Preparation of fullerene-polyyne nanospheres via thermally induced solid-state polymerization

Article abstract of DOI:10.1021/ja054669n

A 1,3,5,7-octatetrayne-linked bis(fullerene) compound has been synthesized. Through a thermally induced solid-state polymerization reaction on a surface, the solid thin film of this compound was transformed into a highly uniform and well organized polymer

Full text of DOI:10.1021/ja054669n

Published on Web 09/23/2005
Preparation of Fullerene-Polyyne Nanospheres via Thermally Induced
Solid-State Polymerization
Ningzhang Zhou, Erika F. Merschrod S., and Yuming Zhao*
Department of Chemistry, Memorial UniVersity of Newfoundland, St. John’s, Newfoundland A1B 3X7, Canada
Received July 13, 2005; E-mail: yuming@mun.ca
Scheme 1
Buckminsterfullerene (C60) is a fascinating building block for
various nanoscale structures and architectures, and covalently
linking it to a variety of conjugated oligomers/polymers has become
an important approach to improve or enhance optoelectronic and
photonic properties for diverse polymer composite systems.1 For
this reason, numerous C60-containing conjugated oligomers and
polymers have been synthesized and characterized in order to seek
potential applications in areas, such as thin-film organic solar
cells,1c,2 nonlinear optical (NLO) and optical limiting materials,3
to name a few. The fullerene-oligomer approach has been known
to be particularly beneficial for fabrication of such devices as bulk
heterojunction organic photovoltaic cells4 and third-order optical
nonlinearity controlled telecommunications apparatus.5
Spherical polymer nanoparticles, especially those containing
fullerene moieties, are very appealing in manufacturing various
nanoscale devices.6 There are a number of methods that can be
used to produce polymer nanospheres, including emulsion poly-
merization, intramolecular cross-linking, and solid-state polymer-
ization. Compared with other methods, little is known about solid-
state polymerization despite the simple preparation and compatibility
with various solid substrates which it affords.7 Conjugated polyynes
are ideal candidates for the preparation of polymer nanospheres
since they are reactive enough to be polymerized in the solid phase
when subjected to heat, UV light, γ-ray irradiation, or pressure.8
Previously, Hirsh and co-workers have reported making C60
nanospheres from emulsion polymerization of a butadiyne-contain-
ing lipofullerene.9 Solid-state polymerization of longer polyynes
bearing fullerene groups has, however, never been achieved.
To explore this possibility, a 1,3,5,7-octatetrayne-bridged bis-
(fullerene) “molecular dumb-bell” 6 was synthesized. This fullerene-
tetrayne species, as expected, may lead to the formation of highly
extended C60-polyenyne networks or supramolecular assemblies,
owing to the unique reactivity of the tetrayne unit in the solid state.
The resulting polymers, like many reported fullerene-oligomer/
polymer hybrids,1-6 could serve as active components in various
organic photovoltaic and NLO devices.
(see Supporting Information). Unlike its precursors, 4 and 5,
fullerene-tetrayne 6 is thermally stable from room temperature up
to ca. 150 °C, as is evident from differential scanning calorimetry
(DSC) analysis; no melting or phase transition appears in the DSC
trace within this range. A broad exothermic peak is observed from
156 to 240 °C, and with a maximum at 211 °C. Apparently, the
tetraynes start solid-state polymerization in this temperature range,
but the broad peak width suggests that the polymerization occurs
in a random manner rather than a regioselective topochemical
polymerization. This can be explained by the relatively large sizes
of fullerene groups prohibiting a tetrayne unit alignment suitable
for topochemical polymerization.12 The DSC trace remains nearly
flat and shows no additional phase transitions from 240 to 350 °C.
The enthalpy for the exothermic process (∆H) is measured to be
77.6 kJ/mol, which is considerably smaller than that of tetrayne 5
(317.7 kJ/mol). It is assumed that a stable polymer was formed
within these temperatures; however, the polymerization degree was
low due to steric effects. The resulting solids after DSC analysis
of 6 were brownish in color and completely insoluble in any organic
solvent. MALDI-TOF mass spectrometric analysis was attempted
on these solids but failed to give any positive identification. IR
analysis, however, shows a dramatically differing vibrational
spectrum than that of 6. A new peak at 1728 cm-1 in the IR
spectrum of the polymer suggests that the polymerization terminates
upon reacting with oxygen to form carbonyl groups. X-ray powder
diffraction analysis shows that the resulting polymer solids are
amorphous.
The synthesis of fullerene-tetrayne 6 is outlined in Scheme 1.
