1,6- and 1,7-Regioisomers of asymmetric and symmetric Perylene Bisimides: Synthesis, characterization and optical properties

Article abstract of DOI:10.3390/molecules19010327

The 1,6- and 1,7-regioisomers of dinitro- (1,6-A and 1,7-A) and diamino-substituted perylene bisimides (1,6-B and 1,7-B), and 1-Amino-6-nitro- and 1-Amino-7-nitroperylene bisimides (1,6-C and 1,7-C) were synthesized. The 1,6-A and 1,7-A regioisomers were

Full text of DOI:10.3390/molecules19010327

Molecules 2014, 19, 327-341; doi:10.3390/molecules19010327
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
1,6- and 1,7-Regioisomers of Asymmetric and Symmetric Perylene
Bisimides: Synthesis, Characterization and Optical Properties
Hsing-Yang Tsai, Che-Wei Chang and Kew-Yu Chen *
Department of Chemical Engineering, Feng Chia University, Taichung 40724, Taiwan;
E-Mails: b77315@hotmail.com (H.-Y.T.); a1981207@hotmail.com (C.-W.C.)
* Author to whom correspondence should be addressed; E-Mail: kyuchen@fcu.edu.tw;
Tel.: +886-4-2451-7250 (ext. 3683); Fax: +886-4-2451-0890.
Received: 25 November 2013; in revised form: 10 December 2013 / Accepted: 11 December 2013 /
Published: 27 December 2013
Abstract: The 1,6- and 1,7-regioisomers of dinitro- (1,6-A and 1,7-A) and diamino-substituted
perylene bisimides (1,6-B and 1,7-B), and 1-amino-6-nitro- and 1-amino-7-nitroperylene
bisimides (1,6-C and 1,7-C) were synthesized. The 1,6-A and 1,7-A regioisomers were
successfully separated by high performance liquid chromatography and characterized by
1
500 MHz H-NMR spectroscopy, and subsequently, their reduction which afforded the
corresponding diaminoperylene bisimides 1,6-B and 1,7-B, respectively. On the other hand,
the monoreduction of 1,6-A and 1,7-A, giving the asymmetric 1-amino-6-nitro (1,6-C) and
1-amino-7-nitroperylene bisimides (1,7-C), respectively, can be performed by shortening
the reaction time from 6 h to 1 h. This is the first time the asymmetric 1,6-disubstituted
perylene bisimide 1,6-C is obtained in pure form. The photophysical properties of 1,6-A
and 1,7-A were found to be almost the same. However, the regioisomers 1,6-C and 1,7-C,
as well as 1,6-B and 1,7-B, exhibit significant differences in their optical characteristics.
Time-dependent density functional theory calculations performed on these dyes are
reported in order to rationalize their electronic structure and absorption spectra.
Keywords: 1-amino-6-nitroperylene bisimide; 1-amino-7-nitroperylene bisimide;
1,6-diaminoperylene bisimide; 1,7-diaminoperylene bisimide; asymmetric perylene
bisimides; symmetric perylene bisimides

