1,6-Dinitroperylene bisimide dyes: Synthesis, characterization and photophysical properties

Article abstract of DOI:10.1002/jccs.201300517

1,6- and 1,7-regioisomers of dinitro-substituted perylene bisimides (1a - 1b and 2a - 2b) were synthesized under mild condition in high yields. The 1,6- and 1,7-regioisomers were successfully isolated from the regioisomeric mixture using conventional meth

Full text of DOI:10.1002/jccs.201300517

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Communication
1,6-Dinitroperylene Bisimide Dyes: Synthesis, Characterization and Photophysical
Properties
Che-Wei Chang, Hsing-Yang Tsai and Kew-Yu Chen*
Department of Chemical Engineering, Feng Chia University, 40724 Taichung, Taiwan
(Received: Oct. 13, 2013; Accepted: Jan. 2, 2014; Published Online: ??; DOI: 10.1002/jccs.201300517)
1,6- and 1,7-regioisomers of dinitro-substituted perylene bisimides (1a–1b and 2a–2b) were synthesized
under mild condition in high yields. The 1,6- and 1,7-regioisomers were successfully isolated from the
regioisomeric mixture using conventional methods of separation and subsequently characterized by 500
MHz 1H NMR spectroscopy. This is the first time when 1,6-dinitroperylene bisimides (1a–1b) are ob-
tained in pure form. Furthermore, the photophysical and electrochemical properties of 1 and 2 were found
to be almost the same. The nitro functionalities provide stability of n-type charge carriers by lowering the
LUMO to resist ambient oxidation, which may offer potential applications in air-stable n-type organic
semiconductors.
Keywords: 1,6-Dinitroperylene bisimide; 1,7-Dinitroperylene bisimide; Organic semiconductors;
DFT calculations.
INTRODUCTION
Perylene bisimides (PBIs) and their related deriva-
this method have been reported.1,42 More recently, cyano-
substituted PBIs were prepared with a high electron mobil-
ity of 0.1–0.6 cm2/V·s in air.1,43 To expand the scope of
PBI-based chromophores available for designing systems
for n-type organic semiconductors, we have synthesized a
series of nitro-substituted (1-nitro and 1,7-dinitroperylene
bisimides) PBIs,44–46 since previous studies have used ei-
ther halogen atoms or cyano substituents as electron-with-
drawing groups. Herein, we report the synthesis and
characterization of 1,6-dinitroperylene bisimides that are
intense red in color and have very similar photophysical
and electrochemical properties to the 1,7-dinitroperylene
bisimides.
tives have attracted much attention due to their potential
applications in molecular electronic and optical devices,
such as organic field-effect transistors (OFETs),1–3 light-
emitting diodes,4–5 photovoltaic cells,6–10 LCD color fil-
ters,11–12 photochromic materials,13–14 molecular wires15–16
and near-infrared absorbing materials.17 These molecules
are advantageous due to their high photochemical and opti-
cal stabilities, ease of synthetic modification and reversible
redox properties.18–38 Furthermore, the electronic charac-
teristics of PBIs can be fine-tuned by the substitution of the
conjugated aromatic core. Although organic field-effect
transistors (OFETs) have been investigated for many de-
cades, most studies have been focused on developing p-
type semiconductors, such as pentacene and hexacene.39–41
For the applications on organic complementary metal-ox-
ide semiconductor (CMOS) circuits, both p- and n-type
semiconductors are required. However, there are several
major drawbacks in most organic n-type materials, such as
their low on/off current ratio, low charge carrier mobility,
instability in air, large barriers to electron injection, and
poor solubility which hampers processes of film-casting.
To date, a promising strategy is to introduce strong elec-
tron-withdrawing substituents onto the main p-chromo-
phore by lowering its potential energy level, so that the
electron charge carriers are less susceptible to oxidation.
Many air-stable n-type organic semiconductors based on
EXPERIMENTAL
Synthesis and characterization: 4a (or 4b) (1.8 mmol),
cerium (IV) ammonium nitrate (CAN) (4.8 g, 8.8 mmol), nitric
acid (8.0 g, 131.1 mmol) and dichloromethane (250 mL) were
stirred at 25 °C under N2 for 50 h. The mixture was neutralized
with 10% KOH and extracted with CH2Cl2. After solvent was re-
moved, the crude product was purified by silica gel column chro-
matography with eluent CH2Cl2 to afford a mixture of 1,7- and
1,6-dinitroperylene bisimides, and 1H NMR (500 MHz) analysis
revealed a 3:1 ratio. Separation of the 1,6 and 1,7 isomers was
performed on a preparative HPLC system equipped with a refrac-
tive index detector and fitted with a macro-HPLC column (Si, 8
mm, 250 × 22 mm). The eluent was 8:1 hexane/ethyl acetate flow-
ing at 12 mL/min. Two fractions were collected from the column;
* Corresponding author. Tel: +886 4 24517250; Fax: +886 4 24510890; E-mail: kyuchen@fcu.edu.tw
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Communication
Chang et al.
the first was pure 1,6 isomer, and the second was pure 1,7 isomer.
