Electrostatically-driven assembly of MWCNTs with a europium complex

Article abstract of DOI:10.1039/c0cc02874f

Luminescent carbon-based materials have been prepared by electrostatic self-assembly of negatively-charged luminescent Eu(iii)-complex with imidazolium-functionalized MWCNTs.

Full text of DOI:10.1039/c0cc02874f

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Electrostatically-driven assembly of MWCNTs with a europium
complexwz
Laura Maggini,a Hassan Traboulsi,a K. Yoosaf,b John Mohanraj,b Johan Wouters,a
Oksana Pietraszkiewicz,c Marek Pietraszkiewicz,c Nicola Armaroli*b and Davide Bonifazi*ad
Received 28th July 2010, Accepted 19th October 2010
DOI: 10.1039/c0cc02874f
Luminescent carbon-based materials have been prepared by
electrostatic self-assembly of negatively-charged luminescent
Eu(III)-complex with imidazolium-functionalized MWCNTs.
complexes are good candidates since they can be electrostatically
coupled to ionic liquids (ILs) as shown by X-ray crystallography.11
This has been further confirmed by recent independent reports by
¸
the groups of Binnemans12 and Goncalves,13 who have confined a
tetrakis(2-naphtoyltrifluoro-acetonato)Eu(III) complex ([EuL4]ꢀ)
on silica supports through coulombic interactions, showing
improved luminescence performance.14
Lanthanide(III) b-diketonate complexes (LnCs) are of great
interest for a wide range of photonic applications1 as they exhibit
high quantum efficiency, narrow emission bands, and high color
purity.2 However, their low thermal and photochemical
stability,3 together with their poor mechanical and conducting
properties and tendency to aggregate, represent important
limitations toward their full exploitation in practical applications.
In order to overcome these drawbacks, LnCs have been dispersed
into material-like composites such as polymers,4 liquid crystals5
or sol–gel6 derived organic–inorganic hybrids. Among others, a
promising class of compounds which has been recently blended
with LnCs are carbon nanotubes (CNTs).7 The appeal of this
approach springs from the possibility of combining the lumines-
cent features of LnCs with the exceptional electronic and
mechanical properties of CNTs.8 In this regard, in a previous
work we have reported a simple method to prepare luminescent
CNT composites by in situ adsorption of a LnC onto oxidized
SWCNTs via hydrophobic interaction from a MeOH solution.9
Despite the very easy and straightforward preparation, the
desorption of the LnC and the difficulty to control the loading
of the complex turned out to be the main limitations toward the
technological exploitation of these composites. However, this
work opened the way for the preparation and spectroscopic
characterization of a new class of luminescent materials in which
the emissive species are non-covalently adsorbed on the CNTs’
outer surface through strong interactions. Inspired by the
coulombic-directed layer-by-layer methodology,10 we have
envisaged to assemble negatively-charged LnCs onto cationic
MWCNTs. Among the different possibilities, anionic Eu(III)
In this communication, we report on the covalent and non-
covalent functionalization of MWCNTs with imidazolium-
based15 ILs (1ꢁBr), bearing a terminal azido function, which
can undergo selective adsorption (Fig. 1) with a negatively-
charged europium complex, through anion exchange reaction
with
a
precursor [EuL4]ꢁNEt4 complex. The synthetic
pathways to obtain the luminescent materials are shown in
Scheme 1. In a typical experiment, pristine MWCNTs
(p-MWCNT) were first dispersed in a solution of 1ꢁBr (20-fold
excess) in mesitylene by ultrasonication. The mixture was
heated at 170 1C to obtain a homogeneous solution of 1ꢁBr.
After 1 h no covalent functionalization of the MWCNTs was
observed (Fig. S9-B, ESIz) and, notably, a uniform IL-coated
material (MWCNT-1ꢁBr) was instead obtained. Prolongation
of the reaction to 14 h in the same conditions finally allowed
the covalent grafting of the N3-terminating ILs trough 1,3-
dipolar cycloaddition reaction, leading to MWCNT-2ꢁBr.
Cycles of sonication in DMF were necessary to remove
unbounded 1ꢁBr (Fig. S9-A, ESIz). In order to perform the
anion exchange reaction, [EuL4]ꢁNEt4 (10-fold excess) was
added to a dispersion of IL-bearing carbon structure in
toluene and stirred for 14 h at rt. To elucidate the chemical
a Department of Chemistry, University of Namur (FUNDP),
Rue de Bruxelles 61, B-5000, Namur, Belgium.
E-mail: lmaggini@fundp.ac.be, davide.bonifazi@fundp.ac.be;
Fax: +32 81 725433
b Molecular Photoscience Group, Istituto per la Sintesi Organica e la
`
Fotoreattivita, Consiglio Nazionale delle Ricerche (CNR-ISOF),
Via Gobetti 101, 40129 Bologna, Italy.
E-mail: nicola.armaroli@isof.cnr.it; Fax: +39 051 6399844
c Institute of Physical Chemistry, Polish Academy of Sciences,
Kasprzaka 44/52, 01-224 Warsaw, Poland. Fax: +48 22 3433333
d Dipartimento di Scienze Farmaceutiche, Universita` degli Studi di
Trieste, Piazzale Europa 1, 34127 Trieste, Italy
w This article is part of the Carbon Nanostructures web-theme issue
for ChemComm.
z Electronic supplementary information (ESI) available: Experimental
details, X-ray analysis, characterization details, XPS and luminescence
characterization. CCDC 787011. See DOI: 10.1039/c0cc02874f
Fig. 1 Representation of the electrostatically-driven self-assembly.
Inset: X-ray structure of precursor [EuL4]ꢁNEt4 complex (for details
see ESIz).
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 1625–1627 1625

