Self-assembly of a tripodal pseudorotaxane on the surface of a titanium dioxide nanoparticle

Article abstract of DOI:10.1021/ja028870z

This paper describes the self-assembly of a heterosupramolecular system consisting of a tripodal viologen, adsorbed at the surface of a titanium dioxide nanoparticle, that threads a crown ether to form a pseudorotaxane. The viologen, a 1,1′-disubstituted-

Full text of DOI:10.1021/ja028870z

Published on Web 04/04/2003
Self-Assembly of a Tripodal Pseudorotaxane on the Surface
of a Titanium Dioxide Nanoparticle
Brenda Long, Kirill Nikitin, and Donald Fitzmaurice*
Contribution from the Department of Chemistry, UniVersity College Dublin,
Belfield, Dublin 4, Ireland
Received October 9, 2002; Revised Manuscript Received February 4, 2003; E-mail: donald.fitzmaurice@ucd.ie
Abstract: This paper describes the self-assembly of a heterosupramolecular system consisting of a tripodal
viologen, adsorbed at the surface of a titanium dioxide nanoparticle, that threads a crown ether to form a
pseudorotaxane. The viologen, a 1,1′-disubstituted-4,4′-bipyridinium salt with a rigid tripodal anchor group,
has been synthesized. This viologen is adsorbed at the surface of a titanium dioxide nanoparticle in solution.
As intended, this tripodal viologen is both oriented normal to and displaced from the surface of the
nanoparticle and threads a crown ether to form the heterosupramolecular complex. The threading of the
crown ether by the tripodal viologen to form the above pseudorotaxane complex at the surface of a titanium
dioxide nanoparticle has been studied by 1H NMR, optical absorption spectroscopy, and cyclic voltammetry.
Introduction
strongly bind the viologen to the surface of a titanium dioxide
nanoparticle. The rigid tripodal arrangement of the linkers
The bottom-up assembly of functional nanoscale architectures
in solution is a goal shared by researchers in many fields.1
Expected benefits include greater control over material proper-
ties and innovative technologies that address unmet needs.2 One
approach envisaged is the self-assembly of nanoscale architec-
tures from molecular and condensed-phase components in
solution and at a surface.3
In considering what molecular and condensed-phase com-
ponents might be suitable,4-6 one is immediately attracted to
functional supermolecules7 and to the rapidly growing number
of nanoparticles whose properties can be tuned by controlling
their size and surface composition.8 It is in this context that we
have sought to synthesize molecular components that are
adsorbed at the surface of a nanoparticle, orientated normal to
the surface of the nanoparticle in order to allow their self-
assembly to form a supermolecule, displaced from the nano-
particle in order to avoid unwanted surface effects, and finally,
sufficiently bulky to preclude unwanted lateral interactions.
A class of functional supermolecule that has attracted a great
deal of attention is the pseudorotaxanes.9 A class of nanoparticle
that has attracted a great deal of attention is metal oxides.10
Accordingly, we have designed and synthesized the tripodal
viologen 9 shown in Scheme 1. The phosphonate linkers
orients the viologen normal to and displaces it from the surface
of the nanoparticle. The inherent bulkiness of the viologen
reduces the possibility of pimerization of the reduced viologen.11
As a consequence, the tripodal viologen 9 complexes the crown
ether 10 at the surface of the titanium dioxide nanoparticle to
form a heteropseudorotaxane 9‚10 shown in Scheme 1.
It should be noted that Fitzmaurice and co-workers12 have
recently described the synthesis of a long-chain alkane thiol
incorporating a terminal crown moiety that is capable of being
adsorbed at the surface of a gold nanoparticle and of threading
a dibenzylammonium cation to form a pseudorotaxane.
It should also be noted that Galoppini and co-workers13 have
recently described the synthesis of tripodal linkers used to adsorb
sensitizer molecules at the surface of the constituent titanium
dioxide nanoparticles of a nanostructured film, forming the
anode of a regenerative photoelectrochemical cell.
Results and Discussion
Design and Synthesis of Molecular Components. The
synthesis of the tripodal viologen 9 was largely based on known
transformations and followed the synthetic strategy outlined in
Scheme 2. The key to the success of this strategy was
minimizing the interaction between any peripheral substituents
and the tertiary carbon in the triarylmethyl alcohol 2. Toward
this end, each leg of molecular tripod was constructed from
(1) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1154-
1196.
(2) Bows, W. InVestors Chronicle 2002, 139, 30-32.
(3) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1212-
1319.
(4) Service, R. F. Science 1997, 277, 1036-1037.
(5) Mann, S.; Shenton, W.; Li, M.; Connolly, S.; Fitzmaurice, D. AdV. Mater.
2000, 12, 147-150.
(6) Niemeyer, C. Angew. Chem., Int. Ed. 2001, 40, 4128.
(7) Lehn, J.-M. Supramolecular Chemistry, Concepts and PerspectiVes;
VCH: Weinheim, Germany, 1995.
(11) Felderhoff, M.; Heinen, S.; Molisho, N.; Webersinn, S.; Walder, L. HelV.
Chim. Acta 2000, 83, 181.
(12) (a) Fitzmaurice, D.; Rao, S. N.; Preece, J. A.; Stoddart, J. F.; Wenger, S.;
Zaccheroni, N. Angew. Chem., Int. Ed. 1999, 38, 1147. (b) Ryan, D.; Rao,
S. N.; Rensmo, H.; Fitzmaurice, D.; Preece, J. A., Wenger, S.; Stoddart J.
F.; Zaccheroni, N. J. Am. Chem. Soc. 2000, 122, 6252. (c) Chia, S.; Cao,
J.; Stoddart, J. F.; Zink, J. I. Angew. Chem., Int. Ed. 2001, 40, 2447.
(13) (a) Galoppini, E.; Guo, W. J. Am. Chem. Soc. 2001, 123, 4342. (b)
Galoppini, E.; Guo, W.; Zhang, W.; Hoetz, P. G.; Qu, P.; Meyer, G. J. J.
Am. Chem. Soc. 2002, 124, 7801.
(8) Alivisatos, A. P. Science 1996, 271, 933-937.
(9) Ashon, P. R.; Glink, P. T.; Stoddart, J. F.; Tasker, P. A.; White, A. J. P.;
Williams, D. Chem. Eur. J. 1996, 2, 729.
(10) Hagfeldt, A.; Gratzel, M. Chem. ReV. 1995, 95, 49.
9
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J. AM. CHEM. SOC. 2003, 125, 5152-5160
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Pseudorotaxane Self-Assembly on a TiO Nanoparticle
A R T I C L E S
2
Scheme 1. (Top) Tripodal Viologen 9 Adsorbed Normal to and Displaced from the Surface of a Titanium Dioxide Nanoparticle and
Complexing 10 to Form a Pseudorotaxane 9‚10, and (bottom) Tripodal Viologen 8b Complexing 10 in the Solution
two p-phenylene groups. This allowed the triarylmethyl deriva-
tive of aniline 3 to be prepared from 2 due to a relatively weak
influence of tertiary carbon on the terminal halogen functional
groups.
