High mobility single-crystal field-effect transistors from bisindoloquinoline semiconductors

Article abstract of DOI:10.1021/ja077444g

A novel heptacyclic bisindoloquinoline-based organic semiconductor has been synthesized, characterized, and used to fabricate single-crystal field-effect transistors. A synthetic route was developed for the synthesis of heptacyclic bis(indolo{1,2-a})quino

Full text of DOI:10.1021/ja077444g

Published on Web 01/03/2008
High Mobility Single-Crystal Field-Effect Transistors from Bisindoloquinoline
Semiconductors
Eilaf Ahmed,† Alejandro L. Briseno,† Younan Xia,† and Samson A. Jenekhe*,†,‡
Departments of Chemistry and Chemical Engineering, UniVersity of Washington, Seattle, Washington 98195-1750
Received September 26, 2007; E-mail: jenekhe@u.washington.edu
Scheme 1. The Synthesis of Bis(indolo{1,2-a})quinolines
Organic semiconductors are of broad fundamental and techno-
logical interests.1-8 They are being explored in device applications
such as organic field-effect transistors (OFETs), photovoltaic cells,
and light-emitting diodes.1-8 There is a great interest in understand-
ing charge transport mechanisms and in establishing the underlying
structure-property relationships in organic semiconductors.1-8
Although there is a vast literature on small-molecule and oligomer-
based semiconductors, such as oligoacenes and oligothiophenes,
the design and synthesis of new materials are necessary to enable
“molecular engineering” of electronic properties for realizing high-
performance organic electronics.1-8
A common method for investigating the intrinsic charge transport
properties of organic semiconductors is the single-crystal field-effect
transistor.7 Organic single crystals are free of grain boundaries or
molecular disorder and are routinely employed as tools for
determiningtheperformancelimitationsofsemiconductormaterials.1c,6
We report herein high-performance single-crystal transistors fab-
ricated from a novel heptacyclic organic semiconductor bearing
nitrogen heteroatoms and a planar molecular backbone, tetraphe-
nylbis(indolo{1,2-a})quinoline (TPBIQ). The newly synthesized
and crystallized TPBIQ exhibits a slipped π-π stacking, a low
ionization potential, and field-effect mobilities as high as 1.0 cm2/
V‚s.
The synthetic pathway to the heptacyclic bis(indolo{1,2-a})-
quinolines (1) involved an intramolecular cyclization reaction on
anthrazoline derivatives (4) as shown in Scheme 1. This new class
of heteroaromatic semiconductors contains seven fused rings,
including five hexagons and two pentagons. Our group has reported
the structure and properties of derivatives of compound 4 which
do not exhibit any significant π-π interactions.8 Utilizing an
intramolecular cyclization reaction on the tricyclic 4, we anticipated
a more conjugated and planar heteroaromatic framework for charge
transport in the heptacyclic 1. The intramolecular cyclization of 4
proceeds in the presence of anhydrous DDQ to afford TPBIQ in
32% yield. Tetraphenylanthrazoline 4 was synthesized from the
diphenylacetone 3 and 2,5-dibenzoyl-1,4-phenylenediamine 2 in a
high yield (75%) via a Friedlander condensation using diphenyl-
phosphate (DPP) as an acid catalyst.9 2,5-Dibenzoyl-1,4-phenylene-
diamine 2 was synthesized according to the literature.10 TPBIQ
was twice recrystallized from a 1:1 THF/MeOH solvent mixture
and is soluble in common organic solvents such as chloroform,
benzene, and toluene. 1H NMR, high-resolution mass spectrometry,
and X-ray crystallography confirmed the structure of TPBIQ. For
example, the singlet resonance at δ 5.81 ppm due to the methine
proton in precursor 4 was completely absent in the 1H NMR
spectrum of TPBIQ. A single molecular ion peak at 658.91 m/z,
which is expected from the calculated molecular weight was
observed in the high-resolution mass spectrum. Thermogravimetric
(Td) at 427 °C. A sharp melting peak (Tm) at 375 °C was observed
by differential scanning calorimetry. The high Td and Tm of TPBIQ
indicate its excellent thermal stability.
Cyclic voltammetry was performed in benzene/acetonitrile (3:1
v/v) solution and on thin films. TPBIQ shows a reversible oxidation
and quasi-reversible reduction in solution (Supporting Information).
The half-wave oxidation (E1/2ox) and reduction (E1/2red) potentials
were 0.79 and -1.80 V (vs SCE), respectively. From the observed
