Chemistry Reference
In-Depth Information
3.2.3.
Incorporation of kinked elements
The most significant consequence of the incorporation of kinked elements
in the molecules is the destruction of the linearity and the shortened length
of the rod-like structural sequence of the molecules. Naturally, the type
and the amount of the kinked elements have to be carefully decided in
the molecular engineering of liquid crystals. For example, Vorlander (1923)
studied the influence of different
−
X
−
elements in the following molecules
3
.
4
:
3
.
4
R
R
CH=N
X
N=CH
the central linking unit
−
X
−
is none, the molecules will have
a linear rod-like shape with a length to diameter ratio somewhat larger
than five. All the compounds, including those with R either being an H, a
methyl, or a methoxy group, were found to be able to form a stable liquid
crystal phase. On the other hand, if
−
X
−
is
−
O
−
,or
−
S
−
,or
−
HCH
−
unit, a kinked structure is introduced (Figure 3.4):
The molecule becomes two short rods linked by the kink. The linearity
is destroyed. The compounds are no longer liquid crystals. In contrast, if
−
X
−
is
−
Hg
−
, the linearity of the molecules can be maintained because the
bond angle at
−
Hg
−
is 180
◦
. The mercury derivatives are all liquid crystals
with the following data for phase-transition temperatures: forR=H,
Tm
=
18
◦
C,
Ti
=22
◦
C; for R = methyl,
Tm
= 217
◦
C,
Ti
= 229
◦
C; for R =
methoxy,
Tm
= 209
◦
C,
Ti
= 285
◦
C.
Kinked units such as
−
O
−
also served as separators between two pheny-
lene rings. As a result, the rotation of the rings about the linking unit
becomes less hindered. Studies on the conformations for diphenyl ether,
diphenyl sulfide, diphenylmethane, and diphenyl ketone have reached the
conclusion that these molecules all have very low energy barriers for inter-
nal rotations. According to the summary of Laupretre and Noel (1991), a
large portion of conformation space lies in the 1 kcal/mol region in these
molecules. Besides, the energy barrier between any of the low-energy regions
is also very small. In other words, the molecules are flexible and can easily
If in
3
.
4
X
Figure 3.4.
Kinked structure destroys chain linearity.
