Liquid-crystalline compounds containing both a strong push-pull azo chromophore and a cholesteryl unit as photoresponsive molecular glass materials

Article abstract of DOI:10.1039/c5tc02126j

A series of liquid-crystalline (LC) compounds (R-Chol, R: CA, CN and NT), containing both a strong push-pull azo chromophore and a cholesteryl unit, was synthesized as novel molecular glass materials. The molecular structures and phases of the condensed state were characterized by spectral analyses, DSC, XRD and POM. The azo compounds existed as glassy solids at room temperature and entered the smectic LC phase above the glass transition temperature. Optical-quality solid films of R-Chol were obtained by spin-coating. The spin-coated films of CA-Chol and CN-Chol existed in the amorphous state and only the NT-Chol film showed weak birefringence and polydomain texture. Upon irradiation with a linearly polarized Ar⁺ laser (488 nm) beam, photoinduced orientation rapidly developed for CA-Chol and CN-Chol films, which showed significant photoinduced birefringence and dichroism. When the irradiating light was switched off, the birefringence did not show decay relaxation as is the usual case, but showed a small increase in the relaxation process. Photoinduced surface-relief-grating (SRG) formation was investigated by irradiating the films with two p-polarized interfering Ar⁺ laser beams. SRGs formed on the CA-Chol films showed the highest inscription rate in the series. The efficiencies of both photoinduced orientation and SRG formation were enhanced for the azo chromophore bearing the carboxylic group as the electron-withdrawing substituent. Two-dimensional (2D) quasi-crystal structures with 10- and 12-fold rotation symmetries were successfully fabricated on the CN-Chol films by using the dual-beam multiple exposure technique. The capability of the materials as photo-storage media was demonstrated by holographic recording.

Full text of DOI:10.1039/c5tc02126j

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Journal Name
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ARTICLE
Liquid-crystalline compounds containing both strong
push-pull azo chromophore and cholesteryl unit as
photoresponsive molecular glass materials
Cite this: DOI: 10.1039/x0xx00000x
a,b
c
a
a
a
Renbo Wei, Zeda Xu, Xiyang Liu, Yaning He, and Xiaogong Wang *
Received 00th January 2015,
Accepted 00th January 2015
A series of liquidꢀcrystalline (LC) compounds (RꢀChol, R: CA, CN and NT), containing
both a strong pushꢀpull azo chromophore and a cholesteryl unit, was synthesized as novel
DOI: 10.1039/x0xx00000x
molecular glass materials. The molecular structures and phases of the condensed state were
characterized by spectral analyses, DSC, XRD and POM. The azo compounds existed as
glassy solid at room temperature and entered smectic LC phase above glass transition
temperature. Opticalꢀquality solid films of RꢀChol were obtained by spinꢀcoating. The spinꢀ
coated films of CAꢀChol and CNꢀChol existed in the amorphous state and only NTꢀChol film
showed weak birefringence and polydomain texture. Upon irradiation with a linearly
www.rsc.org/
+
polarized Ar laser (488 nm) beam, photoinduced orientation rapidly developed for CAꢀChol
and CNꢀChol films, which showed significant photoinduced birefringence and dichroism.
When the irradiating light was switched off, the birefringence did not show decay relaxation
as a usual case, but showed a small increase in the relaxation process. Photoinduced surfaceꢀ
reliefꢀgrating (SRG) formation was investigated by irradiating the films with two pꢀpolarized
+
interfering Ar laser beams. SRGs formed on the CAꢀChol films showed the highest
inscription rate in the series. The efficiencies of both photoinduced orientation and SRG
formation were enhanced for the azo chromophore bearing the carboxylic group as the
electronꢀwithdrawing substituent. Twoꢀdimensional (2D) quasiꢀcrystal structures with 10ꢀ
and 12ꢀfold rotation symmetries were successfully fabricated on the CNꢀChol films by using
the dualꢀbeam multiple exposure technique. The capability of the materials as a photoꢀ
storage medium was demonstrated by holographic recording.
temperature ( g
T ) of the polymers and can be erased by heating
Introduction
[
3,5]
sample to
a
temperature above its Tg
.
Driven by
photoisomerization, azo polymers can show other interesting
photoresponsive functions, such as photoinduced phase
Azobenzene and its derivatives have been widely used as
functional building blocks to construct polymeric materials
[
15]
[16]
[
1ꢀ10]
transition,
spontaneous surface pattern formation,[17ꢀ19p] hotoꢀ
with various photoresponsive properties.
Upon irradiation
mechanical thin film contraction and bending,
photoinduced colloidal deformation.
and
with light at appropriate wavelengths, azo polymers can show a
variety of photoresponsive properties related to the reversible
[
20]
[
1,2,11]
Distinct from the azo polymers, amorphous molecular
materials (also named molecular glass) have recently been
developed as a medium to contain azo chromophores for optical
trans
ꢀ
cis isomerization of azo chromophores.
The
repeated photoꢀisomerization of the azo chromophores can
[
5]
trigger motions at different levels. At the molecular level,
repeated cycles of trans cis trans isomerization drive the azo
chromophores in the polymers to take preferential orientation in
[
21ꢀ25]
and photonic applications.
