Carbon nanotube-fluorenevinylene hybrids: Synthesis and photophysical properties

Article abstract of DOI:10.1016/j.cplett.2009.10.078

Soluble, chromophore-functionalized carbon nanotubes (MWCNTs) have been synthesized via covalent approach with minimal alteration to the characteristic electronic states of MWCNTs. The key step includes incorporating a fluorenevinylene fluorescent moiety onto MWCNTs via the esterification of oxidized MWCNTs, where the solubility of the product is brought about by hexyl groups attached to the fluorene moiety. Electronic properties of the adduct were investigated by UV-Vis and luminescence spectroscopies, whereas its dispersability was monitored by SEM and TEM imaging. Fluorescence of the fluorenevinylene moiety is found to be quenched when bonded to the MWCNT sidewalls, indicating an electronic interaction in the excited state.

Full text of DOI:10.1016/j.cplett.2009.10.078

Chemical Physics Letters 483 (2009) 241–246
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Carbon nanotube–fluorenevinylene hybrids: Synthesis and photophysical properties
J. Mikroyannidis a, K. Papagelis b, M. Fakis c, D. Tasis b,
*
a Department of Chemistry, University of Patras, 26504 Rio Patras, Greece
b Department of Materials Science, University of Patras, 26504 Rio Patras, Greece
c Department of Physics, University of Patras, 26504 Rio Patras, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
Soluble, chromophore-functionalized carbon nanotubes (MWCNTs) have been synthesized via covalent
approach with minimal alteration to the characteristic electronic states of MWCNTs. The key step
includes incorporating a fluorenevinylene fluorescent moiety onto MWCNTs via the esterification of oxi-
dized MWCNTs, where the solubility of the product is brought about by hexyl groups attached to the flu-
orene moiety. Electronic properties of the adduct were investigated by UV–Vis and luminescence
spectroscopies, whereas its dispersability was monitored by SEM and TEM imaging. Fluorescence of
the fluorenevinylene moiety is found to be quenched when bonded to the MWCNT sidewalls, indicating
an electronic interaction in the excited state.
Received 31 August 2009
In final form 24 October 2009
Available online 28 October 2009
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
In the recent years, conjugated polymers have reached the role
of a major, technologically important class of materials. They have
Carbon nanotubes (CNTs) have become one of the most inter-
esting areas of research due to their novel structural, electronic
and optical properties [1]. These outstanding properties have
aroused great potential for implementing CNTs in molecular elec-
tronics applications [2]. In the recent years, attention has been di-
rected to the preparation of donor–acceptor nanohybrids that are
based on CNTs [3]. The general concept is that CNTs can accept
electrons and then transport them under ballistic conditions along
their axis [4]. Yet, it is still difficult to take their full advantages,
since the pristine CNTs show some lack of solubility in any solvent.
In order to overtake this obstacle, CNTs can be chemically mod-
ified, either covalently or non-covalently, allowing enhanced dis-
persion in organic or aqueous media [5]. Moreover, chemical
functionalization enables the efficient decoration of CNT sidewalls
and tips with appropriate electron donors. On one hand, the non-
covalent modification approaches have been used mainly in order
to coat the CNT sidewalls with conjugated polymers and small or-
ganic dyes [6–16].
On the contrary, typical examples of chromophores attached
covalently to both single-walled (SWCNTs) and multi-walled car-
bon nanotubes (MWCNTs) include polyaromatic systems, such as
pyrenes and porphyrins [17–31]. In most cases, SWCNTs and/or
MWCNTs served as efficient electron acceptors that quench the
luminescence of the grafted chromophore. The combination of an
electron donor chromophore with an electron acceptor, the CNT
nanostructure, is expected to result in the development of novel
nanodevices that convert solar energy to electricity.
