Cloverphene: A Clover-Shaped cata-Condensed Nanographene with Sixteen Fused Benzene Rings

Article abstract of DOI:10.1002/anie.201104935

Living in clover: Sequential [4+2] and [2+2+2] aryne cycloadditions allow the isolation of cloverlike and archlike nanographenes (see picture). This approach led to the synthesis of the largest cata-condensed aromatic hydrocarbon that has been obtained to date, a system formed by 16 fused benzene rings (22 benzene rings in total) and 102 sp² carbon atoms.

Full text of DOI:10.1002/anie.201104935

Angewandte
Chemie
DOI: 10.1002/anie.201104935
Arynes
[16]Cloverphene: a Clover-Shaped cata-Condensed Nanographene
with Sixteen Fused Benzene Rings**
Josꢀ M. Alonso, Alba E. Dꢁaz-ꢂlvarez, Alejandro Criado, Dolores Pꢀrez, Diego PeÇa,* and
Enrique Guitiꢃn
In memory of Michael M. Pollard
Since the first isolation of graphene in 2004,[1] a huge amount
of effort has been dedicated to developing efficient method-
ologies to prepare the two-dimensional counterpart of graph-
ite, a material that exhibits intriguing physical properties.[2]
Structurally, graphenes are simply large polycyclic aromatic
hydrocarbons (PAHs): sp2-bonded carbon atoms packed into
a 2D structure of fused benzene rings.[3] This molecular
similarity opens up the possibility of preparing well-defined
nanographenes by bottom-up approaches through organic
synthesis.[4] These chemical methods would avoid the struc-
tural inhomogeneity implicit in the current approaches to
prepare graphenes and they could provide access to nano-
graphenes with different topologies and peripheries. Mꢀllen
and co-workers have reported impressive examples of this
approach, including the preparation of a graphene disk
formed by 91 peri-fused benzene rings through cyclodehy-
drogenation of a suitable precursor.[5] More recently, the same
group reported the preparation of graphene nanoribbons up
to 40 nm in length.[6] It is quite remarkable, however, that
examples of nanographenes exclusively formed by cata-fused
benzene rings are scarce, although this type of condensation
would definitely influence the electronic properties of the
graphene-like material. In fact, the stability of large cata-
fused PAHs strongly depends on the mode of condensation
and number of benzene rings. For example, amongst acenes or
linear cata-condensed PAHs,[7] heptacene is an unstable
molecule under ambient conditions,[8] whereas octacene and
nonacene can only be detected at 30 K in an argon matrix.[9]
Recently, Miller and Anthony independently succeeded at
preparing stable substituted nonacene derivatives.[10] By
contrast, [14]helicene, a cata-condensed PAH with fourteen
ortho-fused benzene rings, has been isolated under ambient
conditions and is the largest cata-condensed PAH that had
been prepared prior to this work.[11]
Scheme 1. Starphenes and cloverphenes.
[*] Dr. J. M. Alonso, Dr. A. E. Dꢀaz-ꢁlvarez, A. Criado, Dr. D. Pꢂrez,
Dr. D. PeÇa, Prof. Dr. E. Guitiꢃn
In 1998, we reported that the cyclotrimerization of
benzyne to give triphenylene (1, Scheme 1) is efficiently
catalyzed by palladium(0) complexes.[12] The scope of this
methodology was soon extended by applying it to the
cyclotrimerization of polycyclic arynes.[13] In this way, large
PAHs such as hexabenzotriphenylene 3 (C42H24)[13a] and
hexabenzotrinaphthylene 4 (C54H30)[13b] were obtained in
reasonable yields (Scheme 1). Herein we report the successful
synthesis of the hexabenzotrianthrycene 5, a threefold
symmetric polyarene with sixteen cata-fused benzene rings
Centro Singular de Investigaciꢄn en Quꢀmica Biolꢄxica e Materiais
Moleculares (CIQUS) and Departamento de Quꢀmica Orgꢃnica,
Facultad de Quꢀmica, Universidad de Santiago de Compostela
15782-Santiago de Compostela (Spain)
E-mail: diego.pena@usc.es
[**] Financial support from the Spanish Ministry of Science and
Education (MICINN, CTQ2010-18208), Xunta de Galicia
(10PXIB2200222PR) and FEDER is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104935.
