Planar chiral azobenzenophanes as chiroptic switches for photon mode reversible reflection color control in induced chiral nematic liquid crystals

Article abstract of DOI:10.1021/ja802472t

In this report, for the first time, a planar chiral photoresponsive compound has been employed in commercially available nematic liquid crystals to achieve phototunable reflection colors. We designed an azobenzenophane compound having conformational restr

Full text of DOI:10.1021/ja802472t

Published on Web 08/02/2008
Planar Chiral Azobenzenophanes as Chiroptic Switches for
Photon Mode Reversible Reflection Color Control in Induced
Chiral Nematic Liquid Crystals
Manoj Mathews and Nobuyuki Tamaoki*
Molecular Smart System Group, Nanotechnology Research Institute, National Institute of
AdVanced Industrial Science and Technology (AIST), 1-1-1, Higashi, Tsukuba,
Ibaraki 305-8565, Japan
Received April 4, 2008; E-mail: n.tamaoki@aist.go.jp
Abstract: In this report, for the first time, a planar chiral photoresponsive compound has been employed
in commercially available nematic liquid crystals to achieve phototunable reflection colors. We designed
an azobenzenophane compound having conformational restriction on the free rotation of naphthalene moiety
to impose an element of planar chirality and the corresponding enantiomers were resolved by HPLC on
chiral column. We have determined the absolute configuration by comparison of density functional theory
(DFT) calculations of its electronic circular dichroism (ECD) spectrum and specific rotation [R]D to
experimental ECD and [R]D data. Enantiomers exhibit photochemically reversible isomerization in solution
without undergoing thermal or photoinduced racemization. As chiroptic switches in different host nematic
liquid crystals, they exhibit good solubility, moderately high helical twisting power, as well as a large change
in helical twisting power due to photoisomerization. A unique feature of these chiral photochromic compounds
is that no other auxiliary chiral agents is required to achieve a fast photon mode reversible full-range color
control in induced cholesterics, that is, both the hypsochromic and bathochromic shift can be obtained
from a single LC formulation by reversible photoisomerization of the single chiral compound.
ogy4 and continue to stimulate innovative research in view of
other possible biomedical applications.5 Most of these applica-
Introduction
Stimuli responsive chiral self-assembling architectures are of
key importance to the existence of life in nature and continue
to emerge as the primary building blocks for the design of
various functional nanoscale assemblies.1 Of particular interest
are the macromolecular and supramolecular systems possessing
chiral photochemical molecular switches because it offers the
advantage of using light as the external stimuli to reversibly
control the molecular and supramolecular chirality.2 For ex-
ample, reversible helical self-assembly of chiral nematic liquid
crystals that reflect specific wavelength of light associated with
its helical pitch are at the forefront in the development of
optoelectronic devices3 and modern color information technol-
tions have in common the necessity to obtain tunable helical
pitch in response to light coupled with other external parameters
such as temperature and electric and magnetic fields and to the
exposure of dopants. Our interest in this topic is for the design
of full-color recording devices that allow quick photon mode
rewritable imaging. In recent years, we have demonstrated that
thermally stable light driven full color recording is possible by
doping the glass forming chiral nematic liquid crystal with
achiral photochromic azobenzene derivatives.6 Alternatively, a
chiral photoresponsive dopant can act as both a chiral agent to
induce a chiral nematic phase in a nematic liquid crystal and a
photoresponsive moiety to control the helical pitch through
photo isomerization.7 When a chiral solute is dissolved in
nematic liquid crystal at the limit of low concentration, the
induced pitch P is correlated with the weight concentration Cw
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10.1021/ja802472t CCC: $40.75
2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 11409–11416 11409

A R T I C L E S
Mathews and Tamaoki
Chart 1. Chemical Structure of the Planar Chiral Dopant E-1 and
the Space Filling Representations of the Corresponding Resolved
Enantiomers
of the dopant (weight of dopant/weight of solution) and its
enantiomeric purity r according to the equation
P
-1 ) ꢀCwr
(1)
The proportionality constant ꢀ is referred to as the helical
twisting power (HTP). Phototunable chiral dopants with not only
high helical twisting power but also with large reversible change
in HTP between its isomeric forms are prized. A great deal of
effort has therefore been dedicated to the design of dopants
incorporating photoresponsive moieties such as azobenzenes,8
fulgides,9 diarylethenes,10 spiropyrans,11 and overcrowded
alkenes12 with point, axial, and helical chirality. Accumulated
experimental7-12 and theoretical results13 show that lower
conformational freedom and higher structural rigidity of both
dopant and the liquid crystalline host engender higher HTP
values, whereas a large change in chiral conformations between
the two distinct interconvertible forms of the chiroptic switch
is desirable for achieving better phototunability of the helical
pitch. In spite of the extensive research, reversible color control
in induced cholesterics without added non photoresponsive chiral
codopants is limited to recent reports employing helically chiral
overcrowded alkenes14 and axially chiral binaphthyl azobenzene
derivatives.8c,15 In both cases, a relatively slow thermal isomer-
ization process of the dopants drives the reversibile bathochro-
mic shift to the initial state, limiting their use in dynamic display-
and variable color filter applications. Therefore, the discovery
of new phototunable chiral dopants is important to broadening
the utility of chiral nematic formulations. We report herein on
a design for the first photoresponsive chiral dopant based on
planar chirality and our success in fast photon mode reversible
reflection color control over the entire visible region for the
first time by employing these chiroptic switches as the only
chiral additive in a commercially available nematic liquid
crystalline host.
