Influence of a change in helical twisting power of photoresponsive chiral dopants on rotational manipulation of micro-objects on the surface of chiral nematic liquid crystalline films

Article abstract of DOI:10.1002/chem.201200836

Herein we report a group of five planar chiral molecules as photon-mode chiral switches for the reversible control of the self-assembled superstructures of doped chiral nematic liquid crystals. The chiral switches are composed of an asymmetrically substituted aromatic moiety and a photoisomerizing azobenzene unit connected in a cyclic manner through methylene spacers of varying lengths. All the molecules show conformational restriction in the rotation of the asymmetrically substituted aromatic moiety in both the E and Z states of the azobenzene units resulting in planar chirality with separable enantiomers. Our newly synthesized compounds in pure enantiomeric form show high helical twisting power (HTP) in addition to an improved change in HTP between the E and Z states. The molecule with a diphenylnaphthalene unit shows the highest ever known initial helical twisting power among chiral dopants with planar chirality. In addition to the reversible tuning of reflection colors, we employed the enantiomers of these five compounds in combination with four nematic liquid crystalline hosts to study their properties as molecular machines; the change in HTP of the chiral dopant upon photoisomerization induces rotation of the texture of the liquid crystal surfaces. Importantly, this study has revealed a linear dependence of the ratio of the difference between HTPs before and after irradiation against the absolute value of the initial HTP, not the absolute value of the change in helical twisting power between two states, on the angle of rotation of micro-objects on chiral nematic liquid crystalline films. This study has also revealed that a change in irradiation intensity does not affect the maximum angle of rotation, but it does affect the speed of rotational reorganization of the cholesteric helix. Copyright

Full text of DOI:10.1002/chem.201200836

FULL PAPER
DOI: 10.1002/chem.201200836
Influence of a Change in Helical Twisting Power of Photoresponsive Chiral
Dopants on Rotational Manipulation of Micro-Objects on the Surface of
Chiral Nematic Liquid Crystalline Films
Reji Thomas,[a] Yohei Yoshida,[a, b] Takehito Akasaka,[a] and Nobuyuki Tamaoki*[a]
Abstract: Herein we report a group of
five planar chiral molecules as photon-
mode chiral switches for the reversible
control of the self-assembled super-
structures of doped chiral nematic
liquid crystals. The chiral switches are
composed of an asymmetrically substi-
tuted aromatic moiety and a photoiso-
merizing azobenzene unit connected in
a cyclic manner through methylene
spacers of varying lengths. All the mol-
ecules show conformational restriction
in the rotation of the asymmetrically
substituted aromatic moiety in both the
E and Z states of the azobenzene units
resulting in planar chirality with sepa-
rable enantiomers. Our newly synthe-
sized compounds in pure enantiomeric
form show high helical twisting power
(HTP) in addition to an improved
change in HTP between the E and Z
states. The molecule with a diphenyl-
naphthalene unit shows the highest
ever known initial helical twisting
power among chiral dopants with
planar chirality. In addition to the re-
versible tuning of reflection colors, we
employed the enantiomers of these five
compounds in combination with four
nematic liquid crystalline hosts to study
their properties as molecular machines;
the change in HTP of the chiral dopant
upon photoisomerization induces rota-
tion of the texture of the liquid crystal
surfaces. Importantly, this study has re-
vealed a linear dependence of the ratio
of the difference between HTPs before
and after irradiation against the abso-
lute value of the initial HTP, not the
absolute value of the change in helical
twisting power between two states, on
the angle of rotation of micro-objects
on chiral nematic liquid crystalline
films. This study has also revealed that
a change in irradiation intensity does
not affect the maximum angle of rota-
tion, but it does affect the speed of ro-
tational reorganization of the cholester-
ic helix.
Keywords: azo compounds · chiral-
ity
· liquid crystals · molecular
machines · photochromism
Introduction
nano-objects have received greater attention as a result of
their expected applications as delivery systems.[9]
The design and construction of motors of ever smaller sizes
have fascinated scientists worldwide owing to the growing
interest in the miniaturization of components for the con-
struction of useful devices, which is an essential feature of
modern technology.[1] From this perspective, several at-
tempts have been made towards the development of syn-
thetic molecular systems that function as shuttles,[2] rotors,[3]
muscles,[4] elevators,[5] and motors,[6–8] with these systems
giving a new outlook to the field of synthetic molecular ma-
chines. Among various reports on molecular machine con-
structs, those generating linear and rotary motion of micro/
Ichimura et al.[10] reported one of the first examples of the
translational motion of objects on surfaces, with the surface
energy gradient formed by the photoisomerization of azo-
benzene units resulting in the movement of droplets on azo-
benzene-modified surfaces. Later, a similar strategy was ex-
tended by other groups employing a molecular shuttle[11] or
a photoactive molecule, like a spiropyran derivative,[12] to
demonstrate the photocontrolled manipulation of liquid
drops on surfaces. Recently, photochromic liquid crystalline
polymers[13] or crystals[14] have found applications as artificial
muscles and actuators, exploiting the structural changes in-
duced upon photoisomerization. Feringa and co-workers[15]
reported the rotational motion of micro glass rods on a
switchable chiral nematic liquid crystalline film induced by
the isomerization of molecular motors in the nematic liquid
crystalline host. In this system, forward rotation was induced
by the photoisomerization of the sterically overcrowded eth-
ylene motor molecule whereas the reverse rotation was ach-
ieved by its thermally induced reverse isomerization. This
study demonstrated how the collective action of molecular-
scale motors embedded in a liquid crystalline host is trans-
lated into the rotational motion of the macroscale object. In
a similar study, Kurihara and co-workers[16] reported a fully
[a] Dr. R. Thomas, Y. Yoshida, T. Akasaka, Prof. Dr. N. Tamaoki
Research Institute for Electronic Science, Hokkaido University
N 20, W 10, Sapporo, Hokkaido 001-0020 (Japan)
Fax : (+81)11-706-9356
E-mail: tamaoki@es.hokudai.ac.jp
[b] Y. Yoshida
Department of Chemistry, Faculty of Science, Toho University
Miyama, 2-2-1, Funabashi-shi, Chiba 274-8510 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200836.
Chem. Eur. J. 2012, 00, 0 – 0
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photocontrolled rotary motion of micro glass rods induced
by the photoisomerization of the central chiral azobenzene
molecule. In both these systems the rotational motion of
micro-objects has been achieved by the rotational reorgani-
zation of the cholesteric helix, which is reversibly switched
by isomerization of the chiral dopants.
It is important to know the relationship between molecu-
lar-scale events such as the isomerization of the chiral
dopant and the macroscopic event of glass rod rotation. The
literature of the last decade contains several studies dedicat-
ed to the design and synthesis of phototunable chiral dop-
ants functionalized with various photochromic units for the
purpose of reversible switching of cholesteric pitch and
thereby color reflections.[17–19] When a chiral solute is dis-
solved in a nematic liquid crystal at the limit of low concen-
tration, the induced helical pitch P is correlated to the
weight concentration Cw of the dopant (weight of dopant/
weight of solution) and its enantiomeric purity r according
to Equation (1).
ported in the literature, those based on an azobenzene pho-
toisomerizing unit have received greater attention due to
their efficiency in bringing about large structural change
upon E/Z photoisomerization.[18,19] Previous studies by our
group have demonstrated the applications of planar chiral
azobenzenes as chiroptical switches for the reversible con-
trol of color reflections,[18d] photocontrolled molecular
brakes,[25] and chiral memory.[26] In this study we have used
modified planar chiral cyclic azobenzenes to gain an under-
standing of the effect of molecular geometry and mesogenic
properties on the helical twisting power of the molecules
and to explore their applications as chiroptical switches in
four structurally different nematic liquid crystalline phases.
