Debundling, selection and release of SWNTs using fluorene-based photocleavable polymers

Article abstract of DOI:10.1039/c1cc11400j

Photocleavable polymers based on 9,9-dialkylfluorene backbone and o-nitrobenzylether were designed and synthesized to obtain stable (n,m) enriched suspensions of semiconducting SWNTs in toluene. Photoirradiation of the suspensions triggered the precipitat

Full text of DOI:10.1039/c1cc11400j

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COMMUNICATION
Debundling, selection and release of SWNTs using fluorene-based
photocleavable polymersw
Fabien Lemasson,a Jana Tittmann,a Frank Hennrich,a Ninette Stu
¨
rzl,a Sharali Malik,a
Manfred M. Kappes*abc and Marcel Mayor*acd
Received 10th March 2011, Accepted 28th April 2011
DOI: 10.1039/c1cc11400j
Photocleavable polymers based on 9,9-dialkylfluorene backbone
and o-nitrobenzylether were designed and synthesized to obtain
stable (n,m) enriched suspensions of semiconducting SWNTs in
toluene. Photoirradiation of the suspensions triggered the precipi-
tation of the SWNTs and TEM images indicate close packing of
SWNTs pointing at partial removal of the coating polymer.
removal of the polymer is troublesome, probably due to the
wrapping of the polymer arround the SWNT being supported
by extensive van der Waals and/or p–p stacking interactions.7
Embedding a photocleavable moiety in the polymer main-chain
would allow us to shorten the polymer length by irradiation and
thus, to render its displacement. Photocleavable subunits in
polymer main-chains have so far been used as linkers in diblock
copolymers11 and star polymers.12 To the best of our knowledge,
this is the first example of an alternating copolymer displaying a
photocleavable moiety in the polymer main-chain.
Single-walled carbon nanotubes (SWNTs) exhibit unique
physical and electronic properties which make them excellent
candidates for micro- and optoelectronics1 as well as for
nanocomposites.2 Nevertheless, limitations in their use arise
from their non-selective synthesis providing a mixture of
semiconducting and metallic tubes with many different (n,m)
chiral indices. Thus, several methods for SWNTs sorting were
developed.3 In particular those which make use of non-covalent
interactions are of interest due to the preservation of the
SWNT intrinsic physical properties.4–7 Among these methods,
polymer wrapping has already demonstrated promising results
and is of great potential due to almost unlimited structural
variation.7 While several polymers displaying various selectivities
for particular SWNTs were reported, their subsequent removal
from the SWNTs’ surface remains a major challenge.8 Recently,
Imahori and coworkers addressed this issue with an azobenzene
based polymer which solubilizes and releases SWNTs bundles
upon photoisomerization.9 Moore, Zang and coworkers reported
the reversible solubilization and release of SWNTs using a
foldamer.10 However, both approaches were not selective for
particular (n,m) SWNT species. Here we report the first example
of debundling, sorting and release of individual SWNT using
fluorene-based photocleavable polymers.
The investigated polymers 6a–e, depicted in Scheme 1, are
composed of fluorene subunits as recognition sites to interact
exclusively with specific (n,m) SWNTs and of an o-nitro-
benzylether as a photocleavable linker. The photocleavable
dibromide 313 was synthesized in 2 steps from aldehyde 1 by
sodium borohydride reduction followed by a Mitsunobu reaction
Poly(9,9-dialkyl)fluorenes are known to suspend efficiently
only a few (n,m) SWNTs in organic media.7a The subsequent
a Karlsruhe Institute of Technology (KIT), Institute for
Nanotechnology, P.O. Box 3640, D-76021 Karlsruhe, Germany
b Institut fur Physikalische Chemie, Karlsruher Institut fur
¨
¨
Technologie, D-76128 Karlsruhe, Germany.
E-mail: manfred.kappes@kit.edu
c DFG Center for Functional Nanostructures, 76028 Karlsruhe,
Germany
Scheme 1 Synthesis of the polymers 6a–e and their building blocks.
Reagents and conditions: (a) NaBH4, THF, 0 1C, 80%; (b) 4-bromo-
phenol, PPh3, DIAD, THF, 0 1C to rt, 68%; (c) PdCl2(dppf), (Bpin)2,
KOAc, dioxane, 80 1C; (d) Pd(PPh3)4, toluene/1 M Na2CO3, 80 1C;
pin: pinacol, DIAD: diisopropyl azodicarboxylate, dppf:
1,10-bis(diphenylphosphino)ferrocene.
d University of Basel, Department of Chemistry, St. Johanns-Ring 19,
CH-4056 Basel, Switzerland. E-mail: marcel.mayor@unibas.ch
w Electronic supplementary information (ESI) available: Synthetic
procedures and characterization of new compounds, additional
TEM images, pictures and UV spectra. See DOI: 10.1039/c1cc11400j
c
7428 Chem. Commun., 2011, 47, 7428–7430
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D2O.16 In contrary, suspension 7c, prepared from polymer 6c
with 4-bromophenol. Its reaction with diboronic esters 5a14
and 5b in a Suzuki coupling led to polymers 6b and 6c,
respectively. Polymers 6d and 6e were synthesized using a
similar coupling protocol but by complementing the
dibromide content with 2,7-dibromofluorene 4a in order to
reach a final stoichiometry fluorene/photocleavable group of
3/1 and 7/1 for polymers 6d and 6e, respectively. Thus, the
o-nitrobenzene moiety was randomly installed in the polymer
chain of 6d and 6e. Also, all of the polymers 6b–e lack
regioselectivity with respect to the orientation of the photo-
cleavable subunit. Polyfluorene 6a was synthesized by Suzuki
coupling of dibromide 4a and diboronic ester 5a. Polymers
6a–e were characterized by NMR spectroscopy and sized
exclusion chromatography (ESIw).
The SWNTs suspensions 7a–e, obtained with polymers
6a–e, respectively, were prepared by ultrasonicating B1 mg
of as prepared HiPco SWNTs and B50 mg of the desired
polymer in 15 mL toluene with a titanium sonotrode.w The
