A photoactive molecular triad as a nanoscale power supply for a supramolecular machine

Article abstract of DOI:10.1002/chem.200500371

A tetrathiafulvalene-porphyrin-fullerene (TTF-P-C₆₀) molecular triad, which generates electrical current by harnessing light energy when self-assembled onto gold electrodes, has been developed. The triad, composed of three unique electroactive

Full text of DOI:10.1002/chem.200500371

DOI: 10.1002/chem.200500371
A Photoactive Molecular Triad as a Nanoscale Power Supply for a
Supramolecular Machine
Sourav Saha, Erik Johansson, Amar H. Flood, Hsian-Rong Tseng, Jeffrey I. Zink,* and
J. Fraser Stoddart*[a]
Abstract: A tetrathiafulvalene–porphy-
rin–fullerene (TTF–P–C60)molecular
triad, which generates electrical current
by harnessing light energy when self-
assembled onto gold electrodes, has
been developed. The triad, composed
of three unique electroactive compo-
nents, namely, 1)an electron-donating
TTF unit, 2)a chromophoric porphyrin
close-packing of the spherical C60 com-
ponent (diameter ~1 nm). In a closed
electronic circuit, a triad-SAM func-
tionalized working-electrode generates
of a p-electron-deficient tetracationic
cyclobis(paraquat-p-phenylene)
(CBPQT4+)cyclophane and a p-elec-
tron-rich
1,5-bis[(2-hydroxyethoxy)
a
switchable
~1.5 mAcmÀ2 when irradiated with a
413 nm Kr-ion laser, wavelength
photocurrent
of
ethoxy]naphthalene (BHEEN)thread.
The redox-induced dethreading of the
a
CBPQT4+
cyclophane from the
which is close to the porphyrin chro-
mophoreꢀs absorption maximum peak
at 420 nm. The electrical energy gener-
ated by the triad at the expense of the
light energy is ultimately exploited to
drive a supramolecular machine in the
form of a [2]pseudorotaxane comprised
BHEEN thread can be monitored by
measuring the increase in the fluores-
cence intensity of the BHEEN unit. A
gradual increase in the fluorescence in-
tensity of the BHEEN unit concomi-
tant with the photocurrent generation,
even at a potential (0 V)much lower
than that required (À300 mV)for the
direct reduction of the CBPQT4+ unit,
confirms that the dethreading process
is driven by the photocurrent generat-
ed by the triad-SAM.
unit, and 3)an electron-accepting C
60
unit, has been synthesized in a modular
fashion. A disulfide-based anchoring
group was tagged to the TTF end of
the molecule in order to allow its self-
assembly on gold surfaces. The surface
coverage by the triad in a self-assem-
bled monolayer (SAM)was estimated
to be 1.4 nm2 per molecule, a density
which is consistent with hexagonal
Keywords: actuation
·
electron
transfer · fluorescence · photocur-
rents · pseudorotaxanes
Introduction
best-qualified candidates for the active supramolecular or
molecular components of artificial molecular machines[7,8] in
fully integrated electronic circuitry. The structural unique-
ness of interlocked molecules involving two or more nonco-
valently bound components allow us to organize, power, and
control their actuation at the molecular level in devices by
employing external stimuli, such as those provided by chem-
ical, electrochemical, and photochemical inputs. To maintain
size and dimension, nanoscale machinery requires a nano-
scale power supply. As a means of incorporating a source of
electrical energy directly into molecular mechanical systems,
we have studied a photoactive donor–chromophore–accept-
or molecular triad[9,10] that mimics[11] the photosynthetic
energy transduction process by harnessing light energy to
generate electrical current. In this program of research, we
have demonstrated that the photoactive molecular triad 1,
whose structural formula is revealed in Figure 1a, can gener-
ate electrical energy at the expense of light energy, the
wavelength of which has been tuned specifically to the ab-
Molecular recognition and self-assembly processes are now
being harnessed[1] under the umbrella of template-directed
synthesis to produce active supramolecular and molecular
components in the rapidly evolving fields of nanoelectron-
ics[2] and nanoelectromechanical systems (NEMS),[3] because
of their interesting stereoelectronic and mechanical proper-
ties[4] that are tunable even at a nanoscale level. Pseudoro-
taxanes[5] and bistable rotaxanes[6] are amongst some of the
[a] S. Saha, E. Johansson, Dr. A. H. Flood, Dr. H.-R. Tseng,
Prof. J. I. Zink, Prof. J. F. Stoddart
The California NanoSystems Institute and
Department of Chemistry and Biochemistry
University of California, Los Angeles
405 Hilgard Avenue, Los Angeles, CA 90095–1569 (USA)
Fax :(+1)310-206-1843
E-mail: zink@chem.ucla.edu
stoddart@chem.ucla.edu
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trode through an electrolyte so-
lution in a complete electronic
circuit. Molecular triads involv-
ing tetrathiafulvalene (TTF)
and p-extended TTF compo-
nents are already known to pro-
duce long-lived (ms)charge-sep-
arated states.[10] This back-
ground provided us with the
impetus to introduce TTF as
the donor unit into our triad
design. Electronic reset by
back-electron transfer (BET)
within the charge-separated
species and/or a unidirectional
electron flow in a closed circuit
restores the neutral ground
state of the triads.
We report here the design
and modular synthesis of
a
TTF–P–C60 based molecular
triad 1 and how it generates
(Figure 1b–d)electrical energy
by harnessing light energy. By
attaching a disulfide-based, ter-
minal anchoring group, the
triad 1 can be self-assembled
onto gold electrode surfaces.
The modular approach adopted
for synthesis could be exploited
to fine-tune the electronic prop-
Figure 1. a)The structural formula of the tetrathiafulvalene-porphyrin-fullerene molecular triad 1 bearing a di-
sulfide anchoring group to enable the formation of a SAM on gold (Au). The triad 1-SAM was employed in a
photoelectrochemical cell to demonstrate the generation of electrical energy from light. b)–d) Proposed vecto-
rial electron transfer mechanism for the photo-response of a photoelectrochemical cell comprised of a SAM of
the triad 1 on an Au electrode in contact with an electrolyte solution that contains a platinum (Pt)counter
electrode (reference electrode not shown)connected externally to a potentiostat (PET =photoinduced elec-
tron transfer; CS=charge shift; BET=back-electron transfer).
sorption maxima of the chromophoric unit. Finally, this mo-
lecular power supply was employed to drive a supramolec-
ular machine, in the form of a donor–acceptor pseudorotax-
ane.[5]
erties—namely, the lifetime of the charge-separated inter-
mediates, redox charges, and so forth—according to
Marcus–Hush theory by subsequent structure-property feed-
back loops. In the triad 1 constitution, a diethylene glycol
spacer unit was incorporated between the TTF and porphy-
rin units in order to separate the donor and acceptor units.