bicyclic azobenzene derivatives as efficient chiroptic switches
with dynamic control over their racemization rate through
trans-cis photoisomerization.18 In the present study, we
designed a small and rigid azobenzenophane consisting of a
1,5-dioxynaphthalene moiety with methylene linkages bonded
at the meta positions of the photoresponsive unit. The incor-
poration of the 1,5-dioxynaphthalene ring system can impart
an element of planar chirality to the molecule, provided there
is no free rotation of naphthalene unit through the cavity of the
macrocycle.19 Molecular modeling indicated that bridges formed
by two atoms (-CH2-CH2-) could impose planar chirality to
such a molecule (Chart 1). Due to molecular constraints, the
orientation of the two aromatic mesogenic core units (long
molecular axis of naphthalene and azobenzene units) is expected
to generate an inherent twist in the molecule, making them
attractive as twist agents or potential chiral dopants in nematic
solvents (Chart 2). The phototunability of helical twisting power
of chiral dopants obtained by appending a photoresponsive azo
moiety to an already existing point or axially chiral unit is
Although planar chirality is well-known in asymmetric
reactions, catalysis, host-guest interactions,16 and for the
development of optically active liquid crystalline materials,17
their employment as phototunable chiral dopants have so far
not been explored. Recently, we first reported on planar chiral
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Planar Chiral Photoresponsive Dopants
A R T I C L E S
Chart 2. Schematic Representation Highlighting the Inherent
Molecular “Twist” and the Switching Process in Chiral Dopant E-1a
Figure 1. Absorption spectral change of E-1 in acetonitrile (1.5 × 10-5
M) upon irradiation at room temperature: (a) red line, initial state before
irradiation; (b) blue line, photostationary state (PSS) after irradiation at 365
nm; and (c) black line, PSS after irradiation at 436 nm (reverse reaction).
a Blue and red structures indicate azobenzene and naphthalene units,
respectively.
mainly attributed to the geometrical changes resulting from
isomerization of rod shaped trans isomer to bent cis isomer. In
the above conventional design, as the photochromic unit is
remotely present from the chiral center, only a weak chiral
conformational changes can be achieved by isomerization. In
contrast, a greater influence of isomerization of the photochromic
unit on both geometrical and chiral conformational changes can
be logically expected with our design of azobenzene moiety
being an integral part of the rigid chiral core unit.
Results and Discussion
Synthesis and Conformational Analysis. We synthesized the
target molecules as a racemic compound in three steps through
reduction of the corresponding dinitro compound under dilute
conditions.20 Silica gel column chromatography followed by
precipitation from CH2Cl2/hexane fetched us the pure trans
isomer of the racemic compound E-1. The NMR and mass
spectra supported their structures. The first indication of the
absence of free rotation of the naphthalene moiety in E-1 was
Figure 2. CD spectra of enantiomers of E-1 in CH3CN: (a) (-)E-1 with
absolute configuration R (red line) and (b) (+)E-1 with absolute configu-
ration S (blue line).
and (+)E-1 as first and second fractions ([R]D (c 0.2, CHCl3)
) -204 and +204) respectively. Absorption spectrum of E-1
and the circular dichroism (CD) spectra of (-)E-1 and (+)E-1
were measured in CH3CN (1.5 × 10-5 M), and the results are
shown in Figures 1 and 2, respectively. The UV spectra of E-1
show maximum absorption in the region of 230 nm (ꢀ/M-1
cm-1 84340) due to the 1Bb transition of the naphthalene
chromophore. The absorption band at 280-370 nm (λmax ) 302
nm, ꢀ/M-1 cm-1 31 306) corresponds to 1La and 1Lb transitions
of naphthyl group and π-π* transition related to azobenzene
1
provided by H NMR studies in solution. A restricted rotation
of naphthalene ring within the NMR time scale through the
azobenzenophane cavity will make the compound resolvable
and allow us to use the different NMR signals of the aliphatic
spacer protons as molecular probes for the study of conforma-
tional processes. Accordingly, the 1H NMR spectra of E-1 (300
MHz, 298 K, CDCl3) show three resonances for the bismeth-
ylene protons [-OCHaHbCHc -]. This is because the two
2
aliphatic -OCHaHb hydrogens become diastereotopic when the
exchange of the enantiomers is rendered slow in the NMR time
scale. One of the two diastereotopic proton resonance is located
at about 4.76 ppm, about 0.16 ppm downfield from its partner.
