Potentiostatic modulation of the lifetime of light-induced charge separation in a heterosupermolecule

Article abstract of DOI:10.1021/jp990414i

A heterosupermolecule has been assembled by covalently linking a TiO₂ nanocrystal, a ruthenium complex, and a viologen. The associated heterosupramolecular function, long-lived light-induced charge separation, has been demonstrated. Potentiostatic modulation of this function has also been demonstrated. The reported findings and associated insights may find practical applications in the area of optical information storage. ? 1999 American Chemical Society.

Full text of DOI:10.1021/jp990414i

J. Phys. Chem. B 1999, 103, 8067-8079
8067
Potentiostatic Modulation of the Lifetime of Light-Induced Charge Separation in a
Heterosupermolecule
Geoffrey Will, Gerrit Boschloo, S. Nagaraja Rao, and Donald Fitzmaurice*
Department of Chemistry, UniVersity College Dublin, Belfield, Dublin 4, Ireland
ReceiVed: February 4, 1999; In Final Form: July 22, 1999
A heterosupermolecule has been assembled by covalently linking a TiO2 nanocrystal, a ruthenium complex,
and a viologen. The associated heterosupramolecular function, long-lived light-induced charge separation,
has been demonstrated. Potentiostatic modulation of this function has also been demonstrated. The reported
findings and associated insights may find practical applications in the area of optical information storage.
Introduction
assembly, therefore, the prospect of effective function modula-
tion is more immediately envisaged. This is because modulation
of a property of a nanometer-scale condensed-phase component
is an increasingly realizable goal.12,13 Two approaches are
envisaged.
A supermolecule possesses the following attributes.1,2 First,
the intrinsic properties of the linked molecular components are
not significantly perturbed, and second, the properties of the
supermolecule are not a simple superposition of the properties
of the molecular components. That is, there exists a well-defined
supramolecular function.
Upon replacement of a molecular component in a supermol-
ecule by a condensed-phase component, a heterosupermolecule
is formed.3-5 By analogy with a supermolecule, a heterosuper-
molecule possesses the following attributes. First, the intrinsic
properties of the linked condensed-phase and molecular com-
ponents are not significantly perturbed, and second, the proper-
ties of the heterosupermolecule are not a simple superposition
of the properties of the condensed-phase and molecular com-
ponents. That is, there exists a well-defined heterosupramo-
lecular function.
The first approach involves organizing the heterosupermol-
ecules of an assembly so that they build their own substrate
(Scheme 2, top), termed an intrinsic substrate. Modulating a
property of the intrinsic substrate, of necessity, modulates the
same property of the condensed-phase component of each of
the heterosupermolecules in the assembly. Since the function
of a heterosupermolecule is determined by the properties of its
components, effective function modulation is expected and has
been observed.14-16 Furthermore, if the property of the intrinsic
substrate can be monitored, then the modulation state of the
heterosupermolecules in the assembly may be inferred.
The second approach involves organizing the heterosuper-
molecules of an assembly so that a property of a condensed-
phase component of each heterosupermolecule may be indi-
vidually modulated (Scheme 2, bottom). With the advent of
many new techniques for manipulating and characterizing
condensed-phase matter on the nanometer scale,12,13 this is an
increasingly realizable approach. Furthermore, if the property
of the condensed-phase component can be monitored, then the
modulation state of each heterosupermolecule in the assembly
may be inferred.
An organized assembly of supermolecules offers the prospect
of addressable supramolecular function (Scheme 1).6-8 By
analogy, an organized assembly of heterosupermolecules offers
the prospect of addressable heterosupramolecular function
(Scheme 1). Both offer the prospect of devices based on function
that is addressable on the nanometer scale.
With respect to devices based on supramolecular function
addressable on the nanometer scale, an important objective is
modulation of the functional state of the constituent supermol-
ecules of an assembly. To fully meet this objective will require
an ability to modulate a chemical or physical property of at
least one of the molecular components of the supermolecules
in the organized assembly. To date, modulation of the function
of the constituent supermolecules of an organized assembly has
been affected by modulating the properties of the assembly
substrate or the contiguous medium.9-11 Neither approach has
proved to be particularly effective.
With respect to devices based on heterosupramolecular
function addressable on the nanometer scale, an equally
important objective is modulation of the functional state of the
constituent heterosupermolecules in an assembly. To fully meet
this objective will require the ability to modulate a chemical or
physical property of at least one of the constituent condensed-
phase or molecular components of the heterosupermolecules in
the organized assembly. By comparison with a supramolecular
The preparation and characterization of a heterosupermolecule
consisting of a covalently linked TiO2 nanocrystal, ruthenium
complex (R), and viologen (V) components have recently been
described (Scheme 3).16 Also described has been the covalent
organization of these heterosupermolecules to form a hetero-
supramolecular assembly, denoted TiO2-RV, in which effective
function modulation via the intrinsic substrate formed by the
TiO2 nanocrystal components proved possible.
Specifically, the direction of light-induced electron transfer
within the heterosupramolecular assembly was modulated by
potentiostatically determining the quasi-Fermi level of the
intrinsic substrate (Scheme 4) and, therefore, also the quasi-
Fermi level of the TiO2 nanocrystal component of each
heterosupermolecule. At positive applied potentials, visible-light
excitation of the R component results in electron transfer to
the TiO2 nanocrystal component (95%). At negative applied
* To whom correspondence should be addressed.
10.1021/jp990414i CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/03/1999

8068 J. Phys. Chem. B, Vol. 103, No. 38, 1999
Will et al.
SCHEME 1
SCHEME 2
potentials, however, visible-light excitation of the R component
results in electron transfer to both the TiO2 nanocrystal (48%)
and V components (52%).
a sacrificial donor, proved unsuccessful because of the fact that
electron transfer from the reduced V component to the oxidized
R component is very fast (12 ps).
Concerning the heterosupramolecular assembly TiO2-RV,
the following limitations were noted. First, modulation of the
direction of light-induced electron transfer at negative applied
potentials is partial. Attempts to fully modulate the direction
of transfer, by applying still more negative potentials, proved
unsuccessful because of desorption of the linked molecular
components from the surface of the TiO2 nanocrystal component
at which they are adsorbed. Second, charge separation is short-
lived. Attempts to inhibit back electron transfer, by addition of
In the course of ongoing studies to address these limitations,
the carboxylic acid groups used to link the TiO2 nanocrystal
and R components were replaced by phosphonic acid groups,
in the expectation that this would prevent desorption of the
molecular components at more negative applied potentials. Also,
the length of the spacer group linking the R and V components
was increased from one to four methylene groups, in the
expectation that this would reduce the rate of back electron
transfer.

