Helical columnar liquid crystals based on dendritic peptides substituted perylene bisimides

Article abstract of DOI:10.1039/c1jm13144c

New columnar liquid crystals based on perylene bisimides (PBIs) were synthesized by introducing dendritic peptides onto the two ends of the imide. The self-assembly behaviour of PBIs in solution and in film was investigated through temperature-dependent and solvent-dependent ultraviolet-visible absorption spectroscopy. The mesomorphic behaviour was studied by polarizing optical microscopy, differential scanning calorimetry, small-angle X-ray diffraction and temperature-dependent Fourier transform infrared spectroscopy. Ordered columnar mesophases were observed for PBIs at room temperature. Circular dichroism studies demonstrate that these materials exhibit supramolecular optical activity in the liquid crystalline state, which is attributed to helical columnar organization. The hydrogen bonds at the periphery of PBIs result in the ordered columnar mesophases structure with small inter-disk distances of 3.4 A

Full text of DOI:10.1039/c1jm13144c

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PAPER
Helical columnar liquid crystals based on dendritic peptides substituted
perylene bisimides†
ab
Defang Xia,a Licui Zhang,a Qianqian Bai,a Libin Bai,ab Tao Yangab and Xinwu Baab
*
Baoxiang Gao,
Received 7th July 2011, Accepted 4th August 2011
DOI: 10.1039/c1jm13144c
New columnar liquid crystals based on perylene bisimides (PBIs) were synthesized by introducing
dendritic peptides onto the two ends of the imide. The self-assembly behaviour of PBIs in solution and
in film was investigated through temperature-dependent and solvent-dependent ultraviolet-visible
absorption spectroscopy. The mesomorphic behaviour was studied by polarizing optical microscopy,
differential scanning calorimetry, small-angle X-ray diffraction and temperature-dependent Fourier
transform infrared spectroscopy. Ordered columnar mesophases were observed for PBIs at room
temperature. Circular dichroism studies demonstrate that these materials exhibit supramolecular
optical activity in the liquid crystalline state, which is attributed to helical columnar organization. The
hydrogen bonds at the periphery of PBIs result in the ordered columnar mesophases structure with
ꢀ
small inter-disk distances of 3.4 A
substitution.12 Jin and co-workers reported crystalline p stacks in the
Introduction
highly ordered columnar PBIs smectic phase.13 Recently, Thelakkat
reported on columnar liquid crystalline PBIs with swallow-tail side-
chains.14 Despite the great interest in columnar liquid crystals with
helical superstructures, only one PBI derivative with chiral side
chains exhibits chiral columnar liquid crystalline structure.15
The unique structural and electronic properties of columnar liquid
crystal materials have attracted considerable attention because of
their potential applications in devices, such as field-effect transistors,
electroluminescent displays, and photovoltaic cells.1 The cofacially
p-stacked structures of the disk-like aromatics in the columnar liquid
crystal phases are widely recognized as an efficient pathway to
promoting one-dimensional electrical conduction.2,3 In particular,
various examples of columnar liquid crystal molecules, such as tri-
phenylenes,4 m-phenylene ethynylene oligomers,5 hexabenzocor-
onenes,6 triindoles,7 and phthalocyanines,8 form pstacks with helical
superstructures that exhibit increased charge-carrier mobilities as
a result of the higher molecular order. Most semiconducting discotics
are good hole-transporting materials (p-type), whereas only a limited
number of good electron-transporting materials (n-type) were
reported.9 Perylene-3,4:9,10-tetracarboxydiimides (PBIs), have
attracted interest as electron-transporting materials with high elec-
tron mobilities.10 However, most of the thermotropic liquid crystal-
line PBI derivatives reported in literature are not columnar liquid
crystals.11 Columnar liquid crystal PBI derivatives are difficult to
design and synthesize because of their low symmetry. The pioneering
Dendrimers and dendrons prove to be particularly versatile
candidates as novel and original scaffoldings for the synthesis of
new liquid crystalline materials.16 These tapered dendritic
molecules aggregate into infinite supramolecular columns with
a polar interior, these columns, which are separated from each
other by the aliphatic medium, self-organize into rectangular or
hexagonal lattices.17 Furthermore, Kato and co-workers showed
that dendritic oligopeptides are useful building blocks for the
synthesis of supramolecular chiral columnar liquid crystals.18
In the present study, novel dendritic peptides substituted
perylene bisimides (DPPBIs) capable of forming liquid crystal-
line assembly are reported. In particular, DPPBIs can form an
ordered hexagonal columnar liquid crystals over
a wide
temperature range, including room temperature. The chiral
peptides induce PBIs to form a helical superstructure. The
addition of hydrogen-bonding motifs to the periphery of PBIs is
carried out in order to study the effect of hydrogen bonding on
the already pronounced columnar stacking.
