This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution 4.0 International License.

Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Creative Commons Namensnennung 4.0 Lizenz.

Nematic Liquid Crystals with High Positive Dielectric Anisotropy

M a g e d A . O s m a n *

B r o w n Boveri Research Center, Chemistry Group, CH-5405 Baden-Dättwil , Switzerland

Z. Naturforsch. 34b, 1092-1095 (1979); received March 23, 1979

Liquid Crystals, Nematic Phases, High Positive Dielectric Anisotropy,

Alkylamino-cyano-b iphenyls , Alkylamino-cyano-phenylbenzoates Alky lamino- and formyl a lkylamino-cyano Schif f 's base, biphenyl and phenylbenzoate

l iquid crystals were synthesized. The mesomorphic behaviour o f these compounds is described. The introduction of the formyl group in the higher homologues lead to the disappearance o f the mesophase. The alkylamino and formylalkylamino substituents are strong electron donating groups and lead to nematic phases with remarkably high posit ive dielectric anisotropy.

I n t e r e s t i n e l e c t r o o p t i c a l d i s p l a y s b a s e d o n t h e

t w i s t e d n e m a t i c e f f e c t [1] h a s l e a d t o a g r o w i n g

d e m a n d f o r n e m a t i c l i q u i d c r y s t a l s w i t h p o s i t i v e

d i e l e c t r i c a n i s o t r o p y . N o n - p o l a r o r a l m o s t n o n - p o l a r

n e m a t i c c o m p o u n d s o f t h e t y p e 1 w e r e f o u n d t o

p o s s e s s a v e r y s m a l l p o s i t i v e d i e l e c t r i c a n i s o t r o p y

(As = 0 . 0 5 - 0 . 5 ) [ 5 - 7 ] ,

X =-CH= N - |21 , - C 0 0 - 131,—N=N— |4|

1

N e m a t i c p h a s e s o f t h e S c h i f f ' s b a s e a n d p h e n y l -

b e n z o a t e t y p e w i t h l a r g e r A a w e r e s y n t h e s i z e d b y A . B o i l e r [8, 9] t h r o u g h r e p l a c e m e n t o f o n e o f t h e

a l k y l g r o u p s b y t h e c y a n o g r o u p w h i c h p o s s e s s a

l a r g e r d i p o l e m o m e n t ( — 4 D ) . T h e r e m a i n i n g a l k y l

g r o u p o n t h e o t h e r e n d o f t h e m o l e c u l e , w h i c h h a s a

d i p o l e m o m e n t o f -f- 0.3 D , w a s n o t c h a n g e d .

R e c e n t l y , G . G r a y [10] a n d R . E i d e n s c h i n k [ 1 1 ]

p r e p a r e d a n a l o g o u s n e m a t i c p h a s e s o f t h e b i p h e n y l

a n d p h e n y l c y c l o h e x a n e t y p e s . A l l t h e s e c o m p o u n d s

h a v e A e v a l u e s f r o m 8 t o 20 [ 1 2 - 1 4 ] w h i c h a l l o w a n o p e r a t i n g v o l t a g e o f 3 v o l t s f o r a t w i s t e d n e m a t i c

d i s p l a y . S i n c e t h e t h r e s h o l d v o l t a g e a n d , c o r r e s p o n d -

i n g l y , t h e o p e r a t i n g v o l t a g e is p r o p o r t i o n a l t o

\J11A e, i t is c l e a r t h a t n e w n e m a t i c l i q u i d c r y s t a l s w i t h l a r g e r d i e l e c t r i c a n i s o t r o p i c s a r e r e q u i r e d i f t h e

o p e r a t i n g v o l t a g e is t o b e s u b s t a n t i a l l y r e d u c e d .