Finally, this nanoscale source of electrical energy was utiliz-
ed[13] to drive[5] the dethreading of a supramolecular ma-
chine in the form of a [2]pseudorotaxane composed of the
A lot of effort has been devoted[9–11] to developing a varie-
ty of donor–chromophore–acceptor-based molecular triads
that are capable of intramolecular photoinduced electron
transfer (PET)from the donor to the acceptor unit. The
photogenerated charge-separated states of two classes of
such molecular triads have already been demonstrated to
generate[9] electrical currents in closed electrical circuits and
to enable active transportation[11] of protons and Ca2+ ions
across artificial lipid membranes. The best known of the full-
erenes, namely C60, is one of the most potent electron ac-
ceptors,[9c] because of its three-dimensional spherical geome-
try that supports very low electronic reorganization energy,
which, in turn, sustains a long-lived and stable charge-sepa-
rated state. Porphyrins (P)are often employed [9–12] as univer-
sal and efficient chromophores. In a donor–chromophore–
acceptor-based triad constitution, a PET from the porphy-
rinꢀs singlet excited state to the C60 unit, followed by a
charge shift (CS)to the donor unit, leads ultimately to a rel-
p-electron-accepting
cyclobis(paraquat-p-phenylene)
(CBPQT4+)cyclophane complexed with the p-donating 1,5-
bis[(2-hydroxyethoxy)ethoxy]naphthalene (BHEEN).
Results and Discussion
Synthesis: A modular approach was adopted (Scheme 1)for
the overall synthesis of the molecular triad 1. The key pre-
cursors were prepared separately from commercially avail-
able starting materials and were subsequently attached to-
gether to build up the dyad, and then finally, the triad 1.
The modularity in the synthetic approach is dependent upon
the generation of three individual components—namely, 1)
an electron-accepting C60 unit, 2)a chromophoric porphyrin
unit, and 3)an electron-donating TTF unit, all having
unique electronic and chemical properties—with ease and
+
atively stable charge-separated intermediate (DC –P–
C60CÀ).[9] The resulting charge-separated state is actually re-
sponsible for the generation of a photocurrent on account of
a unidirectional electron flow from a functionalized working
electrode, through a light-harvesting triad, to a counter elec-
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J. I. Zink, J. F. Stoddart et al.
glycine (2)and the N-boc-protected derivative 3 of 4-amino-
benzaldehyde.[15] Deprotection provided the C60 derivative 5,
bearing a reactive amino group. Scheme 3 outlines the syn-
thesis of the porphyrin unit. The C2-symmetric meso-porphy-
rin 7 was synthesized by using an acid-catalyzed cycliza-
tion[16] of 3,5-di-tert-butylbenzaldehyde with the bispyrrole 6,
which had been obtained previously by an acid-catalyzed re-
action[16] of pyrrole with methyl 4-formylbenzoate. Saponifi-
cation of the methyl ester functions afforded the corre-
sponding porphyrin–dicarboxylic acid 8. One of the two car-
boxylic acid groups in 8 was protected as its benzyl ester in
the presence of N-methylmorpholine, by using the activating
agents, 2-chloro-4,6-dimethoxy-1,3,5-triazine and 4-dimethyl-
aminopyridine (DMAP), to generate the key porphyrin pre-
cursor 9. Scheme 4 outlines the synthesis of the TTF unit.
The TTF diester 10 was prepared by Michael additions of
intermediate sulfur ylides to methylpropiolate.[17] A diisobu-
tyl aluminum hydride (DIBAL-H)-mediated reduction of
the diester 10 to the corresponding diol 11,[17] followed by a
dialkylation of 11 with the protected iodide 13,[18] furnished
the modified TTF-diol 14. Esterification of one of the termi-
nal primary hydroxyl groups in 14 with thioctic acid afford-
ed the alcohol 15.
Scheme 1. Retrosynthetic approach of the triad 1.
simplicity by following known synthetic protocols. Scheme 2
outlines the synthesis of the C60 unit. To begin with, C60 was
functionalized by a 1,3-dipolar cycloaddition[14] of N-phenyl-
Scheme 2. The preparation of the C60 component 5.
Scheme 3. The preparation of the porphyrin component 9.
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Supramolecular Machines
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Scheme 4. The preparation of the TTF component 15.
The manner in which the three pieces—namely, 5, 9, and
15—were brought together to form the triad 1 is illustrated
in Scheme 5. The amine-appended C60 derivative 5 was sub-
jected to an amide bond-forming reaction with the porphy-
rin derivative 9 in the presence of 2-chloro-4,6-dimethoxy-
1,3,5-triazine as the coupling reagent to form the P–C60 dyad
16. BBr3-mediated deprotection[10b] of the benzyl ester pro-
vided the P–C60 dyad 17 containing a free carboxyl group.
Finally, an esterification[10b] of this dyad 17 with the alcohol
15, by using the same coupling conditions, afforded the
target triad 1. This triad, as well as the dyads 16 and 17, and
the alcohol 15, and all their precursors were characterized
1
by mass spectrometry and H NMR spectroscopy.
The triad 1, which contains a terminal disulfide-anchoring
group, was self-assembled on gold wires and foils by im-
mersing them in 0.1mm solutions of the triad in dichloro-
Scheme 5. The synthesis of the triad 1.
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J. I. Zink, J. F. Stoddart et al.
methane for 3–5 days. The SAM-functionalized gold surfa-
ces were characterized by cyclic voltammetry (CV). They
were used subsequently as the working electrodes in the
cells employed for the detailed electrochemical studies de-
scribed later on in this paper.
on a working-electrode surface generates a maximum photo-
current when the SAM is irradiated close to (413 nm)the
porphyrin componentꢀs Soret band at 420 nm. In addition,
photosensitization, coincident with the porphyrin compo-
nentꢀs Q-bands, is observed with excitation at 514 and
568 nm in a process that generates less photocurrent, a find-
ing that is consistent with the smaller excitation coefficients
associated with the Q-bands. Hence, in order to photoexcite
the chromophoric component exclusively and to maximum
effect, an excitation wavelength
UV-visible and photoaction spectra: The absorption spectra
of the triad 1, the P–C60 dyad 16 and the other building
blocks are shown in Figure 2. The triad 1 in CH2Cl2 shows
of 413 nm was chosen for the
subsequent photochemical stud-
ies.