the advantages of low cost and ease of modification of properties
by appropriate substitution. Major applications include the devel-
opment of polymer-based electronic devices such as field-effect
transistors (FETs), solar cells and organic light-emitting diodes
(OLEDs) [32,33]. In addition, oligomers are of significant interest
due to their enhanced solubility in various media. In the recent
years, the synthesis of various well defined conjugated oligomers
derived from phenylene, fluorene, pyrrole and thiophene moieties
has been achieved and reviewed extensively [33]. However, so far
no report is associated with the synthesis and photophysical study
of CNTs grafted with a conjugated oligomer. It would be of great
interest to combine the exotic properties of CNTs with those of
the conjugated oligomers. Such adducts can be incorporated into
fluorenevinylene-based copolymers by condensation reactions
yielding functional electroluminescent composites.
In this work we report on the preparation of a new dispersable
CNT–fluorenevinylene hybrid through esterification reaction be-
tween oxidized MWCNTs and the hydroxylated chromophore
(Fig. 1). Importantly, we complement our work with a detailed
photophysical investigation on ground- and excited state CNT/
chromophore interactions.
2. Materials and methods
All solvents and chemicals were reagent-grade quality, obtained
commercially and used without further purification. The fluorene-
vinylene chromophore was chemically synthesized following a pro-
cedure reported elsewhere [34]. Briefly the diol, the chemical
structure of which is shown in Fig. 1, was synthesized by Heck
coupling of 9,9-dihexyl-2,7-divinylfluorene with 4-bromophenyl
* Corresponding author. Fax: +30 2610 969368.
E-mail address: dtassis@upatras.gr (D. Tasis).
0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2009.10.078

242
J. Mikroyannidis et al. / Chemical Physics Letters 483 (2009) 241–246
Fig. 1. Synthetic scheme of MWCNT–fluorenevinylene hybrid.
acetate in a molar ratio 1:2. This reaction took place in triethyl-
amine utilizing palladium acetate as catalyst. The reaction product
was hydrolyzed in ethanol in the presence of a catalytic amount of
aqueous solution of KOH. Finally, the diol was obtained by acidifi-
cation with hydrochloric acid. In the present investigation, this diol
was selected as chromophore because it is easier to prepare a sym-
metrical derivative compared to a non-symmetrical one. Moreover,
this diol is very soluble in common organic solvents owing to the
two hexyl chains. It is also strongly fluorescent due to the conju-
gated backbone. Multi-walled carbon nanotubes produced by the
chemical vapor deposition method were obtained from Nanocyl
(Belgium). The purity of the pristine material was approximately
95%.
The CNT sample was refluxed in concentrated HNO3 for 48 h, to
remove metal oxide particles remaining from the growth proce-
dure. The mixture was then vacuum filtered through a 0.22 in Mil-
lipore polycarbonate membrane and subsequently washed with
distilled water until the pH of the filtrate was ca. 7. The filtered so-
lid was dried under vacuum for 12 h at 60 °C. The purification pro-
tocol used here is known to add carboxylic groups to the nanotube
edges and defects [35]. These materials will be referred as
MWCNT–COOH. The introduction of the pendant fluorenevinylene
chromophore groups was accomplished, starting from the
MWCNT–COOH material. The synthetic route was the same em-
ployed by Haddon and co-workers to produce soluble SWCNTs
containing alkyl esters as pendant groups [36].
with pore size 0.45 lm. The CNT adduct was washed extensively
with THF to remove the excess of diol and dried in vacuum. This
dispersion–filtration–washing protocol was repeated three times
for more efficient purification of the functionalized tubes. In order
to exclude non-covalent interactions between the organic moieties
and the CNT sidewalls, we performed a blank experiment, in which
HNO3-treated MWCNTs were incubated in a solution of fluorene-
vinylene derivative in triethylamine. The complete chemical route
containing all the modification steps is displayed in Fig. 1.
