Stepwise cyanation of naphthalene diimide for n-channel field-effect transistors

Article abstract of DOI:10.1021/ol300914k

Stepwise cyanation of tetrabromonaphthalenediimide (NDI) 1 gave a series of cyanated NDIs 2-5 with the monocyanated NDI 2 and dicyanated NDI 3 isolated. The tri- and tetracyano- NDIs 4 and 5 show intrinsic instability toward moisture because of their extremely low-lying LUMO energy levels. The partially cyanated intermediates can be utilized as air-stable n-type semiconductors with OFET electron mobility up to 0.05 cm² V^-1 s^-1.

Full text of DOI:10.1021/ol300914k

ORGANIC
LETTERS
2012
Vol. 14, No. 12
2964–2967
Stepwise Cyanation of Naphthalene
Diimide for n-Channel Field-Effect
Transistors
Jingjing Chang,† Qun Ye,† Kuo-Wei Huang,‡ Jie Zhang,§ Zhi-Kuan Chen,§
Jishan Wu,†,§ and Chunyan Chi*,†
Department of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore 117543, KAUST Catalysis Center and Division of Chemical and Life Sciences
and Engineering, 4700 King Abdullah University of Science and Technology,
Thuwal 23955-6900, Kingdom of Saudi Arabia, and Institute of Materials Research and
Engineering, A*Star, 3 Research Link, Singapore 117602
chmcc@nus.edu.sg
Received April 9, 2012
ABSTRACT
Stepwise cyanation of tetrabromonaphthalenediimide (NDI) 1 gave a series of cyanated NDIs 2ꢀ5 with the monocyanated NDI 2 and dicyanated
NDI 3 isolated. The tri- and tetracyano- NDIs 4 and 5 show intrinsic instability toward moisture because of their extremely low-lying LUMO energy
.
levels. The partially cyanated intermediates can be utilized as air-stable n-type semiconductors with OFET electron mobility up to 0.05 cm2 Vꢀ1 sꢀ1
Naphthalene diimide (NDI)1 based molecules have been
intensively investigated as n-type semiconductors for or-
ganic field effect transistors (OFETs). The interest stems
from the early observation of the n-type behavior1c as well
as the versatile tuning of their chemical and electron-
ic properties by varying the substituents at the imide
position2 or on the naphthalene core.3 In order to achieve
applicable electron-transporting materials with high
charge carrier mobility and good air stability, introduction
of electron-withdrawing cyano groups onto the NDI
framework has become an important strategy.4
Our interest herein is to synthesize a series of new cyanated
NDI molecules such as 2ꢀ5 from the tetrabromo-NDI 1
(Scheme 1) and to exploit their applications as new
n-type semiconductors for air-stable OFETs. The dicya-
nated NDI (nonbrominated analog of 3) is known as
an air-stable n-type semiconductor,1f while the syn-
thesis of tetracyano NDI 55 is very challenging because
† National University of Singapore.
‡ King Abdullah University of Science and Technology.
§ Institute of Materials Research and Engineering.
(1) (a) Zhan, X.; Facchetti, A.; Barlow, S.; Marks, T. J.; Rather,
M. A.; Wasielewski, M. R.; Marder, S. R. Adv. Mater. 2011, 33, 268. (b)
Bhosale, S. V.; Jani, C. H.; Langford, S. J. Chem. Soc. Rev. 2008, 37, 331.
(c) Laquindanum, J. G.; Dodabalapur, A.; Lovinger, A. J. J. Am. Chem.
Soc. 1996, 118, 11331. (d) Katz, H. E.; Johnson, J.; Lovinger, A. J.; Li,
W. J. Am. Chem. Soc. 2000, 122, 7787. (e) Katz, H. E.; Lovinger, A. J.;
Johnson, J.; Kloc, C.; Siegrist, T.; Li, W.; Lin, Y. Y.; Dodabalapur, A.
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Wasielewski, M. R. Chem. Mater. 2007, 19, 2703.
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2010, 46, 4225. (b) Chopin, S.; Chaignon, F.; Blart, E.; Odobel, F.
