Synthesis and crystal packing structures of 2,7-diazapyrenes with various alkyl groups at 1,3,6,8-positions

Article abstract of DOI:10.1246/cl.200083

We have developed the synthesis of 1,3,6,8-tetramethoxy-2,7-diazapyrene through reductive aromatization of naphthalene diimide. The methoxy groups were readily converted to a variety of alkyl groups through Ni-catalyzed cross-coupling reaction with alkyl Grignard reagents. The peripheral substituents significantly influenced the packing structures of 2,7-diazapyrenes in the solid state.

Full text of DOI:10.1246/cl.200083

Advance Publication Cover Page
Synthesis and crystal packing structures of 2,7-diazapyrenes with various alkyl groups at
1,3,6,8-positions
Takumi Nakazato, Wakana Matsuda, Tsuneaki Sakurai, Shu Seki,
Hiroshi Shinokubo, and Yoshihiro Miyake*
Advance Publication on the web February 28, 2020
doi:10.1246/cl.200083
©
2020 The Chemical Society of Japan
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Publication manuscripts.

1
Synthesis and crystal packing structures of 2,7-diazapyrenes with various alkyl groups at
,3,6,8-positions
1
1
2
2
2
1
1
Takumi Nakazato, Wakana Matsuda, Tsuneaki Sakurai, Shu Seki, Hiroshi Shinokubo, and Yoshihiro Miyake*
1
Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603,
Japan
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
2
E-mail: miyake@chembio.nagoya-u.ac.jp
1
2
3
4
5
6
7
8
We have developed the synthesis of 1,3,6,8-
tetramethoxy-2,7-diazapyrene through reductive
48 Furthermore, desulfurization only provides 2,7-diazapyrenes
49 with alkyl groups longer than a butyl group.
aromatization of naphthalene diimide. The methoxy groups
were readily converted to a variety of alkyl groups through
Ni-catalyzed cross-coupling reaction with alkyl Grignard
reagents. The peripheral substituents significantly
influenced the packing structures of 2,7-diazapyrenes in the
solid state.
50
To overcome the shortcomings, we have developed a
51 more efficient method for the introduction of various alkyl
52 groups at the peripheral position of 2,7-diazapyrenes. Herein
53 we disclose the reductive aromatization of naphthalene
54 diimide 1 into 1,3,6,8-tetramethoxy-2,7-diazapyrene 5a
55 (Scheme 1c). Furthermore, we have demonstrated the
5
6 transformation of the methoxy groups to alkyl groups via
2
0-22
9
0
Keywords: Diazapyrene, Reductive aromatization,
Cross-coupling
57 cross-coupling reaction with alkyl Grignard reagents.
1
58
H
N
O
O
O
PivO
N
N
OPiv
Ar
Ar
N
Ar
Zn
Piv
cat. [Ni]
ArB(OH)
2
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
Polycyclic aromatic hydrocarbons (PAHs) are widely
2
O
1
-5
investigated for organic electronic materials. To modulate
the electronic and photophysical properties of PAHs,
introduction of heteroatoms to the PAHs flameworks is an
(a)
O
S
N
H
PivO
OPiv
N
Ar
6
effective strategy. Nitrogen is an attractive element to be
