Brominated single walled carbon nanotubes as versatile precursors for covalent sidewall functionalization

Article abstract of DOI:10.1039/c4cc00719k

Herein we report on the facile preparation of brominated SWCNTs based on two complementary reductive activation routes. The respective brominated SWCNTs are highly reactive and can be used in nucleophilic substitution reactions and represent versatile starting materials for the generation of sidewall functionalized SWCNTs with a high density of functional moieties. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc00719k

Showcasing research in Andreas Hirsch’s Laboratory,
Department of Chemistry and Pharmacy,
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Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany
Brominated single walled carbon nanotubes as versatile
precursors for covalent sidewall functionalization
The facile preparation of brominated SWCNTs based on two
complementary reductive activation routes is reported. The
brominated SWCNTs are highly reactive and can be used in
nucleophilic substitution reactions and represent versatile starting
materials for the generation of sidewall functionalized SWCNTs
with a high density of functional moieties.
See Andreas Hirsch et al.,
Chem. Commun., 2014, 50, 6582.
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Brominated single walled carbon nanotubes
as versatile precursors for covalent sidewall
functionalization†
Cite this: Chem. Commun., 2014,
50, 6582
Received 27th January 2014,
Accepted 4th March 2014
Ferdinand Hof, Frank Hauke and Andreas Hirsch*
DOI: 10.1039/c4cc00719k
www.rsc.org/chemcomm
Herein we report on the facile preparation of brominated SWCNTs addition. In principle, other halogenated carbon nanotubes,
based on two complementary reductive activation routes. The like brominated20,21 or iodinated22 SWCNTs, are also accessible
respective brominated SWCNTs are highly reactive and can be used but these systems have not been used for subsequent derivati-
in nucleophilic substitution reactions and represent versatile starting zation reactions.
materials for the generation of sidewall functionalized SWCNTs with
a high density of functional moieties.
Here, we report on the synthesis and unambiguous character-
ization of brominated SWNCTs in combination with a subsequent
nucleophilic substitution approach (Scheme 1). The key step of this
In the last decade, chemistry of single walled carbon nanotubes straightforward synthetic approach was the use of reduced
(SWCNTs) has attracted a lot of interest with respect to covalent SWCNTnÀ as highly reactive intermediates. The reductive activation
framework functionalization and functional group conversion.1,2 of the SWCNT (HiPco) starting material was achieved by two
In this context, remarkable progress has been made, both in the different approaches: (a) molten potassium metal based reduction,
sense of the exploration of the chemical reactivity of SWCNTs3–6 as leading to the formation of the species 1a5 or (b) Birch type
well as in terms of the analytical product characterization.7,8 reduction12,14,23 with potassium dissolved in liquid ammonia,
Nevertheless, the establishment of highly functional organic yielding species 1b.12 This initial reduction step is a fundamental
moieties, covalently connected with the carbon nanotube frame- prerequisite which leads to an efficient SWCNT individualization
work, still remains challenging and is primarily based on stepwise and charge induced reactivity increase for the subsequent bromine
reaction pathways, utilizing carbon nanotube derivatives with addition. The respective brominated SWCNTs 2a, 2b represent
covalently attached anchor groups, e.g. hydroxyl functionalities.9 highly reactive species. We have found that the addition of water
Due to the low intrinsic reactivity of SWCNTs relatively harsh leads to an efficient nucleophilic substitution of the covalently
reaction conditions are needed for covalent framework addition bound sidewall bromine atoms and to the formation of hydroxyl-
reactions. As a consequence, successfully attached functional ated SWCNT derivatives. Because of this pronounced reactivity a
entities are limited to carboxyl groups,10,11 phenyl-12,13 or alkyl detailed Raman spectroscopic characterization of the brominated
addends14 or hydroxyl functionalities.15 In contrast to addition SWCNTs 2a, 2b has been carried out under inert gas conditions,
reactions, nucleophilic SWCNT substitution reactions have hardly following a procedure that we have recently published.5 In order to
been investigated and are solely based on fluorinated carbon gain a statistically significant bulk information about the success of
nanotube precursors.16–18 For instance, Stevens et al. have shown the covalent functionalization of the reductively activated SWCNT
that fluorine substituents on SWCNTs can be substituted by alkyl intermediates with bromine 2a, 2b, Scanning Raman Microscopy
groups from Grignard and alkyllithium reagents.19 The main (SRM) was performed and the respective I(D/G) (intensity ratios of
disadvantage of using fluorinated carbon nanotubes as precursors the D- and G-bands) values of the recorded 625 individual Raman
in SWCNT substitution reactions is their relatively harsh formation spectra were plotted as a distribution function (Fig. 1).
