Highly polarized luminescence from an AIEE-active luminescent liquid crystalline film

Article abstract of DOI:10.1016/j.orgel.2017.07.044

Luminescent liquid crystals (LLCs) have attracted significant interest for organic optoelectronic applications, especially for the generation of linear polarized light. Here, a novel LLC molecule, 2-(4-(nonanealkoxy)phenyl)-3-(4-formamidephenyl)-acrylonitrile (CN-NPFA), is reported, which shows strong fluorescence in the solid state due to the aggregation-induced enhanced emission (AIEE) effect. Moreover, a well-aligned liquid crystalline film using AIEE-active molecules, is obtained using an in-plane electric field with an alignment layer. It exhibits highly polarized luminescence (ρ = 0.74) with a high fluorescence quantum yield. The device is both cheap and easy to fabricate, and has the potential to be used in practical electro-optic applications.

Full text of DOI:10.1016/j.orgel.2017.07.044

Organic Electronics 50 (2017) 177e183
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Highly polarized luminescence from an AIEE-active luminescent liquid
crystalline film
,
b
,
,
Junqing Sha a, Hongbo Lu a *, Mengyi Zhou a, Guo Xia a c, Yong Fang a,
Guobing Zhang a b, Longzhen Qiu a b, Jiaxiang Yang d, Yunsheng Ding b
,
,
a Key Lab of Special Display Technology, Ministry of Education, National Engineering Lab of Special Display Technology, State Key Lab of Advanced Display
Technology, Academy of Opto-Electronic Technology, Hefei University of Technology, Hefei, 230009, People's Republic of China
b Key Laboratory of Advanced Functional Materials and Devices, Anhui Province, School of Chemistry and Chemical Engineering, Hefei University of
Technology, Hefei, Anhui 230009, People's Republic of China
c Key Laboratory of Optical Calibration and Characterization, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, 230031,
China
d College of Chemistry and Chemical Engineering, Anhui University, Hefei, 230601, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Luminescent liquid crystals (LLCs) have attracted significant interest for organic optoelectronic appli-
cations, especially for the generation of linear polarized light. Here, a novel LLC molecule, 2-(4-(non-
anealkoxy)phenyl)-3-(4-formamidephenyl)-acrylonitrile (CN-NPFA), is reported, which shows strong
fluorescence in the solid state due to the aggregation-induced enhanced emission (AIEE) effect. More-
over, a well-aligned liquid crystalline film using AIEE-active molecules, is obtained using an in-plane
Received 22 May 2017
Received in revised form
27 June 2017
Accepted 29 July 2017
Available online 2 August 2017
electric field with an alignment layer. It exhibits highly polarized luminescence (
fluorescence quantum yield. The device is both cheap and easy to fabricate, and has the potential to be
used in practical electro-optic applications.
r
¼ 0.74) with a high
Keywords:
Luminescent liquid crystal
Aggregation-induced enhanced emission
In-plane electric field
© 2017 Elsevier B.V. All rights reserved.
Polarized luminescence
1. Introduction
interesting properties, including the ability to self-assemble. Liquid
crystals form supramolecular assemblies with unidirectional ori-
Polarized luminescence is important because it is useful for
many optoelectronic applications, such as optical information
storage devices, stereoscopic displays, and liquid crystal displays
[1e8]. Emitted light from a single luminescent molecule is usually
polarized; however, light emitted from bulk materials is typically
unpolarized because its constituent molecules are not aligned [2].
Inducing the luminescent molecules into an ordered array is a key
requirement to produce polarized luminescence [1,3,9].
entations of the molecular axis in nature to produce optical
anisotropy [10e13]. Luminescent liquid crystals (LLCs) are rapidly
emerging as an important optical class of materials, which possess
the properties of intrinsic light emission and unique supramolec-
ular organization [14e16]. Currently, the design and synthesis of
luminescent liquid crystals is still a challenging task [9,17,18]. It is
hard to incorporate luminescent functional groups into LCs while
retaining their mesomorphic properties [19,20]. Linear
p-conju-
Liquid crystals (LCs) have thermodynamically stable phases
between the isotropic liquid state and crystalline solid state. The
combination of long-range orientation ordering with relatively free
movement of anisotropic molecules endows liquid crystals with
gated organic luminophores usually show strong luminescence in
diluted solutions; however, their luminescence is often reduced or
even quenched in the condensed phase because of the aggregation-
caused quenching (ACQ) effect [21e23]. Unfortunately, many
practical applications require strong luminescence in the aggre-
gated state because the solid state is preferred for better morpho-
logical stability.
