Synthesis of a water-soluble hexameric metallomacrocycle and its oxidized single-wall carbon nanotube composite

Article abstract of DOI:10.1039/b702797d

Polycationic water-soluble metallomacrocycles possessing 12 positive charges that facilitate ionic coordination with the carboxylate moieties of Oxi-SWNTs form a nanotube composite en route to tailored electronic properties. The multiple ionic pair intera

Full text of DOI:10.1039/b702797d

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis of a water-soluble hexameric metallomacrocycle and its oxidized
single-wall carbon nanotube composite{
Pingshan Wang,a Charles N. Moorefield,b Sinan Li,c Juan Manr´ıquez,d Carol D. Shreiner,a Erika Bustos,d
Aaron L. Hartley,e Luis A. God´ınezd and George R. Newkome*ae
Received 28th February 2007, Accepted 4th May 2007
First published as an Advance Article on the web 16th May 2007
DOI: 10.1039/b702797d
Polycationic water-soluble metallomacrocycles possessing 12 positive charges that facilitate ionic
coordination with the carboxylate moieties of Oxi-SWNTs form a nanotube composite en route to
tailored electronic properties. The multiple ionic pair interactions between the positively charged
metallohexamers and carboxylate modified SWNT are evidenced by TEM and AFM images.
electron-acceptor properties. We herein report the preparation
Introduction
of a water-soluble metallomacrocycle possessing 12 positive
Transition metal-bisterpyridine complexes, specifically with
charges that facilitate ionic coordination with the negatively
Ru(II), have been widely utilized in electron transfer and
charged carboxylate moieties of Oxi-SWNTs thereby forming
photoluminescence research and utilitarian applications.1
a nanotube composite with unique electronic properties.
Crystalline [tpy2RuII] (tpy = 2,29:69,20-terpyridine)2 upon
electrochemical reduction to crystalline [tpy2Ru0] has been
demonstrated3 to undergo a facile reversible, stable, two-
Experimental
electron transfer. The incorporation of this simple monomeric
4,5
General information
unit into stable hexametallomacrocycles [(tpy)6RuII
]
has
6
TEM characterization. An FEI TECNAI 12 transmission
electron microscope (TEM) with an accelerating voltage of
120 kV was used to study the size, distribution, and structure
of the carbon nanotubes and the composite. Samples for
analysis were prepared by adding a droplet of either dilute
solution or suspension to the surface of carbon coated,
400 mesh Cu or Ni grids (available from SPI Supplies),
followed by drying at 25 uC for 48 h.
been achieved via the self-assembly of the ditopic ligand meta-
bis(terpyridinyl)benzene and [Ru(DMSO)4Cl2]. As a result of
the light absorbing properties of these stable hexamers, they
have been utilized as components in photovoltaic devices,6,7
demonstrated to possess photoactive8 and electronic proper-
ties, and employed as polyelectrolytes9,10 by taking advantage
of their donor–acceptor and electrostatic interactions.
Carbon nanotubes11–14 and their corresponding oxidized
single-wall carbon nanotubes (Oxi-SWNT, where ‘Oxi’ is an
abbreviation for oxidized) with their extended p-electron
conjugation are often utilized as electron acceptors. The
Oxi-SWNTs15,16 have been further modified with a wide
variety of useful functional groups by chemical and physical
methods.17,18 Organic or organic/inorganic composites, as well
as biomaterials, have been attached to the ends and sidewalls
of the Oxi-SWNTs by covalent modification in efforts to
enhance their physical and optoelectronic properties.19
Recently, the combination of SWNTs with photoactive electron
donors,20 e.g. porphyrins,21 has been developed for their poten-
tial application as solar energy conversion devices (ca. 4.2%
monochromatic conversion efficiency)22 capitalizing on their
AFM characterization. Droplets of dilute solutions or
suspensions of the commercial Oxi-SWNTs, 2, [(Oxi-
SWNT)n2m] or the disassembled composite, were placed on
the surface of freshly cleaved mica. After drying at 25 uC for
6 h, the samples were characterized by atomic force micro-
scopy (AFM) using a Digital Instruments Nanoscope IIIa
scanning probe microscope equipped with a Multi 75, Force
Modulation Etched Silicon Probe (model MPP-21100) and
operating in tapping mode.
