Synthesis, structure and properties of C3-symmetric heterosuperbenzene with three BN units

Article abstract of DOI:10.1039/c4cc10105g

The parent skeleton of BN heterocoronene with three BN units and C₃ symmetry was synthesized as a model compound of BN-doped graphene. Further investigation of this graphene-type molecule revealed the important role of BN doping in opening the

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Synthesis, Structure and Properties of C3-Symmetric
Heterosuperbenzene with Three BN Units
Cite this: DOI: 10.1039/x0xx00000x
Xiao-Ye Wang,a Fang-Dong Zhuang,a Xin-Chang Wang,b Xiao-Yu Cao,*b Jie-Yu
Wang,*a and Jian Pei*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
The parent skeleton of BN heterocoronene with three BN
units and 3 symmetry was synthesized as a model compound
C
of BN-doped graphene. Further investigation of this
graphene-type molecule revealed the important role of BN
doping in opening the bandgap and modulating the electronic
properties.
BN-doped graphene has attracted great attention recently due to
its unique electronic properties and potential applications in
atomically thin devices and integrated circuitry.1-7 However,
current state-of-art techniques could not control its structure
precisely, such as the doping position and orientation of BN
units, which are essential to determining its properties. In
addition, the hitherto available BN-doped graphenes all exhibit
separate phases of hexagonal boron nitride (h-BN) and
graphene domains, in which the h-BN domain mainly behaves
as insulator (Fig. 1a, Type A).7 In this context, the bottom-up
synthetic approach toward BN-doped graphenes should have
the advantage of controlling the structure precisely and provide
the possibility of constructing the new type of BN-doped
graphene with individual BN units dispersed in graphene
framework (Fig. 1b, Type B), which might exhibit continuous
two-dimensional (2D) charge transport with a desired bandgap.
The rapid advancement of azaborine chemistry8-11 in recent
years has enriched the chemistry of aromatics and provided a
number of interesting monocyclic and polycyclic BN-
containing compounds for wide applications in biological
systems,12 coordination chemistry,13 hydrogen storage,14 and
electronic devices.15-18 Construction of BN-substituted
polycyclic aromatic compounds is not only interesting for their
intriguing optical and electronic properties,19-25 but also
valuable for the long-term goal of bottom-up synthesis of
structurally well-defined BN-doped graphenes. It is possible to
produce such precise structures from suitable precursors in
Fig. 1 Schematic representations of (a, b) two types of BN-doped graphenes, (c)
the previously reported π-extended BN heterocoronene, and (d) the C3-symmetric
BN heterocoronene presented in this work. (red: boron; blue: nitrogen).
Coronene, also known as “superbenzene”, is regarded as the
smallest graphene-type molecule with a zig-zag periphery. We
have previously developed a 2D π-extended BN heterocoronene
skeleton with six fused aromatic rings (Fig. 1c).36 However, the
steric hindrance among the peripheral fused rings resulted in a
curved structure, which made it less resemble the graphene
structure. Moreover, only two BN units were introduced into
the skeleton, endowing the molecule with C2 symmetry. To
increase the doping concentration, which is essential to
modulating the electronic properties of BN-doped graphenes,
we design a new BN heterocoronene skeleton containing three
regioregular BN units and without extra fused rings to provide a
planar parent BN heterocoronene framework (Fig. 1d). This
structure exhibits C3 symmetry, which is more favourable for
being extended to 2D networks than C2-symmetric ones.37-39
combination with the on-surface synthesis or chemical vapour
26-28
deposition method.8,
In addition, it is also of great
importance to construct substructures of BN-doped graphene
and learn message from the model compounds by clear
elucidation of their structures and properties.29-35
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Herein, we present the synthesis, structure, and properties of
this new parent skeleton of BN heterocoronene.
Single crystals suitable for X-ray analysis were obtained by
phase transfer of hexane into a solution of compound in
1
CH2Cl2. As shown in Fig. 3, the BN heterocoronene backbone
exhibits a fully planar geometry. The three bonds connected
with the boron atom (two B—C bonds and one B−N bond) are
also in the same plane, with the sum of the three bond angles of
360°. The adjacent methoxy groups are arranged alternately
above and below the conjugated plane. The peripheral phenyl
rings showed different twisted angles of about 44 ~ 75°. The B
−
N bond lengths are in the range of 1.41 ~ 1.42 Å, indicating a
localized BN double bond character.43 This observation is in
consistence with nucleus-independent chemical shift (NICS)
calculation results (see ESI), which revealed a moderate
aromaticity of the BNC4 ring with a NICS(1) value of –3.7 ppm.
As depicted in Fig. 3c, the molecules are packed into layered
structures, which are arranged in a herringbone fashion. The
distance between the π-conjugated planes is around 4.3 Å, and
the overlap is relatively small due to the large dislocation
between different π-skeletons. These packing characteristics are
attributed to the bulky substituents around the central BN
heterocoronene core.
Scheme 1 Synthetic route to BN heterocoronene 1.
The synthetic route to compound
1 is illustrated in Scheme
1. Hexamethoxytriphenylene
trimerization of o-dimethoxybenzene in the presence of FeCl3.
Triamine was accessed according to a procedure reported by
Wei et al.40 through nitration and further reduction reactions on
. The key step is the threefold aromatic electrophilic
substitution. Compound was simply heated with
dichlorophenylborane and triethylamine in o-dichlorobenzene
(o-DCB) to give the BN heterocoronene in moderate yield.
2 was synthesized through a
3
2
3
1
