Thiophene-fused tetracene diimide with low band gap and ambipolar behavior

Article abstract of DOI:10.1021/ol202357f

The first tetracene diimide derivative fused with four thiophene rings, TT-TDI, was synthesized by an FeCl₃ mediated oxidative cyclodehydrogenation reaction. TT-TDI exhibited a low band gap of 1.52 eV and amphoteric redox behavior. TT-TDI also

Full text of DOI:10.1021/ol202357f

ORGANIC
LETTERS
2011
Vol. 13, No. 22
5960–5963
Thiophene-Fused Tetracene Diimide with
Low Band Gap and Ambipolar Behavior
Qun Ye,† Jingjing Chang,† Kuo-Wei Huang,‡ and Chunyan Chi*,†
Department of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore, 117543, and KAUST Catalysis Center and Division of Chemical and Life
Sciences and Engineering, 4700 King Abdullah University of Science and Technology,
Thuwal 23955-6900, Kingdom of Saudi Arabia
chmcc@nus.edu.sg
Received August 31, 2011
ABSTRACT
The first tetracene diimide derivative fused with four thiophene rings, TT-TDI, was synthesized by an FeCl3 mediated oxidative cyclo-
dehydrogenation reaction. TT-TDI exhibited a low band gap of 1.52 eV and amphoteric redox behavior. TT-TDI also showed a liquid crystalline
property and ambipolar charge transport in thin film field-effect transistors.
Acenes have been intensively investigated for their po-
tential as charge transporting materials.1 However, their
practical applications are usually limited by relatively poor
chemical and photostability.2 Various methods have been
applied to improve their stability which include (1) sub-
stitution by aryl, silylethyne, or thioether groups and (2)
fluorination and substitution with fluorinated groups.2
Alternatively, attachment of electron-withdrawing dicarb-
oximide groups is supposed to be an efficient way to lower
the HOMO energy level of acenes and thus to improve
their stability. Our group recently prepared a series of
stable pentacene diimide derivatives by lateral substitution
with dicarboximide groups.3 Since the most reactive sites of
acenes are located at the zig-zag edges, vertical attachment
of dicarboximide groups is believed to be more efficient to
obtain highly stable acene imides. So far, only naphthalene
mono- or diimides,4 anthracene mono- and diimide,5 and
tetracene monoimides6 with the imide group attached onto
the zig-zag edges have been reported. Synthesis of longer
acenes substituted by more than one vertically attached
imide group is challenging. Herein, we report the first
successful synthesis of a tetrathienyl-fused tetracene diimide
TT-TDI (Scheme 1). Our strategy is to make a core expan-
sion from a smaller naphthalene diimide building block
such as 1 (Scheme 1), and the obtained compound TT-TDI is
expected to show a low band gap and long wavelength abso-
rption due to intramolecular donorꢀacceptor interaction.
† National University of Singapore.
‡ King Abdullah University of Science and Technology.
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r
10.1021/ol202357f
Published on Web 10/14/2011
2011 American Chemical Society