We obtained planar chiral azobenzenes showing the largest
initial HTP or the largest change in HTP among known
planar chiral photoresponsive dopants by introducing suita-
ble substituents into the parent molecule. More importantly,
our results have clearly revealed a linear dependence of the
degree of rotation of micro glass rods on the ratio of the
HTP of the chiral dopant before and after irradiation, not
just the absolute value of change in the helical twisting
power between the initial and final states of the chiral
dopant, against its initial helical twisting power.
Pꢀ1 ¼ bCwr
ð1Þ
The proportionality constant b refers to the helical twist-
ing power (HTP) of the chiral dopant. From the accumulat-
ed experimental[17–23] and theoretical studies[24] on the photo-
controlled reversible tuning of the cholesteric pitch it is
clear that the initial helical twisting power and its difference
between the bi-stable states are the two important parame-
ters that decide the reversible control of the reflection
colors in a wide spectral region. The change in the reflection
wavelength band Dl is related to the change in helical twist-
ing by Equation (2) in which n is the refractive index of the
doped liquid crystal mixture and bini and bfin are defined as
the observed HTP of the chiral dopant before and after pho-
toirradiation, respectively.
Results and Discussion
Synthesis and conformational analysis: In this study we de-
signed and synthesized a series of cyclic azobenzenes (1–5)
that contain disubstituted naphthalene (phenyl or bromine)
or benzene (ethyl ester or methoxy) units linked to an iso-
merizing azobenzene in a cyclic manner through methylene
spacers of varying lengths (Scheme 1). In all the molecules,
the spacers are short enough to avoid the rotation of the dis-
ubstituted aromatic moiety, which in turn imparts an ele-
ment of planar chirality to the molecules. Molecules 1 and 2
differ in the nature of the substituents on the naphthalene
unit, whereas molecules 3 and 4 contain a diethyl ester sub-
stituted benzene unit connected to the azobenzene moiety
but with spacers of different lengths. Among these mole-
cules, 5 is expected to have low structural freedom with the
dimethoxy-substituted dihydroxybenzene unit connected to
the ortho position of the azobenzene unit through short eth-
ylene spacers.
The strategies adopted for the synthesis of molecules 1–5
are shown in Scheme 1. Molecule 1 with the 2,6-diphenyl-
1,5-dihydroxynaphthalene unit was synthesized in four steps
starting from 2,6-dibromo-1,5-dihydroxynaphthalene. The
final product with a cyclic structure was obtained by the re-
duction of the corresponding dinitro compounds under
dilute conditions. A similar cyclization procedure was used
to obtain the cyclic azobenzene 5. In the cases of molecules
2–4 with easily reducible groups such as bromo and ethyl
ester, we used a Mitsunobu cyclization procedure under
dilute conditions to obtain the cyclized products. We found
that the formation of cyclic azobenzenes through Mitsunobu
cyclization reactions under dilute conditions is an efficient
1
1
ð2Þ
Dl ¼ nð Þ
ꢀ
cwrbfin cwrbini
Although there are a few reports on the application of re-
versibly switchable doped chiral nematic liquid crystalline
phases as molecular motors,[15,16] there are no studies that di-
rectly link the nature of the chiral dopant to the degree of
rotation of micro-objects. In their experimental and theoret-
ical studies, Feringa and co-workers examined the mecha-
nism of the rotational reorganization of the cholesteric helix
on the basis of the nature of the liquid crystal host and the
photoisomerization of the chiral dopant.[15b,c] Although it is
apparent that the change in the HTP must be the reason for
the rotation of the glass rods no detailed studies in the liter-
ature that probe the exact parameter of the chiral dopant
that determines the degree of rotation of micro-objects on
these films.
To study the role of chiral dopants in the fully photocon-
trolled rotational reorganization of cholesteric texture, we
synthesized five new planar chiral azobenzenes. Among sev-
eral chiroptical switches with various photochromic units re-
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Photoresponsive Chiral Dopants
FULL PAPER
explained by a lack of rotation
of the asymmetrically substitut-
ed naphthalene moieties, which
results in a diastereotopic envi-
1
ronment. The H NMR spectra
obtained for (E)-3, (E)-4, and
(E)-5 with asymmetrically sub-
stituted benzene units also indi-
cate a lack of rotation of the
disubstituted aromatic moieties
with well-separated resonances
corresponding to the methylene
protons (see Figure S1 in the
Supporting Information).
To obtain further information
on the structures of molecules
(E)-1–5, we performed single-
crystal X-ray diffraction analy-
ses on the structures obtained
(see Figure 2 and Table S1 in
the Supporting Information).
The crystal structures of all the
molecules show that the disub-
stituted aromatic moieties are
placed beside and almost per-
pendicularly to the plane of
azobenzene units. To determine
the strain in the molecules we
measured the angle between
the planes formed by the ben-
zene rings of the azobenzene
moieties in all the molecules.
Molecule (E)-5 shows a maxi-
mum angle of 63.98 between
the planes formed by the ben-
zene rings of the azobenzene
unit, which indicates that the
ortho substitution in the azo-
benzene unit results in high
strain within the molecule. In
contrast, molecule (E)-4 shows
Scheme 1. Synthetic routes to molecules 1–5.
procedure for obtaining similar types of molecules even
with bulky functionalities that are prone to LiAlH4 reduc-
tion.
1
Figure 1 show the H NMR spectra of compounds (E)-1
and (E)-2 in CD2Cl2 and CDCl3, respectively, recorded at
room temperature. The spectrum of (E)-1 displays a multip-
let at d=4.56–4.51 ppm (2H) along with a triplet resonance
at d=4.19 ppm (2H) and doublets of doublets at d=3.26
ꢀ
(2H) and 2.85 ppm (2H) arising from the ethylene spacer
ꢀ
OCH2CH2 . Similarly, the protons in the ethylene spacers of
compound (E)-2 exhibit two separated multiplet resonances
at d=5.34 (2H) and 4.82 ppm (2H) along with a triplet res-
onance at d=3.42 ppm (2H). The split resonances for a
single set of methylene protons in these molecules can be
Figure 1. 1H NMR spectra of (a) (E)-1 and (b) (E)-2 recorded at room
temperature from their solutions in CD2Cl2 and CDCl3, respectively.
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N. Tamaoki et al.
trum also shows a weak absorption band centered at 456 nm
corresponding to n–p* transitions of the azobenzene moiety.
Irradiation with light of 366 nm induced changes in the in-
tensity of the band corresponding to p–p* transitions of azo-
benzene with a concomitant increase in the intensity of the
n–p* transition band leading to the PSS366nm. However, irra-
diation with light of 436 nm induced the recovery of the
spectra leading to PSS436nm. A similar observation can be
made in the case of molecule (E)-1, however, with a slight
shift in the band positions (see Figure S3a in the Supporting
Information). The molecules with asymmetrically substitut-
ed benzene moieties also show isomerization properties with
significant changes in the band intensities corresponding to
the p–p* and n–p* transitions of the azobenzene units (see
Figure S3b,c). Interestingly, molecule (E)-5 with substitution
at the ortho position of the azobenzene unit showed com-
plete E/Z isomerization under visible light (see Figure S4 in
the Supporting Information). In this molecule, the Z-rich
state is attained by irradiation at 436 nm, whereas the re-
verse isomerization (PSS436nm) was carried out by irradiation
with light of wavelength >500 nm (photostationary state,
PSS>500).[19f,27] The E/Z isomer ratios for molecules (E)-1–4
at PSS366nm and PSS436nm were estimated by HPLC analysis.
