An ambipolar transporting naphtho[2,3-c][1,2,5]thiadiazole derivative with high electron and hole mobilities

Article abstract of DOI:10.1021/ol900431a

An ambipolar transporting naphtho[2,3-c][1,2,5]thiadiazole derivative with both high electron and hole mobilities has been synthesized via Suzuki cross-coupling. The electron and hole mobilities are 1.7 × 10^-3 cm²/(V·s) and 1.9 × 10<

Full text of DOI:10.1021/ol900431a

ORGANIC
LETTERS
2009
Vol. 11, No. 10
2069-2072
An Ambipolar Transporting
Naphtho[2,3-c][1,2,5]thiadiazole
Derivative with High Electron and Hole
Mobilities
Yongduo Sun, Lian Duan, Peng Wei, Juan Qiao, Guifang Dong, Liduo Wang, and
Yong Qiu*
Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of
Education, Department of Chemistry, Tsinghua UniVersity,
Beijing 100084, P. R. China
qiuy@mail.tsinghua.edu.cn
Received March 3, 2009
ABSTRACT
An ambipolar transporting naphtho[2,3-c][1,2,5]thiadiazole derivative with both high electron and hole mobilities has been synthesized via
Suzuki cross-coupling. The electron and hole mobilities are 1.7 × 10-3 cm2/ (V·s) and 1.9 × 10-3 cm2/ (V·s) at an electric field of 4.5 × 105
V/cm, respectively, as measured by using time-of-flight technique.
Organic light-emitting diodes (OLEDs) have attracted consider-
able interest due to their potential application in flat-panel
displays and illumination.1 The charge balance in an OLED is
considered to be essential for achieving high device perfor-
mance. In recent years, much attention has been paid to
materials possessing an ambipolar charge transporting property,
which helps to facilitate the balance of electrons and holes,
simplify the device structure, and control the exciton formation
zone for OLEDs.2 A common strategy to achieve ambipolar
property is to combine electron-transporting chromophores and
hole transporting chromophores in a single molecule.2 However,
both electron and hole mobilities of these materials are not very
high. Another strategy is to find chromophores with ambipolar
transport properties. It is reported that a carbazole derivative
exhibited ambipolar property and the electron and hole mobili-
ties are up to 10-4 cm2/(V·s).3 Oligofluorenes are also reported
to possess unusually high electron and hole mobilities which
are close to 1 × 10-3 cm2/(V·s).3
In our previous report, naphtho[2,3-c][1,2,5]thiadiazole
(NTD) derivatives demonstrated good amorphous film-
forming capability and ambipolar transporting property with
mobility values for both charge carriers of more than 10-4
(2) (a) Li, Z.-Y.; Wong, M.-S.; Fukutani, H.; Tao, Y. Org. Lett. 2006,
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69, 3495. (d) Hancock, J. M.; Gifford, A. P.; Tonzola, C. J.; Jenekhe, S. A.
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Era, M.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1995, 66, 3433. (g) Thomas,
K. R. J.; Lin, J. T.; Tao, Y.-T.; Chuena, C. H. J. Mater. Chem. 2002, 12,
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(1) (a) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(b) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.;
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10.1021/ol900431a CCC: $40.75
Published on Web 04/10/2009
 2009 American Chemical Society

