First Nematic Calamitic Liquid Crystals with Negative Birefringence

C h a p t e r 14

First Nematic Calamitic Liquid Crystals with Negative Birefringence Downloaded by UNIV MASSACHUSETTS AMHERST on September 17, 2012 | http://pubs.acs.org Publication Date: November 2, 2001 | doi: 10.1021/bk-2001-0798.ch014

Volker Reiffenrath and Matthias Bremer

Liquid Crystals Division, Merck KGaA, D-64271 Darmstadt, Germany

The combination of aliphatic core structures with lateral highly polarizable groups leads to nematic materials with very l o w or even negative birefringence.

L i q u i d crystalline behavior can be observed i n certain, anisotropically shaped organic compounds. " A liquid crystal can flow like a liquid, but m a y posses other properties, ' such as birefringence, which is normally characteristic o f solid crystals, a prime example being rhombohedral calcite, C a C 0 . Without the birefringence o f the liquid crystal, there w o u l d be no optical response to an applied voltage i n the twisted nematic ( T N ) mode o f a liquid crystal display ( L C D ) . A uniaxial liquid crystal has two principal refractive indices. The ordinary ray n is defined as the light wave with the electric field perpendicular to the optical axis, whereas the extraordinary index n is observed for linearly polarized light with the electric field parallel to the optical axis. In nematic liquid crystals the optical axis is given b y the director w h i c h ideally coincides with the long molecular axis. In nematic l i q u i d crystals k n o w n so far the birefringence (An = n\\ - wj_ = H - n ) is always positive with a range o f about +0.02 to +0.40. W e n o w report the first examples o f nematic, calamitic liquid crystals with extremely small and even negative birefringence. ' 1

3

4 5

3

6

Q

e

C

0

1

7 8

The birefringence is related to the anisotropy o f the molecular polarizability Aa = q - a through the V u k s equation ( l a and l b ) where S is the Saupe orientational order parameter, €o the static dielectric constant and Ν the number o f molecules per unit volume. 9

(

±

(la)

© 2002 American C h e m i c a l Society

In Anisotropic Organic Materials; Glaser, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

195

196 n i - I

AaS

Ν a

2

Downloaded by UNIV MASSACHUSETTS AMHERST on September 17, 2012 | http://pubs.acs.org Publication Date: November 2, 2001 | doi: 10.1021/bk-2001-0798.ch014

η

+ 2

(lb)

3

3ε

0

Since a can be calculated for isolated molecules with a quantum chemical method one can speculate what k i n d o f molecular structure w o u l d lead to small or negative values o f An. In order to derive anisotropic quantities from the calculation, the molecules are oriented so that the smallest molecular moment o f inertia coincides with the χ axis and the larger moments o f inertia with the y and ζ axes, respectively. The molecules are considered to be cylindrically symmetric, i.e. the long molecular axis ideally coincides with the x-axis and the perpendicular components o f the polarizability tensor are averaged. ' G i v e n the fact that nematic liquid crystals are rod-like molecules with a length to breadth ratio o f typically 3 or larger, implying larger polarizability along the long axis, it is not surprising that An is greater than zero. One obvious way o f lowering An is the introduction o f small lateral, highly polarizable groups to a cylindrical core o f l o w refractive power. 10

11

Figure 1. Strategy to achieve negative birefringence

in calamitic

materials

This led to the synthesis o f bicyclohexanes with axial acetylenic substituents starting from axial cyanobicyclohexanes or bicyclohexanones as outlined i n Scheme 1; mesophases and extrapolated ("virtual") clearing points and birefringence are given i n Table I together with calculated ( A M I ) optical anisotropics. 12

13

1 4


197

R'

A=CH A=CN


3

OR'

OH

Scheme 1. Synthesis o f axial alkynylbicyclohexanes. a) 1. D I B A L , Toluene, 25-50 ° C ; 2. H 0 (90%). b) Methyltriphenylphosphoniumbromide, K O t B u , T H F , 0 °C (90%). c) 1. B r , E t 0 , -10 to 0 °C; 2. E t N , R T (65%); 3. K O t B u , t B u O H , 60 °C (30%). d) A = C H : 1. B u L i , T H F , -70 °C; 2. M e l , -70 °C to R T (70%). A = C N : 1. B u L i , T H F , 70 ° C ; 2. T s C N , -70 °C to R T (70%). e) 1. Trimethylsilylacetylene, B u N F 3 H 0 , T H F , -30 ° C ; 2. K F , M e O H , R T (47%). f) 1. B u L i , T H F , -30 °C to -5 ° C ; 2. R ' I , D M S O , -5 °C to R T (73%). +

3

2

2

3

3

4

2

Table L Properties of axially substituted alkynylcyclohexanes Entry

Structure

Phases

Virtual Clp.

