A Hexabenzocoronene-Based Helical Nanographene

Article abstract of DOI:10.1002/chem.202001471

A synthetic route towards a novel hexabenzocoronene-based helical nanographene motif was developed. A hexaphenylbenzene precursor was therefore designed, which cannot undergo, due to steric restrictions, a complete planarization reaction. This precursor was transformed under oxidative cyclodehydrogenation conditions to a π-extended [5]helicene, which was fully characterized including X-ray diffraction analysis.

Full text of DOI:10.1002/chem.202001471

Accepted Article
Accepted Article
Title: A Hexabenzocoronene-Based Helical Nanographene
Authors: Norbert Jux, Max M. Martin, and Frank Hampel
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202001471
Link to VoR: https://doi.org/10.1002/chem.202001471
01/2020

10.1002/chem.202001471
Chemistry - A European Journal
COMMUNICATION
A Hexabenzocoronene-Based Helical Nanographene
Max M. Martin,[a] Frank Hampel[a] and Norbert Jux*[a]
Abstract: A synthetic route towards a novel hexabenzocoronene-
based helical nanographene motif was developed.
different approach, which easily allows to prepare gram quantities
of tolane 4. With this procedure, no precious reagents or
expensive purification techniques, e.g., column chromatography,
were necessary. At first, 3,5-di-tert-butylbenzaldehyde was
reacted under McMurry conditions[26] to stilbene 2 (Scheme 1)
using zinc powder as the reducing agent and TiCl4 as the titanium
species. After aqueous workup and recrystallization from
CH2Cl2/MeOH, product 2 was obtained in suitable yields of 69 %.
Stilbene 2 was subsequently brominated[27] using Br2, which
yielded the double brominated compound 3 after aqueous workup
in almost quantitative yields (95 %). Next, a double HBr
elimination[27] under strong basic conditions (KOtBu in THF)
generated the desired tolane 4, which was isolated after aqueous
workup and recrystallization from CH2Cl2/MeOH in 78 % yield.
With tolane 4 and tetracyclone 5 a standard [4+2] Diels-Alder
reaction was performed. The reaction proceeded smoothly at
260 °C in the microwave reactor, which yielded hexaphenyl-
benzene (HPB) 6 after recrystallization in 81 % yield. Finally, an
oxidative cyclodehydrogenation reaction was applied to form an
extended aromatic -system. However, due to the steric demand
of the tert-butyl groups attached to the phenyl rings drawn in red
(Scheme 1), it seemed impossible to form all six C-C bonds as it
is usually the case for HBCs and related materials. As a result,
the twisted nanographene 7, in which five C-C bonds were formed,
was generated under standard reaction conditions (FeCl3,
CH3NO2/CH2Cl2) and isolated in 80 % yield. The reaction
proceeded smoothly without any side product formation (followed
by TLC).
A
hexaphenylbenzene precursor was therefore designed, which cannot
undergo, due to steric restrictions, a complete planarization reaction.
This precursor was transformed under oxidative cyclo-
dehydrogenation conditions to a -extended [5]helicene, which was
fully characterized including X-ray diffraction analysis.
The first preparation of buckminsterfullerenes that was reported
35 years ago (1985)[1] marked the beginning of the era of synthetic
carbon allotropes.[2] In the following years, intense carbon-based
material research was conducted that resulted in the discovery of
two more members of the carbon family: carbon nanotubes[3] in
1991 and graphene[4] in 2004. The aim to explore and discover
new compositions of pure carbon is still unabated. Only recently
the cyclic sp-hybridized cyclo[18]carbon molecule[5] was
visualized on a surface and confirmed as a member of the carbon
allotrope family.[6] Additionally, hypothetical carbon-only
architectures, such as helical or twisted derivatives of graphene,
toroidal carbon nanotubes or Mackay crystals, are subject of
ongoing research.[7] Important steps towards elusive novel
carbon-only materials were already achieved by the synthesis of
model compounds that possess key characteristics of new carbon
allotropes. Especially, polycyclic aromatic hydrocarbons (PAHs)
and nanographenes with a non-planar, twisted[7-13] or helical[14-23]
topology are becoming increasingly popular in the field of carbon-
based materials.[24] Due to the out-of-plane distortion, a third
dimension is introduced to the nanographenes that results in
unique properties different to the ones of the planar analogues.
