A Large π-Extended Carbon Nanoring Based on Nanographene Units: Bottom-Up Synthesis, Photophysical Properties, and Selective Complexation with Fullerene C70

Article abstract of DOI:10.1002/anie.201608963

Herein we report the organoplatinum-mediated bottom-up synthesis, characterization, and properties of a novel large π-extended carbon nanoring based on a nanographene hexa-peri-hexabenzocoronene (HBC) building unit. This tubular structure can be considered as an example of the longitudinal extension of the cycloparaphenylene scaffold to form a large π-extended carbon nanotube (CNT) segment. The cyclic tetramer of a tetramesityl HBC ([4]CHBC) was synthesized by the reaction of a 2,11-diborylated hexa-peri-hexabenzocoronene with a platinum complex, followed by reductive elimination. The structure of this tubular molecule was further confirmed by physical characterization. Theoretical calculations indicate that the strain energy of this nanoring is as high as 49.18 kcal mol^?1. The selective supramolecular host–guest interaction between [4]CHBC and C₇₀was also investigated.

Full text of DOI:10.1002/anie.201608963

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201608963
German Edition: DOI: 10.1002/ange.201608963
Host–Guest Systems
A Large p-Extended Carbon Nanoring Based on Nanographene Units:
Bottom-Up Synthesis, Photophysical Properties, and Selective
Complexation with Fullerene C70
Dapeng Lu+, Guilin Zhuang+, Haotian Wu, Song Wang, Shangfeng Yang, and Pingwu Du*
Abstract: Herein we report the organoplatinum-mediated
bottom-up synthesis, characterization, and properties of
a novel large p-extended carbon nanoring based on a nano-
graphene hexa-peri-hexabenzocoronene (HBC) building unit.
This tubular structure can be considered as an example of the
longitudinal extension of the cycloparaphenylene scaffold to
form a large p-extended carbon nanotube (CNT) segment. The
cyclic tetramer of a tetramesityl HBC ([4]CHBC) was
synthesized by the reaction of a 2,11-diborylated hexa-peri-
hexabenzocoronene with a platinum complex, followed by
reductive elimination. The structure of this tubular molecule
was further confirmed by physical characterization. Theoret-
ical calculations indicate that the strain energy of this nanoring
is as high as 49.18 kcalmolÀ1. The selective supramolecular
host–guest interaction between [4]CHBC and C70 was also
investigated.
Vçgtle and co-workers, in which they suggested several
ingenious strategies for the synthesis of CPPs,[6] which led to
the successful synthesis of a picotube in 1996.[7] The initial
synthesis of [9]-, [12]-, and [18]CPPs from a curved precursor
was reported in 2008,[8] and several other strategies involving
curved linkers or platinum-mediated catalysis have also been
explored for the production of CPPs.[4a,9]
Further p extension of the CPP system was recently
reported. The incorporation of naphthalene,[10] chrysene,[11]
and pyrene[12] building blocks into the carbon nanohoops are
representative examples. In view of the difficulty in function-
alizing the building blocks to create a fully p-conjugated CNT
sidewall segment, large conjugated systems could also be
formed by using a postconstruction method, such as a cyclo-
dehydrogenation reaction after the formation of the cyclic
structure.[13] Our research group recently introduced two
HBC units as sidewalls into the [18]CPP backbone by
palladium-catalyzed Suzuki coupling.[14]
Despite all of the above progress, a large p-extended
carbon nanoring containing only HBC units has only been
detected by mass spectroscopy,[13b] but no isolated pure
product has been reported so far. Such a cyclic structure
consisting solely of HBC units more closely resembles a CNT
segment than previously reported carbon nanorings[8,9] and
could be a good precursor for the bottom-up synthesis of
uniform CNTs. Therefore, the synthesis of such a structure
and evaluation of its electronic properties is highly desired.
