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Where are we with the discovery and design of biaxial nematics? Geoffrey Luckhurst

Where are we with the discovery and design of biaxial nematics? Geoffrey Luckhurst School of Chemistry, University of Southampton, UK. Praefcke, Kohne, Singer, Demus, Pelzl, Diele, Liq. Cryst., 1990, 7 , 589. Li, Percec and Rosenblatt, Phys. Rev. E, 1993, 48 , R1.

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Where are we with the discovery and design of biaxial nematics? Geoffrey Luckhurst

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  1. Where are we with the discovery and design of biaxial nematics? Geoffrey Luckhurst School of Chemistry, University of Southampton, UK

  2. Praefcke, Kohne, Singer, Demus, Pelzl, Diele, Liq. Cryst., 1990, 7, 589 Li, Percec and Rosenblatt, Phys. Rev. E, 1993, 48, R1

  3. V-shaped molecules: X-ray scattering • B. R. Acharya, A. Primak, and S. Kumar Phys. Rev. Lett. 2004, 92, 145506 • B. R. Acharya, A. Primak, T. J. Dingemans, E. T. Samulski, S. Kumar, Pramana, 2003, 61, 231

  4. The molecules

  5. The scattering patterns

  6. Calculated scattering patterns

  7. V-shaped molecules: structure and optical studies V. Görtz and J.W. Goodby BLCS, Exeter March 2005

  8. Thermotropic Biaxial Nematic Liquid Crystals ● L.A. Madsen, T.J. Dingemans, M. Nakata, E.T. Samulski, Phys. Rev. Lett. 92, 145505 (2004). ● B.R. Acharya, A. Primak, S. Kumar, Phys. Rev. Lett. 92, 145506 (2004). Features: ● core with high bisecting dipole ● rigid bent-core molecule (~140°) ● biaxiality revealed in 2D powder 2H NMR and X-ray diffraction Drawbacks: ● core with high dipole ● bend molecule with rigid core ● i.e. nematic at inexpediently high temperatures ● materials degrade at these high temperatures

  9. Synthesis of Oxadiazoles No R1 R2 R3 R4 Phase Transitions [°C] 8a C12H25O C12H25O H H Iso 203 N 192 SmC 184 SmX 143 SmY 138 SmZ 104 Cr 8b C12H25O C9H19O H H Iso 210 N 182 SmX 157 SmY 149 SmZ 91 Cr 8c C12H25O C8H17O H H Iso 213 N 176 SmX 162 SmY 152 SmZ 77 Cr 8e C12H25O C9H19O H F Iso 205 N 168 SmX 135 SmY 125 SmZ 72 Cr 8f C12H25O C9H19O F F Iso 210 N 197 SmC 186 SmX 155 SmY 150 SmZ 100 Cr 8g C7H15 C7H15 H H Iso 222 N 173 SmX 151 Cr 8h C7H15 C5H11 H H Iso 232 N 164 SmX 149 Cr 8d C12H25O C5H11 H H Iso 215 N 160 SmX 91 Cr

  10. texture of the nematic phase between slide and coverslip at 222 °C observed by rotating the analyser (a) anticlockwise (b) clockwise despite the achiral molecular structure chiral domains in the nematic phase! Textures of the Biaxial Nematic Phase ODBP-P-C7 Iso 222 N 173 SmX 151 Cr schlieren texture of the nematic phase at 202 °C

  11. Textures of the Nematic Phase texture of the nematic phase between slide and coverslip at 202 °C observed by rotating the analyser (a) anticlockwise (b) clockwise ● G. Pelzl, A.Eremin, S.Diele, H. Kresse, W. Weissflog, J.Mat.Chem. 12,2591 (2002). Cr 98 °C (X 80 °C N 95 °C) I ● P19: M. Hird, K.M. Fergusson, Synthesis and Mesomorphic Properties of Novel Unsymmetrical Banana-shaped Esters. C9O-P-ODBP-P-OC12 Iso 210 N 182 SmX 157 SmY 149 SmZ 91 Cr

  12. Textures of the Nematic Phase C5-P-ODBP-P-C7 Iso 232 N 164 SmX 149 Cr nematic phase in an uncovered region on a glass slide at 167 °C, thinner preparation nematic phase in an uncovered region on a glass slide at 173 °C C9O-2F3FP-ODBP-P-OC12 Iso 210 N 197 SmC 186 SmX 155 SmY 150 SmZ 100 Cr ODBP-P-OC12 Iso 203 N 192 SmC 184 SmX 143 SmY 138 SmZ 104 Cr nematic phase in an uncovered region on a glass slide at 189 °C nematic phase in an uncovered region on a glass slide at 189 °C

  13. Possible Explanations: Suggestion I ● G. Pelzl, A.Eremin, S.Diele, H. Kresse, W. Weissflog, J. Mat. Chem. 12, 2591 (2002). R. Memmer, Liq. Cryst. 29, 483 (2002). helical superstructure in a nematic phase of an achiral bent-core molecule can occur due to conical twist-bend deformations

  14. helix-formation via self-assembly of twisted conformers Possible Explanations: Suggestion II possible twisted chiral conformer

  15. Questions ● Are pitch lines really observed in the nematic? ● Are similar effects to be expected for all achiral bent-core materials that have a nematic phase? ● Is there a connection between these observations and the biaxiality of a nematic phase?