Herein, we like to extend the synthetic toolbox of non-planar
nanographenes by a simple and efficient route that is based on
well-established reactions used for hexa-peri-hexabenzo-
coronene (HBC) type chemistry. Precise control over the
formation of the helical unit should be gained by the help of steric
repulsive effects that prevent complete planarization during the
final oxidative cyclodehydrogenation reaction step. This strategy
and all synthetic details are presented in the following paper.
First, a suitable tolane, which is capable of forming a helical HBC-
based product in the upcoming reaction steps, had to be designed.
For that, tetra-tert-butyl-tolane 4 was selected. Although a
synthesis of 4 is already literature known,[25] we employed a
Scheme 1. Synthesis of HBC-based [5]helicene 7. a) Zn, TiCl4, THF, 70 °C,
22 h; b) Br2, CHCl3, rt, 30 min; c) KOtBu, THF, 0 °C, 20 min; d) Ph2O, 260 °C
in microwave reactor, 12 h; e) FeCl3, CH3NO2, CH2Cl2, 0 °C, 100 min.
[a]
Dr. M. M. Martin, Dr. F. Hampel, Prof. Dr. N. Jux
Department of Chemistry and Pharmacy & Interdisciplinary Center
for Molecular Materials (ICMM), Organic Chemistry II
Friedrich-Alexander-University Erlangen-Nuernberg
Nikolaus-Fiebiger-Str. 10, 91058 Erlangen, Germany
e-mail: max.martin@fau.de, norbert.jux@fau.de
Product 7 as well as all of the precursor compounds were carefully
characterized by NMR spectroscopic and mass spectrometric
techniques (for details see SI). X-ray crystallography of 2, 4, 6 and
7 confirmed furthermore the successful synthesis of these
compounds and allowed to get an insight into the solid-state
behavior. Generally, molecules within this project showed a high
tendency to form crystalline structures, as some of the single
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/chem.202001471
Chemistry - A European Journal
COMMUNICATION
crystals, e.g., the ones of 7, grew within 24 h. The structures of 2,
4 and 6 are depicted in the supporting information (Figure S1 –
S4) and a detailed structural analysis of the target compound 7 is
shown in Figure 1. HBC-based [5]helicene 7 crystallizes in a
monoclinic crystal system with the space group P21/n. The helical
character of 7, which is defined, i.e., by the sum of torsion angles
(81.5°) and the interplanar angle (42.2°),[28] was unambiguously
confirmed by the crystal structure. The helicity values of 7
significantly differ to the ones of previously reported [5]helicenes,
e.g., to a carbon[5]helicene (sum of torsion angles: 65.9 – 67.8°;
interplanar angle: 47.3 – 51.3°).[29-30][31] For more details see Table
S6 – S7 in the supporting information. This deviation can be
explained by two major structural variations that affect the helical
properties of the HBC-based [5]helicene. On the one hand, the
rigid aromatic backbone of 7 (Figure 1a, drawn in grey) reduces
the flexibility, especially of the inner rings B, C and D, and
therefore limits the overall twisting capability of the helix. However,
on the other hand, the bulky tert-butyl groups counteract, due to
steric repulsion, to the effects of the stiff backbone and are leading
to an increased distortion of the helical unit, particularly of the
outer rings A and E. The sum of these two effects lead to a helical
unit with an unevenly distributed helical character with areas of in-
and decreased degree of distortion. This is also supported by the
large bond length variations of 1.41 – 1.48 Å between the inner
helical carbon atoms (Figure 1b). The crystal packing (Figure 1d,
1e) reveals that both enantiomers (M) and (P) are present,
therefore forming a racemic crystal. The molecules are, compared
to the crystal packing of, for example, hexa-tert-butyl-HBC[32][33]
relatively loosely packed with distances of ≥5 Å between the
aromatic planes of the molecules. Hence, no – interactions and
only CH– as well as London dispersion interactions were found,
which seem to be the driving force for the preferential formation
of all shown structures in the crystal. Interestingly, layers of a
single chirality are found in the crystal of 7, generating an
alternating pattern of (M) and (P) enantiomer layers (compare
Figure S5).
