Multi-responsible chameleon molecule with chiral naphthyl and azobenzene moieties

Article abstract of DOI:10.1039/c5sm00073d

A photochromic chiral molecule with azobenzene mesogens and a (R)-configuration naphthyl moiety (abbreviated as NCA₂M) was specifically designed and synthesized for the demonstration of chameleon-like color changes responding to multitudinous external stimuli, such as temperature, light and electric field. The basic phase transition behaviors of NCA₂M were first studied by the combination of differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Based on the structure-sensitive X-ray diffraction results obtained at different temperatures, it was comprehended that the NCA₂M molecule exhibited the tilted version of highly ordered smectic crystal phase with 5.45 nm layer thickness. Chiral nematic (N) liquid crystals (LC) with helical superstructures were formed by doping the NCA₂M photochromic chiral molecule in an achiral nematic (N) LC medium. By controlling the helical pitch length of N-LC with respect to temperature, light and electric field, the wavelength of selectively reflected light from the N photonic crystal was finely tuned. The light-induced color change of N-LC film was the most efficient method for covering the whole visible region from blue to green and to red, which allowed us to fabricate remote-controllable photo-responsive devices.

Full text of DOI:10.1039/c5sm00073d

Volume 11 Number 15 21 April 2015 Pages 2897–3084
Soft Matter
www.softmatter.org
ISSN 1744-683X
PAPER
Kwang-Un Jeong et al.
Multi-responsible chameleon molecule with chiral naphthyl
and azobenzene moieties

Soft Matter
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PAPER
Multi-responsible chameleon molecule with chiral
naphthyl and azobenzene moieties†
Cite this: Soft Matter, 2015, 11, 2924
a
a
a
a
b
Dae-Yoon Kim, Sang-A Lee, Minwook Park, Yu-Jin Choi, Shin-Woong Kang
and Kwang-Un Jeong*a
A photochromic chiral molecule with azobenzene mesogens and a (R)-configuration naphthyl moiety
abbreviated as NCA M) was specifically designed and synthesized for the demonstration of chameleon-
like color changes responding to multitudinous external stimuli, such as temperature, light and electric
field. The basic phase transition behaviors of NCA M were first studied by the combination of differential
scanning calorimetry (DSC) and polarized optical microscopy (POM). Based on the structure-sensitive X-
ray diffraction results obtained at different temperatures, it was comprehended that the NCA
(
2
2
2
M
molecule exhibited the tilted version of highly ordered smectic crystal phase with 5.45 nm layer
thickness. Chiral nematic (N*) liquid crystals (LC) with helical superstructures were formed by doping the
2
NCA M photochromic chiral molecule in an achiral nematic (N) LC medium. By controlling the helical
Received 10th January 2015
Accepted 2nd March 2015
pitch length of N*-LC with respect to temperature, light and electric field, the wavelength of selectively
reflected light from the N* photonic crystal was finely tuned. The light-induced color change of N*-LC
film was the most efficient method for covering the whole visible region from blue to green and to red,
which allowed us to fabricate remote-controllable photo-responsive devices.
DOI: 10.1039/c5sm00073d
www.rsc.org/softmatter
When the pitch of N* structure is in the range of the visible
spectrum, iridescent colors can be observed with the naked
Introduction
8
Liquid crystals (LC) are unique so materials showing the eye.
1
mobility of liquid as well as the order of crystal. Calamitic
Therefore, the N*-LC lms, which are also called cholesteric
nematic (N) LC exhibits the angular distribution of long LC (CLC) lms, have attracted considerable interest not only in
2
molecular axis with specic direction and high symmetry.
scientic aspects of partially ordered so materials but also in
9
Since the physical and optical properties of so materials rely engineering point of views of displays and optical applications.
on molecular arrangements, it is essential to control their There have been considerable applications for the N*-LC lms
orientations on the different length scales. The molecular of color tunable and switchable materials. Generally, the
3
10
orientation of so materials showing the N-LC phase can be reection colors in N*-LC depend on the periodicity of the
4

nely controlled with heat, electric eld, and light.
helical superstructure and the refractive index of the system
The chiral nematic (N*) LC phase, which is a chiral version of which can be tuned by the external stimulations of pH, pres-
11
N-LC phase, possesses solely the one-dimensional long-range sure, swelling, temperature, electric eld and light.
molecular orientational order along the long axis of N-LC
Among them, photo-induced reection color controls have
molecule. The N*-LC phase is well-known to exhibit optically unsurpassed advantages for their applications since remote
5
6
active variants and selective reections of light. Because the control is possible in the color-switching systems in N*-LC
12
N*-LC phase is readily formed by introducing chiral molecules mixtures. In this circumstance, a wide range of photo-
into the achiral N-LC medium, the helical pitch length of N*-LC responsive chiral dopants has been investigated including
can be adjusted by the concentration of chiral dopant and by azobenzenes, spiropyrans, fulgides, overcrowded alkenes and
7
13
the helical twisting power change of the chiral dopant itself.
diarylethenes. The azobenzene-based chiral dopants have
received intensive attention because not only the photo-revers-
ible trans 4 cis molecular conformation changes vary the
helical pitch but also the cylindrical molecular shape enhances
the compatibility and solubility in the N*-LC systems. By
simply mixing the azobenzene-based chiral dopants in the N*-
LC phase, a macroscopic chirality can be easily tuned and the
a
Polymer Materials Fusion Research Center & Department of Polymer-Nano Science
and Technology, Chonbuk National University, Jeonju, Jeonbuk, 561-756, Korea.
