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Cite this: Chem. Commun., 2015,
51, 6932
Received 24th January 2015,
Accepted 12th March 2015

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Tetrabenzocircumpyrene: a nanographene
fragment with an embedded peripentacene core†
Ruth Dorel,a Carlos Manzano,b Maricarmen Grisolia,c We-Hyo Soe,b
Christian Joachimc and Antonio M. Echavarren*ad

DOI: 10.1039/c5cc00693g
www.rsc.org/chemcomm

A new disc-shaped highly symmetric C54H20 nanographene fragment,
tetrabenzocircumpyrene, has been synthesized and characterized by
scanning tunnelling microscopy, demonstrating the potential of this
technique for identifying highly insoluble graphenic molecules.

Large polycyclic aromatic hydrocarbons (PAHs) have recently
attracted much attention due to their unique electronic and
self-assembly properties, which provide a wide range of opportunities for the fabrication of novel devices.1 One of the
challenges in this area is the development of efficient synthetic
methodologies for the preparation of well-defined sections of
graphene (nanographenes). In this regard, novel organic protocols
have been developed, although they are always limited by the low
solubility of planar aromatic molecules and the number of side
reactions that may take place, which increase with the molecular
weight.2 Disc-shaped molecules are of particular interest since
they are able to self-organize into columnar phases and therefore
can be used as conducting layers in organic molecular electronics.3
One of the most extensively studied discotic PAH is hexa-peribenzocoronene (HBC), also known as ‘superbenzene’, owing to
its intrinsic charge-carrier ability.4 It is predictable that enlarging
the aromatic framework would enhance the columnar order due to
more intense p–p interactions and, consequently, charge mobility.
The stacking of PAHs as prototypes for graphene multilayers has
been theoretically studied, finding that the binding energies
strongly depend on the size of the PAHs.5 On the other hand,
there is growing interest in acene-based molecules such as

periacenes, which are conjugated systems with two rows of peri-fused
acenes.6 These compounds might serve as biradical materials, and
are believed to be suitable materials for spintronic, electronic,
optical, and magnetic devices. Small periacenes have been known
for a long time, and recently larger analogues such as teranthene and
quarteranthene have been described.7 However, the synthesis of
large periacenes such as peritetracene or peripentacene has not been
achieved yet,8 although peripentacene has been detected by
sublimation of pentacene by mass spectrometry.9
Motivated by its promising properties, we targeted tetrabenzocircumpyrene 1a, a yet unreported highly symmetric disc-shaped
nanographene fragment containing 54 carbon atoms. Polyarene 1a
includes a peripentacene core and can be considered as a ‘expanded
HBC’ (Fig. 1). According to that expected,10,11 DFT calculations
predicted a ca. 30% decrease in the HOMO–LUMO gap in going
from HBC (3.59 eV) to tetrabenzocircumpyrene 1a (2.45 eV).
Our retrosynthetic approach for the synthesis of 1a considers
the formation of the final nanographene fragment by means of

a

¨sos Catalans 16, 43007
Institute of Chemical Research of Catalonia (ICIQ), Av. Paı
Tarragona, Spain. E-mail: aechavarren@iciq.es
b
IMRE, A*STAR (Agency for Science, Technology and Research), 3 Research Link,
117602, Singapore
c
GNS & MANA Satellite, CEMES-CNRS, 29 rue J. Marvig, 31055 Toulouse Cedex,
France
d
´mica Analıtica i Quı
´
´mica Organica, Universitat Rovira i
`
Departament de Quı
Virgili, C/Marcel li Domingo s/n, 43007 Tarragona, Spain
† Electronic supplementary information (ESI) available. CCDC 1035605–1035610.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5cc00693g

6932 | Chem. Commun., 2015, 51, 6932--6935

Fig. 1 Structures of HBC, peripentacene, circumpyrene and tetrabenzocircumpyrene 1a.

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Scheme 1

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Retrosynthetic analysis of tetrabenzocircumpyrene (1a).

