Conjugated polymers based on 1,8-naphthalene monoimide with high electron mobility

Article abstract of DOI:10.1002/pola.28891

1,4,8,9-Naphthalene diimides (NDIs) with strong electron accepting ability and high stability are excellent building blocks for semiconductor polymers. However, 1,8-naphthalene monoimide (NMI) with similar structure and energy levels as that of NDI has never been used to construct conjugated polymers because of synthetic difficulty. Herein, 3,6-dibromo-NMI (DBNMI) with bulky alkyl groups was obtained effectively in a four-step synthesis, and three donor-acceptor (D-A) type conjugated polymers based on NMI were firstly prepared. These polymers have strong absorption in the range of 300–600 nm, low LUMO level of 3.68 eV, and moderate bandgaps of 2.18 eV. Space charge limiting current measurements indicate these polymers are typical electron transporting materials, and the highest electron mobility is up to 5.8 × 10^?3 cm²?V^?1?s^?1, which is close to the star acceptor based on NDI (N2200, 5.0 × 10^?3 cm²?V^?1?s^?1).

Full text of DOI:10.1002/pola.28891

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Conjugated Polymers Based on 1,8-Naphthalene Monoimide with High
Electron Mobility
1,2
Guomin Zhu,1† Youdi Zhang,1,2† Yu Hu,1 Xiaohong Zhao,1 Zhongyi Yuan
,
1,2
Yiwang Chen
1College of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China
2Jiangxi Provincial Key Laboratory of New Energy Chemistry/Institute of Polymers, Nanchang University, 999 Xuefu Avenue,
Nanchang 330031, China
Correspondence to: Z. Yuan (E-mail: yuan@ncu.edu.cn)
Received 4 September 2017; accepted 3 October 2017; published online 00 Month 2017
DOI: 10.1002/pola.28891
ABSTRACT: 1,4,8,9-Naphthalene diimides (NDIs) with strong
electron accepting ability and high stability are excellent build-
ing blocks for semiconductor polymers. However, 1,8-naphtha-
lene monoimide (NMI) with similar structure and energy levels
as that of NDI has never been used to construct conjugated
polymers because of synthetic difficulty. Herein, 3,6-dibromo-
NMI (DBNMI) with bulky alkyl groups was obtained effectively
in a four-step synthesis, and three donor-acceptor (D-A) type
conjugated polymers based on NMI were firstly prepared.
These polymers have strong absorption in the range of
300–600 nm, low LUMO level of 3.68 eV, and moderate bandg-
aps of 2.18 eV. Space charge limiting current measurements
indicate these polymers are typical electron transporting mate-
rials, and the highest electron mobility is up to 5.8 3 1023
cm2 V21 s21, which is close to the star acceptor based on NDI
(N2200, 5.0 3 1023 cm2 V21 s21). 2017 Wiley Periodicals, Inc.
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J. Polym. Sci., Part A: Polym. Chem. 2017, 00, 000–000
KEYWORDS: conjugated polymers; electron acceptor; naphtha-
lene monoimide
thin-film transistors (OTFTs) and organic solar cells.21–25 Imide
groups have been widely used to construct highly efficient
acceptors, such as NDI,26–28 phthalimide,29–34 and perylene
diimides derivatives.35–39 1,8-Naphthalene monoimide (NMI)
with the similar structure and energy level as that of NDI, has
been used to construct small functional molecules which were
used in fluorescent probes, OTFTs and organic solar cells.40–44
However, conjugated polymers based on NMI have never been
reported due to the synthetic difficulty (Fig. 1).
INTRODUCTION Organic semiconductors are the focus of
research in the past decades, because of their various appli-
cations in organic light emitting displays, printable electronic
circuits, and organic solar cells.1–3 Polymeric semiconductors
(conjugated polymers) have drawn special attention com-
pared to small molecule semiconductors, because they have
advantages of good film forming properties, low cost solution
process, and high carrier mobility.4 Acceptors (n-type semi-
conductors, electron transporting materials) are much rarer
compared with their donor counterparts (p-type semicon-
ductors, hole transporting materials), and both of them are
essential in electronic circuits. As for as star acceptors
PCBM,5–7 they have disadvantages of high cost, difficulties of
modification, and weak absorption in a visible region.8–10
Therefore, it is urgent to develop highly efficient polymeric
acceptors.11–15
In 2013, 3,6-dibromo-N-(2,4-diisopropylphenyl)-1,8-naphthali-
mide (DBDIPPNMI) was firstly synthesized by Hiroyuki et al.
with the synthetic route shown in Figure 2.45. However, above
brominated NMI with 2,4-diisopropylphenyl groups has poor
solubility, and it is impossible to be used in conjugated poly-
mers. To increase its solubility, we attempted to obtain DBNMI
with bulky alkyl groups in literature method.46–49 However, tar-
get compound was failed to obtain because of very difficult
purification of crude products, which stimulates us to develop
a new synthetic route for efficient synthesis of soluble DBNMI
for conjugated polymers.