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1. Introduction
Perylene bisimides (PBIs) and their related derivatives have received considerable attention due to
their potential applications in molecular electronic and optical devices, such as light-emitting diodes [1–4],
organic field-effect transistors (OFETs) [5–10], light-harvesting arrays [11,12], photovoltaic cells [13–22],
LCD color filters [23,24], photochromic materials [25,26], and molecular wires [27,28]. PBIs have also
been utilized as building blocks to construct supramolecular or artificial photosynthetic systems [29–31].
These compounds are advantageous due to their high molar absorptivities, excellent thermal and
optical stabilities, ease of synthetic modification and reversible redox properties [32–42]. Additionally,
the electronic characteristics of PBIs can be fine-tuned by the substitution of the conjugated aromatic
core. Based on this concept, more and more perylene bisimide derivatives with either electron-donating
or electron-withdrawing groups have been reported in the literature, including: (a) piperidinyl-substituted
PBIs [43–45]; (b) pyrrolidinyl-substituted PBIs [46–48]; (c) alkylamino-substituted PBIs [49–51];
(d) amino-substituted PBIs [52,53]; (e) alkoxy-substituted PBIs [54–58]; (f) hydroxy-substituted
PBIs [59,60]; (g) aryl-substituted PBIs [61,62]; (h) ferrocenyl-substituted PBIs [63,64]; (i)
alkyl-substituted PBIs [65]; (j) perfluoroalkyl-substituted PBIs [66,67]; (k) boryl-substituted PBIs [68];
(l) cyano-substituted PBIs [69,70]; (m) nitro-substituted PBIs [71–73], etc.
To date, a general strategy for introducing substituents onto the PBIs core is bromination of
perylene dianhydride. Subsequently, nucleophilic substitutions and metal-catalyzed cross-coupling
reactions can then be performed and yield a regioisomeric mixture of 1,7- (major) and 1,6-disubstituted
(minor) PBIs. However, only a few reports of isolation and characterization of both 1,6- and
1,7-disubstituted PBIs have been reported, and still very little is known about the spectroscopic
properties of 1,6-disubstituted PBIs [43,46,63]. To expand the scope of PBI-based chromophores
available for designing systems for colorful dyes and charge transport, we have recently [53] synthesized a
series of symmetric 1,7-dinitro- and 1,7-diaminoperylene bisimides (1,7-A and 1,7-B, Scheme 1).
Herein, we present our research for the synthesis, separation, characterization, optical properties,
and complementary time-dependent density functional theory (TD-DFT) calculations of 1,6- and
1,7-regioisomers of asymmetric and symmetric PBIs.
2. Results and Discussion
2.1. Synthesis
The chemical structures of symmetric (1,6-A, 1,7-A, 1,6-B, and 1,7-B) and asymmetric PBIs (1-A,
1-B, 1,6-C, and 1,7-C) and their synthetic routes are shown in Scheme 1. The synthesis starts from
an imidization of perylene bisanhydride (PBA) by reaction with cyclohexylamine. The cyclohexyl
end-capping groups increase the steric bulkiness to the periphery of molecule, so that the solubility can
be improved. The mononitration can then be achieved by a reaction of perylene bisimide (PBI) with
cerium (IV) ammonium nitrate (CAN) and HNO3 under ambient temperature for 2 h, giving 1-A in
high yields of ca. 95%. The presence of a single nitro substituent can be verified by the presence of
seven signals at δ 8.1~8.8 ppm in the 1H-NMR spectrum and the strong absorption at 1539 cm−1 in the
FT-IR spectrum, corresponding to an asymmetrical stretching of the nitro group (Figure S1). Further
nitration of 1-A using the same reagents at ambient temperature for 46 h gave dinitroperylene

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bisimides in 80% yield. Among the products, a 3:1 mixture of regioisomers (nitrated at the 1,6- or
1,7-positions) was observed by 1H-NMR spectroscopy, a situation similar to the result of bromination
described previously by Rybtchinski, et al. [45]. The regioisomeric 1,6- and 1,7-dinitroperylene
bisimides (1,6-A and 1,7-A) can be separated by high performance liquid chromatography (HPLC).
Pure 1,7-regioisomer (1,7-A) can also be obtained through repetitive crystallizations.
Scheme 1. The synthetic routes to A–C [53].

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It is noteworthy that the characteristic signals of the regioisomers 1,6-A and 1,7-A in the 1H-NMR
spectra (Figure 1), a singlet and two doublets of the perylene core protons, exhibit very small chemical
shift value differences (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 1,6-A and 1,7-A was
performed on the basis of the signal of the cyclohexyl methine protons next to the imide nitrogen at
5.01 ppm. Because they are in the same chemical environment, both cyclohexyl methine protons of
major regioisomer 1,7-A appear as one common multiplet at 5.01 ppm, but the signal splits into double
multiplets for minor regioisomer 1,6-A (protons d1 and d2). Therefore, an unambiguous characterization
has been made successfully on the basis of 500 MHz 1H-NMR.
1
Figure 1. H-NMR (500 MHz, CDCl3) partial spectra of regioisomerically pure perylene
bisimides 1,6-A (top) and 1,7-A (bottom).
The reduction of 1-nitro (1-A), 1,6-dinitro (1,6-A) and 1,7-dinitroperylene bisimides (1,7-A) by tin
(II) chloride dihydrate (SnCl2·2H2O) in refluxing THF (6 h) afforded the 1-amino (1-B), 1,6-diamino
(1,6-B) and 1,7-diaminoperylene bisimides (1,7-B), respectively. The symmetric structures of 1,6- and
1,7-diaminoperylene bisimides can be verified by the presence of three signals (one singlet and two
1
doublet signals) at δ 8.0–9.0 ppm in the H-NMR spectrum, indicating that there are only three
different kinds of protons in the perylene core. However, there are seven sets of signals, including six
doublets and one singlet in the asymmetric 1-aminoperylene bisimide (Figure S2). On the other hand,
the monoreduction of 1,6-A and 1,7-A can be performed by shortening the reaction time (1 h), giving
asymmetric 1-amino-6-nitro (1,6-C) and 1-amino-7-nitroperylene bisimides (1,7-C), respectively. The

