Assembly of an Electronically Switchable Rotaxane on the Surface of a Titanium Dioxide Nanoparticle

Article abstract of DOI:10.1021/ja037592g

This paper describes the self-assembly of a heterosupramolecular system consisting of a tripodal [2]rotaxane adsorbed at the surface of a titanium dioxide nanoparticle. The tripodal [2]rotaxane consists of a dumbbell-shaped molecule, incorporating two electron-poor viologens, threading an electron-rich crown ether. The [2]rotaxane also incorporates a bulky tripodal linker group at one end and a bulky stopper group at the other end. The [2]rotaxane is adsorbed, via the tripodal linker group, at the surface of a titanium dioxide nanoparticle. The structure and function of the resulting hetero[2]rotaxane have been studied in detail by ¹H NMR spectroscopy and cyclic voltammetry. A key finding is that it is possible to electronically address and switch the above hetero[2]rotaxane.

Full text of DOI:10.1021/ja037592g

Published on Web 11/20/2003
Assembly of an Electronically Switchable Rotaxane 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 July 29, 2003; E-mail: donald.fitzmaurice@ucd.ie
Abstract: This paper describes the self-assembly of a heterosupramolecular system consisting of a tripodal
[2]rotaxane adsorbed at the surface of a titanium dioxide nanoparticle. The tripodal [2]rotaxane consists of
a dumbbell-shaped molecule, incorporating two electron-poor viologens, threading an electron-rich crown
ether. The [2]rotaxane also incorporates a bulky tripodal linker group at one end and a bulky stopper group
at the other end. The [2]rotaxane is adsorbed, via the tripodal linker group, at the surface of a titanium
dioxide nanoparticle. The structure and function of the resulting hetero[2]rotaxane have been studied in
detail by 1H NMR spectroscopy and cyclic voltammetry. A key finding is that it is possible to electronically
address and switch the above hetero[2]rotaxane.
Introduction
dioxide nanoparticle. The rigid tripodal arrangement of these
phosphonate moieties orients the viologens normal to and
A goal shared by researchers in many fields is the bottom-
up assembly of functional nanoscale architectures in solution
and at technologically relevant substrates.1 Expected benefits
include greater control over material properties and innovative
technologies that address unmet needs.2 The self-assembly of
molecular and nanoscale condensed phase components in
solution and their self-organization at a suitably patterned
substrate is a possible approach.3
displaces them from the surface of the nanoparticle. The inherent
bulkiness of the tripodal linker reduces the possibility of
pimerization of the reduced viologens.11 As a consequence, the
crown ether is free to shuttle between the viologens. The
structure and function of the resulting hetero[2]rotaxane have
1
been studied in detail by H NMR spectroscopy and cyclic
voltammetry. A key finding is that it is possible to electronically
address and switch the above hetero[2]rotaxane.
In considering what molecular and condensed phase com-
ponents might make such an approach possible;4-6 one is
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; displaced from the
nanoparticle in order to avoid unwanted surface effects; and
finally, sufficiently bulky to preclude unwanted lateral interac-
tions.
A class of functional supermolecule that has attracted a great
deal of attention are the rotaxanes.9 A class of nanoparticle that
has attracted a great deal of attention are metal oxides.10
Accordingly, we have designed and synthesized the tripodal
[2]rotaxane shown in Scheme 1. The phosphonate moieties of
the linker group bind the viologen to the surface of a titanium
It should be noted that Galoppini and Meyer and their
respective co-workers have recently described the synthesis of
tripodal linkers used to adsorb sensitizer molecules at the surface
of the constituent titanium dioxide nanoparticles of a nano-
structured film forming the anode of a regenerative photoelec-
trochemical cell.12
It should also be noted that Fitzmaurice and co-workers have
recently described the synthesis of a tripodal viologen that is
adsorbed at the surface of a titanium dioxide nanoparticle and
is capable of threading a crown ether to from a hetero[2]-
pseudorotaxane.13,14 The syntheses of the tripodal viologen and
related molecular components employed in this study are
described in the Supporting Information.15
Finally, the findings reported here are also significant in the
context of the related findings recently reported by Balzani and
(11) Felderhoff, M.; Heinen, S.; Molisho, N.; Webersinn, S.; Walder, L. HelV.
Chim. Acta 2000, 83, 181.
(12) (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.
(13) (a) Nikitin, K.; Long, B.; Fitzmaurice, D. J. Chem. Soc., Chem. Commun.
2003, 282. (b) Long, B.; Nikitin, K.; Fitzmaurice, D. J. Am. Chem. Soc.
2003, 125, 5152.
(14) (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.
(15) See the Supporting Information.
(1) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1154.
(2) Bows, W. InVestors Chronicle 2002, 139, 30.
(3) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1212.
(4) Service, R. F. Science 1997, 277, 1036.
(5) Mann, S.; Shenton, W.; Li. M.; Connolly, S.; Fitzmaurice, D. AdV. Mater.
2000, 12, 147.
(6) Niemeyer, C. Angew. Chem., Int. Ed. 2001, 40, 4128.
(7) Lehn, J.-M. Supramolecular Chemistry, Concepts and PerspectiVes;
VCH: Weinheim, 1995.
(8) Alivisatos, A. P. Science 1996, 271, 933.
(9) Stoddart and co-workers. J. Am. Chem. Soc. 1996, 118, 4931.
(10) Hagfeldt, A.; Gratzel, M. Chem. ReV. 1995, 95, 49.
9
15490
J. AM. CHEM. SOC. 2003, 125, 15490-15498
10.1021/ja037592g CCC: $25.00 © 2003 American Chemical Society

Assembly of Rotaxane on Surface of TiO Nanoparticle
A R T I C L E S
2
Scheme 1. Electronically Addressable and Switchable Hetero[2]rotaxane
Table 1. Proton Nuclear Magnetic Resonance Data
acid. However, the usefulness of these known methods is limited
in the cases of compounds 1 to 4. Methods involving base
hydrolysis may not be used, as the viologen moieties are not
stable under these conditions. Methods based on acidic hy-
drolysis at elevated temperatures are not applicable, as the
ethoxyethylene moieties are not stable under these conditions.