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Dobreva, D.; Koch, N.; Vollmer, A. Adv. Funct. Mater. 2007, 17, 3715.
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Zhu, D. Chem. Mater. 2011, 23, 1204. (b) Gao, X.; Di, C.; Hu, Y.; Yang,
X.; Fan, H.; Zhang, F.; Liu, Y.; Li, H.; Zhu, D. J. Am. Chem. Soc. 2010,
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2008, 20, 3609. (b) Shukla, D.; Nelson, S. F.; Freeman, D. C.; Rajeswaran,
M.; Ahearn, W. G.; Meyer, D. M.; Carey, J. T. Chem. Mater. 2008, 20,
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Mater. 2009, 21, 94. (d) Kantchev, E. A. B.; Tan, H. S.; Norsten, T. B.;
Sullivan, M. B. Org. Lett. 2011, 13, 5432.
€
r
10.1021/ol300914k
Published on Web 05/25/2012
2012 American Chemical Society

to be carefully controlled. The amount of palladium
catalyst had to be less than 0.03 equiv per bromine and
the amount of ligand less than 0.06 equiv per bromine.
Otherwise, phosphonium salt formation would be ob-
served as the major side reaction, especially at elevated
temperatures.10 By setting the reaction temperature at
50 °C, the tetracyanated product 5 was obtained after
24 h, and its presence was confirmed by MALDI-TOF MS
analysis of the reaction solution (Figure S1 in the Support-
ing Information). However, it was sensitive to moisture
and silica gel, which made the purification unsuccessful.
The decomposition products of 5 in air were analyzed by
MALDI-TOF MS (Figure S1, Supporting Information),
and it was found that hydrolysis happened with 5, most
probably on the cyano groups. This side reaction would
be caused by the low-lying LUMO of 5, which made it
vulnerable to be attacked even by weak nucleophiles such
as water.
Scheme 1. Synthesis of Cyanated Naphthalene Diimides 2ꢀ5a
In order to probe this stability issue, we then carried out
a stepwise cyanation on 1 by running the reaction at room
temperature. All cyanated compounds 2ꢀ5 were detected
duringthe reaction by MALDI-TOFmassspectrometryas
shown in Figure 1. The monocyanated and dicyanated
naphthalene diimides 2 and 3 as major products were
successfully separated in 64% and 61% yield by column
chromatography after reaction for 8 and 16 h, respectively.
After 48h, almostonly the tetracyano-NDI 5 was detected.
However, 4 and 5 could not be separated because of quick
decomposition upon contact with moisture or silica
gel. Interestingly, for the dicyanated compound, only one
isomer was found even though three isomers should be
theoretically possible. The structure of 3 was unambigu-
ously confirmed by the single-crystal structural analysis of
compound 6, which was prepared by nucleophilic attack of
the two bromines on 3 by 4-tert-butylthiophenol (Scheme 1).
This unexpected regioselectivity could be ascribed to two
reasons: first, the second cyanation was directed by the
electron-withdrawing effect of the first cyano group, and
second, the reaction was conducted at room temperature
a Inset: single-crystal structure of 6.
of its expected ultrahigh electron affinity. An attempted
synthesis of analogues of 5 was reported.6 However, the
synthesis was not successful, and intractable results were
obtained. In this work, we challenge the synthesis of 5 by
a stepwise cyanation method. By studying the intermedi-
ates of this cyanation process, the synthetic problems
of 5 would be better understood. The partially cyanated
NDIs, however, are useful as well for air-stable n-channel
OFETs.
The synthesis of the cyanated NDIs 2ꢀ5 is shown in
Scheme 1. The cyanation of tetrabromo-NDI 17 was first
attempted by using CuCN/DMF/reflux protocol,8 which
was reported to be successful for cyanation of other
electron-deficient systems.1f However, under such condi-
tions, the reaction was plagued by severe side reactions and
it generated a mixture of intractable products which are
difficult to identify. We then attempted the palladium-
catalyzed cyanation protocol9 which could be carried out
under milder conditions. The cyanation of 1 was success-
fully carried out with Pd2(dba)3 as Pd source, 1,10-bis-
(diphenylphosphino)ferrocene (dppf) as ligand, CuCN as
cyanide source, and dioxane as solvent. The amount of
catalyst and ligand as well as the reaction temperature had
€
(5) Konemann, M. Naphthalenetetracarboxylic Acid Derivatives
and Their Use As Semiconductors. WO/2007/074137.
