Smectic liquid crystals based on hexaazatriphenylene: Potential organic n-type semiconductor

Article abstract of DOI:10.1039/c0nj00586j

We report the synthesis of dibenzohexaazatriphenylene derivative DBHAT. The addition of electron-withdrawing groups on the dibenzohexaazatriphenylene core leads to a high electron-accepting capability. DBHAT exhibits reversible four-step reduction waves a

Full text of DOI:10.1039/c0nj00586j

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www.rsc.org/njc | New Journal of Chemistry
Smectic liquid crystals based on hexaazatriphenylene: potential
organic n-type semiconductor
Baoxiang Gao,*a Licui Zhang,a Qianqian Bai,a Ying Li,b Junwei Yangb and
Lixiang Wang*b
Received (in Victoria, Australia) 26th July 2010, Accepted 10th September 2010
DOI: 10.1039/c0nj00586j
We report the synthesis of dibenzohexaazatriphenylene derivative
DBHAT. The addition of electron-withdrawing groups on
the dibenzohexaazatriphenylene core leads to a high electron-
accepting capability. DBHAT exhibits reversible four-step
reduction waves at half-wave potentials (E1/2) of À0.42, À0.94,
À1.35 and À1.80 V vs. Ag/AgCl. Furthermore, DBHAT is able
to form a smectic liquid crystalline assembly over a wide tempera-
ture range. With strong dipole–dipole interactions, the smectic
liquid crystals show extremely highly stabilized mesophases.
wide temperature range. With strong dipole–dipole inter-
actions, the smectic liquid crystals show extremely highly
stabilized mesophases.
The synthetic approach of DBHAT is outlined in Scheme 1.
The synthesis of two-condensation intermediate 3 is a key step
to obtain dibenzohexaazatriphenylene derivative DBHAT.
The catechol derivative was successfully realized by adding 2
dropwise into an acetic acid solution of 1 at 20 1C, and then
intermediate 3 was obtained by oxidising the catechol deriva-
tive in dichloromethane. DBHAT was prepared by the con-
densation coupling of 3 and 4. The crude product was purified
by column chromatography on silica gel with chloroform as
Organic conjugated materials that form liquid crystalline (LC)
mesophases have attracted attention for applications in organic
electronic devices.1,2 These materials possess high intrachain
charge carrier mobilities due to high degrees of order and
extensive orbital overlap. Since these materials are typically
composed of conjugated molecules, in which a central planar
redox-active p-delocalised core is surrounded by flexible side
chains,3,4 they are more readily processed from solution or
from an isotropic melt than p-stacked single-crystal materials.
Therefore, extensive efforts have been directed towards
developing and studying LC materials as hole (p-type) and
electron (n-type) transport materials, and n-type organic
conjugated materials, which are capable of transporting holes
and electrons, respectively. However, most semiconducting
LCs are good hole transporting materials (p-type), while only
a limited number of electron transporting LC materials
(n-type), such as heteroaromatic compounds,5–10 and naphthalene
and perylene diimides,11,12 etc., have been reported. Williams
and co-workers have reported the self-assembly of n-type
discotic pyrazine-containing conjugated molecules.13–17 We
also have studied the synthesis and characterization of a series
of n-type pyrazine-containing conjugated molecules, and the
self-assembling properties of n-type pyrazine semiconductors
have been reported.18,19
Here, we have designed and synthesized the dibenzohexaaza-
triphenylene derivative DBHAT. The addition of electron-
withdrawing groups onto the dibenzohexaazatriphenylene
core leads to a high electron-accepting capability. Further-
more, DBHAT is able to form a smectic LC assembly over a
a Key Laboratory of Medicinal Chemistry and Molecular Diagnosis,
Ministry of Education, College of Chemistry and Environmental
Science, Hebei University, Baoding, 071002, P. R. China.
E-mail: bxgao@hbu.edu.cn; Fax: +86 31250793
b State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, P. R. China.
E-mail: lixiang@ciac.jl.cn; Fax: +86 431-85685653
Scheme 1 Synthesis of DBHAT.
c
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the eluent to afford DBHAT as a red solid. The chemical
structure of DBHAT was confirmed by NMR spectra, elemental
analysis and mass spectra. The detailed synthetic procedure
and structure characterization are described in the Experi-
mental section.
The UV-vis absorption and fluorescence spectra of DBHAT
in solution and in a film are shown in Fig. 1. In non-polar
solvents, the absorption bands of DBHAT show a well-
resolved vibrational fine structure at low concentration, which
can be attributed to the vibronic transition of the DBHAT
monomer. The absorption maximum appears at 459 nm, along
with two higher vibronic transitions located at 360 and 323 nm,
which are attributed to the hexaazatriphenylene core p–p*
transition. A noticeable red shift of the absorption is observed
in polar solvents and films. DBHAT exhibits a strong excited
state solvatochromism. The lem value increases as the solvent
polarity increases. The emission wavelength of DBHAT moves
from 596 nm in toluene to 639 nm in dichloromethane, and the
fluorescence quantum yield decreases from 61% in toluene
to 43% in dichloromethane (Fig. 1). This clearly indicates
a strong intramolecular charge transfer from the donor
(decyloxy-benzene) to the acceptor (cyano groups).20,21
The redox properties of DBHAT were studied by cyclic
voltammetry (CV) and differential pulse voltammetry (DPV).