1,4,8,9-Naphthalene diimides (NDIs) with rigid conjugated
cores, high stability, and strong electron accepting ability are
excellent acceptors.16–20 Polymeric acceptors with NDIs as
acceptor parts have shown high performance in organic
†Guomin Zhu and Youdi Zhang contributed equally to this work
Additional Supporting Information may be found in the online version of this article.
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H2SO4 (20 mL) and 68% HNO3 (15 mL) was dropwisely
added to above NMA solution. The reaction was kept at 0 8C
for 1 h, and at 65 8C for extra 1.5 h. The mixture was poured
into the ice, filtered, washed with water and ethanol. The fil-
ter cake was collected and recrystallized from acetic acid.
The crude DNNMA was obtained as a white solid. It was
used directly in the next step without further purification.
Synthesis of DNNMI
FIGURE 1 Structures of NDI, DBNMI, and NMI-T2.
A mixture of DNNMA (5 g, 17.4 mmol), 2-octyl-dodecylamine
(7.6 g, 25.6 mmol), and 100 mL CH3COOH was refluxed for
3 h. The mixture was extracted with CH2Cl2, and purified
through the silica gel column. The eluent is petroleum ether/
CH2Cl2 (1:1, v/v). DNNMI (6.12 g, 62%) was obtained as a
light yellow solid. 1H NMR (400 MHz, CDCl3) d 9.47 (s, 2H),
9.33 (s, 2H), 4.16 (d, 2H), 1.97 (s, 1H), 1.23 (s, 32H), 0.86 (q,
6H). 13C NMR (101 MHz, CDCl3) d 161.7, 147.7, 132.0, 130.7,
130.2, 127.3, 125.3, 77.3, 77.0, 76.7, 45.3, 36.6, 319, 31.6,
30.0, 29.8, 29.3, 26.4, 22.7, 14.1 ppm. MP: 78 8C. MS
(MALDI-TOF): Calcd for C32H45N3O6: 567.33, found: 567.70
(M-).
Herein, a new synthetic route was presented. As shown in
Figure 3, soluble DBNMI was efficiently synthesized with a
four-step route. Three D-A type conjugated polymers based
on NMI were firstly prepared by coupling reactions of
DBNMI with excellent donors 5,50-bis(trimethylstannyl)-2,20-
bithiophene (T2), 5,500-bis(trimethylstannyl)-2,20:50,200-terthio-
phene (T3), and (4,8-bis(5-(2-ethylhexyl) thiophen-2-
yl)benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl)bis(trimethylstan-
nane) (BDT). These polymers are proved to be typical
acceptors, and the highest mobility is as high as 5.8 3 1023
cm2 V21 s21, which is close to star molecule Poly{[N,N’-
bis(2-octy ldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-
2,6-diyl]-alt-5,5’-(2,2-bithio-phene)10 (N2200, Figure S1,
Supporting Information).
Synthesis of DANMI
A mixture of DNNMI (5.0 g, 8.8 mmol), 5% Pd/C (0.6 g), and
20 mL THF was added to the autoclave, and it was filled
with H2, heated at 80 8C for 6 h. Filtrate was collected by fil-
tration, solvent was removed, and the crude product was
purified through the silica gel column with eluent CH2Cl2.
EXPERIMENTAL
Materials and Methods
1
DANMI (3.5 g, 78%) was obtained as a yellow solid. H NMR
All the chemicals in this research are commercially available.