The products 8a or 8b were isolated in the form of a Cl- or
PF6- salt, respectively. The hexafluorophosphate salts of 8a or
8b were used for measurements made in solution and for the
subsequent transformation into corresponding phosphonic acid
9.
The alcohol 2 was prepared from 4,4′-dibromobiphenyl in a
high-yield step that involved lithiation followed by nucleophilic
addition of the resulting intermediate 4-(4-bromophenyl)-
phenyllitium to diethyl carbonate.14 The alcohol 2 was success-
fully converted into the aniline derivative 3 by a known
electrophilic alkylation procedure in acetic acid.15 To convert
the amine 3 into the ester 4, a palladium-catalyzed substitution
was carried out. The conditions were similar to those previously
reported,16 but a more suitable catalyst was employed. To
prepare the monocation 6, an elegant procedure that used the
bipyridinium salt 5 was applied.17 In the course of this ring
opening-closure reaction, the nitrogen atom of 5 is displaced
by a nitrogen originating from the ester 4.
Formation of Pseudorotaxanes in Solution. Threading of
the crown ether 10 by the tripodal viologen 8b‚2PF6 to form
the pseudorotaxane 8b‚10‚2PF6 is shown in Scheme 1. Com-
plexation is accompanied by characteristic changes in the
measured 1H NMR spectra, optical absorption spectra, and cyclic
voltammograms (Figure 1).
The 1H NMR spectra (expansion of the bipyridimium proton
signal range) of the viologen 8b‚2PF6 (5 × 10-3 mol dm-3),
the crown ether 10 (5 × 10-3 mol dm-3) and the corresponding
1:1 8b‚10‚2PF6 complex (5 × 10-3 mol dm-3 each component)
are shown in the top panel of Figure 1. Pronounced changes in
the chemical shifts of the protons in the 3 and 3′ positions of
4,4′-bipyridinium dication, as well as less pronounced changes
for the protons in the 2 and 2′ positions, are observed.19
The alkylation of 6 with EtBr (7a) proceeds in acetonitrile in
high yield. Alkylation with reagent 7b, structurally similar to
the well-known stopper groups, proceeds in benzonitrile with
high yield.18
(16) Gonzalez-Bello, C.; Abell, C.; Leeper, F. J. J. Chem. Soc., Perkin. Trans.
1 1997, 1017.
(17) Hanke, M.; Lutz, C. Synthesis 1980, 31.
(18) Ashton, P. R.; Philp, D.; Spencer, N.; Stoddart, J. F. J. Chem. Soc., Chem.
Commun. 1992, 1124.
(14) Chen, L. S.; Chen, G. J.; Tamborski, C. J. Organomet. Chem. 1983, 251,
139.
(15) Organic Syntheses; Wiley: New York, 1955; Collect. Vol. III, 47.
9
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A R T I C L E S
Long et al.
Scheme 2. Synthetic Scheme for Preparation of Tripodal Viologen
9, Nontripodal Viologen 12, and Stabilizers 13 and 14
The optical absorption spectra of the viologen 8b‚2PF6 (2 ×
10-4 mol dm-3), in the absence and presence of the crown ether
10 (1 × 10-3 mol dm-3), are shown in the middle panel of
Figure 1. The difference between the spectra measured for the
corresponding 1:1 8b‚10‚2PF6 complex and the viologen
8b‚2PF6 is plotted and indicates the presence of a charge-transfer
absorption band at 390 nm with a shoulder at 480 nm.20
The corresponding cyclic voltammograms of the viologen
8b‚2PF6 (1 × 10-3 mol dm-3), in the absence and presence of
the crown ether 10 (5 × 10-3 mol dm-3) are shown in the
bottom panel of Figure 1. It is clear that the first peak reduction
potential of the viologen is shifted to more negative potentials
by 41 mV in the presence of the added crown.21 The corre-
sponding oxidation potential is also shifted to more negative
potentials by 35 mV.
Figure 1. (Top panel) 1H NMR spectra of 8b‚2PF6 (5 × 10-3 mol dm-3),
10 (5 × 10-3 mol dm-3), and an equimolar mixture of 8b‚2PF6 and 10
(5 × 10-3 mol dm-3 each) in acetonitrile at 30° C. (Middle panel) Optical
absorption spectra of (‚‚‚) 8b‚2PF6 (2 × 10-4 mol dm-3), a mixture (s) of
8b‚2PF6 (2 × 10-4 mol dm-3) and 10 (1 × 10-3 mol dm-3), and the
difference (- - -) of these two spectra in acetonitrile at 25° C. (Bottom
panel) Cyclic voltammograms of (- - -) 8b‚2PF6 (1 × 10-3 mol dm-3
)
and a mixture (s) of 8b‚2PF6 (1 × 10-3 mol dm-3) and 10 (5 × 10-3 mol
dm-3) in methanol at 25° C.
The complexation of the nontripodal viologen 11‚2Br with
crown ether 10 to form the pseudorotaxane 11‚10‚2Br was also
studied. As above, complexation is accompanied by changes
1
in the measured H NMR spectra, optical absorption spectra,
and cyclic voltammograms (Figure 2).
The 1H NMR spectra (expansion of the bipyridinium proton
signal range) of the viologen 11‚2Br (5 × 10-3 mol dm-3), the
crown ether (5 × 10-3 mol dm-3), and the corresponding 1:1
11‚10‚2Br complex (5 × 10-3 mol dm-3 each component) are
shown in the top panel of Figure 2. The resonances assigned to
the protons at the 3 and 3′ positions of 4,4′-bipyridinium are
(19) Allwood, B. L.; Spencer, N.; Shahriari-Zavareh, H.; Stoddart, J. F.;
Williams, D. J. J. Chem. Soc., Chem. Commun. 1987, 1064.
(20) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry; Wiley: New York,
2000.
(21) Ashton, P. R.; Ballardini, R.; Balzani, V.; Credi, A.; Ruprecht-Dress, K.;
Ishow, E.; Kleverlaan, C.; Kocian, O.; Preece, J.; Spencer, N.; Stoddart, J.
F.; Venturi, M.; Wenger, S. Chem. Eur. J. 2000, 6, 3558.
9
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Pseudorotaxane Self-Assembly on a TiO Nanoparticle
A R T I C L E S
2
Table 1. Association Constants and Derived Free Energies for
Formation of a Viologen-Crown Complex at 25 °C
viologen
solvent
K (M-1
)
∆G° (kcal mol-1
)
a
8b‚2PF6
8b‚2PF6
8b‚2Cl2
11‚2Br
MeOH/CHCl3
MeCN
MeOH/CHCl3
MeOH/CHCl3
1243 ( 20
795 ( 16
650 ( 8
-4.203
-3.940
-3.821
-3.83
139 ( 5
The corresponding cyclic voltammograms of the viologen 11‚
2Br (1 × 10-3 mol dm-3), in the absence and presence of the
crown ether 10 (5 × 10-3 mol dm-3), are shown in the bottom
panel of Figure 2. It is clear that the first peak reduction potential
of the viologen is shifted to more negative potentials by 30 mV
in the presence of the added crown.21 The corresponding
oxidation potential is shifted to more negative potentials by only
3 mV.