For these reasons, a novel method has been developed for this
step that employs trimethylsilyl bromide in 1,4-dioxane at
ambient temperature. Using this method, the ester groups can
be selectively hydrolyzed.13b The method for separating com-
pounds 1 to 4 from unwanted side products, formed mainly from
1,4-dioxane, exploits their exceptionally high solubility in
concentrated aqueous HCl.
V
1
V
2
2
2′
3
3′
2
2′
2
2′
3
3′
C
TiO2•1R
TiO2•1
TiO2•2
TiO2•3R
TiO2•3
TiO2•4
5
8.85 8.76
8.88 8.78
8.86 8.76
8.71 7.80 7.68 6.44
7.95 7.89 6.24
8.58 7.96 7.88
6.16
6.77
Stoddart and co-workers16 and more recently by Heath and
Stoddart and co-workers.17
Adsorption of Tripodal [2]Rotaxanes at the Surface of a
Nanoparticle. A charge stabilized dispersion of titanium dioxide
nanoparticles (6 nm diameter) was prepared by hydrolysis of
titanium orthotetraisopropoxide in water at pH 2.18 The tripodal
[2]rotaxanes 1 and 3, and the corresponding tripodal axle
components (2 and 4, respectively), readily chelate Ti4+ states
at the surface of the above nanoparticles and are adsorbed. The
number of Ti4+ sites on the surface of each nanoparticle was
estimated, based on literature values for the number of such
states (2 × 1014 cm-2), to be 226.19 An average of 12 tripodal
[2]rotaxanes, or the corresponding tripodal axles, were adsorbed
at the surface of each titanium dioxide nanoparticle. Molecular
modeling reveals that the footprint of a tripodal linker is 144
Å2 and that the area occupied on the surface of a nanoparticle
by 12 adsorbed tripodal viologens is 1728 Å2 corresponding to
15% of the available surface area. The partially deuterated
stabilizer 6 was adsorbed at the remaining Ti4+ sites on the
surface of each nanoparticle.13 These nanoparticles were pre-
cipitated from the above aqueous dispersion by addition of
ammonium hexafluorophosphate, washed with dry methanol,
and redispersed in acetonitrile to form a stable optically
transparent dispersion.
Structural Characterization of Tripodal Hetero[2]rotaxanes
by Proton NMR. The 1H NMR spectra of TiO2•1 and TiO2•2
were recorded with a view to establishing that the tripodal
[2]rotaxane 1 and the corresponding axle component 2 are
adsorbed normal to and displaced from the surface of the TiO2
nanoparticle in TiO2•1 and TiO2•2, respectively. See expanded
aromatic region in Figure 1 and also Scheme 3 and Table 1.
It is concluded, based on the finding that the resonances
assigned to protons of the tripodal linker L and the viologen
V1 adjacent to L in TiO2•1 are suppressed, that 1 is adsorbed at
the surface of the titanium dioxide nanoparticle in TiO2•1.
Specifically, it is found that the following resonances are
Results and Discussion
Design and Synthesis of Tripodal [2]Rotaxanes. The
tripodal [2]rotaxanes (1 and 3) and corresponding tripodal axles
(2 and 4, respectively), along with the crown ether 5 and
nanoparticle stabilizer 6 employed in the present study, are
shown in Scheme 2.
The design and synthesis of tripodal [2]pseudorotaxanes that
are capable of being adsorbed normal to and displaced from
the surface of a titanium dioxide nanoparticle have been
described in detail elsewhere.13 These design principles and
synthetic strategies have been extended to the preparation of
tripodal [2]rotaxanes (and the corresponding tripodal axle
components), which are also capable of being adsorbed normal
to and displaced from the surface of a titanium dioxide
nanoparticle.15
In the case of the tripodal [2]rotaxane 1; the ester analogue
of the tripodal bis-viologen, the crown 5, and the bulky stopper
were all synthesized as described in the Supporting Informa-
tion.15 These subcomponents were subsequently self-assembled
under ambient conditions to form the ester analogue of the
tripodal [2]rotaxane 1 in high yield. The ester analogues of the
tripodal [2]rotaxane 3 and the corresponding tripodal axle
components 2 and 4, respectively, were prepared using a similar
approach.15
The target tripodal [2]rotaxanes 1 or 3 and the corresponding
axle components (2 and 4, respectively) were obtained by
hydrolysis of the phosphonic ester groups at the end of the tripod
legs to yield the corresponding phosphonic acid groups.
There are a number of known methods that can be used to
hydrolyze a phosphonic ester to the corresponding phosphonic
(16) 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.sEur. J. 2000, 6, 3558.
(17) (a) Collier, C. P.; Wong, E. W.; Belohradsky, M.; Raymo, F. M.; Stoddart,
J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391.
(b) Collier, C. P.; Mattersteig, G.; Wong, E. W.; Luo, Y.; Beverly, K.;
Sampaio, J.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. Science 2000,
289, 1172.
(18) O’Reagen, B.; Graetzel, M. Nature 1991, 353, 737.
(19) Finklea, H. O. Semiconductor Electrodes; Elsevier: 1988.
9
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A R T I C L E S
Long et al.
Scheme 2. Molecular Components
suppressed: δ 9.1 (2-position of V1), 8.8 (2′-position of V1),
8.0 (3-position of V1), 7.9 (3′-position of V1), 7.7 (phosphory-
lated p-phenylenes of L), 7.6 (phosphorylated p-phenylenes of
L and p-phenylene link to V1), 7.5 (p-phenylene of L), 7.3 (p-
phenylene of L), 4.8 (methylene group adjacent to V1 of
trimethylene bridge linking viologens), and 2.9 (central meth-
ylene group of trimethylene bridge linking viologens).
orientated normal to and displaced from the surface of the
nanoparticle and that V2 and S of 1 are free to undergo restricted
rotational motion.
