The reductive aromatization of naphthalene diimide: A versatile platform for 2,7-diazapyrenes

Article abstract of DOI:10.1039/c8cc01937a

The reductive aromatization of naphthalene diimide provides tetrapivaloxy-2,7-diazapyrene, which serves as a versatile platform toward peripherally substituted 2,7-diazapyrenes. Time-resolved microwave conductivity measurements demonstrated that the intrinsic electron mobility of 2,7-diazapyrene is significantly higher than that of the corresponding pyrene.

Full text of DOI:10.1039/c8cc01937a

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The reductive aromatization of naphthalene diimide: A versatile
platform for 2,7-diazapyrenes
Takumi Nakazato,a Takuto Kamatsuka,a Junichi Inoue,b Tsuneaki Sakurai,b Shu Seki,b Hiroshi
Shinokuboa and Yoshihiro Miyake*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The reductive aromatization of naphthalene diimide provides properties and functions of pyrenes, 2,7-diazapyrenes
tetrapivaloxy-2,7-diazapyrene, which serves as versatile nitrogen-doped analogue of pyrene, should have versatile
platform toward peripherally substituted 2,7-diazapyrenes. Time- utilities in a wide range of areas. However, studies on remain
1, a
a
1
resolved microwave conductivity measurements demonstrated limited to those of the parent 2,7-diazapyrene (1a), due to the
that the intrinsic electron mobility of 2,7-diazapyrene is lack of efficient synthetic routes to other 2,7-diazapyrene
significantly higher than that of the corresponding pyrene.
derivatives.14-17
Modification of the inner and outer regions in polycyclic
aromatic hydrocarbons (PAHs) is a promising prospect for
tuning their intrinsic properties leading to organic electronic
materials.1 The introduction of peripheral substituents in PAHs
also modulates their electronic and photophysical properties,
as well as their solubility and aggregation behaviour in the
solid state. Consequently, the development of methods for the
peripheral functionalization of PAHs is an important topic in
current organic chemistry.2-6
The incorporation of heteroatoms into the core of PAHs
also dramatically alters their electronic properties.7
Specifically, the replacement of benzene rings in PAHs with
pyridine rings is an efficient method for the modulation of
their electronic properties, as the high electronegativity of the
nitrogen atoms can lower the energy level of the LUMO of the
PAHs.8-16 Furthermore, nitrogen-doped PAHs should find
applications as acid-responsive materials and as supporting
ligands in supramolecular structures due to the ability of the
nitrogen atom on the pyridine ring to coordinate protons and
various metals.10, 12, 14
Scheme 1 Synthesis of 2,7-diazapyrenes.
The synthesis of 1a is based on the reduction of carbonyl
groups and the oxidative aromatization of naphthalene
diimides
2
(Scheme 1a).14, 18 However, the low solubility of 1a
hinders further transformations. Sachdev has reported the
formation of 1,3,6,8-tetrakis(trimethylsiloxy)-2,7-diazapyrene
from the reaction of 2a with butyllithium and trimethylsilyl
chloride.16 Unfortunately, the transformation of the siloxy
groups was unsuccessful due to the low reactivity of the C–O
bonds and the hydrolysis of the Si–O bonds. Herein, we report
the reductive aromatization of 2a to 1,3,6,8-tetrapivaloxy-2,7-
diazapyrene 1b. The latter serves as a platform for the
Pyrenes are simple and fundamental motifs in PAHs, which
have attracted considerable attention as luminescent dyes and
fluorescent probes in material and biological science.5,17
Photophysical properties of pyrene derivatives have been
extensively investigated to date. Considering the excellent
synthesis of a variety of substituted derivatives (1) via Ni-
catalysed cross-coupling reactions (Scheme 1b).19,20 The highly
planar and rigid structure, as well as the incorporation of the
pyridinic nitrogen atoms endow
1
with intriguing
photophysical properties and high electron mobility.
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The synthetic route to diazapyrenes 1b
-
1f is shown in those of 1c (Fig. S1). These dihedral angles in 1c and 1f indicate
-conjugation.
