A Twisted Nanographene Consisting of 96 Carbon Atoms

Article abstract of DOI:10.1002/anie.201703754

Herein we report synthesis, structure and properties of a new type of twisted nanographene, which contains an [8]circulene moiety in a polycyclic framework of 96 sp² carbon atoms. The key steps in this synthesis are the Diels–Alder reaction of a macrocyclic diyne and the subsequent Scholl reaction forming the [8]circulene moiety. Two incompletely cyclized products were isolated from the Scholl reaction, providing insight into the cyclization of the strained octagon. This nanographene is twisted along two directions with end-to-end twists of 142.4° and 140.2° as revealed by X-ray crystallography, and is flexible at room temperature as found from the computational and experimental studies.

Full text of DOI:10.1002/anie.201703754

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Accepted Article
Title: A Twisted Nanographene Consisting of 96 Carbon Atoms
Authors: Kwan Yin Cheung, Chi Kit Chan, Zhifeng Liu, and Qian Miao
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201703754
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A Twisted Nanographene Consisting of 96 Carbon Atoms
Kwan Yin Cheung, Chi Kit Chan, Zhifeng Liu, Qian Miao*
Abstract: Herein we report synthesis, structure and properties of a
new type of twisted nanographene, which contains an [8]circulene
moiety in a polycyclic framework of 96 sp2 carbon atoms. The key
steps in this synthesis are the Diels-Alder reaction of a macrocyclic
diyne and the subsequent Scholl reaction forming the [8]circulene
moiety. Two incompletely cyclized products were isolated from the
Scholl reaction, providing insight into the cyclization of the strained
octagon. This nanographene is twisted along two directions with
end-to-end twists of 142.4 ° and 140.2 ° as revealed by X-ray
crystallography, and is flexible at room temperature as found from
the computational and experimental studies.
Carbon exists as a variety of nanometer-sized allotropes,
where sp2 hybridized carbon atoms are covalently bonded in
[1]
different geometries. The discoveries of fullerenes,
carbon
[2]
[3]
nanotubes
and graphene
have changed the landscape of
materials science and opened avenues to new technologies.[4] In
addition to these well-known nanocarbon materials, many three-
dimensional nanocarbon structures with negative curvature have
been proposed by embedding heptagons or octagons in the
graphitic lattice.[ 5 ] These exotic nanocarbon structures are
predicted to have interesting properties on the basis of
computational studies,[6] but are yet to be synthesized. Organic
synthesis is a promising bottom-up approach to these interesting
but still imaginary nanocarbons as demonstrated by recent
advances in chemistry of negatively curved polycyclic arenes, [7, 8,
Figure 1. (a) Molecular structure of twisted nanographenes 1a–b; (b) fusing
[8]circulene with two units of dibenzo[cd, pd]bisanthene along a C2 axis of
[8]circulene.
9 , 10 , 11 ]
which are not only segments containing structural
In the literature, the [8]circulene framework was constructed
in two different ways, “inward formation of the octagon”[8] and
“outward extension of the octagon”.[9, 10] As shown in Scheme 1a,
1a and 1b were synthesized using the inward strategy from the
corresponding macrocyclic precursors 7a and 7b, respectively.