Specifically, it is found that the following proton resonances
are broadened: δ 8.9 and 8.8 (2- and 2′-positions of V2), 7.8
and 7.7 (3-position of V2), 7.5 (3-position of N-benzyl), 7.3
(phenylenes of S), 7.2-7.0 (2-position of N-benzyl and phe-
nylenes of S), 6.8 (2-position of the alkoxyphenylene group
attached to the ether link), 6.2 (p-hydroxyphenylene of the crown
C), 5.7 (methylene of N-benzyl), 4.8 (methylene group adjacent
It is concluded, based on the finding that the resonances
assigned to protons of the bulky stopper S and the viologen V2
adjacent to S in TiO2•1 are broadened, that L and V1 of 1 are
9
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Assembly of Rotaxane on Surface of TiO Nanoparticle
A R T I C L E S
2
Figure 2. 1H NMR spectra of 3′ (1.4 × 10-3 mol dm-3 molecules) in
chloroform-d and methanol-d4 (20% by vol) at 30 °C and of TiO2•4, TiO2•3
(1.2 × 10-4 mol dm-3 particles and 1.4 × 10-3 mol dm-3 molecules), and
TiO2•4 (1.2 × 10-4 mol dm-3 particles and 1.4 × 10-3 mol dm-3 molecules)
dm-3 particles and 1.4 × 10-3 mol dm-3 molecules) in acetonitrile-d3 at
30 °C was recorded following degassing by bubbling with nitrogen and
reduction addition of zinc metal in the presence of chromium chloride.
Figure 1. 1H NMR spectra of 1′ (1.4 × 10-3 mol dm-3 molecules), TiO2•1
(1.2 × 10-4 mol dm-3 particles and 1.4 × 10-3 mol dm-3 molecules), and
TiO2•2 (1.2 × 10-4 mol dm-3 particles and 1.4 × 10-3 mol dm-3 molecules)
dm-3 particles and 1.4 × 10-3 mol dm-3 molecules) in acetonitrile-d3 at
30 °C was recorded following degassing by bubbling with nitrogen and
reduction addition of zinc metal in the presence of chromium chloride.
1
in acetonitrile-d3 at 30 °C. H NMR spectra of TiO2•3R (1.2 × 10-4 mol
1
in acetonitrile-d3 at 30° C. H NMR spectra of TiO2•1R (1.2 × 10-4 mol
It is concluded, based on the finding that the resonances
assigned to protons of the bulky stropper S and the viologen
V2 adjacent to the bulky stopper in TiO2•2 are broadened, that
L and V1 of 2 are orientated normal to and displaced from the
surface of the nanoparticle and that V2 and S of 2 are free to
undergo restricted rotational motion. Specifically, it is found
that the following resonances are broadened: δ 8.9, 8.8, and
8.6 (the 2- and 2′-positions of V2); 7.9 and 7.8 (the 3-positions
of V2); 7.5 (the 3-position of N-benzyl); δ 7.3 (phenylenes of
S); 7.2-7.0 (2-position of N-benzyl and phenylenes of S); 6.8
(the 2-position of the alkoxyphenylene group of S); 5.7
(methylene of N-benzyl); 4.8 (methylene group adjacent to V2
of trimethylene bridge linking viologens); 4.1 (ether link
positions adjacent to aryloxy groups), 3.8 (rest of ether link
positions); 2.2 (methyl groups of V2); and 1.2 (tert-butyl groups
of S).
Scheme 3. Notation for NMR Assignments
to V2 of trimethylene bridge linking viologens), 4.0 (ether link
positions adjacent to aryloxy groups), 3.8 (rest of ether link
positions and methylenes in C), 3.6-3.5 (methylene protons
of C), 2.2 (methyl groups of V2), and 1.2 (tert-butyl groups
of S).
It is concluded, based on the finding that the resonances
assigned to protons of tripodal linker L and the viologen V1 of
TiO2•2 are suppressed, that 2 is adsorbed at the surface of the
titanium dioxide nanoparticle in TiO2•2. Specifically, it is found
that the following resonances are suppressed: δ 9.0 (2-position
of V1), 8.9 (2′-position of V1), 8.4 (3-position of V1), 8.3 (3′-
position of V1), 7.7 (phosphorylated p-phenylenes of L), 7.6
(phosphorylated p-phenylenes of L and p-phenylene link to V1),
7.4 (p-phenylene of L), 7.3 (p-phenylene of L), 4.7 (methylene
group adjacent to V1 of trimethylene bridge linking viologens),
and 2.9 (central methylene group of trimethylene bridge linking
viologens).
The above findings allow us conclude that both 1 and the
corresponding axle component 2 are adsorbed normal to but
displaced from the surface of the TiO2 nanoparticle in TiO2•1
and TiO2•2, respectively. These findings are consistent with
other recently reported findings.12-14
1
The H NMR spectra of TiO2•3 and TiO2•4 were recorded
with a view to establishing that the tripodal [2]rotaxane 3 and
the corresponding axle component 2 were adsorbed normal to
and displaced from the surface of the TiO2 nanoparticle in
TiO2•3 and TiO2•4, respectively. See expanded aromatic region
in Figure 2 and also Scheme 3 and Table 1.
9
J. AM. CHEM. SOC. VOL. 125, NO. 50, 2003 15493

A R T I C L E S
Long et al.
It is concluded, based on the finding that the resonances
assigned to protons of tripodal linker L and the viologen V1 of
TiO2•3 are suppressed, that 3 is adsorbed at the surface of the
titanium dioxide nanoparticle in TiO2•3. Specifically, it is found
that the following resonances are suppressed: δ 9.2 (2-position
of V1), 8.8 (2′-position of V1), 8.2 (3-position of V1), 7.9 (3′-
position of V1), 7.8 (phosphorylated p-phenylenes of L), 7.7
(phosphorylated p-phenylenes of L and p-phenylene link to V1),
7.6 (p-phenylene of L), 7.5 (the 3-position of N-benzyl), 7.5
(p-phenylene of L), and 5.9 (N-methylene).
