Synthesis of end-cap precursor molecules for (6, 6) armchair and (9, 0) zig-zag single-walled carbon nanotubes

Article abstract of DOI:10.1016/j.tetlet.2010.04.055

We report the synthesis and purification of a C₆₀H₃₀ precursor molecule for the end-cap of a (6, 6) armchair and of a C₅₄H₂₄ precursor molecule for a (9, 0) zig-zag type single-walled nanotube. An approach to controlled growth of single-walled carbon nanotubes is suggested.

Full text of DOI:10.1016/j.tetlet.2010.04.055

Tetrahedron Letters 51 (2010) 3221–3225
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of end-cap precursor molecules for (6, 6) armchair and (9, 0)
zig-zag single-walled carbon nanotubes
*
Andreas Mueller, Konstantin Yu. Amsharov, Martin Jansen
Max Planck Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the synthesis and purification of a C60H30 precursor molecule for the end-cap of a (6, 6) arm-
chair and of a C54H24 precursor molecule for a (9, 0) zig-zag type single-walled nanotube. An approach to
controlled growth of single-walled carbon nanotubes is suggested.
Received 19 March 2010
Accepted 12 April 2010
Available online 24 April 2010
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
known as root-growth. Recently, other groups were able to observe
these two steps, the nucleation of a CNT end-cap and subsequent
Since the advent of carbon nanotubes (CNTs) in 1991,1 these
molecules have attracted a lot of interest in science. They can be
thought of as graphene sheets rolled up to seamless cylinders of
variable diameters and in principle infinite length. Depending on
the number of concentrically arranged tubes, CNTs are denomi-
nated as single-walled (SWCNT), double-walled (DWCNT), or mul-
ti-walled (MWCNT) carbon nanotubes. SWCNTs are either of
semiconducting or metallic type, depending on the orientation of
the hexagonal lattice relative to the tube axis, as classified by the
chiral indices (n, m).2 Their unique molecular structures and the
resulting extraordinary electronic, thermal, and mechanical prop-
erties make them promising candidates for future electronics and
composite materials. However, until now all preparation methods
result in a mix of different diameters and moreover of different chi-
ralities, thus of both semiconducting and metallic types, which is a
serious drawback for the fields of application mentioned. In order
to meet such a strong challenge it is logical to start with directing
the synthesis of CNTs toward the desired species. Herein lies one of
the big challenges of nanotube synthesis. Despite recent advances
in the metal-catalyzed synthesis of SWCNTs with specific struc-
tural characteristics,3–9 complete and rational control of the length,
diameter, and chirality has still not been achieved. Although there
are studies pointing out a dependence of a CNT’s diameter on the
particle size of the used catalytic particle, and thus a certain diam-
eter control can be achieved, it is still not possible to selectively
synthesize isomerically pure CNTs.9
growth in situ in an environmental TEM at temperatures of about
600 °C.13,14 Considering this growth mechanism of CNTs, it appears
to be attractive to avoid the usual nucleation step of CNTs leading
to the formation of an end-cap with accidental geometry by intro-
ducing a predefined end-cap molecule whose structure can be fully
controlled. Subsequent growth will lead to the desired SWCNT spe-
cies as determined by the end-cap geometry. Steps in this direction
were recently reported by Scott’s group by the synthesis of a pos-
sible precursor molecule for the formation of a (6, 6) armchair
nanotube which they aimed to cyclize by flash vacuum pyrolysis
(FVP).15 Although FVP gives reasonable yields of geodesic struc-
tures using precursors of low molecular weight, an increase in
mass is accompanied by a decreasing yield.16–20 Thus corannulene
(C20H12) was obtained with 40% yield and the buckybowl circumt-
rindene (C36H12) was obtained in 27% yield, whereas C60 fullerene
gave just 0.1–1% yield.21 Higher fullerenes were only detected by
mass spectrometry.22,23 Besides low yields, appropriate purifica-
tion which is needed for selective growth of SWCNTs, may pose an-
other problem, especially in case of non or low soluble end-cap
molecules.
The discovery of high efficiency catalytic dehydrogenation
opens new horizons in that direction. Otero et al. and Rim et al. re-
cently demonstrated methods for the cyclodehydrogenation of
polyaromatic hydrocarbons (PAH) to form geodesic polyarenes or
24,25
even C60
.
The former showed that C60 precursor molecules
can be condensed to C60 by catalytic cyclodehydrogenation on a
platinum surface while the latter used a ruthenium surface to con-
dense hexabenzocoronene into a buckybowl. This method for
cyclodehydrogenation of PAHs was reported to have a conversion
efficiency of 100%.
Taking into consideration that SWCNTs can be grown from thin
metal catalyst layers, including Pt-thin films,26 which are prede-
posited on a substrate, the scenario as schematized in Figure 1
appears appealing to us. In a first step the right precursor molecule
According to recent theoretical work by several groups investi-
gating the very first steps of CNT synthesis,10–12 growth of SWCNTs
starts by nucleation of an end-cap fragment on the catalyst particle
followed by subsequent growth through incorporation of carbon
atoms from the metal surface or bulk. This growth mechanism is
* Corresponding author. Tel.: +49 0 711 689 1501; fax: +49 0 711 689 1502.
E-mail address: m.jansen@fkf.mpg.de (M. Jansen).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.04.055

