Core-fluorinated naphthalene diimides: Synthesis, characterization, and application in n-type organic field-effect transistors

Article abstract of DOI:10.1021/acs.orglett.5b03489

A series of difluoro- and tetrafluoro-substituted naphthalene diimides (NDIs) were synthesized by halogen exchange reactions of corresponding bromo-NDIs with CsF in dioxane. Two strong electron acceptor molecules 6 and 8 with low-lying LUMO energy levels of -4.27 and -4.54 eV were obtained, starting from tetrafluoro-NDI. Organic field-effect transistors (OFETs) based on these fluorinated NDIs were fabricated by vapor deposition, exhibiting n-channel field-effect character under ambient conditions with the highest mobility of 0.1 cm² V^-1 s^-1.

Full text of DOI:10.1021/acs.orglett.5b03489

Letter
pubs.acs.org/OrgLett
Core-Fluorinated Naphthalene Diimides: Synthesis, Characterization,
and Application in n‑Type Organic Field-Effect Transistors
Zhongyi Yuan,† Yingjie Ma,† Thomas Geßner,§ Mengmeng Li,† Long Chen,† Michael Eustachi,§
R. Thomas Weitz,§,‡ Chen Li,† and Klaus Mullen*,†
̈
†Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
§BASF SE, 67056 Ludwigshafen, Germany
S
* Supporting Information
ABSTRACT: A series of difluoro- and tetrafluoro-substituted naphthalene diimides (NDIs) were synthesized by halogen
exchange reactions of corresponding bromo-NDIs with CsF in dioxane. Two strong electron acceptor molecules 6 and 8 with
low-lying LUMO energy levels of −4.27 and −4.54 eV were obtained, starting from tetrafluoro-NDI. Organic field-effect
transistors (OFETs) based on these fluorinated NDIs were fabricated by vapor deposition, exhibiting n-channel field-effect
character under ambient conditions with the highest mobility of 0.1 cm2 V−1 s−1.
Scheme 1. Sole Reported Synthesis of Tetrafluoro-NDI
rganic semiconductors are used for high throughput, low-
1
Ocost electronic circuitry on flexible substrates.
High-
performance n-type semiconductors are still rare compared to
their p-type counterparts.2 Besides fullerenes, naphthalene
diimides (NDIs) with compact and electron deficient cores
have demonstrated potential as n-type semiconductors in
organic field-effect transistors (OFETs) and solar cells.3
Although the unsubstituted NDIs display an electron mobility
as high as 6.2 cm2 V−1 s−1, the OFET devices degrade
significantly in air.4 To obtain air-stable semiconducting NDIs,
electron-withdrawing cyano, perfluoroalkyl, and chloro groups
have been introduced to the conjugated cores of NDIs toward
lower-lying LUMO energy levels and better air stability.5
Many chloro and bromo substituted NDIs turned out to be
extraordinary n-type semiconductors.6,7 For example, dichloro-
NDIs and tetrachloro-NDIs have been proven to be air-stable
n-type semiconductors with a high charge carrier mobility of
8.6 cm2 V−1 s−1 based on single crystal OFETs.8 In this regard,
fluorinated NDIs with similar electron affinity are expected to
be air-stable and good substrates for further nucleophilic
substitutions.9 However, to date, only a 1988 patent described
the synthesis of tetrafluoro-NDIs, in that case starting from a
perchlorinated diazapyrene (Scheme 1),10 and neither further
functionalization nor tetrafluoro-NDI derivatives were reported
after that. Another recent paper reported the synthesis of
difluoro-NDI derivatives with the fluoronation method of
perylene diimides.9,11 Herein, we synthesized a series of
tetrafluoro- and difluoro-NDIs with a concise procedure
(Scheme 2). OFETs based on these fluorinated NDIs were
fabricated by vapor deposition. Strong electron acceptor
molecules 6 and 8 were obtained starting from tetrafluoro-
NDIs, and their LUMO energy levels are 0.64 and 0.91 eV
lower than that of unsubstituted NDIs, respectively.
As shown in Scheme 2, fluorinated NDIs 2a−d11 and 4a−b
were obtained as colorless solids by halogen exchange reactions,
starting from dibromo-NDIs and tetrabromo-NDIs, respec-
tively. Our initial attempts with starting material 3a using
common halogen displacement conditions such as potassium
fluoride (KF) in dry DMSO or sulfolane produced no target
compounds, but instead only some dark spots were observed
on TLC plates. When dry dioxane and CsF were employed as
nonnucleophilic solvent and the fluoride source, respectively,
high yields were obtained upon reflux (∼101 °C) or microwave
heating. A much higher temperature (>200 °C) is essential
when KF is used instead of CsF. Tetrafluoro-NDIs 4 are readily
attacked by nucleophiles because of their electron-deficient
cores. For example, they react with water in refluxing dioxane
and are even somewhat unstable to silica gel chromatography.