In the discussion that follows, it is stated that C is localized
at either V1 or V2. Such statements are intended to identify the
thermodynamically favored translational isomer and the most
probable location of C. They are not intended to suggest that C
is stationed at V1 or V2.
Based on the finding that the resonance assigned to the
aromatic protons of C in TiO2•1 is significantly upfield of the
resonance for the same protons of 5 in solution, it is concluded
that C in TiO2•1 is localized at V1 or V2. Specifically, it is
found that the broadened resonance assigned to the aromatic
protons of C is at δ 6.2 for TiO2•1, while it is at δ 6.8 for 5 in
solution. Based on the finding that the resonances assigned to
the aromatic protons of the viologen V2 in TiO2•1 have similar
chemical shifts in TiO2•2, it is concluded that that C is localized
at V1. Specifically, it is found that the broadened resonances
assigned to the 2 and 2′ protons of V2 are observed at δ 8.9, δ
8.8, and δ 8.6 in both TiO2•1 and TiO2•2. It is also found that
that the broadened resonances assigned to the 3 and 3′ protons
of V2 are observed at δ 8.0 and δ 7.9 in TiO2•1 and TiO2•2. It
should be noted that our conclusion C is localized at V1 in
TiO2•1 is consistent with previously report findings13 and with
the fact that the affinity of C for V1 is an order of magnitude
greater than for V2.16
It is concluded, based on the finding that the resonances
assigned to protons of the bulky stopper S in TiO2•3 are
broadened, that 3 is orientated normal to and displaced from
the surface of the nanoparticle and free to undergo restricted
rotational motion. Specifically, it is found that the following
resonances are broadened: δ 7.3 (phenylenes of S), 7.2-7.0
(phenylenes of S), 6.8 (the 2-position of the alkoxyphenylene
group of S), 6.2 (p-hyroxy-phenylene of C), 4.1 (ether link
positions adjacent to aryloxy groups), 3.8 (rest of ether link
positions and methylenes of C), 3.6-3.5 (methylene of C), and
1.2 (tert-butyl groups of S).
It is concluded, based on the finding that the resonances
assigned to protons of tripodal linker L and the viologen V1 of
TiO2•4 are suppressed, that 4 is adsorbed at the surface of the
titanium dioxide nanoparticle in TiO2•4. Specifically, it is found
that the following resonances are suppressed: δ 9.0 (2-position
of V1), 8.9 (2′-position of V1), 8.5 (3-position of V1), 8.4 (3′-
position of V1), 7.8 (phosphorylated p-phenylenes of L), 7.7
(phosphorylated p-phenylenes of L and p-phenylene link to V1),
7.6 (p-phenylene of L), 7.5 (the 3-position of N-benzyl), 7.5
(p-phenylene of L), and 5.7 (N-methylene).
Having established that C is initially localized at V1 in TiO2•1,
our interest turns to what happens when the viologen V1 is
selectively chemically reduced to form TiO2•1R. It might have
been expected, based on previously reported findings,16 that C
would localize at V2, specifically, that the affinity of C for V2
would be greater than for the radical cation of V1.
Based on the finding that the resonance assigned to the
aromatic protons of C in TiO2•1R is significantly upfield of
the same resonance of 5 in solution and significantly downfield
of the same resonance in TiO2•1, it is concluded that C in TiO2•1
is either localized at V2 or, as a result of V1 having been reduced,
is interacting less strongly with the radical cation of V1.
Specifically, it is found that the broadened resonance assigned
to the aromatic protons of C is at δ 6.4 for TiO2•1R, while it
is at δ 6.2 for C in TiO2•1 and at δ 6.8 for 5 in solution.
Based on the finding that the resonances assigned to the 2
and 2′ and the 3 and 3′ protons of the viologen V2 in TiO2•1R
are not and are, respectively, shifted upfield with respect to the
corresponding resonances in TiO2•1 and TiO2•2, it is concluded
that C in TiO2•1R is interacting with V2. Specifically, it is found
that the broadened resonances assigned to the 2 and 2′ protons
of V2 are observed at δ 8.9, δ 8.8, and δ 8.6 in TiO2•1R, TiO2•1,
and TiO2•2. It is also found that that the broadened resonances
assigned to the 3 and 3′ protons of V2 are observed at δ 7.8
and δ 7.7 in TiO2•1R and at δ 8.0 and δ 7.9 in TiO2•1 and
TiO2•2.
These findings permit the conclusion that C in TiO2•1R is
interacting to a greater extent with V2, following reduction of
V1. These findings, however, do not permit the conclusion that
C is localized on V2 and not interacting with V1. In support of
this view, and as discussed in detail below, a dynamic
characterization of this system by cyclic voltammetry suggests
that C still interacts with the radical cation of V1 in TiO2•1R.
It should be noted that this finding is not unexpected for the
following reason: While the association constant for formation
of a complex between C and V2 is an order of magnitude smaller
than the association constant for formation of a complex between
It is concluded, based on the finding that the resonances
assigned to protons of the stopper group S in TiO2•4 are
broadened, that 4 is orientated normal to and displaced from
the surface of the nanoparticle and free to undergo restricted
rotational motion. Specifically, it is found that the following
resonances are broadened: δ 7.3 (phenylenes of S), 7.2-7.0
(phenylenes of S), 6.8 (the 2-position of the alkoxyphenylene
group of S), 4.1 (ether link positions adjacent to aryloxy groups),
3.8 (rest of ether link positions), and 1.2 (tert-butyl groups of
S).