Virtual Δη

Δη (cale)

C 16N35I

21

0.038

0.026

C35N35I

5

0.034

0.020

C45I

-122

0.033

0.028

C55I

-49

0.026

0.004

1 1 C

H

5 11

( 2

1ι

3

C

H

5 11

3

1 s

o'

C H 3

( 4

1I C

H

5 11


198


Note that the calculated values are a l l too small, especially so for the cyanoacetylene 4. One possible explanation lies i n the order parameter S. F o r the calculation a constant value o f 0.7 is assumed, perhaps an oversimplification for compounds 1-4, where S may vary strongly, depending on the lateral perturbation. Furthermore, the standard orientation used for the calculation might not be v a l i d for molecules like 1-4 w h i c h significantly deviate from a cylindrical shape. Figure 2 shows the standard orientation for the saturated bicyclohexane 5, as a reference, and its alkynyl derivatives 6 and 7. The calculated birefringence for the reference compound 5 (0.044) is i n excellent agreement w i t h the extrapolated experimental value (0.043). However, the values o f Table I indicate that, for 1-4 and 6 and 7, using the moments o f inertia to define anisotropies is too crude an approximation to the behavior found i n the condensed phase, where the long molecular axes must be tilted much stronger with respect to the macroscopic director.

Figure 2. Orientation of anisotropies.

of LC molecules with the moments of inertia for the

The z-axis is not shown

definition

explicitly.

A relatively simple way to render the overall molecular shape more symmetrical and thus avoid a tilt o f the molecules i n the nematic phase is shown i n Scheme 2 and Table II. One obtains liquid crystals 8 and 9 with high virtual clearing temperatures, although smectic phases are still present.


199

R

Y ^ ^ R -


OH

Scheme 2. Synthesis of bis(bicyclohexyl)acetylenes. a) 1. BuLi, THF, -70 °C; 2. Add 4-alkyl[bi]cyclohexanone, THF, -70 °C. b) 1. BuLi, THF, -30 °C to -5 °C; 2. R"'I, DMSO, -5 °C to RT (25%). c) 4-alkylcyclohexylcarbonylchloride, CH C1 , pyridine, 0 °C to RT (40%). 2

2

Table II. Properties of unsymmetrical bis(bicyclohexyl)acetylenes Entry

8

Structure

Phases

Virtual Clp.

Virtual Δ η

II

C104S (103)N130I

136

0.029

C 6 6 S 1711

152

0.036

B

9

B

C H„ ^ ^ Λ 5

ο

- ^ Α ^ 0

3

Η

7

15

The acetylenes of types 1 and 3 can be coupled oxidatively to yield symmetrical, and unsymmetrical dimers. These are readily separated by chromatography and the alkoxy-/alkylderivatives 10-16 of Table III now show pure (albeit monotropic) nematic phases. In these materials the extrapolated birefringence is around zero or slightly below.


200


Table III. Properties of symmetrical bis(bicyclohexyl)butadiynes

R

Entry R 10 C H 11 C5H11 12 C7H15 13 CH 0 14 C H 3 O 15 C 2 H 5 O 16 C 2 H 5 O 3

R'

Phases C 151 S 1681 C77S 1981 C61 SB208I C121 Ν (80)I C117 Ν ( 8 7 ) I C 143 Ν ( 1 1 7 ) I C 126 Ν (120)I

C3H7

7

Virtual Clp.

B

C5H11

B

C4H9

C3H7

3

10-16

C5H1, C3H7

CsHu

Virtual Δη

—

—

178 154

-0.004 -0.003

103

0.001

— —

—

— —

—

Table IV. Properties of unsymmetrical bis(bicyclohexyl)butadiynes

Entry 17 18 19 20 21 22 23

R

R'

R"

CH3O CH3O CH3O CH3O C H 0 C H 0 C H 0

C H C H C5H11

C5H11

C5H11

C4H9

C7H14

C4H9

C7H14

3

7

C5H11

3

7

C4H9

C7H14

C H

C,H

3

2

5

C H

2

5

C5H11 C H

2

5

C5H11

3

7

3

7

7

C4H9

C H 3

Phases

Virtual Clp.

C 76 Ν 135 I C 1 0 0 S (63) Ν 128 I C 105 Ν 125 I C90N129I C78 S 134N145I C99 S H 7 N 1 4 2 I C 6 6 S 1 3 5 Ν 1441

116 105 99 110 126 123 130

< 4

First Nematic Calamitic Liquid Crystals with Negative Birefringence

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