Novel photo-polymerizable chiral hydrogen-bonded self-assembled complexes: Preparation, characterization and the utilization as a thermal switching reflective color film

Article abstract of DOI:10.1039/c0jm03810e

In this study, novel photopolymerizable chiral hydrogen-bonded self-assembled complexes (PCHSCs) were fabricated, which were derived from photopolymerizable 4-(6-acryloyloxyhexyloxy) benzoic acid (AHBA, proton donor) and chiral pyridine derivatives (proton acceptor). Their structures were characterized by fourier transform infrared (FT-IR) and the proton nuclear magnetic resonance (¹H-NMR) spectoscopy. The thermal stability, phase behaviors and helical twisted power (HTP) characteristics of the PCHSC were investigated by measuring the variable-temperature FT-IR spectrum, differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and the Cano wedge. The results demonstrate that all the PCHSCs have good thermal stability within a temperature range, and the pitch length of all the cells containing the PCHSCs increases with increasing temperature, which is due to the fact that the HTP values of all the PCHSCs decrease with an increase of temperature. What's more, the introduction of AHBA leads to chiral enhancement of the PCHSCs. Based on the above results, a polymer stabilized cholesteric liquid crystals (PSCLCs) composite with the above PCHSCs was prepared and the thermal-optical characteristics of the PSCLCs film were investigated. The results confirm that the reflective wavelength of the PSCLCs film before and after irradiation can be thermally switched to reflect green and red color from the initial state reflecting a blue/green color with the temperature increasing from 30 °C to 75 °C. On the basis of this mechanism, the novel material in this study can be used as optical/photonic paper, optical sensors and LCs displays, etc.

Full text of DOI:10.1039/c0jm03810e

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PAPER
Novel photo-polymerizable chiral hydrogen-bonded self-assembled complexes:
Preparation, characterization and the utilization as a thermal switching
reflective color film
*
Fengjin Chen, Jinbao Guo, Zhijian Qu and Jie Wei
*
Received 7th November 2010, Accepted 1st April 2011
DOI: 10.1039/c0jm03810e
In this study, novel photopolymerizable chiral hydrogen-bonded self-assembled complexes (PCHSCs)
were fabricated, which were derived from photopolymerizable 4-(6-acryloyloxyhexyloxy) benzoic acid
(AHBA, proton donor) and chiral pyridine derivatives (proton acceptor). Their structures were
characterized by fourier transform infrared (FT-IR) and the proton nuclear magnetic resonance (1H-
NMR) spectoscopy. The thermal stability, phase behaviors and helical twisted power (HTP)
characteristics of the PCHSC were investigated by measuring the variable-temperature FT-IR
spectrum, differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and the Cano
wedge. The results demonstrate that all the PCHSCs have good thermal stability within a temperature
range, and the pitch length of all the cells containing the PCHSCs increases with increasing
temperature, which is due to the fact that the HTP values of all the PCHSCs decrease with an increase
of temperature. What’s more, the introduction of AHBA leads to chiral enhancement of the PCHSCs.
Based on the above results, a polymer stabilized cholesteric liquid crystals (PSCLCs) composite with
the above PCHSCs was prepared and the thermal-optical characteristics of the PSCLCs film were
investigated. The results confirm that the reflective wavelength of the PSCLCs film before and after
irradiation can be thermally switched to reflect green and red color from the initial state reflecting
a blue/green color with the temperature increasing from 30 ꢀC to 75 ꢀC. On the basis of this mechanism,
the novel material in this study can be used as optical/photonic paper, optical sensors and LCs displays,
etc.