conditions, which do not allow easy control of the degree of
In general, the D-band intensity is a measure of the amount
of sp3-defects introduced by covalent addend binding. As it can
be clearly seen from Fig. 1, sidewall brominated SWCNTs are
accessible by both reaction pathways, with a mean I(D/G) ratio of
about 0.5 by solid state reduction with potassium and 0.4 in the
case of Birch type reduction. The corresponding distribution
functions for lexc = 532 nm and lexc = 785 nm are depicted in
Fig. S1 (ESI†). Use of additional excitation energies and organizing
Department of Chemistry and Pharmacy and Institute of Advanced Materials and
¨
¨
Processes (ZMP), Friedrich-Alexander-Universitat Erlangen-Nurnberg (FAU),
Henkestrasse 42, 91054 Erlangen, Germany. E-mail: andreas.hirsch@fau.de;
Fax: +49 9131-8526864; Tel: +49 9131-8522537
† Electronic supplementary information (ESI) available: Experimental details
regarding the synthesis of different SWCNT derivatives, additional Raman
analysis and TG/MS data. See DOI: 10.1039/c4cc00719k
6582 | Chem. Commun., 2014, 50, 6582--6584
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Scheme 1 Sidewall alkoxide functionalization of SWCNTs via (i) reduction with molten potassium (pathway a) or Birch type reduction (pathway b) to the
corresponding carbon nanotubes 1a and 1b followed by (ii) bromination to the synthetic precursor building blocks 2a and 2b and (iii) subsequent
nucleophilic substitution with NaOCH2CF3 yielding alkoxides 3a and 3b, respectively.
Fig. 3 TG/MS profiles of brominated SWCNTs 2b (left) and hydroxylated
SWCNTs 4b (right).
insight into the introduction of sp3-defects, but does not lead to
any information about the chemical nature of the bound
Fig. 1 Statistical Raman analysis of the brominated SWCNT adducts 2a
and 2b; lexc = 633 nm. Inset: median spectra.
addends. For this purpose, the brominated SWCNT derivative
2b was analysed by means of thermogravimetric analysis coupled
with mass spectrometry (TG/MS) – (Fig. 3 – left). Here, a sharp
mass loss (Dwt = 71% – corrected by mass loss of the pristine
starting material – ESI,† Fig. S3) is detected in the region between
250 and 350 1C and can be traced back to the detachment of
bromine from the mass fragments m/z 79, 81 (m/z 80, 82 [HBr])
based on its characteristic isotopic distribution (1 : 1). For 2b, a
degree of functionalization of about 5% can be calculated. The
covalently bound bromine addends can easily and quantitatively
be substituted by the addition of water (Scheme 2).
This reaction sequence provides a fast access route to
hydroxylated SWCNT derivatives 4b. The detached bromide
ions were identified in the aqueous phase by precipitating as silver
bromide. The TG/MS analysis of 4b (Fig. 3 – right) corroborated the
substitution of the bromo addends. The introduced hydroxyl groups
can be identified by their mass fragments m/z 17, 18 (further mass
traces see Fig. S4, ESI†) and a sample mass loss of about 8% (after
Fig. 2 2-D Raman index plot based on the statistical Raman analysis of
the samples 2a, 2b, 3a, 3b at lexc = 532 nm and lexc = 785 nm. – For
detailed data see Table T1 – ESI.†
the measured data in a 2D-diagram (Fig. 2) provide valuable correction – see Fig. S3, ESI†) is detected – no remaining bromine
information about the homogeneity distribution (Raman was found for 4b. Based on these data, the degree of functionaliza-
Homogeneity Index (RHI)) and the electronic type selectivity tion before and after hydrolysis is determined to be about 5% and it
(Raman Selectivity Index (RSI)) of the initial addition reaction
(see also Table T1 – ESI†).8
In contrast to pathway (a), where no selectivity is observed
(RSI = 1.91), the Birch type based addition sequence (b) exhibits
a RSI value of 12.31, indicative of a selectivity with respect to
semiconducting SWCNTs. This kind of electronic type selectivity
has also been observed with other Birch type based functionaliza-
tion sequences.11 The Raman based analysis provides a direct
Scheme 2 Hydrolysis of brominated SWCNT intermediates, yielding side-
wall hydroxylated SWCNT derivatives 4b.