Iodoarene 1 was first desilylated with K2CO3 and then underwent
oxidative coupling with a large excess of trimethylsilylacetylene
(TMSA) in the presence of Hay catalyst10 to afford diyne 2 and a
byproduct, bis(trimethylsilyl)butadiyne. Compound 2 could be easily
isolated by column chromatography and was then reacted with
triisopropylsilylacetylene (TIPSA) under Sonogashira conditions to
yield 3. Selective removal of the TMS group in 3 with K2CO3,
followed by an oxidative homocoupling reaction, gave tetrayne 4
as a relatively unstable solid. Desilylation of 4 with tetrabutylam-
monium fluoride (TBAF) afforded terminal alkyne 5, which was
subsequently converted to the final product 6 using an in situ
ethynylation method.11
The DSC results of 6 are in good agreement with its UV-vis
spectroscopic analyses. Figure 1 shows the UV-vis profiles for
compound 6 measured in both solution and solid films. In toluene,
fullerene-tetrayne 6 shows three distinctive absorption bands at
386, 415, and 454 nm due to the central tetrayne unit, while the
higher-energy bands around 318 nm are due to the exterior
1
The structure and purity of compound 6 were confirmed by H
and 13C NMR spectroscopy and MALDI-TOF mass spectrometry
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10.1021/ja054669n CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
nanosphere is dependent on the thickness of the fullerene-tetrayne
film and its dewetting from the substrate. However, we believe
that the low degree of polymerization could be a principal reason
for the formation of such uniform nanosphere arrays on the surface
because no similar spherical nanoparticles were formed when solid
films of tetrayne 5 were thermally polymerized under the same
experimental conditions.
To our knowledge, this is the first preparation of evenly
distributed and uniform-sized fullerene-containing polymer nano-
spheres via thermally induced solid-state polymerization. In com-
parison to the previous work on fullerene-based nanoaggregates,6a,b,f
the transformation from amorphous solid thin films to orderly
structured nanosphere arrays is indeed remarkable and exceptional.
This method is rather simple, reproducible, and could be useful in
manufacturing various oligomer/polymer-based nanodevices.
Figure 1. UV-vis spectra for compound 6: (a) solid film on quartz without
heating; (b) solid film after thermal annealing at 135 °C for 30 min; (c)
solid film after thermal annealing at 160 °C for 1 h; and (d) in toluene at
room temperature.
Acknowledgment. The authors thank NSERC Canada, the
Canadian Foundation for Innovation, and Memorial University of
Newfoundland for financial support. Prof. D. W. Thompson is
acknowledged for assistance in UV-vis spectroscopic analysis.
Supporting Information Available: Synthetic procedures, 1H and
13C NMR spectra for compounds 2-6, MALDI-TOF mass spectrum,
and detailed DSC, IR, and AFM analyses for 6. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
(1) For recent examples of fullerene-conjugated oligomer hybrids, see: (a)
Zhao, Y.; Shirai, Y.; Slepkov, A. D.; Cheng, L.; Alemany, L. B.; Sasaki,
T.; Hegmann, F. A.; Tour, J. M. Chem.sEur. J. 2005, 11, 3643-3658.
(b) Vail, S. A.; Krawczuk, P. J.; Guldi, D. M.; Palkar, A.; Echegoyen, L.;
Tome´, J. P. C.; Fazio, M. A.; Schuster, D. I. Chem.sEur. J. 2005, 11,
3375-3388. (c) Atienza, C.; Insuasty, B.; Seoane, C.; Mart´ın, N.; Ramey,
J.; Aminur Rahman, G. M. A.; Guldi, D. M. J. Mater. Chem. 2005, 15,
124-132. (d) Nierengarten, J.-F.; Gu, T.; Hadziioannou, G.; Tsamouras,
D.; Krasnikov, V. HelV. Chim. Acta 2004, 87, 2948-2966.
(2) Roncali, J. Chem. Soc. ReV. 2005, 34, 483-495.
Figure 2. AFM images (5 µm × 5 µm) and corresponding line scans of
the thin films of 6: (a) before thermal annealing, and (b) after thermal
annealing at 160 °C for 1 h. White lines indicate where the sections were
taken.
phenylacetylene groups. A broad, weak absorption tail ranging from
480 to 600 nm is observed, arising from the contribution of fullerene
groups. Solid films of 6 were prepared by drop-casting a toluene
solution onto a quartz slide, and UV-vis spectra were measured
before and after being heated at different temperatures. The UV-
vis spectrum of the film after thermal annealing at 135 °C for 30
min shows no significant changes in comparison to the original
film. After heating the film at 160 °C for 1 h, marked distinctions
in UV-vis absorption were observed; the three characteristic bands
due to the tetrayne unit disappeared completely, while the higher-
energy bands below 300 nm and the fullerene absorption tail were
barely altered. These results indicate that intermolecular reactions
have been limited to the tetrayne moieties.