In contrast, difluoro-NDIs 2 with less electron-withdrawing F
Received: December 9, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03489
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Organic Letters
Letter
Scheme 2. Synthesis of Compounds 2a−d, 4a−b, 6, and 8
atoms are chemically more stable than tetrafluoro-NDIs and
can be purified through silica gel columns conveniently.
Therefore, several difluoro-NDIs 2 with different side chains
were synthesized and purified for further applications in
electronic devices.
Taking advantage of the high reactivity of tetrafluorinated
NDIs, nucleophilic substitutions of 4b with 1,2-diaminoben-
zene and 1,2-diamino-4,5-dicyanobenzene afforded 5 and 7,
respectively. Subsequently, strong acceptors 6 and 8 were
obtained by oxidation of 5 or 7 in PbO2 at room temperature in
yields higher than 90%.
The above new compounds 2a, 2b, 2c, 4a, 4b, 6, and 8 were
characterized unambiguously by 1H NMR, 13C NMR, 19F
NMR, and HRMS. The poor solubility of 2c prevented
collection of 13C NMR data while single crystals suitable for X-
ray analysis could be grown. Single crystals of 2b and 4a were
obtained by slow diffusion of methanol into CH2Cl2 solutions.
All three crystals belong to the monoclinic P21/c space group,
and exhibit herringbone arrangements (Figures 1 and S1−
S2).5e Compound 4a with four fluorine atoms at the NDI core
has a nearly planar conjugated core (torsion angles ≤1.6°),
while 2c bears a slightly twisted core with a torsional angle up
to 3.0°. The smaller size of fluorine atoms (atomic radius: F, 0.5
Å vs Cl, 1.0 Å) decreases the repulsion between F and
neighboring O; thus, the twisting within NDI cores is much less
than that of tetrachloro-NDIs and dichloro-NDIs (with a
torsional angle up to 7.2° and 5.1° respectively).5e,12
Considering that bisheptafluorobutyl-substituted 2,6-dichloro-
NDI exhibits the best FET performance in the series of
halogenated NDIs, it is important to compare the packing
behavior of 2c and 4a with that of dichloro-NDI. As shown in
Figure 1, despite their smaller F-substituents, 2c and 4a stack
with larger π−π interaction distances of 3.372 and 3.299 Å,
smaller slipping angles of 48° and 46°, and lower packing
densities of 2.021 and 1.397 g cm−3, respectively, compared
with those of bisheptafluorobutyl-substituted 2,6-dichloro-NDI
Figure 1. (a) Single crystal structure of 2c. (b) Herringbone packing
mode of 2c molecules in the single crystal. (c) Single crystal structure
of 4a. (d) Herringbone packing mode of 4a molecules in the single
crystal.
(packing distance, slipping angle, and packing density are 3.274
Å, 62°, and 2.046 g cm−3, respectively).8a
Cyclic voltammetry was measured for these fluorinated
naphthalene diimides with N,N′-bis(2-ethylhexyl)naphthalene
diimide (RNDI) as a reference. The reduction potentials and
estimated LUMO and HOMO energies are summarized in
Table 1. As shown in Figures S3−S4, all compounds exhibit
two reversible reduction waves but no oxidation waves within
the accessible scanning potential range in CH2Cl2. For all
fluorinated NDI molecules, both the first and the second
reduction waves are at lower potentials (by 0.16−0.91 eV) than
those of RNDI. They are stronger acceptors than core-
unsubstituted NDIs. Notably, the LUMOs of 6 and 8 are as low
as −4.27 and −4.54 eV pointing toward ultrastrong acceptors.
The UV−vis absorption spectra of difluoro-NDI 2b,
tetrafluoro-NDI 4a, RNDI, 6, and 8 in CH2Cl2 are displayed
in Figure 2. Core-fluorinated NDIs (2b and 4a) have no
absorption in the visible range. Optical energy gaps of 2b and
4a are 3.10 and 3.18 eV, close to that of unsubstituted RNDI
B
DOI: 10.1021/acs.orglett.5b03489
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
Table 1. Half-Wave Reduction Potentials (in V vs Fc/Fc+),
Optical Band Gaps, and LUMO and HOMO energies of 2a−
d, 4a−b, 6, 8, and RNDI
a
a
b
c
E (M/M−1
)
E (M−1/M−2
)
LUMO (eV)
HOMO (eV)
2a
2b
2c
−1.01
−1.00
−0.83
−1.07
−0.95
−0.98
−0.53
−0.26
−1.17
−1.50
−1.46
−1.32
−1.58
−1.42
−1.48
−0.96
−0.80
−1.63
−3.79
−3.80
−3.97
−3.73
−3.85
−3.82
−4.27
−4.54
−3.63
−6.87
−6.89
−7.05
−6.81
−7.03
−7.00
−6.26
−6.61
−6.78
2d
4a
4b
6
Figure 3. Transfer curve of compound 2c (left) and 4a (right)
measured in ambient air in the dark.