Electrochemistry: The redox
properties of the triad 1 can be
assigned (Table 1)with refer-
ence to its components and the
P–C60 dyad 16. The CV of each
of the units displays characteris-
tic behavior (Figure 3): the
TTF-based thread is a donor
that is easily oxidized[19] with
two reversible, one-electron
processes observed at +393 and
+847 mV (versus SCE). The
porphyrin is an electroactive
unit that displays[9,10] both re-
Figure 2. The photoaction spectrum (squares connected with a solid line)of a SAM of the triad 1. The UV-
visible spectra of the triad 1 (solid black line), as well as those of its building blocks, the P–C60 dyad (gray
line), a C60 derivative (dotted line)and a TTF-diol (dashed line). The photoaction spectrum of the SAM of the
triad 1 was collected using excitation from a Kr-ion laser and an Ar-ion laser at 34Æ3 mWcmÀ2. The inset dis-
plays the CV (Pt counter electrode, Ag pseudo reference electrode, 0.1m LiClO4, MeCN)of the SAM of 1 on
an Au wire recorded at 100, 300, 500, and 700 mVsÀ1—in the order of increasing peak intensity—illustrating
the linear dependence of the current peak intensity with scan rate.
duction and oxidation, the first
of which occurs at À1176 and
+1027 mV, respectively. The
difference
in
voltage
of
2203 mV between these two
Table 1. Redox potentials (E1/2 [mV] vs SCE)of the triad 1 and its com-
ponents 5, 7, 14, and 16 listed going down the columns from the most
positive to the most negative at 1mm concentration in 0.1m TBAPF6/
CH2Cl2 at room temperature.
absorption features at 253, 305, 326, 420, 451, 516, 551, 590,
and 646 nm. These features are a linear combination[10b,c] of
the absorption bands of each of the components (building
blocks)present in the molecule. The 253 nm peak is attrib-
uted to a C60 absorption, while those at 305 and 326 nm
arise from TTF absorptions. The peaks from 420 through
646 nm are ascribed to the porphyrinꢀs Soret and Q-bands.
The linear combination[10b,c] of the absorption peaks of the
C60, porphyrin, and TTF units in the spectrum of 1 indicates
that there is little or no electronic interaction in the ground
state between any of the components present in the mole-
cule. The absorption maximum of the porphyrinꢀs Soret
band at 420 nm is sufficiently isolated and its absorbance is
much more intense than the other components. These fac-
tors can be exploited to advantage when photoexciting the
porphyrin chromophore selectively into its first singlet excit-
ed state, in which it behaves like a donor.
TTF
Porphyrin C60-ArNH2 P–C60 Dyad
Triad
Assignments
(14)
(7)
(5)
(16)
(1)
–
–
+1322
+1027
–
–
–
–
À1176
À1500
–
–
–
–
–
+1305[a]
+1015
–
–
P2+/+
P+/0
+1027
+847
+393
+825[b] TTF2+/+
–
+403
À672
TTF+/0
0/À1
–
–
–
–
–
À658
À1038
–
À666
À1023
À1186
À1551[c]
À1551[c]
C60
À1/À2
À1032
À1199
C60
P0/À1
–
À1559[c] PÀ1/À2
À2/À3
À1562
À1559[c] C60
[a] An irreversible second oxidation process of the porphyrin component
in the dyad containing C60. [b] A quasi-reversible second oxidation of the
TTF component in the triad containing a contiguous porphyrin unit.
[c] Overlapping reduction potentials.
A photoaction spectrum is generated using a photoelec-
trochemical cell. It is a measure of the photocurrent that
can be generated by a SAM of the triad 1 on a gold elec-
trode when it is irradiated with various wavelengths of light.
Figure 2 illustrates that the fact that the SAM of the triad 1
processes can be correlated to a visible wavelength of
563 nm, reflecting the HOMO-to-LUMO gap of the porphy-
rin unit, and thus correlating to the wavelengths of the Soret
and Q-bands observed in the absorption spectrum. The C60
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Supramolecular Machines
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involved in a charge-transfer (CT)interaction with the C
60
component, as has been observed[9m] for various TTF–C60
dyads in which the two components are sufficiently close to-
gether. However, UV-visible spectra recorded at higher con-
centrations (1mm)of the triad 1 did not display conclusively
any characteristic band around 750 nm that could be as-
cribed to a TTF–C60 CT interaction. This lack of a TTF–C60
CT interaction in a triad is in the line with what Liddell
et al. reported.[10b] These authors have also observed[10b]
a
linear combination of reversible oxidation and/or reduction
peaks of all three electroactive components in the solution-
phase CV investigation of their more rigid TTF–P–C60 triad.
The CV of the 1-SAM on a gold wire electrode shows
[10c]
(Figure 2, inset)a prominent quasi-reversible
peak at
+680 mV, assignable to the two-electron oxidation of the
TTF component. The C60 component displays two closely
spaced reduction processes at À420 and À500 mV.[20] Assum-
ing that the oxidation of the TTF component at +680 mV is
a two-electron process, surface coverage of the triad 1 on
the gold surface is estimated to be 1.4 nm2 per molecule
(1.2 molcmÀ2), a finding that is consistent with hexagonal
close-packing of the spherical C60 component. It is not un-
likely that the variations in the shapes of the CVs of the
triad 1 in solution and it self-assembled onto a surface re-
flect the fact that they exist in different conformations. By
contrast, the CV of SAMs of the Fc–P–C60 triads dis-
played[9c] an absence of the C60-based reduction peaks, while
the Fc-based oxidation peaks were readily observed. The
discrepancies in the CVs of the somewhat similar types of
donor–chromophore–acceptor-based triads reported in the
literature[9,10] may be attributed to the differences in their
detailed structure and conformational arrangements; for ex-
ample, the p-electron donating units, the structure of the
porphyrin units, and the distances between the components
in the triad molecules are different.
Figure 3. Cyclic voltammetry of a)TTF, b)porphyrin, c)C 60, d)the dyad,
and e)the triad. The data were recorded as ~1 mm concentrations in
0.1m TBAPF6 solutions in CH2Cl2 at 200 mVsÀ1 with the standard calo-
mel electrode (SCE)as a reference.
unit displays three one-electron quasi-reversible reduc-
tions,[9d] beginning at À658 mV, that are commensurate with
its high electron-accepting ability. An irreversible oxidation
process is also observed on account of a small amount of
electrode adsorption of the C60 unit throughout the reduc-
tion region. The CV of the P–C60 dyad 16 is a linear combi-
nation of its component parts and each feature in the reduc-
tion region can be assigned. The process at À1551 mV is at-
tributed to the reduction of the porphyrin component to its
dianion and of the C60 component to its trianionic form. The
dyad displays a reversible oxidation that can be assigned to
the porphyrin component. A second irreversible process is
observed that displays an oxidative peak at approximately
+1300 mV that can be attributed to oxidation of the por-
phyrin component to its dication, a process occurring at a
potential similar to that for the irreversible oxidation of the
C60 component. The assignments to the CV response of the
triad 1 follow a similar pattern to that of the dyad through-
out the reduction region. The oxidative region, however, dis-
plays some additional features that are not straightforward
to assign. In particular, the TTF component displays two ox-
idation features (arrows in Figure 3e)that are significantly
lowered in their peak intensities, although they should be
equal in magnitude with all of the other one-electron peaks.