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and quantitatively substituted by water or alcoholates via a
nucleophilic substitution pathway. This process does not lead to
an additional introduction of framework sp3 centers, which is
confirmed by Scanning Raman Microscopy (SRM). In the case of
the Birch type reduction sequence, an electronic type selective
framework functionalization with respect to semiconducting
species is observed. Based on their easy access and high reactivity,
brominated SWCNTs can serve as a versatile starting material for
the generation of multifunctional SWCNT architectures for future
applications. Research with respect to this goal is currently being
pursued in our laboratory.
Fig. 4 TG/MS profiles of final substitution products 3a (left) and 3b (right).
remains practically unchanged which is indicative of a quantitative
substitution of the bromine atoms.
The authors thank the Deutsche Forschungsgemeinschaft
(DFG – SFB 953, Project A1 ‘‘Synthetic Carbon Allotropes’’) and
the Interdisciplinary Center for Molecular Materials (ICMM) for
the financial support.
Hydroxylated carbon nanotubes 4b may serve as highly
functionalized starting materials in subsequent coupling reactions
with carboxylic acid derivatives; alternatively, the functional entity
can directly be brought in by the reaction of brominated SWCNTs
with functional alcoholates. The feasibility of this concept has been
exemplarily proven by the reaction of 2a and 2b with 2,2,2-trifluoro
ethanolate (Scheme 1). The trifluoro ethanol group serves as a
suitable marker for the TG/MS analysis (Fig. 4). In the main mass
loss region (100–400 1C, 9% corrected mass loss) all characteristic
mass fragments of the trifluoro ethanol group were detected: m/z
69 –CF3, m/z 70 HCF3, m/z 99 F3CCH2O–, m/z 100 F3CCH2OH.
Again, in both substitution products 3a and 3b no traces of
bromine were detected by TG/MS, indicative of a quantitative
substitution of the initially bound bromo addends.
This fact is furthermore corroborated by statistical Raman
analysis of the final substitution products 3a and 3b. Here, it can
be shown that the initial density of defects, present in the respective
brominated SWCNT derivatives 2a and 2b, is virtually not increased
after the addition of 2,2,2-trifluoro ethanolate (Fig. 2). Therefore, no
additional carbon lattice sp3 centers are introduced during this part
Notes and references
1 S. A. Hodge, M. K. Bayazit, K. S. Coleman and M. S. P. Shaffer, Chem.
Soc. Rev., 2012, 41, 4409–4429.
2 V. Georgakilas, K. Kordatos, M. Prato, D. M. Guldi, M. Holzinger and
A. Hirsch, J. Am. Chem. Soc., 2002, 124, 760–761.
´
´
3 Z. Syrgiannis, V. La Parola, C. Hadad, M. Lucıo, E. Vazquez,
F. Giacalone and M. Prato, Angew. Chem., Int. Ed., 2013, 52, 6480–6483.
4 B. Vanhorenbeke, C. Vriamont, F. Pennetreau, M. Devillers, O. Riant
and S. Hermans, Chem.–Eur. J., 2013, 19, 852–856.
5 F. Hof, S. Bosch, S. Eigler, F. Hauke and A. Hirsch, J. Am. Chem. Soc.,
2013, 135, 18385–18395.
6 C. Zhao, Y. Song, K. Qu, J. Ren and X. Qu, Chem. Mater., 2010, 22,
5718–5724.
7 M. S. Dresselhaus, A. Jorio, M. Hofmann, G. Dresselhaus and
R. Saito, Nano Lett., 2010, 10, 751–758.
8 F. Hof, S. Bosch, J. M. Englert, F. Hauke and A. Hirsch, Angew.
Chem., Int. Ed., 2012, 51, 11727–11730.