Characterization data: 1a: 1H NMR (500 MHz, CDCl3) d 8.78 (s,
2H), 8.63 (d, J = 8.0 Hz, 2H), 8.30 (d, J = 8.0 Hz, 2H), 5.01 (m,
2H), 2.52 (m, 4H), 1.90 (m, 4H), 1.74 (m, 6H), 1.46 (m, 4H), 1.36
(m, 2H); MS (FAB): m/z (relative intensity) 645 (M+H+, 100);
HRMS calcd. for C36H29O8N4 645.1985, found 645.1983. Se-
cyclohexylamine or octylamine. The cyclohexyl (octyl)
end-capping groups increase the steric bulkiness to the pe-
riphery of molecule, so that improves the solubility without
inducing a pernicious effect on the packing of molecules
during the formation of solid thin films.3 The mono-nitra-
tion can be then achieved by a reaction of perylene bis-
imides (4a–4b) with cerium (IV) ammonium nitrate (CAN)
and HNO3 under ambient temperature for 2 h, giving 3a–3b
in high yields of ca. 90%. Further nitration of 3a–3b using
the same reagents at ambient temperature for 48 h gave 1,6-
and 1,7-dinitroperylene bisimides in 80% yield. The regio-
isomeric 1,6- and 1,7-dinitroperylene bisimides (1a–1b
and 2a–2b) can be separated by high performance liquid
chromatography (HPLC). Pure 1,7-regioisomer (2a or 2b)
can also be obtained through repetitive crystallizations.
It is to be note that the characteristic signals of the
regioisomers 1a (1,6-) and 2a (1,7-) in the 1H NMR spectra
(Fig. 1), one singlet and two doublets of perylene core pro-
tons, exhibit very small differences in the chemical shift
values (0.01 and 0.04 ppm for the doublets at 8.26–8.68).
However, a convenient unequivocal assignment of the
NMR spectrum to the individual regioisomers 1a (1b) and
2a (2b) was performed on the basis of the signal of meth-
anetriyl (methylene) protons next to the imide nitrogen at
5.01 (4.20) ppm. Because of the same chemical environ-
ment, both two methanetriyl (methylene) protons of major
regioisomer 2a (2b) appear as one common multiplet (trip-
let) at 5.01 (4.20) ppm, but the signal splits into double
multiplets (triplets) for minor regioisomer 1a (1b). In this
way, an unambiguous characterization has been made suc-
cessfully on the basis of 500 MHz 1H NMR.
1
lected data for 1b: H NMR (500 MHz, CDCl3) d 8.83 (s, 2H),
8.67 (d, J = 8.0 Hz, 2H), 8.32 (d, J = 8.0 Hz, 2H), 4.23 (m, 4H),
1.76 (m, 4H), 1.25~1.55 (m, 20H), 0.88 (t, J = 6.5, 6H); MS
(FAB): m/z (relative intensity) 705 (M+H+, 100); HRMS calcd.
for C40H41O8N4 705.2924, found 705.2928. Selected data for 2a:
1H NMR (500 MHz, CDCl3) d 8.78 (s, 2H), 8.68 (d, J = 8.5 Hz,
2H), 8.28 (d, J = 8.5 Hz, 2H), 5.01 (m, 2H), 2.51 (m, 4H), 1.92 (m,
4H), 1.74 (m, 6H), 1.46 (m, 4H), 1.36 (m, 2H); MS (FAB): m/z
(relative intensity) 645 (M+H+, 100); HRMS calcd. for
C36H29O8N4 645.1985, found 645.1981. Selected data for 2b: 1H
NMR (500 MHz, CDCl3) d 8.83 (s, 2H), 8.71 (d, J = 8.0 Hz, 2H),
8.31 (d, J = 8.0 Hz, 2H), 4.20 (t, J = 7.5 Hz, 4H), 1.75 (m, 4H),
1.26~1.53 (m, 20H), 0.87 (t, J = 6.5, 6H); MS (FAB): m/z (relative
intensity) 705 (M+H+, 100); HRMS calcd. for C40H41O8N4
705.2924, found 705.2923.