Raman spectra are measured with a ꢀ50 objective at 785 nm
excitation with a Renishaw micro-Raman spectrometer having
1800 grooves/mm grating and spectral resolution ꢁ2 cmꢂ1. The
data were recorded at the laser power of 0.4 mW, measured di-
rectly before the sample, in order to minimize heating effects. To
avoid problems related to inhomogeneities in the samples all spec-
tra were taken on four different spots for each sample. The phonon
characteristics were obtained by fitting Lorentzian lineshapes to
the experimental peaks, after background subtraction.
The UV–Vis-NIR absorption spectra of the CNT samples in THF
were recorded on a Digilab Hitachi U-2800 spectrophotometer.
The morphology of the CNT samples was revealed by Scanning
and Transmission Electron Microscopy imaging. SEM images were
recorded on a Leo 1530 FESEM Gemini scanning microscope. THF
suspensions of MWCNTs were spin casted on mica substrates. All
specimens were sputtered with gold. The TEM images were ob-
tained from the casting of dispersions of modified CNTs in THF
onto Nickel grids (with carbon layer coating, 200 mesh); these
grids were analyzed with a Philips 208 electron microscope at a
100 kV voltage, and the resulting images were collected with an
AMT high-resolution digital imaging camera. Thermogravimetric
studies were performed on a TGA2950 (TA instruments) under
nitrogen at 10 °C/min in temperature range from 30 to 700 °C.
Room temperature photoluminescence (PL) spectra were ob-
tained with a Perkin Elmer LS45 luminescence spectrometer. The
PL spectra were recorded by using an excitation wavelength of
Thus, 80 mg of MWCNT–COOH and 10 mL of thionyl chloride
were mixed and stirred at 80 °C for 24 h. The crude reaction mix-
ture was then evaporated in vacuum to remove the excess of SOCl2
and washed extensively with tetrahydrofuran (THF). In the follow-
ing step, dried acyl-chloride modified CNTs were mixed with ex-
cess of diol in 5 ml of triethylamine. The resulting suspension
was stirred for 24 h at 90 °C. The product was isolated by centrifu-
gation. The functionalized CNT material was redispersed in THF by
bath sonication and was filtered off through a Teflon membrane

J. Mikroyannidis et al. / Chemical Physics Letters 483 (2009) 241–246
243
400 nm. The PL dynamics has been studied with the time-resolved
fluorescence upconversion technique using a mode-locked Ti:sap-
phire femtosecond laser (100 fs pulse duration, 80 MHz repetition
rate) as the light source. The experimental set-up has been de-
scribed in details previously [37,38]. Briefly, the second harmonic
of the femtosecond laser at 400 nm, was used for the excitation
of the samples. The excitation power was below 7 mW and the
fluorescence decays were taken under magic angle conditions.
For comparison, the decays of the MWCNTs–fluorenevinylene
and the fluorenevinylene chromophore alone, in THF solutions,
were detected. The solutions were put into a rotating cell in order
to avoid heating. The fluorescence of the samples was collected and
mixed with the delayed fundamental laser beam (gate) into a BBO
crystal (type I) generating an upconversion ultraviolet beam. This
beam passed through appropriate filters and a monochromator
and it was detected though a photomultiplier as a function of the
temporal delay between the fundamental laser beam and the fluo-
rescence. The temporal resolution of the upconversion spectrome-
ter was 160 fs.
3. Results and discussion
To introduce fluorenevinylene moieties to CNT sidewalls and
tips, acyl-chloride modified carbon nanostructures were reacted
with excess of diol under inert atmosphere to obtain soluble tubes
by esterification reaction [36]. Excess diol was removed by centri-
fugation and repeating dispersion–filtration–washing cycles, as
non-reacted substance simply adsorbed onto CNT sidewalls should
not withstand such treatments.
MWCNT–fluorenevinylene adduct is soluble in common organic
solvents such as tetrahydrofuran, chloroform and dichloromethane
without precipitation for weeks. The solubility is estimated to be
about 0.2 mg/ml in tetrahydrofuran. As
a blank experiment,
Fig. 2. SEM images of (a) MWCNT–COOH and (b) MWCNT–fluorenevinylene
material.