  16. V-shaped molecules: atomistic simulations M. Wilson BLCS, Exeter, March 2005

  17. Bananas are not really bananas! • 4 key dihedrals with low barriers where rotation leads to conformations with radically different structures at a cost of < 2.5 kcal/mol

  18. Min 90/-90 deg Barrier 5 kJ/mol Min 0/180 deg Barrier kJ/mol Min 90/-90 deg Barrier 5 kJ/mol Bananas are not really bananas! • 4 key dihedrals with low barriers were rotation leads to conformations with radically different structures at a cost of < 2.5 kcal/mol

  19. Bulk phase – biaxial? • Fully atomistic simulation of biaxial phase at 468 K • 256 molecules, 3 ns • Colour coding (according to direction of dipole across central ring) (Red + along short axis director blue – along short axis director) • Looks like the formation of biaxial domains but not biaxial phase?

  20. Bulk phase – biaxial? • Fully atomistic simulation of biaxial phase at 468 K • 256 molecules, 3 ns • Colour coding (according to direction of dipole across central ring) (Red + along short axis director blue – along short axis director) • Looks like the formation of biaxial domains but not biaxial phase?

  21. Tetrapodes: The orientational order parameters from IR spectroscopy • K. Merkel, A. Kocot, J. K. Vij, R. Korlacki, G. H. Mehl and T. Meyer • Phys. Rev. Lett. 2004, 92, 145506

  22. Z z x y X Y Orientational Order Parameters XYZ phase principal axes xyz molecular principal axes Major order parameter Molecular biaxiality Phase biaxiality Molecular and phase biaxiality

  23. Order Parameters D/√6 S P/√6 C/6

  24. Tetrapodes: NMR studies • J. L. Figueirinhas, C. Cruz, D. Filip, G. Feio, A. C. Ribeiro, Y. Frère and T. Meyer, G. H. Mehl • Phys. Rev. Lett. 2005, 94, 107802

  25. Molecular structure and organisation

  26. NMR studies

  27. ~

  28. Molecular field theory of biaxial nematics: Relation to molecular structure

  29. Potential of mean torque Uniaxial molecule – uniaxial phase z Z phase director z molecular symmetry axis Z β • Derivation: • Truncated expansion of the pair potential • Variational analysis via dominant order parameter

  30. Potential of mean torque Biaxial molecule – uniaxial phase z Z phase director xyz molecular symmetry axes Z β y x Molecular biaxiality or

  31. X z Z y Y x Potential of mean torque Biaxial molecule – biaxial phase XYZ phase directors xyz molecular symmetry axes β No new parameters

  32. Parameters and molecular structure Straley, Phys.Rev.A, 1974, 10, 1881 u200 = {– 2B(W2 – L2) – 2W(L2 + B2) + L(W2 + B2) +8WBL}/3 u220 = (L2 – BW)(B –W)/√6 u222 = – L(W – B)2/2 n.b. Does not obey the geometric mean rule. B W L

  33. Separability: Molecular field parameters Relation to molecular properties u2mn = u2mu2n Geometric mean approximation u220 = (u200u222)½ Principal axis system u20 = (2uzz – uxx – uyy)/√6 u22 = (uxx – uyy)/2 Analogy to dispersion forces contrast to excluded volume (Luckhurst, Zannoni, Nordio and Segre, Mol Phys., 1975, 30, 1345)

  34. z y x Segmental interactions Segmental anisotropy ua u20 = ua(1 – 3cos)/2 u22 = (3/8)½ua(1 + cos)/2 Biaxiality parameter  = u22/u20 = (3/2)½(1 + cos)/(1 – 3cos) General Uniaxial segments Biaxial segments 

  35. Surface tensor model u20 = (2LB – B2)(1 – 3cos)/2 + B2cos(/2)(1 + sin(/2) u22 = (3/8)½ (2LB – B2)(1 + cos) – 2B2cos(/2)(1 – sin(/2)) n.b. u200 = u20u20 u220 = u22u20 Landau point shifts from ~109º to 105º

  36. Acknowledgements John Goodby Verena Görtz Mark Wilson Daniel Jackson

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