These findings are accounted for by the electron-deficient
viologens forming a stable colored complex with the electron-
rich crown ether. As a result there is a transfer of electron density
to the viologen from the crown. The stabilizing interactions
include a π-π charge transfer between the electron-deficient
1,1′-quaternized pyridinium rings of the viologen and the
electron-rich p-phenylene rings of the crown. This stabilization
is reinforced by a δ+-δ- electrostatic interaction between the
positive nitrogens in the viologen and the electronegative
oxygens in the crown. The pseudorotaxane may be further
stabilized by H-bonding between the 2- and 2′-hydrogens of
the bipyridinium and the oxygens of the crown ether.
Quantifying the collective strength of these noncovalent
interactions requires that the association constant, Ka, for the
complex be determined. The spectrophotometric method used
to determine Ka was that developed by Ray and adapted by
Stoddart for pseudorotaxanes.22,23 The association constants Ka
and derived values for the change in free energy ∆G° are
recorded in Table 1. It is evident that the Ka for the tripodal
viologen, 8b‚2PF6 in acetonitrile, and both 8b‚2PF6 and
8b‚2Br in methanol/chloroform (1:1 by volume), are substan-
tially higher than for the simple viologen 11‚2Br in methanol/
chloroform (1:1 by volume). Since 8b has a phenyl ring
conjugated to the viologen, it is possible that this phenyl ring,
linking the tripod moiety to the viologen, allows the crown ether
10 to adopt a more energetically stable conformation within
the complex formed than is the case for the complex formed
with 11‚2Br. It is also possible that the long-chain (phenoxy-
ethoxy)ethoxy group hinders dissociation of 8b‚10‚2PF6.
Consistent with this finding, it is concluded in the case of
8b‚10‚2PF6 that, following reduction of the pseudorotaxane, the
crown remains associated, albeit more weakly, with the radical
cation of the viologen. This accounts for the finding that both
the first reduction potential and the corresponding oxidation
potential of the viologen are shifted to more negative potentials.
It is also concluded in the case of 11‚10‚2Br that, following
reduction of the pseudorotaxane, the complex dissociates fully.
This accounts for the finding that while the first reduction
potential is shifted to more negative potentials, the corresponding
oxidation potential is not.21
1
Figure 2. (Top panel) H NMR spectra of 11‚2Br (5 × 10-3 mol dm-3),
10 (5 × 10-3 mol dm-3), and an equimolar mixture of 11‚2Br and 10 (5 ×
10-3 mol dm-3 each) in acetonitrile at 30° C. (Middle panel) Optical
absorption spectra of (‚‚‚) 11‚2Br (2 × 10-4 mol dm-3), a mixture (s) of
11‚2Br (2 × 10-4 mol dm-3) and 10 (1 × 10-3 mol dm-3), and the
difference (- - -) of these two spectra in acetonitrile at 25° C. (Bottom
panel) Cyclic voltammograms of (- - -) 11‚2Br (1 × 10-3 mol dm-3
)
and a mixture (s) of 11‚2Br (1 × 10-3 mol dm-3) and 10 (5 × 10-3 mol
dm-3) in methanol at 25° C.
shifted significantly, while those assigned to the protons at 2
and 2′ positions show smaller shifts.19
The optical absorption spectra of the viologen 11‚2Br (2 ×
10-4 mol dm-3) with and without the crown ether 10 (1 × 10-3
mol dm-3) are shown in the middle panel of Figure 2. The
corresponding difference spectrum indicates the presence of a
charge-transfer absorption band at 444 nm.20
Adsorption of Viologens at the Surface of a Nanoparticle.
It has been established that both the tripodal viologen 8b‚2PF6
(22) Ray, A. J. Am. Chem. Soc. 1971, 93, 7146.
(23) Colquhoun, H. M.; Goodings. E. P.; Stoddart; J. F.; Wohlstenholme, J. B.;
Williams, D. J. J. Chem. Soc., Perkin Trans 2 1985, 607.
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A R T I C L E S
Long et al.
and nontripodal viologen 11‚2Br thread the crown ether 10 to
form a pseudorotaxane in solution. What remains to be
established is whether, as expected, the tripodal viologen
9‚2PF6 (acid analogue of 8b‚2PF6) threads the crown ether 10
to form a pseudorotaxane when adsorbed at the surface of a
titanium dioxide nanoparticle. It is not expected that the
nontripodal viologen 12‚2PF6 (acid analogue of 11‚2Br) will
do so.
A charge-stabilized dispersion of titanium dioxide nano-
particles (5 nm diameter) was prepared by hydrolysis of titanium
orthotetraisopropoxide in water at pH 2.24 The tripodal viologen
9‚2Br readily chelates surface Ti4+ states and is adsorbed at
the surface of the above nanoparticles. The number of Ti4+ sites
on the surface of each nanoparticle was estimated, on the basis
of literature values for the number of such states (2 × 1014
cm-2), to be 160.25 An average of 13 viologens 9‚2Br or 39
12‚2Cl, occupying 39 or 13 of the available sites, respectively,
were adsorbed at the surface of each titanium dioxide nano-
particle. Structurally similar stabilizer molecules, 1-(2-phos-
phoethyl)pyridinium bromide (13) or a partially deuterated
analogue, namely, 1-(2-phosphoethyl)pyridinium-d5 bromide
(14), were adsorbed at the remaining Ti4+ sites on the surface
of each nanoparticle. The above nanoparticles were precipitated
by addition of ammonium hexafluorophosphate, washed with
dry methanol, and redispersed in acetonitrile to form a stable
optically transparent dispersion. The concentrations of nano-
particles given below are in moles of particles per cubic
decimeter.
Formation of Pseudorotaxanes on the Surface of a
Titanium Dioxide Nanoparticle. Titanium dioxide nano-
particles modified by the adsorption of the viologen 9‚2PF6 or
12‚2PF6 and stabilized by the adsorption of 13 or 14 were
studied by 1H NMR, optical absorption spectroscopy, and cyclic
voltammetry in the absence and presence of the crown ether
10.
The tripodal viologen 9‚2PF6 (2 × 10-3 mol dm-3) was
adsorbed at the surface of titanium dioxide nanoparticles (1.5
1
× 10-4 mol dm-3). The H NMR spectra of 9‚2PF6 adsorbed
at the surface of the above nanoparticles, in the absence and
presence of a 5-fold excess of 10 (1 × 10-2 mol dm-3), and of
8b‚2PF6 (5 × 10-3 mol dm-3) in solution are shown in the top
panel of Figure 3. It should be noted that the partially deuterated
stabilizer 14 was used in the preparation of these nanoparticles
as it provided the required spectroscopic window.
It is clear that the tripodal viologen 9‚2PF6 is adsorbed at
the surface of the titanium dioxide nanoparticle. That this is
the case may be deduced from the reduction in intensity and
broadening of the resonances assigned to protons in 9‚2PF6.