All the four resonances that can be expected from the -OCHa-
HbCHcHd- protons of the compound was observed for its cis
isomer Z-1. As expected, there were three resonances observed
for the six protons of 1,5-dioxynaphthalene moiety and four
resonances for the eight protons of azobenzene unit.
1
1
chromophore. Bb and Lb bands are assigned to the long axis
1
of the naphthyl group, while the La band corresponds to the
short axis.21 However, only a weak absorption band for n-π*
transition (λmax ) 450 nm) is seen in the absorption spectra. In
contrast, CD spectra exhibit distinct bands for π-π* and n-π*
transitions of the azochromophore. The first eluted enantiomer
(-)E-1 shows a negative band for the n-π* transition (λmax
)
450 nm) followed by relatively weak CD Cotton effects of a
complex patterns between 280 and 370 nm. In the region below
270 nm, the CD spectrum is dominated by a strong positive
Enantiomeric Resolution. The enantiomers of E-1 were
resolved by semipreparative chiral HPLC (Chiralpak IA column
with hexane/EtOAc as an eluent) to give enantiopure (-)E-1
(21) (a) Di Bari, L.; Pescitelli, G.; Salvadori, P. J. Am. Chem. Soc. 1999,
121, 7998–8004. (b) Di Bari, L.; Pescitelli, G.; Marchetti, F.; Salvadori,
P. J. Am. Chem. Soc. 2000, 122, 6395–6398.
(20) See the Supporting Information.
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J. AM. CHEM. SOC. VOL. 130, NO. 34, 2008 11411

A R T I C L E S
Mathews and Tamaoki
Table 1. X-ray Structural Data of trans E-1 and Theoretically Calculated Structural Data for cis Z-1
compound
type
NdN (bond lengths, pm)
CNNC
NNCC (angles, deg)
NNC
E-1
Conformer A
1.241
171.2
8.0, -167.9
168.8, -19.3
16.1, -169.2
-158.9, 25.4
15.6, -170.0
-159.3, 25.5
41.7, -148.9
55.1, -135.2
114.3, 115.1
Conformer B
Conformer C
Calculateda
1.252
1.262
1.249
-171.4
-171.8
10.4
114.8, 113.1
112.7, 114.8
125.2
Z-1
a Theoretically calculated data for the lowest energy conformer at the B3LYP/6-31G(d,p) level.
Figure 3. X-ray crystal structure of E-1 (left) and optimized geometry of its cis isomer Z-1 (right) at the B3LYP/6-31G (d,p) level.
1
exciton couplet centered at 230 nm due to the Bb long-axis
polarized transition of the naphthalene and the B π-π* azo
transition.8c As expected, the second eluted enantiomer (+)E-1
shows a complete mirror image CD spectrum of its enantiomer
(-)E-1. By chiral HPLC analysis we confirmed that racemiza-
tion between the enantiomers (-)E-1 and (+)E-1 does not occur
in solution even at the elevated temperatures (12 h refluxing in
toluene solution).
configurations, due to the racemic nature of the crystal we could
not correlate them to the corresponding (-)- and (+)-enantio-
meric fractions eluted from the chiral column. Resolved
enantiomers showed poor tendency for crystallization. It is
subsequently difficult to apply the conventional exciton chirality
theory, which requires quite accurate knowledge about the
orientation of the relevant transition movements.24 On the other
hand, a time-dependent density functional theory (TDDFT)
method is now routinely used to assign the absolute configura-
tions of rigid molecules.25,26 Therefore, to determine the absolute
configuration of the enantiomers of E-1, we applied the TDDFT
method to calculate the electronic CD curve and specific
rotations on structure of E-1 with the S configuration for
comparison with the experimental ones. On the basis of our
calculations,20 we assigned S absolute configuration to the
second eluted enantiomer (+)E-1 and R absolute configuration
to the first eluted enantiomer (-)E-1.