tetracene diimides. Thus, precursors 5, 6, and 7 with
different substituents were prepared (Scheme 1). Com-
pound 1 underwent similar 4-fold Stille coupling reactions
to give the tetrathiophene-substituted naphthalene diimide
3 in 56% yield. Bromination of 3 with NBS in DMF11
afforded 4 in 87% yield, and subsequent 4-fold Suzuki
coupling reactions provided compounds 5ꢀ7 in good
yields. Precursors 5ꢀ7 were then submitted for ferric
chloride mediated oxidative cyclodehydrogenation. How-
ever, reaction of 5 generated a dark blue mixture which
contained intermolecular coupling products and chlori-
nated side products based on mass spectrometry analysis.
By lowering the oxidant loading or decreasing the reaction
temperature from room temperature to 0 °C, this problem
was not well resolved. We reasoned that the para-position
of the end-capping phenylene ring should be responsible
for the undesired reactivity of this precursor. Therefore, in
6 and 7, the para-positions of the phenylene ring are
blockedbyanelectron-withdrawing trifluoromethyl group
or electron-donating tert-butyl group. For both 6 and 7,
the oxidative cyclization by ferric chloride, however, only
generated one-side closed (i.e., ꢀ2H) products which were
contaminated with starting material and some chlorinated
side products, and separation of the mixture was not
successful. Prolongation of the reaction time or addi-
tion of extra oxidant did not lead to fully cyclized
products but more chlorinated side products. To ad-
dress this problem, we further attempted the cyclization
of 4 with the attempt to synthesize a general functio-
nalizable tetracenediimide building block. Again, cy-
clization of 4 with either ferric chloride or VOF3/
BF3•Et2O only gave partially cyclized product together
with some chlorinated side products.
To better understand the ring closure reaction, the
electrochemical properties of 2 and 4ꢀ7 were studied by
cyclic voltammetry (Figure S1 and Table S1 in the Sup-
porting Information (SI)). All fivecompounds showed two
reversible reduction waves and one irreversible oxidation
wave. The successful substrate 2 was found to have a
HOMO energy level of ꢀ5.73 eV. By changing the dodecyl
chain to an electron-deficient bromine and trifluomethyl
phenyl group, the HOMO energy levels of 4 and 6 were
decreased to ꢀ5.99 and ꢀ5.85 eV, respectively, and this
HOMO energy level decrease would be responsible for
their unsuccessful oxidative cyclization. For 5 and 7,
the HOMO energy level increased to ꢀ5.67 and ꢀ5.59
eV, respectively. With the substituents being electron-
rich, the radical cation generated might delocalize
along the thienyl-phenyl unit and this may lead to
unwanted intermolecular coupling, chlorination, and
low reactivity of the cation. Therefore, the oxidation
potential of the precursor played a key role on the
oxidative cyclization reaction.
Scheme 1. Synthetic Route of TT-TDI and Related Precursors
TT-TDI is also designed to have a rigid core attached with
flexible alkyl chains so that it can show liquid crystalline
properties. Application of TT-TDI in thin-film field-effect
transistors was also investigated.
Synthesis of TT-TDI and related precursors is shown
in Scheme 1. The starting material 2,3,6,7-tetrabromo
naphthalene diimide 1 was synthesized according to re-
ported literature.7 Tetrathienyl-substituted naphthalene
diimide 2 was prepared by a 4-fold Stille coupling of 1
with tributyl(5-dodecylthiophen-2-yl)stannane.8 It is worthy
to note that the catalyst loading of this 4-fold Stille
coupling reaction had to be carefully controlled in order
to avoid any phosphonium salt formation.9 By lowering
the loading of Pd(PPh3)4 to 0.03 equiv per bromine, the
target product 2 was obtained in a 63% yield. Subsequent
ferric chloride mediated oxidative cyclization10 of 2 suc-
cessfully gave the final product TT-TDI in 80% yield. Such
intramolecular cyclization was thought to be challenging
because (1) the electron-deficient naphthalene diimide core
will increase the oxidation potential which makes the
oxidative cyclization hard to occur and (2) there is sig-
nificant steric repulsion between the thiophene units and
the carbonyl group, which gives extra barriers for the
reaction to occur.
The success on TT-TDI provoked us to further extend
this strategy to the synthesis of more tetrathienyl-fused
(7) Gao, X.; Qiu, W.; Yang, X.; Liu, Y.; Wang, Y.; Zhang, H.; Qi, T.;
Liu, Y.; Lu, K.; Du, C.; Shuai, Z.; Yu, G.; Zhu, D. Org. Lett. 2007, 9,
3917.
(8) Luo, J.; Huang, K.; Qu, H.; Zhang, X.; Zhu, L.; Chan, H.; Chi, C.
Org. Lett. 2010, 12, 5660.
(9) (a) The generated high polar byproduct has red color and good
solubility in chloroform if the catalyst loading is greater than 0.03 equiv
per bromine. MS and 1H NMR results indicate that one triphenylpho-
sphine is attached to the core with the loss of one bromine atom. (b)
Melpoder, J. B.; Heck, R. F. J. Org. Chem. 1976, 41, 265. (c) Ziegler,
C. B.; Heck, R. F. J. Org. Chem. 1978, 43, 2941. (d) Marcoux, D.;
Charette, A. B. J. Org. Chem. 2008, 73, 590.
In solution, both 2 and TT-TDI exhibited a strong πꢀπ*
transition with a maximum absorption at ca. 370 nm and
a strong charge transfer band at 494 and 655 nm,
€
(11) Kruger, H.; Janietz, S.; Sainova, D.; Dobreva, D.; Koch, N.;
€
(10) Wu, J.; Pisula, W.; Mullen, K. Chem. Rev. 2007, 107, 718.
Vollmer, A. Adv. Funct. Mater. 2007, 17, 3715.
Org. Lett., Vol. 13, No. 22, 2011
5961

