Chemistry Reference
In-Depth Information
liquid crystal with 34 it was possible to create cholesteric liquid crystal phases of
opposite chirality by irradiation with
l
-CPL or
r
-CPL and return to a nematic phase
using unpolarized light [
218
]. This control and amplification of molecular chirality
by circular polarized light may be used as a data storage system [
218
]. The
reflection color of liquid crystalline films could also be switched between red and
green [
219
].
X
X
CH
3
R
R
R
S
Y
33
X=S, R=CH
3
34
X=O, R=H
35
The structurally related molecules 35, where one of the aromatic rings is
replaced by a methyl group and a stereogenic center is introduced, are light-driven
molecular motors [
18
,
19
]. These molecules consume light energy and convert it
into a unidirectional rotation of the upper half with respect to the lower half
[
19
]. Introduction of a tetrahedral stereogenic center on the aliphatic carbon of
the central ring neighboring the double bond breaks the symmetry. Lifting the
equivalence of right-handed and left-handed rotation introduces a bias for unidi-
rectional rotation about the double bond. These motors were optimized [
220
-
223
]
and it was shown that inverting the absolute configuration at this stereogenic center
reverses the direction of rotation [
224
].
Besides the bi-stable molecular switches, several types of molecules with more
than three states have been reported [
204
,
225
,
226
] as well as immobilized
switches in polymers and on a gold surface [
227
,
228
].
In other applications, BAEs have been introduced into various types of host
molecules or modified to become ligands taking advantage of their spectroscopic,
structural, and functional properties. A dixanthylene core has been incorporated
into a double calix[6]arene [
229
]. BAEs have been incorporated into crown ethers
and their alkali metal binding properties have been studied [
230
-
232
]. Metal
coordination may also occur for 4,5-diazafluorenylidenes 36 and 37 [
64
,
233
,
234
]. The dipyridyl BAE-1 38 forms a BAE with a particularly large bridge Y
and high inversion barrier upon metal complexation [
235
,
236
]. Recently it was
shown that the conformation of the fluorenylidene-xanthene 39 may be switched
from folded (yellow) to twisted (purple) by encapsulation in a suitable cavity [
77
].