In addition, the oxidation processes assigned to the porphy-
rin and C60 components become irreversible ones in the CV
of the triad. We expected that the TTF component would be
Photoelectrochemistry: Photoelectrochemical experiments
were carried out initially in a standard[9c] three-electrode
electrochemical cell by using a gold-foil electrode function-
alized with 1-SAM as the working electrode, a platinum
gauze as the counter electrode, Ag/AgCl (saturated KCl)as
the reference electrode, and 0.1m Na2SO4 (aqueous)as the
electrolyte solution. In this particular set-up, the 1-SAM-
functionalized working electrode generated a cathodic cur-
rent of ~1 mAcmÀ2 (Figure 4a)upon irradiation with a
413 nm laser light (Kr-ion laser, 34Æ3 mWcmÀ2), while the
cell was held with a bias at À500 mV. Bare gold and decane-
thiol-SAM-functionalized gold electrodes did not display
(Figure 4a)any light-gated response. The linear relationship
(Figure 4b)of the magnitudes of the photocurrent with the
laser power confirms that electronic output is a result of
photon input—the larger the input of photons, the higher
the output of electrons.
In a similar experiment, employing 0.1m TBAPF6 in
MeCN as the electrolyte solution and Ag wire as the
pseudo-reference electrode in a more compact electrochemi-
cal cell with shorter distances between the electrodes, led to
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The pseudorotaxane can be considered as a reasonably
stable CT complex between the BHEEN thread and the
CBPQT4+ cyclophane. However, reduction[5a,21] of the
CBPQT4+ to the corresponding CBPQTC/3+ weakens the CT
interaction, which, in turn, results in the decomplexation of
the CBPQTC/3+ unit from the BHEEN thread. The dethread-
ing process can be monitored by measuring the increase in
the fluorescence intensity of the free BHEEN relative to the
weakly fluorescent complexed BHEEN. The required elec-
trochemical potential for the CBPQT4+/ /3+
reduction pro-
C
cess is À300 mV. The introduction of a working electrode
functionalized with the photoactive triad 1 into an electro-
chemical cell containing the pseudorotaxane in the electro-
lyte solution caused a gradual increase in the BHEENꢀs
fluorescence intensity (Figure 5c), even at Eap =0 V while
the functionalized working electrode was being irradiated
with 413 nm laser light during a period of 2900 s. The in-
crease in the BHEENꢀs fluorescence intensity is also accom-
panied (Figure 5d)by the generation of an average photo-
current of 1.1 mAcmÀ2 generated over the excitation period.
However, the BHEEN-based fluorescence remained almost
unaffected at 0 V when the 413 nm excitation was switched
OFF, and consequently, no photocurrent was generated. Ir-
radiation of the 1-SAM with 413 nm light generates the
charge-separated species TTFC+–P–C60CÀ, which releases its
electron from the C60CÀ (Ered =À500 mV)to the pseudoro-
taxane present in the surrounding electrolyte solution.
Therefore, even at an applied potential of 0 V, and in the
presence of a 413 nm laser, the effective cathodic potential
at the working electrode functionalized with the triad 1 is
À500 mV, a potential which is higher than the threshold of
À300 mV that is required for the dethreading process.
The pseudorotaxane was dethreaded deliberately
(Figure 6)in the absence of the 413 nm laser following the
direct electrochemical reduction of the CBPQT4+ cyclo-
phane by applying a À500 mV bias for 1200 s. A time-lag of
~100 s for the increase in fluorescence intensity of the
BHEEN was observed. It arises because of the diffusion of
the free components from the bare gold-foil working elec-
trode to the site of the UV excitation. A gradual increase in
the fluorescence intensity, synchronized with a gradual de-
crease of the cathodic current, was observed until about
750 s, at which point, both of the current and fluorescence
traces reached their corresponding saturation points and re-
mained constant thereafter. An almost constant (750–
1200 s)fluorescence intensity, even when the cell was biased
at À500 mV, leads us to the conclusion that almost all
(~100%)of the pseudorotaxane became dissociated within
the first 750 s. Moreover, the observation of almost no at-
tenuation of the fluorescence intensity within 1200–1800 s
when no bias was applied indicates that the oxidation of the
reduced CBPQTC/3+ to CBPQT4+ by ambient O2 and its sub-
sequent reassociation with the BHEEN is an extremely slow
process.
Figure 4. a)The photocurrent switching cycle (solid line)of the triad 1 in
a SAM on Au, recorded in the photoelectrochemical cell by gating
413 nm excitation ON and OFF every 10 s after the current response has
reached an equilibrium value with polarization at À500 mV (0.1m
Na2SO4, aerated). The bottom two traces were recorded under identical
conditions on a SAM of decanethiol (dashed line)and on bare Au
(dotted line). b) A plot illustrating the linearity of the difference between
the ON and OFF signals in the photocurrent response (DI)with laser in-
tensity, recorded under the same conditions as those described in a).
the generation[21] of a higher and more stable cathodic pho-
tocurrent (~1.4 mAcmÀ2), even at an applied potential (Eap)
of 0 V. No photocurrent was observed when the applied po-
tential was set at or above +700 mV, which is just above the
oxidation potential of the TTF component in the 1-SAM.
An applied potential threshold of +650 mV for the photo-
current generation is roughly concomitant with the oxida-
tion potential (+680 mV)of the TTF component in the 1-
SAM. This phenomenon can be attributed to the unidirec-
tional flow of electrons from the gold-working electrode
through the SAM to the platinum counter electrode through
the electrolyte solution. In an open circuit, a photovoltage
of À12 mV was measured in the presence of a 413 nm laser
(40 mWcmÀ2). The quantum efficiency of the photocurrent
generation by the triad 1 was estimated to be ~1%, based
on the photocurrent output of 1.3 mAcmÀ2 arising from a
413 laser input of 26 mWcmÀ2. In the optimized conditions,
the Fc-donor-based triad was reported[9c] to show 20–25%
photocurrent efficiency.
Dethreading of a pseudorotaxane: Finally, the 1-SAM was
employed (Figure 5a)as a molecular-level energy supply
to power the dethreading of
a
a
[2]pseudorotaxane
(BHEENꢀCBPQT4+)composed of
p-electron-rich
The background signal arising from the scattering of the
413 nm light was determined by comparing (Figure 7)the
fluorescence intensity with the 413 nm laser ON, but with
BHEEN thread and a p-electron-deficient CBPQT4+ cyclo-
phane. The experimental set-up is illustrated in Figure 5b.
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Figure 6. a)Fluorescence of BHEEN when the CBPQT 4+ cyclophane is
directly reduced at À500 mV. The observed plateau for the trace after
750 s, even when the cell is biased at À500 mV for 1200 s indicates com-
plete dethreading after 750 s. b)Current versus time plot at À500 mV.
the BHEEN excitation (UV)turned ON and then OFF.
With the 413 nm laser ON, while the UV was ON or OFF,
no significant signal for the 413 nm irradiation was detected
by the fluorescence detector.
A simple calculation verifies that the percent increase in
the fluorescence intensity is consistent with the percent dis-
sociation of the total pseudorotaxane by the photocurrent.