by changing external factors including electric, magnetic, and
acoustic fields, temperature, and light irradiation.8–19
1. Introduction
The design and development of novel responsive functional
materials, which are able to change their optical and electrical
properties due to changes in the environment, have become the
subject of significant interest in the scientific community.1–6
Cholesteric liquid crystals (Ch–LC) are self-organized materials
capable of forming a planar helical structure when placed
between glass plates coated with rubbed polyimide. Planar Ch–
LC is considered as a one-dimensional photonic band-gap (PBG)
material, which can reflect light with the same sense of polari-
zation as the cholesteric helix. The selective reflection band
(SRB) is centered at the wavelength l ¼ n$P. The width of the
band is Dl ¼ (Dn/n) l, where P is the pitch of the chiral liquid
crystal,7 and n ¼ (ne + no)/2 is the average refractive index of the
cholesteric planes with a birefringence of Dn ¼ ne-no, where ne
and no are the extraordinary and ordinary indices of refraction,
respectively. Ch–LC has the ability to shift the selective reflection
As is well known, a new design of molecules and self-assem-
bled structures from nano to the macro scale play an important
role in improving the functional capabilities of LCs. The concept
of supramolecular chemistry has broadened our horizon in the
design of LCs.20 Supramolecular liquid-crystalline phases with
well-defined structures have been formed by exploiting inter-
molecular hydrogen-bonds (H-bond) since 1989.21,22 Hydrogen-
bonded(H-bonded) thermotropic liquid crystals and supramo-
lecular structures are promising candidates for a variety of
sensors because they are readily fabricated and have the potential
to form a variety of structures with a broad range of new
properties. H-bonding is achieved through the complexation of
two or more molecules containing proton donor and proton
acceptor groups. With H-bonded cholesteric polymer films
containing donor/acceptor side groups, one can expect much
higher sensitivity of the helical supramolecular structure toward
molecules disrupting H-bonds.23–26 A responsive H-bonded
cholesteric polymer film was fabricated by P.V. Shibaev et al.,
which was prepared by photo-polymerizing a H-bonded Ch–LC
composite. The film exhibits optically pumped lasing at various
College of Materials Science and Engineering, Beijing University of
Chemical Technology, Beijing, 100029, P. R. China. E-mail: guojb@
mail.buct.edu.cn; weij@mail.buct.edu.cn
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wavelengths, depending on the SRB position determined by the
pH of the chemical environment, as well as color changing and
SRB shifting responding to the presence of the basic amino acids
in the surrounding aqueous solutions. This phenomenon was
explained as the H-bonds being easily ‘‘turned off’’ and/or
replaced by ion bonding in different pH solutions.27,28 A ther-
mally controllable reflective color paper was fabricated by Yang
group, which is a Ch–LC composite containing a novel
hydrogen-chiral dopant (HCD). The reflective color of Ch–LC
film changes from red to blue, which is due to the HCD breaking
2.2 Synthesis of the H-bonded chiral proton acceptors
Thionyl chloride (50 ml) was added carefully to a stirred mixture
of isonicotinic acid (10.44 g, 0.085 mol) and a few drops of N,
N-dimethylformamide (DMF). A lively gas evolution started,
and all the acid had gone into solution after 1 h. The solution was
continuously stirred at room temperature for 24 h. The final
reaction mixture turned out to be slightly green and was heated
under reflux for a further 1.5 h. Excess thionyl chloride was
removed in vacuo, and ethyl acetate (50 ml) was stirred into the
residue. The crude product was filtered, washed with ethyl
acetate and dried in vacuo at 40 ꢀC. Yield: 14.96 g (99%) of white
crystals sufficiently pure for the preparation of the activated
esters. To a stirred suspension of isonicotinoylchloride hydro-
chloride (14.96 g; 0.084 mol) in THF (100 ml), triethylamine
(40 ml, 0.028 mol) was added slowly. Over 0.5 h a solution of
isosorbide (4.9 g; 0.034 mol), Isomannide (4.9 g, 0.034 mol),
S-(+)-1, 2-propanediol (2.57 g, 0.034 mol) or R-(ꢁ)-1, 2-pro-
panediol (2.55 g, 0.034 mol) dissolved in THF (50 ml) was added
dropwise into the above suspension. The mixture was continu-
ously stirred at room temperature for 48 h, filtered and concen-
trated in vacuo. The residue was washed with distilled water
several times and crystallized in ethyl acetate to give a white solid
powder of slightly yellow color. Fig. 2(a) shows the synthesis
route of the H-bonded chiral acceptors.
