A R T I C L E S
Mathews et al.
1
0
11
12
reflectors and filters, tunable lasers, molecular sensors, and
13
other biomedical applications. The utility of thermal, electrical,
and light-induced color switching and tuning has been reviewed
1
4
by White et al.
The circularly polarized nature of the reflected light is another
remarkable property associated with CLCs. When unpolarized
light impinges on the CLC material, circularly polarized light
2
with the same handedness as the helix is reflected. Circularly
Figure 2. Molecular structure of the axially chiral azobenzenophanes (R)-1
and (R)-2 used as switch molecules in the present study.
polarized light with the opposite handedness is transmitted
through the sample. Outside the reflection band, incident light
is transmitted regardless of its polarization state, as shown in
Figure 1. Selective preparation of right- or left-handed CLC
films is achieved from intrinsically chiral LC molecules or LC
mixtures containing chiral dopants. Enantiomers of a chiral
mesogen or chiral dopant are used to obtain an equal but
1
5
conformations or a shift in the equilibrium between opposite
16
chiralities of multiple chiral centers. For practical applications,
these systems cause some serious drawbacks because of the
requirement of above-ambient-temperature conditions. There-
fore, a light-controlled twist inversion in CLCs is more
attractive. To date, CLC mixtures having phototunable chiral
dopants have been studied as a step toward achieving this goal,
but only a few demonstrations of helicity switching have been
8
opposite twist in the cholesteric phase. On the other hand,
switching of the cholesteric helicity in response to external
stimuli such as temperature, light, or electric field can signifi-
cantly improve and/or widen the area of potential applications
17-19
published.
Such switching requires that the photoresponsive
1
0
of CLC materials.
molecule employed as a dopant either reverses its intrinsic
1
7
In comparison with the large body of literature describing
the tunability of the cholesteric pitch to obtain red, green, and
chirality (e.g., sterically overcrowded alkenes) or forms
different switching states capable of inducing the opposite
cholesteric helix (e.g., trans-cis isomerization in some chiral
3-10
blue reflection colors,
a reversible switching of the choles-
18,19
teric handedness by external stimuli such as temperature and
azobenzenes)
under light irradiation. In these reports, either
1
5-19
light remains a challenging task.
Temperature-dependent
a relatively slow thermal isomerization process of the dopant
16,18
helix inversion can be attributed to either a change in molecular
drives the reversible switching to the initial state,
the helical
handedness inversion is limited to a particular nematic liquid
(
(
4) (a) Eelkema, R.; Feringa, B. L. Org. Biomol. Chem. 2006, 4, 3729–
745. (b) Yoshioka, T.; Ogata, T.; Nonaka, T.; Moritsugu, M.; Kim,
17
crystal (NLC) host, or the helical twisting power (HTP) of
3
1
9
the dopant is low.
S. N.; Kurihara, S. AdV. Mater. 2005, 17, 1226–1229. (c) Mallia, V. A.;
Tamaoki, N. Chem. Soc. ReV. 2004, 33, 76–84. (d) Ichimura, K. Chem.
ReV. 2000, 100, 1847–1873.
Here we report photoinvertible CLCs based on the helical
inversion of chiral cyclic azobenzenophanes (R)-1 and (R)-2
5) (a) Ma, J.; Li, Y.; White, T. J.; Urbas, A.; Li, Q. Chem. Commun
(Figure 2) in three structurally different, commercially available
2
010, 46, 3463–3465. (b) White, T. J.; Bricker, R. L.; Natarajan, L. V.;
Tabiryan, N. V.; Li, Q.; Bunning, T. J. AdV. Funct. Mater. 2009, 19,
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Lett. 2007, 91, 121118. (c) Lu, S.-Y.; Chen, L.-C. Appl. Phys. Lett.
NLC hosts. It is noteworthy that the compounds used here not
only show higher HTP than a structurally similar known chiral
3
20
dopant and reversible control of the selective reflection colors
1
from blue to near-IR (NIR) but also reveal the unique capability
to induce a reversible helical twist inversion in the cholesteric
phase by light irradiation.
(
(
2
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Results and Discussion
Materials Design and Synthesis. Compounds (R)-1 and (R)-2
consist of an axially chiral binaphthyl moiety [(R)-(+)-1,1′-bis(2-
naphthol)] bonded to the meta positions of a photoresponsive
azobenzene through methylene linkages. There are three me-
thylene linking units in (R)-1, while (R)-2 has five linking
methylene groups, making the latter compound conformationally
more flexible. Both compounds were successfully synthesized
by reduction of their corresponding dinitro compounds using
(
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