The E/Z ratios at the PSS366nm were found to be 18:82, 5:95,
16:84, and 22:78, respectively, for (E)-1, (E)-2, (E)-3, and
(E)-4, whereas those at the PSS436nm are 88:12, 89:11, 88:12,
and 79:21. The E/Z isomer ratios for molecule (E)-5 were
estimated to be 57:43 and 85:15 for the photoisomerized Z-
and E-rich states, respectively. For all the molecules, the
photoinduced alternating E/Z isomerizations were repeated
over several cycles without any sign of fatigue.
Figure 2. ORTEP plots showing the structures of molecules (E)-1–5.
a perfectly planar azobenzene unit, attributed to the flexible
ring structure.
Photoisomerization of cyclic azobenzenes: With the aim of
studying the photoisomerization properties of the target
molecules, we carried out UV/Vis and HPLC studies at
room temperature. The E/Z and Z/E photoisomerization re-
actions of all the molecules except for (E)-5 were performed
by irradiating the samples with light of wavelengths of 366
and 436 nm, respectively. Figure 3 shows the UV/Vis spec-
Enantiomeric resolution and chiroptical properties: Follow-
1
ing the indications from the H NMR data, we performed a
chiral resolution and evaluated the chiroptical properties of
molecules (E)-1–5 by using chiral HPLC and circular di-
chroism (CD) spectroscopy. The enantiomers of compounds
(E)-1, (E)-2, and (E)-4 were resolved on a Chiralpak IA
column by using dichloromethane/hexane (40:60) as eluent,
whereas the enantiomers of compounds (E)-3 and (E)-5
were separated on a Chiralpak IB column by using ethyl
acetate/hexane (25:75) and tetrahydrofuran/hexane (30:70),
respectively, as eluents. The first and second eluted enan-
tiomers of all the compounds for the given chiral column
and solvent combination are denoted by the subscripts A
and B, respectively. Figure 4 shows the HPLC traces ob-
tained for molecule (E)-2 with dichloromethane/hexane
(40:60) as eluent. The HPLC trace for racemic (E)-2 ob-
tained before irradiation shows two peaks (retention times,
Rt =13.88 and 17.03 min) corresponding to the first ((E)-2A)
and second eluted ((E)-2B) enantiomers. Irradiation at
366 nm resulted in a reduction in the intensity of the peaks
corresponding to (E)-2A and (E)-2B and the appearance of
two new peaks (Rt =24.46 and 28.44 min) assignable to the
cis enantiomers (Z)-2A and (Z)-2B. The traces obtained after
irradiation with light of 436 nm resulted in the recovery of
the peaks corresponding to (E)-2A and (E)-2B. To confirm
Figure 3. Absorption spectral changes of (E)-2 in THF upon irradiation:
Before irradiation (solid line), at the photostationary state after 366 nm
irradiation (PSS366nm, dashed line), and at the photostationary state after
436 nm irradiation (PSS436nm, dotted line).
trum of compound (E)-2 in THF along with the spectra re-
corded on photoisomerization to the corresponding photo-
stationary states (PSS) by irradiating with light of wave-
lengths of 366 (PSS366nm) and 436 nm (PSS436nm). The spec-
trum recorded before irradiation shows an absorption maxi-
mum at 243 nm corresponding to short-axis polarizations
(1Bb transition) of the naphthalene chromophore along with
a combination band at 278–400 nm corresponding to long-
axis polarization (1La and 1Lb transitions) of the naphthyl
group and p–p* transition of the azobenzene unit. The spec-
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Photoresponsive Chiral Dopants
FULL PAPER
Figure 5. CD spectra of enantiomers of (E)-2 in tetrahydrofuran: (a) (E)-
2A, (b) (E)-2B, (c) PSS366nm from (E)-2A, and (d) PSS366
from (E)-2B.
nm
Inset: The splitting of the n–p* band on irradiation at 366 nm.
Figure 4. Chromatograms of (a) a racemic mixture of (E)-2 before irradi-
ation and at the PSS366nm and PSS436nm, and (b) (E)-2A before irradiation
and at the PSS366nm and PSS436nm (top to bottom) obtained by chiral
HPLC using dichloromethane/hexane (40:60) as eluent at a flow rate of
shows an increase in intensity, as expected, however, with
the band split into a negative band centered at 418 nm and
a positive band at 480 nm (see Figure 5 inset). The spectrum
recorded at PSS436nm shows the recovery of the initial spec-
tral features with isosbestic points at 325 and 383 nm (see
Figure S7b in the Supporting Information). The CD spectra
recorded for (E)-2B under similar conditions show mirror
image spectral features to (E)-2A. The enantiomers of mole-
cule (E)-1 exhibit similar band features to those of (E)-2
with small shifts in the wavelength maxima (see Figure S7a).
The molecules with asymmetrically substituted benzene
units also showed changes in CD band features on photoiso-
merization. The CD spectra recorded for the enantiomers of
molecules (E)-3A and (E)-4A show similar isomerization be-
havior but with opposite signs for the n–p* transition bands
of the azobenzene chromophores at 428 and 458 nm, respec-
tively (see Figure S7c,d). In contrast to the other molecules,
the CD spectra recorded for the enantiomers of (E)-5 at
PSS436nm and PSS>500nm show modifications in the band in-
tensity without significant changes in the band positions (see
Figure S8).
After separating the enantiomers on a preparative scale,
we carried out the crystallization of these enantiomers for a
precise determination of their absolute stereostructures. We
obtained good quality crystals of the separated enantiomers
of (E)-1B and (E)-2B from dichloromethane/hexane (1:1),
whereas (E)-4B crystallized from methanolic solution. From
the X-ray structure of (E)-2B containing the heavy atom bro-
mine, we could successfully determine its absolute configu-
ration with a reliable Flack parameter of 0.04(3).[28] Al-
though X-ray structures of (E)-1B and (E)-4B were obtained,
the absence of heavy elements in these molecules made the
determination of their absolute stereostructures difficult.
From the crystal structure of (E)-2B, we identified the
second eluted enantiomer as the S stereoisomer. Comparing
the absolute structure determined by single-crystal X-ray
diffraction with the CD spectra for (E)-2A and (E)-2B we as-
signed these enantiomers as (R)-(+)-(E)-2 and (S)-(ꢀ)-(E)-
2, respectively.
1.5 mLminꢀ1
.
the stability of the separated enantiomers towards racemiza-
tion under photoisomerization conditions, we carried out
HPLC studies on photoisomerized samples of (E)-2A. Fig-
ure 4b shows the HPLC traces for the enantiomer (E)-2A
before and after photoisomerization. The chromatogram ob-
tained after irradiation with light of 366 nm shows a de-
crease in the intensity of the peak corresponding to (E)-2A
with the selective formation of (Z)-2A. Reverse isomeriza-
tion by irradiation with light of 436 nm showed the recovery
of the peak corresponding to (E)-2A. From the HPLC traces
in Figure 4b it is clear that the enantiomers are stable with-
out racemization under repeated photoisomerization with
radiation of wavelengths 366 and 436 nm. Similar observa-
tions were made for all the other cyclic azobenzenes (see
Figures S5 and S6 in the Supporting Information). The
chiral HPLC results unambiguously proved the complete
lack of rotation of the asymmetrically substituted aromatic
moieties in the cyclic azobenzene cavity that results in re-
solvable enantiomers.