UV characterization. UV-Vis spectra were recorded on an
Ocean Optics, Inc. Chem2000 UV-Vis spectrophotometer.
Cyclic voltammetry. Cyclic voltammograms were recorded
on a BAS C3-Voltammetry Cell Stand with an Epsilon
potentiostat at 20 uC. A three-electrode configuration was
used with a glassy carbon working electrode, a platinum
auxiliary electrode, and an Ag/AgCl reference electrode with a
scan rate of 100 mV s21. The analyte concentration was 1 mg
sample per 10 mL of water. A pH 7 buffer solution (Alfa
Aesar) was used as the supporting electrolyte and solvent.
Depositing a drop of the analyte solution onto the electrode
surface and drying with an infrared lamp formed the film of
the sample. The solvent was purged with argon for 10 minutes
aDepartment of Polymer Science, The University of Akron, Akron,
OH 44325-4717, USA. E-mail: newkome@uakron.edu;
www.dendrimers.com; Fax: (+1) 330-972-2413
bThe Maurice Morton Institute for Polymer Science, The University of
Akron, Akron, OH 44325-4717, USA
cDepartment of Polymer Engineering, The University of Akron, Akron,
OH 44325-4717, USA
dCentro de Investigacio´n y Desarrollo Tecnolo´gico en Electroqu´ımica,
Quere´taro, 76700, Me´xico
eDepartment of Chemistry, The University of Akron, Akron, OH 44325-
4717, USA
{ Electronic supplementary information (ESI) available: TEM and
AFM images. See DOI: 10.1039/b702797d
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prior to the experiment and kept under a blanket of argon
throughout the experiment.
adding NH4PF6 to give (45%) red hexamer 2; mp .300 uC; 1H
NMR (CD3CN) d 9.24 (s, 4H, ArH39,59), 8.81 (d, 4H, PyH3,30),
8.64 (s, 1H, ArH2), 8.01 (s, 2H, ArH4,6), 8.04 (t, 4H, PyH4,40),
7.55 (d, 4H, PyH6,60), 7.28 (s, 4H, PyH5,50), 7.02 (s, 2H, BnH),
5.53 (s, 2H, OCH2Ar), 4.23 (t, 4H, OCH2), 4.15 (t, 4H, OCH2),
3.71–3.46 (m, 40H, OCH2), 3.26 (s, 3H, CH3), 3.31 (s, 6H,
CH3); 13C NMR (CDCl3) d 161.6, 159.3, 156.7, 154.1, 153.6,
148.5, 140.8, 139.3, 133.6, 128.8, 125.9, 123.9, 121.6, 117.2,
108.4, 73.4, 72.7, 71.5, 71.3, 71.2, 71.1, 70.5, 70.0, 59.0; ESI-
MS: m/z: 1276.4 (M 2 7PF6)7+, 1098.1 (M 2 8PF6)8+, 958.3
Experimental procedures
General procedure for synthesis of 1.
A mixture of
di(terpyridinyl)phenol23 (200 mg, 36 mmol), 18-crown-6 ether
(95 mg, 36 mmol) and K2CO3 (49.6 mg, 36 mmol) in DMF
(60 mL) was stirred at 25 uC for 30 min, then the 3,4,5-
tris[(tetraethylene glycol)-functionalized](chloromethyl)ben-
zene24 (40 mmol) was added. The resultant suspension was
stirred at 80 uC for 10 h under N2. The solvent was removed
in vacuo to give a pale yellow oily residue, which was
column chromatographed (Al2O3) eluting with CHCl3–
CH3OH (1 : 10 v/v) to give (90%) the alkylated product, as a
(M 2 9PF6)9+, 849.7 (M 2 10PF6)10+, 758.1 (M 2 11PF6)11+
683.8 (M 2 12PF6)12+
,
.