At À500 mV, an average current of 6.12”10À5 A for 750 s,
which generates 4.7”10À7 mol of electrons, causes complete
(~100%)dissociation of the pseudorotaxane, as shown by
the saturation of the fluorescence intensity after 750 s. Con-
sequently, at 0 V, an average photocurrent of 1.1 mA for
2900 s, which generates 3.3”10À8 mol of electrons, should be
able to dethread 7% of the pseudorotaxane. In the same ex-
periment at 0 V, the BHEENꢀs fluorescence intensity in-
creased[23] by 6.7% with respect to its initial intensity; an ob-
Figure 5. a)Schematic representation illustrating how the light-driven
power supply, that is, the triad 1, provides the electrical energy to deth-
read
a [2]pseudorotaxane ([2]PR)wherein BHEEN is encapsulated
inside the cavity of a CBPQT4+ cyclophane. The curved arrows indicate
the vectorial electron transfer (PET)from the photo-excited porphyrin
chromophore to the C60 component, followed by a charge shift (CS)to
the TTF component, and finally a charge neutralization by the Au elec-
trode in a closed circuit. Subsequently, the electron is transferred to the
pseudorotaxane, leading to dethreading of the reduced CBPQTC/3+
before its electron is passed onto the Pt counter electrode. b)Experi-
mental set-up for the dethreading experiment; the colored arrows indi-
cate the excitation and emission pathways of the respective units. c)The
dethreading process synchronized with a photocurrent generation at an
applied potential of 0 V monitored by the increase in BHEENꢀs flores-
cence (top trace)with excitation at 257 nm. The fluorescence (bottom
trace)of the BHEEN unit in the absence of porphyrin excitation. d)The
photocurrent generation as a result of the photoexcitation by 413 nm
laser.
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J. I. Zink, J. F. Stoddart et al.
Experimental Section
General methods: Starting materials and reagents were purchased from
Aldrich and used as received. The N-boc-4-aminobenzaldehyde deriva-
tive 3 and compounds 10–14 were prepared as described in the litera-
ture.[15, 17,18] All reactions were performed under an argon atmosphere
and in dry solvents unless otherwise noted. Analytical TLC was per-
formed on silica gel 60-F254 (Merck)with detection by fluorescence and/
or by developing in an iodine chamber. Flash chromatography was per-
formed with silica gel 60 (Silicycle). The TTF derivative 15, the P–C60
dyads 16 and 17, and the triad 1 are all mixtures of several diastereoiso-
mers. No attempt was made to isolate them. 1H NMR spectra were re-
corded on Bruker ARX 400 MHz, Bruker Avance 500 MHz, or Bruker
Avance 600 MHz spectrometers at ambient temperature in appropriate
deuterated solvents using tetramethylsilane as an internal reference.
High-resolution matrix-assisted laser desorption ionization mass spectra
(HR-MALDI-MS)were recorded on an IonSpec Ultima 7.0T FT-ICR
MALDI mass spectrometer with dihydroxybenzoic acid (DHB), a-cyano-
hydroxycinnamic acid (a-CN), or terthiophene as a matrix and A19L
oligopeptide (APPPDPDPGP EIKAKRLNL, monoisotopic mass=
2024.1)as an external calibration.
Figure 7. Background correction for the effect of the 413 nm laser scatter-
ing on the BHEEN-based fluorescence intensity. Note that there is
almost no change in the fluorescence intensity (grey trace)concomitant
with the photocurrent generation (black trace)by the 413 nm laser, both
when UV excitation of BHEEN is kept ON and kept OFF.
Compound 4: This compound was prepared by using a slight modifica-
tion of the literature procedure.[16]
A homogeneous solution of C60
servation that is consistent with the percent increase in the
free BHEEN concentration, estimated based on the elec-
tron-count. The decomplexation of the pseudorotaxane by
the photosensitization of the light-harvesting molecular triad
1 at an applied potential of 0 V is therefore a direct result of
the photoreduction of the electron-acceptor C60 component,
which passes its electron onto the pseudototaxanes present
in the adjacent electrolyte solution, causing the electrochem-
ical reduction of the CBPQT4+ cyclophane, resulting in its
dethreading from the BHEEN in a chain process that can
be controlled by a specific wavelength of laser light. Thus,
we have demonstrated how the actuation of the two parts of
a pseudorotaxane can be controlled using light energy by
simply introducing a photoactive molecule into an appropri-
ate supramolecular environment.
(1.44 g, 2 mmol), N-phenylglycine 2 (3.18 g, 20 mmol), and the benzalde-
hyde derivative 3[15] (440 mg, 2 mmol)in 1,2-dichlorobenzene (300 mL)
was heated under reflux for 1.5 h. After cooling down to room tempera-
ture, the reaction mixture was subjected to silica gel (SiO2)column chro-
matography, eluting first with hexanes and then with PhMe to obtain the
C60 adduct 4 (820 mg, 40% yield)as a brown solid. 1H NMR (500 MHz,
CDCl3): d=1.41 (s, 9H), 4.90 (d, J=10 Hz, 1H), 5.58 (d, J=10 Hz, 1H),
5.96 (s, 1H), 6.36 (s, 1H), 7.10 (d, J=9 Hz, 2H), 7.27–7.29 (m, 5H),
7.64 ppm (d, J=9 Hz, 2H); MS (MALDI-TOF): m/z: 1030.7 [M+H]+
(DHB matrix).
Compound 5: A solution of 4 (1.6 g, 1.5 mmol)and trifluoroacetic acid
(40 mL)in PhMe (300 mL)was heated under reflux for 3 h. After cooling
down the reaction mixture, the solvent was removed, and the residue was
re-dissolved in PhMe (50 mL). The remaining acid was quenched with
Et3N (20 mL). The solvents were then evaporated off and the residual
solid was re-dissolved in a minimum volume of PhMe. Column chroma-
tography (SiO2: PhMe)afforded the free amine 5 (1.3 g, 90% yield)as a
dark brown solid. 1H NMR (600 MHz, CDCl3): d=4.89 (d, J=10 Hz,
1H), 5.57 (d, J=10 Hz, 1H), 5.90 (s, 1H), 6.57 (d, J=8 Hz, 2H), 7.27–
7.30 (m, 5H), 7.49 ppm (d, J=8 Hz, 2H); MS (HR-MALDI): m/z:
929.2163 [M+H]+ (DHB matrix).
