Synthesis and characterization of near-infrared absorbing and fluorescent liquid-crystal chromophores

Article abstract of DOI:10.1021/ol801345r

(Chemical Equation Presented) This work demonstrates that the donor-acceptor-donor charge-transfer chromophores can be tailor-made to be near-infrared absorbing and fluorescent, as well as being liquid crystals. The chromophore containing an extremely strong acceptor of benzo[1,2-c:4,5-c]bis([1, 2,5]thiadiazole) can form a columnar mesophase that absorbs at 890 nm and emits at 1160 nm in the solid state. These chromophores are readily soluble in common organic solvents and can form thin films by casting or spin coating, making them suitable for further device applications.

Full text of DOI:10.1021/ol801345r

ORGANIC
LETTERS
2008
Vol. 10, No. 17
3785-3787
Synthesis and Characterization of
Near-Infrared Absorbing and Fluorescent
Liquid-Crystal Chromophores
Xianzhen Li,† Anhua Liu,‡ Shidi Xun,† Wenqiang Qiao,† Xinhua Wan,‡ and
Zhi Yuan Wang*,†,‡
Department of Chemistry, Carleton UniVersity, 1125 Colonel By DriVe, Ottawa,
Ontario, Canada K1S 5B6, and Department of Polymer Science and Engineering,
College of Chemistry and Molecular Engineering, Peking UniVersity, 202 Chengfu
Road, Beijing 100871, People’s Republic of China
wayne_wang@carleton.ca
Received June 27, 2008
ABSTRACT
This work demonstrates that the donor-acceptor-donor charge-transfer chromophores can be tailor-made to be near-infrared absorbing and
fluorescent, as well as being liquid crystals. The chromophore containing an extremely strong acceptor of benzo[1,2-c:4,5-c′]bis([1,2,5]thiadiazole)
can form a columnar mesophase that absorbs at 890 nm and emits at 1160 nm in the solid state. These chromophores are readily soluble in
common organic solvents and can form thin films by casting or spin coating, making them suitable for further device applications.
Recently, columnar mesophase liquid crystals (LC) have
attracted considerable academic and commercial interests
because of their unique structures and properties in organic
semiconductors.1-4 The planar uniaxial alignment with edge-
on orientation of the molecules and columns parallel to the
substrate is needed for field effect transistors (FET) to ensure
the charge migration between the source and the drain,
while the homeotropic alignment with a face-on orientation
of the discs and the columnar axes perpendicular to the
substrate is expected to be beneficial for the performance in
photovoltaic cells (PVC) and light-emitting diodes
(LED).1,5-7 Generally, columnar mesophases are generated
from discotic LC molecules. A large number of discotic LCs
are known and derived from more than 50 different me-
sogens,2 typically being a polyaromatic core such as triph-
enylene, phthalocyanine, and hexabenzocoronene (HBC).8,9
For example, a p/n-type PVC containing a discotic LC HBC
(5) Pisula, W.; Menon, A.; Setpputat, M.; Lieberwirth, I.; Kolh, U.;
Tracz, A.; Sirringhaus, H.; Pakula, T.; Mu¨llen, K. AdV. Mater. 2005, 17,
† Carleton University.
684–689.
‡ Peking University.
(6) Zucchi, G.; Donnio, B.; Geerts, Y. H. Chem. Mater. 2005, 17, 4273–
(1) Sergeyev, S.; Pisula, W.; Geerts, Y. H. Chem. Soc. ReV. 2007, 36,
4277
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1902–1929
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(2) Kumar, S. Chem. Soc. ReV. 2006, 35, 83–109
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D. Liq. Cryst. 1997, 23, 613–617
(8) Herwig, P.; Kayser, C. W.; Mu¨llen, K.; Spiess, H. W. AdV. Mater.
1996, 8, 510–513
(9) Iyer, V. S.; Wehmeier, M.; Brand, J. D.; Keegstra, M. A.; Mu¨llen,
K. Angew. Chem., Int. Ed. Engl. 1997, 36, 1604–1607
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(3) Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.; Ha¨gele, C.;
Scalia, G.; Judele, R.; Kapatsina, E.; Sauer, S.; Schreivogel, A.; Tosoni,
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10.1021/ol801345r CCC: $40.75
Published on Web 08/05/2008
2008 American Chemical Society