Structural Properties. The structure of trans isomer E-1 was
confirmed by X-ray crystallographic analysis. Single crystals
were obtained from a solution of the racemic mixture of E-1 in
CH2Cl2 by vapor diffusion with hexane. The space group of
the crystal was found to be orthorhombic Pna2(1) with 12
molecules per unit cell. There were three crystallographically
independent pairs of enantiomeric molecules, and all of them
adopted the C1 symmetry with slightly different bond lengths,
angles, and torsions from each other (Table 1). One of the X-ray
crystal structure of E-1 is shown in Figure 3.22 The C-NdN-C
torsion angle of about 171° for all the three conformers indicates
that the trans azo unit in the molecule deviate from planarity
by about 9° due to the ring strain. The B3LYP/6-31G(d,p)
optimized structure of cis isomer Z-1 is also shown in Figure
3. The observed deviation in C-NdN angle or the C-NdN-C
dihedral angle for the computationally derived structure of the
cis isomer (Table 1) from that of cis azobenzene23 point to the
evident ring strain in the cyclophane. The trans isomer E-1 was
estimated to be more stable than Z-1 by 16.4 kcal/mol. Analysis
of a single crystal of E-1 by chiral HPLC revealed equal
amounts of the enantiomers as a further support to the racemic
nature of the crystal. Although we assigned the enantiomers in
the X-ray crystal structure with respective R and S absolute
Trans-Cis Isomerization and Chiroptical Properties. After
successful resolution of the enantiomers of E-1, investigation
of their chiroptical properties on photochemical and thermal
isomerizations were carried out by UV-vis and CD spectros-
copy. Figure 1 shows the absorption changes observed upon
(24) (a) Circular Dichroism: Principles and Applications; Nakanishi, K.,
Berova, N., Woody, R. W., Eds.; Wiley-VCH: New York, 2000. (b)
Berova, N.; Di Bari, L.; Pescitelli, G. Chem. Soc. ReV. 2007, 36, 914–
931.
(25) (a) Furche, F.; Ahlrichs, R.; Wachsmann, C.; Weber, E.; Sobanski,
A.; Vogtle, F.; Grimme, S. J. Am. Chem. Soc. 2000, 122, 1717–1724.
(b) Stephens, P. J.; Devlin, F. J.; Cheeseman, J. R.; Frisch, M. J.;
Rosini, C. Org. Lett. 2002, 4, 4595–4598. (c) Pecul, M.; Ruud, K.;
Helgaker, T. Chem. Phys. Lett. 2004, 388, 110–119. (d) Stephens,
P. J.; Devlin, F. J.; Gasparrini, F.; Ciogli, A.; Spinelli, D.; Cosimelli,
B. J. Org. Chem. 2007, 72, 4707–4715.
(22) See the Experimental Section and the Supporting Information for
details.
(23) Mostad, A.; RLmming, C. Acta Chem. Scand. 1971, 25, 3561–3568.
(26) Rappoport, D.; Furche, F. In Time-Dependent Density Functional
Theory; Marques, M., Ullrich, C. A., Nogueira, F., Rubio, A., Burke,
K., Gross, E. K. U., Eds.; Springer: Berlin, 2006; pp 337-354.
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Planar Chiral Photoresponsive Dopants
A R T I C L E S
Scheme 1. Photochemical and Thermal Processes Taking Place in
S and R Enantiomers of Chiroptic Switch Molecule 1
Figure 4. Polarized optical photomicrographs of 0.4 wt% of (S)E-1
dissolved in ZLI-1132 at RT: (a) Chiral nematic fingerprint texture, and
reversible change occurring in pitch by irradiation of sample in a wedge
type cell evidenced as change in distance between the Cano’s lines; (b)
before irradiation, (c) PSSUV, (d) PSSvis; scalebar, 150 µm.
reversible trans-cis isomerization in (R)E-1 to (R)Z-1 and
(S)E-1 to (S)Z-1 proceeds without any racemization. This result
reveals that in both E and Z isomers of 1 the size of the cavity
of the cyclophane is not large enough for the free rotation of
the naphthalene ring through it. This observation is in agreement
with the results obtained by structural analysis of E-1 and Z-1
isomers. The minimum through space distance that is required
for the free rotation of naphthalene ring was estimated to be
about 4.7 Å. Our molecular modeling indicate that the major
steric hindrance for the rotation of the naphthalene moiety arise
from the hydrogen atoms present at the 2,2′ positions of the
azobenzene unit. As shown in Figure 3, the calculated distance
from the rotational axis in naphthalene to the sterically hindering
hydrogens of azo unit in both the trans and cis isomers was
much shorter than the required 4.7 Å for the free rotation of
the naphthalene unit. Scheme 1 summarizes the photoisomer-
ization characteristics of the (R)E-1 and (S)E-1 enantiomers in
solution highlighting their photochemically reversible isomer-
ization to (R)Z-1 and (S)Z-1 without undergoing photoinduced
racemization.