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asymmetric structures of 1-amino-6-nitro (1,6-C) and 1-amino-7-nitroperylene bisimides (1,7-C) can
be verified by the presence of six signals (two singlet and four doublet signals) at δ 8.0–9.0 ppm in the
1H-NMR spectrum, which indicates that there are six different kinds of protons in the perylene core
(Figure 2). Additionally, in the aromatic region four doublets and two singlets of the regioisomers
1,6-C and 1,7-C exhibit significant differences in chemical shift values (ca. 0.3 ppm). More importantly,
these aromatic signals for 1,6-C appear in different order (doublet, doublet, doublet, singlet, singlet,
doublet) compared to that of 1,7-C (doublet, singlet, doublet, doublet, singlet, doublet). This different
pattern of appearance of singlets and doublets makes them easily recognizable by 400 MHz 1H-NMR.
Figure 2. The 1H-NMR (400 MHz, CDCl3) spectra of 1,6-C (top) and 1,7-C (bottom).
2.2. Optical Properties
Figure 3 shows the steady state absorption spectra of A–C in dichloromethane. The longest
wavelength absorption bands of 1-A and 1,6-A (1,7-A) appear at 518 nm and 515 nm, respectively.
These peaks are assigned to the * transitions localized on the perylene core (Figure S3). Moreover,
the absorption spectra of all nitro-substituted PBIs (1-A, 1,6-A, and 1,7-A) are nearly identical with the
spectrum of the non-substituted perylene bisimide (PBI), but they do not exhibit fluorescence.
The reduction of 1,6-A/1,7-A (1-A) to 1,6-B/1,7-B (1-B) switches the substituent from an
electron-withdrawing group to an electron-donating group and causes a large red shift. The spectra of
monoamino-substituted (1-B) and diamino-substituted PBIs (1,6-B and 1,7-B) are dominated by very
broad absorption bands that cover a large part of the visible spectrum (350–750 nm). These broad
bands are representative for perylene bisimide derivatives N-substituted at the bay-core positions, due
to a charge transfer absorption [49]. In contrast to 1,6-A and 1,7-A, the diamino-substituted PBIs 1,6-B
and 1,7-B show significant differences in their absorptive features. In the case of 1,7-B, the lowest
energy band is centered at 620 nm. Whereas for regioisomer 1,6-B, this longest wavelength absorption

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band is slightly broader and blue-shifted by ca. 6 nm with respect to that of 1,7-B and has a small
shoulder at ca. 520 nm. The longest wavelength absorption band of both 1,6-B and 1,7-B is red-shifted
in relation to that of the mono-substituted compound (1-B: 578 nm), which can be explained by the
fact that the addition of amino (electron-donating) groups to the perylene core increases the HOMO
energy level and hence decreases the energy gap. This viewpoint can be further supported by a
theoretical approach based on density functional theory (see 2.3). On the other hand, the longest
wavelength absorption band of the monoreduction adducts 1,6-C and 1,7-C appears at 603 nm and 616 nm,
respectively, which is slightly blue-shifted relative to that of the corresponding direduction compounds
1,6-B (614 nm) and 1,7-B (620 nm). Careful examination of the absorption spectra of 1,6-C and 1,7-C
also reveals that the S0 → S1 electronic transition band (absorption of up to 750 nm) and the S0 → S2
electronic transition band (absorption of up to 475 nm) of 1,7-C are both broader and red-shifted than
those of 1,6-C. These features account for the differences in color observed by the naked eye.
Figure 3. Normalized absorption spectra of 1-A, 1,6-A, 1,7-A, 1-B, 1,6-B, 1,7-B, 1,6-C,
and 1,7-C in dichloromethane [53].
1-A
1,6-A
1.0
1,7-A
1-B
0.8
1,6-B
1,7-B
0.6
1,6-C
1,7-C
0.4
0.2
0.0
300
400
500
600
700
800
Wavelength (nm)
2.3. Quantum Chemistry Computation
To gain deeper insight into the molecular structures and electronic properties of A–C, quantum
chemical calculations were performed using density functional theory (DFT) at the B3LYP/6-31G**
level. The highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals
(LUMOs) of 1,6-B, 1,7-B, 1,6-C, and 1,7-C are shown in Figure 4. The HOMO of all amino-substituted
PBIs (1,6-B, 1,7-B, 1,6-C, and 1,7-C) is delocalized mainly on the amino group and the perylene core.
The LUMO of 1,6-B, and 1,7-B (1,6-diamino and 1,7-diamino) is delocalized from the central
perylene core to the bisimide groups, while the LUMO of 1,6-C and 1,7-C (1-amino-6-nitro and
1-amino-7-nitro) is extended from the central perylene core to the peripheral nitro and the bisimide
groups. The calculated and experimental parameters for perylene bisimide derivatives A–C are
summarized in Table 1. The HOMO/LUMO energy levels of 1,6-B (1,7-B) are −5.43/−3.03
(−5.33/−3.05) eV, and those of 1-B are −5.62/−3.21 eV, yet both are higher than those of 1,6-C

