Aryl-perfluoroaryl substituted tetracene: Induction of face-to-face π-π Stacking and enhancement of charge carrier properties

Article abstract of DOI:10.1021/cm200356y

5-Perfluorophenyl-11-phenyltetracene (FPPT) having both aryl (Ar) and perfluoroaryl (FAr) groups has been designed and synthesized to obtain well-defined n-n stacking structure of the tetracene core. Single-crystal X-ray analyses as well as photoconductiv

Full text of DOI:10.1021/cm200356y

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Aryl-Perfluoroaryl Substituted Tetracene: Induction of Face-to-Face
π-π Stacking and Enhancement of Charge Carrier Properties
||
Toshihiro Okamoto,*,†, Katsumasa Nakahara,† Akinori Saeki,‡ Shu Seki,‡ Joon Hak Oh,§,^
Hylke B. Akkerman,§ Zhenan Bao,§ and Yutaka Matsuo*,†
†School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
‡Division of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
§Department of Chemical Engineering, Stanford University, 381 North-south Mall, Stanford, California 94305-5025, United States
S Supporting Information
b
KEYWORDS: tetracene, aryl-perfluoroaryl interaction, photoconductivity, organic thin-film transistor, crystal engineering
ontrol of molecular ordering and packing of π-conjugated
molecules in the solid state in order to enhance their charge
of tetracene was performed using a previously reported method.6
The Suzuki-Miyaura coupling reaction7 of sterically hindered
5,11-dibromotetracene with phenylboronic acid was successfully
carried out using tris(dibenzylideneacetone)dipalladium(0)
(Pd2(dba)3), bis(2-diphenylphosphinophenyl)ether (DPEphos),
and K2CO3 in a mixture of toluene and ethanol to obtain
monosubstituted 5-bromo-11-phenyltetracene (BrPT) in 57%
yield, together with PPT in 18% yield. The perfluorophenyl
group was introduced by perfluoroarylation8 of BrPT to afford
the target compound (FPPT) in 78% yield. FPPT showed good
solubility in most chlorinated organic solvents as well as in THF
and toluene, which makes it a promising candidate for solution-
processable devices.
The cyclic voltammograms of FPPT and PPT in dichloro-
methane showed reversible one-electron reduction and one-
electron oxidation (Table S1 and Figure S1 in the Supporting
Information). The half-wave potentials for reversible oxidation
and reduction of FPPT were found to be 0.18 and 0.15 V higher
than those of PPT, respectively. These observations were in good
agreement with the results from DFT calculation (Figure S2 in
the Supporting Information). These results suggest that both the
oxidized and reduced species of these compounds are stable in
solution and that one perfluorophenyl group could successfully
lower the HOMO and LUMO levels.
C
transport properties is critical for organic electronics.1 For
instance, the use of bulky substituents to control the packing
density of π-conjugated compounds is an effective means of
tuning the electronic interaction between molecules. This has
been demonstrated with bulky trialkylsilylethynyl-substituted
pentacenes, end-group-substituted oligothiophenes, halogenated
perylene diimide derivatives, and so on.2 These approaches to
crystal engineering have resulted in high-performance organic
field-effect transistors (OFETs).
Herein we report an efficient method for control of molecular
packing, as illustrated in Figure 1. We chose a simple tetracene core
substituted with aryl (Ar) and perfluoroaryl (FAr) groups at the 5-
and 11-positions as a demonstration of concept (Figure 1a).
Regarding our molecular design, we hypothesized that the Ar
and FAr substituents would induce not only an Ar-FAr
interaction3 (face-to-face stacking, Figure 1b) but also a supporting
C-H F interaction(shortlateraldistance, Figure1b,c) between
3 3 3
neighboring molecules.4 Thus, it can be expected that such “strong,
complementary” interactions will lead to a well-defined π-π
stacking structure of charge transporting units (Figure 1c). We
also disclose the photoconductivity and charge carrier mobility of
the compound to discuss the validity of the concept.
In detail, phenyl and perfluorophenyl substituents were se-
lected for the following reasons: (1) Quadrupole moment:
Because phenyl and perfluorophenyl groups each have a large,
permanent quadrupole moment with similar magnitude and
opposite sign,5 the quadrupole moments are expected to effec-
tively offset each other; thus, these groups do not interrupt
charge transport and can serve as charge transport units. (2)
Electronic effect: Despite acene-FAr bonds being twisted
because of steric hindrance, an electron-deficient FAr substituent
on an electron-rich acene should slightly decrease the HOMO
and LUMO energies, thus improving chemical stability. To this
end, we synthesized 5-perfluorophenyl-11-phenyltetracene
(FPPT), together with 5,11-diphenyltetracene (PPT) as a re-
ference compound for comparison (Scheme 1).