Evaluation of Helical Twisting Power. After confirming the
chiroptic properties of (R)E-1 and (S)E-1 in solution, we
investigated their potential as chirality transfer agents in three
commercially available nematic (N) host liquid crystals (LC)
of diverse chemical structures namely ZLI-1132, DON-103 and
5CB.30 The induction of chiral nematic phase was directly
evidenced as a fingerprint texture31 under a polarized optical
microscope (Figure 4). Induced helical pitch P for different
concentrations of dopant was calculated by well-known Cano’s
wedge method.32 The helical sense of the induced N* phase
was determined by contact method33 as well as by measuring
the CD spectra (vide infra). Enantiomers of a chiral solute are
known to induce an equal but opposite twist in the host N phase.
UV and visible light irradiations of E-1 in acetonitrile solution
at 298 K. Upon UV irradiation (λmax ) 365 nm), due to
photochemical trans (E-1) to cis (Z-1) isomerization, there is a
gradual decrease seen in the π-π* transition bands in the
absorption spectra with no concomitant increase in the n-π*
band around 450 nm. In the CD spectra of the (R)E-1 and (S)E-1
enantiomers upon irradiation at 365 nm, the n-π* transition
gradually becomes more intense whereas the exciton couplet
at 230 nm weakens significantly.20 Furthermore, the two weak
exciton couplets between 280 and 370 nm diminish to two weak
positive or negative bands for (R)E-1 and (S)E-1, respectively.
These results clearly indicate the substantial influence of the
photoisomerization on its chiroptical properties. The photosta-
tionary state (PSS) was reached after UV irradiation for about
90 s. Reaction proceeds maintaining isosbestic points through
out the irradiation and we determined the ratio of trans: cis
isomers in the PSSUV as 40:60 by HPLC analysis. Any changes
of the irradiation wavelength did not enhance the cis ratio in
the PSSUV. Upon visible light irradiation (λmax ) 436 nm),
reverse CD and absorption spectral changes occurred. The PSSvis
was obtained in 2 min with 90% of trans isomer E-1. Alternating
irradiations at 365 and 436 nm was repeated several times
without any sign of fatigue. The thermal cis-trans isomerization
was determined by monitoring the increase of absorbance at
302 nm in the absorption spectra. The lifetime (1/k) of cis isomer
in the dark at 298 K was found to be about 2.5 h (k ) 1.10 ×
10-4 s-1), which is much shorter than that of cis-azobenzene
(4.7 days)27 but comparable to other strained cyclic azoben-
zenophanes.28 The first-order rate plots at different temperatures
were used to calculate the activation energy (Ea ) 15.1 kcal
mol-1) of the thermodynamic back relaxation process. Other
thermodynamic parameters such as enthalpy of activation (∆Hq,
15.7 kcal mol-1) and entropy of activation (∆Sq, -1.2 cal K-1
mol-1) were determined according to Eyring equation.29 On
the basis of the chiral HPLC analysis, we found that the
(30) (a) ZLI-1132 (TNI ) 72.3 °C): Mixtures of 4-(4-alkylcyclohexyl)-
benzonitrile and 4-(4-alkylcyclohexyl)-4′-cyanobiphenyl derivatives.
(b) DON-103 (TNI ) 74.2 °C): Mixtures of cyclohexanoic acid phenyl
esters. (c) 5CB (4-cyano-4′-pentylbiphenyl): TNI ) 35.0 °C.
(31) Textures of Liquid Crystals; Dierking, I., Ed.; Wiley-VCH: Weinheim,
2003.
(27) Asano, T.; Okada, T.; Shinkai, S.; Shigematsu, K.; Kusano, Y.;
Manabe, O. J. Am. Chem. Soc. 1981, 103, 5161–5165.
(28) Bassotti, E.; Carbone, P.; Credi, A.; DiStefano, M.; Masiero, S.; Negri,
F.; Orlandi, G.; Spada, G. P. J. Phys. Chem. A 2006, 110, 12385–
12394.
(32) (a) Solladie, G.; Zimmermann, R. G. Angew. Chem., Int. Ed. Engl.
1984, 23, 348–362. (b) Sagisaka, T.; Yokoyama, Y. Bull. Chem. Soc.
Jpn. 2000, 73, 191–196.
(33) Isaert, N.; Soulestin, B.; Malthete, J. Mol. Cryst. Liq. Cryst. 1976,
37, 321–333.
(29) Eyring, H. Chem. ReV. 1935, 17, 65–77.