O
1
2
3
doped into PAHs flameworks because of its high electro-
Piv =
t-Bu
7
-9
negativity. Consequently, nitrogen-doped PAHs are often
S
1
0-12,15,17
examined for n-type semiconductors.
N
H
11
11
C
C
5
N
C
C
5
H
H
11
11
Pyrene is one of the representative PAHs. Numerous
pyrene derivatives have been synthesized and widely
explored in various fields such as photo-, electro- and bio-
Raney Ni
(b)
1
3
chemistry.
prepared.
Nitrogen-doped pyrenes have also been
However, the studies on nitrogen analogous of
N
H
5
N
5
1
4-16
S
S
pyrene have largely lagged behind those of pyrenes because
of the lack in their efficient synthetic methods.
3
a
4
H
N
Recently, we have synthesized 1,3,6,8-tetra(pivaloxy)-
2,7-diazapyrene 2 by reductive aromatization of naphthalene
O
O
MeO
N
OMe
R
N
R
R
Zn
MeOTf
cat. [Ni]
RMgI
1
7a
diimide 1 (Scheme 1a). This strategy enables efficient
introduction of various aryl groups to 2,7-diazapyrenes to
afford tetraaryl-2,7-diazapyrene 3. Unfortunately, however,
tetraalkyl-2,7-diazapyrenes 5 were not accessible because
no coupling reaction of 2 with alkylboronic acids proceeded.
Alkylation of 2 with alkyl Grignard reagents was also not
successful because of the nucleophilic attack of Grignard
reagents to the pivaloxy groups.
(c)
O
N
H
1
O
MeO
N
OMe
R
N
5a
5
5
6
6
6
9
0
1
2
Scheme 1. (a) Synthesis of 2,7-diazapyrene derivatives via
reductive aromatization of 1. (b) Desulfurization of 3a into
4. (c) This work.
Desulfurization of the thienyl groups of 3a with Raney
nickel allowed the indirect introduction of alkyl groups to
the peripheral positions of 2,7-diazapyrene (Scheme 1b).
Field-induced time-resolved microwave conductivity (FI-
63
Scheme
2
shows the synthetic procedure of
64 diazapyrenes 5a–5d. The reaction of naphthalene diimide
65 (1) with 6 equiv of methyl triflate (MeOTf) in the presence
66 of 16 equiv of zinc powder in 1,4-dioxane at 60 °C for 24 h
67 afforded tetramethoxy-2,7-diazapyrene 5a in 41% yield.
68 The methoxy groups in 5a were readily converted to various
69 alkyl groups through the cross-coupling reaction with alkyl
70 Grignard reagents to provide tetraalkyl-2,7-diazapyrenes
71 5b–5d. Treatment of 5a with methylmagnesium iodide in
72 the presence of catalytic amounts of Ni(cod) (cod: 1,5-
73 cyclooctadiene) and bis(dicyclohexylphosphino)ethane
1
8
TRMC) measurements revealed a good electron mobility
1
7a
for 4, demonstrating that tetraalkyl-2,7-diazapyrenes are
promising candidates for n-type semiconductors. To
investigate the potential of 2,7-diazapyrenes for electron
transporting materials, their solid state structures should be
2
1e
1
9
engineered by the proper peripheral alkyl groups. However,
the desulfurization strategy is not straightforward to
synthesize 2,7-diazapyrenes with various alkyl groups.
2
74 (dcype) afforded tetramethyldiazapyrene 5b in 37% yield.