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(22 benzene rings in total), with a diameter of 2.1 nm and the
molecular formula C102H60. To the best of our knowledge, this
is the largest cata-condensed PAH that has been prepared to
date.
In 1968, the term starphene was coined by Clar and
Mullen for benzologues of triphenylene (1), in which the
three branches are annellated to a central ring and radiate
outwards in a linear manner.[14] For example, compound 2 was
named [10]starphene(3.3.3). Since the structure of polyarenes
3, 4, and 5 resembles the threefold symmetry of a three-leaf
clover,[15] we propose the trivial name cloverphene (cloverlike
phene) to refer to hexabenzotriphenylene and its benzo-
logues. Employing this system, compound 5 would have a
[16]cloverphene(5.5.5) aromatic core. The figures in square
brackets give the total number of cata-condensed benzene
rings, while the figures in round brackets give the number of
rings in each of the three branches annellated to the central
ring.
Scheme 3. Synthesis of cloverphene 5.
As shown in Scheme 1, hexaphenyl-substituted [16]clo-
verphene 5 could be obtained in a one-pot procedure by Pd-
catalyzed cyclotrimerization of aryne 6. Based on our
experience in this field, we chose ortho-(trimethylsilyl)aryl
triflate 11 as the precursor of polycyclic aryne 6. The
preparation of this triflate is shown in Scheme 2. Generation
Figure 1. Spectroscopic characterization of cloverphene 5: a) MALDI mass
spectrum; b) simulated mass spectrum for C102H60; c) 13C NMR spectrum;
d) DEPT-135 spectrum; e) 1H NMR spectrum.
Scheme 2. Synthesis of aryne precursor 11.
of monocyclic aryne 9 by treatment of triflate 8 with
tetrabutylammonium fluoride (TBAF) in the presence of
cyclopentadienone 7, followed by refluxing the resulting
mixture in tetrachloroethane for 3 h, afforded benzotriphe-
nylenol 10 in 44% yield. This is a remarkable result bearing in
mind the presence of an unprotected hydroxyl group that
could interact with the highly electrophilic aryne. Apparently,
the intermediate resulting from the Diels–Alder reaction of
aryne 9 and dienone 7 evolves in situ to afford compound 10
by cheletropic extrusion of carbon monoxide. The one-pot
treatment of compound 10 with hexamethyldisilazane
(HMDS) in refluxing THF, followed by successive addition
of nBuLi and Tf2O (Tf = trifluoromethanesulfonyl) at
À1008C, afforded triflate 11 in 70% yield. As expected,
slow generation of aryne 6 by treatment of triflate 11 with CsF
in the presence of 10 mol% of [Pd(PPh3)4] in a mixture of
dichloromethane/MeCN 1:6 at 408C, led to cloverphene 5 in
22% yield as a yellow solid after purification by column
chromatography (Scheme 3).
distribution expected for a hydrocarbon of the composition
C
102H60 (Figure 1). Despite its extremely large size, the
threefold symmetry of this compound led to a simple
13C NMR spectrum, with 8 CH and 7 C aromatic signals,
while the 1H NMR spectrum showed a singlet and a doublet,
which were particularly deshielded at d = 8.71 and 8.30 ppm,
respectively.