€
work of Wurthner and co-workers involved columnar hexagonal
PBIs with tridodecylphenyl substituents, as well as perylene bay
aCollege of Chemistry and Environmental Science, Hebei University,
Baoding, 071002, P. R. China. E-mail: bxgao@hbu.edu.cn; Fax: +86
3125079317
Experimental
bKey Laboratory of Medicinal Chemistry and Molecular Diagnosis,
Ministry of Education, Hebei University, Baoding, 071002, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1jm13144c
General information
Perylenetetracarboxylic acid dianhydride PTCDA was
purchased fromLiaoning Liangang Pigment and Dyestuff
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Chmeicals Co. Ltd. L-Aspartic acid and N-carbobenzyloxy-
L-aspartic acids were purchased from Yangzhou Baosheng Bio-
Chemical Co. Ltd. N-(3-Dimethylaminopropyl)-N0-ethyl-
carbodiimide hydrochloride (EDCI), 4-dimethylaminopyridine
and 1-hydroxybenzotrizole were purchased from Shanghai
Medpep Co. Ltd. Imidazole, and solvents were purchased from
Sinopharm Chemical Reagent Co.Ltd, and used without any
further purification. Solvents used for precipitation and column
chromatography were distilled under normal atmosphere. The
as a white solid (3.13 g, 85%).1H-NMR (400 Hz, CDCl3): d (ppm)
4.14–4.07 (m, 2H), 3.81–3.79(m, 1H), 2.81–2.67 (m, 2 H), 1.63–
1.60 (m, 4H), 1.30–1.26 (m, 36H), 0.88 (t, J ¼ 7.1 Hz, 6H). 13C-
NMR (150 MHz, CDCl3): 174.28, 171.26, 65.46, 65.00, 51.30,
39.02, 29.64, 29.57, 29.51, 29.34, 29.24, 28.55, 25.89, 25.86, 22.68,
14.09. Anal. calcd for C28H55NO4: C, 71.59; H, 11.80; N, 2.98.
found: C, 71.90; H, 11.52; N, 2.71. m/z [MALDI-TOF]: 470.4
(M + H+).
1H NMR spectra were recorded at 20 C on a 400 or 600 MHz
Compound 6 was synthesized according to the reported proce-
dures. Into a 50 mL Schlenk flask were charged L-aspartic acid
1.60 g (12 mmol), 3,4:9,10-perylenetetracarboxyldianhydride
(PTCDA) 1.96 g (5 mmol), and imidazole (20 g). The mixture
was purged with argon for 15 min before being heated at 120 ꢀC
until the reaction mixture was completely soluble in water.
Subsequently, the reaction mixture was cooled to 90 ꢀC.
Deionized water was then added with the protection of argon.
The dark red solution was filtered to remove the trace amount of
unreacted PTCDA. The solution was then acidified with 2 M
HCl aqueous solution to a pH value of 5, the precipitate was
collected by suction-filtration and thoroughly washed with
deionized water until the filtrate was neutral; the red solid was
collected and dried at 75 ꢀC in vacuum oven until constant
weight. Analysis: compound 6 is too insoluble for NMR analysis.
Anal. calcd for C32H18N2O12: C, 61.74; H, 2.91; N, 4.50. found:
C, 62.05; H, 3.04; N, 4.17.
ꢀ
NMR spectrometer (Bruker). Chemical shifts are reported in
ppm at room temperature using CDCl3 as the solvent and tet-
ramethylsilane as an internal standard unless indicated other-
wise. Abbreviations used for splitting patterns are s ¼ singlet, d ¼
doublet, t ¼ triplet, qui ¼ quintet, m ¼ multiplet. Mass spectra
were carried out using MALDI-TOF/TOF matrix assisted laser
desorption ionization mass spectrometry with autoflexIII
smartbeam (Bruker Daltonics Inc). FTIR-spectra were recorded
with a Varian 640-IR in the range of 400–4000 cmꢁ1. UV/Vis
spectra were recorded with a Shimadzu WV-2550 spectropho-
tometer. Differential scanning calorimetry (DSC) was carried out
with a Perkin Elmer differential scanning calorimeter (DSC7)
with heating and cooling rates of 10 K minꢁ1. Phase transitions
were also examined by a polarization optical microscop (POM)
Olympus BX51 with a T95-PE temperature-controlled THMS-
600 hot stage. X-ray diffraction measurements were performed
ꢀ
on a D8 Advance (Bruker AXS Inc.) with Cu-Ka1: 1.54051 A.
