Enhanced photoelectrochemistry in supramolecular CdS-nanoparticle-stoppered pseudorotaxane monolayers assembled on electrodes

Article abstract of DOI:10.1021/jp022102c

A pseudorotaxane monolayer consisting of a supramolecular complex generated between cyclo-bis(paraquet-p-phenylene), (1), and the 1,4-bis(mercapto ethyloxy ethyl)benzene monolayer, (2), that is stoppered by CdS nanoparticles (3-5 nm) is generated on Au electrodes. The surface coverage of the supramolecular complex units and the associated CdS particles corresponds to 2.4 ?± 0.2 ?? 10^-10 mole?·cm^-2 and 3 ?? 10¹² particles?·cm^-2, respectively. The photocurrent generated by the (1)/(2)-CdS pseudorotaxane architecture is 8-fold higher than in a CdS-nanoparticle monolayer system that lacks (1). The enhanced photocurrent in the pseudorotaxane assembly is attributed to vectorial electron transfer of photoexcited conduction-band electrons to the threaded electron acceptor (1), which leads to charge separation and to the retardation of the electron-hole recombination process.

Full text of DOI:10.1021/jp022102c

13094
J. Phys. Chem. B 2002, 106, 13094-13097
Enhanced Photoelectrochemistry in Supramolecular CdS-Nanoparticle-Stoppered
Pseudorotaxane Monolayers Assembled on Electrodes
Laila Sheeney-Haj-Ichia and Itamar Willner*
Institute of Chemistry, The Farkas Center for Light Induced Processes, The Hebrew UniVersity of Jerusalem,
Jerusalem 91904, Israel
ReceiVed: September 19, 2002
A pseudorotaxane monolayer consisting of a supramolecular complex generated between cyclo-bis(paraquat-
p-phenylene), (1), and the 1,4-bis(mercapto ethyloxy ethyloxy ethyl)benzene monolayer, (2), that is stoppered
by CdS nanoparticles (3-5 nm) is generated on Au electrodes. The surface coverage of the supramolecular
complex units and the associated CdS particles corresponds to 2.4 ( 0.2 × 10-10 mole‚cm-2 and 3 × 1012
particles‚cm-2, respectively. The photocurrent generated by the (1)/(2)-CdS pseudorotaxane architecture is
8-fold higher than in a CdS-nanoparticle monolayer system that lacks (1). The enhanced photocurrent in the
pseudorotaxane assembly is attributed to vectorial electron transfer of photoexcited conduction-band electrons
to the threaded electron acceptor (1), which leads to charge separation and to the retardation of the electron-
hole recombination process.
The assembly of semiconductor nanoparticles in monolayer
and thin film systems attracts substantial research efforts directed
to the development of photoelectrochemical systems,1,2 light
emitting diodes,3 and sensor devices.4 For enhanced photocurrent
generation, the retardation of the recombination of the photo-
generated electron-hole pair is essential. The use of molecular
cross-linked semiconductor nanoparticles of controlled chemical
composition5 and the application of core-shell hybrid systems
of different semiconductor materials6 were reported to facilitate
charge separation and enhance photocurrent generation. For
example, a bifunctional nanoporous film consisting of CdS/SnO2
reveals improved photoelectrochemical features due to vectorial
electron transfer of conduction-band electrons from CdS to the
conduction band of SnO2, resulting in the electron-hole
separation in the two particles. Recently, we reported on the
assembly of covalently linked bipyridinium semiconductor
nanoparticles on electrode supports for enhanced photocurrent
generation.7 Vectorial electron transfer of photogenerated
conduction-band electrons to the bipyridinium relay units led
to spatial charge separation and increased photocurrents.
Experimental Section
Microgravimetric Quartz Crystal Microbalance (QCM)
Measurements. Experiments were performed with a QCM
analyzer (Fluke) using Au-quartz crystals (AT-cut 10 MHz).
The geometrical area of the Au electrode was 0.20 ( 0.05 cm2.
Prior to each measurement the modified QCM crystals were
dried under a flow of argon, and the crystal frequencies were
determined in air. The surface coverage of the respective
components was determined by following the frequency change
of the crystals upon the stepwise assembly of the different
components. The surface coverage was estimated using the
Sauerbrey equation12 (eq 1), where ∆m is the mass change, fo
is the resonance frequency of the quartz crystal, A is the
piezoelectrically active area, Fq is the density of quartz (2.648
g‚cm-3), and µq is the shear modulus (2.947 × 1011 dyn‚cm-3
for AT-cut quartz).
∆m
A(µqFq)1/2
∆f ) - 2f 2o
(1)
The tailoring of noncovalent supramolecular architectures on
surfaces is a rapidly developing research area. Threaded catenane
architectures in monolayer structures,8 molecular mechanical
translocation in stoppered rotaxane configurations,9 and layered
receptor- and photosensitizer-relay cross-linked Au nanoparticles
were reported for sensory10 and photoelectrochemical11 applica-
tions. Here we report on the assembly of a pseudorotaxane
monolayer assembly consisting of a CdS-nanoparticle-stoppered
system of cyclo-bis(paraquat-p-phenylene), (1), that is threaded
on a 1,4-bis(mercapto ethyloxy ethyloxy ethyl)benzene mono-
layer, (2). To our knowledge, this is the first example of a
nanoparticle-stoppered pseudorotaxane assembly on a surface.
In addition to the enhanced photocurrents observed in the
nanoarchitectured monolayer, the concept may be broadened
to tailor new supramolecular electronic and optoelectronic
systems in monolayer and thin film configurations.
Photoelectrochemical Measurements. Photoelectrochemical
experiments were performed using a home-built photoelectro-
chemical system that included a 300 W Xe lamp (Oriel, model
6258), a monochromator (Oriel, model 74000), and a chopper
(Oriel, model 76994). The electrical output from the cell was
sampled by a lock-in amplifier (Stanford Research model SR
830 DSP). The shutter chopping frequency was controlled by a
Stanford Research pulse/delay generator model DE 535. The
photoelectrode consisted of the CdS-nanoparticle-stoppered
pseudorotaxane assembled on the Au-quartz crystal or the CdS
nanoparticle-functionalized electrode. A graphite electrode was
used as a counter electrode. The photogenerated current was
measured between the working and counter electrode. The
electrolyte solution consisted of 0.02 M triethanolamine in 0.1
M phosphate buffer, pH 9.5.
Electrochemical Measurements. Cyclic voltammetry was
performed in a standard electrochemical cell using the func-
* Corresponding author. E-mail: willnea@vms.huji.ac.il.
10.1021/jp022102c CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/03/2002