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(−5.89/−3.55 eV) and 1,7-C (−5.92/−3.57 eV). This can be explained by the fact that the nitro (amion)
substituent is an electron-withdrawing (−donating) group and hence decreases (increases) both the
HOMO and LUMO energy levels. The relative band gap energies estimated from the longest
absorption maxima of A–C are in good agreement with the theoretical calculations. The absorption
spectra of 1-B, 1,6-B, 1,7-B, 1,6-C, and 1,7-C were also calculated by time-dependent DFT (TD-DFT)
calculations (Franck–Condon principle, Table S1). The calculated excitation wavelengths for the
S0 → S1 transitions are 547 nm for 1-B, 558 nm for 1,6-B, 579 nm for 1,7-B, 571 nm for 1,6-C, and
573 nm for 1,7-C, which are very close to the experimental results. The slight diversion between the
experimental and calculated values results from the solvation effects for experiments and gas-phase for
theoretical calculations. Furthermore, DFT (B3LYP/6-31G**) calculations show that the ground-state
geometries of the perylene core have two core twist angles, i.e., approximate dihedral angles between
the two naphthalene subunits attached to the central benzene ring; these are ~7.9° and ~15.9° for 1-A,
~17.2 and ~17.3° for 1,6-A, ~17.0° and ~17.1° for 1,7-A, ~9.2° and ~17.5° for 1-B, ~20.0 and ~20.1°
for 1,6-B, ~19.2° and ~19.4° for 1,7-B, ~18.0 and ~19.3° for 1,6-C, and ~17.9° and ~18.5° for 1,7-C
(Table 1 and Figure 5). The core twist angles of the 1,6-disubstituted (amino-substituted) PBIs are
generally larger than those of 1,7-disubstituted (nitro-substituted) compounds.
Figure 4. Computed frontier orbitals of 1,6-B, 1,7-B, 1,6-C, and 1,7-C. The upper graphs
are the LUMOs and the lower ones are the HOMOs [53].
Table 1. Calculated and experimental parameters for perylene bisimide derivatives.
a
b
Compound
1-A
HOMO a
−6.25
−6.55
−6.57
−5.62
−5.43
−5.33
−5.89
−5.92
−5.94
LUMO a
−3.84
−4.07
−4.11
−3.21
−3.03
−3.05
−3.55
−3.57
−3.46
Eg
Eg
Twisting angle (°)
7.9, 15.9
2.41
2.48
2.46
2.41
2.40
2.28
2.34
2.35
2.48
2.39
2.40
2.40
2.24
2.13
2.14
2.19
2.17
2.38
1,6-A
1,7-A
1-B
17.2, 17.3
17.0, 17.1
9.2, 17.5
1,6-B
1,7-B
1,6-C
1,7-C
PBI
20.0, 20.1
19.2, 19.4
18.0, 19.3
17.9, 18.5
0.0
a Calculated by DFT/B3LYP (in eV); b At absorption maxima (Eg = 1240/λmax, in eV).