Charge Separation in Heterosupermolecule
J. Phys. Chem. B, Vol. 103, No. 38, 1999 8069
SCHEME 3
SCHEME 4
formed. Aqueous HCl (40 µL, 1.0 mol dm-3) was added
followed by manual grinding for 15 min. Water (4 mL) and
Carbowax 20000 (0.25 g) were added and dissolved by stirring
for several hours. Finally, Triton X100 surfactant (40 µL) was
added. This sol was spread on a conducting glass substrate
masked with Scotch tape using a glass rod. The resulting gel
film was fired in air at 450 °C for 12 h. All films were stored
in a darkened vacuum desiccator until required for use.
Preparation of Transparent Nanostructured SnO2:Sb
Films. Transparent nanoporous nanocrystalline Sb-doped SnO2
films (3 µm thick, 5 nm diameter nanocrystals) supported on a
fluorine-doped SnO2 glass (0.5 µm, 8 Ω/square, supplied by
Glastron) were prepared as described by Fitzmaurice and co-
workers.19 Briefly, Carbowax 20000 (0.38 g) was dissolved in
an aqueous dispersion of Sb-doped SnO2 colloid (5.0 g, 15 wt
%, Alfa) with average particle size of 5 nm. Acetic acid (2 drops
of a 2 mol dm-3 solution) was added under stirring, resulting
in immediate formation of a gel-like paste. This paste was
diluted by addition of water (1.5 mL) and was used to prepare
a gel film on a conducting glass substrate that was allowed to
air-dry for 30 min and was fired in air at 450 °C for 15 h.
Preparation of Linked Molecular Components. The mo-
lecular components used in the present study, RPVC1 and RPVC4,
were prepared from the ligands shown in Schemes 6 and 7. All
reagents were used as received from Aldrich. All solvents were
Analar grade and also used as received.
Covalent organization of these heterosupermolecules yields
the heterosupramolecular assembly TiO2-RPVC4 (Scheme 5)
for which the associated heterosupramolecular function, long-
lived light-induced charge separation, has been demonstrated.
Potentiostatic modulation of this function has also been dem-
onstrated. These findings and associated insights may find
practical applications in the area of optical information storage.
Experimental Section
Preparation of Transparent Nanostructured TiO2 Films.
Transparent nanoporous nanocrystalline TiO2 films (4 µm thick,
10 nm diameter nanocrystals) supported on fluorine-doped SnO2
glass (0.5 µm, 8 Ω/square, supplied by Glastron) were prepared
as described by Gra¨tzel and co-workers.17 Briefly, a colloidal
dispersion of TiO2 nanocrystals was prepared by hydrolysis of
titanium isopropoxide. The dispersion was autoclaved at 200
°C for 12 h, concentrated (160 g L-1), and Carbowax 20000
(40 wt % equiv of TiO2) added to yield a white viscous sol.
This sol was spread on a conducting glass substrate masked
with Scotch tape using a glass rod. The resulting gel film was
fired in air at 450 °C for 12 h. All films were stored in a
darkened vacuum desiccator until required for use.
Preparation of Transparent Nanostructured Al2O3 Films.
Transparent nanoporous nanocrystalline Al2O3 films (4 µm thick,
14 nm diameter nanocrystals) supported on a microscope glass
slide were prepared as described by Gra¨tzel and co-workers.18
Briefly, the oxide powder (1 g) was ground in an agate mortar
with dropwise addition of distilled water until a viscous paste
All ligands and molecular components were characterized by
1H NMR spectra recorded on a JEOL JNM-GX270 FT
spectrometer. Optical absorption spectra were measured using
a Hewlett-Packard 8452A diode array spectrometer. Optical
emission spectra were measured using a Perkin-Elmer 3000
fluorescence spectrometer. All elemental analysis was performed
by the Chemical Services Unit at University College Dublin.
I: 4,4′-Dicarboxy-2,2′-bipyridine. This reagent was prepared
and characterized as described in detail elsewhere.20

8070 J. Phys. Chem. B, Vol. 103, No. 38, 1999
Will et al.
SCHEME 5
SCHEME 6a
SCHEME 7a
a
(i) K2CrO7/H2SO4; (ii) EtOH, H2SO4, reflux; (iii) NaBH4, EtOH,
RT; (iv) HBr, H2SO4; (v) P(OEt)3.
II: 4.4′-Diethoxycarbonyl-2,2′-bipyridine. This reagent was
prepared and characterized as described in detail elsewhere.21
III: 4,4′-Bis(hydroxymethyl)-2,2′-bipyridine. A solution of
II (6.0 g, 22.1 × 10-3 mol) and sodium borohydride (2.5 g,
66.0 × 10-3 mol) in dry ethanol (300 mL) was stirred at room
temperature for 3 h. Water (300 mL) was added dropwise, and
the solution was stirred at room temperature overnight. The
reaction mixture was extracted with ethyl acetate (4 × 250 mL).
The combined organic layer was dried over MgSO4 and
evaporated to give the crude product, which was recrystallized
from ethanol to yield 4.25 g (89%) of III. 1H NMR (270 MHz,
CDCl3): δ (ppm) 4.62 (s, 4H), 7.27-7.29 (m, unr, 2H), 8.29-
8.30 (d, J ) 0.7 Hz, 2H), 8.49-8.51 (d, J ) 5.5 Hz, 2H).
Calculated for C12H12N2O2: C, 66.65; H, 5.59; N, 12.96.
Found: C, 66.55; H, 5.73; N, 12.84.
a
(i) DMF, reflux; (ii) MeOH, reflux; 20% HCl reflux; (iii) MeOH,
reflux; 20% HCl, reflux.
and concentrated H2SO4 (10 mL). The resulting solution was
refluxed for 6 H and allowed to cool, and water (100 mL) was
added. The pH was adjusted to 7.0 by addition of NaOH, and
the resulting precipitate was filtered, washed with water, and
air-dried. The aqueous filtrate was extracted with ethyl acetate
(2 × 50 mL), the combined organic layer was washed with water
(2 × 50 mL) and dried over MgSO4, and the solvent was
removed. The precipitate and the extracted product were
1
IV: 4,4′-Bis(bromomethyl)-2,2′-bipyridine. III (4.0 g, 11.7
combined to yield 3.5 g (88%) of IV. H NMR (270 MHz,
CDCl3): δ (ppm) 4.51 (s, 4H), 7.37-7.40 (m, unr, 2H), 8.45-
× 10-3 mol) was dissolved in a mixture of HBr (48%, 30 mL)