Letters
J. Phys. Chem. B, Vol. 106, No. 51, 2002 13095
tionalized Au-quartz crystals. A conventional three-electrode
cell, consisting of the Au-quartz electrode, a glassy carbon
auxiliary electrode isolated by a glass frit and a saturated calomel
electrode (SCE) connected to the working volume with a luggin
capillary, was used for the electrochemical measurements. The
potentials measured upon cyclic voltammetry are reported vs
the SCE. The cell was positioned in a grounded Faraday cage.
Cyclic voltammetry was performed using a potentiostat (EG&G,
model 283) connected to the computer (EG&G Software Power
Suite 1.03). All electrochemical measurements were performed
in 0.1 M sodium borate (pH ) 9.2) as background electrolyte.
Materials. All materials were obtained from Aldrich. Cyclo-
bis(paraquat-p-phenylene)tetrahexafluorophosphate was syn-
thesized according to the literature.13
Synthesis of 1,4-Bis(mercapto ethyl oxyethyl oxyethyl)benzene.
Triethylene glycol di-p-tosylate (12 g, 24 mmol) and p-
hydroquinone (1.5 g, 13 mmol) were dissolved in a suspension
of K2CO3 (15 g, 100 mmol) in dry MeCN (150 mL). The
mixture was flushed with Ar, then was stirred under Ar at reflux
for 7 days. The resulting reaction mixture was cooled, filtered,
and the solid was washed with MeCN (20 mL). The solvent
was removed under vacuum, giving a crude residue that was
purified by column chromatography (9:1 dichloromethane/ethyl
acetate on silica gel) to give pure 1,4-bis(tosylate ethyloxy
ethyloxy ethyl)benzene (2 g, 46%). NMR spectra in CDCl3,
1H: δ (ppm) 1.66 (s, 6H), 3.5-4.1 (m, 24H), 6.82 (s, 4H), 7.2
(d, 4H), 7.8 (d, 4H).
Figure 1. Absorption spectrum of AOT-capped CdS nanoparticles
(diameter 3-5 nm).
bis(mercapto ethyloxy ethyloxy ethyl)benzene monolayer (0.02
M in DMSO for 2 h). The functionalized electrodes were treated
for 2 h with 2 mL of a solution of cyclo-bis(paraquat-p-
phenylene)tetrahexafluorophosphate in acetone, 0.02 M, fol-
lowed by the addition of 2 mL of the suspension of CdS
nanoparticles in the reversed micelles. The Au-quartz crystals
were allowed to interact with the CdS nanoparticles for 12 h.
The resulting electrodes were washed with 0.05 sodium chloride
solution.
1,4-Bis(tosylate ethyl oxyethyl oxyethyl)benzene (2 g, 5.5
mmol) and thioacetate (9.4 g, 82 mmol) were dissolved in dry
DMF (100 mL). The mixture was flushed with Ar, then stirred
under Ar at room temperature for 7 days. The solvent was
removed under vacuum, yielding a crude product which was
dissolved in dichloromethane and washed twice with H2O. The
organic phase was dried to yield 1,4-bis(thioacetate ethyl
Results and Discussion
CdS nanoparticles (3-5 nm diameter determined by TEM)
were prepared according to the literature in an AOT (dioctyl-
sulfosuccinate, Na+)/heptane/H2O reverse micellar system.14 The
absorption spectrum of the capped CdS nanoparticles is shown
in Figure 1.
1
oxyethyl oxyethyl)benzene (1 g). H NMR spectra in CDCl3:
δ (ppm) 1.6 (s, 6H), 3.1 (t, 24H), 3.5-3.65 (m, 12H), 3.83 (t,
4H), 4.08 (t, 4H), 6.8 (s, 4H).
1,4-Bis(thioacetate ethyl oxyethyl oxyethyl)benzene (1 g, 2
