Semiconducting 2,3,6,7,10,11-Hexakis{[4-(5-dodecylthiophen-2-yl)phenyl] ethynyl}triphenylene and its discotic liquid crystalline properties

Article abstract of DOI:10.1246/cl.2010.396

We report on the design and synthesis of a new discotic liquid crystalline (LC) semiconducting material based on a triphenylene core. The molecule is readily soluble in common organic solvents to exhibit self-organizing long fiber structures with supramolecularly ordered columnar stacks that lie parallel to the substrate. Compared to all the previously reported triphenylene derivatives, this molecule has a relatively lower band gap energy (E_g = 2.53 eV), which makes it suitable for electronic and optoelectronic applications.

Full text of DOI:10.1246/cl.2010.396

doi:10.1246/cl.2010.396
396
Published on the web March 13, 2010
Semiconducting 2,3,6,7,10,11-Hexakis{[4-(5-dodecylthiophen-2-yl)phenyl]ethynyl}triphenylene
and Its Discotic Liquid Crystalline Properties
Mai Ha Hoang, Min Ju Cho, Kyung Hwan Kim, Tae Wan Lee, Jung-Il Jin, and Dong Hoon Choi*
Department of Chemistry, Advanced Materials Chemistry Research Center, Korea University, Seoul 136-701, Korea
(Received January 21, 2010; CL-100066; E-mail: dhchoi8803@korea.ac.kr)
We report on the design and synthesis of a new discotic
(i)
O
(ii)
S
S
S
B
Br
C12H25
C12H25
Si
C12H25
O
liquid crystalline (LC) semiconducting material based on a
triphenylene core. The molecule is readily soluble in common
organic solvents to exhibit self-organizing long fiber structures
with supramolecularly ordered columnar stacks that lie parallel
to the substrate. Compared to all the previously reported
triphenylene derivatives, this molecule has a relatively lower
band gap energy (Eg = 2.53 eV), which makes it suitable for
electronic and optoelectronic applications.
I
Br
1
2
3
(iii)
Br
Br
Br
Br
Br
C12H25
S
C12H25
5
S
Br
4
(iv)
C12H25
S
C12H25
S
Columnar discotics is an interesting class of materials that
can be potentially used as organic semiconductors in electronic
devices. The corresponding mesogens consist of an aromatic or
fused aromatic core, which can be chemically modified by
peripheral substitution and self-assemble into one-dimensional
columnar superstructures that then arrange in a two-dimensional
lattice. The overlapping of the ³-orbitals of adjacent molecules
within the columns ensures that one-dimensional intra-columnar
charge-carrier transport occurs. While charge carrier mobilities
that are more than sufficient for thin film transistor applications
have been reported, utilization of the discotic liquid crystalline
molecule will be realized only when the pathways for transport
can be uniaxially aligned parallel to the surfaces in thin film
geometries that have length scales of at least 100 nm.1,2
Triphenylenes and their derivatives have been intensively
synthesized and investigated in recent decades. Such molecules
have attracted attention mainly because of their unique tendency
to aggregate into geometrically confined one-dimension colum-
nar arrays that can display LC properties. Hexaalkoxytriphenyl-
enes, hexakis(alkylsulfanyl)triphenylenes, hexaphenyltriphenyl-
