Interactions of functionalized carbon nanotubes with tethered pyrenes in solution

Article abstract of DOI:10.1063/1.1510745

Interactions of functionalized single-wall carbon nanotubes (SWNT) with tethered pyrenes in solution were investigated. Absorption and emission properties of the pyrene moieties tethered to the carbon nanotubes were investigated. The absorption spectra showed that there was no ground-state complexation between the pyrenes and nanotubes. The fluorescence and the fluorescence excitation results showed that 'intramolecular' excimers were formed by the tethered pyrenes and the nature of excimer formation was dynamic.

Full text of DOI:10.1063/1.1510745

Interactions of functionalized carbon nanotubes with tethered pyrenes in
solution
Liangwei Qu, Robert B. Martin, Weijie Huang, Kefu Fu, Daniel Zweifel et al.
Citation: J. Chem. Phys. 117, 8089 (2002); doi: 10.1063/1.1510745
View online: http://dx.doi.org/10.1063/1.1510745
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 117, NUMBER 17
1 NOVEMBER 2002
Interactions of functionalized carbon nanotubes with tethered pyrenes
in solution
Liangwei Qu, Robert B. Martin, Weijie Huang, Kefu Fu, Daniel Zweifel, Yi Lin,
and Ya-Ping Suna)
Department of Chemistry and Center for Advanced Engineering Fibers and Films, Howard L. Hunter
Chemistry Laboratory, Clemson University, Clemson, South Carolina 29634-0973
Christopher E. Bunker, Barbara A. Harruff, and James R. Gord
Air Force Research Laboratory, Propulsion Directorate, WPAFB, Ohio 45433-7103
Lawrence F. Allard
High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge,
Tennessee 37831-6062
͑Received 10 June 2002; accepted 8 August 2002͒
Single-wall carbon nanotubes ͑SWNTs͒ were functionalized by oligomeric species containing
derivatized pyrenes. Absorption and emission properties of the pyrene moieties tethered to the
functionalized SWNTs were studied in homogeneous solution. The absorption spectra suggest no
significant ground-state complexation between the pyrenes and nanotubes. The fluorescence and
fluorescence excitation results show that the tethered pyrenes form ‘‘intramolecular’’
͑intra-nanotube͒ excimers and that the excimer formation is predominantly dynamic in nature. The
time-resolved fluorescence results show that the pyrene monomer and excimer emissions are
significantly quenched by the attached SWNTs. The quenching is explained in terms of a mechanism
in which carbon nanotubes serve as acceptors for excited-state energy transfers from the tethered
pyrene moieties. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1510745͔
INTRODUCTION
from pyrene excimers in the functionalized SWNTs in solu-
tion and that the carbon nanotubes serve as efficient quench-
ers for the pyrene photoexcited states.
Carbon nanotubes have attracted much recent attention
for a variety of potential technological applications.1,2 For
example, carbon nanotubes have been used as physical dop-
ants with conjugated polymers for the development of novel
opto-electronic materials.3,4 The doping effects are strongly
dependent on interactions of aromatic moieties in the poly-
mers with the nanotube graphitic surface. It has also been
reported that pyrene derivatives could be attached to the gra-
phitic surface of a single-wall carbon nanotube ͑SWNT͒ via
␲-stacking type interactions.5 The functional groups on the
nanotube-attached pyrene moieties were used to link natural
proteins for the biological modification of carbon
nanotubes.5 These examples have demonstrated the need for
a fundamental understanding of interactions between carbon
nanotubes and aromatic molecules on the nanotube surface.
