A cycloparaphenylene nanoring with graphenic hexabenzocoronene sidewalls

Article abstract of DOI:10.1039/c6cc03002e

Herein we report the synthesis of a novel hexabenzocoronene-containing cycloparaphenylene carbon nanoring, cyclo[12]-paraphenylene[2]-2,11-hexabenzocoronenylene, by metal-mediated cross-coupling reactions. The nanoring was accomplished by rationally designed palladium-catalyzed coupling of diborylhexabenzocoronene and L-shaped cyclohexane units, followed by nickel-mediated C-Br/C-Br coupling and the aromatization of cyclohexane moieties. The structure was confirmed by NMR and HR-MS. Especially, the cycloparaphenylene structure is firstly observed by STM. The photophysical properties were studied using UV-Vis spectroscopy, photoluminescence (PL) spectroscopy, and theoretical calculations.

Full text of DOI:10.1039/c6cc03002e

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A cycloparaphenylene nanoring with graphenic
hexabenzocoronene sidewalls†
Cite this:DOI: 10.1039/c6cc03002e
Dapeng Lu,a Haotian Wu,a Yafei Dai,b Hong Shi,c Xiang Shao,c Shangfeng Yang,a
Jinlong Yangc and Pingwu Du*a
Received 11th April 2016,
Accepted 3rd May 2016
DOI: 10.1039/c6cc03002e
www.rsc.org/chemcomm
Herein we report the synthesis of a novel hexabenzocoronene-containing as an ideal precursor or seed for bottom-up synthesis of CNTs
cycloparaphenylene carbon nanoring, cyclo[12]-paraphenylene[2]-2,11- with a fixed diameter.1a,6
hexabenzocoronenylene, by metal-mediated cross-coupling reactions.
The main challenge of chemical synthesis of CPPs lies in the
The nanoring was accomplished by rationally designed palladium- high strain energy because of the bending of the planar phenyl
catalyzed coupling of diborylhexabenzocoronene and L-shaped rings. After the synthesis of [10]cyclophenacene via chemical
cyclohexane units, followed by nickel-mediated C–Br/C–Br coupling modification of [60]fullerene for the first time by Nakamura
and the aromatization of cyclohexane moieties. The structure was and coworkers,7 the synthesis of [9]-, [12]-, and [18]CPPs was
confirmed by NMR and HR-MS. Especially, the cycloparaphenylene reported by Jasti and Bertozzi in 2008.3 A bent cyclohexadiene
structure is firstly observed by STM. The photophysical properties can be converted to an aromatic benzene unit by reductive
were studied using UV-Vis spectroscopy, photoluminescence (PL) aromatization and this approach has been applied in the
spectroscopy, and theoretical calculations.
synthesis of CPPs with different benzene rings.4c,5a,8 Itami
and coworkers applied an L-shaped building block based
As the shortest conjugated fragment of armchair carbon nano- on cyclohexane for selective synthesis of CPPs.9 Yamago and
tubes (CNTs), cycloparaphenylenes (CPPs) have recently attracted coworkers developed a strategy based on square-shaped tetra-
increasing attention from scientists.1 CPPs have simple hoop- nuclear platinum complexes for the synthesis of CPPs.10 Several
shaped structures consisting of aromatic rings with para-linkage, other p-extended CPP carbon nanorings have also been success-
structures which were hypothesized half a century ago,2 but which fully synthesized, such as [9]cyclo-1,4-naphthylene ([9]CN)11 and
were synthesized only in the last decade.3 Moreover, these mole- tetraphenyl-substituted [12]CPP.12 Very recently, the Mu¨llen
cular entities have size-dependent, tunable optoelectronic proper- group synthesized dodecaaryl [9]CPP and three dimensional
ties due to their cyclic and curved conjugating structures.3,4 In p-extended polyphenylene cylinders to achieve hexabenzocoronene-
addition, CPPs exhibit strong host–guest interactions4b,5 and they containing cyclic molecules through a final cyclodehydrogenation
can be exploited for supramolecular assemblies or as components reaction.13
of novel carbon materials. Concise synthesis of CNTs through a
In the present study, we report a novel strategy for the synthesis of
stepwise growth process from carbon nanorings is a fantastic a hexabenzocoronene (HBC)-containing cycloparaphenylene carbon
strategy to provide structurally uniform CNTs with high purity. nanoring, cyclo[12]-paraphenylene[2]-2,11-hexabenzocoronenylene, by
Therefore, there is great expectation that CPPs might function Pd- and Ni-mediated cross-coupling reactions. To avoid possible
structural change or incomplete cyclodehydrogenation in the final
a Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences,
