Room-temperature synthesis of soluble, fluorescent carbon nanoparticles from organogel precursors

Article abstract of DOI:10.1039/c2cc35087d

Carbon nanoparticles were obtained at room temperature by irradiating an organogel made from a 1,8-diaryloctatetrayne derivative in chloroform. During the topochemical polymerization, the morphology of the gel changes from fibers to soluble, yellow fluorescent nanoparticles in high yield. Analyses suggest that the resulting nanoparticles are made of amorphous graphitic carbon. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc35087d

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COMMUNICATION
Room-temperature synthesis of soluble, fluorescent carbon nanoparticles
from organogel precursorsw
´
Jules Romeo Neabo, Cecile Vigier-Carriere, Simon Rondeau-Gagne and
Jean-Francois Morin*
´
´
´
¸
Received 16th July 2012, Accepted 17th August 2012
DOI: 10.1039/c2cc35087d
Carbon nanoparticles were obtained at room temperature by
irradiating an organogel made from a 1,8-diaryloctatetrayne
derivative in chloroform. During the topochemical polymerization,
the morphology of the gel changes from fibers to soluble, yellow
fluorescent nanoparticles in high yield. Analyses suggest that the
resulting nanoparticles are made of amorphous graphitic carbon.
The good solubility of electro- and photoactive materials in
common organic solvents represents a significant advantage
for many electronics-related applications that rely on a film-
forming process to cover large substrates such as light-emitting
devices (LED) and organic solar cells.
Herein, we report the synthesis of soluble, fluorescent
carbon nanoparticles using a room-temperature, catalyst-free
process. A 1,8-diaryloctatetrayne derivative (compound 1,
Scheme 1), used as the carbon source, was self-assembled to
form organogels and was irradiated under UV light at 254 nm
at room-temperature. Our initial hypothesis was that by self-
assembling the carbon source into a well-ordered array of
molecules (organogel), the resulting graphitic materials would
retain the gel morphology, namely nano- and microfibers, upon
graphitization reaction at low temperatures. The possibility
of morphology retention has recently been demonstrated for
the graphitization of helical polyacetylene films.6 In our case
however, nanoparticles rather than the expected fibers were
obtained. These nanoparticles, made of amorphous graphitic
carbon, are soluble in common organic solvents and exhibit
intense yellow fluorescence in solution.
Carbon-rich materials like fullerenes, carbon nanotubes and
graphenes have attracted a lot of attention in the past 25 years
because of their exceptional electronic and optical properties.1
Although they are recognized as being very promising materials
for several electronics- and optics-related applications, many
drawbacks associated with their preparation such as the use of
high temperatures (pyrolysis), vacuum and metal catalysts have
slowed down their use in commercial devices. Thus, a wealth of
efforts has been devoted to the development of preparative
methods that allow the synthesis of well-defined, carbon-rich
nanostructures under milder conditions, preferably below 100 1C
and without the use of metal catalysts or complex setups.
One emerging strategy to obtain carbon nanostructures at
relatively low temperatures without a metal catalyst involved
the use of oligoynes as highly reactive precursors.2 In 2004,
Ding and Olesik showed that an amphiphilic tetrayne derivative
can be self-assembled in water to form ultrathin films that
undergo a spontaneous polymerization reaction to form cross-
linked polymer films.3 These films can further be transformed
into nanoparticles by heating the films suspension at 70 1C.
More recently, Frauenrath et al. used a similar approach to
prepare graphitic nanostructures from hexayne-based colloids.4
Less ordered graphitic materials have also been prepared by
Goroff et al. using poly(diiododiacetylene) as the carbon source
and Lewis bases as a catalyst at room temperature.5 However,
to the best of our knowledge, none of the graphitic materials
prepared thus far using room-temperature methods from
reactive carbon-rich precursors are soluble in organic solvents.
The first step toward the preparation of carbon nano-
particles was the synthesis of a tetrayne derivative that bears
amide groups with a long alkyl chain directly attached to the
aryl groups at both ends of the molecule. We7 and others8 have
´
´
Departement de chimie and Centre de Recherche sur les Materiaux
Avance´s (CERMA), Universite´ Laval, 1045 Ave de la Me´decine,
Pavillon A.-Vachon, G1V 0A6, Canada.
