The Precise Synthesis of Phenylene-Extended Cyclic Hexa-peri-hexabenzocoronenes from Polyarylated [n]Cycloparaphenylenes by the Scholl Reaction

Article abstract of DOI:10.1002/anie.201500392

The longitudinal extension of cycloparaphenylenes (CPP) towards ultrashort carbon nanotubes (CNTs) is essential for the solution based bottom-up synthesis of CNTs. Herein, the longitudinal extension of the CPP skeleton by the introduction of hexaphenylbenzene units towards polyarylated [n]CPPs is described. Further, the applicability of the Scholl reaction to selectively form graphenic sidewalls is demonstrated. The ring size and substitution patterns of the polyarylated [n]CPPs were varied to overcome strain-induced side reactions during the oxidative cyclodehydrogenation and cyclic para-hexa-peri-hexabenzocoronene trimers ([3]CHBCs) were selectively obtained. This concept is envisioned as an access to ultrashort carbon nanotubes subject to the condition that further benzene rings with the right connectivity will be inserted.

Full text of DOI:10.1002/anie.201500392

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Chemie
International Edition: DOI: 10.1002/anie.201500392
German Edition: DOI: 10.1002/ange.201500392
Polycyclic Aromatic Hydrocarbons
The Precise Synthesis of Phenylene-Extended Cyclic Hexa-peri-
hexabenzocoronenes from Polyarylated [n]Cycloparaphenylenes by
the Scholl Reaction**
Martin Quernheim, Florian E. Golling, Wen Zhang, Manfred Wagner, Hans-Joachim Räder,
Tomohiko Nishiuchi,* and Klaus Müllen*
Abstract: The longitudinal extension of
cycloparaphenylenes (CPP) towards
ultrashort carbon nanotubes (CNTs) is
essential for the solution based bottom-
up synthesis of CNTs. Herein, the lon-
gitudinal extension of the CPP skeleton
by the introduction of hexaphenylben-
zene units towards polyarylated
[n]CPPs is described. Further, the
applicability of the Scholl reaction to
selectively form graphenic sidewalls is
demonstrated. The ring size and substi-
tution patterns of the polyarylated
[n]CPPs were varied to overcome
strain-induced side reactions during the
oxidative cyclodehydrogenation and
cyclic para-hexa-peri-hexabenzocoro-
nene trimers ([3]CHBCs) were selec-
tively obtained. This concept is envi-
sioned as an access to ultrashort carbon
nanotubes subject to the condition that
further benzene rings with the right
connectivity will be inserted.
Figure 1. Concept of the pre-construction and post-construction methods for the synthesis of
CNT segments with large p sidewalls.
T
he synthesis of monodisperse, length-
and diameter-defined carbon nanotube
(CNT) sidewall segments, as possible seeds for CNT forma-
tion, poses a challenge in organic synthesis and materials
science. The key point for the successful bottom-up synthesis
of CNT segments is the three-dimensional arrangement of
polycyclic aromatic hydrocarbons (PAHs). The properties of
PAHs strongly depend on the size and shape, being controlled
by the bottom-up approach to access one-dimensional (1D) or
2D PAHs, such as graphene nanoribbons (GNRs)[1–5] and
nanographenes (NGs).[6–10] Still, the bottom-up synthesis of
CNTs remains challenging. Surface-mediated strategies, such
as the recently reported growth of [6,6]-CNTs from singly
capped precursors on a platinum surface[11] or the longitudinal
growth of cycloparaphenylene (CPP) precursors[12,13] is sup-
posed to be a promising route to structurally well-defined
CNTs, since chirality and diameter are already predeter-
mined. Regarding the precise structure control and process-
ability, the solution-based bottom-up approach is more
desirable. The pioneering work of Parekh and Guha in
1934[14] and even more so the attempts of Vçgtle and co-
workers[15] nearly sixty years later, led to the successful
synthesis of a picotube in 1996[16] and the CPPs in 2008,[35]
[*] Dr. M. Quernheim, Dr. F. E. Golling, W. Zhang, Dr. M. Wagner,
Dr. H.-J. Räder, Dr. T. Nishiuchi,[+] Prof. Dr. K. Müllen
Max Planck Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
E-mail: tomohiko.nishiuchi@gmail.com
muellen@mpip-mainz.mpg.de
Dr. F. E. Golling
Graduate School Materials Science in Mainz
Staudinger Weg 9, 55128 Mainz (Germany)
[+] Current address: Department of Chemistry
Graduate School of Science
Osaka University
1-1 Machikaneyama, Toyonaka, Osaka 560-0043 (Japan)
[**] This work was supported by the EU FP 7 256617 MOLESOL and the
DFG project MU 334/36-1. F.E.G. thanks the Graduate School
MAINZ for a scholarship.
which are the smallest segments of armchair CNTs.[17–35]
A
further extension of the p system, as an approach towards
CNT segments, can be achieved by pre- and post-construction
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500392.