9
J. AM. CHEM. SOC. VOL. 130, NO. 34, 2008 11413

A R T I C L E S
Mathews and Tamaoki
Figure 6. CD spectrum of an induced chiral nematic film having 10%
w/w mixture of (R)E-1 in ZLI-1132.
Figure 5. Reciprocal helical pitch as a function of concentration of (R)E-1
and (S)E-1 dopants in 5CB (blue line), ZLI-1132 (red line), and DON-103
(green line) nematic host liquid crystals. Positive and negative P-1 values
represent right-and left-handed helical twists obtained for (R)E-1 and (S)E-1
respectively.
behavior in the LC hosts with UV (λmax ) 365 nm) and visible
light (λmax ) 436 nm) irradiations. In the PSSuv, about 60%
conversion of trans isomers (R)E-1 and (S)E-1) to cis form
(R)Z-1 and (S)Z-1 of the dopants in the host nematic LCs were
determined by HPLC analysis. Photoinduced changes in HTP
of the dopants were directly evidenced as change in distance
between the Cano lines when sample in a wedge cell was
observed with a polarized optical microscope under UV or
visible light illumination (Figure 4). Variations in the induced
pitch and the corresponding HTP values were then calculated
using eq 1. It took less than 3 min to reach the photostationary
state under UV light illumination. The changes in HTP values
obtained by irradiation are given in Table 2. The HTP values
of trans dopants in 5CB and ZLI-1132 hosts were decreased
by UV irradiation. On the contrary, dopants in DON-103 had
shown an increase in HTP values with trans to cis photoisomer-
ization. The process was reversed on illumination with visible
light and a PSSvis state in which about 90% of azobenzene units
in trans form was obtained with in 5 min. From the experimental
PSS ratios and the induced helical pitch measurements the
absolute values of ꢀ for pure cis isomer was estimated to be 20
µm-1 in ZLI-1132, 36 µm-1 in 5CB and 21 µm-1 in DON-
103. Due to the large difference in helical twisting powers of
the trans and cis isomers in ZLI-1132 (i.e., +40 µm-1 for
(R)E-1 compared to +20 µm-1 for (R)Z-1), a significant
effective change in the induced pitch ca. 28% by alternate UV
and visible light irradiations was achieved. We also confirmed
that both the trans and cis isomers of the enantiomers have the
same helical twist sense, i.e. right-handed helix for (R)E-1 and
(R)Z-1 and left-handed helix for (S)E-1 and (S)Z-1. Although
the decrease in HTP values for the cis isomers of the dopants
compared to the trans isomers in ZLI-1132 and 5-CB can be
explained based on the better structural compatibility of the
elongated trans azo units of the dopants with the host LC
molecules, a mechanistic interpretation for the increase in HTP
values for the cis isomers in DON-103 requires a better
understanding of the solvent-solute interactions on the chirality
transfer. Because of high twisting ability as well as good
phototunability in HTP values of dopants, we employed them
for achieving photoaddressable LC films in the visible region.
Photoinduced Modulation of Chiral Nematic Reflection
Colors. The reflected colors from chiral nematic phase depends
on the helical pitch P of the phase according to the Bragg’s
law λmax ) Pn, where λmax is the reflection maximum and n is
the refractive index of the system. In mixtures of a nematic
Table 2. Helical Twisting Powers (ꢀ/µm-1) of Dopants in Different
Nematic LC Hosts As Determined by Cano’s Wedge Method and
the Observed Change in Values by Irradiationa
ꢀ (µm-1
)
dopant
host NLC
initial
PSSuv
PSSvis
∆ꢀ [%]b
(R)E-1
5CB
+43
+40
+16
-43
-40
-16
+36
+20
+21
+36
+28
+19
-36
-28
-19
+40
+36
+17
-40
-36
-17
16
30
19
16
30
19
ZLI-1132
DON-103
5CB
ZLI-1132
DON-103
5CB
(S)E-1
(R)Z-1c
ZLI-1132
DON-103
a Positive and negative values represent right- and left-handed helical
twists, respectively. b Percent change in ꢀ observed from initial to
UVpss.
c As determined by HPLC analysis.
Accordingly, chiral dopant (R)E-1 in all the three N host LCs
induced a right-handed helix, while a left-handed helix was
obtained from (S)E-1. The inverse of the pitch (P -1) propor-
tionally increased with increase in the concentration of the
dopant (Figure 5). The absolute values of ꢀ were then
determined by plotting P -1 against concentration of the dopant
and the results are summarized in Table 2. Dopants exhibit
moderately high HTP values and their twisting ability in three
hosts follows the order 5CB (43 µm-1) g ZLI-1132 (40 µm-1
)
> DON-103 (16 µm-1). As expected, these dopants with planar
chirality exhibit higher ꢀ values than that is observed for the
photochromic compounds with central chirality (e10 µm-1) and
are comparable to the moderate-to-high helical twisting powers
reported for axially chiral photochromic compounds. The two
most effective hosts in this work; 5CB and ZLI-1132, have
straight and rigid biaryl moieties as part of their mesogenic
structure with high longitudinal dipole moment, while the least
effective DON-103 is significantly flexible due to ester linkage
with dipole moment perpendicular to the molecular long axis.