To study the chiroptical properties of the separated enan-
tiomers of all the cyclic azobenzenes, we performed CD
spectroscopic analyses of the compounds at room tempera-
ture. Figure 5 shows the CD spectra recorded for the enan-
tiomers (E)-2A and (E)-2B in dilute THF solution. The CD
spectrum of (E)-2A shows distinct bands assignable to the
substituted naphthalene and azobenzene chromophores. The
spectra recorded before irradiation show negative and posi-
tive bands at 343 and 456 nm, respectively, for p–p* and n–
p* transitions of the azobenzene unit. In addition to these
bands, the CD spectra show negative and positive bands, re-
spectively, at 298 and 318 nm. The CD spectra recorded at
PSS366nm show significantly different spectral features. The
bands at 246 and 343 nm show different intensities, whereas
the bands at 298 and 318 nm are replaced by a new positive
band (308 nm). Note that the band at 456 nm corresponding
to the n–p* transitions of the azobenzene chromophore
The results of the chiral HPLC and CD spectroscopic
studies confirm the indications of hindered rotation of the
asymmetrically substituted aromatic moieties obtained from
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N. Tamaoki et al.
Table 1. Helical twisting powers[a] b of dopants in different nematic LC hosts as deter-
the 1H NMR data. These results clearly illustrate
the complete lack of rotation of the disubstituted
aromatic units in all the cyclic azobenzenes, which
in turn gives rise to the planar chirality. The chiral
HPLC studies and CD spectral analyses confirm
that the E/Z isomerization of the separated enan-
tiomers of these molecules occurs without undergo-
ing racemization. By analyzing the CD spectra of
(E)-2B in conjunction with its crystal structure we
could successfully assign the absolute configuration
of the separated enantiomers of molecule (E)-2.
mined by Canoꢁs wedge method and the observed change in values upon irradiation.[b]
Dopant Host NLC
b [mmꢀ1
Initial PSS366nm PSS436nm PSS>500nm
]
bfinꢀbini (bfinꢀbini)/
[c]
[d]
G
]
jbini
j
(E)-1A
(E)-2A
(E)-3A
(E)-4A
(E)-5A
5CB
ZLI-1132 ꢀ59
E-7
ꢀ79
JC-1041XX ꢀ62
5CB
ꢀ45
ZLI-1132 ꢀ35
E-7
ꢀ32
JC-1041XX ꢀ40
5CB
ꢀ18
ZLI-1132 ꢀ24
E-7
ꢀ11
JC-1041XX ꢀ15
5CB
ꢀ19
ZLI-1132 ꢀ21
E-7
ꢀ22
JC-1041XX ꢀ19
ZLI-1132 +5.5
ꢀ72
ꢀ50
ꢀ44
ꢀ64
ꢀ44
ꢀ19
ꢀ18
ꢀ22
ꢀ16
ꢀ9
ꢀ69
ꢀ57
ꢀ75
ꢀ59
ꢀ41
ꢀ33
ꢀ29
ꢀ37
ꢀ16
ꢀ21
ꢀ9
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
+22
+15
+15
+18
+26
+17
+10
+24
+9
+13
+4
+7
+10
+11
+9
+0.31
+0.25
+0.19
+0.29
+0.58
+0.49
+0.31
+0.60
+0.50
+0.54
+0.36
+0.47
+0.53
+0.52
+0.41
+0.47
ꢀ0.38
ꢀ11
ꢀ7
Evaluation of helical twisting power and its photo-
modulation: After confirming the chiroptical prop-
erties of the separated enantiomers of molecules
(E)-1–5 in solution, we examined their potential as
chirality transfer agents in four commercially avail-
able nematic liquid crystalline hosts of different
chemical structures. In this study as mesogenic
hosts we used 4-pentyl-4’-cyanobiphenyl (5CB) and
three other liquid crystalline mixtures, namely, E-7,
ZLI-1132, and JC-1041XX. A preliminary insight
into the chirality induction properties of the sepa-
rated enantiomers was obtained by the formation
of polygonal fingerprint textures of the chiral nem-
ꢀ8
ꢀ10
ꢀ17
ꢀ19
ꢀ20
17
ꢀ9
ꢀ10
ꢀ13
ꢀ10
–
+9
ꢀ2.1
+3.4[d]
+5.4[e]
[a] All the HTP values were calculated based on the weight concentration of the
chiral dopants. [b] Positive and negative values represent right- and left-handed helical
twists, respectively. [c] The change in HTP observed between the initial and final Z-
rich states (bfinꢀbini). [d] The ratio of the difference between the HTPs before and
after irradiation (Z-rich states) against the absolute value of the initial HTP. [e] The
value of bfin in the case of enantiomers of molecule (E)-5 is the HTP at PSS436nm.
atic phases obtained. The induced helical pitches P for all
the chiral nematic phases using enantiomers of compounds
(E)-1–5 at different dopant concentrations were estimated
by using Canoꢁs wedge method.[29] The twist sense of the
chiral nematic phases (N*) for all the dopants were deter-
mined by means of the previously reported color-shift
method[30] along with CD spectroscopy (for compounds (E)-
1B and (E)-2B, see Figure S9 in the Supporting Information).
Figure 6 shows the results for (E)-2A and (E)-2B in various
nematic liquid crystalline hosts, with plots of the reciprocal
helical pitch Pꢀ1 against function concentration. The inverse
of the pitch Pꢀ1 increases linearly with dopant concentra-
tion. The absolute values of the helical twisting power deter-
mined for the first eluted enantiomers of compounds (E)-1–
5 in various nematic liquid crystal hosts are given in Table 1.
For all the molecules, the second eluted enantiomer also
shows the same HTP values as those of the corresponding
enantiomers, but with the opposite twist sense. It is clear
from Table 1 that the molecules with substituted naphtha-
lene units show moderately large helical twisting power
compared with those with disubstituted benzene units. Inter-
estingly, we find a large increase (1.8 times) in the initial
helical twisting power of compound (E)-1 in 5CB (72 mmꢀ1)
compared with the compound with an unsubstituted naph-
thalene unit that we previously reported.[18d] The large in-
crease in the initial helical twisting power of the enantiom-
ers of (E)-1 may be attributed to improved mesogenic prop-
erties resulting from the additional diphenyl substitution on
the naphthalene unit. With the present dopant/liquid crystal
combinations, the liquid crystal hosts 5CB, ZLI-1132, and
JC-1041XX were found to be effective for the enantiomers
of molecules (E)-1–5 in terms of initial HTP and the change
in HTP on photoisomerization. Although the initial HTPs
were comparable to those of the previously[18d] reported
molecule with an unsubstituted naphthalene unit, the enan-
tiomers of (E)-2 show a relatively large change in helical
twisting power between the E- and Z-rich states. The ratios
of the difference between HTPs before and after irradiation
(Z-rich state) against the absolute value of the initial HTP
[(bfinꢀbini)/jbini j] induced by the enantiomer of (E)-2A in
liquid crystalline hosts 5CB and JC-1041XX upon photoiso-
merization were found to be 0.58 and 0.60, respectively. To
the best of our knowledge the initial helical twisting power
shown by the enantiomers of (E)-1 (79 mmꢀ1 in nematic
liquid crystalline host E-7) and the ratio (bfinꢀbini)/jbini j ex-
hibited by the enantiomers of (E)-2 in JC-1041XX are the
highest values reported in the literature for photochromic
Figure 6. Plots of reciprocal helical pitch as a function of concentration of
dopants (E)-2A or (E)-2B in nematic hosts 5CB (&), JC-1041XX (&),
ZLI-1132 (*), and E-7 (*). Negative and positive Pꢀ1 values represent
the left- and right-handed helical twists obtained for (E)-2A and (E)-2B,
respectively.
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Photoresponsive Chiral Dopants
FULL PAPER
chiral dopants with planar chirality. In contrast to the enan-
tiomers of compounds with disubstituted naphthalene units,
those with disubstituted benzene units show low initial HTP
values in the range 5.5–24 mmꢀ1). By comparing the molecu-
lar structure of (E)-5 with other cyclic azobenzenes present-
ed in this study, it is clear that the restricted molecular con-
formation has little effect on the HTP of the chiral dopant,
whereas the mesogenic property plays an important role in
determining the efficiency. Considering the high initial HTP
and the moderately large change in the HTP of the enan-
tiomers of naphthalene-based compounds, we employed
these enantiomers to study the photoswitching of the reflec-
tion colors and rotation of micro-objects on doped chiral
nematic liquid crystalline films.