The packing structure of 1g is different from those of 1c
afforded 1,3,6,8- and 1f (Fig. 1d). While -stacking between the diazapyrene
Scheme 2. Treatment of naphthalene diimide (2a) with 16 the effective expansion of
equiv of zinc powder and 16 equiv of pivalic anhydride (Piv2O)
in 1,4-dioxane at 120 °C for 24
π
h
π
tetrapivaloxy-2,7-diazapyrene (1b) in 85% yield, which was cores was not observed for molecules of 1c and 1f, those of 1g
then successfully transformed into tetraaryl-2,7-diazapyrenes are stacked in a face-to-face manner with an interplanar
1c-f via Ni-catalysed cross-coupling reactions with arylboronic distance of 3.406 Å.
acids.20 The reaction of 1b with 4-tert-butylphenylboronic acid
in the presence of Ni(cod)2 (cod: 1,5-cyclooctadiene),
tricyclohexylphosphine (PCy3), and K3PO4 in toluene at 60 °C
for 18 h afforded the corresponding diazapyrene (1c) in 59%
yield. A variety of arylboronic acids were applicable to this
reaction system, and 1d–1f were obtained in moderate yields.
Unfortunately, attempts to use alkylboronic acids for the
introduction of alkyl groups were not successful. However, we
were able to prepare 1g in 77% yield from the desulfurization
of the thiophene rings of 1f using Raney nickel (Scheme 3).
Diazapyrenes 1b-g are readily soluble in common organic
solvents such as CH2Cl2, CHCl3 and toluene, while 1g is soluble
even in hexane and cyclohexane. The present synthetic route
is simple, but should be useful and promising to solve the
difficulties in the preparation of various functionalized 2,7-
diazapyrenes and tuning their prosperities (vide infra).
Fig. 1 Molecular structures of 1c and 1g. (a) Top and (b) side views of 1c. (c)
Molecular structure and (d) packing structure of 1g. Hydrogen atoms and
tert-butyl groups are omitted for clarity. Thermal ellipsoids are set to 50%
probability.
Fig. 2a shows the electronic absorption spectra of 1c, 1f
and 1g in CH2Cl2. In order to clarify the features of the
electronic spectra, time-dependent density functional theory
(TDDFT) calculations were carried out at the B3LYP/6-
31G+(d,p) level of theory on 1a (as a model for 1g) and 1c. The
simulated absorption spectrum of 1a was consistent with the
experimental one of 1g (Fig. S2). According to the TDDFT
calculations, the lowest energy band of 1g consists of two
similar transitions, which were assigned to a combination of
Scheme
2 Synthesis of 1 via the reductive aromatization of
naphthalene diimide (2a). a) Zn (16 equiv), Piv2O (16 equiv), 1,4-
dioxane, 120 °C, 24 h; b) ArB(OH)2 (16 equiv), Ni(cod)2 (40 mol%), PCy3
(80 mol%), K3PO4 (16 equiv), toluene, 60 °C, 18 h
the HOMO–4
to combination of the HOMO–4
HOMO LUMO transitions. On the other hand, the lowest
→
LUMO and HOMO
→
LUMO+1 transitions, and
a
→
LUMO+1 and
→
energy band of 1c was assigned to the HOMO→LUMO
transition.
Scheme 3 Desulfurization of 1f into 1g
.
In CH2Cl2, diazapyrenes 1c–1g exhibit fluorescence (Φ =
0.13~0.47) spectra, which are the mirror images of the
Recrystallization of 1c (toluene/acetonitrile) and 1g
(acetonitrile) afforded single crystals suitable for X-ray
diffraction analyses. The mean plane deviation of the
diazapyrene core in 1c is 0.079 Å, reflecting the highly planar
conformation of the diazapyrene core (Fig. 1a). The dihedral
angles (~32–33°) between the pyridine moieties and the 4-tert-
buthylphenyl groups in 1c are smaller than those in 1,3,6,8-
tetra(4-tert-buthylphenyl)pyrene (51–55°).21 This result clearly
shows that the replacement of the C–H moieties at the 2 and 7
positions in pyrene with nitrogen atoms reduces the steric
repulsion between the central core and the peripheral aryl
groups. The dihedral angles (~15-18°) in 1f are smaller than
corresponding absorption spectra (Fig. 2b and S10). The
introduction of peripheral aryl groups also exhibits
bathochromic shift of emission bands: The colours of emission
were changed dramatically based on effective -conjugation of
the aryl groups (Fig. 3b, 3c and S10). Furthermore, solid
samples of 1c 1g emit fluorescence ( = 0.01~0.41) (Fig. S10).
