a twisted conformation of the two molecular arms. Similar change is observed in the peak that corresponds to the chemical shift of the carbons in the two alkoxyl groups at the end of the bent core.

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Effect of Conformational Chirality on Optical Activity Observed in a

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