Specifically, the following resonances are significantly re-
duced in intensity and broadened: δ 9.3 (the 2 position of 4,4′-
bipyridinium), 9.2 (the 2′ position of 4,4′-bipyridinium), 8.6 (the
3 position of 4,4′-bipyridinium), 8.5 (the 3′ position of 4,4′-
bipyridinium), 7.9-7.8 (phosphorylated p-phenylenes of tripod
legs and p-phenylene link to bipyridinium), 7.7 (p-phenylene
of tripod legs), 7.5 (p-phenylene of tripod legs and the 3-position
of N-benzyl), 7.1 (the 2 position of N-benzyl), and 5.6
(methylene of N-benzyl). The reason these resonances are
reduced in intensity and broadened is due in part to a
Figure 3. (Top panel) 1H NMR spectra of 8b‚2PF6 (5 × 10-3 mol dm-3
)
in solution and 9‚2PF6 (2 × 10-3 mol dm-3) on titanium dioxide
nanoparticles (1.5 × 10-5 mol dm-3), in the absence and presence of added
10 (1 × 10-2 mol dm-3) in acetonitrile at 30° C. (Middle panel) Optical
absorption spectra of (s) a mixture of 9‚2PF6 (2 × 10-4 mol dm-3) on
titanium dioxide nanoparticles (1.5 × 10-5 mol dm-3) with 10 (1 × 10-3
mol dm-3), (‚‚‚) of 9‚2PF6 (2 × 10-4 mol dm-3) on titanium dioxide
nanoparticles (1.5 × 10-5 mol dm-3), and the difference (- - -) of these
two spectra in acetonitrile at 25° C. (Bottom panel) Cyclic voltammograms
of 9‚2PF6 (6 × 1016 molecules cm-2) in the presence (s) and absence
(- - -) of 10 (1 × 10-2 mol dm-3) in acetonitrile at 25° C.
(24) O’Reagen, B.; Graetzel, M. Nature 1991, 353, 737.
(25) Finklea, H. O. Semiconductor Electrodes; Elsevier: Amsterdam, 1988.
9
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Pseudorotaxane Self-Assembly on a TiO Nanoparticle
A R T I C L E S
2
discontinuity in the paramagnetic susceptibility at the surface
of the titanium dioxide nanoparticle and in part to the restricted
motion of the immobilized tripodal viologen.26
By comparison, the following resonances are not significantly
reduced in intensity or broadened: δ 7.3 (the 3 position of
terminal phenyl), 6.9-7.0 (the 2 and 4 positions of the terminal
phenyl), 4.1 (ether link positions adjacent to aryloxy groups),
and 3.9 (rest of ether link positions). The reason these resonances
largely persist is that the long-chain (phenoxyethoxy)ethoxy
moiety attached to the viologen is both sufficiently far from
the nanoparticle surface and free to rotate.
The proton resonances, which are observed to shift upon
complexation of the crown ether 10 by the tripodal viologen
8b‚2PF6 in solution (Figure 1, top panel), are those assigned to
the protons in the 3,3′ and 2,2′ positions of the 4,4′-bipyridinium
moiety. What is clear from the findings presented here is that
similar shifts are observed upon complexation of the crown ether
10 by the tripodal viologen 9‚2PF6 adsorbed at the surface of a
TiO2 nanoparticle (Figure 3, top panel). It should be noted that
these shifts could only be observed by use of the partially
deuterated stabilizer 14.
The optical absorption spectra of the tripodal viologen
9‚2PF6 (2 × 10-4 mol dm-3) adsorbed at the surface of a
titanium dioxide nanoparticle (1.5 × 10-5 mol dm-3), in the
absence and presence of the crown ether 10 (1 × 10-3 mol
dm-3), are shown in the middle panel of Figure 3. The resulting
difference spectrum is similar to that measured for the ester
analogue 8b‚2PF6 and the crown ether 10 (Figure 1, middle
panel).
The corresponding cyclic voltammograms of the viologen
9‚2PF6 (6 × 1016 molecules cm-2) adsorbed at the surface of
the constituent TiO2 nanoparticles of a nanostructured film, in
the absence and presence of the crown ether 10 (1 × 10-2 mol
dm-3), are shown in the bottom panel of Figure 3. It is found
that the first and second peak reduction potentials of the viologen
are shifted to more negative values by 44 and 48 mV,
respectively, in the presence on the added crown. It is also found
that the first and second peak oxidation potentials are shifted
to more negative values by 25 and 26 mV in the presence of
added crown.
1
Figure 4. (Top panel) H NMR spectra of 11‚2Br (5 × 10-3 mol dm-3
)
in solution and 12‚2PF6 (2 × 10-3 mol dm-3) on titanium dioxide
nanoparticles (1.5 × 10-5 mol dm-3), in the absence and presence (not
shown) of added 10 (1 × 10-2 mol dm-3) in acetonitrile at 30° C. (Middle
panel) Optical absorption spectra of (s) a mixture of 12‚2PF6 (2 × 10-4
mol dm-3) on titanium dioxide nanoparticles (1.5 × 10-5 mol dm-3) with
10 (1 × 10-3 mol dm-3), (‚‚‚) 12‚2PF6 (2 × 10-4 mol dm-3) on titanium
dioxide nanoparticles (1.5 × 10-5 mol dm-3), and the difference (- - -)
of these two spectra in acetonitrile at 25° C. (Bottom panel) Cyclic
voltammograms of 12‚2PF6 (6 × 1016 molecules cm-2) in the presence
(s) and absence (- - -) of 10 (1 · 10-2 mol dm-3) in acetonitrile at
25° C.
On this basis it is concluded that the tripodal viologen
9‚2PF6 adsorbed at the surface of a titanium dioxide nanoparticle
threads the crown ether 10 to form a heteropseudorotaxane. It
is also concluded that transfer of an electron to the viologen to
form the corresponding radical cation does not result in
dissociation of the heteropseudorotaxane at the surface.
It has not been possible to accurately determine Ka for the
complex 9‚10‚2PF6 formed at the surface of a titanium dioxide
nanoparticle. This is because, at the nanoparticle concentrations
used, the sample is not sufficiently transparent. However, it can
be reasonably assumed that the Ka is similar to that measured
in acetonitrile for 8b‚2PF6 (795 ( 16 mol-1 dm). The fact that
the magnitude of the charge-transfer absorption assigned to the
formation of complex is similar, as shown in the middle panels
of Figures 1 (0.027) and 2 (0.024), supports that assumption.
(2 × 10-3 mol dm-3) adsorbed on the nanoparticles (1.5 × 10-4
mol dm-3) and 1H NMR spectrum of the free 12‚2Br (5 × 10-3
mol dm-3) are shown in the top panel of Figure 4. It should be
noted that the partially deuterated stabilizer 14 was used in the
preparation of the nanoparticle dispersion, as it again provided
the required spectroscopic window.
It is clear from the top panel of Figure 4 that the viologen
12‚2PF6 is adsorbed at the surface of the titanium dioxide
nanoparticle. This has been deduced from the significant
reduction in intensity and broadening of the proton resonances.
Specifically, the following resonance is not observed at the
1
The corresponding H NMR and optical absorption spectra
of nontripodal viologen 12‚2PF6 on titanium dioxide nano-
1
particles were measured. The H NMR spectrum of 12‚2PF6
(26) Korgel, B. A.; Fullan, S.; Connoly, S.; Fitzmaurice, D. J. Phys. Chem.