8
d
RNDI
a
Measured in Bu4NPF6 solution in CH2Cl2 with a scan rate of 100
b
mV/s. Ferrocene serves as internal standard. The energy of Fc/Fc+ is
assumed at −4.8 eV relative to vacuum.13 Calculated with the formula
c
of both 4a and 2c are not as good as those of chlorinated NDIs.
Especially, the transfer curve of 4a shows an S-shape. The poor
FET performance indicates the possible poor contact between
the semiconductors and the electrodes. The major reason for
this weak interaction could be due to the small slipping angle of
the stack (48° for 2c, 46° for 4a, but 62° for bisheptafluoro-
butyl-substituted 2,6-dichloro-NDI) and the small dihedral
angle (84° for 2c, 88° for 4a, but 139° for bisheptafluorobutyl-
substituted 2,6-dichloro-NDI) of the edge-to-face herringbone
arrangement in the crystal structure.
d
HOMO = LUMO − optical energy gap. N,N′-Bis(2-ethylhexyl)
naphthalene diimide.
In conclusion, difluoro- and tetrafluoro-NDIs have been
synthesized with a facile procedure. These compounds feature
high planarity, dense packing in the solid, and short interplanar
distances and exhibit a maximum electron mobility μ of 0.1
cm2/(V s). Ultrastrong acceptors 6 and 8 were obtained from
tetrafluoro-NDIs. A number of nucleophilic replacement
reactions may be further used to explore new compounds
based on these electron-deficient fluorinated-NDIs. Similar to
chlorine, the fluorine atoms could also impart sufficient
electron-withdrawing character to the NDI cores to stabilize
injected electrons against environmental traps, and it is possible
to screen more high-performance air-stable n-type semi-
conductors from fluorinated NDIs.
Figure 2. UV−vis absorption of 2b, 4a, RNDI, 6, and 8 in CH2Cl2
(10−5 mol/L).
(3.15 eV), which indicates that F substituents lower the LUMO
and HOMO energies simultaneously.
We evaluated the charge carrier transport properties of the
compounds 2a, 2b, 2c, 4a, 4b, and 6 by fabricating field-effect
transistors (FETs) in a “bottom-contact top-gate” geometry. To
this end, degenerately doped silicon wafers that had been
coated with 30 nm of Al2O3 grown via atomic layer deposition
were functionalized with a tetradecylphosphonic acid self-
assembled monolayer. Subsequently, the organic semiconduc-
tors were thermally evaporated onto these substrates. In order
to tune the morphology of the semiconductors during the
vacuum deposition, the substrates were held at five different
temperatures between 30 and 120 °C during deposition of the
small molecules. Silver source and drain contacts were
subsequently patterned via thermal evaporation through a
shadow mask to define the semiconducting channel (width, 500
μm; length, 50 μm). All electrical measurements were
performed in ambient air in the darkness using a B1500
semiconductor parameter analyzer (Keysight). While com-
pounds 4b and 6 did not show charge transport, FETs based on
compounds 2a and 2b yielded an n-type charge carrier mobility
of around 10−3 cm2/(V s). The best electrical data were
obtained with compounds 4a and 2c, whose transfer curves are
shown in Figure 3. Transistors based on 2c had a maximum
electron mobility μ of 0.1 cm2/(V s), an on/off ratio of close to
104, and a threshold voltage (Vth) of −9 V when deposited at a
substrate temperature of 40 °C. FETs based on 4a had a
maximum electron mobility of 0.02 cm2/(V s), with an on/off
ratio of 102 and a Vth of −9.6 V when deposited at a substrate
temperature of 100 °C. It is worthy to note that the FET results
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03489.
Full experimental details and characterization data (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: muellen@mpip-mainz.mpg.de.
Present Address
‡Department of Physics, LMU Munich, Amalienstr. 54, 80799
Munchen, Germany.
̈
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the DFG
Priority Program SPP 1355 and BASF SE. We also thank Prof.
Mark Watson (Department of Chemistry, University of
Kentucky) for fruitful discussions.
C
DOI: 10.1021/acs.orglett.5b03489
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Organic Letters
Letter
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DOI: 10.1021/acs.orglett.5b03489
Org. Lett. XXXX, XXX, XXX−XXX