HNO3-treated MWCNTs were incubated in a solution of fluorene-
vinylene derivative in triethylamine. After the isolation of the
CNT material by extensive washing, we observed no dispersability
of the tubes in tetrahydrofuran after prolonged bath sonication. In
addition, TEM, Raman and TGA data give solid evidence that the
fluorenevinylene derivative hardly adsorbs onto the CNT sidewalls
(see below).
Observations by SEM and TEM imaging were performed to di-
rectly visualize the morphology of modified CNT material cast from
THF solution. Typical images are presented in Figs. 2 and 3, respec-
tively. Whereas MWCNT–COOH are typically aggregated in bun-
dles (Fig. 2a), images of MWCNT–fluorenevinylene (Fig. 2b)
reveal less bundled nanostructures than what is typically found
for starting material.
This observation is consistent with MWCNT functionalization
with the fluorenevinylene chromophore, where the conjugated
functionality may disrupt the strong interactions between individ-
ual tubes. In addition, the TEM image of the hybrid material (Fig. 3)
shows individually dispersed material. Concerning the dimension
of the modified CNT external diameters, the majority of the tubes
observed in the electron microscopy images have similar diameter
when compared to the starting material (10–20 nm), since the size
of the grafted chromophore is about 3 nm.
Fig. 3. TEM imaging of MWCNT–fluorenevinylene composite material.
Additional evidence for the successful functionalization of
MWCNT material is deduced by thermogravimetric analysis.
Fig. 4 shows the thermal behavior of the starting MWCNT–COOH
together with that of the hybrid material. As shown in a recent
study of our group [35], the thermal degradation of nitric acid-
treated MWCNTs is a multistage process (curve a). The first stage,
up to a temperature of 150 °C, a weight loss is detected for the
highly hydrophilic nitric acid-treated MWCNTs, which corresponds
to the evaporation of the adsorbed water.
The second stage – from 150 to 350 °C – is attributed to the
decarboxylation of the carboxylic groups present on the MWCNT
walls. Thermal degradation in the range between 350 and 500 °C
may be ascribed to the elimination of hydroxyl functionalities, at-
tached to the MWCNT walls. Finally, at the temperatures higher
than 500 °C, the observed degradation corresponds to the thermal
oxidation of the remaining disordered carbon. The TGA curve of
MWCNT–COOH exhibited a weight loss of 10% between 150 and

244
J. Mikroyannidis et al. / Chemical Physics Letters 483 (2009) 241–246
nied with an enhanced absorption for lower wavelengths. Interest-
ingly, as it is evident from Fig. 5, in the spectrum of MWCNT–
fluorenevinylene the absorption of the free molecule is superim-
posed on the rising background that originates from the presence
of MWCNTs. Moreover, the characteristic double peaks in the range
350–370 nm of the free chromophore can hardly be resolved in the
case of the composite material, possibly indicating weak electronic
interactions of the fluorenevinylene and MWCNTs in the ground
state.
Raman spectroscopy provides essential and useful information
for the attachment of functional groups onto the surface of CNTs.
The Raman spectra of MWCNT–COOH, hybrid adduct and material
from the blank experiment excited with the 785 nm are illustrated
in Fig. 6. For all the studied samples three characteristic bands,
namely the D-band at ꢁ1316 cmꢂ1, the G-band at ꢁ1590 cmꢂ1
and the D0-band at ꢁ1614 cmꢂ1 have been detected. The G-band
originates from in-plane tangential stretching of the C–C bonds
in graphene sheets, while the D and D0 bands are disorder activated
modes. It is well documented that an increase of the D-band inten-
sity relative to the G one comprises a fingerprint for successful
sidewall functionalization [39]. Upon functionalization, an increase
of the D-band intensity relative to the G one is clearly observed. In
the case of MWCNT–COOH and the material from the blank exper-
iment, the D- to G- peak intensity ratio [I(D)/I(G)] were estimated
to be 4.2 and 4.3, respectively. For the hybrid material the afore-
mentioned ratio was determined to be about 5.2. This slight in-
crease of the ratio may be connected to an efficient covalent
functionalization through the esterification reaction.