redox potentials, the ionization potential (IP) or HOMO level and
electron affinity (EA) or LUMO level of TPBIQ were estimated11
to be ∼5.19 and 2.60 eV, respectively. Cyclic voltammetry of
TPBIQ thin films gave onset oxidation and reduction potentials
of 0.74 and -1.61 V (vs SCE), respectively, from which we derived
a solid-state IP (or HOMO level) of 5.14 eV and an EA of 2.79
eV.11 The estimated electrochemical band gap (Egel) is thus 2.35
eV.
The optical absorption and photoluminescence (PL) spectra of
TPBIQ in dilute (7.5 × 10-6 M) chloroform solution are shown
in Figure 1. Both spectra show vibronic structure with the 0-0
transition in the absorption at 537 nm, while the orange PL has an
emission maximum at 561 nm. Absorption and PL emission spectra
of vacuum-deposited thin films (∼65 nm thick) similarly showed
vibronic structure with onset absorption at 583 nm and absorption
opt
maximum at 517 nm. The solid-state optical band gap (Eg
)
calculated from the onset absorption is 2.13 eV. The PL emission
of the thin films was orange-red and had a maximum at 625 nm.
Single crystals of TPBIQ were grown by physical vapor transport
sublimation. X-ray crystallography was performed to investigate
the solid-state packing and the intermolecular interactions in the
new organic semiconductor. TPBIQ forms a monoclinic c lattice,
with a space group C2/c and the unit cell dimensions of a )
18.8707(11) Å, b ) 13.9159(8) Å, c ) 12.4345(7) Å, R ) 90°, â
) 95.581°(3), and γ ) 90°. The crystal structure reveals that the
seven fused rings are relatively planar and that the phenyl rings at
C7 and C10 have a twist angle of 42 and 50°, respectively (Figure
2A). Figure 2B shows a slipped face-to-face π-stacking along the
c-axis. In addition, the shortest distance between the molecules is
inclined to the c-axis by ca. 32°. The shortest intermolecular spacing
of N1-N1′ between adjacent molecules is 3.47 Å. The shortest
intermolecular distance between two molecular planes is 3.30 Å
as shown in Figure 2B. This suggests that charge transport is likely
to be most efficient along the crystallographic c-axis due to the
analysis showed
a high onset decomposition temperature
† Department of Chemistry.
‡ Department of Chemical Engineering.
9
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J. AM. CHEM. SOC. 2008, 130, 1118-1119
10.1021/ja077444g CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
the source-drain electrodes, and optical micrographs (Figure 3C)
were recorded to accurately measure W/L values for mobility
calculations. Finally, the growth direction of the crystal was
determined to be along the [001] direction. This long axis was
identified as the c-axis and also the short π-π stacking direction.
Therefore, the carrier mobility of our device from Figure 3C was
measured along c-axis direction.
In conclusion, we have synthesized a new class of heteroaromatic
semiconductors via an intramolecular cyclization that results in a
novel heptacyclic π-conjugated framework showing a planar
geometry with excellent π-π stacking. Single-crystal transistors
yielded mobilities as large as 1.0 cm2/V‚s. In addition, the high
stability from electrochemical and thermal measurements suggests
that this new class of heteroaromatic semiconductors is an excellent
candidate for further studies in organic electronics. Ongoing work
is exploring the synthesis and properties of derivatives of compound
1 with different R groups.
Figure 1. Absorption and photoluminescence (PL) spectra of TPBIQ
solution in chloroform.
Acknowledgment. We acknowledge support from NSF (CTS-
0437912 and DMR-0120967) and AFOSR (F49620-03-1-0162).
A.L.B. acknowledges the Bell Labs Graduate Fellowship. Y.X.
acknowledges support from the David and Lucile Packard Founda-
tion. We thank W. Kaminsky for determining the crystal structure
of TPBIQ.
Figure 2. (A) ORTEP plot of TPBIQ and (B) view down the crystal-
lographic b-axis with indicated intermolecular contacts of 3.30 Å.
Supporting Information Available: Detailed synthetic procedures,
additional data and X-ray crystallographic data in CIF format. This
material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 3. (A) Output and (B) transfer characteristics of a TPBIQ single-
crystal transistor. (C) An optical micrograph of the single-crystal device
corresponding to the data in Figure A,B.
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