Department of Materials Science and Engineering, iChEM (Collaborative
Innovation Center of Chemistry for Energy Materials), University of Science and
Technology of China (USTC), 96 Jinzhai Road, Hefei, Anhui Province, 230026,
P. R. China
b School of Physics Science and Technology, Nanjing Normal University,
1 Wenyuan Road, Nanjing, Jiangsu Province, 210023, P. R. China
c Department of Chemical Physics, Hefei National Laboratory for Physical Sciences
at Microscale, University of Science and Technology of China (USTC), 96 Jinzhai
Road, Hefei, Anhui Province, 230026, P. R. China. E-mail: dupingwu@ustc.edu.cn;
Fax: +86-551-63606207
step,13a,b the HBC moiety was initially introduced into the molecular
scaffold. HBC, which is a fundamental structure of polycyclic aromatic
hydrocarbons (PAHs) and an important segment of two-dimensional
graphene, has received much attention due to its self-assembling
behavior at the molecular level and rich optoelectronic properties.14
CPP carbon nanorings bearing HBCs are thought to be good
precursors for the growth of carbon nanotubes (Fig. 1). Therefore,
it is highly interesting to achieve such molecules. The product was
characterized by high resolution mass spectrometry (HR-MS),
nuclear magnetic resonance (NMR) spectroscopy, steady-state
spectroscopy, and theoretical calculations. In addition, the
† Electronic supplementary information (ESI) available: Experimental details,
figures and so on. See DOI: 10.1039/c6cc03002e
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Fig. 1 Carbon nanoring with graphenic sidewalls as the precursor for the
growth of CNTs.
scanning tunneling microscopy (STM) imaging technique was
successfully applied to view this molecule.
The main challenge of synthesizing cyclo[12]-paraphenylene[2]-
2,11-hexabenzocoronenylene lies in the high strain energy that
results from ring closure in the cyclization step. Inspired by
previous studies,9b,c,15 we chose the L-shaped unit, cis-cyclohexane-
1,4-diyl, as the building block of cyclo[12]-paraphenylene[2]-2,11-
hexabenzocoronenylene to provide the curvature for subsequent
cyclization. As shown in Fig. 2, we hypothesized that 2,11-diboryl
hexabenzocoronenylene can function as a linear unit to construct
Fig. 3 Synthesis procedures of 5. Reaction conditions: (a) Pd(PPh3)4,
Na2CO3, n-Bu4NBr, THF, H2O, 66 1C, 48 h, shielded from light; (b) Ni(cod)2,
2,20-bipyridyl, THF, 66 1C, 72 h, shielded from light; (c) NaHSO4ꢀH2O, DMSO,
the U-shaped dibromide building block by Pd-catalyzed cross- m-xylene, 150 1C, 72 h, in air, shielded from light.
coupling with the L-shaped unit via a sequence of oxidative
addition-transmetalation-reductive elimination processes.16 Then,
the tetramesityl HBC is a suitable building block for the present
Ni-mediated C–Br/C–Br coupling can connect two U-shaped
dibromide building blocks into a macrocycle, followed by the
final aromatization step to achieve the target molecule. It is
noteworthy to mention that it is very difficult to achieve the CPP
product when the cyclodehydrogenation reaction is conducted
after cyclization as in previous studies.13 The cyclodehydrogena-
tion step might be quite difficult in the presence of high ring strain
resulting from macrocyclization and the resulting complicated
mixtures would make the subsequent separation and purification
very difficult. Therefore, it is better to conduct cyclodehydrogena-
tion reaction prior to the macrocyclization reaction.
The synthesis details of the cyclo[12]-paraphenylene[2]-2,11-
hexabenzocoronenylene 5 are provided in Fig. 3. The L-shaped
building block 1 was prepared by adding 4-bromophenyllithium
to cyclohexane-1,4-dione, followed by protecting the hydroxyl
groups with the methoxymethyl (MOM) group.9b The linear unit
2 bearing an HBC structure was prepared from commercially
available reagents and the synthesis details can be found in
the ESI.† We also attempted to use tetra-tert-butyl HBC as the
graphenic unit. However, its poor solubility in most organic
solvents made the subsequent reactions very difficult. With
more trials using other functional groups, we finally found that
work. This tetramesityl HBC unit can be achieved by Kumada
coupling of tetrabromohexaphenylbenzene with mesitylmagne-
sium bromide in THF, followed by cyclodehydrogenation with
FeCl3 in CH2Cl2.17 This highly soluble compound was then
converted to diboryltetramesityl HBC 2 by the iridium-catalyzed
borylation reaction with bis(pinacolato)diboron in a mesitylene/
tert-butyl methyl ether mixed solvent.