The presence of six phenyl groups attached to the
cloverphene core would induce distortion from planarity in
compound 5 in order to minimize steric strain, and this, in
turn, enhances the solubility of this giant PAH and allows its
spectroscopic characterization. In particular, low temperature
NMR experiments suggest that cloverphene 5 adopts a C3-
symmetric molecular-like conformation (Figure 2),[13a] which,
according to computational calculations, is thermodynami-
cally more stable than a C2-symmetric conformation.[15] At the
same time, the presence of these phenyl groups probably
prevents the fast photooxidation of the extended aromatic
core of compound 5.[16]
This compound gave a molecular ion with a mass of m/z
1284.4 in its MALDI mass spectrum with the isotopic
To evaluate compound 5 as a new molecular material
candidate, we studied some of its electronic properties. The
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on two sequential [4+2] aryne cycloadditions with two
molecules of the same diene.[19] The use of a masked
bisbenzyne precursor (8) allowed us to combine two different
sequential cycloadditions: a [4+2] cycloaddition followed by a
cyclotrimerization to afford large PAHs with a threefold
symmetric structure. It should be highlighted that this type of
topology has recently attracted considerable attention in the
field of molecular electronics as single molecule logic gates.[20]
To further explore the utility of aryne 6 to construct large
PAHs, we decided to attempt its cocyclotrimerization with
dimethyl acetylenedicarboxylate (DMAD).[21] Treatment of
triflate 11 with CsF in the presence of DMAD and 10 mol%
of [Pd(PPh3)4] led to the isolation of compound 12 in a
remarkable 59% yield. This compound resulted from the
[2+2+2] cocycloaddition of two arynes and one alkyne
(Scheme 4). Remarkably, this molecule presents a tetraben-
Figure 2. Optimized geometry for a C3-symmetric conformation of
hydrocarbon 5.
Figure 3. Electronic properties of cloverphene 5: a) Absorption (solid
line) and emission (dashed line) spectra in dichloromethane; b) cyclic
voltammogram in dichloromethane (1 mm).
UV/Vis spectrum in dichloromethane exhibited major
absorption bands at 324 nm and 366 nm with shoulders at
383 nm and 420 nm, respectively (Figure 3a). From this
lowest transition we estimated a highest occupied molecular
orbital (HOMO)–lowest unoccupied molecular orbital
(LUMO) energy gap of 2.74 eV, while the gap, computed by
DFT at the B3LYP 6-31G(d) level, was 3.02 eV.[15] The
emission spectrum under excitation at 366 nm showed a
structured fluorescence band at 476 nm with a shoulder at
504 nm, probably due to the structural restrictions of its
nonplanar aromatic core. Notably, in terms of efficiency, we
calculated the quantum yield of compound 5 to be 0.48 in
dichloromethane solution with respect to fluorescein (0.79 in
a 0.1m NaOH solution). As expected, exposure of hydro-
carbon 5 in solution to ambient air and sunlight caused its
degradation, probably by photooxidation. Bearing in mind
the large number of fused benzene rings in its aromatic core,
the photooxidation rate turns out to be relatively low, with a
half-life of 24.6 min.[15] For comparison, the half-life of 6,13-
diphenylpentacene was 8.5 min under similar conditions.[16]
Interestingly, cyclic voltammetry of cloverphene 5 showed
three reversible oxidation waves at 1108 mV, 1302 mV, and
1503 mV vs. Ag/Ag+, respectively (Figure 3b), in accordance
with the presence of three aromatic branches in the molecule.
From the first oxidation process a HOMO energy value of
À5.47 eV can be estimated for cloverphene 5.
Scheme 4. Synthesis of tetrabenzoheptaphene 12.
zoheptaphene core, an angular nanoribbon structure with
very few reported precedents.[22] Crystals of compound 12
were obtained from a chloroform/hexane solution and these
proved suitable for X-ray diffraction analysis. Curiously,
tetrabenzoheptaphene 12 adopts an arch-shaped molecular
structure (Figure 4), with a base 18.6 ꢁ wide and 3.4 ꢁ in
depth.[15] This unusual shape has recently been described by
Pascal and co-workers for a structurally related tetrabenzo-
heptaphene derivative, which also adopts an arch-like struc-
ture in the crystalline state.[23]
The synthetic route described herein to obtain clover-
phene 5 is based on two sequential aryne cycloadditions: a
[4+2] cycloaddition of aryne 9, followed by a [2+2+2]
cycloaddition of aryne 6. This route was inspired by the
bisbenzyne approach to large acenes independently described
by Pascal[17] and Wudl,[18] although their approaches are based
Figure 4. X-ray structure of heptaphene 12 (top and lateral views).
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Keywords: arynes · cycloadditions · hydrocarbons ·
nanographenes · oligoacenes
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