1998, 102, 8379.
9
J. AM. CHEM. SOC. VOL. 125, NO. 17, 2003 5157

A R T I C L E S
Long et al.
Experimental Section
surface: δ 2.6 (the methylene group R to the phosphonic acid).
By comparison, the following resonances, although significantly
reduced in intensity and broadened, are observed: δ 9.0 (the 2
and 2′ positions of 4,4′-bipyridinium), 8.4 (the 3 and 3 positions
of 4,4′-bipyridinium), 4.95 (the methylene group â to the
phosphonic acid), 4.7 (the methylene group of the terminal
ethyl), and 1.7 (methyl of the terminal ethyl, not shown in Figure
4, top panel). Significantly, addition of the crown 10 (2 × 10-3
mol dm-3) to the above dispersion does not result in any
measured change in the spectrum for 12‚2PF6 adsorbed at the
surface of the titanium dioxide nanoparticle (Figure 4, top panel).
This finding suggests that the corresponding heteropseudo-
rotaxane is not formed.
Synthesis of Tripodal Viologens. Reagents and solvents were
purchased from Aldrich Co. and were used as received. All reactions
were performed under nitrogen. The glassware was flame-dried. The
chromatography separations were performed with silica (Merck, 40-
63 µm) and the specified solvent system. Melting points were estimated
with a Gallenkamp melting point device and were not corrected. NMR
spectra were recorded on Varian Inova 300 and 500 spectrometers in
the indicated solvent at 30 °C. Proton NMR spectra were recorded at
299.89 and 499.82 MHz, and phosphorus NMR spectra, at 121.39 MHz.
Mass spectra were recorded on a Finnigan Mat INCOS 50 quadrupole
mass spectrometer.
The viologens 1-{4-[tris[4-(4-dihydrophosphonophenyl)phenyl]-
methyl]phenyl}-1′-{4-[2-(2-phenoxyethoxy)ethoxy]benzyl}-4,4′-bipy-
ridinium dibromide (9b‚2Br) and 1-ethyl-1′-phosphonoethyl-4,4′-
bipyridinium dibromide (12‚2Br) and the stabilizers 1-(2-phosphono-
ethyl)pyridinium bromide (13‚Br) and the partially deuterated analogue
1-(2-phosphonoethyl)pyridinium-d5 bromide (14‚Br) were synthesized
as shown in Scheme 2.
It is interesting to note that the proton resonances assigned
to the viologen moiety of 12‚2PF6, although significantly
1
reduced in intensity and broadened, persist in the H NMR
spectrum of this molecule adsorbed at the surface of a TiO2
nanoparticle. This is comparable behavior to that observed for
the proton resonances assigned to the viologen moiety in
9‚2PF6. This would suggest that the viologen moiety of 12‚
2PF6 is not absorbed flat at the surface of the nanoparticle, as
might have been expected to be the case with a nontripodal
phosphonate linker.
The optical absorption spectrum of the nontripodal viologen
12‚2PF6 (2 × 10-4 mol dm-3) at the surface of a titanium
dioxide nanoparticle (1.5 × 10-5 mol dm-3) in the absence and
presence of crown ether 10 (1 × 10-3 mol dm-3) are shown in
the middle panel of Figure 4. No changes were observed in the
optical absorption spectrum of 12‚2PF6 adsorbed on titanium
dioxide nanoparticles upon addition of 10.
The corresponding cyclic voltammograms of the nontripodal
viologen 12‚2PF6 (6 × 1016 molecules cm-2) adsorbed at the
surface of the constituent TiO2 nanoparticles of a nanostructured
film, in the absence and presence of the crown ether 10 (1 ×
10-2 mol dm-3), are shown in the bottom panel of Figure 4. It
is found that neither the first nor the second peak reduction
potential of the viologen is shifted to a more negative value in
the presence of the added crown. It is also found that neither
the first nor the second peak oxidation potential is shifted to a
more negative value in the presence of added crown.
On the basis of the above findings, it may be concluded that
the nontripodal viologen 12‚2PF6 does not thread the crown
ether 10 to form a pseudorotaxane when adsorbed at the surface
of a nanoparticle. On the basis of the above findings, it may
also be concluded that the viologen moiety is not adsorbed flat
at the surface of the titanium dioxide nanoparticle. Accordingly,
one is led to the conclusion that either the viologen moiety in
12‚2PF6 possesses an average orientation with respect to the
nanoparticle surface that disfavors complexation or it is suf-
ficiently close to the surface that unfavorable interactions
between the crown and the surface dominate and prevent
complexation. Studies are ongoing to address this question.
Tris[4-(4-bromophenyl)phenyl]methanol (2). Diethyl ether (12
cm3) was added to a solution of 1 (1.56 g, 5 mmol) in THF (12 cm3),
the mixture was cooled to -75 °C, and n-butyllithium (2.4 cm3 of 2.5
mol dm-3 in hexane, 6 mmol) was added dropwise over 90 min. The
mixture was stirred for 1 h, after which diethyl carbonate (0.14 g, 1.25
mmol) in THF (1 cm3) was added slowly. The mixture was allowed to
warm to 0 °C (ice bath) and was stirred for 3 h. The mixture was
quenched with methanol (1 cm3) and the solvent was removed under
vacuum. The residue was extracted with ethyl acetate, and the extract
was washed with water, dried (MgSO4), and concentrated. The oily
residue was taken up in chloroform and separated by chromatography
(chloroform-hexane). The product 2 (0.40 g; 44%) was isolated as a
white solid, mp 166 °C: 1H NMR (CDCl3) δ 7.59 (d, 6H, J ) 8.5
Hz), 7.56 (d, 6H, J ) 8.5 Hz), 7.48 (d, 6H, J ) 8.5 Hz), 7.46 (d, 6H,
J ) 8.5 Hz), 2.88 (s, 1H); MS m/z 726 (M+, 81Br, 100%). Anal. Calcd
for C37H25Br3O: C, 61.27; H, 3.47. Found: C, 61.50; H 3.81.
4-{Tris[4-(4-bromophenyl)phenyl]methyl}aniline (3). Acetic acid
(12 cm3), 2 (0.66 g, 0.9 mmol), and aniline hydrochloride (0.21 g, 1.8
mmol) were added to toluene (4 cm3), and the mixture was stirred at
100 °C for 24 h. The solvents were evaporated under vacuum, after
which methanol (20 cm3) and HCl (2 mol dm-3, 1 cm3) were added.
The slurry was refluxed for 24 h and the solvent was evaporated under
vacuum. The residue was dissolved in chloroform and washed with
aqueous NaHCO3. The chloroform extract was dried (MgSO4) and
concentrated under vacuum. The residue was purified by chromatog-
raphy (ethyl acetate-hexane) to give 3 (0.57 g, 78%) as a pale solid:
1
mp 152 °C; H NMR (CDCl3) δ 7.57 (d, 6H, J ) 8.6 Hz), 7.5 (m,
12H), 7.37 (d, 6H, J ) 8.6 Hz), 7.10 (d, 2H, J ) 8.5 Hz), 6.67 (d, 2H,
J ) 8.6 Hz); MS m/z 799 (M+, 79Br, 100%). Anal. Calcd for C43H30-
Br3N: C, 64.52; H, 3.78; N 1.75. Found: C, 64.55; H, 3.79; N, 1.70.