into two chiral dopants at
a
certain temperature.29 We
reported that a thermally controllable SRBs of Ch–LCs
with a double-handed circularly polarized light reflection
band, was achieved by the introduction of an H-bonded
Ch–LC into the polymer template in
Ch–LC film.30
a
single-layer
In this paper, we fabricated four novel PCHSCs, in which
AHBA is the proton donor and chiral pyridine derivative is the
chiral proton acceptor. The thermal stability properties of the
1
PCHSCs were investigated by FT-IR, H-NMR spectroscopy,
DSC, POM analysis and the Cano wedge method. Based on the
above results, a PSCLCs composite with the above PCHSCs was
prepared and the thermal-optical characteristics of the PSCLCs
film were investigated. The main practical purpose of this work is
to develop a kind of thermal switching, reflective color film or
paper with the PCHSCs.
ISIN (isosorbide isonicotinate, 10 g, 50% yield),1H-NMR
(25 ꢀC, 600 MHz, CDCl3) d (ppm), 4.09 (q, 4H, –CH2–), 4.69
(d, 1H, –CH–), 5.08 (t, 1H, –CH–), 5.44 (m, 1H, –CH–), 5.51
(s, 1H, –CH–), 7.82 (d, 2H, Py-H, Py ¼ pyridyl), 7.89 (d, 2H,
Py-H), 8.80 (q, 4H, Py-H). FT-IR (cmꢁ1, KBr pellet), 2936, 2889
(–CH2–), 1735(C]O stretching), 1599, 1562, 1467, 1410 (pyridyl
C]N, C]C), 1275, 1126 (isonicotinate C–O–C), 1346, 1325,
1094, 1063, 1018, 969, 849, 758, 705, 674.
2. Experiments section
2.1 Materials
1
ꢀ
IMIN (isomannide isonicotinate, 10 g, 50% yield), H-NMR
The nematic LC, SLC-1717 (isotropic temperature, Ti ¼ 92 C;
(25 ꢀC, 600 MHz, CDCl3) d (ppm), 4.00 (q, 2H, CH2-), 4.13
(q, 2H, –CH2–), 4.89 (q, 2H, –CH–), 5.35 (d,2H, –CH–), 7.89
(d,4H, Py-H), 8.80 (d,4H, Py-H). FT-IR (cmꢁ1, KBr pellet), 2969,
2864 (–CH2–), 1738 (C]O stretching), 1654, 1599, 1563, 1457,
1412 (pyridyl C]N, C]C), 1278, 1121 (isonicotinate C–O–C),
1361, 1331, 1057, 1033, 1007, 895, 847, 756, 707,678.
20 ꢀC, 589nm, Dn ¼ 0.201; Slichem Liquid Crystal Material Co.,
Ltd.), and the photo-initiator, 2, 2-dimethopxy-1, 2-diphenyle-
hanone (Irgacure651, TCI Co., Ltd) were used. The proton
donor AHBA and nematic diacrylate monomer C6M were
synthesized according to the method suggested by Broer et al.31
The chiral liquid crystal monomer CLCM was purchased from
BASF Co., Ltd. Fig. 1 shows the chemical structures of CLCM,
C6M, AHBA and Irgacure651.
SPIN (S-(+)-1, 2-propanediol isonicotinate, 7.18 g, 41% yield),
1H-NMR (25 ꢀC, 600 MHz, CDCl3) d (ppm), 1.50 (d, 3H, –CH3–),
Fig. 1 The chemical structures of CLCM, C6M, AHBA and Irgacure651.
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Fig. 2 (a) The synthesis route of the H-bonded chiral proton acceptors; (b) the chemical structures of the PCHSCs.