external F-containing impurities.16 No Br-centered resonances
have been detected, evidencing the effectiveness of the anion
exchange reaction. The degree of functionalization of the
materials has been also investigated via thermogravimetric
analysis (TGA) in air (Fig. S3, ESIz). While for MWCNT-2ꢁBr
and MWCNT-2ꢁ[EuL4] no defined weight loss has been observed
due to the low degree of functionalization, in the case of
MWCNT-1ꢁBr and MWCNT-1ꢁ[EuL4], the TGA curves show
a sharp weight loss of about 80% and 60% at 600 1C, respec-
tively, as compared to the p-MWCNTs. The apparent reduced
weight loss for MWCNT-1ꢁ[EuL4] as compared to MWCNT-1ꢁBr
is caused by the presence of Eu(III) atoms, which undergo an
oxidation reaction during the combustion, leading to oxides.
In order to prove the electrostatic interaction in solution
between functionalized MWCNT-1ꢁBr and [EuL4]ꢁNEt4, lumi-
nescence titrations were carried out with the model compound
C60-1ꢁBr.17 Upon adding aliquot amounts of [EuL4]ꢁNEt4 to a
2.3 ꢂ 10ꢀ5 M CH2Cl2 solution of C60-1ꢁBr, first an increase
(up to 0.8 equivalents) and then a decrease of Eu(III)-centered
emission is observed (Fig. 2). Quite similar observations were
obtained for a blank titration experiment carried out by
increasing the concentration of [EuL4]ꢁNEt4 alone in CH2Cl2,
in the absence of C60-1ꢁBr (Fig. S4, ESIz), indicating that the
weird emission intensity trend observed is only due to con-
centration-related optical artifacts.18 Interestingly, even a
single addition of [EuL4]ꢁNEt4 (0.1 eq.) to C60-1ꢁBr resulted
in quenching of the excited state lifetime of [EuL4]ꢁNEt4 from
0.53 ms to 0.20 ms. Futher addition of [EuL4]ꢁNEt4 did not
result in any appreciable changes of the lifetime (Fig. S5,
ESIz). On the contrary, titration experiments carried out by
Scheme 1 Synthetic scheme for the preparation of MWCNT-1,2ꢁ[EuL4]
and C60-1ꢁ[EuL4]. Reagents and conditions: (i) p-MWCNTs, mesitylene,
170 1C, 1 h; (ii) [EuL4]ꢁNEt4, toluene, rt, 14 h; (iii) p-MWCNTs,
mesitylene, 170 1C, 14 h and sonication in DMF; (iv) C60, mesitylene/
DMSO (1 : 1), 140 1C, 14 h, 30%.
composition of the functionalized material (see Table 1), XPS
studies were undertaken. The elemental analysis of the
p-MWCNTs, as expected, presents only the C(1s) and the
O(1s) signals, which appear at 284 and 533 eV, respectively.
Whilst in the case of the covalent material only a small
functionalization was obtained (see Table 1), more interesting
are the results regarding the non-covalent approach. In fact, in
the case of MWCNT-1ꢁBr, the signal of N(1s) centered at
401 eV, indicating the presence of both the azyridyl ring and the
imidazolium functionality in the sample, is present with a high
(12%) atomic ratio, revealing an unprecedent loading for the
sample. The luminescent hybrid assembly MWCNT-1ꢁ[EuL4]
exhibits signals relative to the anionic complex centered on
Eu(3d) (1134 eV) and F(1s) (686 eV), with atomic percentages
of 0.5% and 11% respectively. The amount of F atoms found
in the sample is higher than the expected value (considering the
structure of [EuL4]ꢀ, a ratio of 1 : 12 should be detected),
indicating the presence of some free ligand L, probably
deriving from some decomposition of the complex or some
Fig. 2 (A) Emission spectra of C60-1ꢁBr in the presence of increasing
amounts of [EuL4]ꢁNEt in CH2Cl2. Inset shows the changes in the
Eu(III) centered emission maximum. (B) Emission decay profiles of
C
60-1ꢁBr, [EuL4]ꢁNEt4, a mixture of C60 and [EuL4]ꢁNEt4 (1 : 1), and a
mixture of C60-1ꢁBr and [EuL4]ꢁNEt4 (1 : 1). Emission spectral changes
were recorded with lexc = 370 nm and emission decay profiles were
collected with excitation at 370 nm and monitoring at 612 nm.
Table 1 XPS analysis resultsa for p-MWCNTs, MWCNT-1,2ꢁBr and MWCNT-1,2ꢁ[EuL4]
Sample
C%
O%
N%
F%
Eu%
p-MWCNTs
97 ꢃ 1
2 ꢃ 1
—
—
—
—
11 ꢃ 2
1.7 ꢃ 0.1
—
—
—
—
MWCNTs-1ꢁBr
MWCNTs-2ꢁBr
MWCNTs-1ꢁ[EuL4]
MWCNTs-2ꢁ[EuL4]
Blank testb
73 ꢃ 3
6 ꢃ 2
12 ꢃ 2
0.6 ꢃ 0.1
1.8 ꢃ 0.3
0.7 ꢃ 0.5
—
92.8 ꢃ 0.3
75.2 ꢃ 0.3
90.9 ꢃ 0.8
94 ꢃ 1
6.3 ꢃ 0.3
11 ꢃ 1
0.46 ꢃ 0.02
0.11 ꢃ 0.02
—
6.6 ꢃ 0.7
11.6 ꢃ 0.9
a
b
Each value is the average of three different measurements on the same sample. Blank test: physical dispersion of MWCNTs with [EuL4]ꢁNEt4
after extensive washing in the absence of the ionic liquid functionalities.
c
1626 Chem. Commun., 2011, 47, 1625–1627
This journal is The Royal Society of Chemistry 2011