Conclusion
Compound 6: This compound was prepared by using a slight modifica-
tion of the literature procedure.[16] A solution of methyl-4-formylben-
zoate (17.25 g, 105.1 mmol)in pyrrole (75 mL, 1.06 mol)was degassed by
bubbling Ar for 2 h. After adding trifluoroacetic acid (0.21 mL,
2.8 mmol)to the degassed solution, it was stirred at 20 8C in the dark for
4 h. The reaction mixture was diluted with C6H6 (200 mL)and washed
with 0.1m aqueous NaOH solution and H2O successively. The organic
layer was dried (Na2SO4), and the solvent and the excess of pyrrole were
removed under reduced pressure to afford a viscous brown oil, which
was then coated on SiO2 (20 mL)and purified by flash column chroma-
tography (SiO2: C6H6/EtOAc/Et3N 100:2.5:1.0 to 100:5.0:1.0). The first
pale yellow fraction was collected and the solvent was removed under re-
duced pressure at room temperature. The resulting solid was washed with
cold EtOAc (50 mL)and a subsequent filtration afforded 6 (13.3 g, 42%
yield)as a beige crystalline solid. 1H NMR (400 MHz, CDCl3): d=3.80
(s, 3H), 5.42 (s, 1H), 5.79 (s, 2H), 6.06 (d, J=3 Hz, 2H), 6.61 (d, J=
1 Hz, 2H), 7.17 (t, J=7 Hz, 2H), 7.86 ppm (d, J=8 Hz, 4H).
A variety of donor–chromophore–acceptor-based light-har-
vesting molecular dyads and triads have already been ap-
plied[9–13] in several different ways, such as 1)inducing active
transport on ions across artificial lipid membranes, 2)con-
verting light to electrical energy, and 3)powering nanoscale
machinery. These dyads and triads may be optimized into a
true photocell,[24] generating photovoltages that are propor-
tional to the light power. Moreover, their uses may be ex-
tended even further by utilizing them as nanoscale power
supplies in appropriate environments to drive and control
molecular and supramolecular actuations. Their functions as
power supplies can, in turn, be regulated exclusively by na-
tureꢀs most universal and abundant energy source—light. In
this piece of work, we have demonstrated how a molecular
triad can be employed as a power supply for a supramolec-
ular machine. Clearly, if this molecular triad can be integrat-
ed into a bistable [2]rotaxane, we would have a photodriven
molecular machine based on a light-harvesting system at the
nanoscale level.
Compound 7: This compound was prepared by using a slight modification
of the literature procedure.[16] A solution of 6 (13.30 g, 47 mmol)and 3,5-
di-tert-butylbenzaldehyde (10.36 g, 47 mmol)in CHCl
(4 L)was de-
3
gassed by bubbling Ar for 2 h. Following addition of BF3·Et2O (6.02 mL,
47 mmol)to the degassed solution, it was stirred at 20 8C in the dark for
2 h. Subsequently, p-chloranil (18 g, 76 mmol)was added to the resulting
dark red solution. After stirring for another 12 h, Et3N (20 mL,
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Supramolecular Machines
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150 mmol)was added to the reaction mixture to neutralize the Lewis
acid. The reaction mixture was then concentrated. Flash column chroma-
tography (SiO2: CHCl3)of the crude reaction mixture, followed by the
washing of the resulting purple solid with cold MeOH (50 mL)provided
7 as the major product in a mixture of three different porphyrins. Further
column chromatography (SiO2: hexanes/PhMe 2:1 to 1:3)afforded the
desired meso-porphyrin 7 as the second fraction. Evaporation of the sol-
polar component (activated ester)at the expense of 9. Subsequently, a
solution of the amine-appended C60-adduct 5 (700 mg, 750 mmol)and 4-
dimethylaminopyridine (100 mg)in CH 2Cl2 (100 mL)was added to the
reaction mixture. The resulting reaction mixture was then stirred at 208C
for 4 h. Solvent was removed under a reduced pressure and the dyad 18
was purified by flash column chromatography (SiO2: CH2Cl2/hexanes
90:10 to 95:5)as the second fraction. Evaporation of the solvent from
the second fraction afforded 16 (1.16 g, 80% yield)as a bright purple
solid. 1H NMR (500 MHz, CDCl3): d=À2.82 (s, 2H), 1.55 (s, 36H), 4.83
(d, J=10 Hz, 1H), 5.51 (d, J=10 Hz, 1H), 5.56 (s, 2H), 5.80 (s, 1H),
7.09–7.11 (m, 2H), 7.32–7.43 (m, 4H), 7.46–7.49 (m, 2H), 7.61 (d, J=
8 Hz, 2H), 7.75 (d, J=8 Hz, 2H), 7.80 (s, 4H), 7.88 (d, J=8 Hz, 2H),
8.06 (s, 4H), 8.26 (s, 2H), 8.30 (d, J=8 Hz, 2H), 8.44 (d, J=8 Hz, 2H),
8.77 (d, J=3 Hz, 4H), 8.88–8.91 ppm (dd, J=11, 4.5 Hz, 4H); MS (HR-
MALDI): m/z: 1931.0035 [M+H]+ (no matrix).
1
vent from this fraction yielded a purple solid (3.6 g, 14% yield). H NMR
(600 MHz, CDCl3): d=À2.74 (s, 2H), 1.54 (s, 36H), 4.12 (s, 6H), 7.81 (d,
J=2 Hz, 2H), 8.07 (s, 4H), 8.31 (d, J=8 Hz, 4H), 8.43 (d, J=8 Hz, 4H),
8.79 (d, J=5 Hz, 4H), 8.90 ppm (d, J=5 Hz, 4H).
Compound 8: This compound was prepared by using a slight modifica-
tion of the literature procedure.[16] A solution of 7 (1.01 g, 1.05 mmol)in
a THF/EtOH mixture (1/1, 500 mL)and KOH (5.4 g)in H 2O (50 mL)
was heated under reflux for 12 h. After cooling, the organic solvent was
removed under reduced pressure and the residue was diluted with H2O
(200 mL). The resulting dipotassium salt of the corresponding porphyrin
dicarboxylic acid was then filtered off. An acidification of the solid dipo-
tassium salt with conc. HCl, followed by thorough washing of the residue
with hot H2O provided the dicarboxylic acid 8 (950 mg, 97% yield)as a
purple powder. 1H NMR (500 MHz, CDCl3/(CD3)2SO 3:1): d=À3.62
(brs, 2H), 1.78 (s, 36H), 7.03 (brs, 2H), 7.26 (brs, 4H), 7.52 (m, 4H),
7.64 (m, 4H), 8.05 (brs, 4H), 8.12 ppm (brs, 4H).
Compound 17: A solution of BBr3 (2 mL, excess)in CH 2Cl2 (100 mL)
was added slowly at À788C to a solution of 16 (1.16 g, 601 mmol)in
CH2Cl2 (500 mL). The resulting green solution was allowed to warm up
to 208C and stirred for another 4 h. The reaction mixture was then ex-
tracted with H2O (3”100 mL)to remove any excess of acid. The result-
ing purple solution was dried (Na2SO4)and the organic solvent was
evaporated. Column chromatography (SiO2: CHCl3/MeOH 98:2 to 95:5)
and subsequent evaporation of the solvent from the second fraction af-
forded the dyad 17 (880 mg, 80% yield)as a purple solid. 1H NMR
(500 MHz, CDCl3/(CD3)2SO 4:1): d=À2.88 (brs, 2H), 1.54 (s, 36H), 4.72
(d, J=8 Hz, 1H), 5.40 (d, J=8 Hz, 1H), 5.84 (s, 1H), 6.58 (brs, 1H), 6.93
(d, J=7 Hz, 2H), 7.22 (t, J=6 Hz, 1H), 7.30–7.36 (m, 4H), 7.69 (d, J=
6 Hz, 2H), 7.82 (s, 2H), 8.05 (s, 6H), 8.16 (d, J=6 Hz, 2H), 8.29–8.32 (m,
4H), 8.44 (d, J=6 Hz, 2H), 8.76 (s, 2H), 8.80 (s, 2H), 8.88 (s, 4H),
10.45 ppm (s, 1H); UV/Vis (5.43 mm solution in THF): lmax =251 (C60),
418 (porphyrin Soret band), 512, 546, 588, 644 nm (porphyrin Q-bands);
MS (HR-MALDI): m/z: 1840.9566 [M+H]+ (terthiophene matrix).