Figure 2. Cyclic voltammogram (CV) and differential pulse
voltammogram (DPV) of TT-TDI in dry DCM with 0.1 M
Bu4NPF6 as supporting electrolyte.
energy gap was calculated to be 1.52 eV. It is worth noting
that the HOMO of TT-TDI only changed slightly com-
pared with 2 while the LUMO decreased from ꢀ3.73 eV
for 2 to ꢀ4.10 eV for TT-TDI after cyclization. The low
lying LUMO energy level implies that TT-TDI can serve as
an electron transporting material.
Figure 1. (a) UVꢀvis absorption spectra of 2 and TT-TDI in
chloroform solution (1.0 ꢁ 10ꢀ5 M) and in thin film. Insert:
photos for the dilute solution of 2 and TT-TDI in chloroform.
(b) Calculated three conformers of TT-TDI.
The differential scanning calorimetry (DSC) curve of
TT-TDI showed two endothermic peaks at 13 and 146 °C
upon heating and two exothermic peaks at 2 and 101 °C
upon cooling (Figure 3a). From polarizing optical micro-
scope (POM) analysis, TT-TDI entered an isotropic phase
at 150 °C upon heating, and upon slow cooling from the
isotropic phase, a typical texture for a columnar liquid
crystalline mesophase was observed until room tempera-
ture (insert in Figure 3a). That means TT-TDI is actually a
room-temperature liquid crystal. A powder XRD pattern
of TT-TDI recorded at room temperature revealed a series
of sharp reflection peaks which can be correlated to the
(100), (200), and (300) reflections of a lamellar structure
with an interlayer distance of 2.71 nm (Figure 3b). In
respectively (Figure 1a). Accordingly, the solution color
of 2 and TT-TDI was purple and blue, respectively
(Figure 1a). The absorption maximum of the charge trans-
fer peak was red-shifted about 160 nm after ring closure,
indicating that TT-TDI had a lower band gap and more
extended π-system. In the solid state, TT-TDI showed a
bathochromic shift of 47 nm due to aggregation at the solid
state (Figure 1a). The lowest energy absorption onset of the
solution absorption was at 754 nm, and the optical band
gap was calculated to be 1.64 eV for TT-TDI. TT-TDI
exhibited a very weak emission peak with an emission
maximum at ca. 775 nm when excited at 650 nm (Figure
S2 in SI) due to the charge transfer nature of the molecule.
Time-dependent density function theory (TDDFT at
B3LYP/6-31G*) calculations were conducted for TT-TDI
to understand its electronic and optical properties. Due to
stericcongestion between the fused thiophene rings and the
carbonyl groups, three possible conformers, denoted as
upꢀdown, upꢀup, and twisted, were evaluated (Figure 1b).
The upꢀdown conformer has the lowest energy (1.5ꢀ
3.5 kcal/mol more stable), but all conformers show a
similar calculated absorption spectrum with two major
bands at 347 nm (HOMO to LUMO þ 2) and 627 nm
(HOMOꢀ1 to LUMO) (see SI). The band shapes are in
good agreement with the experimental data.
˚
addition, a reflection peak at 4.48 A was also observed,
which can be correlated to the long-range ordered
π-stacking between the rigid but nonplanar core.
Field-effect transistors were fabricated by spin-coating
the solution of TT-TDI in chloroform onto an octadecyl-
trimethoxysilane (OTMS) modified SiO2/Si substrate in a
top-contact configuration (gold as source/drain electrodes).
The devices were annealed at 90 °C under nitrogen.
Devices based on TT-TDI mainly exhibited n-type behav-
ior when measured under nitrogen (Figure 4aꢀb), with
an average saturation electron mobility μsat(e) = 4.83 ꢁ
10ꢀ3 cm2/(V s), threshold voltage Vth = 56 V, and On/Off
ratio = 5 ꢁ 105. However, in air, the device exhibited
typical ambipolar character (Figure 4cꢀd), with μsat(e) =
7.4 ꢁ 10ꢀ4 cm2/(V s), Vth = 40 V, and On/Off ratio =
103ꢀ104 for n-channel operation and μsat(h) = 9.0 ꢁ 10ꢀ4
cm2/(V s), Vth = ꢀ20 V, and On/Off ratio = 103ꢀ104 for
p-channel operation. The hole transport was enhanced in
ambient perhaps due to doping by molecular oxygen. Such
an ambipolar charge transporting character and the ob-
served amphoteric redox behavior must originate from its
TT-TDI exhibited amphoteric redox properties with one
ox
reversible1eoxidationwavewithhalfwavepotentialE1/2
at 0.94 V (vs Fcþ/Fc), two reversible 1e reduction waves
with half wave potential E1/2red at ꢀ0.77 V and ꢀ0.89 V (vs
Fcþ/Fc), respectively, and one irreversible reduction wave
(Figure 2 and Table S1). A HOMO energy level of ꢀ5.62
eV and a LUMO energy level of ꢀ4.10 eV were estimated
based on the onset potential of the first oxidation wave and
the first reduction wave, respectively. The electrochemical
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Org. Lett., Vol. 13, No. 22, 2011

