Mesogenic Properties of Noncollinear (Bent)

Liquid Crystals

Applications to Environmental Sensing

David Davis, Kent State UniversityAdvisor: Dr. Robert Twieg

Bent Core Liquid Crystals

• Historically, liquid crystal synthesis has been most concerned with structures incorporating a rigid core bearing one or more flexible tails in what’s known as a calamitic (rod-like) liquid crystal.

• Since the early 1990’s much effort has been spent on the synthesis of bent core (noncollinear) liquid crystals, and the study of their properties1. Here the bend exists between ring components in the core.

Calamitic LC

Bent-Core LC

Core

Core

Tail Tail

Tail Tail

Properties of Bent-Core Liquid Crystals• Bent-core LCs exhibit new and

unique but by now very well documented phases.

• Bent core LCs may also possess more controversial phases, e.g. the biaxial nematic phase.

• Posesses increased flexoelectricity- capable of spontaneous electrical polarization in response to strain (2 orders of magnitude higher than calamitic LCs)2.

• Create strain on a liquid crystal layer to achieve a measurable electrical polarization (induce electron flow and measure current), create foundation for a sensor3.

First instance of a Biaxial Nematic phase reported in bent-core liquid crystals (2004)4.

Are There Other Types of Noncollinear Liquid Crystals?

• While studies have focused on the synthesis of materials with the bend in the core, can the bend be placed elsewhere to observe similar properties?

• Current area of study: Synthesis of materials with the bend at the tail-core juncture (e.g., meta-substituted alkoxy tails).

• Target: 28 unique terphenyl-core compounds with differing permutations of meta-, para-, and null substitution of methoxy (C1), hexyloxy (C6), and dodecyloxy (C12) tail lengths.

C6H13O

OC6H13

Bent-core liquid crystal Bend at tail-core juncture

Examples of Materials Synthesized(Identical Length Tails)

C6H13O OC6H13

C6H13O

OC6H13

C6H13O

OC6H13

Cr 67.9 Cr 74.8 Cr 191.6 Sm 225.1 Sm 246.3 Iso LiqIso Liq 244.9 Sm 223.6 Sm 188.5 Cr 72.5 Cr 63.4 Cr

Cr 48.7 Cr 76.8 Iso LiqIso Liq 59.3 Sm 45.1 Cr

Cr 112.7 Iso LiqIso Liq 102.9 Cr

Examples of Materials Synthesized(Non-Identical Length Tails)

C6H13O

OC12H25

C12H25O

OC6H13

C6H13O

OC12H25

C6H13O OC12H25

Phase data TBD Cr 69.2 Iso LiqIso Liq 99.9 Cr

Cr 93.8 Cr 105.2 Iso LiqIso Liq 100.5 Cr 88.2 Cr

Cr 105.9 Iso LiqIso Liq 99.9 Cr

Terphenyl Compound Set

Sample Synthetic Pathways

OC6H13C6H13O

II

B(OH)2H3CO

Pd(PPh3)4, Na2CO3Toluene, EtOH

OCH3H3CO

N

H ClHO OH

C6H13I, K2CO3DMF

HO BrC12H25I, K2CO3

C12H25O Br

HO

BrC6H13I, K2CO3

C6H13O

Br

1) n-BuLi, THF2) B(OMe)3

3) HCl (aq)B(OH)2

C6H13O

Pd(PPh3)4, Na2CO3

Toluene, EtOH

C6H13O

OC12H25

87%

quant.

35%

98%

93% 10%

62%

DMF

DMF

Future Direction of Project• Explore other series of cores for bent molecules to discover possible

mesogenic activity and flexoelectric properties.– Quaterphenyls (work underway Summer ’10)– 1,4-bis(phenylethynyl)benzenes– 1,4-dibenzoyloxybenzenes

RO OR'

RO OR'

RO

O

O

O

O

OR'

References

• 1. Link, D.L. et. al. Spontaneous Formation of Macroscopic Chiral Domains in a Fluid Smectic Phase of Achiral Molecules. Science. 1997, 278, 1924-1927.

• 2. Tschierske, C. Liquid Crystals Beyond Display Applications. J. Mater. Chem. 2008, 18, 2869-2871.

• 3. Jakli, A.I. et. al. Methods and Systems for Determining Flexoelectric Effect in a Liquid Crystal. US Patent Appl. 20100182026, July 22, 2010.

• 4. Madsen, L.A. et. al. Thermotropic Biaxial Nematic Liquid Crystals. Phys. Rev. Lett. 2004, 92, 14505/1-14505/4

Additional References

• Dierking, I. Angew. Chem. Int. Ed. 2010, 49, 29-30.• Lehmann, M. et. al. Chem. Eur. J. 2010, 16, 8275-8279.• Bisoyi, H.K. et. al. Beil. J. Org. Chem. 2009, 5, No. 52.• Reddy, R. A.; Tschierske, C. J. Mat. Chem. 2006, 16,

907-961.• Takezoe, H.; Takanishi, Y. Jpn. J. Appl. Phys. 2006, 45,

597-625.• Etxebarria, J.; Ros, M.B. J. Mat. Chem. 2008, 18, 2919-

2926.