On the other hand, aggregation-induced enhanced emission
(AIEE) has been observed in silole and a-cyanostilbenic derivatives,
which is the opposite of ACQ: these luminophores are highly
emissive after aggregate formation due to the restriction of
* Corresponding author. Key Lab of Special Display Technology, Ministry of Ed-
ucation, National Engineering Lab of Special Display Technology, State Key Lab of
Advanced Display Technology, Academy of Opto-Electronic Technology, Hefei Uni-
versity of Technology, Hefei, 230009, People's Republic of China.
E-mail address: bozhilu@hfut.edu.cn (H. Lu).
http://dx.doi.org/10.1016/j.orgel.2017.07.044
1566-1199/© 2017 Elsevier B.V. All rights reserved.

178
J. Sha et al. / Organic Electronics 50 (2017) 177e183
intramolecular motion (RIM) [24,25]. Based on the AIEE effect, a
variety of new -cyanostilbenic derivatives have been identified.
(s), 1458 (m), 1391 (m), 1247 (s), 1170 (s), 1110 (m), 1017 (s), 898 (m),
806 (s), 758 (m), 714 (m).
a
HRMS (MALDI-TOF) m/z: Found: [MþH]þ 260.1924; molecular
The luminescent liquid crystals, (2Z,-20Z)-2,20-(1,4-phenylene)
bis(3-(4-(dodecyloxy)phenyl) acrylonitrile) e referred to as PDPA,
and (Z)-2-(4-Aminophenyl)-3-(4-(dodecyloxy)phenyl) acrylonitrile
e referred to as (Z)-CN-APHP, exhibit enhanced fluorescence in the
solid states [2,26,27]. The angle between the emission dipole
formula C17H25NO requires [MþH]þ 260.1936.
2.3. Synthesis of 2-(4-(nonanealkoxy)phenyl)-3-(4-
formamidephenyl)-acrylonitrile (CN-NPFA)
moment and long molecular axis of
lower luminescent dichroic ratio.
a-cyanostilbenic LLC produce a
A mixture of T1 (5.18 g, 0.02 mol) and NaOH (0.8 g, 0.02 mol) was
dissolved in ethanol and stirred at 78 ꢀC for 30 min. Then 4-
cyanobenzaldehyde (2.62 g, 0.02 mol) was added, and the
mixture was stirred at 78 ꢀC for 3 h. After the solid was filtered off,
the crude product was performed silica gel column and recrystal-
lized to give a white powder (5.63 g) with a yield of 72.2%.
In this study, a new a-cyanostilbenic LLC, 2-(4-(nonanealkoxy)
phenyl)-3-(4-formamidephenyl)-acrylonitrile (CN-NPFA), was
designed and synthesized, which shows a high fluorescent quan-
tum yield in the solid state (ɸf ¼ 0.44). The in-plane electric field
induce the uniaxial orientation of the CN-NPFA molecules, in which
molecular polar directors are aligned in the direction of the electric
field. Upon excitation, the oriented LLC film emits linearly polarized
light with high degree of polarization.
1H NMR (DMSO, 600 MHz,
d
¼ ppm): 8.05 (s, 1 H, NH) , 7.97 (d,
2 H, ArH, J ¼ 6.0 Hz) , 7.95 (s, 1 H, CH) , 7.92 (d, 2 H, ArH, J ¼ 6.0 Hz) ,
7.69 (d, 2 H, ArH, J ¼ 12.0 Hz) , 7.47 (s, 1 H, NH) , 7.05 (d, 2 H, ArH,
J ¼ 12.0 Hz) , 4.00 (t, 2 H, CH2, J ¼ 6.0 Hz) , 1.70 (m, 2 H, CH2,
J ¼ 6.0 Hz) , 1.39 (m, 2 H, CH2, J ¼ 6.0 Hz) , 1.25 (m, 10 H, CH2,
J ¼ 6.0 Hz) , 0.84 (t, 3 H, CH3, J ¼ 6.0 Hz).
2. Experimental section
13C NMR (DMSO, 150 MHz,
d
¼ ppm): 163.43, 140.86, 139.43,
2.1. Materials and methods
135.22, 132.00, 130.25, 128.40, 120.97, 119.97, 117.79, 115.67, 70.98,
34.51, 32.16, 32.01, 31.89, 31.78, 28.63, 25.31, 21.08, 16.74.