4-{Tris[4-[4-(diethoxyphosphono)phenyl]phenyl]methyl}-
aniline (4). Compound 3 (0.08 g, 0.1 mmol), acetonitrile (2 cm3), and
diethyl phosphite (0.25 cm3, 0.28 g, 2 mmol) were added to dry
potassium carbonate (0.14 g, 1 mmol) followed by bis-1,1′-(diphen-
ylphosphino)ferrocenedichloropalladium complex with dichloromethane
(0.005 g, 0.006 mmol). The mixture was heated under reflux for 15 h,
after which time additional palladium catalyst (0.005 g) was added.
The mixture was refluxed for 20 h, filtered (the precipitate was washed
with an excess of acetone), and concentrated under vacuum. The residue
was purified by chromatography on silica gel (10% MeOH in acetone)
Conclusion
A tripodal viologen, 9‚2PF6, has been designed and synthe-
sized that is adsorbed normal to and displaced from the surface
of a titanium dioxide nanoparticle in solution. Due to the fact
that the tripodal viologen 9 is oriented normal to and displaced
from the surface of the titanium dioxide nanoparticle, it
possesses the ability to thread the crown 10 to form hetero-
pseudorotaxane.
1
to give 4 (0.062 g; 63%) as a solid: mp 132 °C; H NMR (CDCl3) δ
7.89 (dd, 6H, J ) 8.5, 13 Hz), 7.71 (dd, 6H, J ) 8.5, 4 Hz), 7.56 (d,
6H, J ) 8.6 Hz), 7.41 (d, 6H, J ) 8.6 Hz), 7.09 (d, 2H, J ) 8.6 Hz),
6.66 (d, 2H, J ) 8.6 Hz), 4.1-4.3 (m, 12H), 1.36 (t, 18H, J ) 7 Hz);
31P NMR (CDCl3) δ 19.96; MS m/z 971 (M+, 100%). Anal. Calcd for
C55H60NO9P3: C, 67.96; H, 6.22; N, 1.44. Found: C, 67.55; H, 6.03;
N, 1.60.
9
5158 J. AM. CHEM. SOC. VOL. 125, NO. 17, 2003

Pseudorotaxane Self-Assembly on a TiO Nanoparticle
A R T I C L E S
2
1-(2,4-Dinitrophenyl)-4,4′-bipyridinium chloride (5) was obtained
as reported.14
(t, 18H, J ) 7 Hz); 31P NMR (CDCl3-CD3OD) δ 20.19 (s), -143.6
(heptet, J ) 711 Hz); MS (ES) m/z 1382 (8b+, 7%). Anal. Calcd for
C82H85F12N2O12P5: C, 58.85; H, 5.12; N, 1.67. Found: C, 58.12; H,
5.02; N, 1.54.
1-{4-[Tris[4-[4-(diethoxyphosphono)phenyl]phenyl]methyl]phenyl}-
4,4′-bipyridinium Chloride (6‚Cl). Compound 5 (0.18 g 0.5 mmol)
and dry ethanol (10 cm3) were added to 4 (0.35 g, 0.36 mmol), and the
mixture was heated under reflux for 30 h. The solvent was evaporated
under vacuum. The residue was dissolved in chloroform (100 cm3) and
washed with water (50 cm3) to remove excess 5. The organic layer
was dried (MgSO4) and concentrated. The residue was taken up in
acetone and purified by chromatography (methanol-acetic acid) to give
the product 6‚Cl (0.35 g, 85%) as a yellowish glassy solid: 1H NMR
(CD3OD) δ 9.46 (d, 2H, J ) 6 Hz), 8.98 (br s, 2H), 8.74 (d, 2H, J )
6 Hz), 8.26 (d, 2H, J ) 6 Hz), 7.8-7.9 (m, 16H), 7.74 (dd, 6H, J )
8.5, 4 Hz), 7.55 (d, 6H, J ) 8.6 Hz), 4.1-4.3 (m, 12H), 1.36 (t, 18H,
J ) 7 Hz); 31P NMR (CD3OD) δ 20.32.
1-{4-[Tris[4-[4-(dihydrophosphono)phenyl]phenyl]methyl]phenyl}-
1′-{4-[2-(2-phenoxyethoxy)ethoxy]benzyl}-4,4′-bipyridinium Dibro-
mide (9‚2Br). Me3SiBr (0.1 cm3, 0.75 mmol) was added to 8b‚2PF6
(0.033 g, 0.02 mmol) in 1,4-dioxane (2 cm3), and the mixture was heated
at 30 °C for 1 day, after which it was concentrated under vacuum.
Water (0.5 cm3) and 1,4-dioxane (1.5 cm3) were added to the mixture
and then removed under reduced pressure. The crude mixture was
extracted with methanol to give the product (0.026 g, 95%) as a yellow
glass: 1H NMR (CDCl3-CD3OD) δ 9.36 (d, 2H, J ) 6 Hz), 9.20 (d,
2H, J ) 6 Hz), 8.90 (d, 2H, J ) 6 Hz), 8.78 (d, 2H, J ) 6 Hz), 7.8 (m,
6H), 7.6 (m, 10H), 7.5 (m, 8H), 7.4 (m, 6H), 7.18 (t, 2H, 7 Hz), 6.7-
7.0 (m, 5H), 5.81 (s, 2H), 4.1 (m, 4H), 3.8 (m, 4H); 31P NMR
(CDCl3-CD3OD) δ 18.1; MS m/z 1212 (9+, 16%).
The solubility of the 4,4′-bipyridinium monocation 6 depends
strongly upon the counterion of the salt. Since it was found that the
hexafluorophosphates are more soluble in suitable solvents and could
be purified more readily,6 these salts were used in all subsequent
transformations.
Bis(p-phenylene)-34-crown-10 (10) was prepared according to the
procedure reported:27 mp 90 °C (lit.20 mp 88 °C).
1-Ethyl-1′-(diethylphosphono)ethyl-4,4′-bipyridinium bromide
(11‚2Br) and 1-ethyl-1′-phosphonoethyl-4,4′-bipyridinium bromide
(12‚2Br) were prepared as previously reported.21
1-{4-[Tris[4-[4-(diethoxyphosphono)phenyl]phenyl]methyl]phenyl}-
4,4′-bipyridinium Hexafluorophosphate (6‚PF6). An aqueous solution
of NH4PF6 (2 M, 1 cm3) was added to a solution of 6‚Cl (0.100 g,
0.085 mmol) in chloroform (50 cm3). The mixture was shaken for 1
min, and the organic layer was dried (Na2SO4) and concentrated to
give 6‚PF6 (0.100 g, 93%) as a yellow solid: mp 210-220 °C
1-(2-Phosphonoethyl)pyridinium Bromide (13‚Br). Pyridine (3
cm3) was added to diethyl 2-bromoethylphosphonate (0.98 g, 4 mmol)
in acetonitrile (20 cm3), and the mixture was heated at 70° C for 20 h.