Herein we report the synthesis and isolation of a large p-
extended carbon nanoring as a finite model of CNTs based on
four HBC units, [4]cyclo-2,11-para-hexa-peri-hexabenzocor-
onene ([4]CHBC, Figure 1). A platinum-mediated assembly
approach was used, followed by a reductive-elimination
reaction. Platinum-mediated synthesis is a useful method for
the construction of conjugated structures by the assembly of
Carbon nanotubes (CNTs) with a specific diameter and
chirality are important in nanotechnology and nanoelectron-
ics. The synthesis of length- and diameter-specific CNT
segments is a challenge in organic synthesis and materials
science. Surface-mediated strategies have shown great prom-
ise for the preparation of structurally uniform CNTs, such as
the recently reported growth of CNTs on a platinum surface[1]
or on solid alloy catalysts,[2] and the longitudinal growth of
cycloparaphenylene (CPP) precursors.[3] Regarding precise
structural control, the solution-processable bottom-up
approach is another desirable strategy to produce pure
CNTs. CPPs have hoop-shaped structures consisting of
aromatic rings with para linkages and are the shortest
conjugated fragment of armchair nanotubes, which have
been proposed as ideal precursors for the bottom-up synthesis
of structurally uniform CNTs.[3,4] Initial synthetic endeavors
targeting CPPs were reported by Parekh and Guha in 1934.[5]
Modern CPP research can be traced back to a report by
[*] D. Lu,[+] H. Wu, S. Wang, Prof. S. Yang, Prof. P. Du
CAS Key Laboratory of Materials for Energy Conversion
Department of Materials Science and Engineering, Collaborative
Innovation Center of Chemistry for Energy Materials (iChEM)
University of Science and Technology of China (USTC)
Hefei, 230026 (P.R. China)
E-mail: dupingwu@ustc.edu.cn
Prof. G. Zhuang[+]
College of Chemical Engineering, Zhejiang University of Technology
18, Chaowang Road, Hangzhou, Zhejiang Province 310032 (China)
[+] These authors contributed equally.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201608963.
Figure 1. Design of the tubular [4]CHBC carbon nanoring as a finite
model of a carbon nanotube.
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p-conjugated units and subsequent carbon–carbon bond
formation by reductive elimination.[9a,15]
yields. We have also attempted to optimize the reaction
conditions to improve the ultimate yield of our product by
adjusting the solvent, temperature, and base, but no further
improvement was observed in the yield of [4]CHBC.
The molecular weight of [4]CHBC was determined by
MALDI-TOF MS spectrometry. A main peak at m/z 3973.96
was observed (calculated for C312H225 [M+H]+: 3973.79), thus
suggesting the synthesis of the target molecule (Figure 3a).
The 1H NMR spectrum of [4]CHBC featured a set of signals
The HBC unit with tetramesityl groups is a good building
block because of its high solubility in common organic
solvents. Mesityl groups can be introduced at the HBC
periphery by the coupling of tetrabromohexaphenylbenzene
with mesitylmagnesium bromide, followed by cyclodehydro-
genation with FeCl3. This tetramesityl-substituted HBC was
subsequently converted into the 2,11-diborylated tetramesityl
HBC 1 (Figure 2) by treatment with an equimolar amount of
bis(pinacolato)diboron under iridium(I)-catalyzed borylation
conditions.[16] This convenient protocol for the introduction of
À
boryl groups into p-rich hydrocarbons through direct C H
borylation provides a reliable method for the preparation of
large quantities of para-borylated tetramesityl HBC, which is
a key precursor for the synthesis of [4]CHBC.
Figure 3. a) MALDI-TOF MS of [4]CHBC. b) Comparison of the aro-
matic regions of the 1H NMR spectra of [4]CHBC and the tetramesityl
HBC monomer.
Figure 2. Synthetic strategy for [4]CHBC. a) CsF (6.0 equiv),
[Pt(cod)Cl2] (1.1 equiv), THF, reflux, 24 h; b) platinum-mediated reduc-
tive elimination: triphenylphosphine (10 equiv), mesitylene, 1508C.
COD=1,5-cyclooctadiene.
with the signature of only seven singlets, thus indicating its
highly symmetric structure (see the Supporting Information).
Figure 3b shows a comparison of proton signal patterns in the
aromatic region between [4]CHBC and the tetramesityl HBC
monomer. Clear splitting of peaks as a result of spin–spin
coupling between adjacent proton(s) can be observed in the
spectrum of the monomer (peaks a’ and d’ in Figure 3b). In
sharp contrast, these coupling peaks disappear for [4]CHBC,
thus indicating that the HBC units are connected to form
a hoop-shaped and highly symmetric structure. Another
distinct feature is that the signals of the hydrogen atoms at
the peripheries of [4]CHBC are shifted upfield as compared
to the corresponding peaks of the monomer, thus indicating
the influence of adjacent HBC units in the nanoring structure.