features a red-shifted maximum at 367 nm (p-band) with a
significantly decreased molar extinction coefficient compared to
the reference HBC. Furthermore, a reduced resolution of the fine
structure, due to a general broadening of all absorption bands,
was observed. This is also true for the fluorescence spectrum
(Figure 2b), which shows, aside from broadened signals,
hypsochromically shifted emission maxima at 475 and 506 nm for
7. Therefore, the implementation of a helical unit into a
nanographene has clear effects on the photophysical properties,
such as on the position but also on the shape of the respective
absorption and emission bands.
a)
b) Fluorescence
UV-Vis
1,8E5
1.8
1.0 1
0.9
1,6E5
1.6
0,9
1,4E5
1.4
0.8
0,8
tBu-HBC
0.70,7
0.60,6
1,2E5
1.2
11.0E5
8000.800
6000.600
0.5
0,5
7
0.40,4
0.30,3
40000
0.4
0,2
0.2
20000
0.2
0,1
0.1
0
0
0
0
450
500
550
600
600
250
300
350
400
450
450
500
550
250
300
350
400
450
wavelength / nm
wavelength / nm
Figure 2. a) UV-Vis absorption spectra of HBC-based [5]helicene 7 and
reference compound hexa-tert-butyl-HBC[34] in CH2Cl2; b) normalized steady
state fluorescence emission spectra upon irradiation of the absorption maximum.
To conclude, we’ve presented a new procedure for the synthesis
of a HBC-based twisted nanographene that contains a [5]helical
unit. The helicity was introduced with the help of sterically
demanding groups that prevent a complete closure to planar
PAHs during the oxidative cyclodehydrogenation step. These
results complement our previous findings, in which related
nitrogen-doped HBC-like [5]helicenes were formed, however,
controlled by electronic effects.[35] The chiral separation and
characterization as well as the usage of the twisted nanographene
motif of 7 as a platform for further functionalized hybrid materials,
e.g., analogue architectures to our previously reported porphyrin-
HBCs,[36-44] are currently under investigation.
c)
a)
b)
28.1°
21.8°
2.92 Å
42.2°
Acknowledgements
31.6°
E
D
B
Funded by the Deutsche Forschungsgemeinschaft (DFG) –
Projektnummer 182849149 – SFB 953. M.M.M. thanks the Fonds
der Chemischen Industrie (FCI) and the Graduate School
Molecular Science (GSMS) for financial support. We thank David
Reger for helpful discussions and for providing us with the
reference compound hexa-tert-butyl-HBC.
d)
e)
C
A
5.0 Å
Keywords: hexabenzocoronene • nanographene • helicene •
polycyclic aromatic hydrocarbons • carbon allotrope
Figure 1. a) Crystal structure of HBC-based [5]helicene 7. Structure is depicted
as ORTEP model with thermal ellipsoids drawn at a 50 % probability level; b)
torsion angles and bond lengths of the inner helix; c) determination of the
interplanar angle; d), e) packing motif; hydrogen atoms, tert-butyl groups and
solvent molecules are omitted for clarity. Helicenes with the same chirality are
drawn in the same colour. CCDC: 1990061.
[1]
H. W. Kroto, J. R. Heath, S. C. O'Brien, R. F. Curl, R. E. Smalley, Nature
1985, 318, 162-163.
[2]
[3]
[4]
A. Hirsch, Nat. Mater. 2010, 9, 868-871.
The differences in the spectroscopic properties between the HBC-
based [5]helicene 7 and a planar reference compound, hexa-tert-
butyl-HBC,[34] were studied by UV-vis absorption and emission
spectroscopy. The UV-vis absorption spectrum of 7 (Figure 2a)
S. Iijima, Nature 1991, 354, 56-58.
K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V.
