A boron-containing PAH as a substructure of boron-doped graphene

Article abstract of DOI:10.1002/anie.201206699

BBC news: Two boron atoms have been incorporated into a carbon nanosheet by a bottom-up synthesis to give a B-doped nanographene (see picture) with a defined structure. The experimental and theoretical analyses revealed the important role of the two boron atoms in producing its characteristic properties, such as broad absorption bands covering the visible region and reversible redox processes. Copyright

Full text of DOI:10.1002/anie.201206699

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DOI: 10.1002/anie.201206699
Graphene
A Boron-Containing PAH as a Substructure of Boron-Doped
Graphene**
Chuandong Dou, Shohei Saito,* Kyohei Matsuo, Ichiro Hisaki, and Shigehiro Yamaguchi*
Graphene, a two-dimensional (2D) honeycomb lattice of sp2-
hybridized carbon atoms, is a most attractive material because
of its extraordinary thermal, mechanical, and electrical
properties. It holds enormous potential for widespread
applications, such as nanoscale electronic devices, composite
materials, sensors, solar cells, hydrogen storage, and electro-
des for Li-ion batteries.[1] However, its application to nano-
scale transistors is impeded by its intrinsic zero-band-gap
character.[2] Intensive studies have recently been conducted to
address this issue by opening and tuning the band gap, which
endows the graphene with semiconducting properties. One
approach is the synthesis of graphene nanoribbons with
defined widths and edges, which enables its chemical and
electronic properties to be tuned.[3] The other promising
strategy is chemical doping with B or N atoms.[4] In particular,
B-doped graphene has currently attracted increasing atten-
tion as a high-performance field-effect transistor (FET)
material as well as an anode material for high-power Li-ion
batteries.[4a,b]
Despite the intensive theoretical and experimental studies
on the chemistry of B-doped graphene, the synthesis of well-
defined B-doped graphenes remains a challenge. The prep-
aration methods reported to date, such as chemical vapor
deposition, solution-phase reductive coupling, and thermal
annealing, only produce a mixture of B-doped graphenes with
various molecular weights, edge shapes, and doping posi-
tions.[5] In this context, the bottom-up organic synthesis
should have a significant advantage in producing a fragment
of B-doped graphene as a single compound and enable the
precise elucidation of its molecular properties, as is the case of
undoped nanographene flakes.[6] However, the synthesis of
“B-doped nanographene”[7] has been hampered by the
intrinsic instability of tricoordinated organoboranes to
oxygen and moisture. To overcome this issue, N atoms are
often concomitantly introduced into polycyclic skeletons in
the form of B-N units.[8] The interaction between the lone pair
of electrons on the N atom and the empty p orbital of the B
atom enhances the stability, but, at the same time, degrades
the inherent characteristics of the B atom, such as Lewis
acidity and electron-accepting properties. We recently pro-
posed a “structural constraint” as an alternative strategy for
the stabilization. We synthesized a planar triphenylborane
constrained with methylene tethers and demonstrated its
remarkable stability to oxygen and water, despite the absence
of steric protection on the B atom.[9] However, the p conju-
gation is not fully expanded over the entire molecule because
of the sp3 carbon tethers. A breakthrough to compensate for
this shortcoming was made when a triarylborane was
produced in which three aryl groups are directly fused with
one another.[10] The highly planar and conjugated skeleton
indeed enabled it to form the face-to-face p stacking in the
crystalline state and demonstrate intriguing photophysical
properties, although its structure is still far from the nano-
graphene type. Herein, we disclose the first bottom-up
organic synthesis of a B-doped nanographene, which consists
of all sp2-hybridized carbon hexagons with a well-defined B-
embedded structure in terms of the number of B atoms
incorporated as well as their positions. The impact of the
boron doping on the electronic structure of the extended
polycyclic aromatic hydrocarbon (PAH) has been precisely
elucidated.
We have now designed a B-containing PAH 1 as a sub-
structure of B-doped graphene. Its parent honeycomb lattice
2 composed of 15 hexagons should be intriguing due to its
singlet biradical character similar to teranthene (see the
Supporting Information).[11] Since the replacement of a C
atom by a B atom corresponds to a one-electron oxidation,
namely hole doping,[12] the doping of a single B atom into the
skeleton will result in the formation of an unstable open-shell
compound (Scheme 1). Two boron atoms need to be intro-
duced at the same time to produce a stable closed-shell
structure. In compound 1, two B atoms are placed in the
central hexagon, and its closed-shell structure should produce
unique properties that are totally different from those of the
undoped congener 2.