mmol) was added to 20 mL sodium methoxide solution (15 mg
sodium/mL methanol). After standing for 5 min the solvent was
removed under vacuum at 20 °C, giving a crude product which
was dissolved in dichloromethane that was washed with 10%
NaOH. After a second extraction of the dichloromethane
solution with 10% NaOH, the solutions were combined and the
organic phase was discarded. The aqueous layer was acidified
with cold 10% H2SO4, the product was extracted into dichlo-
romethane (3× extractions). The combined dichloromethane
solution was dried over Na2SO4, filtered, and evaporated to give
pure 1,4-bis(mercapto ethyloxy ethyloxy ethyl)benzene. 1H
NMR: δ (ppm) 2.5 (t, 4H), 2.9 (t, 4H), 3.43-3.7 (m, 12H),
4.00 (t, 4H), 6.85 (s, 4H).
Preparation of CdS Nanoparticles.14 An AOT/n-heptane
water-in-oil micromeulsion was prepared by the solubilization
of 2 mL distilled water in 100 mL n-heptane in the presence of
7.0 g of the AOT surfactant (dioctyl sulfosuccinate, sodium salt).
The resulting mixture was separated into 60 mL and 40 mL of
reverse-micelle subvolumes. Aqueous solutions of Cd(ClO4)2
(0.24 mL, 1 M) and Na2S (0.16 mL, 1 M) were added to the 60
mL and 40 mL subvolumes, respectively, and allowed to stir
for 1 h. The two solutions were united and stirred under argon
for 1 h to yield the nanoparticles. The resulting mixture was
stirred for 24 h.
1,4-Bis(mercapto ethyloxy ethyloxy ethyl)benzene, (2), was
synthesized according to the details given in the Experimental
Section. To characterize the composition of the CdS-stoppered
pseudorotaxane, the system was assembled on an Au-quartz
piezoelectric crystal (AT-cut, 10 MHz), Scheme 1(A). The
dithiol (2) was assembled on the Au surface of the Au-quartz
crystal. From the change in the crystal frequency, ∆f ) -90
Hz, we estimate the surface coverage to be 2.8 × 10-10
mole‚cm-2. The resulting monolayer was then interacted with
the bis-bipyridinium cyclophane, (1). The π-donor dialkoxy-
benzene unit in (2) forms a labile acceptor-donor complex on
the Au surface. To the equilibrated system, the CdS nanopar-
ticles in the reverse micellar system were added to yield the
stoppered pseudorotaxane. A related reference system was
generated by the linkage of the CdS nanoparticles to the (2)-
functionalized monolayer in the absence of (1), Scheme 1B.
From the changes in the crystal frequencies upon the assembly
of the two systems, ∆f ) -395 Hz for configuration A and ∆f
) -296 Hz for configuration B, we estimate the surface
coverage of the CdS nanoparticles to be 3 × 1012 particles‚cm-2
and the surface coverage of the threaded (1) to be 2.4 ( 0.2 ×
10-10 mole‚cm-2. Thus, ca. 48 pseudorotaxane units are
stoppered by each CdS nanoparticle.
Figure 2, curve (a), shows the cyclic voltammograms of the
CdS-stoppered (1)-pseudorotaxane that reveals the characteristic
quasi-reversible redox wave of (1). Coulometric assay of the
reduction (or oxidation) wave of (1) indicates a surface coverage
Preparation of CdS-Nanoparticle-Stoppered Pseudorotaxane
Monolayer. The Au-quartz crystals were modified with 1,4-