Charge Separation in Heterosupermolecule
J. Phys. Chem. B, Vol. 103, No. 38, 1999 8071
8.46 (m, unr, 2H), 8.68-8.70 (m, unr, 2H). Calculated for
C12H10N2Br2: C, 42.14; H, 2.95; N, 8.19. Found: C, 42.23; H,
2.97; N, 8.08.
resulting phosphonate ester was refluxed in HCl (20%, 15 mL)
for 16 h. The solvent was removed under vacuum, and the crude
product was precipitated from water (10 mL) using ammonium
hexafluorophosphate and extracted into nitromethane (2 × 5
mL). The crude product was purified on a Sephadex LH20
column (eluent: neutral water) to yield (48%) 0.045 g of RPVC4.
1H NMR (270 MHz, CD3CN): δ (ppm) 1.19-1.21 (t, J ) 7.3
Hz, 3H), 1.49-1.51 (m, unr, 2H), 1.81-1.83 (m, unr, 2H),
2.52-2.55 (m, unr, 2H), 3.16-3.20 (d, J ) 21.5 Hz, 8H), 4.60-
4.66 (m, unr, 4H), 7.20-7.41 (m, unr, 4H), 7.57-8.03 (m, unr,
8H), 8.23-8.49 (m, unr, 11H), 8.99-9.09 (m, unr, 4H).
Calculated for RuC50H56N8F24O12P8: C, 34.01; H, 3.20; N, 6.35.
Found: C, 33.84; H, 3.44; N, 6.46. Absorption (MeCN): 466
nm (ꢀ ) 14 200 mol-1 cm-1). Emission (EtOH): none.
Preparation of Heterosupramolecular Assemblies. A trans-
parent nanostructured TiO2, Al2O3, or SnO2:Sb film was
L1: 4,4′-Bis(diethylmethylphosphonate)-2,2′-bipyridine. A
solution of VI (3.0 g, 8.7 × 10-3 mol) in triethyl phosphite (30
mL) was refluxed for 3 h. The excess phosphite was removed
by distillation under high vacuum. The crude product was
purified by column chromatography on silica column (eluent:
ethyl acetate/methanol 80/20), yielding 2.9 g (73%) of crystalline
1
L1. H NMR (270 MHz, CDCl3): δ (ppm) 1.26-1.31 (t, J )
7.2 Hz, 12H), 3.21-3.29 (d, J ) 22.2 Hz, 4H), 4.03-4.11 (q,
J ) 7.2 Hz, 8H), 7.31-7.35 (m, unr, 2H), 8.35-8.36 (m, unr,
2H), 8.61-8.63 (d, J ) 4.5 Hz, 2H). Calculated for
C20H30N2O6P2: C, 52.63; H, 6.63; N, 6.14. Found: C, 52.37;
H, 6.52; N, 6.42.
L2: 4-((1′-Ethyl-4,4′-bipyridinediium-1-yl)methyl)-4′-methyl-
2,2′-bipyridine) Dihexafluorophosphate. This ligand was pre-
pared and characterized as described in detail elsewhere.16
L3: 5-((1′-Ethyl-4,4′-bipyridinediium-1-yl)-butyl)-2,2′-bipy-
ridine) Dihexafluorophosphate. This ligand was used as received
from Prof. Stoddart and co-workers, then at the University of
Birmingham and now at the University of California at Los
Angeles. The preparation and characterization of this ligand has
been described in detail elsewhere.22
P: cis-Dichloro-bis(4,4′-bis(diethylmethylphosphonate)-2,2′-
bipyridine)ruthenium(II). Ruthenium trichloride (0.092 g, 0.44
× 10-3 mol) and L1 (0.380 g, 0.83 × 10-3 mol) were heated
at 120 °C in N,N-dimethylformamide (40 mL) for 8 h with
continuous bubbling of nitrogen. The solvent was removed by
distillation under high vacuum, and the resulting product was
purified on a silica column (eluent: methanol/acetone 40/60),
yielding 0.144 g (32%) of crystalline P. Calculated for
RuC40H60N4O12Cl2P4‚4H2O: C, 41.78; H, 5.91; N, 4.87.
Found: C, 41.39; H, 5.71; N, 4.58. Absorption (EtOH): 392
and 498 nm. Emission (EtOH): none.
immersed in an ethanolic solution (typically 2 × 10-4 mol dm-3
,
pH 3.0) of RPVC1 or RPVC4 for 2 h. The resulting heterosu-
pramolecular assemblies denoted TiO2-RPVC1, TiO2-RPVC4
,
Al2O3-RPVC1, Al2O3-RPVC4, and SnO2:Sb-RPVC4 were
washed thoroughly with ethanol and stored in a darkened
vacuum desiccator until required for use.
Potential-Dependent Optical Absorption Spectroscopy. A
heterosupramolecular assembly formed the working electrode
of a closed three-electrode single-compartment cell. The counter
electrode was a platinum wire and the reference electrode a
Ag|AgCl|saturated KCl(aq) electrode connected via a salt bridge.
All spectra and electrochemical measurements were undertaken
in an electrolyte solution prepared from a mixture of dry MeCN/
EtOH (70/30 by volume) and containing tetrabutylammonium
perchlorate (TBAP, 0.1 mol dm-3) and triethanolamine (TEOA,
0.05 mol dm-3). All solutions were degassed by bubbling with
argon for 15 min. Potential control was provided by a Solatron
SI 1287 potentiostat controlled by a Lab View program running
on a Power Macintosh. The above cell was incorporated into
the sample compartment of a Hewlett-Packard 8452A diode
array spectrometer. Unless otherwise stated, all spectra were
recorded 3 s after application of the indicated potential against
a background measured at 0.00 V. Importantly, the reported
spectra show no temporal evolution before, during, or after
RPVC1: Bis(4,4′-dimethylphosphonic-2,2′-bipyridine)-(4-((1′-
ethyl-4,4′-bipyridinediium-1-yl)-methyl)-4′-methyl-2,2′-bipyridine)-
ruthenium(II) Tetrahexafluorophosphate. P (0.074 g, 0.07 ×
10-3 mol) and L2 (0.035 g, 0.07 × 10-3 mol) were refluxed in
methanol (10 mL) for 16 h. The solvent was removed under
vacuum, and the compound was precipitated from water (10
mL) with ammonium hexafluorophosphate and extracted into
nitromethane (2 × 5 mL). The nitromethane was removed under
vacuum, and the resulting phosphonate ester was refluxed in
HCl (20%, 15 mL) for 16 h. The solvent was removed under
vacuum, and the resulting crude product was precipitated from
water (10 mL) using ammonium hexafluorophosphate and
extracted into nitromethane (2 × 5 mL). The crude product was
purified on a Sephadex LH20 column (eluent: neutral water)
measurement. The blue-green (all lines) output (200 mW cm-2
)
of a Coherent Ar-ion laser (Innova 70-5) was used as an
excitation source (irradiated area 1 cm2). Unless otherwise stated,
all spectra were recorded 3 s after irradiation against a
background measured at 0.00 V. As above, the reported spectra
show no temporal evolution before, during, or after measure-
ment.
Results and Discussion
1
to yield 0.040 g (35%) of RPVC1. H NMR (270 MHz, CD3-
I. Photophysical Properties of RPVC1 and RPVC4. Shown
in Figure 1 are the optical absorption spectra of RPVC1 and
RPVC4 in a MeCN/EtOH (70/30 by volume) solution (1 × 10-4
CN): δ (ppm) 1.50-1.55 (t, J ) 7.3 Hz, 3H), 2.31 (s, 3H),
3.15-3.21 (d, J ) 18.1 Hz, 8H), 4.55-4.65 (q, J ) 7.3 Hz,
2H), 6.02 (s, 2H), 7.06-7.20 (m, unr, 4H), 7.46-7.84 (m, unr,
8H), 8.26-8.50 (m, unr, 8H), 8.73-8.78 (m, unr, 2H), 8.95-
8.97 (d, J ) 6.2 Hz, 2H), 9.06-9.09 (d, J ) 5.9 Hz, 2H).
Calculated for RuC48H52N8F24O12P8: C, 33.18; H, 3.02; N, 6.45.
Found: C, 33.66; H, 3.11; N, 6.32. Absorption (MeCN): 470
nm (ꢀ ) 13 100 mol-1 cm-1). Emission (EtOH): none.
RPVC4: Bis(4,4′-dimethylphosphonic-2,2′-bipyridine))(5-((1′-
ethyl-4,4′-bipyridinediium-1-yl)butyl)-2,2′-bipyridine)ruthenium-
(II) Tetrahexafluorophosphate. P (0.060 g, 0.05 × 10-3 mol)
and L3 (0.035 g, 0.05 × 10-3 mol) were refluxed in methanol
(10 mL) for 16 h during which a color change to orange was
observed. The solvent was removed under vacuum, and the
mol dm-3) containing the electrolyte TBAP (0.10 mol dm-3
)
and the sacrificial donor TEOA (0.05 mol dm-3). Also shown
in Figure 1 are the optical absorption spectra of RPVC1 and
RPVC4 in a MeCN/EtOH (70/30 by volume) solution (1 × 10-4
mol dm-3) containing the electrolyte TBAP (0.10 mol dm-3
)
and the sacrificial donor TEOA (0.05 mol dm-3) following
irradiation by the blue-green output of an argon-ion laser (200
mW cm-2, 15 s).
In both cases, the following bands are observed between 350
and 800 nm and may be assigned to the R component:23 a weak
band at about 360 nm assigned to a metal-centered (MC) d-d
transition; a strong band at 460 nm (broad) assigned to a spin-