enes, and hexakis(phenylethynyl)triphenylenes3,4 have been
synthesized as its derivatives. However, these derivatives show
high band gap energies that would weaken their potential in
electronic applications.
In a previous study, in an effort to reduce the band gap
energy, we reported semiconducting multibranched conjugated
molecules based on a ³-extended triphenylene core and bearing
six bithiophene groups. Their band gap energies were so low
that they can be used for organic thin film transistor fabrication.
However, the previous triphenylene-based molecules did not
clearly show liquid crystalline properties.5
In this paper, we report the design and synthesis of a
new semiconducting molecule based on a triphenylene core.
This molecule showed a relatively low band gap energy
(Eg = 2.53 eV) and an interesting LC properties.
S
C12H25
S
C12H25
S
6
C12H25
Scheme 1. Synthetic route and optimized geometry of com-
pound 6 (HDTPT). (i) Pd(PPh3)4/Ba(OH)2/H2O/Dimethoxy-
ethane, 80 °C; (ii) trimethylsilyl acetylene, Pd(PPh3)2Cl2, CuI,
Et3N, THF, rt; (iii) K2CO3, CH2Cl2, MeOH, rt; (iv)
Pd(PPh3)2Cl2, CuI, PPh3, Et3N, THF, 90 °C.
before deprotecting the trimethylsilyl group to yield compound
4. 2,3,6,7,10,11-Hexabromotriphenylene (5) was used as the
core moiety in the Sonogashira coupling with compound 4
to efficiently yield compound 6 (HDTPT) (yield, ca. 60%)7
(Scheme 1). Computer calculations using the density functional
theory (DFT) model incorporated with the Spartan program (’06)
showed that the ³-extended core had an absolutely planar
structure, which is a prerequisite for the uniaxial orientation of
columnar discotic liquid crystals.
Figure 1 shows the absorption and emission spectra in
solutions and thin films of this molecule. The absorption of
HDTPT in a dilute chloroform solution showed an absorption
maximum at 382 nm. A drastic spectral change was observed in
the film state of this molecule, which is attributed to a high
degree of intermolecular interaction. Compared to the molecules
of previously reported triphenylene derivatives, molecule 6
showed red-shifted absorption spectral behavior arising from
lower band gap energy in the solid state. HDTPT exhibited
photoluminescence (PL) spectral behaviors with an emission
maximum at 438 nm in the solution state and 501 nm in the film
state. The PL spectrum in the film state is significantly red-
shifted and the color emission was totally changed to green.
The thermal properties of HDTPT were characterized by
Compound 5 was prepared according to a procedure
described in literature.6 Compound 2 was prepared by the
Suzuki coupling reaction of compound 1 with 1-bromo-4-
iodobenzene. Through the Sonogashira coupling with trimethyl-
silyl acetylene, compound 2 was converted to compound 3
differential scanning calorimetry (DSC) and thermogravimetric
¹1
analysis (TGA) (scan rate of 5 °C min
under nitrogen).
HDTPT displayed an endothermic peak at 67 °C and a clear
crystalline transition at 22 °C in the cooling cycle (inset of
Figure 2).
Chem. Lett. 2010, 39, 396-397
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