The glassy materials consist of
ꢀ
ꢀ
some wellꢀdesigned lowꢀmolecularꢀweight organic compounds
to show glassꢀtransition behavior similar to amorphous
[
12]
a direction perpendicular to the light polarization plane. The
orientation can also cause some correlative alignments of
groups and segments adjacent to the azo chromophores. The
irradiationꢀdriven orientation results in the photoinduced
anisotropy (PIA) of solid thin films to show optical dichroism
[
22ꢀ24]
polymers.
Amorphous molecular materials possess wellꢀ
defined structures and often show better reproducible properties
compared with polymers. By using the common spinꢀcoating
method, thin solid films with a smooth surface can be feasibly
prepared by using the amorphous molecular materials. Films of
azo molecular glasses have been used for inscribing SRGs and
[
2,4,5]
and birefringence.
At the micrometer level, the photoꢀ
isomerization induces mass transport of azo polymers. Upon
irradiation with interfering laser beams, surfaceꢀreliefꢀgratings
[
22,24,25]
other surface patterns.
Results show that SRGs can be
[
13,14]
inscribed on azo molecular glass films with a faster rate
compared to those usually observed for azo polymers. Because
of the wellꢀdefined structures, amorphous molecular materials
(
SRGs) can be inscribed on the azo polymer films.
SRGs
are formed at a temperature well below the glass transition
This journal is © The Royal Society of Chemistry 2015
J. Name., 2015, 00, 1-3 | 1

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ARTICLE
5TC02126J
DOI: 10.1039Jo/Curnal Name
could be a desirable type of materials to study the structureꢀ system. Other reagents were commercially available products
property relationship and for future device applications as well.
and used as received without further purification.
Typical amorphous molecular materials are designed to
possess rigid molecular structures to contain one or two was prepared according to a literature procedure.
Succinic acid cholesteryl ester. Succinic acid cholesteryl ester
[
33]
Succinic
azobenzene moieties, which can readily form stable amorphous anhydride (3.3 g, 33 mmol), cholesterol (12.7 g, 33 mmol), and
[
22ꢀ24]
glasses at high temperature.
Recently, threeꢀarm or starꢀ pyridine (0.8 g, 10 mmol) in 300 mL heptane were added to a
shape molecular glasses have been developed for inscribing flask equipped with a magnetic bar. After reaction under reflux
[
26ꢀ30]
SRG and other optical applications.
Flexible linkages have for 24 h, the solution was cooled down to room temperature and
been introduced into the molecular structures and some of the filtrated. The crude product was purified by recrystallization
1
compounds show liquid crystallinity too. In addition to the from acetone. Yield: 70%. H NMR (300 MHz, CDCl ) δ
3
branched molecules, a series of linear molecules linking an azo (ppm): 2.55ꢀ2.70 (m, 4H, OOCCH CH COO), all the other
2
2
1
chromophore to a rigid biphenylene unit through a flexible resonance signals on H NMR spectrum represent protons of
[
31,32]
+
spacer have also been synthesized.
irradiation, photoinduced orientation can be rapidly induced for
the molecular glasses to show significant birefringence and yl)propionate. A solution of succinic acid cholesteryl ester (1.5
dichroism. SRG can also be efficiently inscribed on the film g, 3 mmol), ꢀethylꢀ ꢀhydroxyethylaniline (0.5 g, 3 mmol),
surfaces. According to the spectral feature and isomerization ꢀdicyclohexylcarbodiimide (DCC, 2.4 g, 11 mmol), and 4ꢀ
Upon Ar laser the cholesteryl group.
2-(N-Ethyl-N-phenylamino)ethyl 3’-((cholesteryl)oxycarbon-
N
N
N,
N
behavior, azo chromophores have been classified into dimethylamino pyridine (DMAP, 0.14 g, 1.1 mmol) in 50 mL
azobenzene type, aminoazobenzene type, and pseudoꢀstilbene of dichloromethane was stirred at room temperature for 24 h.
[
11]
type.
The pseudoꢀstilbene
(
push
ꢀ
pull
)
type azo Then, N, Nꢀdicyclohexyl urea was filtered out and the filtrate
chromophores contain strong electronꢀdonating/ꢀwithdrawing was washed with water (150 mL), 5% acetic acid solution (150
substituents at 4 and 4’ positions of the azobenzene moieties. mL), water again (150 mL), and dried over Na SO . After
2
4
These pushꢀpull type azo chromophores possess strong evaporating the solvents, the residue was subjected to column
absorption bands in visible light region due to π→π* electronic chromatography on silica gel with DCM as eluting solvent.
1
transition. Polymers containing such chromophores can show a Yield: 70%. H NMR (600 MHz, CDCl ) δ (ppm): 7.23 (t, 2H,
3
variety of photoresponsive functions caused by the fast trans
ꢀ
ArH), 6.71(m, 3H, ArH), 4.25 (t, 2H, ꢀCH ꢀO), 3.55 (t, 2H, ꢀ
2
[
2,5]
to
ꢀ
cis isomerization and thermal relaxation.