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Figure 5. DFT (B3LYP/6-31G**) geometry-optimized structures of 1,6-C (left) and 1,7-C
334
(right) shown with view along the long axis. For computational purposes, methyl groups
replace the cyclohexyl groups at the imide positions.
3. Experimental
3.1. General
The starting materials such as perylene-3,4,9,10-tetracarboxyldianhydride, cyclohexylamine, acetic
acid, cerium (IV) ammonium nitrate (CAN), 1-methyl-2-pyrrolidinone (NMP), tetrahydrofuran (THF) and
tin (II) chloride dihydrate (SnCl2·2H2O) were purchased from Merck (Whitehouse Station, NJ, USA),
ACROS (Pittsburgh, PA, USA) and Sigma-Aldrich (St. Louis, MO, USA). Solvents were distilled
freshly according to standard procedure. Column chromatography was performed using silica gel Merck
1
Kieselgel si 60 (40–63 mesh). H-NMR spectra were recorded in CDCl3 on a Bruker 400 or 500 MHz
instrument (Palo Alto, CA, USA). Mass spectra were recorded on a VG70-250S mass spectrometer
(Tokyo, Japan). The absorption spectra were measured using a JASCO V-570 UV-Vis
spectrophotometer (Tokyo, Japan). The Gaussian 03 program was used to perform the ab initio
calculation on the molecular structure. Geometry optimizations for compounds A–C were carried out
with the 6-31G** basis set to the B3LYP functional. Vibrational frequencies were also performed
to check whether the optimized geometrical structures for all compounds were at energy minima,
transition states, or higher order saddle points. After obtaining the converged geometries, the
TD-B3LYP/6-31G** was used to calculate the vertical excitation energies.
3.2. Synthesis
3.2.1. Synthesis of 1-Nitroperylene Bisimide (1-A)
Compound PBI (1.8 mmol), cerium (IV) ammonium nitrate (CAN, 1.2 g, 2.2 mmol), nitric acid (2.0 g,
31.7 mmol) and dichloromethane (150 mL) were stirred at 25 °C under N2 for 2 h. The mixture was
neutralized with 10% KOH and extracted with CH2Cl2. After solvent was removed, the crude product
was purified by silica gel column chromatography with eluent CH2Cl2 to afford 1-A in 95% yield.
Characterization data: IR vmax 2928, 2851, 1700, 1659, 1596, 1539, 1401, 1336, 1262, 1245, 1190,
809, 743 cm−1; 1H-NMR (500 MHz, CDCl3) δ 8.74 (1H, d, J = 7.6 Hz,), 8.62~8.69 (4H, m), 8.55 (1H,
d, J = 8.5 Hz), 8.18 (1H, d, J = 7.6 Hz), 5.00 (2H, m), 2.54 (4H, m), 1.91 (4H, m), 1.76 (6H, m), 1.47
(4H, m), 1.34 (2H, m); 13C-NMR (125 MHz, CDCl3) δ 163.4, 163.1, 163.0, 162.1, 147.6, 135.3, 132.7,
132.6, 131.2, 131.0, 129.3, 129.2, 128.9, 127.8, 127.4, 126.5, 126.4, 126.3, 126.2, 125.3, 124.6, 124.3,