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methods (Figure 1). The incorporation of naphthalene,[25]
chrysene,[24,29] anthanthrene,[23] or pyrene[18,20] building
blocks into the carbon hoop are examples for the pre-
construction method (Figure 1). However, longitudinal p ex-
tension and access to fully conjugated CNT segments seem to
be limited, owing to the difficult functionalization of the
different building blocks and the challenge to create a fully p-
conjugated CNT sidewall segment.[28,36]
As an alternative approach, we developed the post-
construction method, similar to the synthesis of GNRs and
NGs, in which, after obtaining the cyclic structure, the large
p surface is formed by the Scholl reaction (Figure 1).[37] This
approach enables us to make variable CNT segments with
large p sidewalls, such as hexa-peri-hexabenzocoronene
(HBC). To obtain these CNT segments, the polyphenylene
macrocycles and cylinders 1 to 4 were synthesized (Figure 2).
However, the high ring strain in these macrocycles hampered
a clean and selective cyclodehydrogenation towards CNT
segments, and resulted in incompletely reacted, chlorinated,
or 1,2-phenyl-shifted compounds.[38] Therefore, we describe
herein the newly designed polyphenylene macrocycles 5a,b
and 6, having a [15]- or [21]-CPP base with alkyl substitutions
at the critical positions as a possible means to prevent a 1,2-
phenyl shift (Scheme 1). In macrocycle 6, 4,5,9,10-tetrahy-
dropyrene units were incorporated to prevent rearrange-
ments in the final step, since they resemble bis(ethylene)-
bridged biphenyl units. Furthermore, we demonstrate the
applicability of the Scholl reaction for the synthesis of [15,15]-
and [21,21]-CNT sidewall segments 7 and 8 by the post-
construction method.
The synthesis of the phenylene-extended and methyl-
functionalized cyclic para-hexaphenylbenzene trimers
([3]CHPBs) 5a,b started from the incorporation of chloro-
1,3-dimethylbenzene into tetraarylbenzoquinone to yield the
syn-diol adduct (for details see the Supporting Information).
In the following steps, alkylation, arylene elongation by
Suzuki cross coupling and iodination with iodine monochlor-
ide yielded the key kinked intermediates 10a,b and 11. These
precursors were cyclized under Yamamoto conditions
(Scheme 1) to primarily afford the cyclic trimers 9a,b;
Figure 2. Polyphenylene macrocycles 1–4 and the alkyl functionalized [15,15]- and [21,21]-CNT segments 7 and 8.
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Scheme 1. Reagents and conditions: a) [Ni(cod)2], cod, 2,2’-bipyridine, DMF, toluene, THF, 808C, 16 h; b) sodium naphthalenide, THF, À788C,
1 h or TiCl4, LiAlH4, THF, 808C, 72 h; cod=1,5-cyclooctadiene, DMF=N,N-dimethylformamide, THF=tetrahydrofuran.
linear side products and oligomers were separated by GPC.
The phenylene-extended [3]CHPBs 5a,b and 6 were obtained
by the reductive aromatization with either sodium naphtha-
lenide or low-valent titanium in THF.
The oxidative cyclodehydrogenation of 6 revealed that the
reaction stopped after a loss of approximately 24 of the
expected 36 hydrogen atoms (details see the Supporting
Information). This implies that the strain in the 15-membered
macrocycles is still too high to afford the desired [3]CHBCs
7a and 8 without any strain-releasing side reactions.
In previous studies, we determined that an extension of
the ring size from a 15- to a 21-membered CPP base
significantly decreases the tendency toward side reactions.[38]
The oxidative cyclodehydrogenation of 5b confirmed this
trend and the desired [3]CHBC 7b was selectively obtained in
80% yield (Scheme 2b). The isotopic mass peak pattern of 7b
in the high-resolution MALDI-TOF-MS is in good agreement
with the simulated one (Figure 3c).