E-mail: kujeong@jbnu.ac.kr
14
b
Department of BIN Fusion Technology, Chonbuk National University, Jeonju, Jeonbuk,
5
61-756, Korea
1
13
†
Electronic supplementary information (ESI) available: H NMR, C NMR, corresponding reection colors are selectively controlled by the
MALDI-ToF, elemental analysis and detailed experimental procedures. See DOI:
15
use of suitable light.
1
0.1039/c5sm00073d
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In this aspect, a novel photochromic chiral molecule MHz, CDCl
3
): d ¼ 7.89 (d, 2H), 7.80 (d, 2H), 7.30 (d, 2H), 7.00 (d,
(
abbreviated as NCA M) was specically designed and synthe- 2H), 4.04 (t, 2H), 3.43 (t, 2H), 2.67 (t, 2H), 1.91 (m, 2H), 1.82 (m,
2
sized by chemically attaching the azobenzene mesogens to the 2H), 1.65 (m, 2H), 1.55 (m, 4H), 1.34 (m, 4H), 0.89 ppm (t, 3H).
naphthyl group with (R)-conguration. The photochromic azo- 3,5-Bis[6-(4-(4-pentylphenylazo)phenoxy)hexyloxy]benzoic
benzene groups in the NCA M molecule can reversibly respond acid ethyl ester (3). A mixture of 2 (4.29 g, 10.0 mmol), DHB
to light, while the presence of a chiral naphthyl group provides (0.60 g, 3.0 mmol), K CO (2.10 g, 15.2 mmol), KI (0.02 g, 0.15
its molecular chirality and breaks the molecular packing mmol), and butanone (50 mL) was heated to reux for 48 h.
2
2
3
16
symmetry. Aer studying the phase transition behaviors of Aer the reaction, solvent was removed in vacuum and the
NCA M using differential scanning calorimetry (DSC) and residue was re-dissolved in chloroform and washed with
2
polarized optical microscopy (POM), the photochemical prop- distilled water three times. It was puried by column chroma-
erties of the NCA M molecule were also investigated in common tography with silica gel by using ethyl acetate : chloroform ¼
2
organic solvents and in a typical N*-LC medium. Furthermore, 1 : 20 and subsequent recrystallization with methanol. The
1
the chameleon-like color changes of the N*-LC with NCA
2
M
resulting product was a yellowish solid (yield: 55%, 1.7 g). H
chiral dopant were demonstrated by tuning temperature, light NMR (400 MHz, CDCl
3
): d ¼ 7.89 (d, 4H), 7.78 (d, 4H), 7.72 (d,
and electric eld.
4H), 7.16 (d, 2H), 6.98 (s, 4H), 6.64 (s, 1H), 4.04 (t, 4H), 3.99 (t,
4
8
H), 3.89 (s, 3H), 2.66 (t, 4H), 1.84 (m, 8H), 1.65 (m, 4H), 1.55 (m,
H), 1.34 (m, 8H), 0.89 ppm (t, 6H).
Experimental section
3
,5-Bis[6-(4-(4-pentylphenylazo)phenoxy)hexyloxy]benzoic
Materials
acid (4). Aqueous KOH (10 M, 10 mL) was added to a stirred
solution of 3 (0.5 g, 0.57 mmol) in ethanol (50 mL). The mixture
was heated at reux for 3 h, and then neutralized with an HCl to
obtain a precipitate. The precipitate was ltered off and washed
with water. The crude product was puried by reprecipitation
from chloroform and ethanol, and dried under vacuum to
4-Pentyl aniline (98%, Aldrich), phenol (99%, Aldrich), 1,6-
dibromohexane (96%, Aldrich), methyl 3,5-dihydroxybenzoate
0
(DHB, 98%, Aldrich), N-(3-dimethylaminopropyl)-N -ethyl-
carbodiimide hydrochloride (EDC, 99%, Aldrich), 4-(dimethy-
lamino)pyridine (DMAP, 99%, Aldrich), (R)-1-(2-naphthyl)
ethylamine (CNE, 99%, Aldrich), anhydrous butanone (99%,
Sigma-Aldrich), anhydrous chloroform (99%, Sigma-Aldrich),
absolute ethanol (99.5%, Sigma-Aldrich), hydrochloric acid
1
afford 4 as a yellow solid (yield: 93%, 0.45 g). H NMR (400 MHz,
CDCl ): d ¼ 7.89 (d, 4H), 7.78 (d, 4H), 7.72 (d, 4H), 7.16 (d, 2H),
3
6
1
.98 (s, 4H), 6.64 (s, 1H), 4.04 (t, 4H), 3.99 (t, 4H), 2.66 (t, 4H),
.84 (m, 8H), 1.65 (m, 4H), 1.55 (m, 8H), 1.34 (m, 8H), 0.89 ppm
(
HCl, 37%, Sigma-Aldrich), sodium hydroxide (NaOH, 93%,
Showa), sodium nitrite (NaNO , 97%, Showa), potassium
carbonate (K CO 99%, Showa), anhydrous magnesium
sulphate (MgSO , 99.5%, Showa), chiral dopant (R-811, Merck
(t, 6H).