It is of particular CH ꢀN), 3.41 (m, 2H, NꢀCH ꢀCH ), 1.17 (t, 3H, ꢀCH ), 2.55ꢀ
2 2 3 3
interest to develop amorphous molecular materials with simple 2.70 (m, 4H, OOCCH CH COO), all the other resonance
2
2
1
structures to contain the push
ꢀpull type azo chromophores in a signals on H NMR spectrum represent protons of the
high density. To our knowledge, there are only few reports cholesteryl group.
available for this type of the molecular glasses.
CN-Chol. 4ꢀAminobenzonitrile (0.6 g, 5 mmol) was dissolved
In this study, a series of compounds containing both a strong in a mixture of acetic acid (10 mL) and sulfuric acid (1 mL) at 0
o
push
ꢀ
pull azo chromophore and a cholesteryl group was
C. NaNO (0.69 g, 10 mmol) dissolved in 1.5 mL water was
2
synthesized. The molecules were constructed by connecting the slowly dripped into the acidified 4ꢀaminobenzonitrile solution.
azo chromophore and cholesteryl unit via a flexible spacer The mixture was stirred in an ice bath for 30 min to obtain the
(
Figure 1). The materials showed liquid crystallinity above Tg
,
solution of diazonium salt. 2ꢀ(NꢀEthylꢀNꢀphenylamino)ethyl 3’ꢀ
as indicated by differential scanning calorimetry (DSC), Xꢀray ((Cholesteryl)oxycarbonyl)propionate (2.5 g, 4 mmol) was
diffraction (XRD) and polarized optical microscopy (POM). On dissolved in 50 mL DMF and the solution was cooled down to
o
the other hand, high transparent solid films could be obtained 0 C. The diazonium salt solution was added dropwise into the
by spinꢀcoating of the azo compound solutions. The properties above DMF solution. After reaction for 5 h, the raw product
of the azo molecular glasses, especially those related with their was obtained by pouring the reaction solution into 100 mL
responses to light irradiation, were investigated in detail.
deionized water. The precipitate was collected by filtration and
washed with excess water until the neutral state was achieved.
Purification by column chromatography (silica gel,
1
dichloromethane) yielded red powder in 90% yield. H NMR
(
600 MHz, CDCl ) δ (ppm): 7.87 (m, 4H, ArH), 7.73 (d, 2H,
3
ArH), 6.78 (d, 2H, ArH), 4.31 (t, 2H, ꢀOCH ꢀ), 3.68 (t, 2H, ꢀ
2
NCH ꢀ), 3.52 (q, 2H, ꢀNCH CH ), 2.60 (m, 4H,
2
2
3
OOCCH CH COO), 1.24 (t, 3H, ꢀCH ), all the other highꢀfield
2
2
3
1
resonance signals in H NMR spectrum represent protons of the
cholesteryl group (Figure S1 in ESI
1
3
)
.
C NMR (150 MHz,
CDCl ) δ (ppm): 11.9, 12.4, 18.8, 19.4, 21.1, 22.6, 22.9, 23.9,
3
2
3
4.3, 27.8, 28.1, 28.3, 29.2, 29.4, 31.9, 32.0, 35.9, 36.3, 36.6,
7.0, 38.1, 39.6, 39.8, 42.4, 45.8, 48.8, 50.1, 56.2, 56.8, 61.7,
Figure 1. Chemical structure of azo molecular glasses (R-Chol).
74.6, 111.5, 112.0, 119.1, 122.8, 126.1, 133.1, 139.6, 143.7,
ꢀ
1
1
51.1, 155.6, 171.6, 172.4. IR (KBr, cm ): 2935, 2902, 2866
(
CꢀH, s), 2224 (C N, m), 1732 (C=O, s), 1597, 1516 (Benz.
≡
Experimental Section
ring, s), 1384 (N=N, s), 1254, 1150 (CꢀOꢀC, s), 1132 (=NꢀAr,
s). MS (ESI): calcd. for C H O N [M + H]+ : m/z =
7
Materials.
NꢀEthylꢀNꢀhydroxyethylaniline (97%) was purchased
4
8
66
4
4
from Tokyo Chemical Industry Co., Ltd. and used as received.
63.5157, found: 763.5157. UVꢀvis: λmax = 447 nm.
CA-Chol. CAꢀChol was similarly prepared as mentioned
4
4
ꢀAminobenzonitrile (98%), 4ꢀaminobenzoic acid (98%), and
ꢀnitroaniline (98%) were purchased from Alfa Aesar and used
above for CNꢀChol synthesis by using the diazonium salt of 4ꢀ
aminobenzoic acid. H NMR (600 MHz, CDCl ) δ (ppm): 8.22
(
1
as received. Tetrahydrofuran (THF) was purified by distillation
with sodium and benzophenone. Deionized water (resistivity
3
d, 2H, ArH), 7.90 (m, 4H, ArH), 6.80 (d, 2H, ArH), 4.33 (t,
>
18 MΩ cm) was obtained from a MilliꢀQ water purification
2
| J. Name., 2015, 00, 1-3
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DOI: 10.1039/C5TC02126J
2
2
H, ꢀOCH ꢀ), 3.69 (t, 2H, ꢀNCH ꢀ), 3.53 (q, 2H, ꢀNCH CH ), incident on the films directly and the other half of the beam was
2 2 2 3
.61 (m, 4H, OOCCH CH COO), 1.26 (t, 3H, ꢀCH ), all the reflected onto the films from the mirror. The diffraction
2
2
3
1
other highꢀfield resonance signals in H NMR spectrum efficiency of the first order diffracted beam from the gratings
represent protons of the cholesteryl group (Figure S1 in ESI). was probed with a low power HeꢀNe laser beam at 632.8 nm in
ꢀ
1
IR (KBr, cm ): 2945, 2902, 2868 (CꢀH, s), 1732, 1687 (C=O, transmission mode. The surface profiles of the films were
s), 1598, 1514 (Benz. ring, s), 1385 (N=N, s), 1244, 1158 (CꢀOꢀ probed by AFM on a Nanoscope V instrument in tapping mode.