Dubonos, I. V. Grigorieva, A. A. Firsov, Science 2004, 306, 666-669.
This article is protected by copyright. All rights reserved.

10.1002/chem.202001471
Chemistry - A European Journal
COMMUNICATION
[5]
F. Diederich, Y. Rubin, C. B. Knobler, R. L. Whetten, K. E. Schriver, K.
N. Houk, Y. Li, Science 1989, 245, 1088-1090.
[24] M. A. Majewski, M. Stepien, Angew. Chem. Int. Ed. 2019, 58, 86-116.
[25] H.-P. Jacquot de Rouville, R. Garbage, R. E. Cook, A. R. Pujol, A. M.
Sirven, G. Rapenne, Chem. Eur. J. 2012, 18, 3023-3031.
[26] K. S. Jeong, Y. S. Kim, Y. J. Kim, E. Lee, J. H. Yoon, W. H. Park, Y. W.
Park, S. J. Jeon, Z. H. Kim, J. Kim, N. Jeong, Angew. Chem. Int. Ed.
2006, 45, 8134-8138.
[6]
K. Kaiser, L. M. Scriven, F. Schulz, P. Gawel, L. Gross, H. L. Anderson,
Science 2019, 365, 1299-1301.
[7]
[8]
S. H. Pun, Q. Miao, Acc. Chem. Res. 2018, 51, 1630-1642.
I. R. Márquez, S. Castro-Fernández, A. Millán, A. G. Campaña, Chem.
Commun. 2018, 54, 6705-6718.
[27] S. Ito, M. Wehmeier, J. D. Brand, C. Kübel, R. Epsch, J. P. Rabe, K.
Müllen, Chem. Eur. J. 2000, 6, 4327-4342.
[9]
K. Y. Cheung, C. K. Chan, Z. Liu, Q. Miao, Angew. Chem. Int. Ed. 2017,
56, 9003-9007.
[28] Y. Shen, C. F. Chen, Chem. Rev. 2012, 112, 1463-1535.
[29] R. Kuroda, J. Chem. Soc., Perkin Trans. 2 1982, 789-794.
[30] A.-C. Bédard, A. Vlassova, A. C. Hernandez-Perez, A. Bessette, G. S.
Hanan, M. A. Heuft, S. K. Collins, Chem. Eur. J. 2013, 19, 16295-16302.
[31] CCDC 933556.
[10] S. H. Pun, C. K. Chan, J. Luo, Z. Liu, Q. Miao, Angew. Chem. Int. Ed.
2018, 57, 1581-1586.
[11] O. Papaianina, V. A. Akhmetov, A. A. Goryunkov, F. Hampel, F. W.
Heinemann, K. Y. Amsharov, Angew. Chem. Int. Ed. 2017, 56, 4834-
4838.
[32] P. T. Herwig, V. Enkelmann, O. Schmelz, K. Müllen, Chem. Eur. J. 2000,
6, 1834-1839.
[12] V. Akhmetov, M. Feofanov, S. Troyanov, K. Amsharov, Chem. Eur. J.
2019, 25, 7607-7612.
[33] CCDC 142008.
[13] J. M. Fernandez-Garcia, P. J. Evans, S. Medina Rivero, I. Fernandez, D.
Garcia-Fresnadillo, J. Perles, J. Casado, N. Martin, J. Am. Chem. Soc.
2018, 140, 17188-17196.
[34] J. Holzwarth, K. Y. Amsharov, D. I. Sharapa, D. Reger, K. Roshchyna,
D. Lungerich, N. Jux, F. Hauke, T. Clark, A. Hirsch, Angew. Chem. Int.
Ed. Engl. 2017, 56, 12184-12190.
[14] T. Fujikawa, Y. Segawa, K. Itami, J. Am. Chem. Soc. 2015, 137, 7763-
7768.