T h i s c a n b e a c h i e v e d b y r e p l a c i n g t h e r e m a i n i n g

a l k y l g r o u p w i t h a n e l e c t r o n d o n a t i n g s u b s t i t u e n t

h a v i n g a s t r o n g e r d i p o l e m o m e n t a l o n g t h e m o l e c u -

l a r a x i s . T h e f i rs t a t t e m p t a l o n g t h i s l i n e w a s m a d e

b y i n t r o d u c i n g t h e d i m e t h y l a m i n o g r o u p ( - ( - 1 . 6 D )

* Reprint requests to Dr . M. A . Osman. 0340-5087/79/0800-1092/$ 01.00/0

a t t h e o t h e r e n d o f t h e b e n z y l i d e n - a m i n o b e n z o -

n i t r i l e m o l e c u l e , b u t t h i s g a v e a n i s o t r o p i c c o m -

p o u n d m e l t i n g a t 183 °C [15] . T h e d i s a p p e a r a n c e o f

t h e l i q u i d c r y s t a l l i n e p r o p e r t i e s is p r o b a b l y d u e t o

t h e h i g h m e l t i n g p o i n t o f t h i s c o m p o u n d a n d t o t h e

b u l k i n e s s o f t h e d i m e t h y l a m i n o g r o u p w h i c h l o w e r s

i t s c l e a r i n g p o i n t . T h e a i m o f t h i s w o r k is t o i n t r o d u c e

t h e less b u l k y m o n o a l k y l a m i n o g r o u p i n m o l e c u l e s

o f t y p e 1 a n d t o s t u d y t h e e f f e c t o f t h i s s u b s t i t u t i o n

o n t h e i r m e s o m o r p h i c b e h a v i o u r a n d o n t h e m a g n i -

t u d e o f t h e d i e l e c t r i c a n i s o t r o p y .

Results and Discussion

N - ( 4 - M e t h y l a m i n o b e n z y l i d e n e ) - 4 ' - a m i n o b e n z o -

n i t r i l e (2) w a s p r e p a r e d b y c o n d e n s i n g 4 - m e t h y l -

a m i n o b e n z a l d e h y d e w i t h 4 - a m i n o b e n z o n i t r i l e i n

b u l k [16].

CH3HN-̂ )-cho+H2N--CN-̂ CH3HN-Q>-CH-N^Q>-CN

z

T h e a l d e h y d e w a s p r e p a r e d b y t h e V i l s m e i e r re-

a c t i o n [17] .

2"c> ~ H j C : N C N H 0 ~ H 3 C H N < > CHO

C o m p o u n d 2 m e l t e d a t 1 6 3 . 1 °C a n d s h o w e d a

m o n o t r o p i c n e m a t i c p h a s e c l e a r i n g a t 138.0 °C.

T h i s c o n f i r m e d t h a t t h e d i s a p p e a r a n c e o f t h e m e s o -

p h a s e i n t h e d i m e t h y l a m i n o d e r i v a t i v e w a s c a u s e d

b y t h e b u l k i n e s s o f t h i s g r o u p . H o w e v e r , t h e m e l t i n g

p o i n t o f c o m p o u n d 2 is t o o h i g h f o r p r a c t i c a l

a p p l i c a t i o n s a n d f o r c o n v e n i e n t l y m e a s u r i n g t h e

d i e l e c t r i c a n i s o t r o p y . T h e h i g h e r h o m o l o g u e s o f

a l k y l a m i n o b e n z a l d e h y d e a n d t h e i r S c h i f f ' s b a s e s

w e r e t h e r e f o r e p r e p a r e d in a n a t t e m p t t o l o w e r t h e

m e l t i n g p o i n t . I t s e e m e d a l s o o f i n t e r e s t t o in-

M. A. Osman • Nematic Liquid Crystals 1093

* > - ® - C H - N - © - C N

X R TM [°C] Tc [°C] Gernerai C [ % ] H [ % ] N [ % ] formula calcd found calcd found calcd found