To synthesize 7a, macrocyclic diene 4 was synthesized by the
Suzuki coupling of 1,2-dibromo-4,5-dimethoxybenzene with
boronic acid pinacol ester 3, which was prepared from (Z)-1,2-
bis(4-bromophenyl)ethene (2).[18] Bromination of 4 followed by
elimination of HBr resulted in macrocyclic diyne 5,[19] whose
strained carbon-carbon triple bonds reacted as dienophiles in
the two-fold Diels-Alder reaction with cyclopentadienone 6 giving
7a in an excellent yield. To tune the solubility of the final product,
7b was prepared by replacing the methyl groups in 7a with
information of three-dimensional nanocarbons but also can be
used as templates or monomer units for further synthesis of
such nanocarbons. [4, 12, 13] Here we report synthesis of a new
twisted nanographene consisting of 96 sp2 carbon atoms. As
derivatives of the nanographene C96H36, compounds 1a–b
(Figure 1a) have a polycyclic framework shaped as a short
twisted ribbon. Unlike the reported twistacenes having
a
longitudinally twisted π-backbone due to steric congestion,[14, 15,
16, 17]
1a–b are twisted because of the embedded [8]circulene
moiety. As found from density functional theory (DFT)
calculation,[8] the ground state of [8]circulene has a shape of tub
with D2d symmetry having three C2 axes and two symmetry
planes (σ) as shown in Figure 1b (one C2 axis is at the
intersection of two symmetry planes). Upon fusing with two units
of dibenzo[cd, pq]bisanthene along a C2 axis of the [8]circulene
moiety, the resulting graphene nanoribbon loses the symmetry
planes and is twisted along the C2 axis. Detailed below are the
synthesis, structural analysis and properties of 1a–b.
propyl groups. The Scholl reaction [20, 21] of 7a in a strongly acidic
[22]
condition (CF3SO3H) with DDQ as oxidant
at 0 °C for 1.5
hours resulted in 1a (13%) with formation of 14 carbon-carbon
bonds, in accompany with incompletely cyclized products 8a
(18%) and 9a (47%) as shown in Scheme 1b. Fortunately, the
three compounds exhibited sufficiently different retardation
factor (Rf) values on thin layer chromatography (TLC), and thus
were conveniently separated by column chromatography on
silica gel. When the Scholl reaction of 7a was prolonged to 4
hours, 1a (18%) and 9a (40%) were isolated by column
chromatography on silica gel while only a trace amount of 8a
was identified by TLC. When 8a was subjected to the Scholl
reaction under similar conditions, 9a was the major product
[*]
K. Y. Cheung, C.K. Chan, Prof. Z. Liu, Prof. Q. Miao
Department of Chemistry
The Chinese University of Hong Kong
Shatin, New Territories, Hong Kong, China
E-mail: miaoqian@cuhk.edu.hk
Supporting information for this article is given via a link at the end of
the document
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10.1002/anie.201703754
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membered ring formed at an early stage in the sequence of
forming 14 carbon-carbon bonds. The Scholl reaction of 7b with
DDQ/CF3SO3H at 0 °C for 4 hours led to 1b in an isolated yield
of 16%, while the attempts to isolate the incompletely cyclized
products 8b and 9b were unfruitful. It was assumed that 8b and
9b exhibited the same Rf values on TLC. The molecular
structures of 1b and 9a were unambiguously determined with
the single crystal structures as detailed in the latter part of this
article, and the structure of 1a was then confirmed by comparing
the 1H and 13C NMR spectra of 1a and 1b. The molecular
structure of 8a was determined on the basis of two-dimensional
1H NMR (NOESY) in addition to conventional 1H and 13C NMR
spectra and high resolution mass spectrum as detailed in the
Supporting Information.
Single crystals of 1b suitable for X-ray crystallography were
obtained by slow diffusion of ethyl acetate and methanol into a
solution of 1b in 1,2-dichlorobenzene, while those of 9a were
obtained by slow evaporation of a solution of 9a in ethyl acetate
and dichloromethane.[23] The crystal of 1b is found to contain a
pair of enantiomers having a twisted ribbon-like π-backbone as
shown in Figure 2, where the substituting groups with disorder
are removed for clarity. The structure of 1b in the crystal slightly
deviates from the ideal D2 symmetry, which is associated with
three mutually perpendicular C2 symmetry axes, C2-x, C2-y and
C2-z as shown in Figure 2. The C–C bonds that are related by
the corresponding ideal C2-axis are found to slightly differ in
length. The polycyclic framework of 1b is 2.03 nm long along C2-
x as measured from the distance between the terminal green
and yellow carbon atoms (Figure 2b), and twists along two
directions. The end-to-end twist along the C2-x axis is 142.4° as
measured from the torsion angel as defined by the terminal
green and yellow carbon atoms (Figure 2b), and the end-to-end
twist along the C2-y axis is 140.2° as measured from the torsion
angel defined by the terminal magenta and blue carbon atoms
(Figure 2c).