[35] F. Ammon, S. T. Sauer, R. Lippert, D. Lungerich, D. Reger, F. Hampel,
N. Jux, Org. Chem. Front. 2017, 4, 861-870.
[15] Y. Nakakuki, T. Hirose, H. Sotome, H. Miyasaka, K. Matsuda, J. Am.
Chem. Soc. 2018, 140, 4317-4326.
[36] J. M. Englert, J. Malig, V. A. Zamolo, A. Hirsch, N. Jux, Chem. Commun.
2013, 49, 4827-4829.
[16] D. Reger, P. Haines, F. W. Heinemann, D. M. Guldi, N. Jux, Angew.
Chem. Int. Ed. 2018, 57, 5938-5942.
[37] D. Lungerich, J. F. Hitzenberger, M. Marcia, F. Hampel, T. Drewello, N.
Jux, Angew. Chem. Int. Ed. 2014, 53, 12231-12235.
[38] D. Lungerich, J. F. Hitzenberger, W. Donaubauer, T. Drewello, N. Jux,
Chem. Eur. J. 2016, 22, 16755-16759.
[17] S. H. Pun, Y. Wang, M. Chu, C. K. Chan, Y. Li, Z. Liu, Q. Miao, J. Am.
Chem. Soc. 2019, 141, 9680-9686.
[18] C. M. Cruz, S. Castro-Fernández, E. Maçôas, J. M. Cuerva, A. Campaña,
Angew. Chem. Int. Ed. 2018, 57, 14782-14786.
[39] D. Lungerich, J. F. Hitzenberger, F. Hampel, T. Drewello, N. Jux, Chem.
Eur. J. 2018, 24, 15818-15824.
[19] A. Campaña, C. Cruz, S. Castro-Fernández, E. Maçôas, A. Millán,
Synlett 2019.
[40] M. M. Martin, N. Jux, J. Porphyrins Phthalocyanines 2018, 22, 454-460.
[41] M. M. Martin, M. Dill, J. Langer, N. Jux, J. Org. Chem. 2019, 84, 1489-
1499.
[20] J. Ma, K. Zhang, K. S. Schellhammer, Y. Fu, H. Komber, C. Xu, A. A.
Popov, F. Hennersdorf, J. J. Weigand, S. Zhou, W. Pisula, F. Ortmann,
R. Berger, J. Liu, X. Feng, Chem Sci 2019, 10, 4025-4031.
[21] P. J. Evans, J. Ouyang, L. Favereau, J. Crassous, I. Fernandez, J. Perles,
N. Martin, Angew. Chem. Int. Ed. 2018, 57, 6774-6779.
[22] Y. Wang, Z. Yin, Y. Zhu, J. Gu, Y. Li, J. Wang, Angew. Chem. Int. Ed.
2019, 58, 587-591.
[42] M. M. Martin, D. Lungerich, P. Haines, F. Hampel, N. Jux, Angew. Chem.
Int. Ed. 2019, 58, 8932-8937.
[43] M. M. Martin, D. Lungerich, F. Hampel, J. Langer, T. K. Ronson, N. Jux,
Chem. Eur. J. 2019, 25, 15083-15090.
[44] M. M. Martin, C. Dusold, A. Hirsch, N. Jux, J. Porphyrins Phthalocyanines
2020, 24, 268-277.
[23] Y. Zhu, X. Guo, Y. Li, J. Wang, J. Am. Chem. Soc. 2019.
This article is protected by copyright. All rights reserved.

10.1002/chem.202001471
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
M. M. Martin, F. Hampel, N. Jux*
Page No. – Page No.
A Hexabenzocoronene-Based Helical
Nanographene
[5]helicene
A twisted nanographene that is based on the well-established hexabenzocoronene
motif is presented. A [5]helical unit was implemented into the nanographene
structure with the help of steric repulsive effects. The-extended helicene was
characterized, i.a., by UV-vis absorption and emission spectroscopy as well as X-ray
crystallography.
This article is protected by copyright. All rights reserved.