Scholl oxidation from 9,20-di(naphthalen-1-yl)tetrabenzo[a,c,j,l]tetracene (2a) in which six C–C bonds would be formed in a
single step (Scheme 1). Polyarene 2a could be obtained by double
nucleophilic addition of the naphthyl moieties to quinone 3,
which could result from Diels–Alder cycloaddition between quinone
4 and o-quinodimethane 5. Quinone 4, the starting point of our
synthesis, has been prepared by oxidation of triphenylene with
excess H2O2, albeit in low yield.12 However, when we tried to scale
up this reaction, the yield dropped significantly to 1–6%. After some
optimization, we found that by using CrO3 as the oxidant in the
presence of 18-crown-6,13 multigram amounts of quinone 4 could
routinely be obtained in up to 41% yield.
We chose 6 as a precursor of diene 5, which was generated
in situ14 and subjected to Diels–Alder cycloaddition with 4 to
form a non-isolable adduct which spontaneously underwent
oxidation to give a mixture of quinones 3 and 3 0 in 70% yield
(Scheme 2). Treatment of this mixture with DDQ did not result
in the complete oxidation of 3 0 to 3, presumably due to the low
solubility of these compounds in standard organic solvents. For
this reason we decided to continue the synthesis using a
mixture of both quinones. Double nucleophilic addition of
1-naphthyllithium to 3/3 0 afforded the crude mixture of diols
7syn, 7anti, 7 0 syn and 7 0 anti, which was directly reduced by
treatment with HI in acetic acid. Without isolation, this mixture
was treated with DDQ in toluene to give key intermediate 2a
as a 5 : 1 mixture of anti : syn rotational isomers (Fig. 2).15 Final
Scholl cyclodehydrogenation of 2a in the presence of excess
of FeCl3 gave the target nanographene fragment 1a in 96%
yield. The synthesis of tetrabenzocircumpyrene (1a) proceeds
in just six steps, requires only two chromatographic purifications, and every step can be conducted on a gram scale
(1.02 g of 1a were prepared in a single round from 6 g of
triphenylene).
The extreme insolubility of this compound precluded its
characterization by NMR spectroscopy. However, we were able
to obtain a clean MALDI+ mass spectrum in which we could
only identify one peak corresponding to the mass and isotopic
pattern of 1a. Furthermore, we attempted the characterization
of 1a by means of scanning tunnelling microscopy (STM). STM

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Scheme 2

Synthesis of 1a.

Fig. 2 ORTEP plot (50% thermal ellipsoids) of the crystal structure of 2a-anti
(left) and 2a-syn (right).

and differential conductance (dI/dV) imaging in conjunction
with tunnelling spectroscopy were used respectively to obtain
topographic images of 1a molecules deposited on an Au(111)
surface and to characterize their molecular electronic states.
dI/dV mapping of single 1a molecules at resonances captured
in the spectra allowed us to imagine the spatial distribution of
molecular electronic states. Electron Scattering Quantum
Chemical (ESQC) calculations including all the elements of
the experimental method, i.e. the substrate, 1a molecule and
STM tip, were done in order to assign the experimental dI/dV
images to the corresponding molecular orbitals.
The topographic, dI/dV and computed images are shown in Fig. 3.
The experimental dI/dV images were taken at voltages corresponding
to two resonances captured in the dI/dV spectra of a single 1a
molecule, namely at À0.75 eV and 1.65 eV (see ESI†). The maps
computed at the energies corresponding to HOMO and LUMO
orbitals showed identical features to those from the experimental

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Fig. 3 (left) Topographic STM images of tetrabenzocircumpyrene (1a)
taken at À0.75 V and 1.65 V, the bias voltages corresponding to the
resonances at positive and negative voltage nearest to the substrate’s
Fermi level. (center) dI/dV maps taken concurrently at the same voltages
showing the spatial distribution of the molecule’s electronic density
(images size: 3.2 Â 2.8 nm). (right) ESQC computed images calculated at
energies corresponding to HOMO and LUMO of 1a.