Compound 9: 4-Methylmorpholine (1 mL, excess)was added to a solu-
tion of 8 (927 mg, 1 mmol)in THF (300 mL)and the reaction mixture
was stirred at 208C for 10 min. Then, 2-chloro-4,6-dimethoxy-1,3,5-tri-
azine (386 mg, 2.2 mmol)was added to it at 0 8C. The resulting solution
was stirred at 08C for 10 min and then at 208C for 4 h. TLC of this solu-
tion showed the appearance of a less polar compound (the activated
ester)at the expense of dicarboxylic acid 8. Subsequently, a solution of
benzyl alcohol (103 mL, 1 mmol)and 4-dimethylaminopyridine (200 mg)
in THF (100 mL)was added dropwise into the solution of the activated
ester. The resulting reaction mixture was then stirred at 208C for 10 h.
After an aqueous workup and extraction with a CHCl3/MeOH mixture
(3”100 mL, 4:1), the combined organic solution was dried (Na2SO4)and
evaporated under reduced pressure. The desired monoester 9 was isolat-
ed by flash column chromatography (SiO2: CHCl3/MeOH 98:2 to 95:5)
as the second fraction. Evaporation of solvent from this fraction afforded
9 (610 mg, 60% yield)as a purple powder. 1H NMR (600 MHz, CDCl3):
d=À2.73 (brs, 2H), 1.54 (s, 36H), 5.56 (s, 2H), 7.42 (d, J=8 Hz, 2H),
7.47 (t, J=8 Hz, 2H), 7.60 (d, J=8 Hz, 2H), 7.81 (brs, 2H), 8.08 (s, 4H),
8.32 (d, J=8 Hz, 2H), 8.35 (d, J=8 Hz, 2H), 8.47 (d, J=8 Hz, 2H), 8.51
(d, J=8 Hz, 2H), 8.80 (d, J=5 Hz, 4H), 8.92 ppm (t, J=5 Hz, 4H); MS
(HR-MALDI): m/z: 1017.5312 [M+H]+ (DHB matrix).
Compound 1: 4-Methylmorpholine (0.1 mL, excess)was added to a solu-
tion of the P–C60 dyad 17 (92 mg, 50 mmol)in a THF/ CH 2Cl2 mixture
(20 mL, 1:2), and the resulting solution was stirred at 208C for 10 min.
Subsequently, 2-chloro-4,6-dimethoxy-1,3,5-triazine (11 mg, 60 mmol)was
added to the reaction mixture at 08C. The resulting solution was stirred
at 08C for 10 min and then at 208C for 3 h. TLC of this solution showed
the appearance of a less polar component (activated ester)at the expense
of the starting material 17. Subsequently, a solution of TTF derivative 15
(3 equiv)and 4-dimethylaminopyridine (100 mg)in CH 2Cl2 (100 mL)was
added to the reaction mixture. The resulting solution was stirred at 208C
for 4 h. After removing the solvent from the crude reaction mixture, the
residue was thoroughly washed with MeOH to remove excess of 15 and
other reagents to obtain the triad 1 (98 mg, 80%). It was further purified
by crystallization from a CH2Cl2 solution to obtain a brownish-purple
solid. 1H NMR (500 MHz, CDCl3): d=À2.81 (s, 2H), 1.58 (brs, 36H),
1.90–1.93 (m, 2H), 2.05 (s, 2H), 2.28–2.30 (m, 2H), 2.34–2.37 (m, 1H),
3.07–3.14 (m, 5H), 3.31–3.34 (m, 1H), 3.40–3.44 (m, 2H), 3.49 (s, 2H),
3.54–3.59 (m, 2H), 3.69–3.75 (m, 4H), 3.80–3.82 (m, 2H), 3.94 (s, 1H),
3.96–3.98 (m, 2H), 4.02 (brs, 1H), 4.12–4.16 (m, 1H), 4.23–4.25 (m, 1H),
4.28 (s, 1H), 4.35 (brs, 1H), 4.68 (brs, 1H), 4.81 (d, J=9 Hz, 1H),[25] 5.50
(d, J=9 Hz, 1H),[25] 5.90 (s, 1H),[25] 6.23 (d, J=5 Hz, 2H),[25] 7.10 (t, J=
7 Hz, 1H), 7.31–7.37 (m, 2H), 7.38 (t, J=7 Hz, 1H), 7.76 (brs, 2H), 7.81
(s, 2H), 7.86 (brs, 2H), 8.06 (s, 4H), 8.26 (s, 4H), 8.31 (brs, 2H), 8.43–
8.50 (m, 3H), 8.78 (brs, 4H), 8.89 ppm (d, J=6 Hz, 4H); MS (HR-
MALDI): m/z: 2451.7211 [M+H]+ (DHB matrix).
Compound 15: 4-Methylmorpholine (0.2 mL, 1.6 mmol)was added to a
solution of thioctic acid (206 mg, 1 mmol)in CH 2Cl2 (50 mL)and the re-
sulting solution was stirred at 208C for 10 min. Then, 2-chloro-4,6-di-
methoxy-1,3,5-triazine (193 mg, 1.1 mmol)was added to the reaction mix-
ture at 08C. The resulting solution was stirred at 08C for 10 min and then
at 208C for 3 h. Subsequently, a solution of 14[17,18] (440 mg, 1 mmol)and
4-dimethylaminopyridine (100 mg)in CH 2Cl2 (20 mL)was added to the
solution of the activated ester in one portion. The resulting reaction mix-
ture was stirred at 208C for 4 h. The solvent was then removed under re-
duced pressure and the crude reaction mixture was coated on neutral alu-
mina. The ester 15 (250 mg, 40% yield)was purified by gradient column
chromatography (SiO2: CH2Cl2/hexanes 90:10 to CH2Cl2/MeOH 97:3)
and was isolated as a yellow oil. 1H NMR (500 MHz, (CD3)2CO): d=
1.46–1.50 (m, 2H), 1.61–1.66 (m, 4H), 1.70–1.75 (m, 1H), 1.87–1.95 (m,
1H), 2.33 (t, J=8 Hz, 2H), 2.45–2.51 (m, 1H), 3.09–3.13 (m, 1H), 3.18–
3.21 (m, 1H), 3.50–3.54 (m, 2H), 3.58–3.64 (m, 12H), 3.65–3.67 (m, 2H),
4.18 (t, J=3 Hz, 2H), 4.33 (s, 4H), 6.56 ppm (s, 2H); MS (HR-MALDI):
m/z: 628.0789 [M]+ (DHB matrix).