C, s), 1136 (=NꢀAr, s). MS (ESI): calcd. for C H O N [M +
Two-dimensional quasi-crystal fabrication. The optical setup
4
8
67
6
3
H]+ : m/z = 782.5108, found: 782.5105. UVꢀvis: λmax = 432 for twoꢀdimensional quasiꢀcrystal fabrication was similar to
[
25]
nm.
those reported previously.
A diodeꢀpumped frequencyꢀ
NT-Chol. NTꢀChol was similarly prepared as mentioned doubled solid state laser beam (size~9 mm) with Gaussian
above for CNꢀChol synthesis by using the diazonium salt of 4ꢀ profile at 532 nm was split into two equalꢀintensity beams by
1
nitroaniline. H NMR (600 MHz, CDCl ) δ (ppm): 8.32 (d, 2H, using a beam splitter. Two λ/2 wave plates were used to
3
ArH), 7.92 (m, 4H, ArH), 6.79 (d, 2H, ArH), 4.32 (t, 2H, ꢀ independently rotate the plane of the polarization of the beams.
OCH ꢀ), 3.69 (t, 2H, ꢀNCH ꢀ), 3.53 (q, 2H, ꢀNCH CH ), 2.60 GlanꢀThompson prisms were applied to ensure that the electric
2
2
2
3
(
m, 4H, OOCCH CH COO), 1.25 (t, 3H, ꢀCH ), all the other fields of the two beams were perpendicular to each other (in x
2 2 3
1
highꢀfield resonance signals in H NMR spectrum represent and yꢀaxis). Two λ/4 wave plates were put in the optical paths
protons of the cholesteryl group (Figure S1 in ESI). IR (KBr, to ensure the interfering beams with opposite circular
ꢀ
1
cm ): 2943, 2903, 2868 (CꢀH, s), 1733 (C=O, s), 1602, 1518 polarizations (the rightꢀ/leftꢀcircularly polarized, RCP/LCP).
2
(
(
Benz. ring, s), 1385 (N=N, s), 1254, 1157 (CꢀOꢀC, s), 1135 The intensity of each beam was 20ꢀ30 mW/cm . The spatial
=NꢀAr, s). MS (ESI): calcd. for C H O N [M + H]+: m/z = period of the interference pattern (Λ) was controlled by
4
7
65
6
4
7
82.4982, found: 782.4980. UVꢀvis: λmax = 477 nm.
Film preparation. The films with a smooth surface were the equation
adjusting the intersection angle of the beams (θ), according to
/(2sin(θ/2)), where λ is the wavelength (532 nm).
λ
2
prepared by spinꢀcoating. The solutions were prepared by The irradiation area was about 0.64 cm . CNꢀChol films
dissolving RꢀChol in anhydrous THF with a concentration of 5ꢀ mounted on a rotation stage were exposed to the interference
2
5% (wt%). The solutions were filtered through 0.45 ꢁm pattern for a fixed period of time. After each exposure, the
membranes and spinꢀcoated onto the glass slides or CaF (for samples were rotated around the surface normal to achieve the
2
the case measuring photoinduced dichroism) with a speed about required orientation of the grating vector relative to the
1
500 rpm/min for 30 s. The spinꢀcoated films were dried at previous one. To fabricate a quasiꢀcrystal structure with 2nꢀfold
room temperature under vacuum for 12 h and then stored in a rotation symmetry, the rotation angle was π/n and n exposure
desiccator for further experiments. The thickness of the films, steps were required. Under typical conditions, the time period
determined by a scratching method on AFM in contact mode, for each exposure was 5ꢀ25 s. The firstꢀorder diffraction
was controlled to be 0.3ꢀ2 ꢁm by adjusting above conditions for efficiency was monitored during each writing step. The exact
different research purposes.
irradiation time for each step was adjusted by controlling the
Photoinduced orientation. The optical setup for photoinduced diffraction efficiency to be the same. The twoꢀdimensional
birefringence experiments was similar to those reported quasiꢀcrystal structures on the films were probed by AFM on a
[
34]
+
previously.
The films were irradiated with an uniform Ar
Nanoscope V instrument in tapping mode.