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123.9, 123.5, 54.5, 54.2, 29.1, 29.0, 26.5, 26.4, 25.3, 25.2; MS (FAB): m/z (relative intensity) 600
[M+H+, 100]; HRMS calcd. for C36H30O6N3 600.2135, found 600.2141.
3.2.2. Synthesis of 1,6- and 1,7-Dinitroperylene Bisimides (1,6-A and 1,7-A)
Compound PBI (1.8 mmol), 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 48 h. The mixture was neutralized with
10% KOH and extracted with CH2Cl2. After solvent was removed, the crude product was purified by silica
gel column chromatography with eluent CH2Cl2 to afford a mixture of 1,7- and -1,6-dinitroperylene
bisimides, and 1H-NMR (400 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 refractive index detector and fitted
with a macro-HPLC column (Si, 8 μm, 250 × 22 mm). The eluent was 8:1 hexane/ethyl acetate flowing
at 12 mL/min. Two fractions were collected from the column; the first was pure 1,6 isomer, and the second
was pure 1,7 isomer. Characterization data: 1,6-A: 1H-NMR (500 MHz, CDCl3) δ 8.78 (2H, s), 8.63 (2H, d,
J = 8.0 Hz), 8.30 (2H, d, J = 8.0 Hz), 5.01 (2H, m), 2.52 (4H, m), 1.90 (4H, m), 1.74 (6H, m),
1.46 (4H, m), 1.36 (2H, m); MS (FAB): m/z (relative intensity) 645 [M+H+, 100]; HRMS calcd. for
1
C36H29O8N4 645.1985, found 645.1983. Selected data for 1,7-A: H-NMR (500 MHz, CDCl3) δ 8.78
(2H, s), 8.68 (2H, d, J = 8.5 Hz), 8.28 (2H, d, J = 8.5 Hz), 5.01 (2H, m), 2.51 (4H, m), 1.92 (4H, m),
1.74 (6H, m), 1.46 (4H, m), 1.36 (2H, m); MS (FAB): m/z (relative intensity) 645 [M+H+, 100];
HRMS calcd. for C36H29O8N4 645.1985, found 645.1981.
3.2.3. Synthesis of 1-Aminoperylene Bisimide (1-B)
Tin chloride dihydrate (5.0 g, 22 mmol), and 1-A (1.0 g, 1.7 mmol) were suspended in THF (50 mL)
and stirred 20 min. The solvent was refluxed 80 °C with stirring for 2 h. THF is removed at the rotary
evaporator, and the residue was dissolved in ethyl acetate and washed with 10% sodium hydrate
solution and brine. The organic layer was dried over anhydrous MgSO4 and the filtrate was concentrated
under reduced pressure. The crude product was purified by silica gel column chromatography with
eluent ethyl acetate/n-hexane (2/3) to afford 1-B in 80% yield. Characterization data: 1-B: IR vmax
1
3346, 3240, 2926, 1694, 1653, 1372, 1338, 1260, 806, 747 cm−1; H-NMR (400 MHz, CDCl3) δ 8.62
(1H, d, J = 8.0 Hz), 8.45 (1H, d, J = 7.6 Hz), 8.38 (1H, d, J = 8.0 Hz), 8.25 (1H, d, J = 7.6 Hz), 8.18
(1H, d, J = 8.0 Hz), 8.10 (1H, d, J = 8.0 Hz), 7.98 (1H, s), 5.03, (2H, s), 4.99 (2H, m), 2.55 (4H, m),
1.91 (4H, m), 1.74 (6H, m), 1.46~1.40 (6H, m); MS (FAB): m/z (relative intensity) 570 [M+H+, 100];
HRMS calcd. for C36H32O4N3 570.2393, found 570.2396.
3.2.4. Synthesis of 1,6- and 1,7-Diaminoperylene Bisimides (1,6-B and 1,7-B)
Tin chloride dihydrate (1.0 g, 4.8 mmol), 1,6- or 1,7-dinitroperylene bisimides (0.5 g, 0.8 mmol)
were suspended in THF (50 mL), and stirred at 25 °C under N2 for 20 min. The solvent was refluxed
80 °C with stirring for 6 h. THF is removed at the rotary evaporator, and the residue was dissolved in
ethyl acetate and washed with 10% sodium hydrate solution and brine. The organic layer was dried
over anhydrous MgSO4 and the filtrate was concentrated under reduced pressure. The crude product
was purified by silica gel column chromatography with eluent ethyl acetate/n-hexane (4/5) to afford