8072 J. Phys. Chem. B, Vol. 103, No. 38, 1999
Will et al.
For a supermolecule closely related to RPVC4, a rate constant
of 6.55 × 108 s-1 for electron transfer to the V component,
corresponding to a lifetime of 1500 ps, and a rate constant of
3.24 × 109 s-1 for electron transfer from the reduced V
component to the oxidized R component, corresponding to a
lifetime of 300 ps, have also been measured by Mallouk and
co-workers.27 A similar value for the rate constant for electron
transfer to the V component (6.2 × 108 s-1) has been measured
by Balzani and co-workers.22
It should be noted that inferring the rate constants for electron
transfer from the V component to the R component in both
RPVC1 and RPVC4 from those measured for closely related
complexes is based on the assumption that the small structural
differences between these complexes do not, as may be the case
for supermolecules assembled using flexible linkers, result in
significant differences in the rates of intramolecular electron
transfer.
Clearly, irradiation under these conditions does not lead to a
measurable change in the optical absorption spectrum of either
RPVC1 or RPVC4. However, this finding is not unexpected, since
the rate constants for back electron transfer from the reduced
V component to the oxidized R component in RPVC1 and RPVC4
are 8.38 × 1010 s-1 and 3.24 × 109 s-1, respectively, and since
the estimated diffusion-limited rate constant for electron transfer
by the sacrificial donor TEOA to the oxidized R component is
about 5 × 104 s-1. Under these conditions, electron transfer by
the reduced V component to the oxidized R component in
RPVC1 and RPVC4 is 6 and 5 orders of magnitude faster than
electron transfer by the sacrificial donor to the oxidized R
component, respectively. This, in turn, ensures that only 1 in
106 and 105 photons absorbed the R component and leads to
Figure 1. Optical absorption spectra of (a) RPVC1 and (b) RPVC4 (1
× 10-4 mol dm-3) in a MeCN/EtOH (70/30 by volume) solution
containing the electrolyte TBAP (0.10 mol dm-3) and the sacrificial
donor TEOA (0.05 mol dm-3) prior to and following irradiation by
the all-lines output of an argon-ion laser (200 mW cm-2, 15 s).
long-lived reduction of a V component in RPVC1 and RPVC4
,
respectively. Consequently, no significant change in the absorp-
tion spectrum is measured, the estimated upper limits for change
of absorbance being between 0.001 and 0.0001, respectively.
If, however, the RPVC4 solution in Figure 1 was irradiated
for 15 min, a long-lived absorption spectrum (not shown) that
may be assigned to the radical cation of the V component is
measured.27 It is noted that the absorbance increase measured
at 600 nm (0.05) was in good agreement with that expected
(about 0.06). Furthermore, Balzani and co-workers have shown
that continuous irradiation of an ethanolic solution of a closely
related supermolecule (3 × 10-5 mol dm-3), also containing
TEOA (1 × 10-2 mol dm-3), with a tungsten lamp (150 W, λ
> 350 nm) results in the long-lived formation of the reduced
form of the viologen.22 In both cases, the reduced form of the
V component persists until oxygen enters the irradiated solution.
allowed metal-to-ligand charge transfer (MLCT) d-π* transi-
tion; and finally, a long-wavelength tail extending to about 600
nm assigned to a spin-forbidden MLCT d-π* transition. No
bands that may be assigned to the V component are observed.
In short, the optical absorption spectra of RPVC1 and RPVC4
and R are indistinguishable.
The band that dominates the visible absorption spectrum of
RPVC1 and RPVC4 is the spin-allowed MLCT transition of the
R component at 460 nm. It has been shown for a complex
similar to the R component that the excited state, in which an
electron is localized on one of the bipyridine ligands,24 is formed
in less than 300 fs25 and is largely triplet in nature.23 It should
be noted that the above electron, although initially localized on
one of the three bipyridine ligands, hops between these ligands
on the picosecond time scale.26 If the R component is not linked
to a V component, the excited state undergoes radiative decay
with a lifetime of about 600 ns to give rise to an emission
spectrum with a maximum at 660 nm.24 If, however, the R
component is linked to a V component, as is the case in RPVC1
and RPVC4, the electron is transferred when localized on the
bipyridine ligand linked to V component and no emission is
observed.24,27,28
II. Photophysical Properties of Al2O3-RPVC1 and Al2O3-
RPVC4. The linked R and V components, RPVC1 and RPVC4
,
were chemisorbed at the surface of the constituent Al2O3
nanocrystals of a nanostructured film to yield the heterosu-
pramolecular assemblies Al2O3-RPVC1 and Al2O3-RPVC4
respectively.
,
Shown in Figure 2 are the optical absorption spectra of
Al2O3-RPVC1 and Al2O3-RPVC4 in a MeCN/EtOH (70/30 by
volume) solution containing the electrolyte TBAP (0.10 mol
dm-3) and the sacrificial donor TEOA (0.05 mol dm-3) prior
to and following irradiation by the blue-green output of an
argon-ion laser (200 mW cm-2, 15 s).
Clearly, the absorption spectra of Al2O3-RPVC1 or Al2O3-
RPVC4 are not significantly different from those of RPVC1 or
RPVC4 in solution. Furthermore, no visible emission is detected
from either Al2O3-RPVC1 or Al2O3-RPVC4. Equally clearly,
irradiation with the blue-green output of an Ar-ion laser (200
For a supermolecule closely related to RPVC1, a rate constant
of 1.38 × 1011 s-1 for electron transfer to the V component,
corresponding to a lifetime of 7 ps, and a rate constant of 8.38
× 1010 s-1 for electron transfer from the reduced V component
to the oxidized R component, corresponding to a lifetime of 12
ps, have been measured by Mallouk and co-workers.27