FT-IR (KBr, cmꢁ1): 3379 (s), 3203 (s), 2920 (s), 2843 (m), 2219
(w), 1655 (s), 1603 (m), 1510 (m), 1422 (s), 1246 (s), 1158 (m), 1022
(vs), 936 (w), 828 (m), 750 (m).
All chemicals were purchased from commercial sources and
used without further purification. The synthetic route for 2-(4-
(nonanealkoxy)phenyl)-3-(4-formamidephenyl)-acrylonitrile (CN-
NPFA) is described in the supporting information (Scheme S1). 1H
NMR (600 MHz) spectra and 13C NMR (150 MHz) spectra were
recorded using an Agilent VNMRS600 spectrometer. Infrared (IR)
spectra were recorded using a Nicolet 380 Fourier Transform IR
(FTIR) spectrometer. Mass spectra were obtained using an Acquity
UPLC LCT Premier XE mass spectrometer. Photoluminescence (PL)
spectra, fluorescent lifetime and quantum yields were recorded
using a Horiba FluoroMax-4 spectrofluorometer. This was done for
samples in both solid and solution states. Thermogravimetric
analysis (TGA) was conducted with a Netzsch STA449F3 TG at a
heating rate of 10 ꢀC minꢁ1. Differential scanning calorimetry (DSC)
was performed with a Mettler 82le/400 DSC at a heating and
cooling rate of 5 ꢀC minꢁ1. Polarizing optical images and fluorescent
images were recorded with an E600POL metallographic microscope
(Leica DM2500M) and a 365 nm excitation laser (Coherent). The
scanning electron microscope (SEM) images were obtained with a
SU8020 SEM. The profile images were recorded using a Zeiss
LSM700 confocal microscope and a NanoMap-PS step-height
measuring instrument.
HRMS (MALDI-TOF) m/z: Found: [MþH]þ 391.2293; molecular
formula C25H30N2O2 requires [MþH]þ 391.2307.
2.4. Fabrication of a highly polarized fluorescent film
A patterned indium tin oxide (ITO) substrate with a 10
mm wide
electrode and a 10 m wide space was prepared. Polyimide (PI)
m
solution was poured onto the patterned ITO substrate to produce
the rubbing alignment layer, see Fig. S1. The original CN-NPFA film
was prepared by spin-coating at lower revolving speed with
approximately 9 mm height. The highly polarized luminescent film
on the substrate was obtained after CN-NPFA cooled down to room
temperature from its nematic phase with a cooling rate of 1 ꢀC
minꢁ1 and an applied direct-current voltage of 200 V.
3. Results and discussion
CN-NPFA emits blue fluorescent light both in solution and solid
states. As shown in Fig. 1(a), CN-NPFA was weakly emissive when
dissolved in a good solvent (DMF) showing moderate quantum
yields (ɸf ¼ 0.17). The luminescence is distinctly enhanced in its
solid-state, which shows high quantum yields (ɸf ¼ 0.44). These
results indicate that CN-NPFA is AIEE active. Fig. 1(b) displays the
luminescence decay profiles of CN-NPFA solution and powders.
According to the calculation results based on density functional
theory (DFT), the optimized geometry of CN-NPFA is rod-like and
twisted in the isolated state due to internal steric repulsions e see
2.2. Synthesis of 4-(nonanealkoxy)phenylacetonitrile (T1)
A mixture of 4-Hydroxyphenylacetonitrile (3.99 g, 0.03 mol) and
potassium carbonate (8 g, 0.06 mol) was dissolved in N,N-dime-
thylformamide (40 mL) and stirred at 90 ꢀC for 15 min. Afterwards,
1-bromononane (6.21 g, 0.03 mol) was added, and the mixture was
stirred at 90 ꢀC for 12 h. The mixture was washed with deionized
water and extracted with ethyl acetate three times and dried over
MgSO4. Finally, after performing silica gel column chromatography,
the product was recrystallized to produce a yellow solid (6.72 g)
with a yield of 86.5%.