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1,6- or 1,7-diaminoperylene bisimides 1,6-B or 1,7-B in 82% yield. Characterization data: 1,6-B:
1H-NMR (400 MHz, CDCl3) δ 8.77 (2H, d, J = 8.0 Hz), 8.51 (2H, d, J = 8.0 Hz), 7.85 (2H, s),
5.05 (2H, m), 4.98 (4H, s), 2.59 (4H, m), 1.92 (4H, m), 1.76 (6H, m), 1.27–1.56 (6H, m); MS (FAB):
m/z (relative intensity) 585 [M+H+, 100]; HRMS calcd. for C36H33O4N4 585.2502, found 585.2508.
Selected data for 1,7-B: 1H-NMR (400 MHz, CDCl3) δ 8.90 (2H, d, J = 8.0 Hz), 8.25 (2H, d, J = 8.0 Hz),
8.14 (2H, s), 5.04, (2H, m), 4.94 (4H, s), 2.61 (4H, m), 1.93 (4H, m), 1.74 (6H, m), 1.36–1.54 (6H, m);
MS (FAB): m/z (relative intensity) 585 (M+H+, 100); HRMS calcd. for C36H33O4N4 585.2502,
found 585.2504.
3.2.5. Synthesis of 1-Amino-6-nitro and 1-Amino-7-nitroperylene Bisimides (1,6-C and 1,7-C)
Tin chloride dihydrate (0.6 g, 3.6 mmol), 1,6- or 1,7-dinitroperylene bisimides (0.4 g, 0.6 mmol)
were suspended in THF (50 mL), and stirred at 25 °C under N2 for 20 min. The solvent was refluxed
80 °C with stirring for 1 h. THF is removed at the rotary evaporator, and the residue was dissolved in
ethyl acetate and washed with 10% sodium hydrate solution and brine. The organic layer was dried
over anhydrous MgSO4 and the filtrate was concentrated under reduced pressure. The crude product
was purified by silica gel column chromatography with eluent ethyl acetate/n-hexane (2/3) to afford
1-amino-6-nitro or 1-amino-7-nitroperylene bisimides (1,6-C or 1,7-C) in 80%. Characterization data:
1
1,6-C: H-NMR (400 MHz, CDCl3) δ 8.85 (1H, d, J = 8.0 Hz), 8.69 (1H, d, J = 8.0 Hz), 8.58 (1H, d,
J = 8.0 Hz), 8.51 (1H, s), 8.22 (1H, s), 8.09 (1H, d, J = 8.0 Hz), 5.31, (2H, s), 5.02 (2H, m),
2.54 (4H, m), 1.93 (4H, m), 1.76 (6H, m), 1.40–1.51 (6H, m); MS (FAB): m/z (relative intensity)
615 [M+H+, 100]; HRMS calcd. for C36H31O6N4 615.2244, found 615.2246. Characterization data:
1,7-C: 1H-NMR (400 MHz, CDCl3) δ 8.95 (1H, d, J = 8.0 Hz), 8.81 (1H, s), 8.73 (1H, d, J = 8.4 Hz),
8.39 (1H, d, J = 8.0 Hz), 8.16 (1H, s), 8.15 (1H, d, J = 8.4 Hz), 5.33, (2H, s), 5.07 (2H, m),
2.55 (4H, m), 1.94 (4H, m), 1.77 (6H, m), 1.36–1.54 (6H, m); MS (FAB): m/z (relative intensity)
615 [M+H+, 100]; HRMS calcd. for C36H31O6N4 615.2244, found 615.2240.
4. Conclusions
We have successfully synthesized, separated, and characterized 1,6- and 1,7-regioisomers of
asymmetric (1,6-C/1,7-C) and symmetric PBIs (1,6-A/1,7-A and 1,6-B/1,7-B). The regioisomers of
dinitro-substituted PBIs (1,6-A and 1,7-A) were separated by conventional high performance liquid
chromatography. Subsequently, the reduction of 1,6-A and 1,7-A afforded the corresponding
diaminoperylene bisimides 1,6-B and 1,7-B, respectively. On the other hand, the monoreduction of
1,6-A and 1,7-A can be executed by reducing the reaction time, giving asymmetric 1-amino-6-nitro
(1,6-C) and 1-amino-7-nitroperylene bisimides (1,7-C), respectively. To our best knowledge, this is
the first time the asymmetric 1,6-disubstituted perylene bisimide 1,6-C has been obtained in pure form.
Our studies have also shown that these 1,6- and 1,7-isomers can readily be characterized by 500 MHz
1H-NMR. The optical properties of 1,6-A and 1,7-A were found to be virtually the same. However, the
regioisomers 1,6-C and 1,7-C, as well as 1,6-B and 1,7-B, exhibit significant differences in their
optical features; the S0 → S1 and the S0 → S2 electronic transition bands of 1,7-C are both broader and
red-shifted than those of 1,6-C, while the absorption spectrum of 1,6-B covers a large part of the
visible region relative to that of 1,7-B. The results offer the potential to synthesize 1,6-disubstituted

Molecules 2014, 19
337
perylene bisimide derivatives with extended optical properties. Working toward their applications
on n-type organic semiconductors (1,6-A and 1,7-A) and organic photovoltaics (1,6-B/1,7-B and
1,6-C/1,7-C) is in progress.
Supplementary Materials
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/19/1/327/s1.
Acknowledgments
The project was supported by the National Science Council (NSC 101-2113-M-035-001-MY2) and
Feng Chia University in Taiwan. The authors appreciate the Precision Instrument Support Center of
Feng Chia University in providing the fabrication and measurement facilities.
Conflicts of Interest
The authors declare no conflict of interest.
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