suspension was filtered to remove large agglomerates and
further purified using density gradient centrifugation (DGC).
It is noteworthy that only DGC provides a complete picture of
the SWNT/polymer suspension as SWNT/polymer complexes
with densities above those of the solvent get lost during
‘‘classical’’ centrifugation. Furthermore, the excess polymer
is removed during DGC.w Stable suspensions 7b–e were
obtained. Obviously, the photocleavable moiety installed in
the polyfluorene backbone does not impair the debundling and
solubilisation processes. Suspensions 7a–e were analyzed by
photoluminescence excitation (PLE) spectroscopy (Fig. 1).
Despite a hypsochromic shift of the signals, the assignment
of each semiconducting (n,m) species is possible.15 The PLE
map of suspension 7b indicates the presence of numerous
different (n,m) SWNTs, reflecting roughly the composition
observed with sodium cholate stabilized HiPco SWNTs in
in which three consecutive fluorene units are present, displayed
a drastic increase in the selectivity, reaching almost that of
polymer 6a. Suspensions 7d and 7e also showed impressive
selectivities despite the randomly installed o-nitrobenzylether
moiety. Based on these results we concluded that subunits of
several adjacent fluorene moieties are required in order to
observe selectivity for particular (n,m) species.
The absorption spectra of suspensions 7b and 7c were recorded
(ESI: Fig. S4).w Intense absorption features of semiconducting
SWNTs (l > 600 nm) were observed for both suspensions.
While minor traces of metallic HiPco SWNTs (300 nm > l >
600 nm) were observed for 7b, 7c disperses exclusively semicon-
ducting SWNTs according to its UV spectrum (Fig. S4, ESIw).
Photoirradiation of suspension 7b for approximately 2 min
with a xenon lamp caused precipitation of SWNTs (ESIw).
Simple heating of the sample did not cause precipitation of
SWNTs corroborating the light induced cleaving of the poly-
mer backbone as the SWNT releasing mechanism. The
SWNTs were collected by centrifugation and analyzed by
Raman spectroscopy.w The low ratio between the defect
induced D band and the tangential G band (D/G band ratio,
Fig. S3, ESIw) indicates that neither defect formation nor side-
wall functionalization of the SWNT occurs during the photo-
cleavage of the o-nitro-benzylether moiety, despite the radical
nature of the process.17 Also the SWNTs with particular (n,m)
indices selected by dispersing with 6c precipitate upon irradiating
7c for 2 minutes (Fig. 2). While increasing the fluorene amount
in the polymers increased its selectivity for particular (n,m)
species, it slowed down the rate of precipitation upon irradiation.
Suspension 7d started to precipitate only 12 hours after
irradiation, and no precipitation at all was observed for
suspension 7e. According to these results, we assume that
over a critical length, the oligofluorenes stemming from the
Fig. 1 PLE maps in toluene of (a) suspension 7a; (b) suspension 7b; (c) suspension 7c; (d) suspension 7d; (e) suspension 7e; (f) HiPco SWNTs
suspended with sodium cholate in D2O.
c
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consecutive fluorene subunits were present in the backbone. The
release of SWNTs after photoirradiation can be varied from
seconds to hours depending on the place and amount of photo-
cleavable moiety in the polymer backbone. To the best of our
knowledge, this is the first study involving controlled precipitation
of solubilized (n,m) enriched semiconducting SWNTs.
We are currently investigating the role of the polymer length
for its dispersing features and we are screening potential
recognition templates for SWNTs.
We thank Peter Gerstel and Christopher Barner-Kowollik
for SEC measurements. We also acknowledge ongoing generous
support by the DFG Center for Functional Nanostructures
(CFN), by the Helmholtz Programme POF NanoMikro,
by the Karlruhe Institute of Technology (KIT) and by the
University of Basel.
Fig. 2 Suspension 7c (a) before and (b) after 2 min irradiation
showing the precipitation of the enriched (n,m) SWNTs.
polymer photodegradation are still able to stabilize the SWNTs.
In a hypothetical full cleavage of all o-nitrobenzylether subunits,
the polymer 6c would release trimers, which are not long enough
to stabilize SWNTs. The polymer 6d on the other hand releases
longer oligomers also, which presumably are able to stabilize
SWNTs. It is worth noting that all these suspensions were stable
for at least 2 months in the dark.
Notes and references
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The precipitated (n,m) enriched SWNTs from suspensions
7c and 7d were collected by centrifugation. TEM images of
SWNTs from suspension 7d displayed residual polymer coating
wrapped arround the tube (Fig. S8, ESIw). The SWNTs from
suspension 7c were washed several times with toluene using a
mild sonication bath. The TEM images in Fig. 3 show bundles
of SWNTs due to their strong p–p stacking interactions. The
close contact between the tubes points at least to partial removal
of the polymer at their surfaces.
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In conclusion, polymers composed of 9,9-dialkylfluorenes as
a SWNT selecting template and o-nitrobenzylether as a photo-
cleavable linker were designed, synthesized and their SWNT
dispersing features were analyzed. Copolymers of both subunits
with HiPco SWNT selecting properties comparable with
poly(9,9-dialkyl)fluorenes were obtained as soon as at least three
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Fig. 3 TEM picture with enhancements of precipitated (n,m) enriched
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7430 Chem. Commun., 2011, 47, 7428–7430
This journal is The Royal Society of Chemistry 2011