Chapter 15
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Liquid Crystalline Trioxadecalins: The Mesogenic Chirality as Sensor for Molecular Conformation and Orientation 1
2
Volkmar Vill , Bruno Bertini , and Denis Sinou
2
1
Institute of Organic Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany Laboratoire de Synthèse Asymetrique, associé au CNRS, CPE Lyon, Université Claude Bernard Lyon 1, 43, boulevard du 11 novembre 1918, 69622 Villeurbanne cédex, France 2
Dedicated
to Professor
Günther Wulff on the occasion of his 65th birthday
T r a d i t i o n a l l y , chirality is introduced to mesogens b y a sterically d i s t u r b i n g substituent. O x y g e n heterocycles offer a different concept: the exchange between isosteric -CH - and -O- groups causes chirality without steric hindrance. T h e macroscopic chiral properties o f t r i o x a d e c a l i n - b a s e d l i q u i d crystals are therefore extremely sensitive to small changes i n the chemical structure and the c h e m i c a l environment. T h e helical inversion phenomenon can be explained b y small changes to the main axis of molecules. 2
C h i r a l i t y is an important factor for molecular properties. T h e l i q u i d crystalline state is a mediator between m i c r o s c o p i c m o l e c u l a r structures a n d m a c r o s c o p i c appearance o f matter. C h i r a l additives to l i q u i d crystalline phases c a n cause the f o l l o w i n g types o f 'mesogenic chirality : helical ordering o f the molecules (helical pitch i n N * , S * , T G B , B P ) , polar properties (ferroelectricity) or phase transitions to phases w h i c h require chirality ( N —» B P , S - » T G B ) . Therefore l i q u i d crystals are w e l l established as sensors for chirality [1,2,3]. The next step is to use this 'mesogenic chirality' as a monitor or sensor for small conformational and orientational changes o f molecules. 1
c
A
A
A
T o d a y , more than 80 0 0 0 mesogenic compounds are k n o w n and 16 000 o f them are chiral [4]. M o s t o f these compounds have a chiral center induced b y the exchange o f a - C H - group by a - C H M e - group i n the flexible w i n g (Figure 1). Thus, chirality is induced by a steric hindrance w h i c h disturbs the mesogenic order. F o r e x a m p l e , compound Β has a clearing temperature 4 7 K lower than compound C . 2
206
© 2002 American Chemical Society
In Anisotropic Organic Materials; Glaser, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
207
A
0
\
Β
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~
—
•
C r 8 4 SCA 118 S
o
C v
o
a X=C1, b X = F, c X=CF3 Figure
3: Synthesis of chiral diols from
giucal.
T h e b o r o n c o m p o u n d s 9 - 1 1 e x h i b i t a clear b e h a v i o r pattern: short-pitch cholesteric phases, blue phases and T G B phases. The cone-shaped compounds 12-13 b e h a v e s i m i l a r l y , w h i l e the t r i o x a d e c a l i n s 6-8 are extremely variable i n their macroscopic chirality. A
Table 1 lists the new compounds together with previously published compounds. C o m p o u n d s a and b show only a cholesteric phase w i t h a short pitch. C o m p o u n d s c, 1, ρ e x h i b i t a cholesteric phase w i t h a temperature-dependent i n v e r s i o n . H o w e v e r , compounds f and q show no inversion at a l l i n the pure form, but o n l y i n mixtures w i t h non-chiral nematics. That is, they induce a helical pitch o f opposite sign i n the mixture to that w h i c h they exhibit themselves in the pure state.
In Anisotropic Organic Materials; Glaser, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
209
6a-e X = C l , 7 a - b , d , g
X = F , 8a4>,e X = C F 3
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5a-c
9a-e X = C1, lua-d X = F, l l a - d X = CF3
ν /?-YC H C0 6
4
2
C