The above findings allow us conclude that both 3 and the
corresponding axle component 4 are adsorbed normal to but
displaced from the surface of the TiO2 nanoparticle in TiO2•3
and TiO2•4, respectively. These findings are consistent with
other recently reported findings.12-14
Dynamic Characterization of Tripodal Hetero[2]rotaxanes
1
by Proton NMR. Shown in Figure 1 are the H NMR spectra
of 1′ in solution, the ester analogue of 1, TiO2•1, and TiO2•2.
Also shown is the spectrum of TiO2•1 following degassing by
bubbling with nitrogen and reduction by addition of zinc metal
in the presence of chromium chloride,20,21 denoted TiO2•1R.
Given in Table 1, where resolved, are the chemical shifts of
the aromatic proton resonances of V1 and V2 and of C.
(20) Evans, A. G.; Evans, J. C.; Baker, M. W. J. Chem. Soc., Perkin Trans. 2
1977, 1781.
(21) 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.
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Assembly of Rotaxane on Surface of TiO Nanoparticle
A R T I C L E S
2
Table 2. Cyclic Voltammetry Data
V
1
2+f V +•
V
1
+•f V ••
V
2
2+f V +•
V
2
+•f V ••
V •• f V +•
V
2
+• f V 2+
V •• f V +•
V
+• f V 2+
1 1
1
1
2
2
2
2
2
1
1
TiO2•1
TiO2•2
∆
TiO2•3
TiO2•4
∆
-0.756
-0.646
-0.110
-0.699
-0.614
-0.085
-1.057
-0.975
-0.082
-1.112
-1.012
-0.100
-1.182
-1.145
-0.037
-1.469
-1.468
-0.001
-1.309
-1.355
+0.044
-1.060
-1.064
+0.004
-0.947
-0.975
+0.028
-1.065
-0.963
-0.102
-0.647
-0.600
-0.047
-0.666
-0.579
-0.087
C, it may be comparable to that for the formation of a complex
between C and the radical cation of V1 and C.16
Further insights are gained by undertaking a similar analysis
of the hetero[2]rotaxane TiO2•3 and the corresponding axle
component TiO2•4, both incorporating a single viologen V1.
Shown in Figure 2 are the 1H NMR spectra of 3′ in solution,
the ester analogue of 3, TiO2•3, and TiO2•4. Also shown is the
spectrum of TiO2•3 following degassing by bubbling with
nitrogen and reduction by addition of zinc metal in the presence
of chromium chloride, denoted TiO2•3R. Given in Table 1,
where resolved, are the chemical shifts of the aromatic proton
resonances of V1 and the aromatic proton resonances of C.
Based on the finding that the resonance assigned to the
aromatic protons of C in TiO2•3 is significantly upfield of the
same resonance of 5 in solution, it is concluded that C in TiO2•3
is localized at V1. Specifically, it is found that the broadened
resonance assigned to the aromatic protons of C is at δ 6.2 for
TiO2•3, while it is at δ 6.8 for 5 in solution. As discussed above,
the resonances assigned to the aromatic protons of the viologen
V1 are not observed due to its proximity to the surface of the
TiO2 nanoparticle.
Having established that C is localized at V1 in TiO2•3, our
interest again turns to what happens when the viologen V1 is
selectively chemically reduced to form TiO2•3R. It might have
been expected, based on previous reports,16 that C would no
longer be localized at V1. In practice, it is found that the
resonance assigned to the aromatic protons of C is no longer
observed.20,21 On this basis, it is concluded that C remains
associated with V1 following its one-electron reduction. It should
be noted that this does not preclude the possibility that C is
more loosely associated with the radical cation of V1. It should
also be noted that these findings are consistent with those
recently reported by us for a related hetero[2]pseudorotaxane13
and with those reported below for the electrochemical charac-
terization of TiO2•3.
Dynamic Characterization of Tripodal Hetero[2]rotaxanes
by Cyclic Voltammetry. Shown in Figure 3 are the cyclic
voltammograms (CVs) of the tripodal hetero[2]rotaxane TiO2•1
and the corresponding axle component adsorbed at the surface
of a TiO2 nanoparticle TiO2•2. The first and second peak
reduction and peak reoxidation potentials of V1 and V2 are given
in Table 2.
It is found that the first and second reduction potentials of
V1 are shifted to more negative values by 110 mV and 82 mV,
respectively, due to the presence of C in TiO2•1. It is also found
that the first and second reduction potentials of V2 are shifted
to more negative values by 37 mV and 1 mV, respectively.
On this basis, it is concluded that initially C interacts with
V1 in TiO2•1. Accordingly, V1 undergoes a first one-electron
reduction to form the corresponding radical cation at more
negative potentials than in the corresponding axle TiO2•2. On
this basis, it is also concluded that C continues to interact with
Figure 3. Cyclic voltammograms of nanostructured TiO2 film (geometric
area of 0.8 cm2, surface roughness of 1000) modified by adsorption of 1
(5 × 1016 molecules) and 2 (5 × 1016 molecules) recorded in acetonitrile
containing added TBAP (0.1 mol dm-3) at 25 °C.
the singly reduced viologen V1. This accounts for the fact that
the radical cation of V1 undergoes a second one-electron
reduction at more negative potentials than the same viologen
in the corresponding axle TiO2•2.
This finding suggests that the affinity of C for V2 is less than
or similar to the affinity of C for the radical cation of V1 and
that C continues to interact with V1 even after this viologen
has been reduced. This is perhaps not surprising when one takes
into account that the affinity of C for V2 is 10 times lower than
its affinity for V1.16 It was nevertheless expected that, following
the two successive one-electron reductions of V1 in TiO2•1, that
C would favor interaction with the doubly positively charged
viologen V2.
Consistent with this expectation is the finding that the first
reduction potential of V2 in TiO2•1 is shifted to more negative
potentials than the same reduction in the corresponding axle
component TiO2•2. An interesting finding is that the second
reduction of V2 is, within experimental error, at the same
potential in TiO2•1 and in TiO2•2. On this basis, it is concluded
that C is not localized at the radical cation of V2.