Scheme 1. a) Synthesis of 1a–b;a b) structures of the incompletely cyclized
products of the Scholl reaction of 7a–b. a Reagents and conditions: (a) (i) BuLi,
THF; (ii) 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. (b) 1,2-
dibromo-4,5-dimethoxybenzene, Pd2(dba)3, SPhos, K2CO3, Bu4NBr,
toluene/H2O, 85oC. (c) (i) Br2, CH2Cl2, 0oC; (ii) t-BuOK, THF. (d) Ph2O, reflux.
(e) (i) BBr3, CH2Cl2, reflux; (ii) 1-bromopropane, K2CO3, DMF, 80oC. (f) (a)
DDQ, TfOH, CH2Cl2, 0oC, 4 hours.
(isolated in a yield of 46%) while only a trace amount of 1a (ca.
1%) was isolated. In contrast, treatment of 9a with
DDQ/CF3SO3H under similar conditions did not lead to further
cyclization but resulted in slow decomposition of 9a. These
results indicate that the formation of 1a does not involve 9a as
an intermediate but likely through a pathway with the eight-
Figure 2. Top view (a), side view (b) and front view (c) for (M)-1b in the crystal
with three C2 axes for the ideal structure of D2 symmetry. (All the substituting
groups and hydrogen atoms removed for clarity and carbon atom positions
shown as ellipsoids at 50% probability level.)
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Table 1. Comparison of the crystal structures of 1b, tetrabenzo[8]circulene
(10) [8] and substituted [8]circulene (11).[9]
As shown in Figure 3a, the crystal structure of 9a reveals
two essentially flat hexa-peri-hexabenzocoronene (HBC)
moieties connected with two phenylene groups. The maximum
deviation of carbon atoms with respect to the mean HBC plane
is only 0.37 Å, and the angle between the two HBC planes is
71.8°. The benzene planes of the two dimethoxyphenylene
groups intersect with the two HBC planes forming angles in the
range of 61.0° to 78.5°. As a result of the V-shaped structure
with two flat HBC moieties, molecules of 9a are arranged in the
crystal with formation of π-stacked dimers as highlighted in
green in Figure 3b with a π-to-π distance of 3.81 Å.
1b
10 [a]
11
C1-C2-C5-C6
C3-C4-C7-C8
C2-C3-C6-C7
C4-C5-C8-C1
C1-C2
67.2
63.7
71.6
Torsion
Angle (°)
67.3
63.7
71.8
2.3
31.9
0.0
0.9
31.9
0.0
Figure 3. Crystal structure of 9a: (a) the π-backbone of 9a with the
substituting groups removed and carbon atom positions shown as 50%
probability ellipsoids; (b) molecular packing of 9a with the unit cell. (Hydrogen
atoms are removed for clarity.)
1.453 (3)
1.421 (3)
1.547 (2)
1.386 (3)
1.462 (3)
1.407 (3)
1.484 (3)
1.398 (3)
1.482 (1)
1.434 (1)
1.482 (1)
1.434 (1)
1.482 (1)
1.434 (1)
1.482 (1)
1.434 (1)
1.450 (2)
1.455 (2)
1.450 (2)
1.447 (2)
1.444 (2)
1.452 (2)
1.444 (2)
1.447 (2)
Bond length
(Å)
C2-C3
C3-C4
C4-C5
C5-C6
Computational study has shown that both [8]circulene and
tetrabenzo[8]circulene (10) are flexible molecules, which
C6-C7
experience fast tub-to-tub inversion at room temperature by
C7-C8
[8, 24]
pseudorotation.