Finally, the thermally induced polymerization process was
examined by atomic force microscopy (AFM). A dilute solution
of 6 in toluene (ca. 10-4 to 10-5 M) was cast on a freshly cleaved
mica surface to afford a thin film. The film was initially disordered
and amorphous with regions made up of single to several molecular
layers, as illustrated in Figure 2a. After thermal annealing of the
same film at 160 °C for 1 h, the surface shows strikingly different
morphology with highly uniform and ordered nanospheres 15-20
nm high, as seen in Figure 2b. Larger images (up to 90 µm × 90
µm) from different areas of the sample showed similar features,
indicating the formation of homogeneously dispersed nanospheres
over a relatively large scale.
(3) (a) Kuang, L.; Chen, Q.; Sargent, E. H.; Wang, Z. Y. J. Am. Chem. Soc.
2003, 125, 13648-13649. (b) Brusatin, G.; Signorini, R. J. Mater. Chem.
2002, 12, 1964-1977. (c) Dai, L. J. Macromol. Sci., Part C 1999, 39,
273-387.
(4) (a) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992,
258, 1474-1476. (b) Ramos, A. M.; Rispens, M. T.; van Duren, J. K. J.;
Hummelen, J. C.; Janssen, R. A. J. J. Am. Chem. Soc. 2001, 123, 6714-
6715.
(5) Chen, Q.; Kuang, L.; Wang, Z. Y.; Sargent, E. H. Nano Lett. 2004, 4,
1673-1675.
(6) (a) Guldi, D. M.; Zerbetto, F.; Georgakilas, V.; Prato, M. Acc. Chem.
Res. 2005, 38, 38-43. (b) Hoppe, H.; Niggemann, M.; Winder, C.; Kraut,
J.; Hiesgen, R.; Hinsch, A.; Meissner, D.; Sariciftci, N. S. AdV. Funct.
Mater. 2004, 14, 1005-1011. (c) Veinot, J. G. C.; Yan, H.; Smith, S.
M.; Cui, J.; Huang, Q.; Marks, T. J. Nano Lett. 2002, 2, 333-335. (d)
Mehta, A.; Kumar, P.; Dadmun, M. D.; Zheng, J.; Dickson, R. M.;
Thundat, T.; Sumpter, B. G.; Barnes, M. D. Nano Lett. 2003, 3, 603-
607. (e) Zhang, P.; Li, J.; Liu, D.; Qin, Y.; Guo, Z.-X.; Zhu, D. Langmuir
2004, 20, 1466-1472. (f) Cassell, A. M.; Asplund, C. L.; Tour, J. M.
Angew. Chem., Int. Ed. 1999, 38, 2403-2405. (g) Nishimura, T.; Takatani,
K.; Sakurai, S.; Maeda, K.; Yashima, E. Angew. Chem., Int. Ed. 2002,
41, 3602-3604.
(7) (a) Ding, L.; Olesik, S. V. Chem. Mater. 2005, 17, 2353-2360. (b) Ding,
L.; Olesik, S. V. Nano Lett. 2004, 4, 2271-2276.
(8) Curtis, S. M.; Le, N.; Nguyen, T.; Ouyang, X.; Tran, T.; Fowler, F. W.;
Lauher, J. W. Supramol. Chem. 2005, 17, 31-36 and references therein.
(9) Hetzer, M.; Clausen-Schaumann, H.; Bayerl, S.; Bayerl, T. M.; Camps,
X.; Vostrowsky, O.; Hirsh, A. Angew. Chem., Int. Ed. 1999, 38, 1962-
1965.
(10) Hay, A. S. J. Org. Chem, 1962, 27, 3320-3321.
(11) (a) Shirai, Y.; Zhao, Y.; Cheng, L.; Tour, J. M. Org. Lett. 2004, 6, 2129-
2132. (b) Caution must be taken when handling polyynes since they are
very reactive in the solid state at elevated temperature or upon irradiation.
(12) (a) Enkelmann, V. In AdVances in Polymer Science; Springer-Verlag:
Berlin, Heidelberg, 1984; Vol. 63, pp 91-136. (b) Baughman, R. H.; Yee,
K. C. J. Polym. Sci.: Macromol. ReV. 1978, 13, 219-239.
In some regions, arrays of larger-sized nanospheres (ca. 120 nm
in height) were observed, as well. It is likely that the size of the
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