3222
A. Mueller et al. / Tetrahedron Letters 51 (2010) 3221–3225
has to be synthesized, which is subsequently deposited on top of
the substrate covered with the appropriate metal catalyst thin-
film, then converted to the end-cap, and finally this way predefined
SWCNTs can be grown. The use of catalytic dehydrogenation of the
precursor molecule directly on the catalytic material instead of
applying flash vacuum pyrolysis has, in our opinion, significant
advantages over the approach as suggested by Scott’s group. The
conversion yield to the end-cap is expected to be close to 100%.
It should also be pointed out that in order to achieve this high yield
there is no need for additional radical promoters on the precursor
molecule which are commonly used to increase the conversion
efficiency in FVP syntheses. This facilitates the synthesis of the pre-
cursor. Further using a flat molecule there is no risk of depositing
the end-cap upside-down on the substrate.
In this work we report the synthesis of two end-cap precursor
molecules for different types of SWCNTs, that is, C60H30 as a possi-
ble precursor for a (6, 6) SWCNT, which is of armchair type, and
C54H24 for a (9, 0) SWCNT, which is of zig-zag type (see Fig. 2). Both
precursor molecules would lead to metallic SWCNTs.
2. Results and discussion
The synthetic route of the (6, 6) precursor (P1) is presented in
Figure 3. 6-Methyl-benzo[c]phenanthrene was prepared according
to Ref. 27 by standard Wittig olefination of 2-acetonaphthone
resulting in the benzostilbene and subsequent Mallory photocycli-
zation using the Katz improvement.28,29 Benzylic bromination
with N-bromosuccinimide and subsequent treatment with
Figure 1. General scheme for the cyclodehydrogenation of the SWCNT end-cap precursor molecules and the subsequent growth of the CNT.
Figure 2. Precursor molecules P1 for a (6, 6) SWCNT end-cap of armchair type and P2 for a (9, 0) SWCNT end-cap of zig-zag type; on the right side the corresponding SWCNT
structures with the end-caps (gray shaded parts) are shown.

A. Mueller et al. / Tetrahedron Letters 51 (2010) 3221–3225
3223
Figure 3. Synthetic route to the C60H30 precursor P1.
aqueous
ethanolic
sodium
cyanide
gave
cya-
et al.31 As for the synthesis of the (6, 6) precursor, aldol trimeriza-
tion under Brønsted acid conditions was used to trimerize the ke-
tone into the desired C54H24 PAH. The product was purified by
nomethylbenzo[c]phenanthrene. The product was hydrolyzed into
acid and was cyclized in two steps by first transforming
benzo[c]phenanthrenylacetic acid into the acid chloride and finally
performing a Friedel–Crafts acylation leading to ring-closure. For
the last synthesis step to turn the monomer into the desired prod-
uct by aldol trimerization either titanium tetrachloride or Brønsted
acid conditions as described by Hill et al. were used.15 In both cases
a reasonable yield of 40% of the precursor P1 was obtained. Under
Brønsted acid conditions the trimerization was performed for dif-
ferent solvent/ketone molar ratios and reaction temperatures. At
a solvent/ketone molar ratio of 284:1 at 180 °C we obtained the tri-
mer as a yellow powder with high purity and good yield. Single
crystals of P1 suitable for X-ray analysis were obtained directly
during trimerization at 160 °C with a solvent/ketone molar ratio
of about 44:1. This allowed us to confirm the molecular structure
of the targeted molecule directly by using X-ray structural analysis
(Fig. 4). However, at these latter synthesis conditions the main
product was the dimer. This can be understood by looking at the
reaction mechanism according to which first the dimer forms
which then reacts with a monomer to the trimer.15,30 In the case
of a low molar ratio solvent/ketone the concentration of the dimer
produced is exceeding its solubility, starts to precipitate, and thus
does not continue to react to the desired trimer.
For the synthesis of the (9, 0) precursor P2 we used a photo-
chemical route presented in Figure 5 including only three synthetic
steps. 1-(Bromoacetyl)pyrene was obtained by a Friedel–Crafts
acylation of bromoacetyl bromide and pyrene and subsequently
cyclized by UV-radiation through a Pyrex filter according to Spijker
Figure 4. Molecular structure of the C60H30 precursor P1 as determined from
single-crystal X-ray diffraction data.

