Room temperature difluoro-tolane and diphenyl-diacetylene liquid crystals with negative dielectric anisotropy

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Room Temperature Difluoro-Tolane and

Diphenyl-Diacetylene Liquid Crystals with Negative Dielectric

Anisotropy

Shin-Tson Wu a , C. S. Hsu b & J. M. Chen b a

Hughes Research Laboratories , 3011 Malibu Canyon Road, Malibu, CA, 90265, USA b

Department of Applied Chemistry , National Chiao Tung University , Hsinchu, Taiwan, ROC Published online: 04 Oct 2006.

To cite this article: Shin-Tson Wu , C. S. Hsu & J. M. Chen (1997) Room Temperature Difluoro-Tolane and Diphenyl-Diacetylene

Liquid Crystals with Negative Dielectric Anisotropy, Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals, 304:1, 441-445, DOI: 10.1080/10587259708046994

To link to this article: http://dx.doi.org/10.1080/10587259708046994

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0 1997 OPA (Overseas Publishers Association) Amsterdam B.V. Published in The Netherlands under license by Gordon and Breach Science Publishers Printed in India

ROOM TEMPERATURE DIFLUORO-TOLANE AND DIPHENYL- DIACETYLENE LIQUID CRYSTALS WITH NEGATIVE DIELECTRIC ANISOTROPY

SHIN-TSON WU

Hughes Research Laboratories

301 1 Malibu Canyon Road, Malibu,

CA

90265. USA

C.

S.

HSU and J. M. CHEN Department of Applied Chemistry National Chiao Tung University Hsinchu, Taiwan,

ROC

Abstract Several laterally difluorinated tolane and diphenyl-diacetylene liquid crystals were synthesized and their physical properties evaluated. These compounds exhibit low melting temperature, small heat fusion enthalpy and large negative dielectric anisotropy. Their potential applications are in the high contrast displays using a homeotropic cell and for infrared spatial light modulators.

1. INTRODUCTION

Homeotmpic liquid crystal cells' are highly desirable for the normally-black display application owing to their excellent contrast ratio. In the voltage-off state, the LC directors

are aligned to be nearly perpendicular to substrate surfaces. The incoming linearly polarized light experiences no phase retardation and is blocked by the crossed analyzer. As a result, an excellent dark state can be achieved. This dark state is insensitive to

dAn

(d=cell gap and An=bireFringence) of the LC cell which provides a large cell gap tolerance during fabrication. As the applied voltage exceeds the M e r i c k s z transition threshold, the LC directors are reoriented by the electric field and phase retardation results.

To realize the useful elecm-optic effect of a homeompic cell, the LC employed must exhibit a negative dielectric anisotmpy besides the general requirements on wide nematic range and low viscosity.

For

active matrix displays, an extra requirement on the LC material is high resistivity. High resistivity enables the LC cell to hold the applied voltage without flickering. To satisfy high resistivity and negative dielectric anisotropy at the same time, laterally fluorinated compounds are often considered.

In this paper, we report on the phase transition temperatures and physical properties of some lateral difluoro-tolane and diphenyl-diacetylene liquid crystals. These LCs possess relatively low melting temperature and high clearing temperature, large and

[2319]/441

4421[2320] S.-T.

wu

et Ul.

negative dielectric anisotropy, and high resistivity. Therefore, they are useful compounds for formulating eutectic mixtures for high c o n m t liquid crystal display applications.

2. (2,3) DIFLUORO TOLANES

The structures of the latexally &fluorinated tolanes we studied are shown below:

F, F

We have synthesized several homologues with n=2-7. Similar tolanes with alkoxy side chains have been reported previously.2 These alkoxy tolanes possess high melting point and large heat fusion enthalpy. Thus, their solubility in a LC mixture is limited even though their high clearing point (T,) is attractive for enhancing the T, of the mixture.

2.1 Phase Transition Temperatures

The phase transition temperatures and heat fusion enthalpy (AH) of the (2,3) difluoro- tolanes we synthesized are listed in Table I.

TABLE

I Phase transition temperatures (in OC) and heat fusion enthalpy (AH in kcal/mol) of the laterally difluoro-tolane LC homologues. Here K , N and I stand for crystalline, nematic and isotropic phase. respectively.

~ n Phase transitions AH 2 K 34.6 N 75.9 I 3.52 3 K 38.5 N 94.2 I 2.28 4 K 22.5 N 105.8 I 3.05 5 K 38.3 N 104.6 I 5.22 6 K 36.8 N 68.9 I 2.60 7 K 37.2

N

86.2 I 2.85

These homologues exhibit relatively low melting temperatures and high clearing point. Normally, the asymmetric side chain groups help to reduce melting tem~erature.~ However, this phenomenon is not so obvious here. The observed low melting temperature is mainly attributed to the molecular width. The lateral difluoro group widens the molecular separation and weakens the inter-molecular association resulting in low melting temperatures.

On

the other hand, the high clearing temperature originates from the cyclohexane ring. The small AH of these homologues is quite desirable for forming

DIPHENYL-DIACETYLENE LC [232 1]/443

eutectic mixtures. According to the Schroder-Van Laar equation:> both low melting temperature and small heat fusion enthalpy play equally important roles in reducing

the

melting temperature of an eutectic mixture. Low melting temperature and high clearing point are critical for practical display applications. Normally, the desirable nematic range of a LC mixture is from

-40

to +85”C.