4.51 (q, 1H, –CH2–), 4.60 (q, 1H, –CH2), 5.58 (m, 1H, –CH–), 7.83
(m, 4H, Py-H), 8.78 (q, 4H, Py-H). FT-IR (cmꢁ1, KBr pellet),
3051, 3025, 2995, 2945 (–CH2–, –CH3), 1729 (C]O streching),
1598, 1562, 1452, 1410 (pyridyl C]N, C]C) 1271, 1124 (iso-
nicotinate C–O–C), 1329, 1065, 999, 934, 853, 757, 707, 684.
RPIN (R-(-)-1, 2-propanediol isonicotinate, 11.56 g, 66% yield),
1H-NMR (25 ꢀC, 600 MHz, CDCl3) d (ppm), 1.50 (d, 3H, –CH3–),
4.51 (q, 1H, –CH2–), 4.60 (q, 1H, –CH2), 5.58 (m, 1H, –CH), 7.85
(q, 4H, Py-H), 8.79 (t, 4H, Py-H). FT-IR (cmꢁ1, KBr pellet), 3065,
3028, 2994, 2965 (–CH2–, –CH3), 1729 (C]O stretching), 1646,
1561, 1457, 1410 (pyridyl C]N, C]C), 1272, 1125 (isonicotinate
C–O–C), 1329, 1064, 999, 929, 852, 757, 707.
2.5 Fabrication of the thermal switching reflective color film
In order to induce planar orientation of LC molecules, the inner
surfaces of glass cells were coated with a 3.0 wt% polyvinyl
alcohol (PVA) aqueous solution. The deposited film was dried at
80 ꢀC for 30 min and subsequently rubbed with a textile
cloth under a pressure of 2.0 g cmꢁ2 in one direction. PET
(polyethylene terephthalate) films of 10–20 mm thickness were
used as the cell spacers. The samples were filled into the cells
by capillary action at an appropriate temperature. To induce
photo-polymerization, the cells containing the PSCLC
composite were irradiated for 5 min with UV light (5.0 mW cmꢁ2
,
365 nm).
2.3 Characterization of the H-bonded chiral proton acceptor
2.6 Measurements
The synthesized H-bonded chiral acceptors were characterized
by FT-IR and 1H-NMR spectroscopy. FT-IR spectra were
recorded on a Nicolet 5700 spectrometer frequencies ranging
The PCHSCs are characterized by FT-IR spectroscopy and
variable-temperature FT-IR spectroscopy. The FT-IR spectra
was recorded on a Nicolet 5700 spectrometer at frequencies
ranging from 400–4000 cmꢁ1. The samples were observed by
POM (Leica, DM2500P) with a hot stage calibrated with an
accuracy of ꢂ0.1 ꢀC (Linkam, THMS-600). DSC analysis was
carried out using a Pyris Diamond DSC instrument with a heat-
ing and cooling rate of 10 ꢀC minꢁ1 under a dry nitrogen purge.
The reflection spectra were examined with AvaSpec-USB2
version 7.3 fiber spectrophotometer in reflection mode. The
morphology of the polymer network was observed by scanning
electron microscopy (SEM, Hitachi, S-4700). The samples for
SEM studies were prepared according to the method described
therein.32
from 400–4000 cmꢁ1. The H-NMR spectroscopy experiments
1
were performed at 600 MHz on a German Bruker AV600 spec-
trometer while CDCl3 was used as the deuterated solvent for the
compounds.
2.4 Preparation of the PCHSCs
PCHSCs were obtained by mixing the proton donor AHBA with
the proton acceptors ISIN, IMIN, SPIN and RPIN in
a proportion of 2 : 1 in THF, followed by slow evaporation.
Fig. 2(b) shows the chemical structures of PCHSCs including
ISIN–AHBA, IMIN–AHBA, SPIN–AHBA and RPIN–AHBA.
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associated with –OH stretching of the H-bond of the chiral
pyridine compound ISIN, the PCHSC of ISIN-AHBA exhibits
two new peaks at 2484.5 and 1888.5 cmꢁ1 as shown in Fig. 3(a).