This finding suggests that the affinity of C for the singly
reduced form of V2 is not sufficient to localize the crown ether
9
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A R T I C L E S
Long et al.
Scheme 4. Electrodynamics of Hetero[2]rotaxane
Figure 4. Cyclic voltammograms of nanostructured TiO2 film (geometric
area of 0.8 cm2, surface roughness of 1000) modified by adsorption of 3 (5
× 1016 molecules) and 4 (5 × 1016 molecules) recorded in acetonitrile
containing added TBAP (0.1 mol dm-3) at 25 °C.
at this viologen. Accordingly, C may be in motion along the
entire length of the axle.
Further insight is provided by considering the difference in
the reoxidation potentials of the viologens in TiO2•1 and TiO2•2.
It is found that the first and second reoxidation potentials of V2
are shifted to more positive potentials by 44 mV and 4 mV,
respectively, due to the presence of C in TiO2•1. It is also found
that the first and second reoxidation potentials of V1 are shifted
to more positive values by 28 mV and more negative values by
47 mV, respectively.
On this basis, it has been concluded that C is not localized
at V2 when this viologen is in either doubly or singly reduced
state; otherwise it would be expected that the first and second
reoxidation potentials would be shifted to more negative values.
This is consistent with the findings above that C is not localized
at either singly or doubly reduced V2. What was not expected
is the magnitude of the observed shifts in the first and second
reoxidation potentials to more positive values. To account for
these findings, we propose that there is a conformational change
associated with the hetero[2]rotaxane TiO2•1 when both violo-
gens are in their doubly reduced state and the crown is free to
move along the axle. This conformational change is currently
the subject of a detailed study whose findings will be reported
in due course.22
On this basis, it has also been concluded that C is not
localized on V1 when it is in its doubly reduced state but is
when it is in its singly reduced state. This accounts for the fact
that the first reoxidation potential is shifted to more positive
potentials, while the second reoxidation potential is shifted to
more negative values. This conclusion is also consistent with
that arrived at based on an analysis of the corresponding
reduction potentials.
Shown in Figure 4 are the CVs of the tripodal hetero-
[2]rotaxane TiO2•3 and the corresponding axle component
adsorbed at the surface of a TiO2 nanoparticle TiO2•4. It is found
that the first and second reduction potentials of V1 are shifted
to more negative values by 85 mV and 100 mV, respectively,
in the presence of C. It is also found that the first and second
reoxidation potentials of V1 are shifted to more negative values
by 102 mV and 87 mV in the presence of added C. On this
basis, it is concluded that transfer of an electron from the TiO2
nanoparticle to V1 to form the corresponding radical cation does
not result in C being completely dissociated from V1. These
findings are as expected and consistent with those reported above
and elsewhere for a closely related system.13
Conclusions
It is concluded that it is possible to electronically address
and switch the hetero[2]rotaxane shown in Scheme 1. Specif-
ically, by applying a suitable potential, it is possible to transfer
between one and four electrons from the conduction band of
the TiO2 nanoparticle to V1 adjacent to L and V2 adjacent to S
of the [2]rotaxane component. As a result, C is localized at either
V1 or the V2.
More specifically, and as shown in Scheme 4, C is initially
localized on V1 adjacent to L. Following transfer of a single
electron to V1, to form the corresponding radical cation, C is
no longer localized on V1 and interacts with both V1 and V2
adjacent to S. Transfer of a second electron to V1, to form the
corresponding diradical, leads to C being localized on V2.
Following transfer of a single electron to V2, to form the
corresponding radical cation, C is no longer localized on V2
and does not interact strongly with either V1 or V2. Transfer of
a second electron to V2, to form the corresponding diradical, is
accompanied by a conformational change. This change in con-
formation is possibly a folding of the fully reduced [2]rotaxane
and a stacking of the diradicals of V1 and V2 and C.
As a consequence of this conformational change, a signifi-
cantly more positive potential than might have otherwise have
been expected, is required to remove the first electron from the
fully reduced [2]rotaxane. The subsequent removal of three
electrons leads to the localization of C, first on V2 and
subsequently in V1 as shown in Scheme 4.
These findings point the way to the preparation and charac-
terization of a related family of hetero[2]rotaxanes and related
(22) Lestini, E.; Nikitin, K.; Fitzmaurice, D. Manuscript in preparation.
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Assembly of Rotaxane on Surface of TiO Nanoparticle
A R T I C L E S
2
(d, 2H, J ) 7 Hz), 9.20 (d, 2H, J ) 7 Hz), 8.76 (d, 2H, J ) 7 Hz),
8.65 (d, 2H, J ) 7 Hz), 7.8 (m, 8 H), 7.65 (m, 6H), 7.57 (m, 6H), 7.4
(m, 8H), 7.18 (d, 6H, J ) 9 Hz), 7.0 (m, 12 H), 6.72 (d, 2 H, J ) 9
Hz), 5.80 (s, 2H), 4.1 (m, 4 H), 3.8 (m, 4H), 1.22 (s, 27 H); 31P NMR
δ 18.4 (s); MS (m/z) 1080 (M+ - 2H).
nanoscale components that can be both electronically and
optically addressed and switched. These nanoscale components
offer potentially significant advantages over purely condensed
phase or molecular components.
Compound 5. Bis-paraphenylene-34-crown-10 was prepared ac-
cording to a known procedure as a white solid:21 mp 90 °C (lit.21 mp
88 °C);1H NMR (CD3CN) δ 6.63 (s, 8H), 3.75 (m, 8H), 3.6 (m, 8H),
3.5 (m, 16 H).