a
π
–
Φ
Interestingly, a new emission band at ~500 nm was observed
with increasing concentration of 1g (Fig. 3a, 3d-e). The lifetime
measured in dilute solution (1.0 × 10–6 M) at 425 nm was 7.6
ns, while it was 19.4 ns when measured at 500 nm in a
2 | J. Name., 2012, 00, 1-3
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concentrated solution (1.0 × 10–2 M). Based on these results, supporting the notion that the LUMO level of
1
is lower than
the new emission at ~500 nm was assigned to the excimer that in
3
.
(b)
(a)
emission.
60000
2.5 eq
0 eq
(b)
(a)
40000
30000
50000
1132 eq
2.5 eq
40000
30000
20000
10000
20000
10000
0
ε
0
0
300
0
300
300
400
500
600
250 300
400
500
600
400
500
400
λ / nm
λ / nm
λ / nm
500
R
λ / nm
Fig. 2 (a) Absorption spectra of 1c (red), 1f (black) and 1g (blue) in CH2Cl2. (b)
Absorption (black solid line: in CH2Cl2; red solid line: in 2,2,2-
trifluoroethanol) and fluorescence (black dashed line: in CH2Cl2; red dashed
line: in 2,2,2-trifluoroethanol) spectra (λex = 360 nm) of 1g.
H
H
N
H
(OTs)2
R
R
N
N
R
R
N
R
R
R
R
R
N
(c)
H+
H+
(c)
(b)
concentration of 1g (M)
1.0 × 10-2
(a)
R
R
R
N
R
N
R
N
H
R
0.8 × 10-2
H
B
0.6 × 10-2
0.4 × 10-2
0.2 × 10-2
1.0 × 10-3
1.0 × 10-4
1.0 × 10-5
1.0 × 10-6
1g (R = C5H11
)
A
4 (R = C5H11)
Fig. 4 Spectral changes in the absorption spectra of 1g in CH2Cl2 (2.7 × 10–5
M) upon addition of TFA (a) (0–2.5 equiv) and (b) (2.5–1132 equiv). (c)
Stepwise protonation of 1g.
(d)
(e)
The low LUMO level motivated us to evaluate the electron
mobility of 1g in the hope to find a new n-type semiconducting
material. In order to minimize the effects of contact resistance
on the electrodes and focus on its intrinsic properties, 1g was
analysed using field-induced time-resolved microwave
conductivity (FI-TRMC) measurements.22 Using FI-TRMC, the
intrinsic carrier mobility at semiconductor-insulator interfaces
was evaluated by monitoring microwaves reflected by a simple
metal-insulator-semiconductor (MIS) device that contains the
target semiconductor materials. Here, a hexane solution of 1g
was spin-coated onto poly(methyl methacrylate)/SiO2 layers to
form a thin film, and gold electrodes were deposited to inject
charge carriers. A powder X-ray diffraction analysis revealed
that 1g exhibits a lamellar periodicity of 14.4 Å (Fig. S14),
suggesting that the 2,7-diazapyrene cores form 2D π-electron
layers parallel to the substrate. Upon applying a gate bias to
the MIS devices, current flows were observed (Fig. S15),
indicating that the injection of electrons into the 1g layer and
the reflected microwave power changed simultaneously (Fig.
S16). The number of charge carriers (Ne) was estimated from
the integration of the flow current, while the
pseudoconductivity (Neμe) was calculated from the reflected
microwave power.22a The Ne and Neμe values were plotted as a
function of the gate bias, and the slope of the resulting plots
describes the electron mobility (Fig. 5a). For 1g, an electron
mobility of μe = 0.6 cm2 V–1 s–1 was estimated. In sharp
0
350 400
500
λ / nm
600
700
Fig. 3 (a) Fluorescence spectra of 1g in methylcyclohexane at various
concentrations (normalized at 425 nm). (b) Photo of cuvette containing 1c in
CH2Cl2 (4.9 × 10–5 M) under illumination (λex = 365 nm). (c) Photo of cuvette
containing 1f in CH2Cl2 (6.9 × 10–5 M) under illumination (λex = 365 nm).