2
2
NCA M (5). EDC (0.3 g, 1.56 mmol) and DMAP (0.2 g, 1.64
2
3
,
mmol) were added to a solution of 4 (0.3 g, 0.35 mmol) in
anhydrous chloroform (30 mL). Aer stirring at room temper-
ature for 30 min, chiral naphthyl ethylamine (0.19 g, 1.11 mmol)
was added as a solid. The solution was stirred for 24 h at room
temperature. Aer the reaction, distilled water was added and
the product was extracted with chloroform. The combined
4
Co.) and achiral nematic liquid crystal (N-LC, MLC 15600-100,
Merck Co.) were used as received.
Synthesis
4
4-(4-Pentylphenylazo)phenol (1). A solution of 4-pentylani- organic layer was dried over anhydrous MgSO . Aer ltration
line (8.1 g, 50 mmol) and HCl (15 mL) in water (100 mL) was and evaporation, the crude product was puried by column
ꢀ
cooled at 0 C and NaNO (3.5 g, 50 mmol) was slowly added in chromatography with silica gel using ethyl acetate : methylene
2
ꢀ
small portions by maintaining the temperature below 5 C. Aer chloride ¼ 1 : 20 to yield 5 as an orange powder (yield: 77%, 0.27
1
stirring for 1 h, the solution of the diazonium salt was added to g). H NMR (400 MHz, CDCl
3
): d ¼ 7.89 (d, 4H), 7.84 (m, 4H),
a cold solution of phenol (4.7 g, 50 mmol) and NaOH (2.5 g in 25 7.78 (d, 4H), 7.48 (m, 3H), 7.72 (d, 4H), 6.98 (d, 4H), 6.88 (d, 2H),
mL water). Here, the temperature was kept below 5 C. The 6.64 (s, 1H), 5.47 (m, 1H), 4.04 (t, 4H), 3.99 (t, 4H), 2.66 (t, 4H),
ꢀ
product was precipitated by the addition of a small portion of 1.84 (m, 11H), 1.67 (m, 4H), 1.55 (m, 8H), 1.34 (m, 8H), 0.89 ppm
1
3
NaCl. By recrystallization of the product in ethanol, the puried (t, 6H). C NMR (400 MHz, CDCl
3
): d ¼ 166.4, 161.3, 160.3,
1
reddish crystal was collected (yield: 71%, 9.5 g). H NMR (400 151.0, 146.9, 145.7, 140.4, 136.7, 133.3, 132.8, 129.0, 128.5,
MHz, DMSO-d): d ¼ 10.2 (s, 1H), 7.76 (d, 2H), 7.72 (d, 2H), 7.35 127.9, 127.6, 126.2, 125.9, 124.7, 124.5, 122.5, 114.6, 105.3,
(
d, 2H), 6.92 (d, 2H), 2.64 (t, 2H), 1.60 (m, 2H), 1.29 (m, 4H), 0.85 104.2, 68.1, 49.3, 35.8, 31.4, 30.9, 29.1, 25.8, 22.5, 21.5, 14.0
+
ppm (t, 3H).
-(6-Bromohexyloxy)phenyl-(4-pentylphenyl)diazene (2).
mixture of 1 (5.0 g, 20 mmol), 1,6-dibromohexane (24.4 g, 100 H 7.70, N 6.95, O 7.93; found: C 77.06, H 7.71, N 6.94.