Charge Separation in Heterosupermolecule
J. Phys. Chem. B, Vol. 103, No. 38, 1999 8073
Figure 3. Optical absorption difference spectra of (a) TiO2-RPVC1
and (b) TiO2-RPVC4 in a MeCN/EtOH (70/30 by volume) solution (1
Figure 2. Optical absorption spectra of (a) Al2O3-RPVC1 and (b)
Al2O3-RPVC4 in a MeCN/EtOH (70/30 by volume) solution containing
the electrolyte TBAP (0.10 mol dm-3) and the sacrificial donor TEOA
(0.05 mol dm-3) prior to and following irradiation by the all-lines output
of an argon-ion laser (200 mW cm-2, 15 s).
× 10-4 mol dm-3) containing the electrolyte TBAP (0.10 mol dm-3
)
and the sacrificial donor TEOA (0.05 mol dm-3) prior to and following
irradiation by the all-lines output of an argon-ion laser (200 mW cm-2
,
15 s). The reference spectra were measured prior to irradiation and are
shown as insets.
mW cm-2) for 15 s does not lead to a measurable change in
the optical absorption spectrum of either Al2O3-RPVC1 or
Al2O3-RPVC4. These findings, however, are not unexpected,
since the band gap of Al2O3 is so large (about 9 eV) that there
are no conduction band states isoenergetic with the electronically
following irradiation by the blue-green output of an argon-ion
laser (200 mW cm-2, 3 s).
As may be seen from Figure 3 (insets), the absorption spectra
of TiO2-RPVC1 and TiO2-RPVC4 are not significantly different
from those of the linked molecular components RPVC1 and
RPVC4 in solution. Furthermore, no visible emission is detected
excited state of the R component of either RPVC1 or RPVC4 18
.
Consequently, photoexcitation of the R component is followed
by electron transfer to the linked V component and, as was the
case for and RPVC1 and RPVC4 in solution, followed in turn by
electron transfer from the reduced V component to the oxidized
R component.
from either TiO2-RPVC1 or TiO2-RPVC4
.
As may also be seen from Figure 3, irradiation of either
TiO2-RPVC1 or TiO2-RPVC4 does result in a significant change
in the measured absorption spectrum at wavelengths longer than
520 nm. No such similar change is observed for RPVC1 and
RPVC4 or for Al2O3-RPVC1 and Al2O3-RPVC4 under similar
conditions (see Figures 1 and 2). These spectral changes, which
in the absence of oxygen are long-lived, may readily be assigned
to formation of the radical cation of the V component.27
These findings, together with those for RPVC1 and RPVC4
and for Al2O3-RPVC1 and Al2O3-RPVC4, strongly suggest that
the conduction band and trap states of the TiO2 nanocrystal
component participate in reduction of the V component of
It should be noted that extended irradiation of Al2O3-RPVC1
or Al2O3-RPVC4 for 15 min with the blue-green output of an
Ar-ion laser (200 mW cm-2) results, as was the case for RPVC1
and RPVC4 in solution, in a measurable absorption by the radical
cation of the V component.
In short, the behavior of Al2O3-RPVC1 and Al2O3-RPVC4
and RPVC1 and RPVC4 in solution is the same.
III. Photophysical Properties of TiO2-RPVC1 and TiO -
2
RPVC4. The linked R and V components, RPVC1 and RPVC4
,
TiO2-RPVC1 and TiO2-RPVC4 30
Specifically, irradiation of
.
RPVC1, RPVC4, Al2O3-RPVC1, or Al2O3-RPVC4 for 15 s does
not result in long-lived formation of the radical cation of V,
while irradiation of TiO2-RPVC1 and TiO2-RPVC4 under the
same conditions does. On this basis, it may be asserted that the
conduction band and trap states of the TiO2 nanocrystal, known
to be isoenergetic with the electronically excited state of the R
component,31 participate in reduction of the V component of
were chemisorbed at the surface of the constituent TiO2
nanocrystals of the nanostructured TiO2 film to yield the
heterosupramolecular assemblies TiO2-RPVC1 and TiO2-
RPVC4, respectively.
Shown in Figure 3 are the optical absorption spectra of TiO2-
RPVC1 and TiO2-RPVC4 in a MeCN/EtOH (70/30 by volume)
solution containing the electrolyte TBAP (0.10 mol dm-3) and
the sacrificial donor TEOA (0.05 mol dm-3) prior to and
TiO2-RPVC1 and TiO2-RPVC4
.

8074 J. Phys. Chem. B, Vol. 103, No. 38, 1999
Will et al.
SCHEME 8
In a similar experiment for an unmodified nanostructured
TiO2 film (not shown), the absorbance measured at 600 nm
remained constant between 0.00 and -1.20 V, although there
was a sharp increase in absorbance at -1.20 V. The reverse
sweep showed a decrease in absorbance between -1.50 and
-1.20 V. No other absorbance changes were observed.
The potential-dependent optical absorption spectroscopy and
band energetics of nanostructured TiO2 films have previously
been studied in a range of protic and aprotic solvents.32 On this
basis, the reversible increase in absorbance observed at applied
potentials more negative than -1.20 V is assigned to the
accumulation of electrons in available states of the conduction
band. Also on this basis, the potential of the conduction band
edge at the semiconductor-liquid electrolyte interface (SLI) is
determined to be -1.20 V, while, consistent with the measured
onset for band gap absorption (380 nm), the potential of the
valence band edge at the SLI is determined to be +2.06 V.
Furthermore, it is known that there are two populations of
intraband states in nanostructured TiO2 films.30 The first of these
lies about 0.5 eV below the conduction band edge and is
assigned to surface TiIV atoms. The second lies about midgap,
i.e., about 1.63 eV below the conduction band edge, and is
assigned to surface peroxy species. From the above and from
the known half-wave potentials of the R and V components in
Figure 4. Potential dependence of absorbance by (a) TiO2-RPVC1
and (b) TiO2-RPVC4 at 600 nm in a MeCN/EtOH (70/30 by volume)
solution containing the electrolyte TBAP (0.10 mol dm-3) and the
sacrificial donor TEOA (0.05 mol dm-3). The reference absorbance
was that measured at 0.00 V.
If the above is the case, we can state the following. First, to
fully account for these findings, it will be necessary to have a
quantitative description of the trap and conduction band state
energetics of the TiO2 nanocrystal component. Second, if the
occupancy of these state may be controlled by the potential
applied to the constituent TiO2 nanocrystal component of TiO2-
RPVC1 and TiO2-RPVC4, then the extent to which visible-light
irradiation results in long-lived charge separation may be
potentiostatically modulated.
IV. Potential-Dependent Optical Absorption Spectroscopy
of TiO2-RPVC1 and TiO2-RPVC4. The condensed-phase and
molecular component energetics of TiO2-RPVC1 and TiO2-
RPVC4 in a MeCN/EtOH (70/30 by volume) solution containing
the electrolyte TBAP (0.10 mol dm-3) and the sacrificial donor
TEOA (0.05 mol dm-3) were elucidated by potential-dependent
optical absorption spectroscopy.
As may be seen from Figure 4, on sweeping the applied
potential to TiO2-RPVC1 and TiO2-RPVC4 from 0.00 to -1.50
V the following is observed: a gradual increase in absorbance
between -0.35 and -0.65 V, an increase in absorbance between
-0.65 and -0.75 V, a decrease in absorbance between -0.75
and -0.95 V, and an increase in absorbance at -1.20 V. During
the reverse sweep the following is observed: a decrease in
absorbance between -1.50 and -1.20 V, an increase in
absorbance between -0.95 and -0.75 V, an essentially constant
absorbance between -0.75 and -0.35 V, and a gradual decrease
in absorbance between -0.35 and 0.00 V. It should be noted,
as discussed in section VI, that the residual absorbance at 0.00
V eventually decreases to zero.
RPVC1 and RPVC4 33
structed (Scheme 8).
,
an energy level diagram may be con-
By use of this energy level diagram, the potential-dependent
spectroscopy of RPVC1 and RPVC4 may be accounted for. It
has been established that there is a population of surface TiIV
states at -0.70 V and that the half-wave potential for the first
reduction of the V component is -0.41 V.33 It has also been
established, by optical absorption spectroscopy (not shown), that
the increase in absorbance at 600 nm between -0.35 and -0.75
V is due to formation of the radical cation of the V component
in TiO2-RPVC1 and TiO2-RPVC4 29
On this basis, it is
.
concluded that reduction of the V component is mediated by
surface TiIV states.34 It should be noted that only 38% and 45%
of V components are reduced in TiO2-RPVC1 and TiO2-
RPVC4, respectively, suggesting that the orientation of the
molecular components with respect to the nanocrystal compo-
nent may be important. These values have been calculated as
follows. In the case of TiO2-RPVC1 the absorbance by the R
component at 470 nm in Figure 3a is 0.54. From the measured
extinction coefficient of R at 470 nm (13 100 mol cm-1) the