Photoinduced modulation of chiral nematic reflection
colors: To determine the basic properties of the photocon-
trolled chiral dopants, we used the separated enantiomers of
some of these molecules to reversibly tune the color reflec-
tions. For this purpose we used the enantiomers of (E)-1 or
(E)-2 in combination with nematic host 5CB as a result of
the high initial HTP and the large change in helical twisting
power on photoisomerization. Importantly, all the cyclic
azobenzenes presented in this study show high miscibility
with the nematic liquid crystalline hosts used (see Table S2
in the Supporting Information). To demonstrate the reversi-
ble tuning of the reflection colors, we first prepared liquid
crystalline mixtures by doping either one of the enantiomers
of (E)-1 or (E)-2 in 5CB at concentrations of 5.2 and
10.7 wt%, respectively. The mixtures of 5CB with chiral
dopant were added to a glass cell formed by two glass plates
coated with a polyimide alignment layer with a cell gap of
10 mm. The film formed by the liquid crystalline mixture of
5CB doped with 5.2 wt% of (E)-1A shows a chiral nematic
phase at room temperature with a selective reflection band
of blue color at 450 nm (see Figures 7a and 8a). Irradiation
of this film with light of wavelength 366 nm resulted in a
shift of the selective reflection band towards longer wave-
lengths, as shown in Figure 7a. On the other hand, irradia-
tion with light of 436 nm resulted in a reversal of the move-
ment of the reflection band back to a shorter wavelength
region, the final band position being at 480 nm (see Fig-
ure 7b). The corresponding reflection colors recorded at dif-
ferent intervals of photoirradiation at 366 and 436 nm are
shown in Figure 8a. Similarly, we recorded the reflection
spectra for the film formed by the liquid crystalline mixture
of 5CB doped with (E)-2A, in which the selective reflection
band was at 430 nm with a violet color (see Figure 8b). Irra-
diation with light of 366 nm resulted in a shift of the selec-
tive reflection band in both the visible and near-IR regions.
Irradiation at 366 nm for 80 s resulted in a PSS366nm of the
liquid crystalline film with the selective reflection band posi-
tioned at 920 nm (Figure 7c). Subsequent irradiation of the
film with light of 436 nm again resulted in the reversal of
the movement of the selective reflection band to 450 nm
(see Figure 7d) with the reflection colors as shown in Fig-
ure 8b.
Figure 7. Transmittance spectra recorded for doped chiral nematic liquid
crystalline films showing changes in the selective reflection bands upon
photoirradiation. Spectra of the film formed by doping 5.2 wt% of (E)-
1A in 5CB upon irradiation at a) 366 and b) 436 nm and the film formed
by doping 10.7 wt% (E)-2A in 5CB upon irradiation at c) 366 and
d) 436 nm.
Photocontrolled rotational motion of solid objects on chiral
nematic films: To study the effect of chiral dopants on the
previously reported[15] rotational motion of micro-objects on
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N. Tamaoki et al.
tion of microscale objects as
well as the rotational reorgani-
zation of the texture, we sprin-
kled micro glass rods of diame-
ter 5 mm and approximate
length 25 mm on to the films
with polygonal fingerprint tex-
ture. Figure 9 shows a represen-
tative example of the rotation
of a glass rod on the chiral
nematic film formed by 5CB
doped with (E)-2A. The glass
rod underwent a rotation of
277(ꢁ5)8 in an anticlockwise di-
rection upon irradiation at
366 nm; subsequent irradiation
with light of wavelength 436 nm
resulted in a clockwise rotation
leading to the original position
of the glass rod. A similar ex-
periment with enantiomer (E)-
2B showed an equal degree of
Figure 8. Changes in the reflection color of the chiral nematic liquid crystalline films upon irradiation at 366
and 436 nm: a) Film consisting of 5.2 wt% of (E)-1A in 5CB and b) film consisting of 10.7 wt% of (E)-2A in
5CB.
the surface of doped chiral nematic liquid crystalline films,
we employed enantiomers of the newly synthesized chiral
cyclic azobenzenes as chiral dopants in various nematic
liquid crystalline hosts. In this study our aim was to analyze
the influence of parameters such as the helical twisting
power of the chiral dopants, the intensity of photoirradia-
tion, and the concentration of dopant on the rotation of mi-
croscale objects on the doped chiral nematic liquid crystal-
line films. To examine the generality of the rotational behav-
ior of the micro-objects, we prepared liquid crystalline mix-
tures of 5CB, E-7, ZLI-1132, and JC-1041XX doped with
the enantiomers of (E)-1–5 at different concentrations. The
films of the doped liquid crystalline mixtures were obtained
by drop casting the mixtures on glass plates coated with a
unidirectionally rubbed polyimide alignment layer. As ex-
pected, all the films of the doped liquid crystals show a pol-
ygonal fingerprint texture by aligning the axis of the choles-
teric helix parallel to the substrate at the interface between
the liquid crystal and air. The linewidth varies in a given
nematic host depending on the HTP and the concentration
of the chiral dopants. The nematic liquid crystals doped with
enantiomers (E)-1 and (E)-2 show shorter pitch, whereas
those doped with the enantiomers (E)-3–5 show a relatively
wider pitch at the same concentration. The drop-casted
films with cholesteric texture have thicknesses of
30(ꢁ5) mm, as estimated by UV/Vis spectroscopy.
Figure 9. Optical micrographs showing the rotation of a glass rod on the
surface of a chiral nematic liquid crystalline film formed by doping with
(E)-2A (1 wt%) in 5CB. Rotation of a glass rod upon irradiation at (a–
d) 366 and (e–h) 436 nm recorded at intervals of 15 s.
rotation but with opposite directions of rotation. To obtain a
clear idea of the rotational behavior of micro glass rods on
chiral nematic liquid crystalline films doped with cyclic azo-
benzenes, we carried out our experiments on all the enan-
tiomers of the cyclic azobenzenes presented in this study.
Similar to the observations with the enantiomers of (E)-2 in
5CB, the chiral nematic liquid crystalline films formed by
doping with enantiomers of other cyclic azobenzenes (com-
binations made by doping enantiomers of (E)-1–4 in nemat-
ic liquid crystal hosts 5CB, E7, ZLI-1132, and JC-1041XX)
also showed rotation of the glass rods induced by the rota-
tional reorganization of the cholesteric texture upon irradia-
tion at 366 and 436 nm. By using the chiral nematic films
formed by doping with enantiomers of molecule (E)-5, we
could control the rotation of the glass rods completely
under visible-light irradiation. In this system, the forward
and reverse rotations are generated by irradiation with
After confirming the formation of the polygonal finger-
print texture under an optical microscope with polarizer, we
irradiated the films at 366 or 436 nm (for molecule (E)-5) to
attain the Z-rich state of the chiral dopants. On photoirra-
diation, all the films showed rotational reorganization of the
polygonal fingerprint texture induced by the photoisomeri-
zation of the cyclic azobenzene dopants. To show the rota-
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Photoresponsive Chiral Dopants
FULL PAPER
wavelengths of 436 and >500 nm, respectively. In all cases,
the films formed by doping with enantiomers showing a neg-
ative change in HTP from initial to Z-rich states, (bfinꢀbini)<
0, induced clockwise rotation of the glass rods, whereas
those with a positive change induced anticlockwise rotation,
as observed by Feringa and co-workers,[15b] the changes in
HTP values being calculated on the basis of both the values
and signs of bfin and bini.