mmol), K CO (5.9 g, 36 mmol) and butanone (150 mL) was
ppm. MS (MALDI-ToF) for m/z calcd 1008.34; [M + Na] found:
4
A
1031.23. Elemental analysis calcd (%) for C65 : C 77.42,
H N O
77 5 5
2
3
stirred and heated to reux for 24 h. Aer the reaction, the
solvent was removed in vacuum and the residue was dissolved
in chloroform and extracted with distilled water several times. Proton ( H) and carbon-13 ( C) nuclear magnetic resonance
Characterization
1
13
Yellow crystals were obtained by recrystallization of the crude (NMR) spectra were recorded on a JNMEX400 spectrometer in
1
oil in methanol and acetone (yield: 76%, 6.1 g). H NMR (400 deuterated chloroform (CDCl
3
). Matrix assisted laser desorption
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ionization time-of-ight mass spectroscopy (MALDI-ToF MS,
Voyager-DE STR Biospectrometry Workstation) and elemental
analysis (Vario EL) experiments were conducted to identify the
chemical structure and purity. The ultraviolet-visible (UV-Vis)
absorption spectra in chloroform solution was measured by the
UV-Vis spectrophotometer (Jasco, ARSN-733). Optical switching
behaviors were characterized by ber-optic spectrophotometer
(
Ocean Optics, QP1000) equipped with reection mode of
2
optical microscopy. Additionally, the Cerius simulation so-
ware from Accelrys (version 4.6) was used to calculate the
minimal-energy geometry in the isolated gas-phase utilizing the
COMPASS force eld.
The thermal behavior was monitored by using a PerkinElmer
PYRIS Diamond differential scanning calorimetry (DSC)
equipped with an Intracooler 2P apparatus. The transition
temperatures were determined by measuring the onset
temperatures obtained during the cooling and heating scans.
The change of optical textures at a given temperature was
observed by using cross-polarized optical microscopy (POM,
Nikon ECLIPSE E600POL) coupled with a LINKAM LTS 350
heating stage.
One-dimensional (1D) wide-angle X-ray diffraction (WAXD)
experiments were conducted using Rigaku 12 kW rotating-
anode X-ray (CuKa radiation) generator coupled with a diffrac-
tometer. The diffraction peak positions and widths were cali-
brated with silicon crystals. Samples were scanned across a 2q-
Scheme 1 Synthetic procedures of photo-responsive chiral molecule,
NCA M.
2
19
molecular packing symmetries within the N-LC media. The
ꢀ
ꢀ
ꢀ
ꢁ1
angle range of 1.5 to 35 at a scanning rate of 2 min . The 2D
WAXD patterns were obtained using a Rigaku X-ray imaging
system with an 18 kW rotating anode X-ray generator. Silicon
crystal powder, used as an internal reference, shows a diffrac-
target compounds were puried by column chromatography.
Chemical structures and purities of NCA
ates were conrmed by thin-layer chromatography (TLC),
NMR spectroscopy (see Fig. S1–S5 in the ESI†), C NMR spec-
troscopy (Fig. S6†), MALDI-ToF (Fig. S7†) and elemental analysis
2
M and its intermedi-
1
H
13
ꢀ
tion ring at a 2q value of 28.466 . For the 2D WAXD experiment,
at least 10 min of exposure time was required for a high-quality
pattern. In both 1D and 2D WAXD experiments, background
scatterings were subtracted from the sample scans.
(
Fig. S8†).
In order to investigate the absorption properties of the
NCA M compounds, UV-Vis absorption measurements in
2
ꢁ
2
chloroform solution (2 ꢂ 10 mM) were conducted. As indi-
cated in Fig. 1a, the absorption band between two isosbestic
points (312 and 412 nm) corresponding to the p–p* transition
is originated from the trans-form of azobenzene, whereas the
ground state (n) of the cis conformer of azobenzene is excited to
the p* state by absorbing the UV-light in the range of 410–540
nm. As shown in Fig. 2a, upon irradiating the UV-light with a
Result and discussion
Programmed photochromic chiral molecule
As illustrated in Scheme 1, a naphthalene-based chiral molecule
2
(abbreviated as NCA M) containing two azobenzene mesogens
with alkyl chains was newly designed and synthesized. The
detailed synthetic procedures are explained in the Experimental
2
section of this paper. The NCA M molecule was obtained by the
amidation coupling reaction of chiral naphthyl ethylamine with
two trans-azobenzene derivatives in the presence of EDC and
DMAP. The azobenzene derivatives are specically introduced
for the photochromic groups to increase the compatibility
2
between NCA M and N-LC host medium. Furthermore, the
hydrophobic alkyl chains covalently connecting to the both side
of azobenzene and the di-hydroxybenzoate (DHB) group also
17
enhance the compatibility with N-LC.
Since the photochromic azobenzene moiety is applied as the
mesogenic group in NCA M, the molecular conformations and
phase structures can be reversibly switched with respect to
2
18
temperature and light. Here, the naphthyl group is specically
chosen for the chiral core, because the chiral compound derived
2
Fig. 1 UV-Vis absorption spectra of NCA M by irradiating (a) 365 nm
from naphthyl ethylamine are effective for breaking the and (b) 450 nm light, respectively.