9 T. Palacin, H. L. Khanh, B. Jousselme, P. Jegou, A. Filoramo, C. Ehli,
D. M. Guldi and S. Campidelli, J. Am. Chem. Soc., 2009, 131,
15394–15402.
of the functionalization sequence. Moreover, as can be clearly seen 10 H. Yu, Y. Jin, F. Peng, H. Wang and J. Yang, J. Phys. Chem. C, 2008,
112, 6758–6763.
in the 2D-Raman index plot, the initial electronic type discrimina-
tion with respect to semiconducting SWCNT species (represented
11 B. Gebhardt, F. Hof, C. Backes, M. Mu¨ller, T. Plocke, J. Maultzsch,
C. Thomsen, F. Hauke and A. Hirsch, J. Am. Chem. Soc., 2011, 133,
by the RSI values – Table T1, ESI†) remains unchanged by the
subsequent substitution reaction. In a reference experiment we have
also investigated whether the observed data can be traced back to a
19459–19473.
12 Y. Ying, R. K. Saini, F. Liang, A. K. Sadana and W. E. Billups, Org.
Lett., 2003, 5, 1471–1473.
13 C. A. Dyke and J. M. Tour, J. Phys. Chem. A, 2004, 108, 11151–11159.
direct nucleophilic sidewall addition of 2,2,2-trifluoro ethanolate 14 F. Liang, A. K. Sadana, A. Peera, J. Chattopadhyay, Z. Gu,
R. H. Hauge and W. E. Billups, Nano Lett., 2004, 4, 1257–1260.
15 B. Gebhardt, Z. Syrgiannis, C. Backes, R. Graupner, F. Hauke and
(Scheme S1, ESI†). Therefore, pristine HiPco SWCNTs were reacted
with the alcoholate under the same reaction conditions as those
A. Hirsch, J. Am. Chem. Soc., 2011, 133, 7985–7995.
used for the bromo substitution sequence. The respective Raman 16 L. Valentini, J. Macan, I. Armentano, F. Mengoni and J. M. Kenny,
Carbon, 2006, 44, 2196–2201.
17 L. Zhang, V. U. Kiny, H. Peng, J. Zhu, R. F. M. Lobo, J. L. Margrave
spectra and TG/MS data (Fig. S5, ESI†) unequivocally show that
attachment of 2,2,2-trifluoro ethanolate does not take place. This
and V. N. Khabashesku, Chem. Mater., 2004, 16, 2055–2061.
clearly demonstrates that a nucleophilic substitution mecha- 18 M. X. Pulikkathara, O. V. Kuznetsov and V. N. Khabashesku, Chem.
Mater., 2008, 20, 2685–2695.
19 J. L. Stevens, A. Y. Huang, H. Peng, I. W. Chiang, V. N. Khabashesku
nism of brominated SWCNTs is responsible for the formation
of the alkoxylated tubes 3a, 3b.
and J. L. Margrave, Nano Lett., 2003, 3, 331–336.
Brominated carbon nanotubes, accessible by a direct reductive 20 J. F. Colomer, R. Marega, H. Traboulsi, M. Meneghetti, G. Van
Tendeloo and D. Bonifazi, Chem. Mater., 2009, 21, 4747–4749.
21 W. Z. Wang, A. S. Mahasin, P. Q. Gao, K. H. Lim and M. B. Chan-
bromination sequence, serve as versatile building blocks for the
facile generation of sidewall functionalized SWCNT derivatives.
Park, J. Phys. Chem. C, 2012, 116, 23027–23035.
Two complementary routes were used for the synthesis of these 22 K. S. Coleman, A. K. Chakraborty, S. R. Bailey, J. Sloan and
M. Alexander, Chem. Mater., 2007, 19, 1076–1081.
23 J. Chattopadhyay, S. Chakraborty, A. Mukherjee, R. Wang,
highly reactive intermediates: (a) solid state reduction of SWCNTs
with potassium and (b) Birch type reduction with a subsequent
P. S. Engel and W. E. Billups, J. Phys. Chem. C, 2007, 111,
reaction with bromine, respectively. The bromo addends are easily
17928–17932.
6584 | Chem. Commun., 2014, 50, 6582--6584
This journal is ©The Royal Society of Chemistry 2014