cm2/(V·s),4,2i and the theoretical calculation showed that the
ambipolar property was mainly due to the overlap of frontier
orbitals on NTD core. We think that we could further
enhance the mobilities of NTD derivatives through introduc-
ing planar side substituents, since the twisted substituents
would weaken the charge transport ability. In this paper, we
report a new compound, 4,9-bis(4-(benzo[d]thiazol-2-yl)phe-
nyl)naphtho[2,3-c][1,2,5]thiadiazole (BBTPNTD), which ex-
hibits very high and close electron and hole mobilities of
1.7 × 10-3 cm2/ (V·s) and 1.9 × 10-3 cm2/ (V·s) at an electric
field of 4.5 × 105 V/cm, respectively.
calorimetry (DSC). BBTPNTD displays high thermal stability
with the temperature for a 5% weight loss of 448, the
decomposition temperature of 512 °C, and the melting
temperature of 411 °C. The electrochemical properties of
BBTPNTD were studied by cyclic voltammetry (CV). As
shown in Figure 1., we detected that one quasireversible
As shown in Scheme 1, BBTPNTD was synthesized from
4,9-dibromonaphtho[2,3-c][1,2,5]thiadiazole and dimethyl
Scheme 1. Synthetic Route for BBTPNTD
Figure 1. Cyclic voltammogram of BBTPNTD. Scan rate: 150 mV
s-1; working electrode: Pt disk electrode; reference electrode: Ag/
AgCl. Oxidation CV was performed in dichloromethane containing
with 0.1 M n-Bu4NBF4 as the supporting electrolyte. Reduction
CV was performed in acetonitrile containing with 0.1 M n-Bu4NBF4
as the supporting electrolyte.
oxidation potential occurred at 0.87 V and one reversible
reduction potential at -1.53 V, which indicates that both
the radical cations and radical anions are stable entities.6
The highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) of BBTP-
NTD are -5.61 and -3.21 eV, respectively, which were
calculated using a linear correlation with dicyclopenta-
dienyl iron.
The BBTPNTD film was prepared by vacuum sublimation
on glass and was characterized by X-ray diffraction. There
was no diffraction peaks except for the peaks from glass,
indicating that the film was amorphous. The absorption
spectra and photoluminescence (PL) spectra of BBTPNTD
in CHCl3 (2 × 10-5 M) and as thermally evaporated film
are show in Figure 2. BBTPNTD displays absorption peaks
at 485 and 509 nm in solution and as thin film, respectively.
And its PL emission peaks are at 587 and 621 nm in solution
and as thin film, respectively.
The carrier mobilities of BBTPNTD in amorphous thin
film were characterized by the time-of-flight (TOF) transient
photocurrent technique.7,3b,c The configuration of the device
is ITO/Ag (60 nm)/BBTPNTD (2.0 µm)/ Ag (200 nm). The
carrier mobility was calculated according to the following
formula: µ ) D/(TtE)) D2/(VTt), where V is the applied bias,
Tt is the charge transit time, and D is the thickness of the
organic layer.
4-(benzo[d]thiazol-2-yl)phenylboronate by a palladium-
catalyzed Suzuki-coupling reaction.5 Dimethyl 4-(benzo[d]thi-
azol-2-yl)phenylboronate was synthesized from 2-ami-
nothiophenol and 4-bromobenzoyl chloride.5 All the materials
used in our experiments were purified by vacuum train
sublimation.
The thermal stability of BBTPNTD was estimated by the
thermogravimetric analysis (TGA) and differential scanning
(3) (a) Shen, J.-Y.; Yang, X.-Y.; Huang, T.-H.; Lin, J. T.; Ke, T.-H.;
Chen, L.-Y.; Wu, C.-C.; Yeh, M.-C. P. AdV. Funct. Mater. 2007, 17, 983.
(b) Liao, Y.-L.; Lin, C.-Y.; Liu, Y.-H.; Wong, K.-T.; Hung, W.-Y.; Chen,
W.-J. Chem. Commun. 2007, 1831. (c) Wu, C.-C.; Liu, T.-L.; Hung, W.-
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Chen, C. H. Jpn. J. Appl. Phys. 2006, 45, 555. (e) Lai, M.-Y.; Chen, C.-H.;
Huang, W.-S.; Lin, J.-T.; Ke, T.-H.; Chen, L.i-Y.; Tsai, M.-H.; Wu, C.-C.
Angew. Chem. 2008, 120, 591;Angew. Chem., Int. Ed. 2008, 47, 581. (f)
Tsai, T.-C.; Hung, W.-Y.; Chi, L.-C.; Wong, K.-T.; Hsieh, C.-C.; Chou,
P.-T. Org. Electron. 2009, 10, 158.
(4) (a) Qiu, Y.; Wei, P.; Zhang, D.; Qiao, J.; Duan, L.; Li, Y.; Gao, Y.;
Wang, L. AdV. Mater. 2006, 18, 1607. (b) Wei, P.; Duan, L.; Zhang, D.;
Qiao, J.; Qiu, Y. J. Mater. Chem. 2008, 18, 806.
(5) (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (b) Neeff,
R.; Bayer, O. Chem. Ber. 1957, 90, 1137. (c) Wu, F.-I.; Shih, P.-I.; Tseng,
Y.-H.; Chen, G.-Y.; Chien, C.-H.; Tung, Y.-L. J. Mater. Chem. 2005, 15,
4753. (d) Laskar, I. R.; Chen, T.-M. Chem. Mater. 2004, 16, 111. (e)
Moghaddam, F.-M.; Bardajee, G.-R.; Ismaili, H.; Taimoory, S.-M. Synth.
Commun. 2006, 36, 2543. (f) Kato, S.-I.; Matsumoto, T.; Ishi-i, T.;
Thiemann, T.; Shigeiwa, M.; Gorohmaru, H.; Maeda, S.; Yamashita, Y.;
Mataka, S. Chem. Commun. 2004, 2342.
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2003, 15, 1080. (b) Shirota, Y.; Kinoshita, N.; Noda, T.; Okumoto, K.;
Ohara, T. J. Am. Chem. Soc. 2000, 122, 1102.
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Org. Lett., Vol. 11, No. 10, 2009