The solubilization of carbon nanotubes via chemical
functionalization offers unique opportunities to study the
nanotube-molecule interactions in homogeneous solution.6
Carbon nanotubes are known to contain surface-bound car-
boxylic acids,7–9 which have been used to attach functional
groups via amide or ester linkages.6,9–20 We have previously
shown that carbon nanotubes can be functionalized with li-
pophilic and hydrophilic dendron species to be made soluble
in a variety of common solvent systems.16,17 Here we report
the preparation and spectroscopic study of the functionalized
SWNTs with tethered pyrene moieties. The fluorescence re-
sults show that there are significant emission contributions
EXPERIMENT
Materials
Methyl 3,5-dihydroxybenzoate ͑98%͒ and 18-crown-6
͑98%͒ were purchased from Avocado Research Chemical
Ltd., 1-bromohexadecane ͑99%͒ and thionyl chloride
͑99.5ϩ%͒ from Acros, 1-pyrenemethanol ͑98%͒ from Ald-
rich, and lithium tetrahydridoaluminate ͑95%͒ from Alfa Ae-
sar. Solvent grade THF was distilled over sodium before use,
and other solvents were either of spectrophotometry/HPLC
grade or purified via simple distillation. Poly͑methyl meth-
acrylate͒ ͑PMMA͒ with an average molecular weight of
350 000 was also purchased from Acros. Deuterated NMR
solvents were obtained from Cambridge Isotope Laborato-
ries.
The SWNT sample was supplied by Professor A. M. Rao
of Physics Department, Clemson University.21 It was purified
by using an established procedure already reported in the
literature.7,22
Methyl 3-hydroxy-5-hexadecoxy benzoate „I…
1-Bromohexadecane ͑1.83 g, 6 mmol͒ was added to a
solution of methyl 3,5-dihydroxybenzoate ͑2 g, 11.9 mmol͒,
potassium carbonate ͑8.1 g, 58.6 mmol͒, and 18-crown-6
͑0.25 g, 0.94 mmol͒ in THF-acetonitrile ͑150 mL–30 mL͒.
After the mixture was refluxed for 10 h, the solvent was
removed on a rotary evaporator. The sample was redissolved
a͒
Author to whom correspondence should be addressed. Electronic mail:
syaping@clemson.edu
0021-9606/2002/117(17)/8089/6/$19.00
8089
© 2002 American Institute of Physics
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J. Chem. Phys., Vol. 117, No. 17, 1 November 2002
Qu et al.
in chloroform. The chloroform solution was washed with
water several times and then dried with anhydrous MgSO4.
The crude product was purified using silica gel column chro-
matography with chloroform as eluting solvent ͑43% yield͒.
1H-NMR ͑500 MHz, CDCl3): ␦ϭ0.88 ͑t, Jϭ7.10 Hz, 3H͒,
1.20–1.40 ͑m, 24H͒, 1.40–1.55 ͑m, 2H͒, 1.7–1.79 ͑m, 2H͒,
3.90 ͑s, 3H͒, 3.97 ͑t, Jϭ6.9 Hz, 2H͒, 5.22 ͑s, 1H͒, 6.60 ͑s,
1H͒, 7.11 ͑s, 1H͒, 7.15 ͑s, 1H͒ ppm; 13C-NMR ͑125 MHz,
CDCl3): ␦ϭ14.21, 22.78, 26.00, 29.45, 29.65, 29.68, 29.74,
29.78, 32.01, 52.36, 68.45, 107.05, 107.91, 108.95, 132.11,
156.65, 160.50, 166.92 ppm.
IPy-SWNT
The functionalization of SWNTs with IPy was accom-
plished via the esterification of the nanotube-bound carboxy-
lic acids.16,17 In a typical experiment, a purified SWNT
sample ͑15 mg͒ was treated in concentrated HCl solution to
fully recover the carboxylic acid groups on the nanotube
surface, followed by refluxing the sample in thionyl chloride
for 24 h to convert the carboxylic acids into acyl chlorides.
After a complete removal of residual thionyl chloride on a
rotary evaporator with a vacuum pump, the nanotube sample
was mixed well with carefully dried IPy ͑150 mg, 0.26
mmol͒ in a flask, heated to 90 °C, and vigorously stirred for
24 h under nitrogen protection. The reaction mixture was
extracted repeatedly with chloroform to obtain a dark-
colored homogeneous solution. After the evaporation of sol-
vent chloroform, the mixture was purified to remove unre-
acted IPy by Soxhlet extraction with acetone for 48 h. The
IPy-SWNT sample was obtained by drying under vacuum at
1-Pyrenemethanchloride „II…
Thionyl chloride ͑0.98 g, 7.6 mmol͒ was added to a so-
lution of 1-pyrenemethanol ͑1.5 g, 6.4 mmol͒ in toluene ͑50
mL͒. After it was refluxed for 12 h, the solution was washed
with water several times and then dried with anhydrous
MgSO4. The product was obtained after the removal of tolu-
1
1
ene on a rotary evaporator ͑ϳ100% yield͒. H-NMR ͑300
60 °C for 24 h. H-NMR ͑500 MHz, CDCl3): ␦ϭ0.4–1.9
MHz, CDCl3): ␦ϭ5.33 ͑s, 2H͒, 8.0–8.40 ͑m, 9H͒. 13C-NMR
͑75 MHz, CDCl3): ␦ϭ44.98, 123.0, 124.96, 125.88, 126.00,
127.50, 127.80, 128.25, 128.50, 129.0, 130.8, 131.0, 131.5,
132.0 ppm.