Circular dichroism spectra were recorded on a Jasco J-810
spectropolarimeter, and the date of every sample is the average of
there times scan.
Compound SAPBI. Into a 10 ml Schlenk flask were charged
compound 3 (1.03 g, 2.2 mmol), PTCDA (0.39 g, 1.0 mmol) and
imidazole (8 g). The mixture was purged with argon for 15 min
before being heated to 120 ꢀC for 2 h. It was then allowed to cool
to room temperature before 20 mL deionized water was added.
The precipitate was collected by suction-filtration and thor-
oughly washed with deionized water and ethanol. The crude
product was purified by column chromatography on silica gel
with methylene chloride as the eluent to afford SAPBI as a red
Synthesis and characterization of PBIs
Compound 2. 2.67 g of N-carbobenzyloxyl-aspartic acid
(10.0 mmol) was dissolved in 60 mL of DCM, 4.79 g EDCI
ꢀ
(25.0 mmol) was added, and the mixture was stirred at 0 C for
1
30 min. 3.72 g 1-Dodecanol (20.0 mmol) and 0.25 g DMAP
(2.0 mmol) were added and the mixture stirred at room
temperature for 24 h. Solvent was removed by rotary evapora-
tion. The residue was purified by column chromatography on
silica gel with methylene chloride as the eluent to afford
solid (0.67 g, 52%). H-NMR (600 Hz, CDCl3) d (ppm) 8.72 (d,
J ¼ 7.9 Hz, 4H), 8.65 (d, J ¼ 8.1 Hz, 4H), 6.27 (t, J ¼ 7.2 Hz,
2 H), 4.22ꢁ4.04 (m, 8 H), 3.56 (dd, J ¼ 6.5 Hz, 2 H), 3.06 (dd, J ¼
7.6 Hz, 2 H), 1.62–1.55 (m, 8H), 1.25–1.11 (m, 72H), 0.89–0.83
(m, 12H). 13C NMR (150 MHz, CDCl3): 170.83, 170.72, 155.96,
136.23, 128.52, 128.17, 128.06, 67.04, 66.01, 65.25, 50.49, 36.71,
31.92, 29.64, 29.59, 29.52, 29.36, 29.24, 29.22, 28.52, 28.45, 25.85,
25.79, 22.69, 14.11. Anal. calcd for C80H114N2O12: C, 74.15; H,
8.87; N, 2.16. found: C, 74.50; H, 8.56; N, 1.97. m/z [MALDI-
TOF]: 1317.8 (M + Na+).
1
compound 2 as a white solid (5.32 g, 88%). H-NMR (300 Hz,
CDCl3): d (ppm) 7.36–7.30(m, 5H), 5.76 (d, J ¼ 8.5 Hz, 1H), 5.14
(s, 2H), 4.63 (dd, J ¼ 8.7 Hz, 1H), 4.13(d, J ¼ 3.6 Hz 2H), 4.06 (t,
J ¼ 6.8 Hz, 2H), 3.04–2.82 (m, 2H), 1.63–1.57 (m, 4H), 1.30–1.26
(m, 36H), 0.88 (t, J ¼ 7.1 Hz, 6H). 13C-NMR (150 MHz, CDCl3):
170.83, 170.72, 155.96, 136.23, 128.52, 128.17, 128.06, 67.04,
66.01, 65.25, 50.49, 36.71, 31.92, 29.64, 29.59, 29.52, 29.36, 29.24,
29.22, 28.52, 28.45, 25.85, 25.79, 22.69, 14.11. Anal. calcd for
C36H61NO6: C, 71.60; H, 10.18; N, 2.32. found: C, 71.83; H,
10.02; N, 2.01. m/z [MALDI-TOF]: 604.7 (MH+).