H CH3 163.2 138.0 C15H13N3 76.57 76.46 5.57 5.61 17.86 17.68 H C2H5 154.6 129.1 C16H15N3 77.08 77.02 6.06 6.04 16.86 16.86 H C3H7 151.7 — C17H17N3 77.53 77.51 6.51 6.55 15.91 15.96 H C4H9 129.6 — Ci8HI9N3 77.94 77.76 6.91 6.88 15.15 15.06 H C6H13 97.8 78.6 CsoHagNg 78.65 78.78 7.59 7.72 13.76 13.73 H C8H17 95.8 78.1 C22H27N3 79.24 79.36 8.16 8.28 12.60 12.45 CHO CH3 159.1 134.8 C16H13N3O 72.98 72.83 4.98 5.10 15.96 15.92 CHO C2H5 132.3 — C17H15N3O 73.63 73.49 5.45 5.55 15.15 15.00 CHO C4H9 81.2 — C19H19N3O 74.73 74.55 6.27 6.32 13.76 13.60

v e s t i g a t e t h e e f f e c t o f i n t r o d u c i n g t h e f o r m y l g r o u p

i n t h e a l k y l a m i n o S c h i f ' s b a s e s .

T h e m e l t i n g a n d c l e a r i n g p o i n t s o f t h e S c h i f f ' s

b a s e s a r e g i v e n i n T a b l e I .

A s i t w a s e x p e c t e d t h e m e l t i n g p o i n t s f e l l r a p i d l y

f r o m 1 6 3 . 1 °C t o 9 7 . 8 °C w i t h g r o w i n g c h a i n l e n g t h

u p t o CÖ- H o w e v e r , e x t e n s i o n o f t h e a l k y l c h a i n t o C 8 d i d n o t m a r k e d l y l o w e r t h e m e l t i n g p o i n t .

U n f o r t u n a t e l y t h e c l e a r i n g p o i n t s s u f f e r e d a s i m i l a r

d e c r e a s e s o t h a t a l l t h e s y n t h e s i z e d a l k y l a m i n o

S c h i f f ' s b a s e s s h o w e d o n l y m o n o t r o p i c n e m a t i c

p h a s e s . T h e n e m a t i c t o i s o t r o p i c t r a n s i t i o n t e m -

p e r a t u r e s i n t h i s s e r i e s a r e m u c h h i g h e r t h a n t h o s e

o f t h e c o r r e s p o n d i n g a l k y l d e r i v a t i v e s [9]. T h e

i n t r o d u c t i o n o f t h e f o r m y l g r o u p i n c o m p o u n d 2 d i d

n o t h a v e a s i g n i f i c a n t e f f e c t o n e i t h e r i t s m e l t i n g

p o i n t o r o n i t s c l e a r i n g p o i n t . H o w e v e r , i t d i d c a u s e

a r e m a r k a b l e d e c r e a s e o f t h e m e l t i n g p o i n t s as w e l l

a s o f t h e c l e a r i n g p o i n t s o f t h e h i g h e r h o m o l o g u e s s o

t h a t t h e m e s o m o r p h i c p r o p e r t i e s d i s a p p e a r e d i n

t h e s e c o m p o u n d s . T h e n e m a t i c p h a s e i n t h i s ser ies

w a s o n l y m o n o t r o p i c a n d t h e m e a s u r e m e n t s o f t h e

d i e l e c t r i c a n i s o t r o p y i n t h e s u p e r c o o l e d s t a t e w a s

p r a c t i c a l l y i m p o s s i b l e .

T h e c h e m i c a l l y m o r e s t a b l e 4 - a l k y l a m i n o - 4 ' -

c y a n o b i p h e n y l s w e r e p r e p a r e d a c c o r d i n g t o t h e

f o l l o w i n g s c h e m e :

jsn/HCl

T h e 4 - b r o m o - 4 ' - n i t r o b i p h e n y l w h i c h w a s o b t a i n e d

b y b r o m i n a t i n g 4 - n i t r