The next step was performed to construct the U-shaped
HBC-containing building block 3. It can be prepared in a yield
of B68% as a yellow solid by the Suzuki–Miyaura cross-coupling
reaction of diboryltetramesityl HBC 2 (1 equiv.) with an excess
amount of 1 (10.3 equiv.) in the presence of Pd(PPh3)4 catalyst
(8.4 mol%), Na2CO3 (7.9 equiv.), and n-Bu4NBr (1.3 equiv.) in
THF/H2O (v/v = 7 : 1). With the U-shaped building block 3 in
hand, we next investigated the cyclization reaction to synthesize
macrocycle 4. The Ni(cod)2-mediated Yamamoto coupling reac-
tion was applied for the synthesis of 4 in the presence of 3
(1 equiv.), Ni(cod)2 (2.5 equiv.), and 2,20-bipyridyl (2.7 equiv.) in
dry THF.8c,11,13a,15 The good planarity and high rigidity of the
HBC fragment in the backbone of the building precursor seemed
to have a deleterious effect on the conformation for cyclization,
since the yield for this step was relatively low.
Subsequently, the crude product obtained via the macro-
cyclization step was directly subjected to aromatization without
thorough purification. The cyclohexane-1,4-diyl moieties were
converted to the corresponding benzene rings with NaHSO4ꢀH2O
in mixed solvents of DMSO and m-xylene at 150 1C for 2 days in air.
After extensive purification initially by flash column chromato-
graphy and then by preparative thin layer chromatography, the target
molecule 5 was obtained in B2% yield over two steps as a yellow
solid. This compound was characterized by NMR and HR-MS.
Fig. 2 Synthesis strategy for target molecule 5.
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Its isotopic pattern obtained in mass spectrometry was in good
agreement with the simulated values (Fig. S1, ESI†).
The cyclic structure of 5 was further confirmed by the STM
imaging technique at the submolecular level. A direct view of
the molecular profile was obtained in real space with the
molecular film deposited on an Au(111) surface by the drop
casting method. Fig. 4 shows the STM images of the molecular
film after annealing treatment. A large-scale STM image with
singly dispersed carbon nanoring molecules is shown in
Fig. 4a. These molecules were adsorbed onto the surface in
an unordered fashion. In order to have a clearer view of the
morphology of these molecules, a higher resolution image was
also collected (Fig. 4b), from which the individual molecule
can be recognized and the molecular structure was clearly
identified. Two graphenic HBC fragments in each macrocyclic
molecule can be clearly resolved with a flat-flying configuration
on the surface, embraced by two curved poly-benzene linkers as
Fig. 5 (a) UV-Vis absorption (solid lines) and fluorescence (dashed lines)
spectra of 5 (green line) and tetramesityl HBC (blue line) in CH2Cl2 with a
concentration of 5 ꢁ 10ꢂ6 M; (b) fluorescence of 5 in CH2Cl2 (5 ꢁ 10ꢂ6 M)
under 365 nm irradiation using a hand-held UV lamp.
two bead chains. Note that each benzene group corresponds to at 471 nm, 491 nm, and 534 nm, respectively. The fluorescence
one bright spot, but not all the benzene rings can be resolved quantum yield was determined to be FF = 7.5% using anthracene
due to the twisted configuration. The result shows the nanoring- in ethanol as the reference (FF = 30%). This value is lower than
like structure lying on the substrate, which matches the mole- that of tetramesityl HBC (FF = 14.8%), which might be due to the
cular model of 5 very well, further confirming the carbon effect of cyclization. Compound 5 shows intense green-yellow
nanoring structure.
photoluminescence in solution under UV irradiation (Fig. 5b).