2926 | Soft Matter, 2015, 11, 2924–2933
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range. Upon increasing the cooling rate, the onset temperature
shis to lower temperatures and the heat (DH) release at tran-
ꢀ
ꢁ1
sition is reduced. At the 2.5 C min cooling rate, the onset
temperature of the exothermic transition and corresponding
ꢀ
ꢁ1
ꢁ1
heat are 94.2 C and ꢁ36.7 J g (ꢁ37.0 kJ mol ), respectively,
ꢀ
ꢁ1
ꢁ1
which are reduced to 85.7 C and ꢁ33.1 J g (ꢁ33.4 kJ mol ) at
ꢀ
ꢁ1
2
0 C min . From the linear dependence on the cooling rate,
2
Fig. 2 Absorbance at 365 nm of NCA M in chloroform at room
temperature under (a) UV light for 100 s and (b) Vis light for 100 min,
respectively.
the onset transition temperature and heat at equilibrium are
ꢀ
ꢁ1
estimated to be 95 C and ꢁ37 kJ mol , respectively. The
variation of the transition temperature depending on the cool-
22
ing rate implies that the heat is associated with crystallization.
maximum intensity at 365 nm, the thermodynamically stable Hence, the endothermic thermal transition detected upon
trans conformer of azobenzene transforms to its metastable cis subsequent heating is ascribed to the crystal melting. The
conformer and reaches into a new photo-stationary state (PSS365 thermodynamic properties obtained from the subsequent
nm) in 60 s (Fig. 2a). On the other hand, upon subsequently heating process are consistent with those from the cooling
irradiating Vis-light (450 nm), the amount of trans conforma- process.
tional azobenzene isomer is increased by concomitantly
Although the DSC technique is sensitive to heat absorption
decreasing the intensity and area of the absorption band for the and release events at thermal transitions and serves quantita-
trans/cis isomers of azobenzene (Fig. 1b). Absorption bands tive thermodynamic properties, it does not provide direct
23
recover the initial state (PSS450 nm) aer 60 min (Fig. 2b). Photo- information regarding the molecular order at transitions. 1D
isomerization rate constants of p–p* and n–p* transitions are WAXD is used to investigate the temperature-dependent struc-
ꢁ
2
ꢁ1
ꢁ2
ꢁ1
K
(
t
¼ 6.72 ꢂ 10
s
and K
c
¼ 9.78 ꢂ 10 min , respectively tural development of NCA
2
M. Fig. 4b shows a set of 1D WAXD
M compound obtained at a cooling rate of
Fig. S9†). Transition between two photo-stationary states is a patterns of the NCA
fully reversible process by alternating UV- and Vis-light irradi- 2.5 C min . Above 95 C, the NCA
2
ꢀ
ꢁ1
ꢀ
2
M molecules are in the
ations, which allows us to fabricate a reversible photo-modu- isotropic state (I), exhibiting only two amorphous halos at 2q ¼
20
ꢀ
ꢀ
lating device.
2.25 (d-spacing ¼ 3.93 nm) and 2q ¼ 19.38 (d-spacing ¼ 0.45
nm), which correspond to the average periodicity of electron
density uctuations among the nanophase-separated NCA M
and to the average distance between the amorphous mesogens,
2
Thermodynamic transition and its corresponding structural
evolution of NCA M molecule
2
respectively. The isotropic melt above T was further conrmed
by the observation of the complete dark state at 120 C under
the polarized optical microscopy (POM) due to the crystal
i
ꢀ
To determine the thermal transition temperature and quanti-
tative thermodynamic properties of the NCA M compound, DSC
2
2
1
melting.
experiments were conducted. As shown in Fig. 3, DSC ther-
ꢀ
Just below the isotropization temperature (T ¼ 95 C), ve
mograms exhibit a single exothermic peak at different cooling
i
ꢀ
ꢀ
ꢁ1
apparent low-angle reections at 2q ¼ 1.62 (d ¼ 5.45 nm), 2q ¼
rates from 2.5 to 20 C min in the investigated temperature
ꢀ
ꢀ
3
.33 (d ¼ 2.65 nm), 2q ¼ 4.89ꢀ (d ¼ 1.80 nm), 2q ¼ 6.51
Fig. 3 Sets of DSC heating (a) and cooling (b) thermal diagrams for Fig. 4 (a) POM images of NCA
2
M during the cooling process; (b) 1D
ꢀ
ꢁ1
NCA M at different scanning rates ranging from 2.5 to 20 C min .
2
WAXD patterns at different temperatures.
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ꢀ
(
d ¼ 1.35 nm) and 2q ¼ 8.12 (d ¼ 1.08 nm) start to develop. In (0011) and (0013), respectively. However, based on the global
addition to the changes of the low 2q-angle region, a sudden equilibrium geometry of NCA M calculated at 0 K utilizing the
2
ꢀ
2
narrowing of the high 2q-scattering halo occurs at 2q ¼ 17.93 , COMPASS force eld of Cerius 4.6 soware, the theoretical
ꢀ
ꢀ
2
1.19 and 23.03 , which their corresponding d-spacings are length of NCA M is 3.41 nm (Fig. S10†). Although the calcula-
2
0.49, 0.42 and 0.38 nm, respectively. This result means that the tion is based on the all-trans conformation of alkyl and azo-
highly ordered phase is emerged. The formation of the highly benzene groups, the calculated molecular length (l) is
ꢀ
ordered phase is also monitored under POM at 30 C, as shown signicantly smaller than the detected layer spacing (d). Note
2
in Fig. 4a. Therefore, based on the POM results combined with that the bigger layer spacing of NCA M molecule indicates that
those of DSC and 1D WAXD, it is realized that there is one a bilayer structure is formed with an interdigitation and the
ꢀ
ordered phase below the T ¼ 95 C.