Synthesis of the [(Oxi-SWNT)n-2m] composite. The oxidized
single-wall carbon nanotubes were purchased from Sigma-
Aldrich Co. and used without further purification. An aqueous
solution of Ru-hexamer 2 (2.5 mg) with five drops of MeCN
was added to an aqueous solution of basic, oxidized single-wall
carbon nanotubes (8 mg) at 25 uC with agitation to afford a
black precipitate. Filtration through a nylon filter, followed by
washing with water then MeOH–H2O (1 : 3) gave (9.1 mg) the
black solid nanotube composite.
1
colorless oil. H NMR (CDCl3) d 8.77 (s, 4H, ArH39,59), 8.71
(d, 4H, PyH3,30), 8.66 (d, 4H, PyH6,60), 7.93 (s, 1H, ArH4), 7.84
(t, 4H, PyH4,40), 7.55 (s, 2H, ArH2,6), 7.32 (t, 4H, PyH5,50), 6.77
(s, 2H, BnH), 5.16 (s, 2H, OCH2Ar), 4.20 (t, 4H, OCH2), 4.16
(t, 2H, OCH2), 3.84 (t, 4H, OCH2), 3.79 (t, 2H, OCH2),
3.71–3.46 (m, 36H, OCH2), 3.34 (s, 3H, CH3), 3.31 (s, 6H,
CH3). 13C NMR (CDCl3) d 159.2, 156.2, 156.1, 152.9, 150.1,
149.2, 141.1, 136.9, 132.1, 123.9, 121.5, 119.3, 114.5, 107.3,
72.4, 72.0, 71.9, 70.9, 70.7, 70.6, 70.56, 70.53, 69.8, 69.0, 59.0;
ESI-MS (m/z, %): 1266.0 (M+ + H, 100%), 1287.9 (M+ + Na)
and 1373.9 (M+ + Ag).
Disassembly of the composite. 2 mg of the [(Oxi-SWNT)n-2m]
composite was suspended in MeCN–H2O (6 ml, 1 : 4 v/v) then
NH4PF6 (1.5 mg) was added; after 5 minutes sonication
at 25 uC, the black solid was completely dissolved and the
solution became red.
Synthesis of Ru-hexamer 2. To a solution of ligand 1 (80 mg)
dissolved in MeOH (20 mL) was slowly added dropwise a
25
mixture of Ru(DMSO)4Cl2 (one equiv.) in MeOH (100 mL)
Results and discussion
at 25 uC. The resultant red solution was refluxed for 12 h then
filtered through a celite layer, and concentrated in vacuo to
give a deep red residue. After column chromatography (SiO2,
CH3Cl–CH3OH 10 : 1), the counterions were exchanged by
The water-soluble bis(terpyridinyl) monomer 1 (Scheme 1)
was prepared (85%) by alkylation of meta-bis(terpyri-
dinyl)phenol23,26 with the known tris-3,4,5-(tetraethylene
Scheme 1 Synthesis of hexarutheniomacrocycle 2a, which is represented by cartoon 2b.
3024 | J. Mater. Chem., 2007, 17, 3023–3029
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Scheme 2 Illustration of multi-ion pair association of Ru-hexamer 2 with Oxi-SWNT.
glycol)-functionalized chloromethylbenzene.24 The structure of
monomer 1 was confirmed (1H and 13C NMR) by the singlets
at 5.16 and 8.77 ppm assigned to the methylene group and
39,59-tpyHs, respectively, as well as the ESI-MS peaks at m/z
1266.0 [M + H+] and 1287.9 [M + Na+]. Self-assembly of 1
was achieved by treatment with one equivalent of
[Ru(DMSO)4Cl2]25 in refluxing MeOH for 18 h to give the
desired [(1)6Ru6(PF6)12] (2), which was column chromato-
graphed (SiO2) with a CHCl3–MeOH (10 : 1) mixture.
of regenerated 2 [(1)6Ru6(PF6)12] was superimposable with the
original sample.