The mixture was concentrated under reduced pressure, dissolved in
water (20 cm3), and washed with dichloromethane (2 × 20 cm3). The
aqueous layer was concentrated and the residue was dissolved in HCl
(6 mol dm-3, 20 cm3). The mixture was heated under reflux for 20 h
and evaporated under reduced pressure to give 13‚Br (0.73 g, 65%) as
1
(decomp); H NMR (CD3OD) δ 8.84 (d, 2H, J ) 6 Hz), 8.66 (d, 2H,
J ) 5 Hz), 8.29 (d, 2H, J ) 6 Hz), 7.7-7.9 (m, 6H), 7.6 (m, 12H),
7.50 (d, 6H, J ) 8.5 Hz), 7.33 (d, 6H, J ) 8.6 Hz), 4.0-4.2 (m, 12H),
1.25 (t, 18H, J ) 7 Hz)); 31P NMR (CD3OD) δ 19.74 (s), -143.46
(heptet, J ) 711 Hz); MS m/z 1111 (6+, 100%). Anal. Calcd for
C65H66F6N2O9P4: C, 62.10; H, 5.29; N, 2.23. Found: C, 61.90; H, 5.17;
N, 2.11.
1
a white solid: mp 152 °C; H NMR (CD3OD) δ 9.09 (d, 2H, J ) 6
Hz), 8.67 (t, 1H, J ) 7 Hz), 8.16 (m, 2H), 4.92 (q, 2H, J ) 7 Hz), 4.8
(s, 2H), 2.6 (m, 2H); 31P NMR (CD3OD) δ 25.2 (s). Anal. Calcd for
C7H11BrNO3P: C, 31.37; H, 4.14; N, 5.23; Br, 29.81. Found: C, 31.19;
H, 3.97; N, 5.16; Br, 29.81.
1-{4-[Tris[4-[4-(diethoxyphosphono)phenyl]phenyl]methyl]phenyl}-
1′-ethyl-4,4′ -bipyridinium Bis(hexafluorophosphate) (8a‚2PF6).
Acetonitrile (1 cm3) and ethyl bromide 7a (0.6 g, 5 mmol) were added
to 6‚PF6 (0.025 g, 0.02 mmol), and the mixture was heated at 80 °C in
a sealed tube for 24 h. The mixture was concentrated and the product
was purified by chromatography [methanol-nitromethane-aqueous
NH4PF6 (2 mol dm-3) in the ratio 80:19:1 by volume] to give 8a‚2PF6
(0.027 g, 95%) as a yellow solid: mp 210-220 °C (decomposition);
1H NMR (CD3OD) δ 9.07 (d, 2H, J ) 7 Hz), 8.95 (d, 2H, J ) 6.4
Hz), 8.55 (d, 2H, J ) 7 Hz), 8.43 (d, 2H, J ) 6.4 Hz), 7.6-7.9 (m,
16H), 7.53 (d, 6H, J ) 8.5 Hz), 7.35 (d, 6H, J ) 8.5 Hz), 4.63 (q, 2H,
J ) 7.2 Hz), 4.0-4.2 (m, 12H), 1.63 (t, 3H, J ) 7.2 Hz), 1.26 (t, 18H,
J ) 7 Hz); 31P NMR (CD3OD) δ 20.24(s), -143.68 (heptet); MS m/z
1-(2-Phosphonoethyl)perdeuteriopyridinium Bromide (14‚Br).
Pyridine-d5 (0.336 g, 4 mmol) was added to diethyl 2-bromoethylphos-
phonate (1.22 g, 5 mmol) in acetonitrile (5 cm3), and the mixture was
heated under reflux for 25 h. The mixture was concentrated under
reduced pressure, dissolved in water (20 cm3), and washed with
dichloromethane (2 · 20 cm3). The aqueous layer was concentrated and
the residue was dissolved in HCl (6 mol dm-3, 10 cm3). The mixture
was heated under reflux for 20 h and evaporated under reduced pressure
1
to give 14‚Br (0.88 g, 80%) as a white solid: mp 154 °C; H NMR
(CD3OD) δ 9.09 (trace), 8.67 (trace), 8.16 (trace), 4.92 (q, 2H, J ) 7
Hz), 4.8 (s, 2H, exchangeable signal), 2.6 (m, 2H); 31P NMR (CD3-
OD) δ 22.1 (s). Anal. Calcd for C7H6D5BrNO3P: C, 30.79; H, 5.90;
N, 5.13; Br, 29.26. Found: C, 31.15; H, 5.96; N, 5.16; Br, 29.08.
1139 (8a+
- , 100%). Anal. Calcd for
H, 92%), 570 (8a2+
C67H71F12N2O9P5: C, 56.23; H, 5.00; N, 1.96. Found: C, 55.88; H,
4.75; N, 1.92.
4-[2-(2-Phenoxyethoxy)ethoxy]benzyl Bromide (7b) was obtained
Determination of Association Constants. The method used to
determine Ka was developed by Ray and adapted by Stoddart for
pseudorotaxanes.15,16 It involves measuring the optical absorption
spectrum of different solutions containing equal concentrations of, for
example, the viologen 8b‚2PF6 and the crown ether 10. This method
assumes that the complex formed has a 1:1 stoichioimetry and allows
Ka to be defined as
1
similarly to the procedure reported.13 Colorless solid: mp 58 °C; H
NMR (CD3OD) δ 7.2-7.35 (m, 4H), 6.8-7.0 (d, 5H, J ) 8.5 Hz),
4.42 (s, 2H), 4.08 (t, 4H, J ) 5 Hz), 3.85 (t, 4H, J ) 5 Hz). Anal.
Calcd for C17H19BrO3 C, 58.10; H, 5.45; Br, 22.75. Found C, 57.90;
H, 5.55; Br, 22.87.
1-{4-[Tris[4-[4-(diethoxyphosphono)phenyl]phenyl]methyl]phenyl}-
1′-{4-[2-(2- phenoxyethoxy)ethoxy]benzyl}-4,4′-bipyridinium Bis-
(hexafluorophosphate) (8b‚2PF6). Compound 7b (0.017 g, 0.05 mmol)
and benzonitrile (0.1 cm3) were added to 6‚PF6 (0.025 g, 0.02 mmol),
and the mixture was heated at 30 °C for 1 day. The crude mixture was
purified by chromatography (MeOH/MeNO2/NH4PF6) to give the
product 8b‚2PF6 (0.033 g, 99%) as a bright yellow solid: mp 118 °C;
1H NMR (CDCl3-CD3OD) δ 8.99 (d, 2H, J ) 7 Hz), 8.81 (d, 2H, J )
7 Hz), 8.43 (d, 2H, J ) 7 Hz), 8.31 (d, 2H, J ) 7 Hz), 7.8 (m, 6H), 7.6
(m, 10H), 7.53 (d, 6H, J ) 9 Hz), 7.35 (m, 8H), 7.15 (t, 2H, J ) 7
Hz), 6.75-6.9 (m, 5H), 5.63 (s, 2H), 4.1 (m, 16H), 3.8 (m, 4H), 1.26
[8b‚10‚2PF6]
Ka )
(1)
[8b‚2PF6][10]
(27) Anelli, P. L.; Ashton, P. R.; Ballardini, R.; Balzani, V.; Delgado, M.;
Gandolfi, M. T.; Goodnow, T. T.; Kaifer, A. E.; Philp, D.; Pietraszkiewicz,
M.; Prodi, L.; Reddington, M. V.; Slavin, A. M. Z.; Spencer, N.; Stoddart,
J. F.; Vincenti, C.; Williams, D. J. J. Am. Chem. Soc. 1992, 114, 193-
218.