The position of these peaks almost coincides with the position of
–OH stretching of H-bond between the –OH and pyridyl group
(2500 and 1900 cmꢁ1).22 A significant decrease in the intensity of
peaks at 2563 and 2673 cmꢁ1 means that the H-bonds between
carboxylic acid and pyridyl groups successfully replaced that of
AHBA dimmers. A similar phenomenon can also be observed
from the PCHSCs of SPIN-AHBA and RPIN-AHBA as shown
in Fig. 3(c) and (d). However H-bonds between carboxylic acid
and pyridyl groups can’t be seen clearly in IMIN-AHBA as
shown in Fig. 3(b) through the same fabrication process.
However, when IMIN-AHBA was heated to the melting point
and then cooled to the room temperature, two peaks centered at
2535 and 1925 cmꢁ1 appear as shown in Fig. 4(b). It suggests that
the thermal effect plays an important role in the fabrication of
the H-bonds between IMIN and AHBA.
3. Results and discussions
3.1 Characterization of PCHSCs
PCHSCs were obtained by mixing the proton donor and the
proton acceptors in THF, followed by removal of the solvent.
During the self-assembly process, there are two strong H-bond
self-assembly processes, the –O–H/N from ISIN-AHBA,
IMIN-AHBA, SPIN-AHBA or RPIN-AHBA and the –O–H/
O]C– from AHBA dimers. Owing to the H bonds of –O–H/N
being relatively stronger than those of –O–H/O]C–,22 in
a proportion of ISIN(IMIN, SPIN or RPIN)/AHBA ¼ 1/2, the
chiral pyridine derivative molecules should easily combine with
AHBA molecules prior to the formation of carboxylic acid
dimers. Infrared spectrometry is a powerful method to distin-
guish the existence and intensity of H-bonding. Therefore, in this
work, the formation and thermal stability of the H-bonds were
mainly studied by FT-IR spectroscopy.
Fig. 3 shows the FT-IR spectra of the PCHSCs and the
precursors. The formation of H-bonds was detected through the
modification of the –OH stretching band of the AHBA. Before
self-assembly, the AHBA has peaks at 2850–2980 cmꢁ1 associ-
ated with –CH2- stretching and two peaks at 2563 and 2673 cmꢁ1
The thermal property and liquid cystallinity of the precursors
and the PCHSCs were studied by DSC and POM and the results
were summarized in Table 1. SPIN and RPIN are the right and
left hand enantiomers, respectively, and so they have almost the
same melting temperatures, 57.38 ꢀC and 57.04 ꢀC, respectively.
ISIN and IMIN are diastereomers of right hand. Since the dia-
stereomers possess different physical properties, the melting
temperature of IMIN (164.58 ꢀC) is much higher than that of
ISIN (138.98 ꢀC). This is also the reason why the melting
temperature of IMIN-AHBA is higher than that of ISIN-AHBA.
The DSC measurements show that the four H-bonded chiral
proton acceptors are crystalline materials with a single melting
temperature. The phase transition temperature for AHBA is
ꢀ
96.05 C, 112.08 ꢀC and 138.36 ꢀC corresponding to Cr-SmA,
SmA-N and N-I, respectively. After the complexion with ISIN,
IMIN, SPIN, and RPIN, no clear liquid crystalline phases were
observed in the PCHSCs by DSC and POM. All the four
PCHSCs exhibit crystallinity characteristics with a melting
ꢀ
ꢀ
temperature of 87.25 C for ISIN-AHBA, 90.58 C for IMIN-
AHBA, 83.00 ꢀC for SPIN-AHBA and 82.56 ꢀC for RPIN-
AHBA, respectively. We believe that the heterotopic faces of the
isosorbide chiral center and the flexible propanediol chiral
center, located in the middle of the PCHSCs molecule, strongly
influence the molecular linearity as a result of no clear liquid
crystalline phases. However the complexes exhibit a single
melting point that is quite different from those of their individual
components, which is attributed to the formation of H-bonds
between the AHBA and pyridine derivatives.