Next, the macrocyclization reaction was performed by the
platinum-mediated cyclization method.[17] In the presence of
cesium fluoride, the HBC precursor 1 reacted with an
equimolar amount of [Pt(COD)Cl2] in dry THF under
reflux. The resulting mixture was directly subjected to
reductive elimination by heating in the presence of triphe-
nylphosphine (10 equiv) in dry mesitylene for 48 h under
nitrogen (Figure 2). After extensive purification by initial
flash column chromatography and then preparative thin-layer
chromatography, the target nanoring molecule [4]CHBC was
obtained as a yellow solid in 5.4% yield over two steps, which
is comparable to the yields of many similar reactions.[17a,18]
Although other cyclic oligomers with more than four HBC
units (such as [5]CHBC) could also be produced, it would be
very difficult to characterize them owing to their much lower
1
The H and 13C NMR data and UV absorption and emission
spectra (see below) confirmed the successful synthesis of
[4]CHBC.
2
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The photophysical properties of [4]CHBC were studied
theory (DFT) calculations with the DMol3 software.[21] For
by UV/Vis absorption spectroscopy, steady-state fluorescence
spectroscopy, and time-resolved fluorescence decay. The
tetramesityl HBC monomer was used as a reference com-
pound for comparison. The absorption bands of [4]CHBC
were observed between 300 and 500 nm, and the maximum
absorption peak (lmax) was observed at 375 nm with the
molecular absorption coefficient e = 5.1 ꢀ 105 cmÀ1mÀ1 (Fig-
ure 4a). Two moderate absorption peaks were also observed
with maxima at 431 and 458 nm. Notably, the absorption
feature of [4]CHBC showed a clear redshift in comparison
with those for the tetramesityl HBC and other CPPs.[19] This
redshift could be ascribed to the larger p-conjugation system
from the HBC units and the cyclic structure.
simplification, we replaced all the peripheral methyl groups
with hydrogen atoms in the initial geometrical optimization
owing their negligible influence on the conjugated framework
of this structure (Figures 5a,b). The structure features D2
symmetry with all atoms relaxed without constraints. Each
À
HBC monomer is linked to adjacent monomers by a C C
single bond of about 1.485 ꢁ in length with a torsion angle of
approximately 35.988. Inspecting the frontier molecular
orbitals (MOs; Figures 5c,d; see also Figure S2), we can
conclude that both HOMO-1 and LUMO + 1 exhibit a p-type
antibonding orbital, the two of which are located at two sides
of this macrocycle rather than at the equator. The energy gap
between HOMO and LUMO is 1.81 eV, thus suggesting that
this structure is quite stable.
Furthermore, the strain energy
of [4]CHBC was calculated to
be 49.18 kcalmolÀ1 according to
the reported method (see
Table S1 in the Supporting
Information).[22] This value is
comparable to that of [12]CPP
(55 kcalmolÀ1).[23]
Considering the concave
cavity of [4]CHBC, it should
act as a host for p-conjugated
molecules with a convex sur-
face, such as fullerenes. We next
Figure 4. a) UV/Vis absorption (solid lines) and fluorescence spectra (dash lines) of [4]CHBC (green) and
tetramesityl HBC (pink) in CH2Cl2. b) Emission lifetime measured at 585 nm for [4]CHBC in CH2Cl2.
c) Photograph showing the fluorescence of [4]CHBC in CH2Cl2 under a UV lamp. PL=photolumines-
cence.
investigated the supramolecular
host–guest interaction behavior
between [4]CHBC and fuller-
ene C70. Their interaction was
first observed when C70 was
The emission spectrum of [4]CHBC showed multiple
emission bands with maximum peaks at 468, 506, and 586 nm
(lex = 400 nm). The edge emission was even extended to
750 nm. Similarly, these emission bands were significantly
shifted towards lower energy as compared with the emission
spectrum of the monomer, which is consistent with the UV/
Vis result and confirms the p-expanded nature in [4]CHBC.