After verifying the photoinduced rotation of glass rods on
the chiral nematic liquid crystalline films formed by doping
with enantiomers of (E)-1–5, we examined the factors influ-
encing the rotational reorganization of the polygonal finger-
print texture. We found that the degree of rotation of the
glass rods is highly dependent upon the thickness of the
liquid crystalline films. On increasing the film thickness, the
glass rods rotated for several cycles upon photoirradiation.
Films with a thickness of 30(ꢁ5) mm show a uniform color
under a cross polarizer whereas thick and non-uniform films
show varying colors. Throughout this study we used thin
films as the film thickness is easy to maintain.
sity or from other unknown factors that depend on the in-
tensity of the irradiation. To study the effect of light-intensi-
ty on the isomer ratio at a given energy, two chiral nematic
liquid crystalline films (1 wt% (E)-2A in 5CB) of the same
thickness (30 mm) were prepared and irradiated to the Z-
rich state of the chiral dopant. The Z-rich films were then
exposed to light of 436 nm with an intensity of 30.4 or
2.8 mWcmꢀ2 until the energy reached 575 J. At this energy,
the E/Z ratios of the films irradiated with light of intensity
30.4 (19 s) and 2.8 mWcmꢀ2 (205 s) were found to be 42:58
and 52:48, respectively, as estimated by HPLC analysis.
Thus, it is clear that at a given energy the chiral dopant
shows different efficiencies of photoisomerization. The
lower isomerization efficiency at a given energy observed
under light of higher intensity (see Figure 10) may be due to
the shorter irradiation time, which would make the diffusion
speed of the azobenzene dopant in the liquid crystalline
host affect the photochemistry.
We also investigated the influence of the HTP of the
chiral dopants on the fully photoinduced rotational reorgan-
ization of the polygonal fingerprint texture of chiral nematic
films. The possible dependence of rotational angle on the
difference in the HTP of the chiral dopant between two
stable photoisomerizable chiral states has been reported
previously.[15b] To examine the effect of the change in HTP
on the rotation of the glass rods we plotted the maximum
rotation angle obtained (E to Z) for films with 1 wt% of the
dopant in a single liquid crystal host (ZLI-1132) against the
difference in HTP, bfinꢀbini, with bini and bfin the HTP before
and after irradiation, respectively. However, the plot did not
show any correlation even in a single nematic liquid crystal-
line host (see Figure S10 in the Supporting Information). In
contrast, the degree of rotation plotted against the ratio of
the difference between HTPs before and after irradiation
To study the dependence of rotation angle on irradiation
intensity we carried out rotation experiments with a glass
rod on a chiral nematic liquid crystalline film (1 wt% (E)-2A
in 5CB) starting from the Z-rich state and by using light of
436 nm of different intensities. The curves in Figure 10 show
against the absolute value of the initial HTP, (bfinꢀbini)/jbini
j
, shows a linear trend at a concentration of 1 wt% of the
nematic host. Figure 11 shows the maximum rotation angle
of the glass rod observed upon irradiation at 366 nm of the
initial E-rich state to the Z-rich state for various combina-
tions of chiral dopants and nematic hosts at different con-
centrations plotted against the product of the ratio
(bfinꢀbini)/jbini j and dopant concentration. The plot exhibits
a linear trend that clearly indicates a dependence of the ro-
tation angle on both the ratio (bfinꢀbini)/jbini j and the dopant
concentration. This linear trend between the ratio (bfinꢀbini)/
jbini j and the rotation angle of the glass rods holds for both
enantiomers of the compounds (see Figure 11). Another in-
teresting feature of the relationship between the ratio
(bfinꢀbini)/jbini j and the rotation angle of the glass rod is that
the line does not pass through the (0,0) point. Even with a
Figure 10. Energy-dependent variation of the angle of rotation of a glass
rod on a chiral nematic liquid crystalline film formed by doping 1 wt%
of (E)-2A in 5CB on irradiation with light of 436 nm at different light in-
ꢀ2
&
*
*
( ).
tensities 30.4 ( ), 14.4 (&), 7.9 ( ) and 2.8 mWcm
the variation in the rotation angle of the micro glass rod
under irradiation with light of 436 nm with intensities of
30.4, 14.4, 7.9, and 2.8 mWcmꢀ2. In these experiments, the
glass rod shows the same rotation angle maxima irrespective
of the light intensity of the irradiation. Note that at a given
energy, the glass rods show different rotation angles, which
indicates that the efficiency of rotation, defined as degree of
rotation per irradiation energy, is influenced by the intensity
of the photoirradiation. Irradiation with light of 436 nm at
an intensity of 2.8 mWcmꢀ2 induced the highest efficiency
of rotation of the glass rod. This unexpected result may orig-
inate either from differences in the isomer ratios of the
chiral dopants upon irradiation with light of different inten-
very low value of 0.031 for the product of (bfinꢀbini)/jbini
j
and the dopant concentration, the glass rod on the chiral
nematic films formed by mixtures of 5CB doped with the
enantiomers of (E)-1 (0.1 wt%) showed a rotation angle of
878. The mechanism for this phenomenon is not clear and
will be the subject of future work.
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N. Tamaoki et al.
before and after irradiation against the absolute value of the
initial HTP, not just the difference in the HTPs, on the
degree of rotation of micro-objects. Our results also illus-
trate that the speed of the rotational reorganization of the
cholesteric texture is determined by the rate of E/Z photoi-
somerization and that the maximum degree of rotation is
not influenced by the intensity of the irradiation.
Experimental Section
Nematic liquid crystal hosts E-7, 5CB (TCI chemical, Japan), ZLI-1132
(Merck), and JC-1041XX (Chisso Co. Ltd.) were obtained from the sour-
ces indicated. All the solvents and chemicals were purchased from com-
mercial sources and used without further purification unless otherwise
stated. 1H and 13C NMR spectra were recorded with a JEOL ECX
400 MHz spectrometer by using tetramethylsilane as an internal standard.
MALDI-TOF MS was performed with an Applied Biosystems Voyager-
DE pro instrument. Absorption spectra were recorded with an Agilent
8453 spectrophotometer and the CD spectra were recorded on a JASCO
J-S720 CD spectrophotometer. Photoisomerization studies were conduct-
ed by using radiation from a super-high-pressure mercury lamp (500 W,
USHIO Inc.) after passing through appropriate filters (366, 436, and
>500 nm). High-pressure liquid chromatography (HPLC) was conducted
with a Hitachi Elite La Chrome HPLC system by using either CHIRAL-
PAK IA (for compounds 1, 2, and 4) or CHIRALPAK IB (for com-
pounds 3 and 5; DAICEL Chemical Industries Ltd.) columns to separate
the enantiomers and to analyze the E/Z and enantiomeric ratios. Solvent
mixtures of dichloromethane/hexane (40:60, for compounds 1, 2, and 4),
ethyl acetate/hexane (25:75, compound 3), and tetrahydrofuran/hexane
(30:70, for compound 5) were used as eluents in the HPLC experiments.
The E/Z isomeric ratios at the photostationary states were determined by
HPLC monitored at common isosbestic points of 284 (for 1, 2, and 5)
and 278 nm (for 3 and 4). Microscopic analyses were performed with an
OLYMPUS BX-60 optical microscope equipped with a SONY DXC-950
3CCD color video camera.
Figure 11. Angle of rotation of glass rods on doped liquid crystalline
films plotted against the product of the ratio (bfinꢀbini)/jbini j and the con-
centration of the chiral dopant. The positive and negative values of the
angles of rotation indicate clockwise and anticlockwise rotation of the
glass rods, respectively. The molecules are represented as (E)-1 (square),
(E)-2 (dot), (E)-3 (triangle), (E)-4 (diamond), and (E)-5 (star). The
liquid crystalline hosts are represented by different colors: 5CB (black),
ZLI-1132 (red), E-7 (blue), and JC-1041XX (green). The data with stand-
ard error bars are provided in the Supporting Information.