͑broad͒, 3.0–7.0 ͑broad͒, 7.0–9.0 ͑broad͒ ppm; 13C-NMR
͑500 MHz, CDCl3): ␦ϭ14.22, 22.8, 26.0, 29.47, 29.80, 32.0,
120.0–134.0 ͑broad͒ ppm.
Methyl 3-hexadecoxy-5-pyrenemethoxyl benzonate
„III…
A solution of I ͑1 g, 2.6 mmol͒, II ͑0.64 g, 2.6 mmol͒,
potassium carbonate ͑0.42 g, 3 mmol͒, and 18-crown-6 ͑0.2
g, 0.75 mmol͒ in THF-acetonitrile ͑20 mL–100 mL͒ was
refluxed for 20 h. After the removal of solvent on a rotary
evaporator, the solid sample was redissolved in chloroform.
The chloroform solution was washed with water several
times and then dried with anhydrous MgSO4. The crude
product was purified using silica gel column chromatography
with chloroform as eluting solvent ͑98% yield͒. 1H-NMR
͑500 MHz, CDCl3): ␦ϭ0.88 ͑t, Jϭ7.05, 3H͒, 1.25–1.50 ͑m,
26H͒, 1.70–1.90 ͑m, 2H͒ 3.85–4.10 ͑m, 5H͒, 5.75 ͑s, 2H͒,
6.83 ͑s, 1H͒, 7.25 ͑s, 1H͒, 7.43 ͑s, 1H͒, 7.90–8.40 ͑m, 9H͒
ppm, 13C-NMR ͑125 MHz, CDCl3); ␦ϭ14.22, 22.79, 26.00,
29.46, 29.66, 29.69, 29.79, 32.02, 52.36, 68.48, 69.14,
107.12, 107.88, 108.47, 123.08, 124.75, 125.04, 125.52,
125.56, 126.14, 127.07, 127.46, 127.82, 128.24, 129.39,
130.82, 131.30, 131.75, 132.10, 159.97, 160.36, 167.01 ppm.
3-Decyloxy-5-pyrenyloxyphenylmethan-1-ol „IPy
…
Lithium tetrahydridoaluminate ͑0.12 g, 3.1 mmol͒ was
added to a solution of III ͑1.6 g, 2.6 mmol͒ in dry diethyl
ether ͑25 mL͒. After the solution was refluxed for 8 h, water
͑2 mL͒ was added to quench the reaction. The reaction mix-
ture was filtered to yield the crude product, followed by pu-
rification using silica gel column chromatography with chlo-
The IPy-SWNT sample is soluble in common organic
solvents such as chloroform, THF, and toluene. Results from
the characterization using transmission electron microscopy
͑TEM, Fig. 1͒, defunctionalization in thermal gravimetric
analysis ͑TGA͒, and Raman spectroscopy confirm that the
sample contains functionalized SWNTs.
1
roform as eluting solvent ͑93% yield͒. H-NMR ͑500 MHz,
CDCl3): ␦ϭ0.88 ͑t,Jϭ7.0 Hz, 3H͒, 1.25–1.44 ͑m, 26H͒,
1.72–1.79 ͑m, 2H͒, 3.94 ͑m, 2H͒, 4.66 ͑s, 2H͒, 5.72 ͑s, 2H͒,
6.59–6.74 ͑m, 3H͒, 7.90–8.60 ͑m, 9H͒ ppm; 13C-NMR ͑125
MHz, CDCl3): ␦ϭ14.24, 22.80, 26.00, 29.47, 29.68, 29.71,
29.80, 32.03, 65.51, 68.23, 68.90, 101.10, 105.36, 105.78,
122.0–132.0 ͑15C͒, 143.48, 160.37, 160.72 ppm.