Compound DPPBI. 0.31 g of compound 6 (0.5 mmol) was
dissolved in 60 mL of DMF, 0.48 g EDCI(2.5 mmol) was added,
and the mixture was stirred at 0 ꢀC for 30min. 1.17 g of
compound 3 (2.5 mmol) and 0.38 g HOBt (2.5 mmol) were added
and the mixture stirred at room temperature for 24 h. Solvent
was removed by rotary evaporation. The crude product was
purified by column chromatography on silica gel with methylene
chloride: ethanol (100 : 1) as an eluent to afford DPPBI as a red
Compound 3. 4.86 g of compound 2 (8.0 mmol) dissolved in
110 mL ethanol and 0.50 g of 10% Pd/C catalyst was added. The
mixture was hydrogenated overnight in a Parr shaker at 2 psi H2.
The reaction mixture was then filtered and the solvent was
removed under reduced pressure to give a white solid. The crude
product was purified by column chromatography on silica gel
with methylene chloride: ethanol (100 : 1) as an eluent to afford 3
1
solid (1.04 g, 86%). H-NMR (600 Hz, CDCl3) d (ppm) 8.61 (s,
4H), 8.60(s, 4H), 8.40 (br, CONH, 4H), 6.7–614 (br, 2H), 4.90
(br,4H), 4.35–3.96 (m, 16H), 3.76–3.60 (m, 4H), 3.03–2.92 (m,
8H), 1.62–1.55 (m, 16H),1.25–1.23 (m, 144H), 0.87 (m, 24H).
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13C-NMR (150 MHz, CDCl3): 170.95, 170.88, 170.66, 170.50,
168.10, 167.98, 162.90, 134.28, 131.57, 129.04, 125.77, 123.08,
122.95, 65.99, 65.83, 65.23, 65.16, 51.56, 49.29, 49.03, 48.81,
36.67, 36.20, 36.08, 31.93, 29.66, 29.59, 29.37, 29.27, 28.57, 28.45,
25.93, 25.83, 25.80, 22.69, 14.11. Anal. calcd for C144H230N6O24:
C, 71.19; H, 9.54; N, 3.46. found: C, 71.37; H, 9.26; N, 3.17. m/z
[MALDI-TOF]: 2450.7 (M + Na+).
Results and discussion
The chemical structures of PBIs are shown in Scheme 1. The
swallow-tail amino acid-substituted perylene bisimides (SAPBI)
were synthesized using 3,4:9,10-perylenetetracarboxyldianhy-
dride and L-aspartic acid dodecyl ester according to the reported
method.19 For the synthesis of DPPBI, a two-fold condensation
between 3,4:9,10-perylenetetracarboxyldianhydride and L-
aspartic acid in imidazole at 120 ꢀC for 1 h produced N,N0-di((S)-
1,4-dicarboxy)-3,4,9,10-perylenetetracarboxyldiimide; DPPBI
was obtained by the coupling reaction of N,N0-di((S)-1,4-dicar-
boxy)-3,4,9,10-perylenetetracarboxyldiimide and L-aspartic acid
dodecyl ester in the presence of 1-ethyl-3-(3-dimethylamino-
propyl) carbodiimide.
The self-assembly behaviour of PBI in solution was investi-
gated via temperature-dependent and solvent-dependent ultra-
violet-visible (UV-Vis) absorption spectroscopy (Fig. 1). In
cyclohexane, at high temperature (80 ꢀC), the spectrum of
SAPBI displayed an absorption band between 400 and 550 nm,
which corresponds to the S0 / S1 transition of the PBI with
a well-resolved vibronic structure that can be attributed to
a breathing vibration of the perylene skeleton (Fig. 1 top).
Aggregation took place upon cooling, and the absorption coef-
ficients drastically decreased, with a concomitant blue-shift of the
absorption maximum, as well as a pronounced shoulder at
a longer wavelength (575 nm). These spectral features are char-
acteristic of J-aggregate PBI.11 The aggregation is attributed to
the poor solvency of cyclohexane. With a good solvent for the
perylene skeleton, such as toluene and 1,4-dioxane, SAPBI
Fig. 1 Temperature dependent UV-vis absorption spectra of SAPBI
(top) and DPPBI (bottom) with 1.0 ꢂ 10ꢁ4 M.
displayed a UV-Vis absorption of non-aggregated structures at
a wide temperature range. DPPBI exhibiꢀted strong aggregation
in cyclohexane at a low temperature (20 C). When the temper-
ature was increased from 20 to 80 ꢀC, the aggregation of DPPBI
was completely suppressed. DPPBI in toluene showed strong
aggregation at a low temperature. However, aggregation did not
occur in 1,4-dioxane (Fig. 1 bottom).