[*] Dr. C. Dou, Dr. S. Saito, K. Matsuo, Prof. Dr. S. Yamaguchi
Department of Chemistry
Graduate School of Science
Nagoya University, and
CREST (Japan) Science and Technology Agency
Furo, Chikusa, Nagoya 464-8602 (Japan)
E-mail: s_saito@chem.nagoya-u.ac.jp
yamaguchi@chem.nagoya-u.ac.jp
Dr. I. Hisaki
Department of Material and Life Science, Graduate School of
Engineering, Osaka University (Japan)
[**] This work was supported by CREST and JST (S.Y.), by a Grant-in-Aid
for Young Scientists (B) (24750038, to S.S.), and a Grant-in-Aid for
Young Scientists (A) (24685026, to I.H.). Crystallographic data
collection was performed at the BL38B1 in the SPring-8 at the
BL38B1 with approval of JASRI (proposal No. 2012A1809). We thank
Prof. T. Kubo, A. Konishi (Osaka University), Prof. H. Shinokubo,
and Dr. S. Hiroto (Nagoya University) for their approval to use their
spectral data. PAH=polycyclic aromatic hydrocarbon.
Compound 1a was synthesized by the oxidative cyclo-
dehydrogenation (intramolecular Scholl reaction) of a 6,13-
dihydro-6,13-diborapentacene precursor 3 bearing an orthog-
onal anthryl group on each boron atom (Scheme 2). Bulky
mesityloxy substituents are introduced at the 4,5-positions of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206699.
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pressure chemical ionization time-of-flight (APCI-TOF) MS
showed a parent ion signal for 1a at m/z 1157.4942 (calcd for
C84H63O4B2 [M+H]+, m/z 1157.4931; see the Supporting
Information). Two sets of coupled signals (Ha and Hb, and
Hc and Hd in Figure 1) were observed at 6.96 and 9.12 ppm,
and 7.76 and 9.02 ppm, respectively in the 1H NMR spectrum
Scheme 1. Stepwise boron doping of an extended polyaromatic hydro-
carbon.
Figure 1. 1H NMR spectrum of 1a in [D2]tetrachloroethane at 353 K.
of 1a in [D2]tetrachloroethane at 353 K (Figure 1). The
relatively downfield chemical shifts of the Hb and Hd signals
are attributed to the deshielding effect by the ring current of
the neighboring benzene rings in the cove region. The other
deshielded singlet signal at 10.85 ppm corresponds to the He
atom, which reflects the close contact with the oxygen atoms
(see the Supporting Information).[13] Variable-temperature
1H NMR measurements from 193 to 353 K did not show any
significant change. This temperature independency indicates
a large energy gap between the singlet closed-shell ground
state and a triplet excited state. The gap was calculated
theoretically to be 34.9 kcalmolꢀ1 for 1a at the B3LYP/6-
31G* level, which is far larger than that of the parent undoped
nanographene 2 (1.5 kcalmolꢀ1) with an open-shell ground
state (see the Supporting Information). The broad 11B NMR
signal of 1a at 58.0 ppm is typical of tricoordinated boron
compounds.
The single crystals were obtained by slow diffusion of
heptane into a solution of 1a in chlorobenzene. The X-ray
crystallographic analysis revealed a contorted polycyclic
skeleton of the B-doped nanographene 1a composed of 48
sp2-hybridized C atoms and two tricoordinated B atoms
(Figure 2).[15] Fifteen six-membered rings are fused to form
the nanographene sheet with four cove regions and two zigzag
edges. As a consequence of steric overcrowding of the Hb and
Hd atoms in the cove regions, the p-conjugated core skeleton
is distorted away from planarity. Although the distance
between the most deviated C atoms and the C48B2 mean
plane is 1.02 ꢀ, the dihedral angles between the most
contorted benzene rings and the central C4B2 ring is 19.78.