The 1H and 13C NMR spectra were measured on a NMR
spectrometer with CDCl3 as a solvent and TMS as an internal
reference. Cyclic voltammetry (CV) of acceptors in the thin
film was performed with a standard commercial electro-
chemical analyzer in a three electrode single component cell
under argon. Working electrode: glassy carbon; reference
electrode: Ag/Ag1, auxiliary electrode: Pt wire; supporting
electrolyte: 0.1 M tetrabutylammonium hexafluorophosphate
(Bu4NPF6) in acetonitrile at a scan rate of 100 mV s21, inter-
nal standard: ferrocene (Fc). UV–vis absorption spectra were
taken on a Hitachi U-3100 UV–vis spectrophotometer. Ther-
mogravimetric analysis (TGA) measurement was performed
on a TA Instruments, TGA-2050. By the GPC method, and
polystyrene was used as a standard by using 1,2,4-trichloro-
benzene as eluent under 160 8C.
(400 MHz, CDCl3) d 7.75 (d, 2H), 7.05 (d, 2H), 4.05 (d, 6H),
1.96 (d, 1H), 1.25 (d, 32H), 0.86 (q, 6H). 13C NMR (101 MHz,
CDCl3) d 164.8, 145.5, 135.2, 123.5, 118.5, 117.4, 111.9,
77.3, 77.0, 76.7, 44.4, 36.6, 31.9, 31.6, 30.1, 29.6, 29.3, 26.5,
22.7, 14.1 ppm. MP: 132 8C.HRMS (MALDI-TOF): Calcd for
C32H49N3O2: 507.3825, found: 507.3816 (M–).
Synthesis of DBNMI
DANMI (2.0 g, 3.9 mmol) and 40% aqueous HBr solution
(200 mL) were mixed at 25 8C; 0.4 mol/L NaNO2 aqueous
solution (120 mL) was dropwisely added to above mixture.
Then, the mixture of CuBr (3.0 g, 20.9 mmol) and 40%
aqueous HBr solution (120 mL) was added at 25 8C for
3 h. The mixture was extracted with CH2Cl2, and purified
through the silica gel column. The eluent is petroleum
ether/CH2Cl2 (2:1, v/v). DBNMI (0.7g, 27%) was obtained
as a light yellow solid. 1H NMR (400 MHz, CDCl3) d 8.64
(s, 2H), 8.27 (s, 2H), 4.08 (d, 2H), 1.98 (d, 1H), 1.24 (d,
32H), 0.99 – 0.69 (q, 6H). 13C NMR (101 MHz, CDCl3) d
162.8, 134.1, 124.4, 122.3, 77.3, 76.9, 76.6, 44.9, 36.5, 31.8,
29.9, 29.6, 29.4, 26.4, 22.6, 14.0 ppm. MP: 92 8C. MS
Synthesis of DNNMA
1,8-Naphthalic anhydride (NMA; 10 g, 50.5 mmol) was dis-
solved in 35 mL 98% H2SO4 at 0 8C, and the mixture of 98%
(MALDI-TOF): Calcd for
633.23 (M–).
C32H45Br2NO2: 633.18, found:
Synthesis of NMI-T2
A mixture of DBNMI (299.3 mg, 0.5 mmol), T2 (232.2 mg,
0.5 mmol), Pd(PPh3)4 (32.6 mg, 0.03 mmol), toluene
(10 mL), and DMF (1 mL) was refluxed for 24 h under
argon. The crude product precipitated in methanol and was
collected by filtration. This solid purified through the silica
FIGURE 2 Synthetic route for DBDIPPNMI in literature.
2
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FIGURE 3 Synthetic route of DBNMI and related polymers. [Color figure can be viewed at wileyonlinelibrary.com]
gel column, and the eluent is CHCl3. NMI-T2 (446.5 mg, 84%
yield) was obtained as a dark red solid. Mn 5 5.9 KDa, Mw/
Mn 5 1.32; UV–vis (CHCl3): kmax (E) 5 370 nm (23,100
M21 cm21).
Synthesis of NMI-BDT
A mixture of DBNMI (191.4 mg, 0.3 mmol), BDT (271.0 mg,
0.3 mmol), Pd(PPh3)4 (20.8 mg, 0.02 mmol), toluene
(10 mL), and DMF (1 mL) was refluxed for 24 h under
argon. The crude product precipitated in methanol and was
collected by filtration. This solid purified through the silica
gel column, and the eluent is CHCl3. NMI-BDT (379.2 mg,
82% yield) was obtained as a dark red solid. Mn 5 52.1 KDa,
Synthesis of NMI-T3
A mixture of DBNMI (298.7 mg, 0.5 mmol), T3 (270.9 mg,
0.5 mmol), Pd(PPh3)4 (32.6 mg, 0.03 mmol), toluene
(10 mL), and DMF (1 mL) was refluxed for 24 h under
argon. The crude product precipitated in methanol and was
collected by filtration. This solid purified through the silica
gel column, and the eluent is CHCl3. NMI-T3 (484.2 mg, 85%
yield) was obtained as a dark red solid. Mn 5 4.8 KDa, Mw/
Mn 5 1.62; UV–vis (CHCl3): kmax (E) 5 414 nm (38,900
M21 cm21).