13096 J. Phys. Chem. B, Vol. 106, No. 51, 2002
Letters
Figure 3. Photocurrent spectra of nanoparticle-functionalized Au-
quartz crystals: (a) CdS-nanoparticle monolayer; (b) CdS-nanoparticle-
stoppered (1) pseudorotaxane system. Data recorded in phosphate buffer
solution 0.1 M, pH ) 9.5, that included triethanolamine, 2 × 10-2 M.
Electrodes are biased at E ) 0.0 V vs SCE. Inset: Photocurrents (upon
irradiation at 420 nm) as a function of applied potential on the electrode.
Figure 2. Cyclic voltammetry of (a) CdS-stoppered (1) pseudorotaxane
monolayer-functionalized Au-quartz crystal. (b) CdS-nanoparticle
monolayer-functionalized Au-quartz crystal. Data recorded in 0.1 M
sodium borate solution, pH ) 9.2 under Ar, scan rate 100 mV‚s-1
Inset: Cathodic peak current of (1) at different scan rates.
.
of 2.0 ( 0.2 × 10-10 mole‚cm-2, consistent with the value
extracted from the microgravimetric quartz crystal microbalance
experiments. The peak current of the redox-wave of (1) relates
directly to the scan rate (cf. Figure 2, inset), consistent with a
surface-confined configuration of (1). Figure 2, curve (b), shows
the cyclic voltammogram of the CdS-nanoparticle monolayer
linked to the electrode surface with the dithiol wire (2). The
latter configuration was treated with (1). As the CdS nanopar-
ticles are capped with a negatively charged protecting monolayer
of AOT surfactant, the electrostatic binding of (1) to the particles
may occur. Indeed, we find that (1) associates with the CdS
nanoparticles by electrostatic interactions, but the surface-bound
(1) is completely washed-off with 0.05 M sodium chloride
solution. In contrast, the (1)-pseudorotaxane configuration is
not affected when rinsing the system with the sodium chloride
solution. This result further confirms that a supramolecular CdS-
stoppered rotaxane is generated on the electrode.
Figure 3, curve (a), shows the photocurrent spectrum observed
upon illumination of the CdS-monolayer electrode without (1),
whereas curve (b) shows the photocurrent action spectrum
resulting in the irradiation of the supramolecular (1)-pseudo-
rotaxane CdS system. Clearly, the photocurrent generated in
the latter system is ca. 8-fold higher than the photocurrent
observed in the CdS monolayer only. The photocurrent action
spectrum follows the absorbance band of the CdS nanoparticles
(λmax ) 420 nm), indicating that it originates from the
photoexcitation of the nanoparticles. The light-to-current con-
version quantum efficiency in the (1)-pseudorotaxane-CdS
system corresponds to φ ) 1.55%. The enhanced photocurrent
observed in the presence of the (1)-pseudorotaxane system is
attributed to vectorial electron transfer in the system that yields
charge separation, retardation of recombination processes, and
SCHEME 1: (A) Assembly of the Pseudorotaxane Monolayer Consisting of CdS Nanoparticles on Au-Quartz Crystal;
(B) Assembly of the CdS-Nanoparticle Monolayer on the Au-Quartz Crystal in the Absence of (1)

Letters
J. Phys. Chem. B, Vol. 106, No. 51, 2002 13097
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consisting of CdS nanoparticles without (1), rapid electron-
hole recombination perturbs electron ejection into the electrode
and the generation of the photocurrent. On the other hand, in
the supramolecular assembly, the transfer of the conduction-
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separation and the retardation of the back electron transfer.
Subsequent electron transfer from the reduced relay to the
electrode leads to the effective generation of the photocurrent.
This explanation is supported by the fact that application of a
potential on the electrode that reduces (1) to the respective
radical cation, E ) -0.55 vs SCE, blocks the effective
photocurrent generation. That is, reduction of (1) perturbs the
relay functions of the threaded element, Figure 3, inset.
In conclusion, we have described a novel approach to
assemble supramolecular pseudorotaxane-CdS-nanoparticle
architectures on surfaces for enhanced photocurrent generation.
Vectorial electron transfer in the system, which results in charge
separation, leads to the improved photoelectrochemical functions
of the system.
Acknowledgment. This research is supported by the Enrique
Berman Fund, The Hebrew University of Jerusalem.
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