Figure 4. Output (a) and transfer (bꢀd) characteristic of the
OFETs based on TT-TDI. (aꢀb) Measured under nitrogen;
(cꢀd) measured in air.
gap due to intramolecular donorꢀacceptor interaction as
well as extended π-conjugation, and it displays amphoteric
redox behavior. It is worth noting that, although with a
nonplanar core structure, the TT-TDI showed ordered
self-assembly and exhibited room-temperature liquid crys-
talline properties. Ambipolar charge transport was also
observed in thin film field-effect transistors.
Figure 3. (a) The first cooling and the second heating DSC
curves of TT-TDI measured under a nitrogen atmosphere with
heating/cooling rate of 10 °C/min. Insert is the polarizing optical
microscope image recorded at 136 °C during cooling. (b)
Powder XRD pattern of TT-TDI recorded at 30 °C. Insert is
the proposed layer-like packing mode.
small band gap. Atomic force microscope (AFM) mea-
surements showed a highly ordered liquid crystalline thin
film after thermal annealing, which is in accordance with
the XRD results. The thin-film XRD pattern indicated an
ordered packing with up to five orders of diffraction. And
the out of plane d-spacing of 2.71 nm in thin film corre-
sponds to the interlayer distance which suggests a layer-
like superstructure (see SI).
In summary, we have successfully synthesized the first
tetracene diimide TT-TDI fused with four electron-rich
thiophene units. It was found that the HOMO energy level
of the precursors is crucial for successful oxidative cycliza-
tion mediated by ferric chloride. TT-TDI has a small band
Acknowledgment. C.C. acknowledges financial support
from the National University of Singapore under MOE
AcRF FRC Grants R-143-000-444-112 and Start Up
Grant R-143-000-486-133. K.-W.H. acknowledges the
financial support from KAUST.
Supporting Information Available. Synthetic proce-
dures and characterization data for all new compounds,
PL spectrum of TT-TDI, additional CV data, theoretical
calculation details, and more data on device fabrication
and characterization. This material is available free of
charge via the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 22, 2011
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