TABLE 1 Molecular Weights of the Polymersa
Polymer
Mn (KDa)
Mw (KDa)
Mw/Mn
NMI-T2
NMI-T3
NMI-BDT
5.9
7.8
1.32
1.62
1.91
4.8
7.8
52.1
99.3
FIGURE 4 Absorption spectra of NMI-T2, NMI-T3, and NMI-
BDT in solid film. [Color figure can be viewed at wileyonlineli-
brary.com]
a
1,2,4-Trichlorobenzene as eluent, under 160 8C, and polystyrene as
standard.
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The key intermediate 3,6-dibromo-N-(2-octyldodecyl)-1,8-
naphthalimide (DBNMI) was obtained by bromination of
DANMI. The product can be rapidly separated by a silica gel
column.
With DBNMI as acceptor part, three D-A type conjugated pol-
ymers were obtained by Stille coupling reactions. The cou-
pling reactions occur smoothly in refluxed toluene at the
presence of catalyst Pd(PPh3)4, and the yield is over 80%. To
improve the activity of the catalyst, a small amount of DMF
was added.
The structures of new compounds DNNMI, DANMI, and
DBNMI were well characterized by 1H NMR, 13C NMR, and
MS. The polymers NMI-T2, NMI-T3, and NMI-BDT were char-
acterized by gel permeation chromatography (GPC).
FIGURE 5 Cyclic voltammograms of NMI-T2, NMI-T3, and NMI-
BDT. [Color figure can be viewed at wileyonlinelibrary.com]
Number-average molecular weight (Mn) of NMI-T2, NMI-T3,
and NMI-BDT are 5.9, 4.8, 52.1 KDa respectively, which was
measured by GPC using 1,2,4-trichlorobenzene as eluent
under 160 8C and polystyrene as the standard. The molecu-
lar weight of NMI-T2 and NMI-T3 is low because of their
poor solubility. High molecular weight of NMI-BDT was
obtained, because the donor monomer BDT contains soluble
alkyl chains. All three polymers are soluble in chloroform,
chlorobenzene and other organic solvents, which guarantees
their solution process (Table 1).
Mw/Mn 5 1.91; UV–vis (CHCl3): kmax (E) 5 410 nm (49,300
M21 cm21).
RESULTS AND DISCUSSION
Synthesis
As shown in Figure 2, we failed to get pure DBNMI in litera-
ture method, because the crude product contains a main
side product, which has a very similar retention factor value
in TLC as that of DBNMI. Therefore, we adopted a new syn-
thetic route. The key intermediate DBNMI was obtained
through a four-step synthesis.
Optical Properties
The UV–vis absorption spectra of NMI-T2, NMI-T3, and NMI-
BDT in thin film are illustrated in Figure 4. These polymers
have strong absorption in the range of 350–550 nm. Com-
pared to the NDI polymers, maximum absorption of polymer
NMI-T2 show significant blue shift by 286 nm, and other pol-
ymers also have similar blue shifts,52 because of less elec-
tron withdrawing imide groups. In solution (Figure S2,
Supporting Information), the maximum extinction coeffi-
cients (Emax) of NMI-T2, NMI-T3, and NMI-BDT are around
23,000–50,000 M21 cm21 located at 370, 414, and 410 nm,
respectively. Compared with solution absorption, solid
absorption redshifts more than 50 nm, indicating strong
aggregation of NMI polymers in solid state. The optical band
gaps of NMI-T2, NMI-T3, and NMI-BDT were estimated from
the absorption onset edge of the thin films, and the values
are of 2.18, 2.06, and 2.16 eV, respectively. Therefore, these
polymers are moderate bandgaps acceptors. Medium
3,6-Dinitro-1,8-naphthalic anhydride (DNNMA) was obtained
by nitration of commercial NMA at 70 8C in the mixture of
98% H2SO4 and 68% HNO3.The low reaction temperature is
necessary, which could improve the yield of DNNMA.