The thermal stability of the PCHSCs was studied by the
variable-temperature FT-IR spectroscopy as shown in Fig. 4,
where the stability of the H-bonds of the PCHSCs can be directly
confirmed. When the temperature increases above the melting
point, the two H-bonded characteristic peaks of the four
PCHSCs at about 2500 and 1900 cmꢁ1 still exist, which indicates
the hydrogen bonds between carboxylic acid and pyridyl groups
are fairly stable as shown in Fig. 4(a) and (b).33 Additionally, the
intensity of these two peaks decreases with the increasing
temperature, which suggests that the H-bond interaction
between the proton donor and acceptor becomes weaker as an
effect of heat. However, on the whole, the PCHSCs possess good
thermal stability in the measured temperature range.
Fig. 3 FT-IR spectra of the PCHSCs and the precursors (a) ISIN-
AHBA, ISIN and AHBA; (b) IMIN-AHBA, IMIN and AHBA; (c)
SPIN-AHBA, SPIN and AHBA; (d) RPIN-AHBA, RPIN and AHBA.
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Fig. 4 Variable-temperature FT-IR spectra of the PCHSCs (a) ISIN-AHBA; (b) IMIN-AHBA; (c) SPIN-AHBA; (d) RPIN-AHBA.
Table 1 Phase transition temperature of the precursors and the
PCHSCsa
the samples and the corresponding POM micrographs taken in
the Cano wedge at a certain temperature. It can be seen that the
pitch length of the samples increase with increasing temperature,
as well as the visible wider bright-dark stripes, which is due to the
HTP of the PCHSCs decrease with increasing temperature.
However, PCHSCs with different types of stereoisomer exhibit
quite different HTP characteristics. ISIN & IMIN are diaste-
reomers and are both right hand chiral molecules. Therefore, the
HTP of ISIN-AHBA and IMIN-AHBA have almost the same
variation tendency, as shown in Fig. 5(a) and (b). SPIN & RPIN
are enantiomers and SPIN is right handed while RPIN is left
handed. RPIN-AHBA has a bigger original pitch length
(26.0 mm) and shows greater sensitivity to temperature than
SPIN-AHBA (8.1 mm). We believe that two compound that are
enantiomers of each other possess different optical activity. It
can be concluded that the different type of stereoisomers result in
quite different HTP properties.
Phase
transition temperature
Compound
ISIN
IMIN
SPIN
RPIN
AHBA
ISIN-AHBA
IMIN-AHBA
SPIN-AHBA
RPIN-AHBA
Cr 138.98 ꢀC I
Cr 164.58 ꢀC I
Cr 57.38 ꢀC I
Cr 57.04 ꢀC I
Cr 96.05 ꢀC SmA 112.08 ꢀC N 138.36 ꢀC I
Cr 87.25 ꢀC I
Cr 90.58 ꢀC I
Cr 83.00 ꢀC I
Cr 82.56 ꢀC I
a
Note: Cr-crytal; SmA- smectic A; N-nematic; I-isotropic.
The HTP and the correlative influencing factor are important
parameters which decide the SRB and the responsiveness of the
chiral nematic liquid crystals. Fig. 5 and 6 shows the HTP
measurement of the PCHSCs and the precursors.
To make sure that the H-bonds really contributes to the
original HTP and the HTP temperature dependence, the HTP
properties of the precursor ISIN and RPIN were also studied as
shown in Fig. 6(a) and (b). Two samples were prepared in the
same method as described above. The corresponding weight
ratios are ISIN: SLC1717 ¼ 1 : 99 and RPIN: SLC1717 ¼ 7 : 93,
respectively. It can be seen that the original pitch length of ISIN
(20.1 mm) is greater than that of ISIN-AHBA (12.1 mm) with the
same percentage adding to SLC 1717 (the mol ratio of ISIN to
ISIN-AHBA is 2.64) as shown in Fig. 5(a) and 6(a). According to
HTP ¼ 1/P$C, in which P is the value of the pitch length, and C is
the concentration of the PCHSCs, the ISIN-AHBA has greater
HTP than ISIN. This result can also be concluded from the
In order to study the HTP’s temperature dependence of the
PCHSCs, four samples were prepared by mixing the PCHSCs
with SLC1717 in CH2Cl2, followed by the removal of solvent, the
weight ratios of which are corresponding to ISIN-AHBA:
SLC1717 ¼ 1 : 99, IMIN-AHBA: SLC1717 ¼ 1 : 99, SPIN-
AHBA: SLC1717 ¼ 7 : 93, and RPIN-AHBA: SLC1717 ¼ 7 : 93
(these weight ratios show the actual concentration, i.e., 1% of
ISIN-AHBA, 99% of SLC1717, etc.), respectively. The HTP
value is measured according to the Cano wedge method.34 Fig. 5
(a)–(d) show the temperature dependence of the pitch length of
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of RPIN would be much greater than that of RPIN-AHBA.