(28) Rao, S. N.; Fitzmaurice, D. HelV. Chim. Acta 1998, 81 (5), 902.
9
J. AM. CHEM. SOC. VOL. 125, NO. 17, 2003 5159

A R T I C L E S
Long et al.
A series of refinements to eq 1 leads to a plot of c/A against 1/(A)1/2
according to
Preparation of Transparent Nanostructured TiO2 Films. Trans-
parent nanostructured TiO2 films were deposited on F-doped tin oxide
glass substrates (10 Ω units, 0.5 µm thick, supplied by Glastron).
Specifically, a colloidal dispersion of TiO2 was prepared as described
above. It was then autoclaved at 200 °C for 12 h to yield a dispersion
of 12 nm diameter nanocrystals. Concentration of this dispersion (160
g L-1) and addition of Carbowax 20000 (40 wt % equivalent of TiO2)
yields a viscous paste. This paste was spread with a glass rod on the
conducting glass substrate masked by scotch tape. Following drying
in air for 1 h, the film was fired, also in air at 450 °C for 12 h. The
resulting transparent nanostructured electrodes are 4 µm thick.
Modification of Transparent Nanostructured TiO2 Films. A
transparent nanostructured TiO2 film was immersed in a dry acetonitrilic
solution of the required molecular component (0.5 mmol) for 1 h at
room temperature. The modified film was removed from the above
solution, washed thoroughly with dry acetonitrile, and stored in a
desiccator until required for use. The active area of the film is 0.8 cm2
(geometric area) × 1000 (surface roughness), which is 800 cm2. The
density of sites at which a phosponate linker may be adsorbed is 2 ×
c
A
1
1
1
ꢀl
)
+
(2)
(Kaꢀl)1/2 (A)1/2
A is the absorbance of the complex at λmax, c is the concentration of
the viologen and of the crown ether, ꢀ is the extinction coefficient of
the CT band, and l is the optical path length, which is 1 cm. The slope
of a plot of c/A against 1/(A)1/2 yields (1/Kaꢀl)1/2, while the intercept of
the same plot yields (1/ꢀl).
All optical absorption spectra were recorded at 25 °C on a Hewlett-
Packard 8452A diode array spectrometer controlled by a LabView
program running on a Macintosh Power PC. A standard solution
containing equimolar amounts of 8b‚2PF6 and 10 (8 × 10-3 mol dm-3
)
in a 2 cm3 volumetric flask was prepared. The absorption spectrum of
the standard solution was recorded and the absorbance at λmax was
determined. The solution was diluted accurately and the solution’s new
absorbance was measured. This process of dilution and measurement
was repeated. In total, 15 data points were obtained. Accurate dilution
factors and concentrations were determined gravimetrically for the
above solutions. Association constants, Ka, and derived free energies
of complexation, -∆G°, were obtained. Complexation of the nontri-
podal viologen 11‚2Br with crown ether 10 was also studied by the
same method.
1014 cm-2 25
. On this basis it is estimated that there are approximately
5 × 1016 tripodal viologens and 1.5 × 1017 nontripodal viologens
adsorbed at the surface of the TiO2 film.
Electrochemical Characterization. All cyclic voltammograms were
recorded in solution under the following conditions: The working
electrode (WE) was an isolated platinum wire. The counterelectrode
(CE) was an isolated platinum gauze. The reference electrode (RE)
was a nonaqueous Ag/Ag+ electrode filled with 10 mM AgNO3 in the
electrolyte solution. The electrolyte solution consisted of 0.1 M
tetrabutylammonium perchlorate (TBAP) in dry acetonitrile. When
stated, a 5-fold excess of crown ether was added to all viologen samples,
thereby ensuring that virtually all the viologen was present in the
complexed form. The solutions were bubbled with argon gas for 20
min prior to measurement. All cyclic voltammograms were recorded
on a Solartron SI 1287 potentiostat controlled by a LabView program
running on a Machintosh Power PC at a scan rate of 20 mV s-1 unless
otherwise stated.
Preparation of an Aqueous Dispersion of Titanium Dioxide
Nanoparticles. An aqueous dispersion of titanium dioxide nanoparticles
was prepared as previously reported by hydrolysis of titanium tetra-
isopropoxide at pH 2.19 Heating for 8 h evaporates the unwanted side
product 2-propanol and promotes crystallization of the nanoparticles.
The average diameter of the TiO2 particles was determined to be 5 nm
by TEM. X-ray diffraction confirmed that the crystal structure was
anatase. The final concentration of TiO2 as determined by dry weight
measurements was 3.2% (w/w).
Preparation of Modified Nanoparticles. An aqueous dispersion
of TiO2 nanoparticles (1 cm3) was transferred to a centrifuge tube and
was diluted with methanol (to 10 cm3). Viologen 9‚2Br (3.2 mg, 2
µmol) or 12‚2Br (0.9 mg, 2 µmol) in CHCl3/MeOH (1:1 v/v) was added
dropwise to a vigorously stirred solution of TiO2. This was stirred for
a further 10 min after addition was completed. Either 13‚Br or 14‚Br
(10 mg, 30 µmol) in MeOH (1 cm3) was added to this solution, which
was then stirred for 1 h. To induce counterion exchange and precipita-
tion, NH4PF6 was added in large excess (0.1 mmol). The precipitate
was centrifuged and washed with dry MeOH (5 cm3). The last step
was repeated three times to remove all the residual water. Finally the
precipitate is then transferred into acetonitrile (3 cm3) and sonicated
for 1 h to redisperse the particles. The resulting solution is an optically
transparent dispersion of modified TiO2 nanoparticles.
All cyclic voltammograms were recorded for nanostructured TiO2
films under the following conditions: The working electrode (WE)
was a nanostructured TiO2 film. The counterelectrode (CE) was isolated
platinum gauze. The reference electrode (RE) was a nonaqueous Ag/
Ag+ electrode filled with 10 mM AgNO3 in the electrolyte solution.
The electrolyte solution consisted of 0.1 M TBAP in dry acetonitrile.
When stated, a 10 mM solution of crown ether was added to the
electrolyte solution. The solutions were rigorously degassed by the
freeze-thaw method under nitrogen gas. As above, all cyclic voltam-
mograms were recorded on a Solartron SI 1287 potentiostat controlled
by a LabView program running on a Machintosh Power PC at a scan
rate of 20 mV s-1 unless otherwise stated.
JA028870Z
9
5160 J. AM. CHEM. SOC. VOL. 125, NO. 17, 2003