(6) Dawson, R. E.; Hennig, A.; Weimann, D. P.; Emery, D.; Ravikumar,
V.; Montenegro, J.; Takeuchi, T.; Gabutti, S.; Mayor, M.; Mareda, J.;
Schalley, C. A.; Matile, S. Nat. Chem. 2010, 2, 533.
Figure 1. MALDI-TOF mass spectrum of the reaction mixture
after reaction at room temperature for 24 h.
€
€
(7) (a) Roger, C.; Wurthner, F. J. Org. Chem. 2007, 72, 8070. (b) Gao,
X.; Qiu, W.; Yang, X.; Liu, Y.; Wang, Y.; Zhang, H.; Qi, T.; Liu, Y.; Lu,
K.; Du, C.; Shuai, Z.; Yu, G.; Zhu, D. Org. Lett. 2007, 9, 3917.
(8) Ellis, G. P.; Romney-Alexander, T. M. Chem. Rev. 1987, 87, 779.
(9) Qu, H.; Cui, W.; Li, J.; Shao, J.; Chi, C. Org. Lett. 2011, 13, 924.
(10) Ye, Q.; Chang, J.; Huang, K. W.; Chi, C. Org. Lett. 2011, 13,
5960.
Org. Lett., Vol. 14, No. 12, 2012
2965

with a slow rate, and hence, the thermodynamically con-
trolled cyanation would occur at sites with least steric
hindrance.
Solutions of 1ꢀ3 in chloroform show well-resolved
absorption bands at 300ꢀ460 nm, with only a slight shift
of the wavelength upon cyanation (Figure S4, Supporting
Information). The optical energy gaps of 1ꢀ3 estimated
from the absorption onset at the longest wavelength are
nearly the same, i.e., 2.77 eV. Compound 6 shows an
entirely different absorption spectrum, with a large broad
absorption peak at 620 nm that was believed due to the
intramolecular charge transfer.
Field effect transistors with 1ꢀ3 as active compounds
were fabricated in a bottom-gate, top-contact configura-
tion (see details in the Supporting Information). A thin
film of semiconductor materials was deposited at different
substratetemperatureTd by vapor deposition onto a pþ-Si
wafer with 200 nm ofthermallygrownSiO2 asthedielectric
layer. The SiO2 substrate was modified by either octade-
cyltrimethoxysilane (OTMS) or CYTOP (poly(perfluoro-
butenylvinylether)) amorphous fluoropolymer or without
any modification. The typical transfer and output
curves measured in N2 on OTMS-modified sub-
strate with Td = 60 °C are shown in Figure 3, and the
device characteristics data at various Td temperatures
are collected in Table 1. All of the devices exhibit
typical n-channel behavior, and the highest FET elec-
The electrochemical properties of 1ꢀ3 were investigated
by cyclic voltammetry and differential pulse voltammetry
(Figure 2a and Figure S2, Supporting Information). Com-
pound 2 exhibited two reversible reduction waves with half-
wave potential E1/2red at ꢀ0.66 and ꢀ1.11 V (vs Fcþ/Fc).