Measurements
UV/vis absorption spectra were recorded on either a Shi-
madzu UV2101-PC or a Perkin Elmer Lambda 900 spectro-
photometer. Fluorescence spectra were measured on either a
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J. Chem. Phys., Vol. 117, No. 17, 1 November 2002
Interactions of carbon nanotubes with pyrenes in solution
8091
FIG. 2. Absorption and fluorescence ͑FLSC, excitation wavelength 337 nm͒
spectra of IPy ͑-.-.-.-͒ and IPy-SWNT ͑—͒ in room-temperature toluene so-
lutions. Shown in the inset is a comparison of the fluorescence excitation
͑emission wavelength 490 nm͒ spectrum of IPy-SWNT ͑-----͒ with the cor-
responding absorption spectrum.
criminators, a 457 biased time-to-amplitude converter, and a
916 A multichannel analyzer. The instrument response func-
tion of the setup was ϳ2 ns ͑FWHM͒.
All solutions used in emission measurements were
deoxygenated in repeated freeze–pump–thaw cycles.
FIG. 1. TEM images of IPy-SWNT on a stainless steel grid coated with
holey LaCrO3. The sample on the grid was heat-treated in air at 400 °C for
30 min before imaging. Both scale bars represent 5 nm.
RESULTS AND DISCUSSION
Spex Fluorolog-2 or a Spex Fluorolog-3 photon-counting
emission spectrometer. The Fluorolog-2 is equipped with a
450-W xenon source, a Spex 340S dual-grating and dual-exit
emission monochromator and two detectors. The two grat-
ings are blazed at 500 nm ͑1200 grooves/mm͒ and 1000 nm
͑600 grooves/mm͒. The room-temperature detector consists
of a Hamamatsu R928P PMT operated at Ϫ950 V, and the
thermoelectrically cooled detector consists of a near-
infrared-sensitive Hamamatsu R5108 PMT operated at
Ϫ1500 V. The Fluorolog-3 is equipped with a 450-W xenon
source, double monochromators for excitation and emission,
and a Hamamatsu R928P PMT operated at Ϫ950 V as detec-
tor. Unless specified otherwise, all emission spectra were
corrected for nonlinear instrumental response by use of pre-
determined correction factors.
Fluorescence decays were measured using the time-
correlated single photon counting ͑TCSPC͒ method. The
TCSPC setup consists of a nitrogen flash lamp ͑Edinburgh
Instruments͒. The 337 nm light was isolated using a band-
pass filter ͑10 nm FWHM͒. Fluorescence decays were moni-
tored through band-pass filters. The detector consists of a
Phillips XP2254/B ͑red-sensitive version of XP2020͒ PMT
in a thermoelectrically cooled housing. The PMT was oper-
ated at Ϫ2 kV using an EG&G Ortec 556 power supply. The
detector electronics from EG&G Ortec include two 9307 dis-
The absorption spectra of IPy and IPy-SWNT in room-
temperature toluene solutions are compared in Fig. 2. Domi-
nating both spectra is the characteristic absorption of deriva-
tized pyrene in the 300–400 nm region. There is no evidence
for any significant absorption contribution from pyrene
ground-state complexes in the IPy-SWNT sample, namely
that the pyrene moieties in IPy and IPy-SWNT samples have
similar molecular environments, resembling simple pyrene
derivatives. It has been reported in the literature that the
planar pyrene structure interacts strongly ͑␲-stacking͒ with
the nanotube graphitic surface in the solid state.5 The results
presented here seem to indicate that the interactions between
carbon nanotubes and the tethered pyrene species in solution
are not strong enough to introduce any substantial changes to
the pyrene absorption spectrum.