This solution also presented an intense greenish photolumi-
nescence under irradiation by a hand-held UV lamp (Fig-
ure 4c).[20] The photoluminescence quantum yield was deter-
mined to be FF = 8.3% by using anthracene as the reference
(FF = 30% in ethanol). This value is lower than those of
tetramesityl HBC and CPPs (for tetramesityl HBC, FF =
14.8%, and for [9]–[16]CPP, the FF values range from 73 to
88%),[19] probably as a result of the high proportion of
nonradiative decay in the low-energy emission.
added to a solution of [4]CHBC in dichloromethane, where-
upon the solution underwent a clear color change from yellow
to brown. Accordingly, there was reduced fluorescence
intensity under UV light at 365 nm. This complexation
between [4]CHBC and C70 was then confirmed by MS
Upon excitation at 390 nm, the luminescence lifetime (ts)
of [4]CHBC was determined to be approximately 10.1 ns at
585 nm by single-exponential decay fitting (Figure 4b). As for
the tetramesityl HBC, a similar single-exponential decay was
observed with ts ꢀ 18.0 ns at 520 nm at the same excitation
wavelength (see Figure S1 in the Supporting Information).
The fluorescence lifetime of [4]CHBC is longer than that of
[12]CPP (ts = 2.2 ns).[19]
Figure 5. Optimized geometry and frontier MOs of [4]CHBC: a) Top
view of the molecular structure; b) side view of the molecular
structure; c) HOMO of [4]CHBC; d) LUMO of [4]CHBC.
To investigate the structure and electronic properties of
the nanoring [4]CHBC, we performed density functional
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spectrometry. The peak observed at m/z 4814.85 (calculated
for C382H225 [C70ꢁM + H]+: 4814.77; Figure 6a) suggests the
formation of a 1:1 complex. We next carried out fluorescence
measurements with an excitation wavelength at 400 nm. The
main emission peaks (554, 586, and 617 nm) showed a signifi-
cant blueshift as compared with pure [4]CHBC, and the
emission intensity decreased upon the addition of C70 to the
solution of [4]CHBC in toluene at 258C (Figure 6b; see also
Figure S3a). On the basis of the quenching results, the binding
constant (Ka) in toluene was estimated to be approximately
1.07 ꢀ 106 mÀ1 (see Figure S3b). The 1:1 complexation
between [4]CHBC and C70 was also suggested by a UV/Vis
titration experiment (Figure 6c). A Job plot of the UV/Vis
spectra at 375 nm in toluene showed a maximum absorption
change when the ratio of [4]CHBC and C70 reached approx-
imately 1:1 (Figure 6d). We also tried to use C60 as the guest to
investigate the supramolecular properties of [4]CHBC, but no
obvious change was observed in the photophysical experi-
ments (see Figure S4), possibly because the sizes of [4]CHBC
and C60 are incompatible (see Figure S5).
In conclusion, the large p-extended molecular carbon
nanoring [4]CHBC was synthesized by a platinum-mediated
reductive-elimination reaction. The electronic absorption and
fluorescent behavior of [4]CHBC revealed its unique opto-
electronic properties. Furthermore, the tentative investiga-
tion of its interaction with C70 showed that a 1:1 complex can
be formed. [4]CHBC would be useful for the bottom-up
synthesis of structurally uniform carbon nanotubes. Further
challenges, including the design and synthesis of highly
conjugated carbon nanorings more similar to long CNT
segments and the production of size-specific CNTs are
subjects of research currently in progress in our laboratory.
Acknowledgements
This research was supported financially by the National
Natural Science Foundation of China (21271166, 21473170),
the Fundamental Research Funds for Central Universities
(WK2060140015, WK2060190026, WK3430000001), and the
Program for New Century Excellent Talents in University
(NCET).
Keywords: carbon nanorings · carbon nanotubes · fullerenes ·
host–guest systems · p conjugation
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Manuscript received: September 13, 2016
Final Article published: && &&, &&&&
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Communications
Host–Guest Systems
A decent slice of a carbon nanotube was
synthesized in the form of a large p-
extended carbon nanoring based only on
hexa-peri-hexabenzocoronene units (see
picture). The photophysical properties of
the nanoring were investigated by both
steady-state and time-resolved fluores-
cence spectroscopy, and a selective
supramolecular host–guest interaction
with C70 was identified.
D. Lu, G. Zhuang, H. Wu, S. Wang,
S. Yang, P. Du*
&&&& — &&&&
A Large p-Extended Carbon Nanoring
Based on Nanographene Units: Bottom-
Up Synthesis, Photophysical Properties,
and Selective Complexation with
Fullerene C70
6
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
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