Experimental Section
Synthesis of Molecular Components. Synthesis of the ester
analogues and immediate precursors of the molecular components 1
to 4 shown in Scheme 2 has been described in detail elsewhere.15 The
molecular components 1 to 4 were obtained from their ester precursors
as follows: bromotrimethylsilane (0.05 mL) in dioxane (0.25 mL) was
added to the ester (3 µmol) in dry 1,4-dioxane (1.00 mL). The resulting
mixture was kept at room temperature for 20 h and extracted with
concentrated HCl, and the extract was washed with ether (2 × 2 mL)
and concentrated under reduced pressure to give the acid as a glassy
solid (3 µmol, 100%). The crown ether 5 was synthesized following
literature methods.21 The stabilizer 6 was synthesized as described in
detail elsewhere.13b
Compound 6. 1-Phosphonoethylperdeuteriopyridinium bromide was
1
obtained as a white solid (yield 80%):13 mp 154 °C; H NMR (CD3-
OD) δ 9.09 (trace), 8.67 (trace), 8.16 (trace), 4.92 (q, 2H, J ) 7 Hz),
4.8 (s, 2H), 2.6 (m, 2H); 31P NMR (CD3OD) 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, 5.17; Br, 29.08.
Preparation of Titanium Dioxide Nanoparticle Aqueous Disper-
sions. An aqueous dispersion of titanium dioxide nanoparticles was
prepared by the hydrolysis of titanium tetraisopropoxide at pH 2 as
described in detail elsewhere.18 Refluxing the initially formed dispersion
at 80 °C for 8 h evaporates the principal side product 2-propanol and
promotes crystallization of the nanoparticles. The average diameter of
the TiO2 nanoparticles was determined by TEM to be 6 nm. X-ray
diffraction confirmed their crystal structure to be anatase. The final
concentration of TiO2 nanoparticles was determined by dry weight
measurements to be 3.2% w/w.
Preparation of Modified Nanoparticles. An aqueous dispersion
of TiO2 nanoparticles (1 mL) was transferred to a glass centrifuge tube
and was diluted with methanol (to 10 mL) yielding a dispersion of a
known particle concentration (1.2 × 10-5 mol dm-3). A known amount
(1.5 × 10-6 mol) of the tripodal [2]rotaxane 1 or 3 (or the corresponding
tripodal axle 2 or 4) was dissolved in CHCl3/MeOH (1:1 by vol) and
added dropwise to the above dispersion of TiO2 nanoparticles while
stirring vigorously. Stirring was continued for 10 min. A known weight
(2.4 × 10-5 mol) of the stabilizer 6 was dissolved in MeOH (1 mL)
and added dropwise to the above stirring dispersion. Stirring was
continued for 1 h. To induce counterion exchange and precipitation of
the modified nanoparticles, NH4PF6 was added in large excess (1.0 ×
10-3 mol). The precipitate was centrifuged and washed with dry MeOH
(5 mL). The last step was repeated 3 times to remove all the residual
water. Finally the precipitate was sonicated in acetonitrile (3 mL) for
1 h to redisperse the nanoparticles. The resulting dispersion was
optically transparent.
The results of the characterization of these compounds are given
below. Melting points were estimated using a Gallenkamp melting point
device and were not corrected. NMR spectra were recorded using 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 Micromass LCT KC 420 mass spectrometer.
Compound 1. 1-{4-[Tris{4-(4-dihydrophosphonophenyl)-pheny}-
methyl]phenyl}-1′-(3-[3,3′-dimethyl-1′-(4-[bis{4-tert-butylphenyl}-4-
ethylphenylmethyl]phenoxyethoxyethoxyphenylmethyl)-4,4′-bipyridyl-
1-yl]propyl)-4,4′-bipyridinium 34-crown-10 tetrachloride: 1H NMR
(CD3OD) δ 9.34 (s, 1H), 9.33 (d, 2H, J ) 7 Hz), 9.15 (d, 2H, J ) 7
Hz), 9.15 (d, 1H, J ) 7 Hz), 9.10 (s, 1H), 8.90 (d, 1H, J ) 7 Hz), 8.34
(d, 2H, J ) 7 Hz), 8.13 (d, 1H, J ) 7 Hz), 8.11 (d, 2H, J ) 7 Hz),
8.05 (d, 1H, J ) 7 Hz), 7.90 (d, 1H, J ) 7 Hz), 7.8 (m, 10 H), 7.65
(m, 6H), 7.55 (d, 6H, J ) 9 Hz), 7.38 (d, 6H, J ) 9 Hz), 7.15 (d, 4H,
J ) 9 Hz), 7.0 (m, 14H), 6.69 (d, 2 H, J ) 9 Hz), 6.20 (s, 8 H), 5.75
(s, 2H), 5.1 (m, 4 Hz), 4.1 (m, 4 H), 3.8 (m, 4H), 3.6 (m, 24H), 3.5
(m, 8H), 3.1 (m, 2H), 2.50 (q, 2H, J ) 7 Hz), 2.32 (s, 6H), 1.20 (s, 18
H), 1.12 (t, 3H, J ) 7 Hz); 31P NMR δ 18.4 (s); MS (m/z) 2468
(MCl32+).