2.2 Dielectric anisotropy:

The dielectric constants of the n=4 homologue were measured by the capacitance method.6 Results are: E, =I0 and E, =4 at 1 kHz and T=23”C. This large negative

dielectric anisotropy originates from the lateral fluoro groups. From mean-field theory, the dielectric constants of a LC compound are determined by the dipole moment and its relative position with respect to the principal molecular axis as:

’

E//= N h F ( < w

+

(Fp2/3kT) [l

-

(1-3c0s28)S])

E L = NhF(<al>

+

(Fp2/3kT) [l

+

(1/2)(1-3cos28)S])

A& = NhF((af

- ad

-

(Fp2/2kT)(1- 3 cos2B))S

(1) (2)

(3) where N is the molecular packing density, h = 3E/(2~ + I ) is the cavity field factor, &= (&I

+2&1)/3 is the averaged dielectric constant, F is the Onsager reaction field, at and

at

are

the principal elements of the molecular polarizability tensor, 8 is the angle between the dipole moment

p

and the principal molecular axis, and

S

is the order parameter of the second rank.

The effective dipole moment of the two fluoro groups is perpendicular to the principal molecular axis.

Thus,

the dielectric anisotropy (E,

-

E ~ ) is large but negative. The threshold voltage of the LC cell was measured to be 2.4

.

,

V

From the relationship V, = x [K,JAe]”, the bend elastic constant (K3J of the n=4 homologue is calculated to

be 3 ~ 1 0 . ~ dyne.

2.3 Birefringence:

The birefringence of the n=4 homologue was determined to be 0.20 at X=633 nm by measuring the voltage-dependent phase retardation of a homeotropically-aligned ce11.6 The observed relatively high birefringence is due to linear molecular conjugation of the

tolane core. The tolane structure offers reasonably good photostability. Therefore, this series of LC compounds can be employed for high contrast display applications.

444/[2322] s.-T.

wu

el ul.

3.

(2,3)

DIFLUORO DIPHENYL-DIACETYLENES

The structure of the (2,3) laterally difluorinated diphenyl-diacetylene liquid crystals we

studied is shown below:

C.H2"+,-(=J-C

EC

-

c

=c

The compounds we synthesized have n=3-6. Their phase transition temperatures and heat

fusion enthalpy are listed in Table 11.

TABLE

II

Phase transition temperatures (in OC), dielecmc anisotropy (A& at

T=25"C and 1 kHz), birefringence (An at 1 4 3 3 nm) and heat fusion

enthalpy (AH in kcal/mol) of the laterally difluoro diphenyl-diacetylene LC homologues. Here K, N and I stand for crystalline, nematic and isotropic phase, respectively. n Phase transitions A E A n m 3 K 37.5 N 71.1 I 4.13 4 K 23.6 N 47.2 I

-

6.4 0.28 4.25 5 K 34.6 N 58.7 I 4.46 6 K 19.6 I

---

Normally, the melting temperature of such a highly conjugated liquid crystal is quite high. However, due to the lateral substitutions, their melting point is reduced to near room temperature.

'Ihe physical propemes of the n=4 homologue was characterized and results are

included in Table 11. 'Ihe high An results from long molecular conjugation and the large negative dielectric anisotropy from lateral dipole moment.

The absorption edge of the diphenyl-diacetylene LCs extends to 350 nm.6 Thus, their photostability in the blue

spectral

region is a concern and not so desirable for visible display application. However, there are other displays or light modulation in the infrared and microwave regions where photostability of these LCs is not a problem. The high

birefringence

and

negative dielecmc anisotropy of these compounds turn out to'be attractive for such applications. To improve the lifetime of a LC device, the cell should be sealed peripherally in order to keep moistures and oxygen out. Otherwise, moistures and oxygen tend to interact with LC molecules and cause alignment deterioration. As a result, the contrast ratio of the LC device is degraded gradually.

DIPHENYL-DIACETYLENE LC [2323]/445

4. CONCLUSION

The laterally fluorinated tolane and diphenyl-diacetylene liquid crystals exhibit large but negative dielectric anisotropy, high birefringence and low melting temperature. Owing to

their low melting temperature and small heat fusion enthalpy, a wide nematic range eutectic mixture consisting entirely of these homologues can be formulated. These LCs would also possess a relatively high resistivity (estimated to be -lo'* &XI) for active

matrix display applications. When these mixtures are used in a horntropically aligned cell, a contrast ratio higher than 1ooO:l can be obtained under the crossed-polarizer configuration.

ACKNOWLEDGMENT

The Hughes group is supported by

Air

Force Office for Scientific Reseaxh under a

contract F49620-94-C-0078. STW is indebted to W. H. Smith, Jr. of

HRL

for his technical assistance.

REFERENCES

1 . M. F. Schiekel and K. Fahrenschon,

-fi,

391 (1971).

2. V. Reiffenrath et al,

SPIE

m,

84 (1990). 3. S. T. Wu, et al, ,&f- 630 (1992). 4. L. Schrder, 11.449 (1 893). 5. J. J.

Van

Laar,

216 (1908). 6 . I. C. Khoo and S. T.Wu,

(World Scientific, Singapore, 1993).

7 . W. Maier and G. Meier, w u r f o r s h . &I '

A

16,262 (1961).

. .