The doping positions of the two B atoms were determined
Scheme 2. Synthesis of B-doped nanographene 1a. Reagents and
conditions: a) 4, nBuLi, Et2O, from 08C to 258C, then 5, toluene, from
08C to 258C; b) FeCl3, CH3NO2 and CH2Cl2.
the anthracene moieties for the cyclization to occur selec-
tively at the 1,8-positions and to prevent the strong aggrega-
tion of the resulting PAH p skeleton. This anthryl group was
originally reported by Anderson and coworkers, and is widely
used for the synthesis of expanded p skeletons.[13]
The precursor 3 was prepared in 54% yield by the
lithiation of 9-bromobis(mesityloxy)anthracene 4[13] with
nBuLi, followed by treatment with dibromodiborapentacene
5.[14] Compound 3 showed high stability to water and oxygen
as a result of the steric protection of the boron atoms by the
bulky anthryl groups. The cyclodehydrogenation of 3 with an
excess of FeCl3 proceeded successfully to form 1a in 51%
yield as a deep purple solid. As expected, the doubly B-doped
nanographene 1a is stable enough to handle in air and was
isolated by column chromatography on silica gel without any
special precautions. Compound 1a is sufficiently soluble in
common organic solvents, such as chlorobenzene
(4.8 mgmLꢀ1) and ortho-dichlorobenzene (10.8 mgmLꢀ1),
thus demonstrating its processability in solution.
ꢀ
unambiguously, as the B C bond lengths of 1.507(2), 1.531(2),
and 1.535(2) ꢀ are significantly longer than those of the other
ꢀ
ꢀ
The structure of 1 was unambiguously characterized by
mass spectrometry, NMR spectroscopy (Figure 1), and finally
X-ray crystallography (Figure 2). These analyses revealed 1a
to be a single compound. High-resolution atmospheric
C C bonds (1.37–1.48 ꢀ). Notably, these B C bonds are
much shorter than those of the nonfused triphenylborane
(1.57–1.59 ꢀ).[16] This structural characteristic has already
been observed in the other planarized triarylboranes pre-
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PAHs of similar sizes, such as hexabenzocoronene (42 sp2-
hybridized C atoms; labs = 361 and 392 nm),[18] the B-contain-
ing PAH 1a has a significantly red-shifted labs value (see the
Supporting Information). On the other hand, the labs value of
1a is blue-shifted when compared to those of reactive PAHs
with zigzag edges, such as extended acenes,[19] rylenes,[20] and
anthenes.[11,21] In particular, teranthene, which has a singlet
biradical character and whose spectrum should be similar to
that of the undoped congener 2, shows absorption bands in
the near-IR region (labs = 878 and 1054 nm),[11] but it gradu-
ally decomposes under ambient conditions (see the Support-
ing Information). Taking these comparisons into consider-
ation, one may conclude that “B doping” results in the
nanographenes having broad absorption bands in the long-
wavelength visible region, while maintaining their stability.
The other important feature of the B-doped nanogra-
phene is its fluorescence. The emission spectrum of com-
pound 1a shows a broad fluorescence band in the visible/near-
IR region with a maximum (lem) at 679 nm, although the
quantum yield is low (F = 0.04). Fluorescence reaching the
near-IR region is rare in undoped nanographenes. Fluores-
cences comparable to that of 1a have only been reported for
Figure 2. X-ray crystal structure of 1a (50% probability for thermal
ellipsoids).
viously synthesized by our research group.[9,10] The B atoms
take an ideal trigonal planar geometry, with the sum of the
three C-B-C angles being 360.08. p stacking of the nano-
graphene skeletons was not observed in the packing structure
because of the steric hindrance of the bulky mesityl groups.
The mesityl groups are almost perpendicular to the p frame-
work, which suppresses the strong aggregation and guaran-
tees the sufficient solubility in organic solvents.
supernaphthalene (lem = 611 nm)[22] and quaterrylene (lem
=
680 nm).[23] The emission spectra of 1a as well as its
absorption spectra showed negligible solvent effects (see the
Supporting Information).
DFT calculations were performed at the B3LYP/6-31G*
level of theory to more precisely elucidate the electronic
structure of 1 (Figure 4). The optimized structure reproduced
The photophysical properties of 1a help to understand the
impact of replacing C atoms with B atoms in extended PAHs.