E-mail: jean-francois.morin@chm.ulaval.ca; Fax: +1-418-656-7916;
Tel: +1-418-656-2812
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for all new compounds, XRD,
FTIR, TEM and gelation properties. See DOI: 10.1039/c2cc35087d
Scheme 1 Synthesis of the reactive 1,8-diaryloctatetrayne derivative.
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10144 Chem. Commun., 2012, 48, 10144–10146
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previously shown that amide groups are ideal to drive
organogel formation through intermolecular hydrogen bonds
while providing the optimal intermolecular distance for topo-
chemical polymerization of butadiyne moieties. Moreover,
long alkyl chains are not only necessary to make the precursors
soluble and to drive the self-assembly process through van der
Waals interactions, but also to make the final graphitic materials
soluble in organic solvents. Thus, based on our previous study on
the topochemical polymerization of 1,4-diarylbutadiyne in the
gel state,7 we designed and prepared compound 1 as depicted in
Scheme 1. Starting from compound 2,7 an amidation reaction
with dodecylamine followed by an alcohol protection reaction
using tert-butyldimethylsilyl chloride (TBDMSCl) gave compound
3 in 53% yield over two steps. It is worth mentioning that the
protection of the primary alcohol was necessary to reduce the
polarity of the substrate, thus enabling further purification
steps by column chromatography. Compound 3 was then
subjected to Sonogashira coupling with an excess of trimethyl-
silyl-1,3-butadiyne9 under standard conditions to give compound 4
in excellent yield. Afterward, the terminal alkyne was deprotected
and the resulting relatively unstable product was immediately
subjected to a Hay coupling without further purification to
give compound 5 in good yield. Finally, the alcohol groups
were deprotected using tetrabutylammonium fluoride (TBAF)
to yield compound 1.
Nonetheless, the solution was kept under UV irradiation for
72 hours before the resulting solutions were analyzed by size-
exclusion chromatography (SEC) (see Fig. S9 in ESIw).
Unexpectedly, the chromatogram obtained for the toluene
solution is very different than that obtained for the chloroform
one. On the one hand, the crude material obtained from the
toluene gel contains high molecular weight fraction, but also a
fairly large amount of very low molecular weight portion that
could be attributed to the decomposition of compound 1 into
smaller fragments, although we have no direct proof of such
decomposition. On the other hand, the chromatogram of the
chloroform solution does not show any trace of decomposed
materials. Moreover, more than 90% of the starting compound 1
was converted to higher molecular weight materials. The
decomposition of compound 1 upon irradiation in toluene is
not understood at this stage. Thus, based on this result, we
decided to investigate only the materials produced from the
chloroform-based gel.
To assess whether or not the nano- and microfiber morphology
was retained, a SEM analysis was performed on the crude
material (Fig. 1b). Surprisingly, both nanoparticles and nano-
fibers were observed in the unpurified material. Since the
nanofibers appear very similar to the ones observed in the
organogel of the starting materials, we attributed the presence
of fibers in the unpurified material to unreacted starting
materials. To verify our hypothesis, we proceeded towards
the purification of the material in order to extract the high-
molecular weight fractions. The purification was achieved
through successive precipitations in methanol. Since compound 1
has a similar solubility to the high molecular weight material in all
the solvents we tested, we were not able to remove all the starting
material from the crude product. Thus, we performed semi-
preparative SEC purification using THF and collected only
the desired peak. High molecular weight material with only
trace amounts of starting material was thus recovered.
Compound 1 shows very good gelation properties in many
solvents (see Table S1 in the ESIw), especially in chloroform,
toluene, benzene and chlorobenzene. In most cases, freestand-
ing, translucent organogels can be obtained at concentration
as low as 0.5 wt%. SEM images obtained after drying a
toluene-based gel under ambient conditions on a metallic
substrate showed a highly dense structure made of micron-long
fibers (Fig. 1) indicative of very strong interfiber interactions.7,10
To obtain better insight into the molecular arrangement, the
same dried gel was subjected to XRD analysis between 11 and
201 (see Fig. S8 in ESIw). Three rather broad peaks at 2.83, 0.65
and 0.45 nm characterize the diffraction pattern, suggesting a
rather disordered arrangement with a face-to-face intermolecular
distance of 4.5 A, which is consistent with intermolecular
hydrogen bonds attributed to the amide groups.