Fluorescence decay measurements were performed using a
The photophysical properties of cyclo[12]-paraphenylene[2]-
2,11-hexabenzocoronenylene 5 were studied using UV-Vis absorp- nanosecond pulsed laser system in degassed CH2Cl2 solution at
tion spectroscopy and steady-state fluorescence spectroscopy room temperature. The lifetime (ts) of 5 was determined to be
(Fig. 5a). Tetramesityl HBC was used as a reference compound B14.2 ns at 544 nm with single-exponential decay when excited
for comparison. The absorption bands of 5 are observed at 300– at 390 nm (Fig. S2, ESI†). As for tetramesityl HBC, a similar
420 nm and the strongest maximum absorption peak is located at single-exponential decay was observed with ts = 18.0 ns at
l
max = 371 nm with the absorption coefficient e = 1.0 ꢁ 105 cmꢂ1 Mꢂ1
.
520 nm at the same excitation wavelength (Fig. S3, ESI†).
Two moderate absorption peaks were also observed with maxima at Compared to CPPs (ts = 10.6 ns for [9]CPP and ts = 2.2 ns for
l
max = 354 nm and 425 nm. It is noteworthy to mention that lmax of [12]CPP18), the fluorescence lifetime of 5 is longer, which could
the absorption bands have obvious redshift in comparison with the be attributed to the contribution of the HBC units.
reference compound (tetramesityl HBC) and those of other CPPs.18
Finally, we performed a density functional theory (DFT)
This can be ascribed to a much larger p conjugation system of study at the B3LYP/6-31G level to reveal the electronic nature
the graphenic HBC units in the structure. The fluorescence of the cyclo[12]-paraphenylene[2]-2,11-hexabenzocoronenylene
emission spectrum of 5 was studied at room temperature under 5. The optimized geometry and pictorial representations of
an excitation at 370 nm. Multiple emission peaks were observed frontier molecular orbitals (MOs) of 5 are shown in Fig. 6.
The optimized geometry shows a C2h symmetry with a highly
twisted CPP core. The diameters were calculated to be around
19.5 Å and 26.6 Å from Cartesian coordinates (Table S1, ESI†).
Two more side views of the optimized structure of 5 are shown
in Fig. S4 (ESI†). The energy levels of the HOMO and the LUMO
are ꢂ5.10 eV (HOMO) and ꢂ1.88 eV (LUMO), respectively. The
energy diagrams of the frontier MOs of 5 are shown in Fig. S5
(ESI†). Based on the calculation results, the spatial distribution
of the HOMO and the LUMO of 5 is mainly localized on two
HBC moieties. The fact that frontier MOs of 5 are dominated by
HBC structures is likely the main reason why the shapes of its
UV-Vis absorption and fluorescence spectra show similarity
with those of tetramesityl HBC. The strongest maximum absorp-
tion peak for 5 observed at lmax = 371 nm can be assigned to the
HOMOꢂ4 - LUMO 3, HOMOꢂ3 - LUMO 4, HOMOꢂ1 -
LUMO 5, and HOMOꢂ5 - LUMO 1 p–p* transitions at 379 nm
and 378 nm with oscillator strengths f = 2.4562 and f = 2.3971,
respectively (Table S2, ESI†).
Fig. 4 STM images of the sample deposited on Au(111). The sample was
prepared by the drop casting method and the images were obtained in
UHV. (a) A STM image showing the singly dispersed carbon nanoring
molecule 5 adsorbed on the Au(111) surface in an unordered fashion. (b)
A smaller area STM image showing the existing morphology of the target
molecules with the cyclic-shaped structure. The molecular configuration
was illustrated by matching the molecular models on the STM image.
Imaging conditions: U = 1.5 V, I = 1 nA.
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In conclusion, we synthesized an HBC-containing cyclopara-
phenylene, cyclo[12]-paraphenylene[2]-2,11-hexabenzocoronenylene.
A palladium-catalyzed cross-coupling reaction was used to construct
our building block 3 containing hexabenzocoronene and L-shaped
cyclohexane units. The macrocyclization step was achieved by
Ni-catalyzed Yamamoto coupling, followed by aromatization to
transform cyclohexane moieties into phenyl units. This synthesis
route provides a good strategy to overcome the problem of
cyclodehydrogenation in the presence of high ring strain and
successfully achieved the target molecule. This molecule was
characterized by HR-MS, NMR, and STM techniques. The direct
view of the molecular profile further confirmed its structure.
Further challenges including designing and synthesizing carbon
nanorings more similar to CNT segments, and producing size-
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