To investigate whether the polycyclic
C8-C1
framework of 1a–b has similar flexibility, 1a’, a simplified
molecule with eight methyl groups replacing the t-butyl groups in
1a to reduce computational cost, was studied with density
functional theory (DFT) calculation. Molecular geometry
optimization at the B3LYP/6-31G(d) level of DFT reveals twised-
1a’ as the global energy minimum and saddle-1a’ as the local
energy minimum as well as a transition state (TS-1a’) between
twist-1a’ and saddle-1a’ as shown in Figure 4. Twised-1a’ has
D2 symmetry with three C2 symmetry axes, two of which are
shown in Figure 4a, while saddle-1a’ has C2v symmetry with two
symmetry planes (σyz and σxz in Figure 4b) and one C2 symmetry
axis. The twisted polycyclic framework of 1a’ is essentially the
same as that of 1b as found in the crystal structure. Twisted-1a’
is found more stable than saddle-1a’ by 17.2 kJ/mol, which
corresponds to an equilibrium constant of 1.0×103 at 25 °C for
the conversion of saddle-1a’ to twist-1a’. The interconversion of
the two conformers occurs by pseudorotation through TS-1a’,
which is shaped as a twisted saddle of C2 symmetry as shown in
Figure 4c. The energy barrier from twisted-1a’ to TS-1a’ is 25.9
kJ/mol, which is slightly lower than that calculated for
tetrabenzo[8]circulene (30.6 kJ/mol) [8] and corresponds to a rate
constant of 1.8×108 s−1 at 25 °C, as estimated using the Eyring
equation k = κ(kBT/h)exp(−ΔG‡/RT) and assuming a value of
unity for the transmission coefficient (κ).[25] Furthermore, saddle-
1a’ can be converted to another enantiomer of twisted-1a’
[a] The crystal structure of 10 contains two conformers (A and B as named in
Ref. 8), which exhibit slightly different dihedral angles and bond lengths. The
values of 10 here refer to conformer A.
To better understand the geometry of the [8]circulene moiety
in 1b, the crystal structures of 1b, tetrabenzo[8]circulene (10) [8]
and substituted [8]circulene (11) [9] are compared in Table 1. It is
found that the [8]circulene moiety of 1b is similar to that of 11
having the C2-C3 and C6-C7 bonds almost parallel to each
other and the C4-C5 and C8-C1 bonds almost parallel to each
other with the corresponding torsion angles (C2-C3-C6-C7 and
C4-C5-C8-C1) close to 0°. With these almost parallel carbon-
carbon bonds, the [8]circulene moiety of 1b is close to the D2d
symmetry. Unlike 1b and 11, the [8]circulene moiety of 10 is
twisted with torsion angles C2-C3-C6-C7 and C4-C5-C8-C1 both
of 31.9°, and thus largely deviates from the D2d symmetry but
adopts the S4 symmetry. Such twist in 10 was attributed to
crystal-packing force. [8, 10] Another finding from the comparison
in Table 1 is that the octagon of 1b exhibits a larger variation in
carbon-carbon bond length than those in 10 and 11 presumably
as a result of extra benzene rings fused to the [8]circulene
moiety. The C3-C4 and C7-C8 bonds in 1b are particularly long
in agreement with the twist of the whole polycyclic framework
along the C2-x axis.