constructed bilayer building block is tilted away from the layer
i
25
ꢀ
normal. The reection in the quadrant at 2q ¼ 23.03 (d ¼ 0.38
ꢀ
nm) is found 35 away from the meridian (layer normal direc-
Identication of molecular packing structure of NCA M
molecule
2
tion). This diffraction result clearly indicates that the bilayered
ꢀ
and interdigitated NCA
2
M building blocks are tilted 55 away
from the layer normal direction, as schematically illustrated in
Fig. 5b. Note that the scattering halo detected at 2q ¼ 23.03 in
To obtain the structural and symmetry information of the
ordered phase in detail, 2D WAXD experiment from the
ꢀ
24
the quadrant is originated from the average periodicity of
electron density uctuations between the nanophase separated
azobenzene mesogens with a tilted arrangement. On the
equator of 2D WAXD along the SD, a pair of diffraction arcs is
detected at 2q ¼ 17.93ꢀ (d ¼ 0.49 nm). Diffractions on the
equator as well as in the quadrants in the wide angle region
means that a highly ordered structure is formed in the 3D by the
lateral molecular close packing. Even though the quality of 2D
WAXD pattern is not excellent and the diffraction information is
oriented NCA
the 2D WAXD pattern of an oriented NCA
macroscopically oriented NCA
mechanically shearing at 90 C and subsequent annealing by a
2
M samples must be conducted. Fig. 5a shows
M sample. The
M sample was prepared by
2
2
ꢀ
ꢀ
slow cooling from 90 C to room temperature. The incident X-
ray beam direction is perpendicular to the shear direction (SD,
parallel to the equator). Diffraction arc positions and widths in
Fig. 5a are calibrated based on the diffraction ring of silicon
ꢀ
power crystal at 2q ¼ 28.466 (d ¼ 0.314 nm). In this 2D WAXD
limited, it is obvious that the NCA M molecules are self-
pattern, diffraction information of NCA
2
M molecular arrange-
2
assembled to make the bilayer building blocks which are
further laterally self-organized with tilted fashion in the layered
superstructure. To understand in more detail the molecular
ments appears on two different length scales. One is on the
ꢀ
nanometer length scale in the low 2q-angle region between 1.5
ꢀ
and 10 , in which the self-assembled information for the
2
packing structure of NCA M compound, this 2D WAXD result
layered structure of NCA M can be obtained. Diffractions on the
2
ꢀ
ꢀ
should be analyzed by the support of the electron diffractions of
single crystals and the computer simulations in the real and
reciprocal spaces.
sub-nanometer length scale between 10 and 30 can provide
information about the lateral molecular packing structures
between the nanophase separated azobenzene mesogens and
naphthyl groups.
ꢀ
In Fig. 5a, a strong diffraction at 2q ¼ 1.62 (d ¼ 5.45 nm) is
Temperature-dependent color change of N*-LC lm
detected on the meridian with its high order diffractions at 2q ¼
ꢀ
ꢀ
ꢀ
The mixtures of N-LC medium with a chiral dopant selectively
reect different colors according to the Bragg's law, lmax ¼ n ꢂ p,
where lmax is the centre wavelength for the maximally reected
light, n is the refraction index of LC, and p is the helical pitch in
3
1
.33 (d ¼ 2.65 nm), 2q ¼ 4.89 (d ¼ 1.80 nm), 2q ¼ 6.51 (d ¼
ꢀ
ꢀ
.35 nm), 2q ¼ 8.12 (d ¼ 1.08 nm), 2q ¼ 17.93 (d ¼ 0.49 nm)
ꢀ
and 2q ¼ 21.19 (d ¼ 0.42 nm). This result indicates that the
highly ordered structure of NCA
2
M is basically a layer structure
26
micrometres. However, to realize its potential in the practical
applications for optical devices, the reection band of N*-LC
lm is required to be in visible wavelength. It is noteworthy to
remind that the pitch length can be controlled by tuning the
with the layer d-spacing of 5.45 nm. Therefore, Miller indices of
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
the diffractions at 2q ¼ 1.62 , 3.33 , 4.89 , 6.51 , 8.12 , 17.93
ꢀ
and 21.19 are identied as (001), (002), (003), (004), (005),
27
concentration of the chiral dopant with different handedness.