Photos of cuvettes containing 1g in methylcyclohexane under illumination
(λex = 365 nm) at (d) 1.0 × 10–6 M and (e) 1.0 × 10–2 M.
We also discovered that the use of protic solvents induces
significant changes in the photophysical properties of 1g
.
When we measured the absorption spectrum of 1g in 2,2,2-
trifluoroethanol, a new absorption band was observed at ~420
nm (Fig. 3a). In addition, the emission spectrum was
bathochromically shifted. In sharp contrast, such a change was
not observed for 1,3,6,8-tetrapentylpyrene (3). In order to
investigate the potential of 1g to serve as an acid-responsive
material, protonation experiments of 1g in CH2Cl2 were carried
out (Fig. 4). The addition of trifluoroacetic acid (TFA) to a
CH2Cl2 solution of 1g resulted in the appearance of a new
absorption band at 427 nm (Fig. 4a). The observation of
isosbestic points supports the direct transformation of 1g into
the monoprotonated species
A (Fig. 4c). After 2.5 equiv of TFA
had been added, the absorption band bathochromically shifted
to 444 nm (Fig. 4b). For the sake of comparison, we prepared
the diprotonated compound
4 (OTs = p-toluenesulfonate),
which exhibited a similar absorption band at 441 nm; this
result confirms that the absorption band at 444 nm in Fig. 4b
contrast, pyrene
3 showed little microwave response for both
holes and electrons, i.e., the mobilities were below the
instrument detection limit (< 0.1 cm2 V–1 s–1) (Fig. 5b and S17).
This clear contrast suggests that nitrogen-doping PAHs
dramatically alters their electronic properties, inducing e.g.
electron mobility in the solid state. It should be noted that the
originates from the diprotonated species
B. Similar changes
were also observed in the fluorescence spectra of 1g upon
protonation (Fig. S13).
The electrochemical properties of 1g and
3 were examined
by cyclic voltammetry (Fig. S6–S9). Diazapyrene 1g exhibits one
reversible reduction potential (–2.56 V; vs [Fc]/[Fc]+), while a
devices were prepared by spin-coating from
a non-
halogenated solvent, the use of which has not been
reduction peak was not observed for pyrene
3 up to –3.0 V,
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In summary, we have developed a novel protocol for the
synthesis of various 2,7-diazapyrenes using a combination of a
reductive aromatization of naphthalene diimide and Ni-
catalysed cross-coupling reactions. We have also discovered
that 1g exhibits an excimer emission in concentrated solution
as well as acid-responsive changes in the absorption and
, 7269.
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confirmed that the electronegativity of the nitrogen atoms
lowers the LUMO level. Furthermore, FI-TRMC measurements
clearly demonstrated
compared to pyrene, suggesting that diazapyrenes
become promising n-type organic semiconductors.23 The
present method is potentially applicable for the synthesis of
a
higher electron mobility for 1g
1
may
7
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π
-
extended pyridine-doped PAHs by the reductive aromatization
of various arylene diimides. Consequently, this protocol will
open up a new field of larger nitrogen-doped PAHs based on
precise control of structure and electronic nature. Further
investigations into the synthesis of new pyridine-annulated
PAHs using reductive aromatization are currently in progress in
our group.
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This work was supported by the Grants-in-Aid for Scientific
Research on Innovative Areas “pi-System Figuration (No.
2601)” (JSPS KAKENHI grants 26102003 and 261020110) and
“Precisely Designed Catalysts with Customized Scaffolding (No.
2702)” (JSPS KAKENHI grant 16H01013) from MEXT (Japan).
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Conflicts of interest
There are no conflicts to declare.
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R
R
N
N
R
R
H
N
O
O
O
O
Reductive
Aromatization
N
H
• Easy access to nitrogen-fused PAHs
• Tuning of packing structures
• Acid-responsive emission
• n-Type conductivity