A solution of 1a in toluene has a purple color (Figure 3). The
Figure 3. UV/Vis absorption (black line) and Vis/near-IR fluorescence
(red line, lex =570 nm) spectra of 1a in toluene, along with the
oscillator strengths (blue bars) obtained by TD-DFT (B3LYP/6-31G*)
calculations. Inset: photograph of 1a in toluene.
Figure 4. Kohn–Sham molecular orbitals of 1a, based on calculations
at the B3LYP/6-31G* level.
broad absorption bands in the UV/Vis absorption spectrum
recorded in toluene cover the entire visible region of 400–
700 nm, with maxima (labs) at 564 and 487 nm, and with
a shoulder band at 640 nm. In general, the absorption spectra
of polyaromatic hydrocarbons (PAHs) depend on the size and
the edge shapes.[17] The stable PAHs with armchair-type edges
(so-called fully benzenoid PAHs) have relatively shorter
labs values for their sizes. When compared to stable benzonoid
well the distorted structure of the cove regions observed in
the crystal structure (see the Supporting Information). The
calculation showed that the p orbitals of the B atoms
contribute to the HOMO-1 and LUMO to a significant
extent. The transition energies and oscillator strengths
simulated by the TD-DFT (B3LYP/6-31G*) calculations
showed a good agreement with the observed absorption
spectrum of 1a (Figure 3 and see the Supporting Informa-
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tion). The absorption bands observed at 640, 564, and 487 nm
are assignable to the electronic transitions HOMO!LUMO,
HOMO-1!LUMO + 1, and HOMO-2!LUMO, with rea-
sonable oscillator strengths of 0.103, 0.262 and 0.328, respec-
tively. It is notable that all these transitions involve either the
HOMO-1 or LUMO. Namely, the contribution of the
p orbitals of the B atoms is responsible for the intense and
broad absorption bands observed in the visible region.
Since the doubly B-doped nanographene 1a is isoelec-
tronic with the dicationic species of the undoped nano-
graphene 22+, we compared their electronic structures theo-
retically (see Figure S19 in the Supporting Information).
Although the energy levels are significantly different from
each other, because of the positive charge of 22+, the
molecular orbitals of these compounds are similarly distrib-
uted, except for the LUMO and HOMO-1 in 1a. Since these
orbitals have large orbital coefficients at the doping positions,
the replacement of the C atoms with B atoms perturbs the
energy levels of these orbitals. The replacement tends to
relatively elevatetheir energy levels. As a result, the HOMO-
1 and HOMO as well as the LUMO and LUMO + 1 in 1a are
closer in energy than those in 22+. It is also worth noting that
the molecular orbital distributions of 1a are not perturbed by
the introduction of the mesityloxy substituents, although their
energy levels are elevated by 0.3–0.5 eV (see the Supporting
Information).
In summary, we have achieved the bottom-up organic
synthesis of a stable “B-doped nanographene” as a single
closed-shell compound. In contrast to B-doped graphenes
prepared by conventional methods, this compound has a well-
defined structure, including the doping positions of the B
atoms. The crystal structure revealed it to deviate from
planarity because of steric overcrowding in the cove regions.
The most important impact of the “B doping” is the
significant contributions of the p orbitals of the B atoms to
both the relevant unoccupied and occupied orbitals, which
play important roles in the broad absorption over the entire
visible region as well as the fluorescence in the near-IR
region. The two B atoms are also crucial for gaining the
reversible multiredox properties. This study will provide
a benchmark for future structural engineering of B-doped
graphenes.[4] More detailed analyses of this B-doped nano-
graphene are currently underway, including studies of the
Lewis acidity, the carrier mobility, and its performance as
a Li-ion battery electrode.
Received: August 18, 2012
Published online: October 19, 2012
Keywords: boron · fluorescence · graphene · nanostructures ·
polycyclic aromatic hydrocarbons
.
The electrochemical properties of the B-doped nano-
graphene are important for applications in nanoscale tran-
sistors or Li-ion battery electrodes. The sufficient solubility of
1a enabled cyclic voltammetry measurements in THF
(Figure 5). Regardless of the scan rates, reversible redox
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