Compound 3 showed two similar reversible reduction
waves with E1/2red at ꢀ0.45 and ꢀ1.00 V, and one quasi-
red
reversible reduction wave with E1/2 at ꢀ1.56 V. Com-
pound 1 showed three quasi-reversible reduction waves
red
with E1/2 at ꢀ0.89, ꢀ1.26, and ꢀ1.48 V. The LUMO
energy levels of 1, 2, and 3 were estimated to be ꢀ4.02,
ꢀ4.21, and ꢀ4.42 eV, respectively, based on the onset
potential of the first reduction wave. It was worthy to note
that after replacement of one bromine atom with a cyano
group, the LUMO energy level was reduced by ca. 0.2 eV,
showing a good linear relationship with the number of CN
groups (Figure 2b). DFT (B3LYP/6-31G*) calculations
predicted that the LUMO energy levels of 1ꢀ5 are ꢀ3.69,
ꢀ3.95, ꢀ4.28, ꢀ4.54, and ꢀ4.82 eV (Figure S3, Supporting
Information), which also exhibit a good linear relationship
with the number of CN groups (Figure 2b). Thus, it is
reasonable to make a linear extrapolation of the experi-
mental plot and the LUMO energy levels of 4 and 5 are
estimated to be ꢀ4.62 and ꢀ4.82 eV, respectively. The very
low lying LUMO energy levels reflected the highly elec-
tron-deficient nature of 4 and 5, which would be the most
possible origin of their instability.
tron mobility measured in N2 is 0.018, 5.1 ꢁ 10ꢀ4
,
and 0.050 cm2 Vꢀ1 sꢀ1 for 1, 2, and 3, respectively.
The electron mobilities are dependent on Td and
dielectric surface treatment. For example, the mobil-
ity of 3 increased from 0.018 cm2 Vꢀ1 sꢀ1 at rt to
0.050 cm2 Vꢀ1 sꢀ1 for thin film deposited at 100 °C.
The thin film on OTMS/SiO2 exhibited higher perfor-
mance (0.050 cm2 Vꢀ1 ꢀ1) compared to bare SiO2
s
substrates which only gave a low mobility of 2.4 ꢁ
10ꢀ5 cm2 Vꢀ1 sꢀ1. The threshold voltage (Vth) shifts to
more negative with increasing the number of cyano
groups from 1 to 2 and to 3, indicating the higher the
electron affinity the easier it is to create mobile elec-
trons at low gate bias.
Thin films of 1ꢀ3 deposited at different Td exhibited a
similar X-ray diffraction pattern which can be correlated
to a lamellar packing mode (Figure S5ꢀ7, Supporting
˚
Information) with a d(001) spacing of 17.0, 18.4, and 18.5 A,
respectively. Thin film of 2 showed less intense and broader
diffraction peaks compared with 1 and 3, indicating a less
ordered microstructure. AFM images revealed highly crystal-
line surface microstructure and the crystallite size tended
to increase with increasing Td for all compounds (Figure
S8ꢀ10, Supporting Information), which can explain the
higher mobility at the higher substrate temperature due to
minimized grain boundaries. A thin film of 2 exhibited small-
er grains and much more grain boundaries, which limit the
efficient charge transport and correspond to lower charge
carrier mobility.
The OFET devices operated in ambient conditions showed
different degradation (Table S1, Supporting Information).
Compound1 withthe lowestelectron-affinity exhibitedthe
largest devicevariations(∼1000ꢁ), while 3withthehighest
electron-affinity showed a smaller decrease in charge
carrier mobility by a factor of ∼10ꢁ. This trend gave a
Figure 2. (a) Cyclic voltammograms of 1ꢀ3 measured in dry
CH2Cl2. (b) Plots of the calculated (DFT at B3LYP/6-31G*
level) and experimental LUMO energy levels of 1ꢀ5 with the
number of CN groups.
2966
Org. Lett., Vol. 14, No. 12, 2012

slightly shifted to negative voltage maybe due to uninten-
tional doping by electron-rich chemical functionalities
and/or the local electric field.11 The electron trap sites like
silanolgroupsonSiO2 could be neglectedwhenthe LUMO
of semiconductor is lower than ꢀ3.8 eV,12 which may also
explain the good device stability of 2 and 3.