Fig. S2. The relatively higher non-radiative rate-constant (knr
¼
(1 ꢁ ɸf)/tF ¼ 2.8 ꢂ 109
s
ꢁ1) than radiative rate-constant (kr ¼ ɸf/
tF ¼ 2.1 ꢂ 108
s
ꢁ1) of CN-NPFA in DMF solution is mainly a
consequence of the nonplanarity and torsional relaxation of the
CN-NPFA molecule in the isolated state. On the other hand, for the
1H NMR (CDCl3, 600 MHz,
d
¼ ppm): 7.21 (d, 2 H, ArH, J ¼ 6.0 Hz)
, 6.88 (d, 2 H, ArH, J ¼ 6.0 Hz) , 3.94 (t, 2 H, CH2, J ¼ 6.0 Hz) , 3.67 (s,
2 H, CH2) , 1.77 (m, 2 H, CH2, J ¼ 6.0 Hz) , 1.30 (m, 12 H, CH2,
J ¼ 6.0 Hz) , 0.88 (t, 3 H, CH3, J ¼ 6.0 Hz).
aggregation
state,
the
non-radiative
rate-constant
(knr ¼ 2.4 ꢂ 108 sꢁ1) is one order of magnitude smaller than that in
solution, while the radiative rate-constant (kr ¼ 1.9 ꢂ 108
s
ꢁ1) is
13C NMR (CDCl3, 150 MHz,
d
¼ ppm): 161.54, 131.68, 124.10,
about the same order of magnitude as in solution, which gives the
peculiar AIEE effect (see Fig. 2).
120.93, 117.70, 70.78, 34.53, 32.19, 32.05, 31.92, 31.85, 28.67, 25.50,
25.34, 16.79.
To confirm the visual observation, the photoluminescence (PL)
spectra of CN-NPFA in DMF solution and DMF/water mixtures were
FT-IR (KBr, cmꢁ1): 2918 (s), 2850 (s), 2245 (w), 1602 (m), 1506

J. Sha et al. / Organic Electronics 50 (2017) 177e183
179
Fig. 1. (a) PL spectra of CN-NPFA solution and powder. Insets are the photo of CN-NPFA in solution and solid state under natural and UV (365 nm) light. (b) The fluorescent lifetime
of CN-NPFA in solution and condensed states.
Fig. 2. (a) PL intensity spectra of CN-NPFA in DMF and H2O/DMF mixtures. (b) PL peak intensity as a function of water content.
investigated. Water was used because it does not dissolve CN-NPFA,
hence, the luminescent molecules will aggregate in the aqueous
mixtures with high water fractions. With the addition of pure water
to the DMF solution, the PL intensity increases accordingly and the
PL peak red-shifts slowly to 462 nm from 442 nm. The highest
emission was obtained in a DMF/water mixture with 30% water. In a
good solvent (DMF), the benzene rings of the CN-NPFA molecule
can rotate freely around a single-link, which consumes the energy
of the excited state and results in weak fluorescence emission. For
the DMF/water mixtures, CN-NPFA intramolecular rotation is
restricted due to space constraints, which suppresses the non-
radiative decay of the excited state and enhances the fluorescent
intensity. The solubility of CN-NPFA decreases and its concentration
decreases, which leads to the reduction of fluorescence intensity
after the moisture content exceeds 30%.
stability. As shown in Fig. S3, CN-NPFA is thermally very stable with
an onset decomposition temperature as high as 308 ꢀC. The
mesomorphic properties of CN-NPFA were characterized using
both differential scanning calorimetry (DSC) and polarized optical
microscopy (POM). The phase-transition temperatures during first
cooling and subsequent heating at a scan rate of 5 ꢀC/min are
depicted in the DSC curves e see Fig. 3(a). Four exothermic peaks
were observed at 182 ꢀC, 153 ꢀC, 124 ꢀC and 116 ꢀC during the
cooling process, whereas the endothermic peaks during the sub-
sequent heating process were at 125 ꢀC, 136 ꢀC, 163 ꢀC and 184 ꢀC.
Clearly, the highest transition temperature during cooling is rather
close to that observed during heating, which indicates such tran-
sitions approach thermodynamic equilibrium, usually associated
with liquid crystal transitions. The POM observations are in good
agreement with the DSC results. Liquid crystalline phases were
observed enantiotropically during both heating and cooling cycles.