2
1
2
3
4
5
6
7
The use of ethyl and propyl Grignard reagents furnished
tetraethyldiazapyrene 5c and tetrapropyldiazapyrene 5d in
16% and 12% yields, respectively. Unfortunately, the
introduction of longer alkyl groups was sluggish under the
optimized conditions. Diazapyrenes 5a–5d are readily
26 In sharp contrast, 5b forms face-to-face stacking in the
27 crystal with an interplanar distance of 3.385 Å (Figure 1b).
28 A brickwork packing structure was observed in 5a (Figure
29 S1b). The distance between π-planes in 5a (3.448 Å) is
30 longer than those of 5b and 5c having alkyl groups. It is
31 noteworthy that such small differences in peripheral
32 substituents dramatically changed the morphology of the
2 2 3
soluble in common organic solvents such as CH Cl , CHCl ,
and toluene.
3
3 solid state.
H
O
O
N
O
O
MeO
MeO
N
OMe
OMe
R
N
R
(a)
(b)
4
.0
.0
a)
b)
3
N
H
1
N
R
N
R
2.0
1.0
0
5a
5b (R = CH3): 37%
c (R = C2H5): 16%
d (R = C3H7): 12%
5
5
8
9
0
1
2
Scheme 2. Synthesis of tetralkyldiazapyrenes. (a) Zn (16
equiv), MeOTf (6 equiv), 1,4-dioxane, 60 °C, 24 h; (b)
RMgI (6 equiv), Ni(cod)
2
50 300
400
λ / nm
500
350 400
500
λ / nm
600
1
1
1
2
(40 mol%), dcype (40 mol%),
toluene, 80 °C, 24 h.
Concentration of 5b (M)
(c)
(d)
(e)
2
.0 × 10–2
2
.0 × 10–5
(b)
(a)
Top
Side
3
80 400
500
λ / nm
600
700
(f)
(g)
(h)
Concentration of 4 (M)
2
.0 × 10–2
2
.0 × 10–5
(c)
(d)
Top
3
80 400
500
λ / nm
600
700
Side
34
3
3
3
3
3
4
4
4
5
6
7
8
9
0
1
2
Figure 2. (a) UV-vis absorption spectra of 5a (black line)
and 5b (red line) in CH Cl . (b) Fluorescence spectra of 5a
(black line) and 5b (red line) in CH Cl . (c) Emission
spectra of 5b in CH Cl . Photo images of 5b in CH Cl
(2.0×10 M) under excitation (λex = 360 nm) at (d) 2.0×10
2
2
2
2
2
2
2
2
–5
–
5
–
2
M and (e) 2.0×10 M. (f) Emission spectra of 4 in CH
2 2
Cl .
1
1
1
1
1
3
4
5
6
7
Photo images of 4 in CH Cl under excitation (λex = 360 nm)
2
2
Figure 1. (a) Molecular structure and (b) packing structure
of 5b and (c) molecular structure and (d) packing structure
of 5c. Hydrogen atoms are omitted for clarity. Thermal
ellipsoids are set to 50% probability.
–
5
–2
at (g) 2.0×10 M and (h) 2.0×10 M.
43
Figure 2a and Figure S7 show UV-vis absorption
44 spectra of 5a–5d in CH Cl . While the change of the alkyl
2
2
4
4
4
4
4
5
5
5
5
5
6
7
8
9
0
1
2
3
groups did not alter the absorption spectra of
tetraalkyldiazapyrene 5b–5d, bathochromic shift was
observed in tetramethoxydiazapyrene 5a. Diazapyrenes 5a–
1
1
2
2
2
2
2
2
8
9
0
1
2
3
4
5
We successfully obtained single crystals of 5a
3
(dichloroethane/hexane), 5b (CHCl /hexane), and 5c
(CHCl /acetonitrile) suitable for X-ray analysis (Figures 1
3
2 2
5d show fluorescence (Φ = 0.28–0.49) in CH Cl . In
and S1). The diazapyrene core in 5a, 5b, and 5c adopts
highly planar conformation. Each molecule of 5c is arranged
in a slipped stacking manner in the one-dimensional
columnar structure. The distance between π-planes is 3.364
Å (Figure 1d). This packing structure is similar to that of 4.
accordance with the absorption spectra, the emission band
of 5a also bathochromically shifted compared to 5b–5d
(Figures 2b and S7). As the concentration increased, a new
emission band was observed around 500 nm in 5b (Figure.
2
3
2c). The fluorescence lifetime measured at 425 nm in a