The fluorescence spectra of IPy and IPy-SWNT in room
temperature toluene solutions are obviously different, with
the spectrum of IPy-SWNT containing substantial contribu-
tion from the pyrene excimer emission.23,24 The ratio of ex-
cimer to monomer emission intensities is independent of the
solution concentration according to a dilution experiment
͑Fig. 3͒. The results suggest that the excimer formation is
‘‘intramolecular’’ ͑intra-nanotube͒, due to neighboring IPy
moieties that are attached to the same piece of SWNT. This
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J. Chem. Phys., Vol. 117, No. 17, 1 November 2002
Qu et al.
FIG. 4. Absorption and fluorescence spectra of IPy-SWNT in toluene
at 77 K ͑—͒ are compared with those in room-temperature toluene solution
͑-.-.-.-͒.
FIG. 3. Results from a dilution experiment on the concentration indepen-
dence of pyrene fluorescence for IPy-SWNT in room-temperature toluene.
The fluorescence spectra are normalized.
face and perhaps undergo dynamic excimer formulation via
diffusional motions along the surface.
Fluorescence decays of IPy and IPy-SWNT were re-
corded and measured to evaluate the role of carbon nano-
tubes in the deactivation of the pyrene excited states. As
shown in Fig. 5, the fluorescence of IPy in room-temperature
toluene is long-lived, with the lifetime ͑ϳ150 ns͒ compa-
rable to those of other mono-functionalized pyrene mol-
ecules. After IPy is attached to nanotube to form IPy-SWNT,
the fluorescence decays of the pyrene moieties undergo sig-
nificant changes. Both the monomer and excimer fluores-
is conceptually similar to the formation of pyrene excimers
in pyrene-tether-pyrene molecules25 and in linear polymers
with pendantly attached pyrene moieties.26
The fluorescence excitation spectrum of IPy-SWNT
monitored at the excimer emission maximum ͑490 nm͒ is
rather similar to the absorption spectrum ͑Fig. 2͒. This agree-
ment between absorption and excitation spectra suggests that
the intramolecular excimer formation in solution is dynamic
in nature, involving diffusional motions of the participating
pyrene moieties that are not pre-associated in the ground
state.23,25,26 The predominantly dynamic nature of the exci-
mer formation is confirmed by the results obtained from
fluorescence measurements in a frozen solution at low tem-
perature. The excimer emission is largely suppressed for
IPy-SWNT in toluene at 77 K ͑Fig. 4͒,27 because the frozen
solution hinders the diffusion of pyrene moieties required for
the excimer formation.
A SWNT may be considered as a substrate with a large
surface area. The pyrene species tethered to the functional-
ized SWNTs may be compared to the pyrenes adsorbed on a
solid surface. For pyrene derivatives on various surfaces
͑such as silica͒, there is typically strong static pyrene exci-
mer emission due to significant aggregation ͑pre-association͒
of pyrene structures in the ground state.28,29 For IPy-SWNT,
the absorption and emission results suggest that the pyrene
structures in neighboring IPy units ͑attached to the same
piece of SWNT͒ are apparently not stacked together on the
nanotube surface. This might be a result of steric hindrance
in IPy, due to the presence of a long alkyl chain and the
relatively short tether linking the pyrene moiety. However,
the spectroscopic results presented here do not necessarily
preclude the possibility that the pyrene moieties in
IPy-SWNT interact strongly with the nanotube graphitic sur-
FIG. 5. Fluorescence decays of IPy and IPy-SWNT ͑monomer at emission
wavelength 390 nm and excimer at emission wavelength 490 nm͒ in room-
temperature toluene solutions.
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J. Chem. Phys., Vol. 117, No. 17, 1 November 2002
Interactions of carbon nanotubes with pyrenes in solution
8093
FIG. 6. Absorption and fluorescence spectra of IPy ͑-.-.-.-͒ and IPy-SWNT
͑—͒ in PMMA polymer thin film matrix at room temperature.