Charge Separation in Heterosupermolecule
J. Phys. Chem. B, Vol. 103, No. 38, 1999 8075
surface concentration of RPVC1 was calculated to be 2.5 × 1016
studies have shown that molecules similar to the ruthenium
complex component in RPVC1 and RPVC4 occupy an area of
85 Å2 (absolute) at the air-water interface.37 On this basis, the
extent of loading in TiO2-RPVC1 and TiO2-RPVC4 is estimated
to 22% and 18%, respectively.
molecules cm-2 (geometric). From the known extinction of the
radical cation of the V component at 600 nm (10 400 mol cm-1
)
an absorbance of 0.59 is expected if all V components are
reduced. The measured absorbance at 600 nm in Figure 4a is
0.19, indicating that 38% of all viologens are reduced. In the
case of TiO2-RPVC4 the absorbance by the R component at
470 nm in Figure 3b is 0.50. From the measured extinction
coefficient of R at 470 nm (14 200 mol cm-1) the surface
concentration of RPVC1 was calculated to be 2.0 × 1016
molecules cm-2 (projected area). From the known extinction
of the radical cation of the V component at 600 nm (14 000
mol cm-1) an absorbance of 0.49 is expected if all V
components are reduced. The measured absorbance at 600 nm
in Figure 4b is 0.22, indicating that 45% of all viologens are
reduced.
V. Potential-Dependent Light-Induced Charge Separation
in TiO2-RPVC1 and TiO2-RPVC4. As discussed in section III,
irradiation of TiO2-RPVC1 and TiO2-RPVC4 under the stated
conditions at the open-circuit potential results in long-lived
formation of the radical cation of the V component. Furthermore,
it is likely that the conduction band and trap states of the TiO2
nanocrystal component mediated this process. As discussed in
section IV, the occupancy of the conduction band and trap states
of the TiO2 nanocrystal component may be controlled poten-
tiostatically. Taken together, these findings suggested that the
effect of irradiation of TiO2-RPVC1 and TiO2-RPVC4 might
be modulated potentiostatically.
For this reason TiO2-RPVC1 and TiO2-RPVC4 in a MeCN/
EtOH (70/30 by volume) solution containing the electrolyte
TBAP (0.10 mol dm-3) and the sacrificial donor TEOA (0.05
mol dm-3) were irradiated at different applied potentials by the
green-blue output of an argon-ion laser (200 mW cm-2, 3 s).
The potential dependence of the effect of irradiation of on TiO2-
RPVC4 is shown in Figures 5 and 6.
At an applied potential of 0.00 V, irradiation of TiO2-RPVC1
and TiO2-RPVC4 leads to reduction of 14% and 8% of the V
components, respectively. It should be noted, however, that
radical cations formed are not long-lived. At an applied potential
of -0.45 V, irradiation of TiO2-RPVC1 and TiO2-RPVC4 leads
to reduction of 20% and 14% the V component, respectively.
Furthermore, the radical cations formed are longer-lived. At an
applied potential of -0.60 V, 32% and 26% of the V
components of TiO2-RPVC1 and TiO2-RPVC4 are reduced, a
fraction that is not increased by irradiation. It should be noted,
as discussed in section IV, that at an applied potential as negative
as -0.75 V only 38% and 45% of the V components were
reduced in TiO2-RPVC1 and TiO2-RPVC4, respectively.
The above findings, and those in section IV, are consistent
with the following mechanism for long-lived light-induced
charge separation (Scheme 9). At 0.00 V absorption of a visible
photon by the R component of either TiO2-RPVC1 or TiO2-
RPVC4 results in electron transfer to a conduction band state of
the TiO2 nanocrystal component with 95% efficiency.16 In the
presence of the added sacrificial donor TEOA a significant
fraction of the injected electrons, about 20% under the reported
conditions,38 are not transferred back to the oxidized R
component but occupy the available trap states at the surface
of the TiO2 nanocrystal component. In short, there is a shift of
the quasi-Fermi level to a potential that is sufficiently negative
to reduce the V components in both TiO2-RPVC1 and TiO2-
RPVC4. Furthermore, since the radical cation of the viologen
component is formed only where the heterosupramolecular
assembly is irradiated, the shift of the quasi-Fermi level to more
negative potentials is apparently localized. At an applied
potential of 0.00 V, trap states are being filled more quickly
than they are being emptied under irradiation. Once irradiation
ceases, however, these trap states are gradually emptied,
resulting in the slow reoxidation of the reduced viologen
components.
It has been established that there is a population of surface
TiIV states at -0.70 V and that the half-wave potential for the
second reduction of the V component is -0.81 V.33 It has also
been established, by optical absorption spectroscopy, that the
decrease in absorbance at 600 nm between -0.75 and -0.95
V is due to formation of the neutral dication of the V component
in TiO2-RPVC1 and TiO2-RPVC4. On this basis, it is concluded
that second reduction of the V component is also mediated by
surface TiIV states.34
At still more negative potentials, that is, more negative than
-1.20, there is a sharp increase in absorbance that may be
assigned, by examination of the corresponding absorption
spectrum (not shown),32 mainly to accumulation of electrons
in the available states of the conduction band. By comparison
with the findings outlined above for an unmodified nanostruc-
tured TiO2 film, it is concluded that there is no change in the
potential of the conduction band edge at the SLI following
adsorption of RPVC1 and RPVC4 to form the heterosupramo-
lecular assemblies TiO2-RPVC1 and TiO2-RPVC4, respectively.
When the applied potential is reversed, the filled conduction
band states are emptied, leading to a sharp decrease in
absorbance between -1.50 and -1.20 V. Subsequently, the
intraband states are emptied, leading to a sharp increase in
absorbance, as the neutral diradical of the V component in
TiO2-RPVC1 and TiO2-RPVC4 is reoxidized to form the highly
colored radical cation. Because the concentration of intraband
trap states is small at potentials more positive than -0.41 V,
the half-wave potential for the first reduction of the V
component, this process is kinetically limited and the absorbance
assigned to this species persists for some minutes even at 0.00
V (see also section VI).
The findings that chemisorption of RPVC1 and RPVC4 at the
surface of the TiO2 nanocrystal film to form TiO2-RPVC1 and
TiO2-RPVC4, respectively, does not affect the energy of the
valence or conduction bands of the TiO2 nanocrystal components
are consistent with previously reported findings.32 However,
these and closely related studies have also established that
chemisorption of the R component via phosphonic acid groups
at surface TiIV atoms result in their promotion into the
conduction band and elimination as trap sites.32,34,35 From Figure
3 the concentrations of adsorbed RPVC1 and RPVC4 are estimated
to be 2.5 × 1016 molecules cm-2 (projected area) and 2.1 ×
1016 molecules cm-2 (projected area), respectively. This finding
implies, assuming a surface roughness of 1000 17 and a
concentration of TiIV surface states of 2 × 1013 TiIV sites per
cm2 (absolute area),36 that most of the available surface states
are occupied, as might have been expected from the relatively
slow rates of oxidation and reduction. Langmuir-Blodgett
At an applied potential of -0.45 V, irradiation of both TiO2-
RPVC1 and TiO2-RPVC4 leads to reduction of a larger fraction
of the V components, while the resulting radical cations are
longer-lived. These findings, however, are entirely consistent
with the mechanism proposed above for light-induced charge