Formation of [(Oxi-SWNT)n(2)m] was further demonstrated
by transmission electron microscopy (TEM), whereby a dilute
sample suspended in H2O was cast onto a carbon-covered
grid (Cu or Ni, 400 mesh). The TEM analysis of [(Oxi-
SWNT)n(2)m] visibly proved the morphologies at different
magnifications, as shown in Fig. 1. Comparison of the
diameter of commercial Oxi-SWNT (Fig. 1A) with that of
[(Oxi-SWNT)n(2)m] showed a diameter increase to ca. 100 nm,
this increase arose from [(Oxi-SWNT)n(2)m] being sandwiched
together and held in place by electrostatic interactions. Higher
magnification (Fig. 1D) gave further information about the
surface of the composite by revealing uniform round spots
(4.1 nm ¡ 0.2 nm) attributed to the Ru-hexamer 2, where the
measured dimensions are consistent with the pertinent dimen-
sions of the hexaruthenium ring, which possesses an edge-to-
Counterion exchange from Cl2 to PF6 via treatment with
2
an aqueous solution of NH4PF6 gave pure red microcrystals.
Formation of the macrocycle was confirmed by the diagnostic
peaks of 6,60-tpyH at 7.55 ppm and 39,59-tpyH at 9.24 ppm
(1H NMR), which exhibited a characteristic low-field shift
(Dd = + 0.47 ppm) and a high-field shift (Dd = 2 1.16 ppm),
respectively, when compared to that of the ligand 1. Multi-
charged peaks from +7 to +12 were observed in the mass
2
˚
˚
spectrum (ESI-MS) of 2 indicating the loss of either PF6 or
edge diameter of 39.5 A and an internal void region of 17.5 A
as determined by molecular modeling studies.
PF5 giving further evidence of macrocyclization.
Our preliminary investigation27 concerning nanocomposites
derived from [(bistpy)6Fe6(PF6)12] functionalized with dendri-
tic light-absorbing antennae exhibited relatively strong photo-
current responses ranging from y0.5 to 2.5 mA cm22 upon
exposure to white light (40 mJ cm22) for the unoptimized
devices. In order to evaluate and demonstrate a ‘proof-of-
concept’ with respect to the capture of photons as well as
energy transfer to an accepter species, the water-soluble
Ru-hexamer 2 was used. When the deep red solution of 2
(2.5 mg in 2 ml of deionized water) was added to the aqueous
solution of Oxi-SWNT (8 mg, pH # 9.0) with agitation at
60 uC, a red cotton-like precipitate immediately started to
form. After 3 h, the reaction was complete resulting in a
notably colorless solution, which lacked either Oxi-SWNT or
Ru-hexamer 2, as determined by UV-vis spectroscopy,
remained. Further evidence for the formation of the nano-
composite was obtained on this clear aqueous solution, in
which a seven peak pattern (31P NMR) for NaPF6, the
combined by-product, was observed. The physical properties
of the nanocomposite [(Oxi-SWNT)n(2)m] were tested after
filtering through a nylon filter (0.2 mm), followed by washing
initially with copious amounts of water to remove any residual
water-soluble components, then with MeOH–H2O (1 : 3 v/v),
and lastly drying in vacuo.
The atomic force microscopy (AFM) images were derived
from one droplet of suspension of [(Oxi-SWNT)n(2)m] on
a freshly cleaved mica surface and dried under ambient
conditions for 6 h (see ESI{). In Fig. 2A, the AFM image of
starting Oxi-SWNT shows the clean, smooth carbon nano-
tubes; however, in Fig. 2B, the AFM image of [(Oxi-
SWNT)n(2)m] indicated that the Ru-hexamer 2 was complexed
onto the walls/ends of the SWNTs. The diameter of the
SWNTs measured 1–1.5 nm (the radius of curvature of the
AFM tip was calculated to be 10.5–11 nm),28 and after
calibration, the diameter and height of Ru-hexamer 2 were
measured at ca. 4–5 nm and 0.6–0.9 nm, respectively, which
was similar to the simulated size of the metallohexamer.5 Some
larger particles were discernible from the sidewall/ends of
SWNTs (see ESI{) due to some aggregation of Ru-hexamer 2.
After the disassociation of [(Oxi-SWNT)n(2)m] by adding
aqueous NH4PF6, the AFM images (see sESI{) showed that
the surface/ends of Oxi-SWNTs were free of Ru-hexamer 2,
which gave further evidence for the ion-pair interactions.