Characterization. H NMR and C NMR spectra were
2
1
13
laser beam (488 nm, 60 mW/cm ). To provide a homogeneous
beam profile over the irradiated area of the film, the laser beam obtained on a JEOL JNMꢀECA300 or JEOL JNMꢀECA600
was expanded through a spatial filter and collimated. The NMR spectrometer with tetramethylsilane (TMS) as the
photoinduced change in the refractive index was probed in a internal standard at ambient temperature in CDCl . FTꢀIR
3
realꢀtime manner by a lowꢀintensity beam from a HeꢀNe laser spectra were collected on a Nicolet 560ꢀIR spectrometer, the
(
632.8 nm). The probe HeꢀNe laser beam was transmitted samples were mixed with KBr and then pressed into thin IRꢀ
through a pair of crossed polarizers, whose polarization transparent disks. Mass spectra were determined using
o
directions were set at ±45 with respect to the polarization of Thermofisher LTQ system, the sample were dissolved in
the exciting beam. When the light intensity variation reached chloroform. Thermal analyses of the compounds were carried
the saturated level, the exciting beam was turned off to out using TA Instruments DSC Q2000 and TGA Q5000
o
investigate the relaxation process.
systems with a heating rate of 10 C/min in a nitrogen
Photoinduced dichroism. Photoinduced dichroism was atmosphere. The UVꢀvis spectra of the samples (THF solutions)
investigated by polarized FTꢀIR spectroscopy using the spinꢀ were measured by using an Agilent 8453 UVꢀvis
[
35]
coating films on calcium fluoride disks.
After irradiating spectrophotometer. The XRD experiments were performed on a
+
2
with the linearly polarized Ar laser (488 nm, 60 mW/cm ) for RINT2000 vertical goniometer, 185 with a 3 kW ceramic tube
0 min to induce the orientation, polarized infrared spectra of as the Xꢀray source (Cu Kα). Polarizing microscopic
3
the samples were obtained at room temperature using the observations were conducted on a Nikon LV 1000 POL
Nicolet 560ꢀIR FTIR spectrophotometer equipped with a wire microscope equipped with a Nikon DSꢀFi2 CCD camera, Nikon
grid polarizer (KRSꢀ5). The orientation order parameter (S) was DSꢀU3 digital sight, and Linkam LTS420E hot stage. The
calculated by the following equation,
S = (A A )/(A +2A||)
and A are the absorbance perpendicular and parallel
surface images of SRGs and twoꢀdimensional quasiꢀcrystal
⊥
−
||
⊥
(1) structures were monitored using an atomic force microscope
(
AFM, Nanoscope V) in the tapping mode.
where A
⊥
|
|
[
35]
to the polarization direction of the laser beam.
Photoinduced SRG formation. SGRs were inscribed by using Results and discussion
[
13,14]
2
the Lloyd mirror setꢀup according to the literature.
A
+
Synthesis and characterization
linearly polarized Ar laser beam (488 nm, 100 mW/cm ) was
used as the light source. After the
pꢀpolarized laser beam was The chemical structure of the newly synthesized azo
expanded and collimated, half of the collimated beam was compounds is shown in Figure 1 and the synthetic route is
This journal is © The Royal Society of Chemistry 2015
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DOI: 10.1039Jo/Curnal Name
given in Scheme S1 (ESI). The molecules contain both an azo crystal (LC) depending on the temperature. The glass transition
chromophore and a cholesteryl group. The molecules differ can be seen when the temperature increases. After glass
from each other only in the chromophoric electronꢀwithdrawing transition, the materials enter the liquid crystalline phase as
groups, which are carboxyl, cyano, and nitro for CAꢀChol, CNꢀ indicated by POM and XRD. The phase transition occurs in the
Chol, and NTꢀChol, respectively. All the target molecules were high temperature range of the DSC curves is the liquid
obtained from the same precursor, 2ꢀ(NꢀethylꢀNꢀphenylꢀ crystal/isotropic (LC/I) transition. The Tg and TLC/I obtained
amino)ethyl 3’ꢀ((cholesteryl)oxycarbonyl)propionate. It was from the DSC are summarized in Table 1. CAꢀChol containing
then reacted with the diazonium salts of the corresponding the 4ꢀcarboxylazobenzene moiety shows the highest Tg and
amino compounds through the azoꢀcoupling reaction. This TLC/I values among the compounds. It can be attributed to the
reaction route can introduce azo chromophores with different hydrogen bonding between the COOH groups as confirmed by
ꢀ1
electronꢀwithdrawing groups at the final stage of the IR from the wavenumber shift of the peak at 1687 cm (Figure
3
6
preparation under mild conditions, which is suitable to prepare S2 in ESI). The liquid crystal textures of RꢀChol (R = CA, CN
a series of similar azo molecules bearing different electronꢀ or NT) observed with POM are given in Figure S3 (ESI). The
1
withdrawing groups. The H NMR spectra of the target XRD measurements indicate that the RꢀChol form the smectic
molecules CAꢀChol, CNꢀChol and NTꢀChol are given in phase in the LC state (Figure S4 in ESI).
Figure S1 (ESI) with the resonance signal assignments. The
Table 1. The glass transition temperature (T
g
), liquid crystal/isotropic
multiple resonance signals at the high magnetic field are from
the protons of the cholesteryl unit. The other resonances can be
transition temperature ( LCꢀI) and λmax of RꢀChol.
T
well assigned to the chemical structure as given in Figure 1
The structures of the newly synthesized molecules are also
verified by FTꢀIR, MS and UVꢀVis.