As shown in Fig. 3(b), a nematic liquid crystalline texture was
observed using POM when it was cooled from its isotropic state to
175 ꢀC. It can be identified by the Schlieren texture and indicates
that CN-NPFA can behave as a liquid crystal. Moreover, after further
From
a geometrical viewpoint, the a-cyanostilbene is of
calamitic shape and can form a liquid crystalline phase after
chemically endowing it with flexible tails. Prior to testing whether
CN-NPFA is mesomorphic or not, a thermogravimetric analysis
(TGA) measurement was conducted to evaluate its thermal

180
J. Sha et al. / Organic Electronics 50 (2017) 177e183
Fig. 3. (a) DSC curves of CN-NPFA at a heating/cooling rate of 5 ꢀC per minute. (bec) LC textures were recorded by POM at 175 ꢀC, 150 ꢀC, 120 ꢀC as CN-NPFA first cooled from the
isotropic melt.
cooling from the mesomorphic phase to 150 ꢀC, a typical focal-conic
fan-shaped texture of smectic C was formed e see Fig. 3(c). As the
sample was cooled to a temperature lower than the third transition
temperature, a smectic C* phase was observed e see Fig. 3(d). It can
be identified by the striated fan-shaped texture. With the sample
was continuously cooled down to room temperature, crystalliza-
tion of CN-NPFA occurred. When CN-NPFA was heated to its
isotropic state, smectic C and nematic liquid crystal textures
emerged, followed by smectic C* phase at 136 ꢀC, 163 ꢀC.
The alignment of the luminescent molecules plays a critical role
in obtaining polarized fluorescent emission with a high dichroic
ratio. Various methods have been developed to prepare the aligned
luminescent layer, and some excellent reviews of preparation
methods of aligned films are published. Mechanical friction is often
used to prepare aligned films by rubbing the liquid crystal film with
a cloth rod or by depositing the liquid crystal materials onto a
rubbed aligned substrate, e.g. polyimide. In pursuit of highly
polarized fluorescent emission, CN-NPFA molecules were first filled
polarizer. Both the contrast ratio (R) and the degree of linear po-
larization ( ) were determined from the spectra using Equations (1)
and (2), respectively.
r
.
R ¼ I I⊥
(1)
(2)
k
ꢀ
ꢁ.ꢀ
ꢁ
r
¼
I ꢁ I⊥
k
I þ I⊥
k
Here I and I⊥ represent the luminescent intensities when the
k
polarizer-axes are parallel and perpendicular to the rubbing
orientation, respectively. A contrast ratio of R ¼ 1.36 was obtained
for the CN-NPFA molecules in the rubbing cell, which indicates a
lower linear polarization degree (
r
¼ 0.15). It is difficult to induce a
smectic C* phase into an ordered arrangement via rubbing align-
ment. In addition, there is a significant angle (approximately 27ꢀ)
between the emission dipole moment and the long molecular axis
of the CN-NPFA molecule according to the DFT calculations e see
Fig. S5. These produce the lower dichroism of the CN-NPFA film.
In order to obtain a single domain of the smectic C* phase, an
electric field was applied using in-plane electrodes. A patterned ITO
into 10 mm gap cells with antiparallel-rubbed polyimide alignment
layers by capillarity action. The cell thickness was controlled with
dispersion beads. Upon cooling to room temperature from the
isotropic state with lower cooling rate (1 ꢀC minꢁ1), an inhomo-
geneous optical texture of CN-NPFA was observed using POM e see
Fig. S4(a). More precise observations were obtained using fluores-
cent microscopy (FM), by rotating the direction of the polarizer.
During the FM analysis, we found the luminescence intensity does
not vary significantly when the rubbing orientation was parallel or
perpendicular to the polarizer e see Fig. S4(b). In order to study the
luminescence anisotropy of the CN-NPFA films, we measured the
polarized luminescence spectra, by rotating the direction of the
substrate was prepared with in-plane striped electrodes of 10
mm
width and 10 m spacing e see Fig. S6. When the CN-NPFA was
m
cooled down to 174 ꢀC from its isotropic state, a voltage of 200 V
was applied, and a vague outline of LLC stripes was formed e see
Fig. S7(a). The LLC domains are clearly aligned with the electrodes
when the temperature dropped to 147 ꢀC, which can be seen in
Fig. S7(b). After the temperature decreased to 119 ꢀC, the orienta-
tion of the LLC molecules becomes more ordered and the stripes
gradually more homogeneous e see Fig. S7(c). Under the electric
field, the LLC molecules rotate to form an ordered arrangement

J. Sha et al. / Organic Electronics 50 (2017) 177e183
181
where the polarization direction of the CN-NPFA molecules co-
incides with the electric field direction.