Self-assembled monolayer formation: The triad 1 was self-assembled on
Piranha-cleaned gold wires and foils from solutions of 1 in CH2Cl2 (0.1m)
in the absence of light over a period of 72 h. The resulting SAM-modified
gold surfaces were cleaned by rinsing with appropriate solvents (CH2Cl2
and EtOH)and used as the working electrode in the electrochemical and
photoelectrochemical experiments.
Compound 16: 4-Methylmorpholine (0.5 mL, excess)was added to a so-
lution of 9 (765 mg, 752 mmol)in THF/ CH Cl2 (1/3, 100 mL), and the re-
Physical studies: The UV-visible absorption spectra were recorded on a
Varian Cary 100 Bio in CH2Cl2. The extinction coefficients were obtained
from a single-point determination. Electrochemical experiments were
carried out at room temperature in argon-purged solutions of the samples
in MeCN, with a Princeton Applied Research 263 A Multipurpose instru-
2
sulting solution was stirred at 208C for 10 min. Then, 2-chloro-4,6-di-
methoxy-1,3,5-triazine (145 mg, 900 mmol)was added to the reaction mix-
ture at 08C. The resulting solution was stirred at 08C for 10 min and then
at 208C for 3 h. TLC of this solution showed the appearance of a less
Chem. Eur. J. 2005, 11, 6846 – 6858
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6855

J. I. Zink, J. F. Stoddart et al.
ment interfaced to a PC (EG&G software). Cyclic voltammetric (CV)
experiments were performed using a glassy carbon working electrode
(0.018 cm2, Cypress Systems); its surface was polished routinely with a
0.05 mm alumina-water slurry on a felt surface immediately before use.
The counter electrode was a Pt wire and the reference electrode was
either SCE or Ag/AgCl. Tetrabutylammonium hexafluorophosphate
(0.1m)was added as supporting electrolyte. Cyclic voltammograms were
obtained at scan rates ranging from 2 to 1000 mVsÀ1. For reversible proc-
esses, E1/2 was calculated from an average of the cathodic and anodic
cyclic voltammetric peaks. To establish the reversibility of a process, we
used the criteria of 1)60 mV between cathodic and anodic peaks and 2)
close to unity ratio of the intensities of the cathodic and anodic currents.
Experimental errors: potential values, Æ10 mV; absorption maxima,
Æ2 nm. The SAM-functionalized Au electrode (Au coil, 0.5 diameter,
2.262 cm2 working area)for the CV experiments was washed thoroughly
before use by rinsing in CH2Cl2, then in THF, and finally in MeCN.
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Phototoelectrochemical measurements: A standard three-electrode cell,
consisting of Au-foil functionalized with a SAM of the triad 1, a Pt gauze
counter electrode (in the shape of a bucket), and a Ag/AgCl (saturated
KCl)reference electrode were used. The Au foil was located at the
center of the Pt gauze bucket (diameter ~3 cm)counter electrode. An
aqueous 0.1m Na2SO4 solution (aerated)was employed as the electrolyte.
Single-wavelength laser light from a Krypton ion laser (Coherent, Innova
300C, 407, 413 and 568 nm)or an Argon ion laser (Coherent Innova
90C-5, 355, 457, 466, 476, 488, 496, 501 and 514 nm)was aligned and dis-
persed into the photoelectrochemical cell to irradiate a ~1 cm2 area of
the Au foil. All laser powers were measured at the source and were cor-
rected for attenuation by the intervening optics. A galvanostat experi-
ment was performed to detect the voltage (À500 mV)at which the dark
current was closest to zero. The photoaction spectrum was generated by
measuring the photocurrent at À500 mV after equilibration at 100 s gen-
erated by excitation at different wavelengths at
a constant power
(40 mWcmÀ2). Control experiments were conducted on bare Au foil or a
decanethiol (Aldrich)and butanethiol (Aldrich)SAMs coated Au elec-
trode. In an optimized system, a triad 1-SAM/Au working electrode, Pt
wire in a fritted tube as the counter electrode, Ag wire-pseudoreference
electrode, and 0.1m TBAPF6 solution in MeCN as supporting electrolyte
in a 1”1”3 cm3 quartz cuvette was used as the electrochemical cell. The
same laser source and power as mentioned earlier was used for photoex-
citation.
Dethreading experiments: The dethreading experiments were conducted
under the same conditions as the photoelectrochemistry by using solu-
tions of the sample in MeCN with 0.1m TBAPF6 as supporting electrolyte
and with an Ag wire as a pseudo-reference electrode in a 3 mL quartz
cell. The initial concentration of the pseudorotaxane at equilibrium was
0.37mm, based on
a 1:1 mixture of 0.5mm concentrations of both
BHEEN and CBPQT4+ units and their association constant[26] of Ka =
2.53”104 mÀ1 (MeCN, 298 K). The detection of fluorescence from the
BHEEN has been described elsewhere.[3a] Briefly, UV excitation (Ar ion
laser, Innova 300C FReD, lex =257 nm, 10 mWcmÀ1À2)was used and the
florescence was detected at 458 through a monochromator connected to
an intensified CCD camera (ICCD, PI-MAX, Princeton Instruments).
The florescence versus time trace was obtained by integrating the area
under the spectrum from 320 to 370 nm every 0.5 s. The florescence spec-
tra are an average of 50 spectra obtained every 0.5 s.
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Acknowledgements
This research was supported by a National Science Foundation (NSF)
Nanoscale Interdisciplinary Research Team (NIRT ECS-0103559)fund,
an Office of Naval Research (ONR-contract number N00014–00–1–0216)
grant, an NSF Equipment Grant (CHE-9974928), an NIH Shared Instru-
ment Grant (S10RR1S9S2)and the Center for Cell Mimetic Space Ex-
ploration (CMISE)—a NASA University Research, Engineering and
Technology Institute (URETI), the award number being NCC2–1364.
6856
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ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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À560 mV in a solution-phase CV of a TTF–P–C60 based triad.
[21] Z. M. Liu, A. A. Yasseri, J. S. Lindsey, D. F. Bocian, Science 2003,
302, 1543–1545.
acteristic chemical shifts and splitting patterns are exhibited by the
protons on the fused pyrrolidine ring (3H)on the C unit and on
60
the TTF ring (2H). Their integrated intensity, however, does not
correlate perfectly on account of the shielding effect of the C60 unit
and also, because of the complexity of the triad constitution, which
generates a number of diastereoisomers.
[26] R. Castro, K. R. Nixon, J. D. Evanseck, A. E. Kaifer, J. Org. Chem.
1996, 61, 7298–7303.
[22] On average, the cyclophane is assumed to be monoreduced, on ac-
count of the 100-fold excess of the pseudorotaxane (bulk concentra-
tion 0.5mm)compared to the dethreaded CBPQT 2C/2+ cyclophane
Received: April 3, 2005
Published online: August 5, 2005
6858
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Chem. Eur. J. 2005, 11, 6846 – 6858