.
o
a
b
Sample
T
g
( C)
T
(
LC1/LC2
T
( C)
171
76
119
LC2/I
λ
max (nm)
o
a
o
a
C)
ꢀ
63
ꢀ
CAꢀChol
54
31
30
433
448
477
The thermal transitions and phase behaviour of RꢀChol CNꢀChol
(
R=CA, CN or NT) were investigated by DSC, XRD and POM. NTꢀChol
a
b
From the heating scans. THF solutions.
Figure 2 shows the DSC curves of RꢀChol on the secondꢀ
heating and the secondꢀcooling scans. RꢀChol (R=CA, CN or
NT) shows both the properties of glassy materials and liquid
Figure 2. DSC curves of R-Chol series compounds for the second heating and second cooling scans.
The UVꢀvis spectra of the azo compounds in THF solutions
are shown in Figure 3, which possess typical spectral features
of pseudoꢀstilbeneꢀtype azo compounds. The absorption bands
corresponding to the π→π* transition appear at the longest
wavelength and the azo compounds show strong absorption in
the visible region. The positions of the absorption bands are
strongly affected by the electronꢀwithdrawing groups on the
azobenzene moieties. The λmaxs of CAꢀChol, CNꢀChol and NTꢀ
Chol are 433, 448, and 477 nm (Table 1). This tendency reflects
their inductive and conjugative strength for both the ground and
the excited states.
Photoinduced birefringence and dichroism
The solid thin films with smooth surface were prepared by spinꢀ
coating for all the following investigations. As THF was
quickly evaporated during the spinꢀcoating process, only NTꢀ
Chol film shows weak birefringence and polydomain texture as
Figure 3. UV-vis spectra of the azo compounds in THF.
4
| J. Name., 2015, 00, 1-3
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Page 5 of 9
Journal Name
Journal of Materials Chemistry C
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DOI: 10.1039/C5TC
A
02
R
1
T
2
I
6
C
J
LE
[
29,31]
indicated by the POM observation (Figure S5 in ESI). For the the azobenzene moieties.
For both amorphous molecular
other two compounds, no birefringence could be observed. The materials and azo polymers, the similar tendency has been
DSC measurements were performed by using solid samples observed for these electronꢀwithdrawing groups to affect the
[
31,32,36,38]
collected from the spinꢀcoated RꢀChol films and the DSC birefringence growth and saturated values.
It reflects
curves are given in Figure S6 (ESI). Only NTꢀChol shows the the influence of the electronꢀwithdrawing groups on the
endothermic transition corresponding to TLC/I transition. These photoinduced orientation ability at the molecular level. On the
results indicate that the CAꢀChol and CNꢀChol exist in the other hand, the extremely low degree of the photoinduced
amorphous state for the spinꢀcoated films under the processing birefringence of NTꢀChol could also be related with the liquid
conditions. Only NTꢀChol shows liquid crystallinity with the crystallinity of the film. If the alignment of azo chromophores
local low orientational order.
is perpendicular to film surface (homeotropical orientation), the
Photoinduced birefringence of CAꢀChol, CNꢀChol and NTꢀ light absorption should be weak as the transition moment is
Chol were investigated in a realꢀtime manner by irradiating the perpendicular to the vibration direction of the electric field of
solid thin films of CAꢀChol, CNꢀChol and NTꢀChol with a the light wave. Therefore, it will result in a low isomerization
2
linearly polarized laser beam (60 mW/cm ) at 488 nm as degree of the azo chromophores. Moreover, the photoinduced
described in the experimental section. The photoinduced isomerization of the azobenzene moieties in NTꢀChol film
birefringence was calculated from the detected transmittance of could be less efficient due to the tight packing of the
[
37]
the probing laser by the equation,
chromophores to form the LC phase.
For azo polymers and related materials, the photoinduced ꢀn
2
I
=
I0 sin (πꢀnd
/
λ
)
(2)
where
crossed polarizer, I0 is the intensity of the probe beam is switched off.
transmitted through the first polarizer,
is the wavelength of the relaxation, no birefringence decrease is observed for CAꢀChol
I
is the intensity of the light transmitted through the usually decreases in some degree when the writing laser beam
[2,4,5,33,36,38ꢀ40
] In contrast to the typical decay
λ
probe wave (633 nm),
birefringence.
d
is the film thickness, and ꢀn is the and CNꢀChol after switching off the irradiating light (Figure 4).
Even a slight increase in the birefringence can be observed after
switching off the irradiating light, which has also been observed
for other amorphous molecular materials with a linear
[
31,32]
molecular structure.
Figure 4. The variations of the photoinduced birefringence of R-Chol thin films
with the irradiation time and relaxation time.