3
2
–1 –1
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
dilute solution was 6.5 ns, while the lifetime measured at
500 nm in a concentrated solution was 26 ns. These results
clearly suggest that the newly observed emission was the
excimer emission. Interestingly, the stronger excimer
emission of 5b was observed than that of 4 (Figure. 2f). This
result clearly indicates that 5b readily forms an excimer
because of the smaller steric hindrance of the methyl group
than the pentyl group.
53 recording μ
e
= 0.06 cm V s (Figure. S9). These values
54 are almost same as the electron mobility of 4 measured
55 under the similar conditions (μ = 0.09 cm V s ) (Figure.
e
2
–1 –1
56 S10). Powder X-ray diffraction (PXRD) measurements of
57 the spincoated film of 5b revealed that the packing structure
58 in the film was close to that of the single crystal (Figure.
59 S11a). On the other hand, the patterns obtained from the
60 spincoated film and simulation from the single crystal data
61 were different for 5c (Figure. S11b). Control of the packing
62 structures and the degree of crystallinity in the film state are
63 worthy of further investigation. Nevertheless, the
64 comparable electron mobility values evaluated for 5b and 5c
65 suggests that the overlaps of low LUMOs of the 2,7-
66 diazapyrene derivatives contribute to the observed high
67 electron mobility for 5b and 5c.
The electrochemical properties of 5a–5d were
investigated by cyclic voltammetry and differential-pulse
voltammetry (Table 1). Diazapyrene 5b exhibits one
1
1
1
1
1
1
1
1
1
+
reversible reduction potential (–2.58 V; vs [Fc]/[Fc] ),
which is similar to those of 5c and 5d. While no reduction
wave was observed in 5a up to –3.0 V. This result indicates
that the introduction of methoxy groups to the diazapyrene
core raises the LUMO level. The first oxidation potential of
5a was also observed at a lower potential (0.16 V) than
those of 5b–5d.
68
In summary, we have developed the synthesis of
69 1,3,6,8-tetramethoxy-2,7-diazapyrene 5a through reductive
70 aromatization of naphthalene diimide. The methoxy groups
7
7
1
2
were converted to a variety of alkyl groups through Ni-
catalyzed cross-coupling reaction with alkyl Grignard
a
1
9
Table 1. Redox potentials of 5a–5d.
1
1
73 reagents. The X-ray crystallographic analysis of
elucidated that the length of the alkyl groups dramatically
75 affected their packing structures in the solid state.
5
Diazapyrene
E
ox (V)
Ered (V)
7
4
5a
0.16
–
b
5b
0.81
0.83
0.83
–2.58
–2.56
–2.56
7
7
6
7
Tetramethyl-2,7-diazapyrene 5b exhibited more distinct
excimer emission as compared to tetrapentyl-2,7-
5c
5d
a
78 diazapyrene 4, indicating that the length of the alkyl groups
2
2
2
2
2
2
0
1
2
3
4
5
Solvent: CH
2
Cl
2
(oxidation) or THF (reduction); supporting
(0.1 M); working electrode: glassy carbon
electrode; counter electrode: platinum wire; reference electrode:
Ag/Ag . All potentials are referenced to the potential of
ferrocene/ferrocenium couple. Determined by differential-
7
8
9
0
modulates the intermolecular interactions in solution.
Furthermore, FI-TRMC measurements clearly revealed the
electrolyte: Bu
4
NPF
6
+
81 intrinsic high electron mobility of 5b and 5c. Further
82 investigations for the application of 2,7-diazapyrenes to
b
8
8
8
8
8
8
8
9
9
9
9
3
4
5
6
7
8
9
0
1
2
3
OFETs are currently in progress in our group.
pulse voltammetry.
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
The unique face-to-face packing structure of 5b in its
single crystal stimulated us to investigate its conductivity.
The field-induced time-resolved microwave conductivity
(FI-TRMC) technique was employed to investigate the
local-scale charge transport properties for both positive
(hole) and negative (electron) carriers. A CHCl
of 5b was spincoated to form a thin film on polyimide-
coated SiO insulating layers on gold electrode-patterned
This work was supported by Grant-in-Aids for Scientific
Research on Innovative Areas “Precisely Designed
Catalysts with Customized Scaffolding (No. 2702)” (JSPS
KAKENHI Grant 16H01013) from MEXT, Japan. We thank
Prof. Satoru Hiroto (Kyoto University) for the excited state
lifetime measurements.
1
8
3
solution
2
Supporting
Information
is
available
on
quartz substrate. A top gold electrode was deposited on the
5b layer to fabricate the metal–insulator–semiconductor
http://dx.doi.org/10.1246/cl.******.
(MIS) device. The MIS device, placed in the resonant cavity, 94 References and Notes
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96
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e
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2
–1 –1
(Figure. S8d), yielding μ
e
= 0.07 cm V
s
for the film of 103
1
1
1
04
05
06
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Triangular fluorescence cell (T-81 TOSOH QUARTS) was used
for the measurement of the excimer emission.
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