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Oton et al.
852 Chem. Mater., Vol. 23, No. 3, 2011
from solution of dithiophene-tetrathiafulvalene (DT-TTF,
μ
semiconductors such as oligothiophenes, acenes or tetra-
thiafulvalenes.19-24 Moreover, the presence of electron-
withdrawing groups, such as phthalimides, among
others,26 leads to the decrease of both the HOMO and
LUMO energy levels that is known to improve the air
stability and reliability of the prepared devices.27-29
In this paper we report the synthesis of four new bis-
phthalimide substituted TTF derivatives as novel ambi-
polar semiconductors. The electron-withdrawing groups
attached to the TTF molecule increase the electron affin-
ity of the materials but keep the HOMO energies suf-
ficiently unaltered to guarantee hole injection.26 These
materials were fully characterized in solution as well as in
solid state. Also, a careful theoretical investigation was
carried out to study the potential of these materials for
charge transport in terms of energy levels and crystal
packing. Single crystal OFETs were fabricated, achieving
a maximum hole mobility of 0.33 cm2 V-1 s-1. Further-
more, the charge carrier mobility of the materials was also
investigated by electrical time of flight (EToF) and hole,
and electron mobilities as high as 2.0 cm2 V-1 s-1 have
been found for some of the materials.
max = 3.6 cm2 V-1 s-1),11 hexamethylene-tetrathiaful-
valene (HM-TTF, μmax=10 cm2 V-1 s-1),12-14 dibenzo-
tetrathiafulvalene (DB-TTF, μmax = 1 cm2 V-1 s-1),15
and a parent TTF compound (μmax = 1.2 cm2 V-1 s-1).16
Due to the strong donor character of these molecules, in
most of the reported devices the TTF analogues behave as
p-type semiconductors (i.e., hole conduction), with the
exception of only two TTF derivatives with strong electron-
withdrawing substituents that have been shown to behave
as n-type semiconductors (i.e., electron conduction).17
N-type and ambipolar (i.e., hole and electron conduction)
semiconductors are of special importance for the fabrica-
tion of p-n junctions and complementary circuits.18-24
However, despite the intense work devoted to the devel-
opment of these materials, their performance is still far
from the one found with p-type organic semiconductors.
This can be partly caused by electron trapping in ambient
air and/or at the dielectric surface sites but also by the lack
of stable materials. Another major issue to achieve elec-
tron conduction is the electron injection. Indeed, organic
semiconductors should have LUMO energy levels which
match the work function of the source-drain electrodes
to decrease the electron injection barrier, a value that is
commonly assumed to be lower than -4.0 eV.25 Similarly,
hole conduction is ensured when the materials have a HOMO
level aligned with the work function of the hole injecting
electrode. Therefore, one approach to preparing ambi-
polar organic semiconductors is to introduce electron
acceptor units to the molecular cores of well-known p-type
Experimental Section
Materials and Methods. All reactions were carried out under
Ar using solvents which were dried following routine proce-
dures. Bis-(bromomethyl)dithiolone30 was synthesized using
procedures reported in the literature. Graphite paste XC-12
was purchased from Dotite, and thermally grown silicon dioxide
was purchased from Si-Mat. Chemical reagents obtained from
commercial sources were used without further purification.
Column chromatography was performed using silica gel (60 A
C.C. 35-70 μm, sds) as the stationary phase. The MALDI-TOF
MS spectra were recorded on a Bruker Ultraflex II TOF spec-
trometer. Infrared spectra were recorded on a Perkin-Elmer FT-
IR Spectrum One spectrophotometer. UV-vis spectra were
performed in o-dichlorobenzene heated at 100 ꢀC (c = 1 ꢀ
10-4 M) using a VARIAN CARY 5000 spectrophotometer.
Elemental analyses were carried out on a Carlo Erba CE 1108
elemental analyzer. Cyclic voltammograms (CV) were per-
formed with a conventional three-electrode configuration con-
sisting of platinum wires as working and auxiliary electrodes
and Ag as pseudoreference electrode. These experiments were
carried out in a 10-3 M solution of the corresponding TTF
derivative in o-dichlorobenzene, thermostatted at 150 ꢀC to be
able to solubilize the materials, and containing 0.1 M of (n-
C4H9)4PF6 (TBAHP) as supporting electrolyte. Deoxygenation
of the solutions was performed previously to the experiments by
bubbling nitrogen for at least 10 min, and the working electrode
was cleaned after each run. The CVs were recorded with an
increasing scan rate from 0.05 to 0.50 V s-1. Ferrocene was used
as an internal reference both for potential calibration and for
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