Figure 5. The polar plots of the relative infrared absorbance of stretching
-
1
-1
■), 1597 cm for
Figure 4 shows variations of the photoinduced birefringence vibration bands of the ν(C=C) (aromatic, 1599 cm for CA-Chol (
-1
CN-Chol (●), and 1601 cm for NT-Chol (▲)) of the R-Chol films.
of the RꢀChol thin films with the irradiation time. Before the
excitation laser was switched on, no transmittance of the
probing laser beam can be detected by the photodiode, because
The photoinduced dichroisms of the RꢀChol films were
the sample placed between the crossed polarizers is isotropic. It characterized using polarized FTꢀIR spectroscopy. Figure 5
indicates that although mesogenic groups in NTꢀChol possess gives the polar plots of the relative infrared absorbance of the
local orientational order, the film is polydomain and isotropic in stretching vibration bands of ν(C=C, aromatic) for the RꢀChol
ꢀ
1
ꢀ1
macroscopic level in the film plane. As homeotropical films, which are 1599 cm for CAꢀChol, 1597 cm for CNꢀ
ꢀ
1
alignment is typical in smectic LC, the predominant Chol, and 1601 cm for NTꢀChol. In this experiment, the Rꢀ
orientational direction of the mesogenic groups could be Chol films were irradiated with a linearly polarized Ar laser
perpendicular to the substrate surface. When the writing laser beam (488 nm, 60 mW/cm ) till the saturation and then
+
2
beam is switched on, the birefringence increases rapidly at the polarized FTꢀIR spectra were obtained at different angles
initial stage and is then gradually saturated after the rapid relative to the polarization plane. The orientation order
increase. The saturated birefringence of CAꢀChol is the highest parameter (S) was calculated by Equation 1. The orientation
in the series, which can reach 0.21. This value is much higher order parameters derived from the angularꢀdependent IR
[
2,4,5]
compared with those for most azo polymers.
The saturated absorption are 0.461, 0.199 and 0.001 for CAꢀChol, CNꢀChol
birefringence of NTꢀChol is much smaller compared with the and NTꢀChol, respectively. The orientation order parameters
values of its counterparts, which is only 0.02. The photoinduced show the same order as indicated by the birefringence
birefringence behaviour of the azo molecular glasses is closely measurements. The orientation order parameter of CAꢀChol is
[
2,4,5,33,38]
correlated with the effect of the electronꢀwithdrawing groups on much higher than the ordinary values of azo polymers.
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J. Name., 2015, 00, 1-3 | 5

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ARTICLE
5TC02126J
DOI: 10.1039Jo/Curnal Name
The low orientational order parameters of NTꢀChol could be growth rate for SGR formation in the series. This tendency is
attributed to the same reasons discussed above.
the same as the photoinduced birefringence growth rate (Figure
).
Twoꢀdimension (2D) quasiꢀcrystal surface patterns on RꢀChol
4
Photoinduced SRGs and surface quasi-crystal structures
films were fabricated by dualꢀbeam multiple exposure
The properties of the amorphous molecular materials to form
surface relief gratings (SRGs) and other surface relief patterns
were investigated. SRGs were inscribed by exposing the spinꢀ
coated solid films of RꢀChol to the interference patterns formed
[25]
technique. In each step, the spinꢀcoated films of RꢀChol were
exposed to the interference patterns formed by beams with
opposite circular polarizations (LCP/RCP). A diodeꢀpumped
frequencyꢀdoubled solid state laser beam (size~9 mm) with
by the two
p
ꢀpolarized laser beams (polarization in the plane of
2
Gaussian profile at 532 nm (30 mW/cm ) was used as the
[
14]
incidence), which is typically used in previous study.
The
excitation light. After each exposure, the films were rotated
around the surface normal to achieve the required orientation of
the grating vector relative to the previous one. The profiles and
trough depths of the surface quasiꢀcrystal structures were
detected by AFM.
+
2
Ar laser beam with the wavelength of 488 nm (100 mW/cm )
was used as the writing light and a low intensity HeꢀNe laser
beam (632.8 nm) was used as the probe beam. The profiles and
trough depths of the gratings were detected by AFM.
Figure 7a and 7b give the AFM images of the quasiꢀcrystal
patterns with 12ꢀfold symmetry on the CNꢀChol film. The
quasiꢀcrystal structures on the surfaces were fabricated through
6
exposure steps by using the interference pattern with the
period of 6.2 ꢁm. Figure 7c and 7d give the typical optical
micrograph of the quasiꢀcrystal structures in the transmittance
and refractive modes respectively, which show the quasiꢀcrystal
patterns in a large scale. Figure 7e gives the diffraction pattern
of a HeꢀNe laser beam with normal incidence to the sample.
The quasiꢀcrystal structures with other folds of the symmetry
are shown in Figure S7 (ESI). The quasiꢀcrystal structures with
different rotation symmetry can be prepared on CNꢀChol films
through this procedure. Although the CAꢀChol film has the
higher efficiency to form SRG than that of CNꢀChol, the quasiꢀ
crystal structures with such high quality cannot be inscribed on
CAꢀChol film under the same condition. It indicates that
although both SRG and quasiꢀcrystal surface pattern are formed
by the photoinduced mass transport, the fabrication of the
quasiꢀcrystal surface patterns shows stricter requirements for
other material properties, such as the mechanical response
parameters to the deformation.
Holographic recording
The performance of the amorphous molecular materials as
holographic recording medium was tested by using typical twoꢀ
beam interference technique. The scheme of the holographic
recording is given in Figure S8 (ESI). A diodeꢀpumped
frequencyꢀdoubled solid state laser (532 nm) was used as the
light source. A piece of the spinꢀcoated film of CAꢀChol was
used as the recording medium. The film with the smooth
surface was exposed to the interference pattern formed by the
object wave and the coherent reference wave. In the process,
the laser beam with Gaussian profile was split into two equalꢀ
intensity beams by using a beam splitter. Both beams were
Figure 6. a) and b), typical AFM images of the sinusoidal surface relief structures
on the films of CA-Chol after irradiation for 30 min; c), the first order diffraction
efficiency as a function of the irradiation time in the process of SRG formation on
the films of R-Chol.