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A. Mueller et al. / Tetrahedron Letters 51 (2010) 3221–3225
In summary we have synthesized end-cap precursor molecules
for two types of metallic single-wall carbon nanotubes, one for a (6,
6) nanotube of armchair type and a second for a (9, 0) nanotube of
zig-zag type. Further we show that these molecules can be ob-
tained with high purity by applying high temperature sublimation.
The synthesized precursor molecules will be used for the proposed
SWCNT synthesis scenario.
3. Experimental section
Mass spectra were recorded with a Shimadzu AXIMA resonance
spectrometer in positive mode. The single crystal data were col-
lected with a dual wavelength diffractometer system using an
Incoatec microfocus X-ray source
I
lS
(Cu
Ka radiation,
k = 1.54178 Å). More details of the diffractometer setup can be
found elsewhere.32 The structure was solved by direct methods
and refined by fullmatrix least-squares fitting with the SHELXTL soft-
ware package.33 IR spectra were measured with a Bruker IFS 113 V
FT-IR spectrometer.
Experimental procedures for the synthetic pathways up to oxo-
6,7-acebenzo[c]phenanthrene and cyclopenta[cd]pyren-3(4H)-one
can be found in Supplementary data.
Figure 5. Synthetic route to the C54H24 precursor P2.
sublimation. The LDI-TOF mass spectrometry analysis gave almost
exclusively a signal at 672.26 m/z which strongly indicates the for-
mation of the desired trimer (Fig. 6).
3.1. Trimerization of oxo-6,7-acebenzo[c]phenanthrene (P1)
A
mixture of oxo-6,7-acebenzo[c]phenanthrene (20 mg,
For the purpose that the two molecules are intended, their pur-
ity is a very crucial issue. Both precursors were found to be non sol-
uble in common solvents and poorly soluble in 1,2,4-
trichlorobenzene or hot o-dichlorobenzene which prevented puri-
fication by chromatographic methods or recrystallization. Never-
theless it was found that high temperature sublimation (400 °C)
using rather low vacuum (10À2 mbar) of both compounds gave
the desired products in high purity as was confirmed by LDI-TOF
analysis (see Supplementary data Figs. S2 and S4). This is indicative
of a high thermostability of the desired products compared to the
byproducts like dimer, tetramer, or polymers.
0.075 mmol), p-toluene sulfonic acid monohydrate (54 mg,
0.284 mmol), propionic acid (0.02 ml), and o-dichlorobenzene
(2.4 ml) was transferred in a glass ampoule of 1 cm in diameter.
The glass ampoule was evacuated and sealed by melting while
the reaction mixture was frozen by liquid nitrogen to prevent the
solvent from evaporation. The mixture was heated at 180 °C for
16 h. After cooling down the glass ampoule a black liquid was
yielded and a yellow precipitate could be observed. The mixture
was poured into 50 ml of methanol. Addition of aqueous NaOH
solution resulted in a precipitate which was subsequently vacuum
filtrated and washed with methanol, H2O, acetone, CH2Cl2, and
Figure 6. LDI-TOF analysis of purified precursor P2 after trimerization of cyclopenta[cd]pyren-3(4H)-one, positive mode.

A. Mueller et al. / Tetrahedron Letters 51 (2010) 3221–3225
3225
petroleumether resulting in a yellow powder (8 mg, 40% yield).
References and notes
Further purification was done by sublimation at 400 °C and 10À2
mbar giving a yellow sublimate. LDI-TOF MS: m/z = 750.31 [M]+
(Exact Mass: 750.2348). UV–vis: kmax (1,2,4-trichlorobenzene)/
[nm] = 330, 382, 406, 431. Crystal data: monoclinic space group
C2/c, a = 33.444(3), b = 9.5755(9), c = 27.512(2) Å, b = 119.131(4)°,
V = 7696.1(12) Å3, Z = 8 2hmax = 130.01°, À39 < h < 37, À9 < k < 10,
À31 < l < 32, k = 1.54178 Å, T = 100(2) K, final R1 = 0.0608
(R2(w) = 0.1809). CCDC-761361 contains the supplementary crys-
tallographic data for this Letter. These data can be obtained free
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