adding aliquot amounts of C60-1ꢁBr into a 3.4 ꢂ 10ꢀ7
M
of the mechanical and charge conductivity properties of the
active layers within devices.
solution of [EuL4]ꢁNEt4 resulted in a continuous decrease of
the lifetime up to the value of 0.25 ms (Fig. S6, ESIz). A test
experiment with uncharged pristine C60 gave a longer lifetime
of 0.35 ms at the highest concentration level (Fig. 2B), as a
consequence of diffusional quenching. These results suggest
the existence of an interaction between C60-1ꢁBr and [EuL4]ꢁ
NEt4 which, albeit reducing the excited state lifetime of the
emitter, preserves an intense luminescence signal. Notably,
within the experimental error, no changes have been observed
when luminescence titrations of [EuL4]ꢁNEt4 were carried out
with the imidazolium linker molecule, namely 1ꢁBr.
This work was supported by EU (ITN ‘‘FINELUMEN’’
PITN-GA-2008-215399), the CNR (PM.P04.010, MACOL),
the FRS-FNRS, (no. 2.4.625.08.F and F.4.505.10.F), the
‘‘Loterie Nationale’’, the Region Wallonne through the ‘‘SOL-
´
WATT’’ program (SUNTUBE, no. 850551), the ‘TINTIN’
ARC project (no. 09/14-023) and the University of Namur
(internal funding). DB especially aknowledges Nanocyl for the
Nanocylt NC7000 MWCNTs.
Notes and references
The luminescence studies showed that MWCNT-1ꢁ[EuL4] is
a bright red luminescent material (upon UV excitation) where
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ESIz). In solid state (KBr matrix) MWCNT-1ꢁ[EuL4] showed
an intense and sharp emission peak at 615 nm and weaker
features at 593, 654 and 702 nm, characteristic of Eu(III) ions
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suggests high loading of the luminophore on the surface of
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Fig. 3 (A) Emission spectra of hybrid MWCNT-1ꢁ[EuL4] recorded in
KBr upon excitation at 370 nm, (B) confocal image of spin-coated
MWCNT-1ꢁ[EuL4] onto glass substrate and (C) emission spectra
recorded from (a) bright particles and (b) background; confocal
images were recorded after exciting with 400 nm laser and the emission
signal was collected by using a 450 nm cut-off filter.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 1625–1627 1627