Stability under ambient conditions is also an important
criterion for organic semiconductors. We evaluated the stability
of FPPT by UV-vis spectroscopy in chloroform, under white-
light illumination in air (Figures S3 and S4 in the Supporting
Information). FPPT was found to have the highest stability
among investigated compounds: its half-life was six-, six-, and
twofold longer than that of PPT, rubrene, and tetracene,
respectively. This observation indicates that the electron-with-
drawing perfluorophenyl group is a useful moiety for improving
the oxidative stability of organic semiconducting materials. In
addition, FPPT was thermally stable up to 300 ꢀC in nitrogen
Received: February 3, 2011
FPPT was synthesized in three steps starting from commer-
cially available tetracene, as shown in Scheme 1. Dibromination
Revised:
Published: March 11, 2011
February 28, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/cm200356y Chem. Mater. 2011, 23, 1646–1649
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Figure 1. Molecular design and concept for the enhancement of π-π
stacking between neighboring charge transporting units by the intro-
duction of Ar and FAr substituents.
Scheme 1. Synthesis of FPPTa
Figure 2. Crystal structure of FPPT. (a) Molecular structure with
dihedral angles between planes A, B, C, and D. (b) Two-site inter-
molecular interactions (Ar-FAr interaction and C-H F inter-
3 3 3
action). (c) C-H
F
interactions between substituents. (d)
3 3 3
Distance between tetracene units. (e) CPK model of the packing
structure.
a Reagents and Conditions: (a) Phenylboronic acid, Pd2(dba)3, DPE-
phos, 2 M K2CO3 aq, toluene-EtOH, 95 ꢀC, 4.5 h, 57%. (b) Penta-
fluorobenzene, Pd(OAc)2, S-Phos, K2CO3, iPrOAc, 80 ꢀC, 43 h, 78%.
The third involves close π-π interorbital interaction between
the tetracene cores. The interplanar distance between tetracene
cores ranged from 3.30 to 3.35 Å, shorter than the sum of the
(Figure S5 in the Supporting Information) and tolerated vacuum
deposition in our test.
C
C van der Waals radii (3.40 Å) (Figure 2d). Therefore,
3 3 3
efficient charge transport can be expected from the strong
electronic coupling between the adjacent molecules
(Figure 2e). On the other hand, π-π stacking of the tetracene
core of PPT was not observed (Supporting Information). Only
To reveal the intermolecular interactions, we performed
single-crystal X-ray structure analyses of FPPT and PPT
(Figures 2 and S6-S11 in the Supporting Information). In the
molecular structure of FPPT, the tetracene core has an almost
flat configuration with an interplanar angle of 0.25ꢀ between
the mean planes of rings B and C (Figure 2a). The dihedral
angles between the tetracene core and substituents were
roughly perpendicular (Figure 2a). There are three types of
intermolecular interactions. One type is the Ar-FAr interaction
(Figure 2b) in the face-to-face stacking mode of phenyl and
perfluorophenyl groups with an interplanar distance of 3.37 Å, a
similar value to that reported for benzene-d6 and perfluoro-
the C-H π interaction can be seen in the single-crystal
3 3 3
structure. These data indicate that the Ar and FAr groups play
an essential role in the molecular ordering in the single crystal.
As judged from the X-ray structure data, the molecular packing
structure of FPPT should be highly beneficial to charge transport
properties because the system has a well-ordered face-to-face π-
π stacking structure, which can create effective electronic overlap
of the π-orbitals. To measure the conductive properties of FPPT,
we carried out flash-photolysis time-resolved microwave con-
ductivity (FP-TRMC) measurements, which can be used to
predict the nanometer-scale conductivity of charge carriers
generated by laser pulse irradiation under a low-frequency
microwave electric field.11 Figure 3a,b shows the photoconduc-
tivity (φ∑μ)12 and its peak value for microcrystalline samples of
benzene (3.36 Å).9 The second is the C-H F interactions,
3 3 3
seen in the following short contacts: C-H1 F1, C-
3 3 3
H2 F2, and C-H3 F3, as shown in Figure 2b,c. In
3 3 3
3 3 3
particular, C-H1 F1 exhibits the shortest distance of 2.40
3 3 3
Å, which might be critical to forming this packing structure.4,10
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interactions between Ar and FAr play an effective role in
controlling the thin-film morphology, as well as in enhancing
the charge carrier mobility.