Thus the RPIN-AHBA also has greater HTP than RPIN. This
phenomenon can be demonstrated by the molecular model, as
shown in Fig. 7, which is a stereoscopic numbering scheme of
ISIN-AHBA and RPIN-AHBA. By self-assembly of the
H-bonds of the H-bonded chiral proton acceptor with AHBA,
benzene rings of AHBA are introduced to the two pyridine ends
of ISIN or RPIN. The relatively strong carboxylic acid/pyridine
interaction leads to the formation of a stable conjugation
structure on the two sides of the chiral center in PCHSCs. It is
possible that the stable conjugation structure leads to the
enhancement of chiral center as a result of greater HTP. The
result indicates the introduction of aromatic acid could lead to
self-assembled supramolecular architectures with perceptible
enhancements of chiral properties.
On the other hand, the pitch length of ISIN decreases while the
pitch length of ISIN-AHBA increases with increasing tempera-
ture, which confirms that the H-bond between ISIN and AHBA
are successfully fabricated. This can also be suggested by the
PCHSCs of RPIN-AHBA. From 30 ꢀC to 60 ꢀC, the pitch length
of RPIN-AHBA shows a much greater increase from 25.0 mm to
50.0 mm compared to that of RPIN which increases from 19.3 mm
to 25.0 mm, as shown in Fig. 5(d) and 6(b). Based on the above
two experimental facts, the chiral enhancement phenomenon and
the different variation tendency between the PCHSCs and the
precursors, it can be concluded that the H-bonds between the
carboxylic acid and pyridyl group actually play a very important
role in the original HTP and the HTP’s temperature dependence
in the PCHSCs.
Fig. 5 Temperature dependence of the pitch length of the samples with
PCHSCs and the corresponding POM micrographs taken in the Cano
wedges at a certain temperature defined on each graph (a) ISIN-AHBA;
(b) IMIN-AHBA; (c) SPIN-AHBA; (d) RPIN-AHBA.
3.2 The application of the PCHSCs in thermal switching
reflective color paper
Based on the above characteristics of the PCHSCs, polymer
stabilized cholesteric liquid crystal (PSCLC) composites were
prepared and the thermal-optical characteristics of the PSCLCs
films were investigated. The PSCLC composites were prepared
by mixing 12% of ISIN-AHBA, 6.6% of CLCM, 1% of C6M,
80% of SLC1717 and 0.4% of Irgacure651 in CH2Cl2, followed
by removal of the solvent. CLCM was used to adjust the
reflective wavelength of the PSCLC film to the visible region. It
should be noted that the pitch length of the CLCM increases
a little with increasing temperature, which contributes a little to
the temperature sensitivity of PSCLC composite. The PSCLC
composite was placed into the planar treated cell, which was
called cell A. Fig. 8 represents the temperature dependence of the
reflection spectra of cell A before polymerization. The reflection
wavelength exhibits an obvious red shift from 450 nm to 640 nm
when thꢀe temperature was incrementally increasing from 30.0 ꢀC
to 75.0 C. The color cell A reflects changes from blue to green
and finally to red. This result is in agreement with the above HTP
test. The insets in Fig. 8 are the photographs of cell A taken at
different temperatures.
Fig. 6 Temperature dependence of the pitch length of ISIN and RPIN,
and the corresponding POM micrographs taken in the Cano wedges at
a certain temperature defined on each graph (a) ISIN; (b) RPIN.