Furthermore, height analysis performed using the AFMs
revealed average hexamer and CNT heights of 8.7 and
˚
13.5 A, respectively. This corresponds well with the average
˚
measured height of the hexamer/CNT composite of 22.5 A (see
ESI{ for details).
The disassembly of [(Oxi-SWNT)n(2)m] was readily induced
by its suspension (2 mg) in MeCN/H2O (3 ml, 1/4 v/v) and
adding NH4PF6 (1.5 mg), followed by 5 minutes of sonication
at 25 uC. The appearance of a red solution with a black
suspension supported the dissociation; the UV-vis absorption
X-Ray photoelectron spectroscopy (XPS; monochromatic
Mg Ka radiation at a power of 250 W, 93.90 eV) for bare Oxi-
SWNT (Fig. 3A) showed typical graphite-carbon-like peaks29
(C1s) at 285 eV and O1s (from oxidized carbon) at 531 eV. The
complexed [(Oxi-SWNT)n(2)m] (Fig. 3B) revealed new peaks
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Fig. 1 TEM images of A) commercial Oxi-SWNT, and the ion-pair association of the [(Oxi-SWNT)n(2)m] composite at different magnifications:
B) scale bar 1 mm, C) scale bar 100 nm, D) scale bar 20 nm.
or PF26 (Na1s is at 1072 eV and P2p at 130.6 eV), supporting
the reaction, SWNT-COONa + [(2)6Ru6(PF6)12] which pro-
duced [(Oxi-SWNT)n2m]; the by-product, NaPF6, was easily
removed by multiple water washings.
IR spectra of Ru-hexamer 2, commercial Oxi-SWNT, and
[(Oxi-SWNT)n(2)m] showed the stretching vibration nP-F of
2
PF6 at 841 cm21 for the starting Ru-hexamer 2 (Fig. 4,
curve C); after ion-pair assembly with Oxi-SWNT (Fig. 4,
curve A), the nP-F stretch disappeared (Fig. 4, curve B) and a
new broad vibration at ca. 1590 cm21 attributed to the
Fig. 2 Typical AFM images of (A) commercially Oxi-SWNTs (scan
area: 1.3 mm 6 1.3 mm, z scale: 0–20 nm), and (B) the hybrid [(Oxi-
SWNT)n(2)m] structure afforded by multi-ion pair assembly (scan area:
6.4 mm x 6.4 mm, z scale: 0–50 nm).
carboxylates of [(Oxi-SWNT)n(2)m] was observed. This shift
from that observed for the uncomplexed Oxi-SWNTs
(1730 cm21, Fig. 4A) further supports the electrostatic com-
plexation. The stretching vibration nC-H attributed to the PEG
chains (2910 cm21, Fig. 4C) still remained after complexation
(see Fig. 4B).
attributed to N1s at 398 eV along with other new peaks
assigned to Ru (3d3/5 at 384 eV and 3d5/5 at 280 eV) confirming
the presence of 2. The atomic O/C ratio (17.6) of neat Oxi-
SWNT was increased to 26.5 after association with 2 affording
further evidence for the complexation and the presence of
bulky PEG chains. The XPS showed no evidence of either Na+
In Fig. 5, comparisons of UV-vis spectra of the starting
Oxi-SWNT, complex 2, and [(Oxi-SWNT)n(2)m] exhibited a
slight hypsochromic shift (ca. 1–2 nm) for the metal–ligand
absorbance between 2 and [(Oxi-SWNT)n(2)m], which supports
Fig. 3 XPS spectra of (A) commercial Oxi-SWNT, and (B) multi-ion pair association of [(Oxi-SWNT)n(2)m].
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Fig. 4 FTIR spectra of A) commercial Oxi-SWNT, the peak at
1730 cm21 is attributed to the surface carboxylic acid groups, B)
assembled ion-pair complex, the new vibration at ca. 1590 cm21 is
attributed to the carboxylates from Oxi-SWNT, along with the
disappearance of nP-F at 841 cm21, and C) solid Ru-hexamer 2, the
Fig. 6 Raman spectra of Oxi-SWNT (black line) and [(Oxi-
SWNT)n(2)m] (grey line). Insert: the breathing mode fields at ca.