Compound 2. 1-{4-[Tris{4-(4-dihydrophosphonophenyl)pheny}-
methyl]phenyl}-1′-(3-[3,3′-dimethyl-1′-(4-[bis{4-tert-butylphenyl}-4-
ethylphenylmethyl]phenoxyethoxyethoxyphenylmethyl)-4,4′-bipyridyl-
1-yl]propyl)-4,4′-bipyridinium tetrachloride: 1H NMR (CD3OD) δ 9.53
(d, 2H, J ) 7 Hz), 9.38 (d, 2H, J ) 7 Hz), 9.36 (s, 1H), 9.20 (d, 1H,
J ) 7 Hz), 9.10 (s, 1H), 8.89 (d, 1H, J ) 7 Hz), 8.79 (d, 2H, J ) 7
Hz), 8.65 (d, 2H, 7 Hz), 8.07 (d, 1H, J ) 7 Hz), 7.91(d, 1H, J ) 7
Hz), 7.8 (m, 10 H), 7.65 (m, 6H), 7.58 (m, 6H, J ) 9 Hz), 7.38 (d, 6H,
J ) 9 Hz), 7.18 (d, 4H, J ) 9 Hz), 7.0 (m, 14H), 6.72 (d, 2 H, J ) 9
Hz), 5.77 (s, 2H), 5.1 (t, 2H, J ) 6 Hz), 5.00 (t, 2H, J ) 6 Hz), 4.1
(m, 4 H), 3.8 (m, 4H), 2.95 (m, 2H), 2. 55 (q, 2H, J ) 7 Hz), 2.29 (s,
6H), 1.22 (s, 18 H), 1.20 (t, 3H, J ) 7 Hz); 31P NMR δ 18.4 (s); MS
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, an aqueous colloidal dispersion of TiO2 was prepared as
described above and autoclaved at 200 °C for 12 h to yield a dispersion
of 12 nm diameter nanocrystals.18 Concentration of this dispersion (160
g L-1) and addition of Carbowax 20000 (40 wt % equiv of TiO2) yields
a viscous paste. This paste was spread using 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 (1 × 10-3 mol dm-3) of the tripodal [2] rotaxane 1 or 3 (or
the corresponding axle component 2 or 4, respectively) for 1 h at room
temperature. The modified film was removed from the above solution
and washed thoroughly with dry ethanol. The modified films were dried
using a hot air gun (at approximately 80 °C) before being 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 ×
+
(m/z) 1929 (MCl3 - H).
Compound 3. 1-{4-[Tris{4-(4-dihydrophosphonophenyl)pheny}-
methyl]phenyl}-1′-4-(4-[tris{4-tert-butylphenyl}methyl]phenoxymethyl)-
phenylmethyl)-4,4′-bipyridinium bis-paraphenylene-34-crown-10 dichlo-
ride: 1H NMR (CD3OD) δ 9.36 (d, 2H, J ) 7 Hz), 8.86 (d, 2H, J )
7 Hz), 8.36 (d, 2H, J ) 7 Hz), 8.11 (d, 2H, J ) 7 Hz), 7.8 (m, 8 H),
7.65 (m, 8H), 7.58 (m, 6H), 7.45 (m, 6H), 7.18 (d, 6H, J ) 9 Hz), 7.0
(m, 12 H), 6.70 (d, 2 H, J ) 9 Hz), 6.10 (s, 8H), 5.86 (s, 2H), 4.1 (m,
4 H), 3.8 (m, 4H), 3.5 (m, 16H), 3.4 (m, 8H), 1.22 (s, 27 H); 31P NMR
δ 18.4 (s); MS (m/z) 1622 (M2+ - 2H).
Compound 4. 1-{4-[Tris{4-(4-dihydrophosphonophenyl)pheny}-
methyl]phenyl}-1′-4-(4-[tris{4-tert-butylphenyl}methyl]phenoxymethyl)-
phenylmethyl)-4,4′-bipyridinium dichloride: 1H NMR (CD3OD) δ 9.36
1014 cm-2 19
. On this basis, it is estimated that there are approximately
5 × 1016 tripodal [2]rotaxanes (or the corresponding axle components)
adsorbed at the surface of the TiO2 film.
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A R T I C L E S
Long et al.
Paramagnetic Suppression Nuclear Magnetic Resonance Spec-
troscopy. To measure a PASSY (paramagnetic suppression) NMR
spectrum of a solution of a given molecular component in acetonitrile,23
a known volume of the sample (1 mL), was first deoxygenated by
bubbling with nitrogen. Zinc metal powder (200 mesh, 10 mg) was
then added.20,21 The sample continued to be degassed by bubbling with
nitrogen. After 30 min, the dispersion became deeply colored indicating
formation of a radical cation of the viologen, and the proton NMR
spectrum was measured.
electrode (WE) was an isolated platinum wire. The counter electrode
(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 was tetrabutylammonium
perchlorate (TBAP, 0.1 mol dm-3) in dry acetonitrile. All solutions
were bubbled with argon gas for 20 min prior to measurement. All
cyclic voltammograms were recorded on a Solartron SI 1287 poten-
tiostat controlled by a LabView program running on a Machintosh
Power PC at a scan rate of 20 mV s-1 unless otherwise stated.
To measure a PASSY (paramagnetic suppression) NMR spectrum
of a dispersion of modified nanoparticles in acetonitrile,23 a known
volume of the sample (1 mL) was first deoxygenated by bubbling with
nitrogen. Chromium trichloride (0.2 mg) was then added and solubilized
by sonication for 30 min, followed by Zn metal powder (200 mesh, 10
mg).20,21 The sample continued to be degassed by bubbling with
nitrogen. After 30 min, the dispersion became deeply colored indicating
formation of a radical cation of the viologen, and the proton NMR
spectrum was measured.
Optical Absorption Spectroscopy. All optical absorption spectra
were recorded at 25 °C using a Hewlett-Packard 8452A diode array
spectrometer controlled by a LabView program running on a Macintosh
Power PC. All samples were contained in a 1.0 cm quartz cuvette.
Cyclic Voltammetry. All cyclic voltammograms (CVs) were
recorded in solution under the following conditions: The working
All CVs were recorded for nanostructured TiO2 films under the
following conditions: The WE was a nanostructured TiO2 film. The
CE was isolated platinum gauze. The RE was a nonaqueous Ag/Ag+
electrode filled with 10 mM AgNO3 in the electrolyte solution. The
electrolyte solution was TBAP (0.1 mol dm-3) in dry acetonitrile. All
solutions were bubbled with argon gas for 20 min prior to measurement.
As above all CVs 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.
Supporting Information Available: Syntheses of the tripodal
viologen and related molecular components employed in this
study. This material is available free of charge via the Internet
at http://pubs.acs.org.
(23) Carrington, A.; McLachlan, A. D. Introduction to Magnetic Resonance;
Harper and Row: New York, 1967.
JA037592G
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15498 J. AM. CHEM. SOC. VOL. 125, NO. 50, 2003