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respectively. The absorption spectrum of 9a is very similar to
that of HBC showing intense peaks at 392 nm and 362 nm with
a very weak absorption band at about 442 nm due to the
forbidden 0-0 transition of the D6h symmetric HBC.[26, 27] This is in
good agreement with the structure of 9a having two flat HBC
moieties linked together with nearly orthogonal phenylene
groups as found in the crystal structure. The orange solution of
1a in dichloromethane appears almost non-luminescent upon
irradiation with UV light, while the yellow solutions of 8a and 9a
exhibit green luminescence upon irradiation with UV light. The
non-luminescent nature of 1a in solution can be attributed to its
flexible polycyclic framework, whose conformational change as
found from the above calculation consumes the energy of
excited state resulting in radiationless internal conversion. In
agreement with this explanation, 1a exhibited enhanced
photoluminescence when water was added into its solution in
THF to cause aggregation as detailed in the Supporting
Information. This is attributable to aggregation induced emission
(AIE) due to restriction of intramolecular motions.[28] The cyclic
voltammogram of 1a in CH2Cl2 exhibits one reversible oxidation
wave with the half-wave oxidation potential of 0.27 V versus
ferrocenium/ferrocene (Fc+/Fc), while those of 8a and 9a both
exhibit two reversible oxidation waves. The half-wave oxidation
potentials of 8a are 0.54 V and 0.68 V versus Fc+/Fc, and those
of 9a are 0.68 V and 0.77 V versus Fc+/Fc. From the first half-
wave oxidation potentials, the HOMO energy levels of 1a, 8a
and 9a are estimated as −5.37 eV, −5.64 eV and −5.78 eV
respectively.[29]
Figure 4. (a) Molecular model of twisted-1a’; (b) molecular model of saddle-
1a’; (c) the transition state and relative Gibbs free energy for the
interconversion between twist-1a’ and saddle-1a’. (Geometry of these models
were calculated at the B3LYP/6-31G(d) level of DFT, and hydrogen atoms are
removed in the models for clarity.)
through a transition state that is the enantiomer of TS-1a’.
Based on this calculation, it can be concluded that the polycyclic
framework of 1a–b is even more flexible than that of 10, and
racemization of twisted 1a–b can occur very rapidly through
pseudorotation via the saddle conformer as the intermediate.
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels of twisted-
1a’ are calculated as −4.67 eV and −1.70 eV, respectively, while
the HOMO and LUMO energy levels of saddle-1a’ are calculated
as −4.29 eV and −2.41 eV, respectively. This indicates that
twisted-1a’ is also electronically more stable than saddle-1a’.
Figure 5. Absorption spectrum of 1a, 8a and 9a in CH2Cl2 (5 ×
10−6 mol/L).
In summary, a new type of twisted nanographene (1a–b)
containing an [8]circulene moiety was successfully synthesized
using macrocyclic diyne 5 as a key intermediate. This synthesis
involved the Diels-Alder reaction of 5 followed by the Scholl
reaction, which resulted in the [8]circulene moiety with formation
of 14 carbon-carbon bonds. Two incompletely cyclized products
(8a and 9a) were isolated and identified, revealing some insights
of the Scholl reaction. X-ray crystallography of 1b has revealed
a polycyclic framework twisted along two directions with end-to-
end twists of 142.4° and 140.2°. DFT calculation indicates that
this twisted nanographene is flexible at room temperature, in
The optical and electrochemical properties of 1a were
studied with UV-vis absorption spectroscopy and cyclic
voltammetry (CV) in comparison to those of 8a and 9a. As
shown in Figure 5, the longest-wavelength absorption maximum
of 1a occurs at 422 nm in accompany with a wide shoulder until
about 520 nm. Both 8a and 9a exhibit blue-shifted absorption
relative to that of 1a in agreement with their less conjugated π-
backbones. As estimated from the absorption edge, the HOMO-
LUMO gaps of 1a, 8a and 9a are 2.38 eV, 2.59 eV and 2.74 eV,
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agreement with the observed aggregation induced emission.
The macrocyclic diyne 5 can in principle react with bis-diene,
such as the reported biscyclopentadienone,[30] yielding polymeric
or oligomeric precursors for synthesis of longer twisted
graphene nanoribbons. Research in this direction is in progress
in our laboratory.
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Acknowledgements
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10.1002/anie.201703754
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
A new type of twisted nanographene
containing an [8]circulene moiety in a
polycyclic framework of 96 sp2 carbon
atoms was synthesized from a
macrocyclic diyne. Its structure was
studied with X-ray crystallography and
DFT calculation.
Kwan Yin Cheung, Chi Kit Chan, Zhifeng
Liu, Qian Miao*
Page No. – Page No.
A Twisted Nanographene Consisting
of 96 Carbon Atoms
This article is protected by copyright. All rights reserved.