165 cm21
.
peak at 841 cm21 indicates the stretching vibration of nP-F from
RBM was observed, which suggested a debundling effect by
the hexamer 2. Notably, the AFM also showed that the height
of the bare tubes in the [(Oxi-SWNT)n2m] composite was in the
range of 1–1.5 nm, which is smaller than that of the Oxi-
SWNT bundles.
2
counterion PF6
.
Upon complexation with 2, the diameter-dependent breath-
ing mode at 165 cm21 moved slightly by ca. 2–3 cm21 (see
Fig. 6 insert).5 The tangential mode (G-band) of the complex
at ca. 1590 cm21 remained unchanged compared to that of
the Oxi-SWNTs. Since ionic coordination does not induce
additional sp3 hybridized carbon defects, there is no significant
change of the intensity ratio of the D and G bands (ID/IG).30
Electrochemical measurements were conducted in order to
follow the construction of the nanocomposite. Electrochemical
experiments were performed using a potentiostat connected to
a three-electrode cell. A glassy carbon disk (diameter = 1 mm)
was used as the working electrode; Pt and Ag wires were used
as counter and pseudo-reference electrodes (all potentials were
referenced to the ferrocenium|ferrocene couple, denoted as
Fc+|Fc), respectively. All experiments were obtained using a
scan rate of 100 mV s21 and 0.1 M Bu4NPF6 in DMF as an
electrolytic solution, previously purged with ultrapure N2,
at 298 K.
Fig. 5 UV-vis absorption spectra in water of (A) commercial
Oxi-SWNT; (B) water-soluble Ru-hexamer 2; (C) complex
[(Oxi-SWNT)n(2)m].
the electronic association.20 The intensity was suppressed after
complexation. A new absorption at ca. 462 nm arose due to the
electrostatic interaction between 2 and the Oxi-SWNTs or
by the partial aggregation of 2 on the sidewalls or ends of
Oxi-SWNT.
The Raman spectrum of Oxi-SWNT (Fig. 6, obtained using
a krypton laser with a wavelength of 647 nm at 4 mW power)
showed a small disorder mode or sp3 mode (D-band, black
line) at ca. 1293 cm21. The frequency of the radial breathing
mode (RBM) is inversely proportional to the diameter of the
carbon nanotubes, therefore an upshift of the RBM suggests a
diameter decrease.30 The commercial Oxi-SWNTs used are
bundles with diameters of 4–5 nm; this is partly due to the
hydrogen bonding interactions between the oxygenated
functionalities (i.e., the carboxylic acid groups) of individual
nanotubes.31,32Amidation of Oxi-SWNTs with octadecylamine
(ODA) resulted in an upshift of the RBM frequency, which
can be attributed to the destruction of hydrogen bonding
and nanotube debundling to afford CNTs with smaller
diameters (1.3 nm).33 During our study, hexamer 2 was used
to intercalate Oxi-SWNT bundles and ionically coordinate
with the carboxylic acid groups thereby further limiting
hydrogen bonding. A slight upshift, ca. 2–3 cm21, of the
The cyclic voltammetry (CV) response of Ru-hexamer
metallomacrocycle (2, 0.1 mM) was obtained in DMF
(Fig. 7A) at 298 K. The CV reveals two reversible peaks at
21.61 and 21.86 V, associated with the redox processes of the
terpyridine moieties.34 The anodic waves, detected between
0.2–0.7 V, are related to the oxidation of the aryl ether groups
anchored to 2.35 Also, another anodic wave was observed
at a more positive potential (0.90 V) corresponding to the
irreversible oxidation of the ruthenium atoms suggesting that
the redox centers in 2 were buried by the large ethylene glycol
chains. Therefore, electron transfer between
electrode surface is reasonably slow.36
2 and the
Since Oxi-SWNT37 and [(Oxi-SWNT)n(2)m] showed low
solubility in the electrolytic medium, their CVs were obtained
by preparing modified electrodes, as follows: 150 mL cm22 of
Oxi-SWNT suspension (1.4 mg per mL of pure DMF) or
[(Oxi-SWNT)n(2)m] previously precipitated in pure water,
transferred using a micropipette and carefully positioned on
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Surface coverage of 2 in the modified electrode with
[(Oxi-SWNT)n(2)m] nanocomposite was estimated to be
1.24 6 1028 mol cm22 using the terpyridine-related cathodic
peak and eqn (1),33
Q
C~
(1)
zFA
in which Q is the integrated charge below the peak (Fig. 7C), z
is the number of electrons transferred per mol of studied
sample, F is Faraday’s constant (96500 C equiv21), and A
represents the electrode area in cm2.