Figure 2. Absorption and PL emission spectra of CHCl3 solution
and thermally evaporated film of BBTPNTD.
The TOF transients of electrons and holes for BBTPNTD
in air at room temperature are shown in Figure 3. The
electron and hole mobilities of BBTPNTD are very close
and extremely high, reaching 1.7 × 10-3 cm2/(V·s) and 1.9
× 10-3 cm2/(V·s) at an electric field of 4.5 × 105 V/cm,
respectively. No plateau is observed in Figure 3, indicating
dispersive transport for electrons and holes in BBTPNTD.
The field-dependence of the electron and hole mobilities of
BBTPNTD are shown in Figure 4. The field-dependence of
both the electron and hole are well-agreed with the Poole-
Frenkel (PF) relationship, µ ) exp(ꢀE1/2), where ꢀ is the
Poole-Frenkel factor.
To acquire a better understanding of the ambipolar
property, theoretical calculations on the electronic states of
BBTPNTD were carried out at the B3LYP/6-13G(d) level
in the Gaussian 98 program.8 The electron-density distribu-
tions of HOMO and LUMO as well as the schematic
representation of the packing of the adjacent BBTPNTD
molecules are shown in Figure 5. It is noted that both HOMO
and LUMO are mainly located on the NTD chromophore in
BBTPNTD; thus, electrons and holes can perform intermo-
lecular hopping in similar spatial extent between the adjacent
NTD chromophores through the π-π stacking interaction.
That is, BBTPNTD molecules exhibit similar charge-transfer
integrals for electrons and holes. The electron and hole
reorganization energies were also calculated to be very close
(0.32 eV for electrons and 0.25 eV for holes), indicating that
the abilities of the molecules to accept and donate electrons
are similar. According to the Marcus theory, similar charge-
transfer integrals and reorganization energies for electrons
Figure 3. TOF transients for BBTPNTD at the electric field of 4.5
× 105 V/cm: (a) electrons; (b) holes. The insets display the
corresponding double-logarithmic plots.
and holes will lead to the ambipolar transport property.8
Moreover, as the result of theoretical calculations, the
dihedral angle between the phenyl and benzothiaol is only
(7) (a) Kepler, R. G. Phys. ReV. 1960, 119, 1226. (b) Fong, H. H.; Lun,
K. C.; So, S. K. Chem. Phys. Lett. 2002, 353, 407.
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W. T.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (c) Frisch, M. J.; Trucks,
G. W.; Schlegel, H. B. G.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.;
Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.;
Raghavachari, K.; AlLaham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.;
Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.;
Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.;
Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.;
Binkley, J. S.; Defrees, D. Baker, J.; Stewart, J. P.; Head-Gordon, M.;
Gonzalez, ; Pople, C. J. A. Gaussian 98, Revision A.9, Gaussian: Pittsburgh,
PA 1998.
Figure 4.
Electron and hole mobilities vs E1/2 for BBTPNTD.
Org. Lett., Vol. 11, No. 10, 2009
2071

interaction between the adjacent molecules, finally improving
the carrier transporting property. On the other hand, the
whole molecule is twisted with the dihedral angle of 54°
between the NTD group and the (benzol[d]thiazol-2-yl)phe-
nyl group, which is similar to those of other NTD deriva-
tives.4
In summary, we have synthesized and characterized a new
naphtho[2,3-c][1,2,5]thiadiazole derivative which possesses
ambipolar carrier transporting property. Both the electron
and hole mobilities reached as high as 10-3 cm2/(V·s) at room
temperature. The unusual ambipolar carrier transporting
property in the amorphous state is explained according to
the molecular reorganization energy, the distribution of
electrons on frontier orbitals as well as the configuration of
the molecule.
Acknowledgment. We thank the National Natural Science
Foundation of China (No. 60877026) and the National Key
Basic Research and Development Program of China under
Grant No. 2006CB806203 for support.
Figure 5. Molecular orbital surfaces of HOMO and LUMO of
BBTPNTD obtained at the B3LYP/6-31G* level. All the MO
surfaces correspond to an isocontour value of Ψ ) (0.0025.
Supporting Information Available: Spectroscopic char-
acterization of BBTPNTD. This material is available free
of charge via the Internet at http://pubs.acs.org.
0.5°, which means that all the atoms in the (benzothiaol-2-
yl)phenyl are nearly on the same plane. The good planarity
of the side groups facilitates reducing the distance between
the adjacent molecules and enhancing the π-π staking
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