In this research, two chiral molecules with (R)-conguration are
used to induce N*-LC phase: one is a laboratory synthesized
novel chiral naphthyl NCA M molecule with a photo-responsive
2
azobenzene group and the other is a commercially available
chiral dopant (R)-811 bearing high helical twisting power. By
simply dissolving the chiral molecules in the N-LC host, the
N*-LC mixtures are prepared and loaded into standard electro-
optic cells.
For example, the N*-LC mixtures of 1.0 wt% of NCA M and
2
2
9.0 wt% of (R)-811 in N-LC host MLC 15600-100 are capillary-
lled into a 10 mm thick glass cell coated with polyimide planar
temperature (a) and molecular packing models in the crystalline phase (b). alignment layer (see Fig. 6). The aligned N*-LC texture is

2
Fig. 5 2D WAXD pattern of the crystalline phase of NCA M at room
2928 | Soft Matter, 2015, 11, 2924–2933
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extended helical pitch to the longer wavelengths and the
consequently limited repetition of the helical twist at the xed
layer thickness. As seen in Fig. 7c, the oily streaks are
completely absent and no strong reection band is observed in
the entire spectral range. The helical twisting power of chiral
molecules abruptly decreases during the phase transition from
28
cholesteric to isotropic phases.
Spectral shi of selective reection under electric elds
To investigate an electric eld induced reection shi, the N*-
LC mixtures are lled into the 10 mm thick electro-optic cell with
planar anchoring boundaries. At rst, to understand switching
behaviors of the N*-LC under applied voltages, the electro-optic
measurements have been carried out at room temperature.
Fig. 6 Materials information: host nematic liquid crystal (N-LC) MLC
1
5600-100, chiral dopant (R)-811, NCA
2
M molecule and their weight Fig. 8a presents the POM images of N*-LC imposed by various
amplitudes of square wave voltages at 1.0 kHz. The initial state
ratio in the mixture.
exhibits characteristic planar texture with oily streaks as seen in
Fig. 8a. Without the electric eld, the reection of the cell
observed at room temperature under POM (Fig. 7a). As expec- corresponds to a blue color with a reection maximum around
ted, a characteristic oily-streak texture for the N*-LC phase is 480 nm (Fig. 7 and 8). As the voltage is increased above 15 Vpp
,
observed. By adjusting the chiral concentration, the pitch the planar texture transforms to the focal conic (FC) state. At 80
length of the N*-LC mixture is optimized to reect colors in the Vpp, the cholesteric helix is completely unwound and the light-
visible region. At the initial state, as shown in Fig. 7a and d, the scattering FC state turns to a transparent homeotropic state
cell exhibits a reection band at ca. 480 nm at room tempera- (Fig. 8). The dark POM image and conoscopic gure in the inset
ture. Fig. 7d shows the temperature dependence of selective clearly show that the cholesteric helix is unwound and the
reections measured during the heating process.
uniaxial optic axis is aligned perpendicular to the substrate
As seen in Fig. 7d, the wavelength of the reection peak is plane (i.e., homeotropic state). Since no signicant reection
red-shied by increasing temperature while the reection effi- from the N*-LC lm is observed above 15 Vpp, the spectral shi
ciency decreases with temperature. It is obvious that the for selective reection has been investigated as a function of
cholesteric pitch increases with temperature, indicating that applied voltage prior to the transition to the FC state.
the helical twisting power (HTP) of the chiral additive is
Fig. 9 presents the spectral shi controlled by an external
inversely proportional to temperature. Although the mixture electric eld in the planar state. No signicant change has been
ꢀ
retains its cholesteric phase at temperatures above 55 C up to observed below 12 Vpp. However, the reection band sensitively
ꢀ
the TNI (63 C), the N*-LC lm does not show any characteristic responds to the external eld in the range from 12 to 15 Vpp. The
reection band in the visible region. It is likely due to the initial blue reection at 480 nm shis to green at 530 nm as
seen in Fig. 8b and 9. The inset in Fig. 9 shows conspicuous
broadening and weakening of selective reection. These char-
acteristics are attributed to the reorientation of LC molecules
Fig. 7 POM images of the N*-LC phase of the 1.0 wt% NCA
2
2
M with
9.0 wt% (R)-811 in N-LC host at 25 C (a), 55 C (b), 60 C (c), and their Fig. 8 POM images of electric field induced state for the 1.0 wt%
spectral change in the selective reflection, caused by temperature NCA M with 29.0 wt% (R)-811 in N-LC host (a) and their corresponding
real cell images (b).
ꢀ
ꢀ
ꢀ
2
change (d).
