with unsatisfactory field-effect performance.4 However,
n-type semiconductors are envisioned as key components of
organic p-n junction, bipolar transistors, ring oscillators, and
complementary integrated circuits.5 Therefore, the design and
synthesis of novel n-type organic semiconductor materials
are crucial for the development of practical organic electrics.
In order to obtain high electron mobility, the organic
semiconductor layer should be highly ordered with strong
intermolecular interaction and should also have a proper
energy level of the lowest unoccupied molecular orbital
(LUMO) near the work functions of source/drain electrodes.5
Besides, the electron affinity, which is defined as the energy
difference between the LUMO and vacuum level, could
influence the stability of n-type materials to a very large
extent. In general, large electron affinity is necessary to obtain
air-stable n-type materials. As a result, exploration of organic
semiconductors with high electron affinity is an effective way
to realize high performance stable n-type OFETs. As an
example, introducing fluoro or fluoroalkyl substituents into
pentacene and oligothiophene derivatives could lead to high
electron affinity and result in high performance n-type
organic semiconductors.6 Perfluoropentacene has positively
shifted redox potentials relative to those of the nonfluorinated
compound and was found to adopt the same herringbone
structure as pentacene.7 In the herringbone structures, organic
molecules are packed more or less edge-to-face in two
dimensional layers, minimizing the π-overlap between
adjacent molecules. π-Stacks in a face-to-face structure can
increase the intermolecular interactions, facilitating charge
transport.8 On the other hand, planar molecules are consid-
ered to be favorable for a large transfer integral between
two neighboring molecules and less reorganization energy
upon ionization, which are required for high mobility.9 Here
we have designed and synthesized planar molecules with
trifluoromethyl groups 3a and 3b, which have a planar
geometry similar to that of pentacene. Molecules of 3a self-
assemble in the solid state to give efficient intermolecular
interactions in all three dimensions. The OFETs based on
compounds 3a and 3b were air-stable and showed excellent
n-type device performance.
The synthetic routes to the novel n-type transport materials
are shown in Scheme 1. 3-Hydroxybenzotrifluoride was
Scheme 1. Synthesis of Compounds 3a and 3b
nitrified to give 2-nitro-5-trifluoromethylphenol (1a). 2-Nitro-
4-trifluoromethylphenol (1b) was prepared by the reaction
of sodium hydroxide with 2-nitro-4-trifluoromethylchlo-
robenzene in dimethyl sulfoxide. Pd/C-catalyzed reduction
reaction of the nitrophenols 1a and 1b was carried out in a
H2 atmosphere followed by treating with dry HCl gas, to
give the 2-amino-5-trifluoromethylphenol hydrochloride (2a)
and 2-amino-4-trifluoromethylphenol one (2b), respectively.
The obtained 2-aminophenol hydrochlorides 2a and 2b were
treated with 2,5-dihydroxy-[1,4]benzoquinone in the presence
of acetic acid, to give the 3,10-ditrifluoromethyl-tripheno-
dioxazine (3a) in 94% yield and 2,9-ditrifluoromethyl-
triphenodioxazine (3b) with a yield of 67%. The compounds
3a and 3b were purified by sublimation, and their molecular
structures were determined by spectral data along with
elemental analysis.
A crystal of 3a suitable for single-crystal X-ray diffraction
studies was grown in a physical vapor growth process. The
crystal structure is based on π-stacking along the a-axis
direction in which the plane-to-plane is ca. 3.44 Å. The close
face-to-face π-π stacking lies in the direction normal to the
molecular plane. Pentacene crystallizes into a herringbone
arrangement with no intermolecular cofacial interactions.10
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