dI/dV images, clearly confirming the structure assigned to tetrabenzocircumpyrene 1a. The experimental 2.40 eV HOMO–LUMO gap
determined on 1a deposited on Au(111) is fully consistent with the
calculated value of 2.45 eV in the gas phase (DFT, B3LYP/6-31G(d)).
We also considered the extension of this synthetic route
to prepare tetrabenzocircumpyrene derivatives by using substituted 1-bromonaphthalenes as the starting materials for the
double nucleophilic addition to the mixture of 3 and 3 0
(Scheme 3). Thus, we prepared dialkyltetrabenzocircumpyrenes
1b and 1c, as well as the higher analogue hexabenzocircumpyrene
1d. The structure of the direct precursors of nanographene fragments
2b–d could be confirmed by X-ray diffraction (Fig. 4).15

Scheme 3 Synthesis of tetrabenzocircumpyrene derivatives. (a) ArBr,
n-BuLi, Et2O/toluene, rt, 2 h (2b, 32%; 2c, 26%; 2d, 38%); (b) HI, AcOH,
100 1C, 16 h; (c) DDQ, toluene, 70 1C, 1 h; and (d) FeCl3, CH3NO2/CH2Cl2,
rt, 16 h (1b, 85%; 1c, 54%; 1d, 64%).

6934 | Chem. Commun., 2015, 51, 6932--6935

Fig. 4 ORTEP plot (50% thermal ellipsoids) of the crystal structures of 2b
(top), 2c (center) and 2d (bottom).

We observed that when we treated alkyl-substituted precursors
of nanographene fragments with FeCl3 two products were formed.
We decided to use 2b as a model substrate to examine this
transformation in detail (Scheme 4). The major product was the
expected dialkyltetrabenzocircumpyrene 1b, which could be easily
separated by filtration. After meticulous purification, we were able
to isolate from the filtrate a brown solid slightly more soluble in
organic solvents at temperatures over 100 1C. This solid could be
crystallized and the structure was confirmed to be 8 by X-ray
diffraction (Fig. 5).15 This partially fused polyarene shows a
contorted aromatic surface, which might also be of interest in
material science.16 To prove if partially cyclized 8 was an intermediate
in the formation of 1b, we resubjected 8 to the oxidative cyclization
conditions. However, after 24 h only unchanged 8 could be detected
by mass spectrometry, demonstrating that this polyarene is not an
intermediate in the formation of 1b, but rather a product resulting
from a competitive pathway. This result further demonstrates that
not all the conceivable pathways in Scholl cyclodehydrogenation
processes are equally productive and underscores that the factors
controlling the successive formation of C–C bonds under the
conditions of the Scholl reaction are still not well understood.17
In conclusion, tetrabenzocircumpyrene 1a, a new discshaped nanographene cutout containing 54 carbon atoms,
has been efficiently prepared in a simple and practical manner
(6 steps from commercially available starting materials), and
fully characterized by means of STM. The potential of the route
to obtain other related nanographene fragments has also

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2

3

Scheme 4

Scholl cyclodehydrogenation of 2b.

4

5
6
7

Fig. 5

ORTEP plot (50% thermal ellipsoids) of the crystal structure of 8.

been demonstrated. Application of this strategy for the synthesis
of larger analogues and other polyrenes with contorted aromatic
surfaces is underway.
We thank ATMOL (Contract No. FP7-270028), the European
Research Council (Advanced Grant No. 321066), the MINECO
(Severo Ochoa Excellence Accreditation 2014-2018 (SEV-2013-0319)
and project CTQ2013-42106-P), the AGAUR (2014 SGR 818), and
the ICIQ Foundation. We gratefully acknowledge the ICIQ X-ray
diffraction, NMR, and Mass Spectrometry units, and SGIker of
UPV/EHU for technical support.

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