In conclusion, we have demonstrated that the introduction of
Ar and FAr groups as substituents onto tetracene at the 5- and 11-
positions induced a well-defined, close face-to-face stacking of
the tetracene cores. This approach is promising for inducing π-
π stacking among charge transport units. Additionally, the
proposed synthetic methodology can be easily used to prepare
other organic semiconducting cores starting from the corre-
sponding dibromo compounds. Photoconductivity and OTFT
measurements clearly demonstrated that the proposed system
can control the molecular packing as well as the film morphology,
resulting in enhanced charge transport properties. Further
investigations on the synthesis of new derivatives as well as
OFET measurements on single-crystal FETs are currently under-
way in our laboratory.
’ ASSOCIATED CONTENT
S
Supporting Information. Supporting Information with
b
detailed synthetic procedures, electrochemical and thermal data,
stability test data, theoretical calculations, and thin-film morphol-
ogy and crystallinity data for FPPT and PPT (PDF). X-ray crystal
structure data for FPPT and PPT are also included (CIF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 3. Photoconductivity of FPPT. (a) Transient photoconductivity
for FPPT, PPT, and rubrene (red, blue, and black lines, respectively)
microcrystals. (b) Comparison of maximum photoconductivity. (c)
Crystal axes and indices for axis-dependent photoconductive mea-
surement on a FPPT single crystal. (d) Result of the anisotropic
photoconductivity.
’ AUTHOR INFORMATION
FPPT, PPT, and rubrene. Rubrene is an appropriate material for
comparison because it has the same tetracene core with phenyl
substituents and was reported to have the highest hole mobility
among organic single-crystal FETs.13 Notably, FPPT exhibited
the same order of photoconductivity and kinetics behavior as
rubrene, and both have considerably higher photoconductivity
than PPT.
We next investigated the anisotropic charge transport proper-
ties of a FPPT single crystal. The overlapping π-orbitals of the
tetracene cores are aligned face-to-face along the a and c axes (see
Figure 2d,e). The microwave electric field was applied parallel (a
and b axes) and perpendicular (c axis) to the quartz plate
(Figure 3c) to evaluate photoconductivity for each axis. Peak
conductivity was found to be ca. twofold larger in the a and c axes
than in the b axis. These results suggest that suitable molecular
arrangement will provide the charge carrier pathway for good
mobility in future works.
Finally, we evaluated thin-film charge transport properties and
investigated thin-film morphologies to discuss the effects of Ar
and FAr groups. Perfluoroaryl- or perfluoroalkyl-containing
organic thin-film transistors (OTFTs) have been paid much
attention.14 OTFTs based on FPPT and PPT were fabricated in a
top-contact, bottom-gate configuration. Notably, FPPT was
found to exhibit an average hole mobility that reached 4.2 Â
10-2 cm2/(V s) with an on/off ratio greater than 105 on an n-
octadecyltrimethoxysilane (OTS)-treated SiO2 substrate (Table
S2 and Figure S12 in the Supporting Information).15 Under the
same conditions, however, PPT exhibited values smaller by 2
orders of magnitude (3.1 Â 10-4 cm2/(V s)).15 These results
were supported by AFM images of thin films of FPPT and PPT,
which displayed smooth two-dimensional and bumpy three-
dimensional crystal growth, respectively (Figures S13-16 in
the Supporting Information). The results suggest that the
Corresponding Author
*E-mail: tokamo@sanken.osaka-u.ac.jp (T.O.); matsuo@
chem.s.u-tokyo.ac.jp (Y.M.).
Present Addresses
Department of Advanced Electron Devices, The Institute of
Scientific and Industrial Research (ISIR), 8-1 Mihogaoka, Ibaraki,
Osaka 567-0047, Japan.
^School of Nano-Bioscience & Chemical Engineering, Ulsan
National Institute of Science & Technology (UNIST), Ulsan
689-798, Korea.
’ ACKNOWLEDGMENT
The authors thank Prof. Eiichi Nakamura and Dr. Anatoliy N.
Sokolov for valuable discussion. This work was supported
by JSPS Grant-in-Aid for Young Scientists (Start-up) (No.
20850036). H.B.A. acknowledges the Netherlands Organization
for Scientific Research (NWO) for support. This work was
partially supported by Strategic Promotion of Innovative Re-
search and Development from Japan Science and Technology
Agency, JST.
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