derivative and a perylene diimide derivative exhibited an
external quantum efficiency of more than 34% near 490 nm
and power efficiency of up to 2%.10 To capture the infrared
energy, low band gap or NIR-absorbing organic columnar
mesophase LC semiconductors are urgently needed for PVCs.
NIR materials have potential applications for telecom-
munications, thermal imaging, and biological imaging. The
NIR light enables a deeper depth (5 cm) of detection than
the visible light in tissue imaging and reduces the autofluo-
rescence and absorption of biological species and water.11
All the known columnar mesophase LC compounds absorb
and emit light strongly only in the visible spectral region.4
Herein, we report the design and synthesis of low band
gap donor-acceptor-donor (D-A-D) charge-transfer (CT)
LC chromophores (Scheme 1). Among many low band gap
thiophene spacer, in order to facilitate a stronger intramo-
lecular charge transfer and thus lower the band gap energy.
Considering the molecular geometry, most of discotic LC
molecules typically consist of a large disk-like core that is
surrounded by several flexible peripheral chains. However,
a long rod-like rigid core with two or three paraffinic chains
at both ends is also known to generate columnar mesophases
by assembling into groups of 3-4 molecules that generate
one slice in a columnar stack.19,20 Thus, in our design, the
rigid rod-like D-A-D moiety is deemed to be a suitable
mesogen and is attached with three long aliphatic tails at
each side.
LC-BT and LC-BBT were synthesized by the Stille
coupling reaction of the dibromo-BT and dibromo-BBT with
the tributyltin derivative of 2, respectively (Scheme 1). The
reaction of 1 with tributyl(2-thienyl)stannane gave 2 in 97%
yield (see the Supporting Information). Compound 2 was
first converted to the tributyltin intermediate that was then
used without further purification in the Stille coupling
reactions for making LC-BT and LC-BBT. Both final
products are characterized by spectroscopic means and their
structures are consistent with the spectral data. They are
found to be quite soluble in common organic solvents such
as chloroform, tetrahydrofuran, toluene, and xylene. Ther-
mogravimetric analysis indicates that the onset temperatures
for 5% weight loss in air are 327 and 336 °C for LC-BT
and LC-BBT, respectively (Figures S9 and S10, Supporting
Information).
Scheme 1. Synthesis of LC-BT and LC-BBT Chromophores
Birefractive phenomena were obviously observed for LC-
BT by polarized optical microscopy (POM) when cooling
to 103 °C from its isotropic phase, but no clear changes of
the textures appeared when further cooling to room temper-
ature (Figure S11, Supporting Information). The mesophase
transition of LC-BT could not detected by differential
scanning calorimetry (DSC) (Figure S16 and Table S2,
Supporting Information) and X-ray diffraction (Figure S17,
Supporting Information) at a heating or cooling rate of 5
deg/min. Thus, although LC-BT seems to a liquid crystal,
its liquid crystal behavior is not very obvious, which may
be due to the small molecular size and low structural
symmetry of the benzothiadiazole core.
LC-BBT was then subjected to studies by DSC (Figure 1
and Table S2 in the Supporting Information) and POM
(Figure 2). During the cooling run, DSC reveals two
transitions: One takes place from isotropic to liquid crystal
phase at 163.5 °C and another from liquid crystal phase to
another phase at 138.5 °C. In the heating process two
transitions occur at 158.3 and 165.7 °C. Upon cooling, fluid
and birefringent phases are observed below 168 °C by POM
(Figure S12, Supporting Information). Figure 2, taken from
POM at 160 °C on cooling from the isotropic phase, shows
clearly a pseudo-focal-conic texture, typical of a columnar
mesophase. At temperature below 140 °C a large change in
the optical texture took place (Figure S13, Supporting
organic compounds, the D-A type of chromophores are
particularly useful as a potential NIR chromophore, because
their band gap levels and other properties can be readily
tuned by using a variety of donors and acceptors.12-14 Thus,
in our design for low band gap LC chromophores, a strong
electron-withdrawing heterocyclic quinoid, benzo[1,2-c:4,5-
c′]bis([1,2,5]thiadiazole) (BBT), is selected as an acceptor.
A relatively less powerful electron-withdrawing acceptor,
benzo[c][1,2,5]thiadiazole (BT), was also selected for com-
parison. The BBT-type unit is known to possess a substantial
quinoidal character, allowing for greater electron delocal-
ization and thus lowering the bandgap.15-18 The two
alkoxyphenyl donors are linked to the acceptor via a
(10) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.;
Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119–1122.
(11) Wang, S.; Li, X.; Xun, S.; Wan, X.; Wang, Z. Y. Macromolecules
2006, 39, 7502–7507.
(12) Roncali, J. Chem. ReV. 1997, 97, 173–206
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(13) van Mullekom, H. A. M.; Vekemans, J. A. J. M.; Havinga, E. E.;
Meijer, E. W. Mater. Sci. Eng., R 2001, 32, 1–40
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(14) Wudl, F.; Kobayashi, M.; Heeger, A. J. J. Org. Chem. 1984, 49,
3382–3384
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(15) Ono, K.; Tanaka, S.; Yamashita, Y. Angew. Chem., Int. Ed. Engl.
1994, 33, 1977–1979
(16) Karikomi, M.; Kitamura, C.; Tanaka, S.; Yamashita, Y. J. Am.
Chem. Soc. 1995, 117, 6791–6792
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(19) Ribeiro, A. C.; Heinrich, B.; Cruz, C.; Nguyen, H. T.; Diele, S.;
(17) Kitamura, C.; Tanaka, S.; Yamashita, Y. Chem. Mater. 1996, 8,
Schro¨der, M. W.; Guillon, D. Eur. Phys. J. E 2003, 10, 143–151.
570–578
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(20) Lincker, F.; Bourgun, P.; Masson, P.; Didier, P.; Guidoni, L.; Bigot,
(18) Yamashita, Y.; Ono, K.; Tomura, M.; Tanaka, S. Tetrahedron 1997,
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53, 10169–10178
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Org. Lett., Vol. 10, No. 17, 2008