On the basis of the fruitful color changes of the PSCLCs film
before polymerization, cell A was irradiated with UV light
(5.0 mW cmꢁ2, 365 nm) at 30 ꢀC for about 5 min and the
reflection spectra were shown in Fig. 9. Increasing the tempera-
ture from 30 ꢀC to 75 ꢀC, the reflection wavelength of cell A after
polymerization shows a red shift from 575 nm to 650 nm and the
PCHSCs of RPIN-AHBA. With the same percentage adding to
SLC 1717(the mol ratio of RPIN to RPIN-AHBA is 3.04), the
original pitch length of RPIN is 19.3 mm compared with that of
RPIN-AHBA (25.1 mm) as shown in Fig. 5(d) and 6(b). If the
same mol ratio is added into SLC1717, the original pitch length
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Fig. 7 Stereoscopic numbering scheme of ISIN-AHBA and RPIN-AHBA; the grey, red, and white balls indicate carbon, oxygen, and hydrogen atoms;
the red dashes are hydrogen bonds.
Fig. 8 Temperature dependence of the reflection spectra of cell A; the
Fig. 9 Temperature dependence of the reflection spectra of cell A before
polymerization at 30 ꢀC (curve 1), after polymerization at 30 ꢀC (curve 2)
and at 75 ꢀC (curve 3); the insets are the corresponding photographs.
insets are the photographs of the cell A taken at different temperatures.
respective color changes from green to red, which is due to the
HTP of ISIN-AHBA decreasing with increasing temperature.
The obvious red shift of reflection band from 460 nm to 560 nm
of the PSCLCs film before and after polymerization was attrib-
uted to the polymerization conditions and the special H-bond
Ch–LC system containing ISIN-AHBA and CLCM, while the
broadened distribution of the reflection wavelength is derived
from the density inhomogeneity of the polymer network, as
described in previous studies.35–37 The insets in Fig. 9 are the
photographs of cell A taken at 30 ꢀC before (blue) and after
polymerization (green), as well as cell A at 75 ꢀC after poly-
merization (red). It can be concluded that the PSCLC film after
polymerization also possesses good temperature dependence. It
is noteworthy that the reflection intensity of the PSCLC film
before or after polymerization decreases when the cell is heated
up to 75 ꢀC, which is because this temperature is close to the clear
point of the PSCLC composite.
addressed in the green background of the PSCLCs film. Fig. 10
(a1) and (a2) represent the photographs of the PSCLC film after
polymerization and the character T written by a hot pen on the
film at 75 ꢀC. It is obvious that T is as clear as we can see. Fig. 10
(b1) and (b2) represent the POM micrographs of the textures in
different regions of the PSCLC film after polymerization. It can
be seen that the thermally addressed regions and the ones far
away from them are both planar textures. Fig. 10(c1) and (c2)
show the SEM of the polymer networks formed in the film at
magnifications of 5000 and 50000, it is clear that an obvious
polymer grain can be observed. The polymer network formed in
the PSCLC film plays an important role in the resolution of the
letter T. This is because the polymer network helps restrict the
heating conduction. Additionally, it has been demonstrated that
the state of information written can stay stable for about 3 h at
room temperature without any external field. Then the red T
automatically disappeared and recovered to the original state as
a result of strengthened H-bond interactions. This indicates that
According to the thermal-optical characteristics of the PSCLC
film after polymerization, a red character T could be thermally
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temperature without any external field and then automatically
disappeared and recovered to the original state as a result of
strengthened H-bond interactions. The technique developed in
this study has great applications such in optical/photonic paper,
optical sensors, LCs displays, etc.
Acknowledgements
This work is financially supported by the National Natural
Science Foundations (Grant 50903004).
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In summary, we fabricated four novel PCHSCs, which are
named ISIN-AHBA, IMIN-AHBA, SPIN-AHBA and RPIN-
AHBA. The measurements via variable-temperature FT-IR
spectroscopy suggested that all the PCHSCs have good thermal
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