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Paper
Fig.
obtained from the 1.0 wt% NCA
at room temperature. The inset shows the measured reflection spectra of the cell with 1.0 wt% NCA M and 29.0 wt% (R)-811 in N-LC host
9
Center of reflection wavelength versus applied voltage
2
M and 29.0 wt% (R)-811 in N-LC host Fig. 10 Macroscopic cell images and corresponding reflection spectra
2
under different voltages.
upon UV-irradiation at 365 nm for 0 s (blue), 5 s (green) and 10 s (red),
respectively.
and subsequent deformation of the cholesteric helix. With the
increase of applied electric eld, LC molecules with positive towards longer wavelengths and eventually the reection satu-
dielectric anisotropy (D3 > 0) begin to rotate for a minimized rates in red (R, ꢃ620 nm) as shown in Fig. 10 and 11. At the
dielectric energy. Therefore, LC molecules initially on the plane given condition, the spectral change reaches the equilibrium
of substrates partially tilt toward substrate normal (i.e., parallel state in approximately 10 s of UV-illumination (see Fig. 11). The
to the applied electric eld). Consequently, the cholesteric helix saturation of color shi occurs likely due to a photo-stationary
is slightly deformed by retaining the average helical axis normal state of the isomerization. Therefore, the saturation wavelength
to the substrate. This results in a red shi of reection due to an and time may be further adjusted by controlling conditions
oblique reection and a gradual broadening of the reection such as the irradiation intensity and the content of the chiral
29
band. Above 15 Vpp, however, the cholesteric helix is dopant. In the absence of UV-illumination, a reection color
completely frustrated and thus turns to a light-scattering FC gradually blue-shis and restores its pristine blue color within
state as in Fig. 8a (center). As seen in the middle of Fig. 8b, the 20 s. The results evidently show the reversible color tuning,
green reection at 530 nm is originated from the tilting of a manipulated by external light. These observations are mainly
24
cholesteric helix from the layer normal. The colors between attributed to the photochromic isomerization of the chiral
blue and green reection can be reversible and continuously additive. As a result of trans- to cis- isomerization, HTP of the
manipulated by tuning the applied voltage from 10 to 15 Vpp
.
chiral dopant decreases signicantly and the subsequent
lengthening of cholesteric pitch results in a continuous red-
shi of spectral reection as a function of irradiation time. In
addition, the broadening of reection bands is evident during
the spectral shi of selective reections as seen in Fig. 10. This
can be understood by the intensity dependent photo-stationary
Remote-controlled reection lights of N*-LC lm by light
Since the NCA M chiral additive contains azobenzene moieties
2
(see Scheme 1), photochromic isomerization can be easily
2
state of isomerization. Since the chiral NCA M molecules
facilitated by irradiating either UV-light for trans- to cis-isom-
erization or Vis-light for cis- to trans-isomerization (see Fig. 1
and 2). It is well understood that HTP is sensitively affected by
30
chemical conformations of chiral additives. It is also known
that the photo-responsive chiral materials can be adopted for
31
the manipulation of cholesteric pitch. Therefore, it may be
possible to reversibly change the reected colors of N*-LC lms.
The LC cells with 10 mm thickness are prepared by loading the
2
mixture with 1.0 wt% NCA M and 29.0 wt% (R)-811 in N-LC
host. The inner surfaces of cells are treated by rubbed polyimide
for a homogeneous alignment. The cells are covered with black
paint on one side and placed under 365 nm UV-light
illumination.
Initially, a strong blue (B) reection peak at ꢃ480 nm is
observed prior to UV-light illumination as shown in Fig. 10.
Upon irradiating at 365 nm UV-light with the intensity of 100
Fig. 11 The spectral shift of selective reflections for the cell with
ꢁ
2
mW cm , the reection color shis to green (G) at ꢃ545 nm in
s. The prolonged illumination further shis a reection band UV-irradiation.
1
2
.0 wt% NCA M and 29.0 wt% (R)-811 in N-LC host under continuous
5
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strongly absorb UV-light, the effective irradiation intensity for
trans- to cis-isomerization depends on the penetration depth of
UV-light. UV-light intensity exponentially decays as the path-
length increases in the LC medium. As a result, the population
of cis-isomer with a lower HTP is higher near the UV-side,
compared to the far UV-side. This population gradient of
isomers and corresponding HTP gradient along UV-propagation
direction create subsequent pitch distribution. This eventually
instigates a broadening of the reection band as observed in
Notes and references
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Fig. 10.
Although the reected colors of N*-LC can be controlled by
the external stimuli, such as temperature and electric eld, the
range of wavelength is rather limited. At conditions for these
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Conclusions
The naphthalene-based chiral molecule (abbreviated as
NCA M) containing two photochromic azobenzene mesogens
2
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sensitive X-ray diffraction results combined with thermal and
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2
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with a layered structure. Since NCA
solubility in typical N-LC media, the NCA
2
M molecules exhibit good
M can be a promising
2
photo-responsive chiral dopant. The reection colors from the
N*-LC phase were reversibly tuned by external multi-stimuli,
such as temperature, electric eld, and light. A consequence of
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V. P. Tondiglia, R. L. Sutherland, S. A. Siwecki and
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Program (2013R1A1A2007238), SW Fusion Components R&D 11 A. C. Arsenault, T. J. Clark, G. von Freymann, L. Cademartiri,
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2Z04320) and BK21 Plus Program, Korea. D.-Y.K. appreciates
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