OFETs on CYTOP/SiO2 bilayer substrates showed
comparable performance to that on OTMS-modified sub-
strates, showing the highest electron mobilities of 0.017,
5.1 ꢁ 10ꢀ4, and 0.047 cm2 Vꢀ1
s
ꢀ1 for 1, 2, and 3, respec-
tively, when measured in N2 (Figure S11 and Table S2,
Supporting Information). The OFET devices also gave a
more positive Vth and a larger Ion/Ioff ratio. When the
devices were operated in ambient conditions, the mobility
showed a smaller decrease (∼2ꢁ). Thin films of 1ꢀ3
deposited on CYTOP/SiO2 exhibited similar XRD pattern
but with enhanced reflection intensity (Figure S12ꢀ14,
Supporting Information) compared to the OTMS/SiO2
substrate, indicating more ordered packing. The surface
morphology also changes slightly (Figure S15ꢀ17, Sup-
porting Information). Solution-processed OFET devices
were fabricated by spin-coating 3 in chloroform onto
OTMS/SiO2 substrate, and the device showed an average
FET electron mobility of 0.02 cm2 Vꢀ1
s
ꢀ1 with good air
stability (Figure S18ꢀ20, Supporting Information), which
is comparable to vapor deposited devices.
In summary, stepwise cyanation of tetrabromonaphtha-
lene diimide 1 gave a series of cyanated compounds 2ꢀ5.
The electron affinity showed a good linear relationship
with the number of the cyano groups and the extremely
low-lying LUMO energy level of 4 and 5, making them
unstable to moisture. The mono- and dicyanated NDIs 2
and 3, however, can be obtained as stable n-type semicon-
ductors and used for air-stable n-channel OFETs with
moderate electron mobilities. This research implies that in
the search for new high-performance n-type semiconduc-
tors we need to find a good balance between the materials’
stability and device stability by careful control of the
electron affinity.13
Figure 3. Representative OFET transfer and output character-
istics of 1 (a, b), 2(c, d), and 3 (e, f) thin films (Td = 60 °C) on
OTMS-modified substrates in N2.
Table 1. OFET Characteristics for 1ꢀ3 Measured in N2 on
OTMS-Modified Substrates
Td (°C)
μ (e) [cm2 Vꢀ1 sꢀ1
]
VT [V]
on/off
Acknowledgment. C.C. acknowledges financial support
from the National University of Singapore start-up grant
R-143-000-486-133 and MOE AcRF FRC grant R-143-
000-444-112. K.-W.H. acknowledges financial support
from KAUST. We thank Dr. Tan Geok Kheng at NUS
for crystallographic analysis.
1
2
3
rt
60
0.011
19ꢀ24
15ꢀ20
14ꢀ19
12ꢀ18
13ꢀ17
12ꢀ17
3ꢀ6
106
106
105
105
105
105
105
105
104
0.018
100
rt
0.012
3.7 ꢁ 10ꢀ4
3.9 ꢁ 10ꢀ4
5.1 ꢁ 10ꢀ4
0.018
60
100
rt
60
0.036
3ꢀ7
Supporting Information Available. Synthetic proce-
dures and characterization data. Crystallographic data
of compound 6. Electrochemical data. DFT calculation
data. Details of FET fabrication and characterizations.
This material is free of charge via Internet at http://pubs.
acs.org.
100
0.050
4ꢀ7
sign that the n-type ambient sensitivity could be decreased
with increasing of electron affinity. The Vth of 1 was
significantly shifted to positive with about 15ꢀ30 V due
to electron traps at the interface between the semiconduc-
tor and dielectric. However, the Vth of 2 and 3 were both
(12) Yoon, M.-H.; Kim, C.; Facchetti, A.; Marks, T. J. J. Am. Chem.
Soc. 2006, 128, 12851.
(13) Jones, B. A.; Facchetti, A.; Wasielewski, M. R.; Marks, T. J.
J. Am. Chem. Soc. 2007, 129, 15259.
(11) (a) Bao, Z.; Lovinger, A. J.; Brown, J. J. Am. Chem. Soc. 1998,
120, 207. (b) Jones, B. A.; Ahrens, M. J.; Yoon, M.-H.; Facchetti, A.;
Marks, T. J.; Wasielewski, M. R. Angew. Chem., Int. Ed. 2004, 43, 6363.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 12, 2012
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