Additional Lewis acid (such as AlCl3) was not necessary in this
step due to the high electron density on the benzene rings.
Compound
1 was stable enough to be isolated by column
chromatography on silica gel and recrystallization from
CHCl3/MeOH to give a white solid. The excellent thermal
stability was demonstrated by thermogravimetric analysis
(TGA), which revealed a high decomposition temperature (5%
weight loss) of 341 °C (Fig. S1).
Fig. 3 Single crystal structure of BN heterocoronene
1. (a) Top view, (b) side
view, and (c) packing mode. Thermal ellipsoids were shown at 50% probability.
Hydrogen atoms were omitted for clarity. Methoxy groups were also omitted in
(b), (c) for clarity. Gray: carbon; red: boron; blue: nitrogen; purple: oxygen.
The absorption and emission spectra of compound
recorded in CH2Cl2 solution. As depicted in Fig. 4, compound
1 were
Fig. 2 1H NMR spectrum of compound
1 in CDCl3. (*residual solvent)
1
showed an absorption maximum at 321 nm, which is blue-
shifted compared to that of the similar carbon analogue (345
nm).42 Several weak absorption bands extend to 404 nm,
showing an optical bandgap of 3.08 eV estimated from the low-
energy absorption onset. The absorption spectrum of compound
The chemical structure of compound
1 was unambiguously
verified by NMR spectroscopy, high-resolution MS, and finally
single-crystal X-ray analysis. As shown in Fig. 2, the protons
on nitrogen atoms showed a sharp singlet signal at 9.02 ppm,
which is significantly different from the broad signal at 4.58
1
in thin film showed a slight red-shifted absorption maximum
ppm of the amino groups in compound 3 41
, but almost identical
peaked at 325 nm, indicating weak intermolecular electronic
couplings in the solid state (Fig. S2). The emission spectra
showed a well-resolved fine structure peaking at 403, 424, and
448 nm, which are also significantly blue-shifted relative to the
carbon analogue (470, 498, 534 nm).42 These characteristics
indicate that the BN heterocoronene possesses a larger bandgap
than the carbon skeleton. The fluorescence quantum yield was
determined to be 0.21 with the integrating sphere, which is
much higher than similar carbon analogues (less than 0.1).42
Cyclic voltammetry (CV) measurement in CH2Cl2 revealed that
to the signal (9.01 ~ 9.03 ppm) of protons at the same position
in its carbon analogue.42 This observation indicates that the
protons on nitrogen atoms in compound
protons, rather than amine-type protons. In addition, two singlet
signals were observed at 4.16 and 3.60 ppm, indicating that
1 behave as aromatic
there are two different kinds of methyl groups in compound 1,
which reveals its C3-symmetric character. In 11B NMR, only a
broad resonance at 37.2 ppm was observed, further
demonstrating the C3-symmetric structure of compound 1.
the HOMO level of compound
1 is about −5.40 eV. Thin film
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of
1 exhibited a work function of −5.65 eV determined by continuity of the whole 2D electronic system. That means this
photoelectron spectroscopy (PES) measurement.
new type of BN-doped graphene could be endowed with
semiconducting properties without scarifying its 2D charge
transport character.
In summary, we have synthesized the parent skeleton of
BN-substituted coronene containing three BN units with
defined positions and orientations. The single crystal structure,
optical and electronic properties were fully investigated. The
BN heterocoronene showed an enlarged bandgap of over 0.3 eV
compared to its carbon analogue. Message from this graphene-
type molecule reveals that individual BN doping is efficient for
opening a bandgap of graphene, while at the same time
maintains the character of 2D π-electron continuity. This
strategy presented here can also be applied to the bottom-up
peripheral decoration of (nano)graphenes to modulate their
properties.45 In addition, the C3-symmetry of this BN
heterocoronene skeleton makes it a potential candidate for 2D
extension to achieve the bottom-up fabrication of atomically
precise BN-doped graphene. Further modification of this
skeleton is ongoing in our lab to pursue suitable precursors for
this approach.
Fig. 4 Absorption and emission spectra of compound
1
in CH2Cl2 (1 × 10–5 M).
Acknowledgements
This work was supported by the Major State Basic Research
Development Program (No. 2013CB933501) from the Ministry
of Science and Technology, National Natural Science
Foundation of China (51273003, 21190031, 21302009), Beijing
Natural Science Foundation (2144049), and Doctoral Program
Foundation from Ministry of Education of China
(20130001120018).
Notes and references
a Beijing National Laboratory for Molecular Sciences, Key Laboratory of
Bioorganic Chemistry and Molecular Engineering of Ministry of
Education, Center for Soft Matter Science and Engineering, College of
Chemistry and Molecular Engineering, Peking University, Beijing
100871, P. R. China. Email: jieyuwang@pku.edu.cn; jianpei@pku.edu.cn
Fig.
heterocoronene
B3LYP/6-311G(d,p) level of theory.
5
DFT-calculated frontier molecular orbitals and energy levels of BN
(right) and its corresponding carbon analogue (left) at the
1
b
College of Chemistry and Chemical Engineering, Xiamen University,
Xiamen 361005, P. R. China. Email: xcao@xmu.edu.cn
To further understand the electronic properties and the
effect of BN doping in this graphene-type molecule, we Electronic Supplementary Information (ESI) available: Experimental
performed density functional theory (DFT) calculations on BN
heterocoronene and its carbon analogue at the B3LYP/6-
311G(d,p) level (Fig. 5). The optimized structure of is in good
agreement with the single crystal data, showing an essentially
planar backbone. Both compound and its carbon analogue
displayed degenerate HOMO and LUMO energy levels (see
ESI). The HOMO and LUMO orbitals of both and its carbon
details, synthesis, characterization, single crystal data, computational
studies, and NMR spectra. CCDC 1039514. See DOI: 10.1039/c000000x/
1
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