Further analysis by NMR spectroscopy confirmed the
successful synthesis of 7b (spectra are in the Supporting
Information). Characteristic singlets at d = 9.27 ppm and
8.94 ppm in the 1H NMR spectrum of 7b confirmed the
formation of HBC units during the cyclodehydrogenation of
5b. Furthermore, sharp singlets at d = 2.96 ppm and d =
As a final step to form graphenic sidewalls, the macro-
cycles 5a,b and 6 were subjected to cyclodehydrogenation
with iron chloride at room temperature (Scheme 2).[39–44] In
previous studies a 1,2-phenyl shift was observed for the non-
methylated [3]CHPBs 3 (Scheme 2a). Yet by having alkyl
chains at these critical positions, the selective synthesis of
phenylene extended cyclic hexa-peri-hexabenzocoronene
trimers ([3]CHBCs) 7a,b and 8 is expected, even if the ring
strain and distortion were still high for 5a,b and 6.[45]
The analysis of the cyclodehydrogenation products
revealed an extra loss of two to six hydrogen atoms for 5a.
Two fractions with an additional loss of two hydrogen atoms
(fraction 1) or four hydrogen atoms (fraction 2) were isolated
by high-performance liquid chromatography purification and
analyzed by MALDI-TOF mass spectrometry (Figure 3a,b),
indicating strain-induced rearrangements.
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Scheme 2. Cyclodehydrogenation reactions.
1.74 ppm assigned to the methyl and tert-butyl substituents
verified the highly symmetric character of the structure of 7b.
2D-NMR techniques were used to elucidate the whole
structure (see Supporting Information).
350 nm because the rigid tetrahydropyrene units decrease the
distortion of the CPP base, which enhances the p conjugation
in comparison to 5a.[48]
The emission maxima of 5a, 5b, and 6 were found at
420 nm, 403 nm, and 420 nm, respectively. Compared with the
Stokes shift of 5b (8190 cmÀ1), and 6 (4760 cmÀ1), 5a showed
a quite large Stokes shift of 11900 cmÀ1. Since 5a has a highly
distorted CPP base in the ground state, the strain and
distortion are released in the excited state, resulting in
a large Stokes shift.[38,46,49] After cyclodeydrogenation, the
electronic absorption and emission spectra of 7b showed
distinctive patterns, which are observed in substituted HBCs,
with weak a- and p-band absorptions at 421, 400, and 350 nm,
respectively, and a strong b-band at 367 nm, as well as the
emission maxima at 479, 508, and 545 nm.[50,51]
The electronic absorption and emission spectra also
reflect the strain and distortion in the [3]CHPB macrocycles.
In the UV/Vis spectra, significant differences could be
observed for the absorption maxima of 5a, 5b, and 6
(Figure 4). The absorption maxima of CPPs appear at
around 340 nm, independent of size, except for ring sizes
below [8]CPP.[46] For the phenyl- and alkyl-substituted
[3]CHPBs, an absorption maximum at 280 nm was found for
5a, whereas for 5b the maximum was bathochromically
shifted to approximately 300 nm, indicating a decrease in ring
strain and the associated distortion.[47] An even larger red-
shift was observed for 6 with an absorption maximum at
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Figure 3. High-resolution MALDI-TOF MS spectra of 7b and 5a after cyclodehydrogenation and the simulated spectra, a) HPLC fraction 1 of 5a;
b) HPLC fraction 2 of 5a; c) MALDI-TOF spectrum of 7b and the corresponding structure.
Figure 4. UV/Vis (solid line) and fluorescence spectra (dashed line) of a) 5a,b and 6 and b) 5b, 7b in CH2Cl2.
In summary, we demonstrated the precise synthesis of the
[3]CHBC macrocycle 7b. The increase of the ring size
combined with a blocking of the critical positions decreased
the strain-releasing side reactions and selectively afforded
[3]CHBCs. Furthermore, it was confirmed that the formation
of graphenic sidewalls in strained cycles was possible by
applying the Scholl reaction. Since a p extension of the
[3]CHBC skeleton would lead to CNT sidewall segments, our
post-construction method represents a promising way to the
bottom-up synthesis of CNTs. However, the ring-strain in the
15-membered macrocycles 5 and 6 was still too high to
undergo a selective and complete dehydrogenation and the
scale-up is limited. Therefore, the synthesis of polyphenylene
macrocycles of sufficient size and structure to quantitatively
form conjugated graphenic sidewalls is the future challenge
for the efficient bottom-up synthesis of CNTs.
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Keywords: carbon nanotubes · cycloparaphenylene ·
macrocycles · polycyclic aromatic hydrocarbons · Scholl reaction
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Received: January 15, 2015
Revised: April 14, 2015
Published online: June 25, 2015
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