Fig. 2 shows the CV and DPV curves of DBHAT. The CV scan
for DBHAT with anodic scanning from 0 to 2 V shows no
peaks. Upon cathodic scanning, DBHAT exhibits four reversible
reduction waves, which were further confirmed by the DPV
scan, and the half-wave potentials (E1/2) of these reduction
waves are located at À0.42, À0.94, À1.35 and À1.80 V vs.
Ag/AgCl. These results indicate the n-type nature of DBHAT.
It is noteworthy that the multistep redox behavior of DBHAT
is comparable with the results for C60 fullerene reported by
Echegoyen and co-workers.22 Furthermore, the first reduction
of DBHAT occurs at a less negative potential than that of C60
fullerene, indicating the extremely high electron-accepting
capability of DBHAT.
Fig. 2 Cyclic voltammograms of DBHAT in dichloromethane at a
scan rate of 0.1 V sÀ1
.
showed a large band gap, 2.21 eV, which was estimated from
the onset of UV-vis absorption in a solid film.
Thermogravimetric analysis (TGA) performed under nitrogen
revealed that DBHAT is thermally stable with a weight loss
temperature beyond 350 1C. The differential scanning calorimetry
(DSC) traces of DBHAT are illustrated in Fig. 3, and the
transition temperatures and enthalpies are summarized. The
first heating trace is significantly different from the second, but
no further changes were observed upon subsequent cooling
and heating. There is only one endothermic transition at 144 1C,
and the corresponding transition enthalpy is 8.55 KJ molÀ1
.
It corresponds to the crystal phase to LC phase transition,
based on polarized light microscopy (PLM) observations.
The LUMO (lowest unoccupied molecular orbital) energy
levels and band gap of DBHAT can be estimated from the CV
and UV/vis spectroscopic data. The LUMO energy level
of DBHAT, estimated from the reduction onset potentials,
was À3.98 eV, comparable to the well-known n-type perylene-
tetracarboxylic acid diimide material.23 Moreover, DBHAT
Fig. 1 UV-vis absorption and photoluminescence spectra of DBHAT.
Fig. 3 Thermogravimetric (A) and DSC (B) curves of DBHAT.
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2736 New J. Chem., 2010, 34, 2735–2738 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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extremely high stabilized mesophases. These results suggest
that dibenzohexaazatriphenylene derivative DBHAT is a
potential organic n-type semiconductor.
This work was supported by the National Natural Science
Foundation of China (nos. 20804012 and 20574067), the
Science Research Project of Department of Education of
Hebei Province (no. 2007413) and the National Natural
Science Foundation of Hebei University (no. 2007105). The
authors thank the reviewers for good suggestions. The authors
also thank Dr Shi Di Xun of Carleton University, Canada for
his help with manuscript revision.
Experimental
Fig. 4 Polarized optical micrographs of DBHAT at 200 1C.
General information
Upon heating from room temperature to 200 1C, typical conical
fan-shaped textures characteristic of a smectic phase are observed
for DBHAT under a polarizing microscope, as shown in Fig. 4.
Within the temperature range from 150 1C to the decomposition
temperature (350 1C), the fan-shaped textures of the smectic
phase do not change, suggesting that the smectic liquid crystals
show extremely highly stabilized mesophases.
The starting materials, diaminomaleonitrile, rhodizonic acid
dihydrate and solvents were purchased from Acros and used
without any further purification. Compound 1 was synthesized
according to a reported method.18 1H NMR spectra were
recorded at 25 1C on a 300 MHz NMR spectrometer (Bruker).
Chemical shifts are reported in ppm at room temperature
using CDCl3 as the solvent and tetramethylsilane as the
internal standard unless indicated otherwise. Abbreviations
used for splitting patterns are s = singlet, d = doublet, t =
triplet and m = multiplet. Mass spectra were carried out using
MALDI-TOF/TOF matrix assisted laser desorption ionization
mass spectrometry with an autoflexIII smartbeam (Bruker
Daltonics Inc.). Elemental analyses were carried out on an
Eager 300 elemental analyzer.
Fig. 5 shows the XRD scan collected for the birefringent
phase of DBHAT at 200 1C. The birefringent phase of
DBHAT was confirmed as a smectic phase through assignment
of the reflections. In the small-angle region, the XRD profile
of DBHAT shows two reflection peaks corresponding to
d-spacings of 25.94 and 12.91 A, which were indexed in sequence
as (100) and (200). No sharp peaks are observed in the wide-
angle region, although one diffuse halo is centered around
d-spacings of ca. 5 A, which is attributed to liquid-like correla-
tions between adjacent molecules. The data indicate that DBHAT
adopts a smectic mesophase.