8076 J. Phys. Chem. B, Vol. 103, No. 38, 1999
Will et al.
Figure 5. Optical absorption difference spectra of TiO2-RPVC1 at (a)
0.00 V, (b) -0.45 V, and (c) -0.60 V in a MeCN/EtOH (70/30 by
volume) solution containing the electrolyte TBAP (0.10 mol dm-3) and
the sacrificial donor TEOA (0.05 mol dm-3) prior to and immediately
following irradiation by the all-lines output of an argon-ion laser (200
mW cm-2, 3 s) and 60 s later. The reference spectra were measured
prior to irradiation.
Figure 6. Optical absorption difference spectra of TiO2-RPVC4 at (a)
0.00 V, (b) -0.45 V, and (c) -0.60 V in a MeCN/EtOH (70/30 by
volume) solution containing the electrolyte TBAP (0.10 mol dm-3) and
the sacrificial donor TEOA (0.05 mol dm-3) prior to and immediately
following irradiation by the all-lines output of an argon-ion laser (200
mW cm-2, 3 s) and 60 s later. The reference spectra were measured
prior to irradiation.
viologen component in TiO2-RPVC1 or TiO2-RPVC4 to the
TiO2 nanocrystal component and charge separation is long-lived.
At an applied potential of -0.60 V, irradiation of either
TiO2-RPVC1 or TiO2-RPVC4 does not result in an increase in
the intensity of the absorption spectrum assigned to the radical
cation of the V components. Again, this finding is entirely
consistent with the mechanism proposed. Specifically, at suf-
ficiently negative applied potentials all those V components with
the requisite geometry have been reduced and optically pumping
the quasi-Fermi potential to more negative potentials will not
result in an increase in the fraction of viologen components that
are reduced.
separation at 0.00 V. Specifically, as a more negative potential
is applied to the TiO2 nanocrystal component of TiO2-RPVC1
and TiO2-RPVC4, more of the surface TiIV states are filled
initially and irradiation results in a localization of the quasi-
Fermi level to still more negative potentials. This, in turn, results
in a larger fraction of the V components in TiO2-RPVC1 and
TiO2-RPVC4 being reduced. Furthermore, once irradiation
ceases, only those trap states at potentials more negative than
the potentiostatically applied potential, -0.45 V, are emptied.
As a consequence, there are no empty surface TiIV states
available to mediate back electron transfer from the reduced

Charge Separation in Heterosupermolecule
J. Phys. Chem. B, Vol. 103, No. 38, 1999 8077
SCHEME 9
Figure 7. Natural logarithm of the absorbance at 600 nm at the
indicated applied potential plotted against elapsed time for (a) TiO2-
RPVC1 and (b) TiO2-RPVC4 in a MeCN/EtOH (70/30 by volume)
solution containing the electrolyte TBAP (0.10 mol dm-3) and the
sacrificial donor TEOA (0.05 mol dm-3) following irradiation by the
all-lines output of an argon-ion laser (200 mW cm-2, 3 s) at -0.45 V.
The reference spectra were measured prior to irradiation.
It will be noted that the findings presented do not preclude a
mechanism where the photoinjected electrons are transferred
directly to an oxidized R component formed by light-induced
electron transfer to the V component to which it is covalently
linked. In this context, it is noted that there is increasing
evidence to support the view that the rate of electron transfer
to the oxidized R component increases significantly as the
concentration of electrons present in the nanostructured TiO2
film increases.38 As a consequence, more detailed time-resolved
studies are necessary and are being undertaken.39
It should also be noted that direct electron transfer by the
electronically excited R component, even at negative applied
potentials, does not lead to long-lived charge separation. This
is because, as has been discussed in detail above, back electron
transfer from the reduced V component to the oxidized R
component is between 5 and 6 orders of magnitude faster than
electron transfer from the sacrificial donor.
VI. Charge Recombination Kinetics in TiO2-RPVC1 and
TiO2-RPVC4. As discussed above, irradiation of TiO2-RPVC1
or TiO2-RPVC4 at an applied potential of -0.45 V results in
electron transfer from the photoexcited R component to the TiO2
nanocrystal component. This, in turn, results in a localized shift
of the quasi-Fermi level to more negative potentials and to
electron transfer from the TiO2 nanocrystal component to the
V component. Reduction of the V component yields the visible
spectrum characteristic of the corresponding radical cation and,
more specifically, an increase in the absorbance measured at
600 nm.
TABLE 1: Rate Constants for Reoxidation of Radical
Cation of Viologen Components in TiO2-RPVC1 and
TiO2-RPVC4
applied potential
+1.00 V
0.00 V
-0.45 V
TiO2-RPVC1 1.7 × 10-3 s-1
1.5 × 10-3 s-1 0.5 × 10-3 s-1
TiO2-RPVC4 34.0 × 10-3 s-1 5.8 × 10-3 s-1 1.0 × 10-3 s-1
be measured (see Figure 7). Similar measurements were
performed in which TiO2-RPVC1 and TiO2-RPVC4 were
irradiation at -0.45 V and the rate of decay of the radical cation
formed measured at applied potentials of 0.00 and +1.00 V.
The decay of the absorbance assigned to the radical cation of
the viologen component is monoexponential, with the corre-
sponding first-order rate constants being given in Table 1.
From the findings summarized in Table 1, it is evident that,
in the case of both TiO2-RPVC1 and TiO2-RPVC4, the rate of
reoxidation of the V component increases at more positive
applied potentials. Again, this is consistent with the mechanism
proposed above and is explained by the availability of a larger
number of empty surface TiIV states at more positive potentials
capable of mediating electron transfer.5,29
From the findings summarized in Table 1, it is also evident
that at a given applied potential the rate of reoxidation of the V
component in TiO2-RPVC1 is significantly slower than that of
the same component in TiO2-RPVC4. It should be noted that
this result is consistent with the findings discussed in section
III. It is tentatively suggested that this be accounted for by the
By subsequent monitoring of the decrease in the absorbance
measured at 600 nm, the rate of reoxidation of the reduced V
component in TiO2-RPVC1 and TiO2-RPVC4 at -0.45 V can