In summary, this study highlights the role of the chiral
dopant in determining the rotational reorganization of the
polygonal fingerprint texture of the doped chiral nematic
liquid crystalline films. Our experiments show that the inten-
sity of the irradiation only influences the speed of rotation
of the glass rod by changing the speed of isomerization of
the chiral dopants. However, the ratio of the difference be-
tween the HTPs before and after irradiation against the ab-
solute value of the initial HTP, (bfinꢀbini)/jbini j, determines
the degree of rotation of micro-objects on the chiral nematic
films doped with enantiomers of the cyclic azobenzenes (E)-
1–5.
Measurement of helical pitch and handedness: The chiral nematic liquid
crystal was prepared by weighing the required amount of the host liquid
crystal and the dopant into a vial and mixing them following the addition
of a few drops of dichloromethane. The solvent was evaporated under re-
duced pressure and the mixture was introduced into the wedge cell by ca-
pillary action at room temperature. The pitch was determined by measur-
ing the intervals between the Cano lines on the surfaces of the wedge
cells (EHC, KCRK-07, tanq=0.0196). The relationship between the dis-
tances between the Cano lines and the helical pitch is given by Equa-
tion (3) in which R is the distance between the Cano lines and q is the
angle between the cells.
Conclusion
A new set of planar chiral azobenzene-based molecular
switches have been designed and synthesized to study the
effect of structure on helical twisting power. The enantiom-
ers of (E)-1 show the highest known initial helical twisting
power for planar chiral molecules and is complimented by
the mesogenic property of the diphenylnaphthalene moiety,
whereas the enantiomers of (E)-2 with a dibromo-substitut-
ed naphthalene unit exhibit a relatively high initial HTP
along with a large change in helical twisting power. By using
substituted naphthalene-based compounds we could achieve
better tunability of color reflections covering the range from
visible to the near-IR region. Molecules with substituted
benzene units show a poor initial HTP (5.5–24) even with
their higher structural rigidity and conformational restric-
tion. By employing the enantiomers of all the cyclic azoben-
zenes we could achieve the complete photocontrolled rota-
tional motion of micro glass rods on doped nematic liquid
crystalline films. By using the chiral dopants in combination
with various nematic liquid crystalline hosts we determined
the effect of the ratio of the difference between HTPs
P ¼ 2Rtan q
ð3Þ
The twist sense of the helix was determined by the color-shift method
using a polarizing microscope. Under the illuminated microscope, wedge
cells filled with \ doped liquid crystalline mixture showed color stripes be-
sides the disclination lines. The twist sense of the helix can be determined
from the direction of movement of the color stripes upon rotation of the
polarizer. If the colors are shifted towards the thicker region of the
wedge upon turning the analyzer in a clockwise direction (located oppo-
site side of the light source with respect to the cell), the helix is right-
handed or (+). By convention, a right-handed twist sense corresponds to
a positive sign of the pitch and vice versa.
General procedure for the synthesis of compounds (E)-1 and (E)-5: A
solution of dinitro compound 7 or 13 (1 mmol) in dry THF (400 mL) was
added over 6 h to a stirred solution of LiAlH4 (379.5 mg, 10 mmol) in
THF (100 mL) at 858C under argon. After the addition of the dinitro
compound the reaction mixture was stirred overnight at room tempera-
ture. The reaction mixture was carefully quenched by the addition of
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Photoresponsive Chiral Dopants
FULL PAPER
water with the reaction vessel placed in an ice bath. The precipitate
formed was filtered off and the THF was evaporated on a rotary evapo-
rator. The residue was extracted with ethyl acetate, evaporated, and puri-
fied by column chromatography on silica gel using a mixture of ethyl ace-
tate and hexanes as eluent to afford the desired products.
Compound (E)-1: Orange solid; yield: 7%; 1H NMR (400 MHz, CD2Cl2,
258C, TMS): d=7.98 (d, J=8.5 Hz, 2H), 7.62 (d, J=8.4 Hz, 2H), 7.39–
7.38 (m, 12H), 7.17 (d, J=8.5 Hz, 2H), 7.04 (d, J=7.6 Hz, 2H), 7.0 (s,
2H), 4.56–4.51 (m, 2H), 4.19 (t, J=9.8 Hz, 2H), 3.26 (dd, J=9.3, 9.2 Hz,
2H), 2.85 ppm (dd, J=6.0, 6.5 Hz, 2H); 13C NMR (400 MHz, CD2Cl2,
258C, TMS): d=153.3, 152.5, 141.0, 140.12, 131.8, 129.9, 129.2, 129.1,
128.9, 128.5, 127.3, 123.2, 119.6, 118.4, 72.8, 36.3 ppm; MS (MALDI-
TOF): m/z calcd for C38H30N2O2: 546.23 [M+H]+; found: 547.43.
8.16 (d, J=8.2 Hz, 2H), 7.73 (d, J=8.0 Hz, 2H), 7.53 (t, J=7.9 Hz, 2H),
7.46 (dd, J=8.9, 8.9 Hz, 4H), 4.31 (t, J=13.0 Hz, 4H), 3.35 ppm (t, J=
13.0 Hz, 4H).
Synthesis of compound 7: Na2CO3 (4 mL, 2 mol) was added to a solution
of 6 (700 mg, 1.15 mmol), phenylboronic acid (561 mg, 4.6 mmol), and
tetrakis(triphenylphosphine)palladium(0) (116 mg, 0.1 mmol) in dime-
thoxyethane (DME, 15 mL) under argon and heated at reflux for 48 h.
The reaction mixture was cooled to room temperature and poured into
water (200 mL), filtered, and washed with excess ethyl acetate to obtain
the desired product as a white powder (600 mg, 85% yield). 1H NMR
(400 MHz, CDCl3, 258C, TMS): d=8.09 (d, J=7.8 Hz, 2H), 7.96 (s, 2H),
7.79 (d, J=8.5 Hz, 2H), 7.56 (d, J=9.3 Hz, 2H), 7.46–7.41 (m, 6H), 7.38–
7.32 (m, 6H), 3.87 (t, J=12.8 Hz, 4H), 3.00 ppm (t, J=12.8 Hz, 4H).
Compound (E)-5: Orange crystalline solid; yield: 3%; 1H NMR
(400 MHz, [D6]acetone, 258C, TMS): d=7.64 (d, J=7.7 Hz, 2H), 7.47 (t,
J=7.6 Hz, 2H), 7.25 (t, J=7.7 Hz, 2H), 7.10 (d, J=8.1 Hz, 2H), 5.98 (s,
2H), 4.75 (m, 2H), 4.64 (m, 2H), 3.75 (m, 2H), 3.51 (m, 2H), 3.19 ppm
(s, 6H); 13C NMR (400 MHz, CDCl3, 258C, TMS): d=153.0, 145.5, 140.0,
138.8, 131.3, 130.7, 127.3, 116.3, 106.3, 67.6, 56.5, 27.8 ppm; MS (MALDI-
TOF): m/z calcd for C24H24N2O4: 404.17 [M+H]+; found: 405.33.
General procedure for the synthesis of compounds 8 and 12: Compound
11 or 3-nitrophenethyl alcohol (5 mmol) was heated at reflux with zinc
dust (30 mmol) and NaOH (30 mmol) in a solvent mixture of ethanol/
water (2:5; 35 mL) until the red coloration had completely disappeared.
The hot reaction mixture was filtered and purged with air for 30 min.
The solvent was evaporated and the residue was filtered, dissolved in
ethyl acetate, and washed with water to remove the inorganic salts. Ethyl
acetate was evaporated to obtain the desired products as orange solids.