The self-assembly behavior of PBI in solution was further
investigated via concentration-dependent UV-Vis and circular
dichroism (CD) spectroscopies (Fig. 2). The maximum absorp-
tion of SAPBI appeared at 529 nm, along with two higher
vibronic transitions located at 495 and 462 nm corresponding to
a typical absorption of dissolved PBI molecule (Fig. 2A). The
absence of a CD signal in toluene suggests that the aggregation of
SAPBI was greatly suppressed by the bulky swallow-tail
N-substituted groups (Fig. 2C). By contrast, the absorption
bands of DPPBI show a well-resolved vibrational fine structure
at a low concentration, which can be attributed to the vibronic
transition of the PBI monomer. When the concentration was
increased from 1.0 ꢂ 10ꢁ6 to 1.0 ꢂ 10ꢁ4 mol Lꢁ1, the absorption
coefficient of DPPBI dramatically decreased by 65%, and a blue
shift of the absorption maximum to 491 nm was observed
(Fig. 2B). The intensity reversal observed for the 0–0 and 0–1
bands indicates that the Franck–Condon factors favored the
higher (0–1) excited vibronic state and suggests the formation of
H-type cofacial p–p stacking between the PBI units.20 Concen-
tration-dependent CD measurements provide insight into the
helical self-assembly of DPPBI in toluene. At a concentration of
10ꢁ3 mol Lꢁ1, the CD spectra of DPPBI show a negative Cotton
effect, followed by a positive Cotton effect at a higher wave-
length, with the CD signal passing through zero near the
Scheme 1 The synthesis of PBIs.
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ꢁ27 ꢀC and then disappeared at 181 ꢀC to form an isotropic melt
(Fig. 4A). The high isotropization temperature of DPPBI is the
result of p–p stacking in the PBI columns and of hydrogen bond
interactions of the amide groups, which reinforced the columnar
organization. Columnar liquid crystals with a mesophase at
room temperature, mesophase stability over a wide temperature
range, and a single mesophase structure are desired for potential
applications.13
As DPPBI cooled from their clearing temperatures to room
temperature (20 ꢀC), typical texture characteristics of the
columnar phase were observed under a polarizing microscope
(Fig. 3B). DPPBI exhibited a focal texture with straight linear
defects, which is characteristic of ordered columnar
mesophases.23
Fig. 4B shows the XRD profiles collected for the birefringent
phase of PBIs at room temperature. The PBI birefringent phases
were confirmed as a Colh phase through the assignment of the
reflections. In the small-angle region, the XRD profile of DPPBI
shows three reflection peaks corresponding to d spacings of 31.2,
Fig. 2 Concentration-dependent UV-vis spectra of SAPBI (A) and
DPPBI (B) in toluene at 20 ꢀC and the concentration-dependent CD
spectra of SAPBI (C) DPPBI (D) in toluene at 20 ꢀC.
ꢀ
18.1, and 15.2 A, which are indexed in the sequence as (100),
(110), and (200), respectively. In comparison, DPPBI, with
absorption maximum of DPPBI. As the concentration was
reduced, the intensity of the signal continued to decrease until it
reached the baseline at 10ꢁ5 mol Lꢁ1 (Fig. 3D). These results
indicate the formation of helical superstructures at a high
concentration.21
dendritic peptide substituents, exhibited a smaller d100 value of
ꢀ
31.2 A compared with SAPBI, which has swallow-tail amino acid
ꢀ
substituents (d100 ¼ 40.6 A). In addition, the packing of the
cores within the columns improved, as indicated by the sharp
(001) reflection at 2q ¼ 26.25ꢀ. This reflection corresponds to an
ꢀ
intermolecular distance of 3.4 A between the aromatic cores of
The large difference in the self-assembly behaviors of DPPBI
and SAPBI suggests that a multivalent H-bonding interaction is
crucial for the solution assembly. The hydrophilicity of the
solvent has a pronounced effect on the self-assembly behavior of
DPPBI, which shows a strong aggregation behavior in hydro-
phobic solvents, such as toluene and cyclohexane. However,
aggregation does not occur in a hydrophilic solvent such as
1,4-dioxane, tetrahyrofuran, or dimethyl formamide (Figures S1
and S2 in the Supporting Information†). Hydrophilic solvents,
which are usually hydrogen donors, interfere with the intermo-
lecular hydrogen bonding between peptides.22
DPPBI, which is smaller than the spacing in the LC phase of
ꢀ
SAPBI (3.5 A), where the reflection is significantly broader. The
XRD results show that DPPBI self-assembled into highly
ordered columnar structures, in contrast to SAPBI, which is less
ordered. Furthermore, the present study reports a new columnar
PBI mesophase, in which hydrogen bond formation resulted in
Thermal and mesomorphic properties
The thermotropic behavior of PBIs was investigated using
a combination of differential scanning calorimetry (DSC),
polarization optical microscopy, and X-ray diffraction (XRD)
experiments. With a DSC scan from ꢁ60 to 200 ꢀC, SAPBI only
showed a phase transition at 150 ꢀC, which is attributed to
a phase transition from liquid crystalline mesophase to isotropic
liquid state. The DSC analysis of DPPBI shows a phase
behavior, where a liquid crystalline mesophase appeared at
Fig. 4 (A) DSC of SAPBI and DPPBI, (B) XRD patterns of SAPBI and
DPPBI at 20 ꢀC.