These results are in consistent with the observations that lower
conformational freedom and higher structural rigidity of the
chiral dopant and the structural similarity between the host and
dopant lead to higher HTP values.
Photomodulation of Helical Twisting Power. Next we inves-
tigated the effect of isomerization of the dopants on the HTP
values. Dopants displayed efficient reversible photoisomerization
9
11414 J. AM. CHEM. SOC. VOL. 130, NO. 34, 2008

Planar Chiral Photoresponsive Dopants
A R T I C L E S
Figure 7. Changes in selective reflection spectra of the chiral nematic LC film consisting of 12 wt% of (R)E-1 in ZLI-1132: (a) on UV irradiation and (b)
on visible light irradiation at room temperature.
with no other auxiliary chiral agents. The reversible blue shift
achieved by 436 nm irradiation in this study is about 10 times
faster than the switching times reported so far for the thermally
driven systems.8c,14,15 It is true that the change of color in
minutes is still too slow for the application to memory or active
display media, although it is acceptable for rewritable full-color
display media that changes the image several times a day. A
further significant improvement in the phototunability of HTP
and response times can be achieved by obtaining a higher trans
Figure 8. Changes in reflection color of the chiral nematic LC film
to cis isomerization ratio at the PSSuv than that is reported here.
Therefore studies are in progress to develop more efficient planar
chiral photoresponsive dopants with high helical twisting power
and faster photo responses.
consisting of 12 wt% of (R)E-1 in ZLI-1132 by varying UV irradiation
time: 0 s (left), 30 s (middle), and 120 s (right).
liquid crystal with a chiral additive, the pitch length can be
increased or decreased with concentration of the chiral com-
pound. Therefore, we prepared liquid crystal mixtures with
sufficient amount of (R)E-1 and (S)E-1 chiral compounds for
the pitch to be in the visible region. Dopants not only have
excellent phototunability of helical twisting power due to rigid
planar chiral photochromic core structure but also show good
miscibility toward the nematic host LCs. A mixture consisting
of 10 wt% of (R)E-1 in ZLI-1132 resulted in a reddish LC film.
Figure 6 shows its CD spectrum (recorded at normal incidence)
having a large negative band corresponding to the selective
reflection of the right-handed circularly polarized light (SR-
RCPL) in consistent with the right-handedness of the induced
chiral nematic phase.34 A mixture consisting of compound
(R)E-1 and ZLI-1132 (12:88 by weight) inside a glass cell
consisting of two glass plates coated with polyimide alignment
layer with a 5 µm cell gap, showed a chiral nematic phase at
room temperature with a selective reflection band around 450
nm. Irradiation with 365 nm light resulted in the shift of the
selective reflection band to a longer wavelength red reflection
band through green reflection color as shown in Figure 7a and
Figure 8. The reflection band returned to the initial state after
3 h in the dark due to thermal cis to trans isomerization of the
azo compound. However, a rapid reversible shift of the reflection
color band was achieved by visible-light irradiation as shown
in Figure 7b. This process was repeated several times to confirm
the fatigue resistance. Thus, a fast reversible photon mode
reflection color control with blue, green, and red colors was
achieved in an induced chiral nematic film by alternating UV
and visible-light irradiation employing the planar chiral dopant
Conclusions
Photochromic compounds having planar chirality were em-
ployed to reversibly control the selective reflection colors in an
induced chiral nematic liquid crystalline film. For this purpose,
a chiral azobenzenophane was designed and synthesized and
the enantiomers were resolved by HPLC on a chiral column.
These enantiomers showed photochemically reversible isomer-
ization in solution without undergoing thermal or photoinduced
racemization. Employing these chiroptic switches as dopants
in different host nematic LCs not only exhibit good solubility,
reversible photoisomerization with fatigue resistance, and
moderately high helical twisting powers but also bring about a
large photon mode reversible change in HTP due to cis-trans
isomerization. Furthermore, for the first time, we achieved a
fast reversible control of the three primary colors that are both
hypsochromically and bathochromically shifted by the action
of light in an induced chiral nematic LC film having photo
tunable dopant as the only source of molecular chirality. The
synthetic achievements and conformational knowledge obtained
herein will allow us to design new members of this type of
compound with planar chirality and is expected to expand the
field of phototunable chiral dopants as well as the chiroptic
switches for various applications.