-e(Ered + 4.4) (eV)] to be -5.02 and -3.96 eV, respectively.
In comparison, LC-BBT has a much lower band gap (1.06
eV) than LC-BT (2.02 eV), representing the lowest band gap
LC chromophore reported to date.
LC-BBT is strongly NIR-absorbing with peaks at 836 nm
in solution and at 890 nm in film (Figure 3), which fills up
Figure 1. DSC of LC-BBT (heating/cooling rate: 5 deg/min).
Information) and so we could not confirm whether or not
LC-BBT was in the solid crystalline phase.
On the basis of X-ray diffraction analysis (Figure 2), the
mesogen is in the isotropic phase at 170 °C. When cooling
Figure 3. Normalized absorption and PL spectra (excited at 780
nm) of LC-BBT in CHCl3 (10-5 M) and in film.
the NIR spectral gap (e.g., 700-900 nm) left by most organic
PVCs. The optical band gap deducted from the absorption
onset for LC-BT and LC-BBT films is 1.95 and 1.09 eV,
respectively. Thus, PL of LC-BBT is expected to be in the
NIR region as well, as is evident from the emission peaks at
1060 nm in solution and 1160 nm in film (Figure 3). In
comparison, LC-BT film has a maximum absorption at 533
nm and emits a deep red color at 665 nm (Figure S15,
Supporting Information).
In summary, a NIR-absorbing and fluorescent columnar
mesophase LC chromophore has been successfully designed
and obtained. This work implies that a variety of low band
gap columnar mesophase LC having a rigid D-A-D core can
be realized. LC-BBT should be useful for exploration in
tandem solar cells and NIR LC display.
Figure 2. Focal-conic texture (left) at 160 °C and X-ray diffraction
patterns (right) of LC-BBT at different temperatures on cooling
from the isotropic phase.
to 160 and 150 °C, two diffraction peaks appear at 2θ )
25° and 2.44°. Together with POM analysis, it can be
confirmed that the mesophase is columnar. The wide-angle
diffraction data are consistent with intercolumnar distance
(l ) 3.4 nm) and cofacial distance (dc ) 3.5 Å). With further
cooling to 140 °C, two sharp peaks at 2θ ) 2.49° and 6.75°
appeared. The d-spacing ratio of the two diffractions was
about 1:1/ꢀ7, while in the wide angle region, only a
scattering peak was present, indicating a hexagonal columnar
mesophase (Colh) arrangement of the molecules. The poly-
catenar calamitic molecules, such as LC-BBT, are able to
develop columnar mesophase by having several calamitic
rigid cores disposed side by side and surrounded by the
corresponding disorganized aliphatic end chains.19,20
Acknowledgment. This work was supported by the
Natural Sciences and Engineering Research Council of
Canada and the National Natural Science Foundation of
China (Nos. 20674001 and 20325415).
Supporting Information Available: Experimental pro-
cedures; 1H and 13C NMR spectra of 1, 2, LC-BT, and LC-
BBT; CV, optical textures and thermal behaviors of LC-BT
and LC-BBT; absorption and PL spectra; DSC trace; and
variable-temperature X-ray diffraction patterns of LC-BT.
This material is available free of charge via the Internet at
http://pubs.acs.org.
The electrochemical behavior of LC-BBT was investigated
with cyclic voltammetry (Figure S14, Supporting Informa-
tion). One quasireversible oxidation/reduction process was
found for LC-BBT, with the oxidation onset at 0.62 V and
the reduction onset at -0.44 V. Thus, the EHOMO and ELUMO
are calculated [EHOMO ) -e(Eox + 4.4) (eV) and ELUMO
)
OL801345R
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