FIG. 7. Fluorescence decays of IPy and IPy-SWNT ͑emission wavelengths of
390 nm and 490 nm͒ in PMMA polymer thin film matrix at room tempera-
ture.
cence emissions ͑monitored at 380 nm and 490 nm, respec-
tively͒ for IPy-SWNT in room-temperature toluene are
nonexponential, suggesting contributions from a distribution
of emitting pyrene moieties with complicated excited state
processes.23 This is consistent with the known results for the
intramolecular excimer formation and decay in pyrene-
tether-pyrene molecules.25 The observed decay curves can
barely be deconvoluted using a three-exponential function,
yielding lifetimes of ϳ40 ns and ϳ60 ns for the longest-
living components in the monomer and excimer decays,
respectively.30 Despite the difficulty in the deconvolution of
decay curves even with multi-exponential functions, it is ob-
vious from the direct comparison of decays in Fig. 5 that the
pyrene excited states become considerably shorter-lived
upon the attachment of IPy species to SWNTs.30,31 However,
an unambiguous mechanistic consideration of the fluores-
cence decays is complicated by the fact that there are exci-
mer formation and decay processes in IPy-SWNT but not in
IPy. More desirable would be a comparison of the floures-
cence decay results of IPy before and after its attachment to
SWNT without the complication associated with the excimer
formation. As discussed above, the excimer formation in
IPy-SWNT could be suppressed in a solid-like medium.
Thus, polymer matrix was used for such a purpose.
The IPy and IPy-SWNT samples were dispersed in poly-
mer thin films. In the film preparation, to a toluene solution
of IPy or IPy-SWNT was added PMMA to form a highly
viscous polymer blend, followed by casting on a slide. The
fluorescence spectrum of IPy-SWNT in PMMA thin film at
room temperature is broad, with the pyrene excimer emission
largely suppressed ͑Fig. 6͒.27 This is consistent with the re-
sults obtained in frozen solution as discussed above, because
here the diffusional motions of the pyrene moieties are hin-
dered by the solid-state polymer matrix. Shown in Fig. 7 is a
comparison of the fluorescence decays of IPy and IPy-SWNT
in PMMA thin films at room temperature. Obviously, in the
absence of any significant contributions associated with
pyrene excimers in the polymer matrix, the fluorescence de-
cay of IPy-SWNT is again considerably faster than that of
IPy. On the other hand, the decays of IPy in toluene solution
and in PMMA thin film are quite similar, suggesting no fun-
damental medium dependence of pyrene excited state pro-
cesses in IPy. The results of IPy-SWNT, however, point to the
presence of additional excited state decay pathways in the
SWNT-attached pyrene moieties, which are not available in
known pyrene-tether-pyrene molecules.
One likely additional decay pathway is the quenching of
pyrene excited states by the attached carbon nanotube, which
may serve as an energy sink in an excited state energy trans-
fer mechanism. Since SWNTs have significant absorptions in
the visible and near-IR regions, there should be plenty of
low-lying excited states with the nanotubes. As a result, the
coupling between energy levels of the pyrene moieties and
the attached SWNT might be responsible for the proposed
energy transfer quenching of pyrene excited state decays in
IPy-SWNT. However, structural details on the interactions
between excited pyrene species and the attached nanotube
͑for example, the stacking of pyrene species on the nanotube
surface vs other longer-distance couplings͒ remain to be ex-
plored. It seems less likely that ␲-stacking type interactions
play a dominating role in the quenching of pyrene excited
states by the attached nanotube, because such strong and
specific interactions would probably have resulted in more
significant static quenching. Long-distance coupling might
be considered as a more likely mode of interactions between
excited pyrene species and the nanotube graphitic surface in
IPy-SWNT. Further experimental investigations are required.
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J. Chem. Phys., Vol. 117, No. 17, 1 November 2002
Qu et al.
ACKNOWLEDGMENTS
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We are grateful to Professor A. M. Rao and his students
for providing the SWNT sample. Y.P.S. thanks NSF ͑CHE-
9727506 and, in part, EPS-9977797͒, NASA ͑NCC1-01036,
NGT1-52238, and NATl-01036͒, the South Carolina Space
Grant Consortium, and the Center for Advanced Engineering
Fibers and Films ͑NSF-ERC at Clemson University͒ for fi-
nancial support. C.E.B. and J.R.G. thank AFOSR and Dr. J.
Tishkoff for financial support. We also acknowledge the
sponsorship by the Assistant Secretary for Energy Efficiency
and Renewable Energy, Office of Transportation Technolo-
gies, as part of the High Temperature Materials Laboratory
User Program, Oak Ridge National Laboratory, managed by
UT-Battelle, LLC, for the Department of Energy ͑DE-AC05-
00OR22725͒.
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