397
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
(b)
(c)
(d)
(a)
Figure 3. Optical micrographs of HDTPT annealed at 140 °C.
(A) Film on normal glass slide and (B) film on rubbed polyimide
film.
300
400
500
Wavelength/nm
600
700
ordered LC columnar phase of HDTPT (©400 times). The
molecule can be processed from solution to form self-organizing
long fibers with supramolecularly ordered columnar stacks that
lie parallel to the substrate and can be oriented uniaxially onto
normal glass slides. An optical micrograph of the texture on a
rubbed polyimide film clearly displays the uniaxial alignment of
the molecules.
Figure 1. UV-vis absorption and PL emission spectra of
HDTPT. (a) Absorption spectrum in solution state, (b)
absorption spectrum in film state, (c) PL spectrum in solution
state, and (d) PL spectrum in film state.
6
L52-First Heating
L52-First Cooling
4
2
The intermolecular coupling of ³ electrons on adjacent
molecules along the columnar stack in discotic LCs has been
recognized as a promising structure that can allow easy charge
movement along the stack. The fabrication of a thin film
transistor using highly ordered HDTPT is now in progress.
We have successfully synthesized and characterized a new
discotic LC semiconducting material, based on a triphenylene
core. The molecule can be oriented uniaxially to form self-
organizing long fibers that lie parallel to the substrate. The
findings reported in this paper are expected to contribute to the
design of new discotic LC semiconductors based on ³-extended
triphenylene for future electronic and optoelectronic applica-
tions.
0.20
0.15
0.10
0
2
θ
= 2.66°
-2
-4
d = 33.23 Å
2θ = 2.60°
d = 34.00 Å
50
100
150
Temperature/°C
2
θ
= 3.37°
d = 26.23 Å
70°C
100°C
160°C
100°C
70°C
25°C
0.05
0.00
10
20
30
θ
40
Figure 2. Temperature dependence of X-ray diffraction pat-
terns of HDTPT.
This research work was fully supported by Korea Research
Foundation (No. KRF-2007-313-C00499, 2008-2009) and by
Korea Science and Engineering Foundation, (No. KOSEFR-
0120070001128402008). Particularly, Prof. D.H. Choi is grate-
ful for the financial support from the Seoul R&BD Program
(2009-2010).
To confirm the crystalline property of this LC molecule,
X-ray diffraction (XRD) experiments were performed at varying
temperatures using the synchrotron radiation (1.542 ¡) of the
3C2 beam line. The X-ray diffraction scans of HDTPT film
annealed at different temperatures reveal the temperature at
which the highest crystalline order is achieved (Figure 2). From
the peaks at 2ª = 3.37°, the interlayer spacing is evaluated to be
26.23 ¡. The peak shifts to a lower 2ª direction above the
melting point (67 °C), reflecting a crystalline-crystalline phase
transition. The diffraction intensity increased drastically above
this temperature, and the peak position shifted to 2ª = 2.66°
(d = 33.23 ¡), which is the intercolumnar distance of the
nematic phase. The liquid crystalline mesogenic phase of
discotic LC was maintained in a broad temperature range from
67 to 160 °C. Although the diffraction intensity decreased
significantly when the temperature reached 160 °C, an ordered
phase was retained upon cooling. The presence of peaks in the
3.8-5.2 ¡ [(010) reflection] region showed the stacking perio-
dicity of molecules in a nematic columnar phase.
References and Notes
1
W. Pisula, A. Menon, M. Stepputat, I. Lieberwirth, U. Kolb, A.
Tracz, H. Sirringhaus, T. Pakula, K. Müllen, Adv. Mater. 2005, 17,
684.
2
3
S. Sergeyev, W. Pisula, Y. H. Geerts, Chem. Soc. Rev. 2007, 36,
1902.
N. Boden, R. J. Bushby, G. Headdock, O. R. Lozman, A. Wood, Liq.
Cryst. 2001, 28, 139.
4
5
K. Praefcke, B. Kohne, D. Singer, Angew. Chem. 1990, 102, 200.
M. H. Hoang, M. J. Cho, K. H. Kim, M. Y. Cho, J.-S. Joo, D. H.
Choi, Thin Solid Films 2009, 518, 501.
6
7
R. Breslow, B. Jaun, R. Q. Kluttz, C.-Z. Xia, Tetrahedron 1982, 38,
863.
HDTPT: 1H NMR (400 MHz, CDCl3): ¤ 8.10 (s, 6H), 7.43 (d,
J = 8.2 Hz, 12H), 7.29 (d, J = 8.2 Hz, 12H), 7.01 (d, J = 3.2 Hz,
6H), 6.69 (d, J = 3.2 Hz, 6H), 2.81 (t, 12H), 1.74 (m, 12H), 1.30 (m,
108H), 0.90 (t, 18H). 13C NMR (400 MHz, CDCl3): ¤ 146.14,
141.45, 134.40, 132.70, 127.47, 125.26, 125.10, 124.62, 124.35,
122.07, 94.79, 90.06, 32.24, 31.98, 30.67, 30.06, 30.02, 30.00,
29.82, 29.70, 29.66, 22.30, 14.43. MS (MALDI-TOF) m/z: calcd
By using a hot-stage optical microscope (Mettler Co.), the
LC properties of the molecule were observed under crossed
polarizers. Depending on the temperature for annealing, we
could observe different LC textures. Figure 3 shows the highly
C
C
162H192S6 [M+] 2329.33; found 2329.12 Anal. Calcd for
162H192S6: C, 83.45; H, 8.30; S, 8.25%. Found: C, 83.43; H,
8.30; S, 8.25%.
Chem. Lett. 2010, 39, 396-397
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/