8078 J. Phys. Chem. B, Vol. 103, No. 38, 1999
Will et al.
The associated heterosupramolecular function, long-lived light-
induced charge separation, has been demonstrated.
A mechanism, supported by detailed studies of TiO2-RPVC4
and the closely related heterosupramolecular assemblies Al2O3-
RPVC4 and SnO2:Sb-RPVC4, has been proposed. In summary,
electron transfer directly from the R component to the TiO2
nanocrystal component results, upon trapping of the injected
electron in a surface TiIV state, in a localized shift of the quasi-
Fermi level to more negative potentials and in electron transfer
to the V component. Charge separation is long-lived because
the sacrificial donor in solution competes with electron transfer
from the TiO2 nanocrystal component to the oxidized R
component and because electron transfer from the reduced V
component to the oxidized sacrificial donor does not occur. If
electrons are transferred directly from the R component to the
V component, charge separation is not long-lived because the
sacrificial donor in solution does not compete with electron
transfer from the reduced V component to the oxidized R
component.
Figure 8. Optical absorption difference spectra of SnO2:Sb-RPVC4
at (a) 0.00 V, (b) -0.45 V, and (c) -0.60 V in a MeCN/EtOH (70/30
by volume) solution containing the electrolyte TBAP (0.10 mol dm-3
)
and the sacrificial donor TEOA (0.05 mol dm-3) prior to and
immediately following irradiation by the all-lines output of an argon-
ion laser (200 mW cm-2, 3 s). The reference spectra were measured
prior to irradiation under the above conditions. The spectrum of SnO2:
Sb-RPVC4 measured at the open-circuit potential under the above
conditions is shown as an inset.
Because the electron transfer that results in long-lived light-
induced charge separation is from a surface TiIV state to the V
component, potentiostatic control of the occupancy of these
states has been used to modulate the effect of irradiation. In
summary, at an applied potential of -0.45 V the surface TiIV
states are potentiostatically filled to a potential just positive of
the potential at which the V component is reduced. As a
consequence, irradiation leads to reduction of a significant
fraction of these components. Furthermore, since there are no
empty surface TiIV states at potentials positive of the potential
at which the V component is oxidized, charge separation is long-
lived. At an applied potential of 0.00 V, however, the surface
TiIV states are potentiostatically filled to a potential significantly
positive of the potential at which the V component is reduced
and irradiation leads to reduction of a substantially smaller
fraction of these components. Furthermore, since there are empty
surface TiIV states at potentials positive of the potential at which
the V component is oxidized, charge separation is not long-
lived. At an applied potential of +1.00 V no reduction of the
V component is observed upon irradiation, while any previously
reduced V component is rapidly reoxidized. In short, the above
heterosupramolecular assembly may be written using green light
at -0.45 V, read using red light, and erased by application of
+1.00 V.
It is important to note the limitations of the above heterosu-
pramolecular assembly. First, it is not an organized heterosu-
pramolecular assembly, as a consequence of which the con-
stituent heterosupermolecules are not individually addressable.
Second, the constituent heterosupermolecules most probably do
not act fully independently, i.e., there is likely electron transfer
between viologens adsorbed at the same or adjacent nanocrys-
tals. Third, because the constituent condensed-phase and mo-
lecular components are covalently linked, they may not be self-
assembled.
reduced configurational freedom of the V component in TiO2-
RPVC1 compared with that of the same component in TiO2-
RPVC4
.
It should be noted that it is not possible to say whether there
is transfer of electrons between adjacent viologens in TiO2-
RPVC1 and TiO2-RPVC4. However, it should also be noted that
while this may affect the absolute rate constants measured by
providing a pathway by which electrons localized on a viologen
component can migrate to a surface state that mediates their
transfer to the TiO2 nanocrystal component, the proposed
mechanism and the proposed interpretation of the effect of the
applied potential are appropriate in either case.
VII. Potential-Dependent Light-Induced Charge Separa-
tion in SnO2:Sb-RPVC4. Further support for the proposed
mechanism was obtained by comparing the effect of applied
potential on light-induced charge separation in SnO2:Sb-RPVC4.
Shown in Figure 8 are the optical absorption spectra of SnO2:
Sb-RPVC4 in a MeCN/EtOH (70/30 by volume) solution
containing the electrolyte TBAP (0.10 mol dm-3) and the
sacrificial donor TEOA (0.05 mol dm-3) prior to and following
irradiation by the blue-green output of an argon-ion laser (200
mW cm-2, 3 s) at the indicated applied potentials.
Regardless of the applied potential, irradiation of SnO2:Sb-
RPVC4 does not lead to a measurable increase in the absorbance
assigned to the radical cation of the V component. This finding,
however, is not unexpected, since the electrons injected into
the SnO2:Sb nanocrystal component move freely throughout the
nanostructured film in the absence of low-energy states capable
of acting as traps.18 That is, an injected electron can either be
transferred back to the oxidized R component or be quickly
diffused toward the back contact, as a consequence of which
there is no significant localized shift of the quasi-Fermi level
to more negative potentials and no detectable light-induced
reduction of the V component.
Nevertheless, the findings reported here demonstrate that
effective function modulation may indeed be achieved by
organizing the condensed-phase components of a heterosuper-
molecule to form an intrinsic substrate. In the near future, optical
write-read-erase devices based on the above heterosupramo-
lecular assembly will be described.40
Conclusions
A heterosupermolecule has been assembled by covalently
linking a TiO2 nanocrystal, a ruthenium complex, and a
viologen. The covalent organization of these heterosupermol-
Acknowledgment. These studies were supported by a grant
from the Commission of the European Union under the Training
and Mobility of Researchers (Network) Program (Contract
FMRX CT96-0076).
ecules yields a heterosupramolecular assembly TiO2-RPVC4
.

Charge Separation in Heterosupermolecule
J. Phys. Chem. B, Vol. 103, No. 38, 1999 8079
Launikomis, A.; Lay, P.; Mau, A.; Sargeson, A.; Sasse, W. Aust. J. Chem.
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