orientation. According to calculations, the resulting
r
(0.71) in-
dicates that the light is highly polarized. This is comparable to other
linear polarized illumination systems like the electroluminescent
system and fluorescent dye-doped system. In general, the molec-
ular orientation affects the degree of linear polarization. Addi-
tionally, the order parameter (S) of the liquid crystalline directors
can be determined using the following equation:
To confirm this molecular orientation, we analyze the POM of
the aligned CN-NPFA films. As shown in Fig. S8, a distinct alterna-
tion of bright and dark fields was observed in the POM, when the
films were rotated between the crossed polarizers. This indicates
that the liquid crystal molecules were oriented uniaxially. Under an
electric field, CN-NPFA molecules with a smectic layering structure
rotate the title cone to form a ferroelectric structure due to
dielectric polarization. As expected from this, the polar axis of the
liquid crystal molecules is uniaxially aligned and parallel to the in-
plane field between the electrodes. The optical axis of the CN-NPFA
film was tilted towards the electric field and the angle between the
electric field and the optical axis was approximately 30ꢀ. This is
consistent with the angle between the dipole moment and the long
molecular axis of the CN-NPFA molecule.
ꢀ
ꢁ.ꢀ
ꢁ
S ¼ I ꢁ I⊥
I
k
þ 2I⊥ ¼ ðR ꢁ 1Þ ðR ꢁ 2Þ
=
(3)
k
In our case, the value of S is 0.63, which is slightly higher than
for the nematic LC phase. This means that the smectic-based illu-
minator can be used in a linear polarized illumination systems.
The surface profile of the aligned CN-NPFA film was investigated
using confocal microscopy and a step profiler. As shown in Fig. 5(a)
and (b), the LLC stripes shape of approximately 15 mm height were
To demonstrate the versatility of our method, the luminescence
from the aligned CN-NPFA films was investigated. As shown in the
FM images in Fig. 4(a), a linearly polarized luminescent signal is
observed with strong polarization-angle dependence. Relatively
bright blue light appears when the electric field is parallel to the
observed due to the dielectrophoretic effect. When CN-NPFA mol-
ecules are subjected to a non-uniform electric field, the CN-NPFA
molecules are pulled toward the region where the gradient of the
electric field is the highest. According to the simulation results
shown in Fig. 5(c), when a voltage is applied across the ITO elec-
trodes, the electric field is strongest near the edges of the ITO
stripes. The largest gradient electric field and dielectric force would
point toward this region. Thus, the CN-NPFA molecule prefers
filling this region and two LLC lines are formed at the edges of the
ITO stripe e see Fig. S9(b). Then, CN-NPFA in the two regions can
grow up quickly and combine together due to the surface tension e
see Figs. S9(c) and (d)). This surface profile of the stripes was
confirmed using a scanning electron microscope (Fig. S10).
polarizer axis, showing
a high fluorescent quantum yield
(ɸf ¼ 0.32). Many studies have been conducted of the development
of polarized luminescent illumination, which often required pre-
patterned templates and complex processing steps. In this study,
linearly polarized illumination could be generated from lumines-
cent liquid crystals films, easily.
A further investigation of the orientation of the CN-NPFA mol-
ecules was carried out using polarized luminescence spectra. The
aligned CN-NPFA film shows luminescent emission with
a
To further improve the linear polarization, a rubbing alignment
layer was prepared on the patterned ITO substrate. The angle be-
tween the rubbing direction and the electric field was about 27ꢀ.
Table 1 shows the contrast ratio, the degree of linear polarization
and the order parameter of CN-NPFA for different orientation
methods. The results show a better aligned LLC film with a high
maximum wavelength at 458 nm when excited with light at a
wavelength of 365 nm. The luminescence intensity from the
aligned CN-NPFA molecules decreases as the azimuthal angle be-
tween the applied electric field and the polarizer changes from 0 to
90ꢀ e see Fig. 4(b). This enables sensitive probing of the molecular
Fig. 4. (a) Photos of the luminescence for the field-induced uniaxial orientation of CN-NPFA film. The subscripts indicate the angle between the electric field and the orientation of
the polarizer. (b) PL intensity as a function of wavelength for different polarizer axes. (c) The PL intensity profile with different polarizer axes for a wavelength of 458 nm.

182
J. Sha et al. / Organic Electronics 50 (2017) 177e183
Fig. 5. (a) Surface morphology of the aligned CN-NPFA film detect using confocal microscopy. (b) Profile characterization of the aligned CN-NPFA film recorded on a step height
measuring instrument. (c) Simulated electric-field distribution using the three-dimensional electromagnetic field simulation software CST.
Table 1
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4. Conclusion
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