Redox properties of ground and electronically excited states: [Ru(bpy)2Qbpy]2+ monolayers

Article abstract of DOI:10.1021/jp9828890

Dense monolayers of [Ru(bpy)₂Qbpy]²⁺, where bpy is 2,2′-bipyridyl and Qbpy is 2,2′:4,4″:4′4″-quarterpyridyl, have been formed by spontaneous adsorption onto clean platinum microelectrodes. Cyclic voltammetry of these monolayers is nearly ideal, and five redox states are accessible over the potential range from +1.3 to -2.0 V. Chronoamperometry conducted on a microsecond time scale has been used to measure the heterogeneous electron-transfer rate constant, k, for both metal- and ligand-based redox reactions. Heterogeneous electron transfer is characterized by a single unimolecular rate constant (k/s^-1). Standard heterogeneous electron-transfer rate constants, k°, have been evaluated by extrapolating Tafel plots of In k vs overpotential, η, to zero driving force to yield values of (5.1 ± 0.3) × 10⁵ s^-1, (3.0 ± 0.1) × 10⁶ s^-1, and (3.4 ± 0.2) × 10⁶ s^-1 for k°_3+/2+, k°_2+/1+, and k°_1+/0, respectively. Temperature-resolved measurements of k reveal that the electrochemical activation enthalpy, ΔH^?, decreases from 12.1 ± 1.7 kJ mol^-1 for the 3+/2+ reaction to 7.5 ± 0.8 kJ mol^-1 for the 2+/1+ process. Probing the temperature dependence of the formal potential gives the reaction entropy, ΔS_rc°. Significantly, the free energy of activation is constant at 6.9 ± 0.6 kJ mol^-1 for all three redox couples investigated. The electronic transmission coefficient, K_el, describing the probability of electron transfer once the transition state has been reached, is considerably less than unity for all three redox processes. Following photoexitation using a laser pulse at 355 nm, emission is observed from the monolayers with an excited-state lifetime (6.2 μs) that exceeds that of the complex in solution (1.4 μs). It appears that weak electronic coupling between the adsorbates and the electrode means that the excited states are not completely deactivated by radiationless energy transfer to the metal. For the first time, we have used voltammetry conducted at megavolt per second scan rates to directly probe the redox potentials and electron-transfer characteristics of electronically excited species.