Photoluminescence (PL) spectroscopy is a valuable tool for
studying the interaction between the fluorenevinylene chromo-
phore and MWCNTs in the excited state. In Fig. 7, the steady
state fluorescence spectra of the fluorenevinylene moieties and
MWCNT–fluorenevinylene composite material in THF solutions
are depicted. The concentrations of both solutions have been
adjusted to an optical density (O.D.) of 0.1 at 400 nm. The
fluorenevinylene chromophore emits light in the blue spectral
region with maximum at 447 nm exhibiting a quantum yield
Fig. 4. TGA profiles of MWCNT–COOH (curve a) and hybrid material (curve b).
500 °C, followed by a rapid weight loss of 12% up to 700 °C at
which the amount of the MWCNT residue is about 72%.
The onset temperature for the thermal pyrolysis of parent diol is
about 400 °C, whereas a residue of 40% is obtained at 700 °C (spec-
trum not shown) [34]. By assuming that at 700 °C the fluorenevin-
ylene residues remaining in the hybrid material have the same
wt.% as that of the neat diol, and the wt.% of oxygen-containing
groups/disordered carbon is close to zero at this temperature, the
wt.% of the fluorenevinylene moieties in the hybrid material
(Fig. 4, curve b) may be roughly calculated and was found to be
ꢁ10%. Therefore, the chromophore/carbon ratio was calculated to
be approximately 1:370. Analogous phenomena of relatively low
functionalization degrees have also been observed by other groups
[28]. The TGA profile of the blank experiment adduct (not shown)
was almost identical with the one of MWCNT–COOH. This means
that non-covalent interactions between the chromophore func-
tions and the CNT sidewalls should be ruled out.
(
Uf) of 55%. However, as it is evident from Fig. 7, a significant
fluorescence quenching is observed for the composite samples
Uf = 2.5%) suggesting the presence of electronic interactions be-
In order to confirm further the proposed functionalization
scheme, UV–Vis-NIR absorption spectra were recorded at room
temperature in THF solution. Fig. 5 shows the typical absorption
spectra of free fluorenevinylene, oxidized MWCNTs and the func-
tionalized material. The spectrum of free fluorenevinylene shows
intense features in the range between 350 and 400 nm accompa-
(
tween the oligomer and the MWCNTs which can be identified
as electron or energy transfer. In the inset of Fig. 7, the PL
spectra are shown in normalized units. It is worth stressing that
no significant differences were observed for the linewidth and
Fig. 5. Absorption spectra of neat fluorenevinylene, MWCNT–COOH and hybrid
Fig. 6. Raman spectra of starting material MWCNT–COOH, hydrid adduct and
material in THF solutions.
material from the blank experiment, after background subtraction.

J. Mikroyannidis et al. / Chemical Physics Letters 483 (2009) 241–246
245
concentration of the solutions has been adjusted to an O.D. of 0.7 at
400 nm. The excitation wavelength was 400 nm while the decay
rate was monitored at 440 nm, which is close to PL maximum of
fluorenevinylene chromophore.
As shown in Fig. 8a, the PL decay for the MWCNT–fluorenevin-
ylene hybrid is overall faster than that of the chromophore moiety
strongly indicating electronic interaction between the fluorenevin-
ylene and the MWCNTs. More specifically, the PL decay of the com-
posite is best fitted by a three-exponential decay function with an
ultrafast component of 600 fs (15%), a fast component of 5 ps (25%)
and a slow component of >300 ps (60%). On the other hand, the PL
decay of the fluorenevinylene molecule is best fitted by a bi-expo-
nential function with time constants of 5.3 ps (15%) and >300 ps
(85%).