TEM analysis of the resulting material was performed in
order to gain better insight into the morphological features of
the nanoparticles (Fig. 2). Remarkably, only spherical nano-
particles with diameters between 130 and 350 nm were
obtained. The change in morphology (from fibers to nano-
particles) during the course of the reaction is not yet well
understood, but can partly be attributed to a decrease in the
The topochemical polymerization was performed on organogels
of toluene and chloroform at a concentration of 10 wt% under
irradiation at 254 nm for 72 hours. After only a few hours of
irradiation, the gel state was lost and dark solutions were obtained
in both cases. The loss of the gel state can be attributed to the
conformational changes within the gel upon photopolymerization,
thus disrupting the intermolecular hydrogen bond network.7,10,11
Fig. 1 SEM images of organogel from compound 1 in chloroform
before (a) and after (b) irradiation (10 mg mLÀ1). The scale bars are (a)
50 mm and (b) 1 mm, respectively.
Fig. 2 TEM image of the SEC-purified material obtained after
irradiation of compound 1 in the gel state for 72 hours at a concen-
tration of 1.0 wt% in chloroform. The scale bar is 0.5 mm.
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the emission band (FWHM E 100 nm) is very similar to the
ones observed for similar materials.13
In conclusion, soluble graphitic nanoparticles exhibiting yellow
fluorescence in solution were prepared at room temperature
through topochemical polymerization of an amide-containing
1,8-diaryloctatetrayne in the gel state. Studies are currently
underway to prepare nanoparticles with different sizes and
shapes by changing the organogel morphology.
The authors would like to thank the National Sciences and
Engineering Research Council of Canada (NSERC) for financial
Fig. 3 UV-visible (a) and fluorescence (b) spectra of compound 1
(black) and SEC-purified nanoparticles (blue) in THF solution.
support and Jean-Franc¸ ois Rioux, Richard Janvier and Rodica
free surface energy of the material during the gel-to-solution
process.
Plesu for their help in characterization.
FTIR and Raman spectroscopy analyses were performed on
the SEC-purified materials and the results are shown in
Fig. S10 in ESI.w The bands at 2199 and 2126 cmÀ1 associated
to the internal alkyne units in the FTIR spectrum of compound 1
vanished with time upon irradiation meaning that the sp carbon
atoms were progressively transformed into other types of
carbon, presumably sp2 carbon atoms. However, the complete
disappearance of those peaks has never been observed under
the reaction conditions used to make these carbon nano-
particles. The strong peaks associated to alkyl chains at 2918
and 2849 cmÀ1 are still present in the nanoparticles spectrum,
proving that our approach is useful for the preparation of
soluble, carbon nanoparticles bearing alkyl chains.
Notes and references
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Raman spectroscopy was performed to assess the presence
of graphitic carbon in the nanoparticles. The band associated
to the tetrayne moiety at 2105 cmÀ1 disappeared and was
replaced by two very broad and intense bands at 1352 cmÀ1
and 1581 cmÀ1, associated to the D and G bands of graphite-
like materials, respectively (see Fig. S10 in ESIw). The rather
broad signals suggest that nanoparticles are mostly composed
of amorphous, disordered graphitic materials and that the
graphitization reaction is probably incomplete as observed by
FTIR analysis.5,6,12
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The optical properties of the carbon nanoparticles were also
determined and the results are shown in Fig. 3. A diluted
solution of carbon nanoparticles in THF exhibits a yellowish
orange color with a featureless UV-visible spectrum peaking
at around 300 nm (Fig. 3a). This is consistent with other
graphitic nanomaterials reported recently.13,14 This solution
also presents intense yellow fluorescence peaking at 541 nm
when excited with UV light at 350 nm, representing a Stoke
shift of ca. 140 nm (Fig. 3b). The longer wavelength emission of
our materials can be attributed to their bigger size compared to
those already published (o100 nm). Nonetheless, the width of
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10146 Chem. Commun., 2012, 48, 10144–10146
This journal is The Royal Society of Chemistry 2012