Experimental Section
Instrumentation. 1H and 13C NMR spectra were recorded on a
Varian Gemini 300. The chemical shifts are reported in δ (ppm)
relative to tetramethylsilane as internal standard. UV-vis spectra
were recorded on a JACSO V-570 spectrometer. CD spectra and
optical rotations were recorded on a JASCO J-830 spectropola-
rimeter. High-performance liquid chromatography (HPLC) was
(34) (a) de Gennes, P. G.; Prost, J. The Physics of Liquid Crystals, 2nd
ed.; Oxford: New York, 1993. (b) Yoshida, J.; Sato, H.; Yamagishi,
A.; Hoshino, N. J. Am. Chem. Soc. 2005, 127, 8453–8456.
9
J. AM. CHEM. SOC. VOL. 130, NO. 34, 2008 11415

A R T I C L E S
Mathews and Tamaoki
carried out on PD-8020, TOSH with CHIRALPAK IA columns.
Matrix-assisted laser desorption ionization time-of-flight mass
spectrometry (MALDI-TOF MS) was conducted using an Applied
Biosystems Voyager-DE pro instrument operated in reflector mode
and samples were prepared through deposition of THF solutions
of the compounds and a-cyano-4-hydroxycinnamic acid. Micro-
scopic analyses were performed with an OLYMPUS IMT-2
equipped with a digital camera (CoolPix 995, NIKON) and a Mettler
FP82HT hot stage. Photoisomerization was carried out with a
superhigh-pressure mercury lamp (500 W, USHIO Inc.) through
an appropriate filter (356 or 436 nm).
Materials. Nematic liquid crystal hosts ZLI-1132 and DON-
103 were purchased from Merck while 5CB was obtained from
TCI chemicals, Japan. Enantiomeric chiral agents, R-1011 (right-
handed cholesterics) and S-1011 (left-handed cholesterics) with
known helical handedness induction abilities in the above nematic
solvents were purchased from Merck. All solvents, reagents and
other chemicals were obtained from commercial sources and used
without further purification, unless otherwise noted.
Single-Crystal X-ray Diffraction data for E-1: C26H22N2O2,
Fw ) 394.46, orthorhombic, space group Pna2(1), a ) 15.6790(14)
Å, b ) 9.3310(8) Å, c ) 41.624(4) Å, V ) 6089.6(9) Å3, Z ) 12,
µ ) 0.082 mm-1, T (K) ) 183(2), Fcalcd ) 1.291 mg/m3, GOF on
F2 ) 1.023, R1 ) 0.0522, and wR2 ) 0.1153 (F2 all data). Crystal
size 0.40 × 0.30 × 0.05 mm3. Diffraction data were collected on
a Bruker Smart Apex diffractometer with Mo KR radiation. The
data reduction was carried out with SAINTPLUS V6.22.35 Semiem-
pirical absorption correction was applied with SADABS.36 The
structure determination was done with direct methods and refine-
ments with full-matrix least-squares on F2 with SHELXTL version
6.12.37
Measurement of Helical Pitch and Handedness. The chiral
nematic liquid crystal was prepared by weighing appropriate amount
of host liquid crystal and the dopant into a vial followed by mixing
them with the addition of a few drops of dichloromethane. After
evaporation of the solvent under reduced pressure, the mixture was
loaded into the wedge cell by capillary action at room temperature.
The pitch was then determined by measuring the intervals of Cano’s
lines appearing on the surfaces of wedge-type liquid crystalline cells
(EHC, KCRK-07, tan θ ) 0.0194). The relationship between the
distances of Cano’s lines and the helical pitch P is P ) 2R tan θ,
where R represents the distance between Cano’s lines, and θ is the
angle between the cells.
Acknowledgment. This work was supported by a grant-in-aid
for science research in a priority area ”New Frontiers in Photo-
chromism (No. 471)” from the Ministry of Education, Culture,
Sports, Science, and Technology (MEXT), Japan.
Supporting Information Available: Synthesis and character-
ization data (1H NMR, 13C NMR, HPLC, and X-ray crystal-
lography file of the compound E-1), details of computational
study. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA802472T
(35) SMART, version 5.625, and SAINTPLUS, version 6.22; Bruker Axs
Inc.: Madison, WI, 2000.
(36) Sheldrick, G. M. SHELXTL, version 6.12; Bruker AXS: Madison, WI,
(37) Sheldrick, G. M. SADABS; University of Go¨ttingen: Go¨ttingen,
2000.
Germany, 1996.
9
11416 J. AM. CHEM. SOC. VOL. 130, NO. 34, 2008