Scheme I The synthetic routes of 1a–3a and 1b–3b
RESULTS AND DISCUSSION
Scheme I shows the chemical structures of 1,6-dinitro
(1a–1b) and 1,7-dinitroperylene bisimides (2a–2b) and
their synthetic routes. The synthesis starts from an imidiza-
tion of perylene bisanhydride (5) by reacting with either
Fig. 1. 1H NMR (500 MHz, CDCl3) partial spectra of
regioisomerically pure perylene bisimides 1a
(top) and 2a (bottom).
2
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1,6-Dinitroperylene Bisimide Dyes
Table 1. Summary of half-wave redox potentials, HOMO and
Fig. 2 shows the steady state absorption spectra of
1a–4a in THF. The absorption spectra of 1,6-dinitro- (1a),
1,7-dinitro- (2a) and 1-nitroperylene bisimides (3a) are
nearly identical with the spectrum of the non-substituted
perylene bisimide (4a), but they do not exhibit fluores-
cence. The longest wavelength absorption bands of 1a–4a
appear at 514, 515, 518, and 520 nm, respectively. These
peaks are assigned to the p-p* transitions localized on the
perylene core.47 In contrast to dipyrrolidinyl-substituted
(electron-donating substituted) regioisomers (1,6- and
1,7-),48 the shapes of the absorption spectra of 1a (1,6-
dinitro) and 2a (1,7-dinitro) are very similar in the region
from about 300 nm to 600 nm. Furthermore, the vibronic
progressions at 450–525 nm diminish gradually with an in-
creasing number of the nitro groups, which can be ex-
plained by the decrease of molecular rigidity due to en-
hanced steric hindrance induced by the nitro groups.
LUMO energy levels for 1a–4a
b
Compound E(1)a (V) E(2)a (V) EHOMO (eV) ELUMOb (eV)
1a
2a
3a
4a
-0.11
-0.09
-0.19
-0.46
-0.37
-0.34
-0.51
-0.76
-6.73
-6.75
-6.64
-6.36
-4.33
-4.35
-4.25
-3.98
a Measured in a solution of 0.1 M tetrabutylammonium hexa-
fluorophosphate (TBAPF6) in dichloromethane versus SCE.
b Estimated versus vacuum level from ELUMO = -4.44 – E(1),
EHOMO = ELUMO – Eg.
electronic properties of 1–4, quantum chemical calcula-
tions were performed using density functional theory
(DFT) at the B3LYP/6-31G** level.49–51 The highest occu-
pied molecular orbitals (HOMOs) and the lowest unoccu-
The cyclic voltammograms of 1a–4a are shown in
Fig. 3. These molecules undergo two quasi-reversible one-
electron reductions in THF at modest potentials. Table 1
summarizes the redox potentials and the HOMO and
LUMO energy levels estimated from cyclic voltammetry
(CV) for 1a–4a. It appears that the first reduction potential
is shifted toward more positive values with increasing
number of substituted nitro groups, while both the HOMO
and the LUMO energy levels decrease with the trend. The
HOMO/LUMO energy levels of 1a, 2a, 3a, and 4a are esti-
mated to be -6.73/-4.33, -6.75/-4.35, -6.64/-4.25, and
-6.36/-3.98 eV, respectively. Note that the LUMO energy
levels of 1a and 2a are quite similar to those of dicyano-
substituted PBIs,44 which may offer potential applications
in air-stable n-type organic semiconductors.
Fig. 3. The cyclic voltammograms of 4a (green line),
3a (blue line), 2a (red line), and 1a (black line)
measured in THF solution, at 200 mV/s.
For deeper insight into the molecular structures and
Fig. 4. Computed frontier orbitals of 1a and 2a. The
upper graphs are the LUMOs and the lower
ones are the HOMOs.
Fig. 2. Normalized absorption spectra of 1a–4a in
THF.
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Communication
Chang et al.
Table 2. Calculated (DFT/B3LYP) and experimental parameters
Twisting
ACKNOWLEDGEMENTS
The project was supported by the National Science
Council (grant No. NSC 101-2113-M-035-001-MY2) and
Feng Chia University in Taiwan. The authors appreciate
the Precision Instrument Support Center of Feng Chia Uni-
versity in providing the fabrication and measurement facil-
ities.
Compound HOMOa LUMOa
Eg
Eg
a
b
angle (o)a
1a
1b
2a
2b
3a
3b
4a
4b
-6.55
-6.58
-6.57
-6.60
-6.25
-6.29
-5.94
-5.97
-4.07
-4.10
-4.11
-4.11
-3.84
-3.84
-3.46
-3.49
2.48
2.48
2.46
2.49
2.41
2.45
2.48
2.48
2.40 17.22, 17.34
2.40 17.33, 17.59
2.40 17.02, 17.12
2.41 17.02, 17.30
2.39
2.39
2.38
2.39
7.89, 15.87
7.90, 16.02
0.00, 0.00
0.00, 0.00
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