DNNMA and 2-octyldodecylamine was refluxed in acetic acid
to give 3,6-dinitro-N-(2-octyldodecyl)-1,8-naphthalene imide
(DNNMI) in 62% yield.50 Since the nitro group can be
substituted by amino groups, we tried different solvents,
such as ethanol and N,N-dimethylformamide, and found that
acetic acid is the best solvent.
3,6-Diamino-N-(2-octyldodecyl)-1,8-naphthalimide (DANMI)
was obtained by hydrogenation of DNNMI in THF with cata-
lyst 5% Pd/C, and the isolated yield is up to 77%.51 The
product can be separated by neutral alumina column, or sep-
arated by a silica column treated with trimethylamine.
TABLE 2 Optical and Electrochemical Data of NMI-T2, NMI-T3, and NMI-BDT
a
b
c
Polymers
kabs (nm)
Solution
370
Emax (M21cm21
)
kon set (nm)
Eg (eV)
LUMO (eV)
HOMO (eV)
Film
NMI-T2
NMI-T3
NMI-BDT
392
428
330/414/519
23,100
38,900
49,300
569
602
574
2.18
2.06
2.16
23.68
23.67
23.66
25.86
25.73
25.82
414
329/410/515
a
c
The molar extinction coefficient at kmax in solution.
Measured in thin films.
Optical band gap.
b
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FIGURE 6 Plots for the measurements of electron mobility of NMI-T2, NMI-T3, and NMI-BDT. [Color figure can be viewed at
wileyonlinelibrary.com]
bandgap acceptors with excellent photovoltaic properties are
highly desired in the organic solar cells.53,54
Figure 6. The electron mobility of NMI-T2, NMI-T3, and NMI-
BDT is 4.9 (60.9) 31023, 1.0 (60.3) 31025, 3.1 (60.3)
31023 cm2 V21 s21 respectively. The highest electron mobil-
ity is up to 5.8 3 1023 cm2ꢁV21 s21, which is close to star
N2200 (5 3 1023 cm2 V21 s21, SCLC method).56
Electrochemical Properties
To study the energy levels of these polymers, their redox
properties in the solid state were measured by the CV. All of
them show a semi-reversible reduction peak indicating their
ability to accepting at least one electron (Fig. 5). The LUMO
levels were estimated based on the assumption that the
energy level of Fc/Fc1 is 24.8 eV relative to vacuum,55 and
all three polymers show similar LUMO energy levels in
23.66ꢀ–3.68 eV. The reason is that all the polymers have
the same first electron accepting position (O atom in the car-
bonyl group), and the carbonyl groups in these polymers
have little energy difference. The HOMO energy levels were
calculated by equation of HOMO 5 LUMO – Eg, and the values
are 25.86, 25.73, and 25.82 eV for NMI-T2, NMI-T3, and
NMI-BDT, respectively. Compared with the NDI polymers,
LUMO levels of NMI polymers increased by about 0.25 eV,
which lies in less electron withdrawing imide groups of NMI
polymers (Table 2).
Thermal Stability
As shown in Figure 7, all three polymers exhibit good ther-
mal stability in TGA, and 5% weight loss temperature is
higher than 330 8C under the air. Different scanning calorim-
etry traces (Figure S3, Supporting Information) of NMI-T2,
NMI-T3 and NMI-BDT show no phase transition from room
temperature to 250 8C.
CONCLUSIONS
In conclusion, soluble DBNMI was obtained efficiently
through a new synthetic route. Three D-A types conjugated
polymers based on NMI was first prepared. These polymers
have low LUMO levels, moderate bandgaps, and strong
absorption. They show typical characters of n-type semicon-
ductors in SCLC measurements, and the highest electron
mobility is up to 5.8 3 1023 cm2 V21 s21. These polymers
are a class of excellent electron transport materials and are
expected to be widely used in the organic optoelectronics.
Their applications in OTFTs and organic solar cells are in
progress in our laboratory.
Charge Mobility
Space charge limiting current (SCLC) is a widely used
method to estimate the electron transport properties of
semiconductors. In this research, the electron mobility was
measured by SCLC devices with the structure of ITO/ZnO/
polymer film/Al. The current-voltage curves were shown in
ACKNOWLEDGMENTS
The work was supported by grants National Natural Science
Foundation of China (No. 21562031 and 21602150), and Natu-
ral Science Foundation of Jiangxi Province in China (No.
20161BAB213064, 20171ACB21012, 20161BAB213063, and
20142BAB213005). The authors declare no competing finan-
cial interest.
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