Anodic waves related to oxidation of the aryl ether groups
were not observed (Fig. 7C). A probable explanation for this is
that aryl ether groups can be oxidized in polar media, such as
DMF (dielectric constant 36.7) or MeCN (37.5),29 but the
oxidation process is more difficult in non-polar media, such as
CH2Cl2 (8.93)31 or SWNT (3.24).35 It is noteworthy that the
low dielectric constant for SWNT is also consistent with the
capacitive behavior observed for the modified electrodes,
based on Oxi-SWNT (Fig. 7B).
Conclusion
In conclusion, the multi-ion pair association of Ru-hexamer 2
and Oxi-SWNT was successfully prepared; the self-assembly
phenomenon to form the complex was verified by the
immediate precipitation, as well as the TEM and AFM
images, XPS, IR, UV-vis spectroscopy, and CV. Competing
counterion reagents, such as NH4PF6 and KPF6, can readily
disassemble the non-covalently linked composite at 25 uC. This
self-assembly method shows potential for use in solar cell
conversion or electronic nanodevices, since the components of
the nanocomposites afford entry into the formation of tailored
molecular surfaces capable of yielding dense electron storage
and release devices as well as components for photovoltaic
devices due to the ability of the macrocycles to act as
sensitizers; such studies are currently in progress.
Fig. 7 Cyclic voltammograms obtained in DMF + 0.1 M Bu4NPF6
for (A) 0.1 mM of water soluble Ru-hexamer 2, and modified
electrodes with a film of (B) Oxi-SWNT or (C) [(Oxi-SWNT)n(2)m]
nanocomposite. All experiments were conducted at 298 K and using a
scan rate of 100 mV s21
.
the electrode surface. Once the solvent was removed, the
Oxi-SWNT or [(Oxi-SWNT)n(2)m]-modified electrodes were
immediately immersed in the electrolytic medium.
While voltammograms obtained for Oxi-SWNT films
(Fig. 7B) revealed a capacitive behavior for the electrodes,
based on carbon nanotubes;34 those obtained for [(Oxi-
SWNT)n(2)m] films (Fig. 7C) showed one cathodic peak at
21.13 V, associated with the quasi-reversible redox processes
of the terpyridine moieties (during the inverse potential scan
this peak can be observed at 0 V). This observation can be
explained by invoking the electronic properties of carbon
nanotubes.10–13 According to previous TEM images obtained
for the nanocomposites (Fig. 1C and D), it is possible to
consider carbon nanotubes as the main support of hexamer-
RuII units. In this way, during the cathodic scan, p-electron
conjugation in the carbon nanotubes easily allows the
reduction of terpyridine moieties (Fig. 7C); however, during
the inverse potential scan, the oxidation of terpyridines needs
additional energy, since carbon nanotubes are poor electron
donors (see their capacitive behavior in Fig. 7B).10–13 In
essence, the oxidation of terpyridines in the nanocomposite
should be energetically disfavored. A similar effect is observed
for RuII oxidation in [(Oxi-SWNT)n(2)m], since the corre-
sponding potential (1.35 V, Fig. 7C) was measured at a more
positive value than that previously observed for 2 in solution
(0.90 V, Fig. 7A).
Acknowledgements
The authors gratefully thank the National Science Foundation
[DMR-0401780, INT-0405242], the Air Force Office of
Scientific Research (F49620-02-1-0428, 02), and the Ohio
Board of Regents for financial support.
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