In the inset of Fig. 8a, the PL dynamics of the two samples is pre-
sented on a longer time scale where the slow decay component is
evident. The ultrafast 600 fs component, existing only in the
dynamics of the composite, is attributed to interactions between
fluorenevinylene and MWCNTs. According to previous reports
[40,41], the electron transfer from conjugated molecules to CNTs
is an ultrafast process; faster than the energy transfer. Therefore,
it is considered that the 600 fs component is due to a photoinduced
electron transfer from the excited state of fluorenevinylene chro-
mophores to MWCNTs. This is reminiscent of the behavior encoun-
tered in other covalently modified CNT/conjugated chromophore
systems [25,42–46]. Using the results of the PL dynamics, a charge
separation rate of kcs ꢁ 2 ꢀ 109 sꢂ1 has been calculated through the
Fig. 7. PL spectra of free fluorenevinylene and MWCNT–fluorenevinylene compos-
ite in THF solutions. In the inset, the spectra are shown in normalized units.
peak position of the spectra in Fig. 7, indicating that the pro-
posed functionalization scheme does not alter the chromophore
architecture.
The PL dynamics of the MWCNT–fluorenevinylene composites,
studied via the femtosecond upconversion technique, provides evi-
dence for the electronic interactions between the fluorenevinylene
and MWCNTs. Fig. 8a shows the PL decays of the fluorenevinylene
and the MWCNT–fluorenevinylene composite in THF solutions. The
equation kcs = 1/hshybriꢂ1/h
s
i, where hshybri and h i are the ampli-
s
tude averaged decay constants of the hybrid and the free fluorene-
vinylene, respectively. In a very recent work, Cordella et al. [46]
have found extremely high charge separation rate (5 ꢀ 1010 sꢂ1
)
in an aminofluorene-functionalized SWCNT composite which was
attributed to the large energy level displacement between the
two moieties.
To determine whether the electronic interaction between the
chromophores and the CNT sidewalls occurs within the same
tube or through electron hopping from the chromophores to
adjacent nanotubes, we have examined the PL decays of the
MWCNT–fluorenevinylene composites for two different concen-
trations corresponding to O.D. of 0.7 and 1.4 at 400 nm excitation
wavelength. The decay curves have been shifted for clarity and
both are presented in Fig. 8b. As the concentration increases,
more chromophores should be found in close proximity to nano-
tubes leading to faster decays if a hopping mechanism governs
the dynamics. The observed similarity in PL decays for these
two different concentrations strongly suggests that within the
studied concentration range the electronic interaction occurs
through the chemical bridge between the chromophores and
the MWCNTs.
Finally, the 5 ps component found in both fluorenevinylene
solution and composite material could be potentially attributed
to vibronic or solvent relaxation while the slow decay component
can be attributed to radiative population decay. The existence of
these long decay components in the composite material is due
to the fluorenevinylene units which do not interact with the
nanotubes, thus exhibiting dynamics similar to the free
chromophores.
Efficient charge separation in the chromophore-functionalized
CNT composites is a prerequisite in developing CNTs based solar
cells. Additionally, enhanced absorption throughout the visible
spectrum and efficient transport of the carriers towards the elec-
trodes are also needed. Due to their defect-free one dimensional
structure, CNTs provide excellent channels for ballistic electron
transport. Although promising results have been published [47],
more systematic work has to be carried out to develop CNT-based
organic solar cells.
Fig. 8. (a) PL decays of (i) fluorenevinylene and (ii) MWCNT–fluorenevinylene
composite in THF. The inset in (a) shows the PL decays in longer timescale. (b) PL
decay of MWCNT–fluorenevinylene composite in THF solution for O.D. 0.7 and 1.4.
The detection wavelength for each curve was 440 nm.

246
J. Mikroyannidis et al. / Chemical Physics Letters 483 (2009) 241–246
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