Synthesis of compound (E)-2: Diisopropyl azodicarboxylate (DIAD;
808 mg, 4 mmol) was added dropwise over 7 h to a mixture of 2,6-dibro-
mo-1,5-dihydroxynaphthalene (318 mg, 1 mmol), 8 (270 mg, 1 mmol), and
triphenylphosphine (786 mg, 3 mmol) in THF (250 mL) maintained at
08C under argon. The reaction mixture was slowly brought to room tem-
perature and stirring was continued for 4 d. The solvent was evaporated
and the residue was directly purified by chromatography over silica gel
using ethyl acetate/hexane (20:80) as eluent to obtain 2 as an orange
crystalline solid (27.6 mg, 5% yield). 1H NMR (400 MHz, CDCl3, 258C,
TMS): d=7.65 (d, J=7.8 Hz, 2H), 7.60 (d, J=9 Hz, 2H), 7.40 (t, J=
7.7 Hz, 2H), 7.33 (d, J=8.3 Hz, 2H), 7.25 (d, J=8.1 Hz, 2H), 6.93 (s,
2H), 5.34 (m, 2H), 4.82 (m, 2H), 3.42 ppm (t, J=4.9 Hz, 4H); 13C NMR
(400 MHz, CDCl3, 258C, TMS): d=153.8, 152.9, 139.4, 131.8, 130.8,
129.4, 128.8, 122.0, 119.9, 119.2, 111.7, 73.3, 36.0 ppm; MS (MALDI-
TOF): m/z calcd for C26H20Br2N2O2: 549.99 [M+H]+; found: 551.17.
Compound 8: Orange solid; yield: 70%; 1H NMR (400 MHz, CDCl3,
258C, TMS): d=7.81–779 (m, 4H), 7.47 (t, J=8.0 Hz, 2H), 7.36 (d, J=
7.6 Hz, 2H), 3.95 (dd, J=6.5, 6.5 Hz, 4H), 2.99 (t, J=6.5 Hz, 4H),
1.44 ppm (t, 5.9 Hz, 2H); MS (MALDI-TOF): m/z calcd for C16H18N2O2:
270.14 [M+H]+; found: 271.30.
Compound 12: Orange solid; yield: 74%; 1H NMR (400 MHz, CDCl3,
258C, TMS): d=7.56 (d, J=9.5 Hz, 2H), 7.46 (t, J=2.4 Hz, 2H), 7.43 (t,
J=7.9 Hz, 2H), 7.05 (d, J=9.8 Hz, 2H), 4.23 (t, J=6.0 Hz, 4H), 3.91
(dd, J=5.7, 5.7 Hz, 4H), 2.10 (m, 4H), 1.70 ppm (t, J=5.4 Hz, 4H); MS
(MALDI-TOF): m/z calcd for C16H18N2O2: 330.16 [M+H]+; found:
331.08.
Synthesis of compound 13: DIAD (2.5 g, 12.36 mmol) was added drop-
wise to a vigorously stirred solution of 2,5-dimethoxy-1,4-dihydroxyben-
zene (700 mg, 4.12 mmol), 2-nitrophenethyl alcohol (1.65 g, 10 mmol),
and triphenylphosphine (3.24 g, 12.36 mmol) in tetrahydrofuran (5 mL)
at 08C under argon. The stirring was continued for 24 h at 08C on com-
pletion of the addition of DIAD. The reaction mixture was poured into
cold water (200 mL), filtered, and washed thoroughly with excess ethyl
acetate. The residue was purified by column chromatography over silica
gel using methylene dichloride as eluent to obtain the desired compound
as a white powder (88 mg, 4.6% yield). 1H NMR (400 MHz, [D6]DMSO,
258C, TMS): d=7.96 (d, J=8.1 Hz, 2H), 7.66 (m, 4H), 7.51 (t, J=7.7 Hz,
2H), 6.64 (s, 2H), 4.17 (t, J=6.8 Hz, 4H), 3.64 (s, 6H), 3.34 ppm (t, J=
6.7 Hz, 4H); MS (MALDI-TOF): m/z calcd for C28H28N2O8: 468.15
[M+H]+; found: 469.21.
General procedure for the synthesis of compounds (E)-3 and (E)-4: Di-
ACHTUNGTRENNUNGisopropyl azodicarboxylate (DIAD; 8 mmol) was added dropwise over
7 h to a mixture 10[31] or 12 (2 mmol), diethyl 2,5-dihydroxyterepthalate
(2 mmol), and triphenylphosphine (1.97 g, 7.5 mmol) in THF (400 mL)
maintained at 08C under argon. The solvent was evaporated and the resi-
due was extracted with ethyl acetate, washed with water (ꢂ2), and dried
over MgSO4. The ethyl acetate was evaporated and the residue was puri-
fied by chromatography over silica gel using ethyl acetate/hexane (30:70)
as eluent to obtain the desired products.
Compound (E)-3: Orange crystalline solid; yield: 4%; 1H NMR
(400 MHz, CDCl3, 258C, TMS): d=7.50 (d, J=10.0 Hz, 2H), 7.47 (s,
2H), 7.37 (t, J=8.0 Hz, 2H), 7.04 (dd, J=2.7, 1.7 Hz, 2H), 6.99 (s, 2H),
4.66 (m, 2H), 4.52 (m, 2H), 4.37 (m, 2H), 4.18 (dd, J=7.2, 7.2 Hz, 4H),
1.20 ppm (t, J=7.2, 6H); 13C NMR (400 MHz, CDCl3, 258C, TMS): d=
165.1, 158.7, 153.0, 151.2, 130.0, 126.4, 120.9, 118.4, 115.7, 108.1, 67.9,
64.8, 61.4, 14.0 ppm; MS (MALDI-TOF): m/z calcd for C28H28N2O8:
520.18 [M+H]+; found: 521.45.
Compounds (E)-4: Orange crystalline solid; yield: 5%; 1H NMR
(400 MHz, CD2Cl2, 258C, TMS): d=7.37 (m, 4H), 7.26 (s, 2H), 7.12 (s,
2H), 7.05 (m, 2H), 4.53 (m, 2H), 4.32 (t, J=11.6, 2H), 4.23 (m, 8H), 2.2
(m, 4H), 1.24 ppm (t, J=8.9, 6H); 13C NMR (400 MHz, CD2Cl2, 258C,
TMS): d=165.7, 160.7, 154.3, 151.6, 130.0, 125.0, 121.1, 116.9, 116.5,
108.3, 66.3, 65.4, 61.5, 30.9, 14.3 ppm; MS (MALDI-TOF): m/z calcd for
C30H32N2O8: 548.22 [M+H]+; found: 549.37.
Acknowledgements
We thank Dr. Shin-ichiro Noro, RIES, Hokkaido University, and Bruker
Diffraction Laboratory, JNCASR, Bangalore, India, for helping with the
X-ray diffraction experiments and P. K. Hashim for useful discussions.
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Received: March 13, 2012
Revised: June 25, 2012
Published online: && &&, 0000
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Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Photoresponsive Chiral Dopants
FULL PAPER
Molecular machines: The ratio of the
difference between the helical twisting
powers before and after irradiation
against the absolute value of the initial
helical twisting power along with the
concentration of the dopant in the
nematic liquid crystalline host deter-
mine the rotation of micro-objects on
the liquid crystalline films (see figure).
Molecular Machines
R. Thomas, Y. Yoshida, T. Akasaka,
N. Tamaoki* .................. &&&& — &&&&
Influence of a Change in Helical
Twisting Power of Photoresponsive
Chiral Dopants on Rotational Manipu-
lation of Micro-Objects on the Surface
of Chiral Nematic Liquid Crystalline
Films
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!