Figure 6a and 6b show typical AFM images of the sinusoidal
surface relief structures. The SRGs with regular spaces are
formed on the films of CAꢀChol after irradiated with the
+
linearly polarized to possess the sꢀpolarization (polarization
interfering Ar laser beams (488 nm) for 30 min. The sinusoidal
perpendicular to the plane of incidence). One beam whose size
was controlled by a lens irradiated onto the CAꢀChol film
directly. The image of the panda was recorded by the other
beam after expanded by two lenses. The object wave then
irradiated onto the same area of the film after collimated by the
third lens. The whole holographic recording process was
completed in 5ꢀ20 s. After recording the image, a HeꢀNe laser
surface patterns with a period about 1 ꢁm, which depends on
the wavelength and the incident angle of the writing
[
13,14]
beams,
can be seen from the images. The average
amplitude of the SRG inscribed on the surface of the CAꢀChol
film can reach 1 ꢁm for the film with a thickness about 2 ꢁm.
The SRG formation was also monitored by measuring the
diffraction efficiencies of the probe beam in a realꢀtime manner.
Figure 6c presents the first order diffraction efficiency as a
function of the irradiation time in the process of SRG formation
on the films of CAꢀChol, CNꢀChol and NTꢀChol. The
diffraction efficiencies for the CAꢀChol, CNꢀChol and NTꢀChol
gratings are 0.17, 0.14 and 0.01 after irradiation for 1000 s,
respectively. The result shows that CAꢀChol has the highest
(
633 nm) was used as the reference wave to read out the
holographic pattern recorded in the film.
Figure 8 gives the original picture and the image read out
by the reference wave. It can be seen that the panda image can
be vividly recorded in the hologram. The image is recorded by
the photoinduced volume birefringence to achieve the
polarization holographic recording. As surface modulation can
6
| J. Name., 2015, 00, 1-3
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Journal of Materials Chemistry C
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ARTICLE
DOI: 10.1039/C5TC02126J
also be induced on the film, the image becomes blurred after films of CAꢀChol and CNꢀChol exist in the amorphous state
irradiation for a longer time. As both processes possess very and only NTꢀChol film shows weak birefringence and
different time scales, where the polarization grating formation polydomain texture. At the room temperature, the amorphous
is completed in a much shorter time period, the polarization molecular glass films show a very sensitive response to the
holographic recording can be achieved by controlling the light irradiation as exhibited by the photoinduced birefringence,
writing time. This problem can also be solved by sandwiching dichroism and SRG formation. The birefringence and the
[
41]
the thin film in twoꢀpieces of the glass slides.
orientation order parameters of the CNꢀChol and CAꢀChol
films are much higher than those of typical azo polymers and
NTꢀChol. When the irradiating light is switched off, no
orientation relaxation occurs and the birefringence even
Conclusions
A series of liquidꢀcrystalline compounds containing strong increases slightly. The SRGs can be formed on CNꢀChol and
push pull azo chromophores and cholesteryl unit (RꢀChol, R: CAꢀChol films with the high efficiency. Twoꢀdimensional (2D)
ꢀ
CA, CN and NT) was developed as novel molecular glasses. quasiꢀcrystal structures with 10ꢀ and 12ꢀ rotation symmetries
The azo compounds exist as glassy solid at room temperature, are successfully fabricated on the CNꢀChol films by using the
whose phases of the condensed states depend on molecular dualꢀbeam multiple exposure technique. By using one of the
structures and processing conditions. In the thermodynamic molecular glasses, CAꢀChol, the application of the materials as
equilibrium state, the molecular materials show smectic liquid holographic medium is demonstrated.
crystallinity above their glass transition temperature. On the
other hand, by using a volatile solvent THF, the spinꢀcoated
Figure 7. The 12-fold quasi-crystal produced with the Λ = 6.2 μm interference pattern on the film of CN-Chol: (a) AFM 2D-view image, (b) AFM 3D-view image, (c)
optical micrograph in transmittance mode, (d) optical micrograph in refractive mode, (e) photograph of the He-Ne laser diffraction pattern.
Figure 8. The original panda image and the image read out from hologram by the reference wave.
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DOI: 10.1039/C5TC02126J
Journal Name
RSCPublishing
ARTICLE
Acknowledgements
The financial support from NSFC under project 51233002 is
gratefully acknowledged.
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Journal of Materials Chemistry C
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DOI: 10.1039/C5TC02126J
Graphical Abstract
Liquid-Crystalline Compounds Containing Both Strong Push-Pull
Azo Chromophore and Cholesteryl Unit as Photoresponsive
Molecular Glass Materials
a
b
a
a
a
Renbo Wei, Zeda Xu , Xiyang Liu, Yaning He, and Xiaogong Wang *
a
Department of Chemical Engineering, Laboratory of Advanced Materials
(
MOE), Tsinghua University, Beijing 100084, P. R. China.
b
Department of Physics, School of Science, Beijing University of Aeronautic and
Astronautic, Beijing 100083, P. R. China