Cyclic voltammetry (CV) was performed on a CHI660b
electrochemical analyzer with a three-electrode cell in a
5.0 Â 10À4 mol LÀ1 DBHAT dichloromethane solution with
0.1 M tetrabutylammonium perchlorate at a scan rate of
100 mV sÀ1. A glass carbon disk (2 mm diameter) was used
as a working electrode with a Pt wire as the counterelectrode
and an Ag/AgCl electrode as the reference electrode. The
redox potential was calibrated with ferrocene/ferrocenium
(0.44 V vs. Ag/AgCl in DCM). LUMO energy levels were
calculated according to LUMO = (4.4 + Eonsetred).
In summary, we have reported the dibenzohexaazatri-
phenylene derivative DBHAT. The addition of electron-
withdrawing groups onto the dibenzohexaazatriphenylene
core leads to a high electron-accepting capability. DBHAT
exhibits four reversible reduction waves at half-wave potentials
(E1/2) of À0.42, À0.94, À1.35 and À1.80 V vs. Ag/AgCl. The
LUMO energy level of DBHAT was À3.98 eV, comparable to
the well-known n-type perylenetetracarboxylic acid diimide
material. Furthermore, DBHAT is able to form a smectic LC
assembly over a wide temperature range. With strong dipole–
dipole interactions, the smectic liquid crystals showed
UV/vis spectra were recorded by a Shimadzu WV-2550
spectrophotometer. TGA measurements were performed on
a Perkin-Elmer TGA7 thermal analyzer. Differential scanning
calorimetry (DSC) was carried out using a Perkin-Elmer
differential scanning calorimeter (DSC7) with heating and cooling
rates of 10 K minÀ1. Phase transitions were also examined by a
polarization optical microscope (POM) Olympus BX51 with a
T95-PE temperature-controlled THMS-600 hot stage. X-Ray
diffraction measurements were performed on a D8 Advance
instrument (Bruker AXS Inc.) with Cu-Ka1: 1.54051 A.
Synthesis and characterization of DBHAT
Into a stirred solution of 1 (1.140 g, 2.0 mmol) in 80 mL acetic
acid was added compound 2 dropwise (0.206 g, 1.0 mmol) in
10 mL water at 20 1C. The mixture was stirred overnight and
the precipitated product filtered. The precipitated product
was dissolved in 100 mL dichloromethane, 2.50 g NaIO4 in
30 mL water was added and the mixture stirred overnight.
The organic layer was washed with water (100 mL Â 2) and
Fig. 5 X-Ray diffraction patterns of DBHAT at 200 1C.
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brine (100 mL Â 2), dried over anhydrous Na2SO4 and
evaporated in vacuo to dryness. The crude product was dried
under vacuum and purified by column chromatography on
silica gel with chloroform as the eluent to afford compound 3
as a red solid (0.72 g, 58%). 1H NMR (300 Hz, CDCl3, 298 K):
d (ppm) 8.53 (s, 2 H), 8.42 (s, 2 H), 7.23 (t, J = 7.5 Hz, 8 H),
6.88 (m, 8 H), 4.00 (t, J = 6.2 Hz, 8 H), 1.88–1.79 (m, 8 H),
1.50–1.32 (m, 28 H), 0.92 (t, J = 6.5 Hz, 12 H). Anal. calc. for
C84H104N4O6: C, 79.32; H, 8.44; N, 4.51. Found: C, 79.03; H,
8.65; N, 4.82. m/z [MALDI-TOF]: 1241.9 (MH+).
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A mixture of compound 3 (0.620 g, 0.5 mmol) and com-
pound 4 (0.108 g 1.0 mmol) in 60 mL acetic acid was heated to
120 1C for 24 h under a nitrogen atmosphere. After the
reaction mixture had cooled to room temperature, it was poured
into water (200 mL) and extracted with dichloromethane
(30 mL Â 3). The organic layer was washed with saturated
aqueous sodium hydrogen carbonate solution (100 mL Â 2)
and brine (100 mL Â 2), dried over anhydrous Na2SO4 and
evaporated in vacuo to dryness. The residue was purified by
column chromatography on silica gel with dichloromethane as
the eluent to afford DBHAT as a deep red solid (0.58 g, 88%).
1H NMR (300 Hz, CDCl3, 298 K): d (ppm) 8.73 (s, 2 H), 8.64
(s, 2 H), 7.32 (d, J = 7.5 Hz, 8 H), 6.93 (d, J = 8.7 Hz, 8 H),
4.03 (t, J = 6.6 Hz, 8 H), 1.88–1.81 (m, 8 H), 1.38–1.33
(m, 28 H), 0.93 (t, J = 6.3 Hz, 12 H). 13C NMR (75 Hz,
CDCl3, 298 K): d (ppm) 159.28, 145.94, 143.99, 143.21, 132.60,
131.53, 131.35, 114.67, 68.45, 32.31, 29.98, 29.85, 29.74,
26.51, 23.08, 14.50. Anal. calc. for C86H104N8O4: C, 78.62;
H, 7.98; N, 8.53. Found: C, 78.53; H, 7.74; N, 8.76. m/z
[MALDI-TOF]: 1314.7 (MH+).
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2738 New J. Chem., 2010, 34, 2735–2738 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010