Fig. 3 (A) POMs of SAPBI, (B) POMs of DPPBI at 20 ꢀC.
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ꢀ
the small inter-disk distance (3.4 A) found in the columnar PBI
liquid crystals.
The aggregation behavior of PBIs in the liquid crystalline
mesophase was further investigated using temperature-depen-
dent UV-Vis absorption spectroscopy on PBIs films (Fig. 5).
From 170 (isotropic liquid state) to 20 ꢀC (liquid crystalline
state), SAPBI shows the J-aggregate, a considerably blue-shifted
absorption band at around 400–500 nm, in addition to the
aforementioned red-shifted band at 570 nm (Fig. 4, top).24 By
contrast, cooling from the clearing temperature of DPPBI to
room temperature, the decreased absorption intensity of the 0–0
and increased absorption intensity of the 0–1 were observed,
suggesting the formation of H-type cofacial p–p stacking
between the DPPBI units. The formation of H-type cofacial p–p
aggregation is ascribed to the hydrogen bonds between the
DPPBI units in the liquid crystalline phases.
The presence of the hydrogen bonds in the liquid-crystalline
phases of DPPBI was demonstrated by temperature-dependent
Fourier transform infrared spectroscopic (FT-IR) studies
(Fig. 6A). Cooling from 190 (isotropic state) to 20 ꢀC (liquid
crystalline state) resulted in a decline in the absorption inten-
sities of the free amide groups at 1693 cmꢁ1. On the other hand,
the absorption intensities of the hydrogen bonds of the amide
groups at 1660 cmꢁ1 increase. Furthermore, the N–H stretching
frequency of DPPBI shifts from 3378 cmꢁ1 (190 ꢀC) to 3368
cmꢁ1 (20 ꢀC), indicating hydrogen bond formation in the
mesophase. With the high selectivity and directionality of
hydrogen bonds, the formation of the hydrogen bonds should
contribute to the induction of the columnar liquid crystalline
properties.
Fig. 6 (A) Temperature dependent FT-IR of DPPBI and (B) CD spectra
of DPPBI in thin film states at 20 ꢀC.
Conclusions
The present study succeeded in developing novel liquid crystal-
based PBIs. The dendritic moieties, acting as N-substituents,
induced the formation of a columnar liquid crystalline structure.
The hydrogen bonds at the PBI peripheries resulted in the
ordered columnar mesophases and small inter-disk distance (3.4
The higher helical order in the liquid crystal phase was further
demonstrated by CD measurements. The CD spectra of DPPBI
in the thin-film states show the same Cotton effect observed in
a solution of DPPBI (Fig. 6B). The CD signal for the fresh film
shows a weak intensity. However, the intensity increased after
heating to 180 ꢀC and then cooling to 20 ꢀC. These results
demonstrate the formation of helical superstructures in the liquid
crystal phase.
ꢀ
A) in the columnar PBIs liquid crystalline structure. Meanwhile,
the chiral peptides induced PBIs to assemble helical superstruc-
tures in the liquid crystalline mesophase. This property, along
with the room temperature liquid crystalline structure and
inherent optoelectronic properties of the PBIs core, makes
dendritic peptide-substituted PBIs a promising material for use
in optoelectronic devices.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Nos.20804012), the National Natural
Science Foundation of Hebei Province (Nos. B2009000169), the
Science Research Project of Department of Education of Hebei
Province (Nos.2007413), the National Natural Science Founda-
tion of Hebei University (Nos.2007105).
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