Full text of DOI:10.1021/jp9828890

10004
J. Phys. Chem. B 1998, 102, 10004-10012
Redox Properties of Ground and Electronically Excited States: [Ru(bpy)2Qbpy]2+
Monolayers
Robert J. Forster* and Tia E. Keyes†
School of Chemical Sciences, Inorganic Chemistry Research Centre, Dublin City UniVersity, Dublin 9, Ireland
ReceiVed: July 7, 1998; In Final Form: September 30, 1998
2+
2
Dense monolayers of [Ru(bpy) Qbpy] , where bpy is 2,2′-bipyridyl and Qbpy is 2,2′:4,4′′:4′4′′-quarterpyridyl,
have been formed by spontaneous adsorption onto clean platinum microelectrodes. Cyclic voltammetry of
these monolayers is nearly ideal, and five redox states are accessible over the potential range from +1.3 to
-
2.0 V. Chronoamperometry conducted on a microsecond time scale has been used to measure the
heterogeneous electron-transfer rate constant, k, for both metal- and ligand-based redox reactions.
-
1
Heterogeneous electron transfer is characterized by a single unimolecular rate constant (k/s ). Standard
heterogeneous electron-transfer rate constants, k°, have been evaluated by extrapolating Tafel plots of ln k vs
5
-1
6 -1
overpotential, η, to zero driving force to yield values of (5.1 ( 0.3) × 10 s , (3.0 ( 0.1) × 10 s , and (3.4
6
-1
(
0.2) × 10 s for k°3+/2+, k°2+/1+, and k°1+/0, respectively. Temperature-resolved measurements of k reveal
q
-1
that the electrochemical activation enthalpy, ∆H , decreases from 12.1 ( 1.7 kJ mol for the 3+/2+ reaction
-
1
to 7.5 ( 0.8 kJ mol for the 2+/1+ process. Probing the temperature dependence of the formal potential
gives the reaction entropy, ∆Src°. Significantly, the free energy of activation is constant at 6.9 ( 0.6 kJ
-
1
mol for all three redox couples investigated. The electronic transmission coefficient, κel, describing the
probability of electron transfer once the transition state has been reached, is considerably less than unity for
all three redox processes. Following photoexitation using a laser pulse at 355 nm, emission is observed from
the monolayers with an excited-state lifetime (6.2 µs) that exceeds that of the complex in solution (1.4 µs).
It appears that weak electronic coupling between the adsorbates and the electrode means that the excited
states are not completely deactivated by radiationless energy transfer to the metal. For the first time, we
have used voltammetry conducted at megavolt per second scan rates to directly probe the redox potentials
and electron-transfer characteristics of electronically excited species.
5
Introduction
dreds of nanoseconds, the slow time scale of diffusion prevents
the measurement of dynamic parameters. This diffusion limita-
Electronically excited states play pivotal roles in areas as
diverse as dye sensitization of semiconductors for solar energy
tion can be eliminated by forming a spontaneously adsorbed
monolayer. Electrochemical measurements on excited states
1
2,3
conversion to photosynthesis.
Photoexcited reactants have
8
,9
within monolayers are not often performed because, beyond
the reductive or oxidative quenching of the excited state that
we wish to investigate, other deactivation pathways exist. First,
if the monolayers are formed on mirror smooth surfaces, then
energy transfer from the excited state to the electrode is expected
to be efficient if the electron transfer distance is sufficiently
short to allow heterogeneous electron transfer to occur within
been used extensively to study electron transfer because excited
states, created upon absorption of a photon, are simultaneously
better electron donors and acceptors than their ground-state
4
precursors. However, while our understanding of heteroge-
neous electron transfer in the ground state has evolved to a high
degree on both theoretical and experimental fronts, there have
been few reports on direct measurements of exited-state redox
1
0,11
potentials.5 Moreover, the direct measurement of oxidation and
the lifetime of the electronically excited state.
These
apparently contradictory demands, i.e., a short electron-transfer
distance for fast heterogeneous electron transfer, vs a large
separation of the excited state from the surface to prevent energy
transfer, coupled with the extreme demands on experimental
time scale, represent significant barriers to progress in this area.
However, it is important to note that efficient interaction between
the excited molecules and metallic states of the electrode is only
expected for near perfect metallic mirrors. Indeed, as has been
reported many times, depending on the length of the bridging
ligand, and the quantum efficiency for fluorescence, excitation
of the electronic plasma resonance within the metal can cause
reduction kinetics of electronically excited states remains
completely unexplored. This situation has arisen largely because
electronically excited states are transient species and typically
have submicrosecond lifetimes. Conventional electrochemical
methods cannot provide a meaningful insight into the redox
properties of these fleeting species since traditional voltammetry
is restricted to millisecond, or longer, time scales.6 However,
with the advent of ultrafast electrochemical techniques and
microelectrodes, it is now possible to directly probe the prop-
erties of species having submicrosecond lifetimes.7
While Fox and co-workers have demonstrated that it is pos-
sible to directly measure the excited-state redox potentials for
solution-phase species with lifetimes on the order of hun-
1
2,13
fluorescence enhancement at rough electrodes.
Second,
because the surface concentration of the adsorbates can be high,
lateral energy or electron transfer may lead to quenching of the
excited states. Therefore, the ability to control the surface
coverage is important.
†
Present address: School of Chemistry, Dublin Institute of Technology,
Kevin Street, Dublin 2, Ireland.
1
0.1021/jp9828890 CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/11/1998

Ru(bpy)2Qbpy]2 Monolayers
+
J. Phys. Chem. B, Vol. 102, No. 49, 1998 10005
[
CHART 1
using nitrogen, and a blanket of nitrogen was maintained over
the solution during all experiments.
In high-speed chronoamperometry,16,17 a custom-built func-
tion generator-potentiostat, with a rise time of less than 10 ns,
was used to apply potential steps of variable pulse width and
amplitude directly to a two-electrode cell. A Pt foil and an
Ag/AgCl reference electrode were combined to form a counter
electrode. The foil lowered the resistance and provided a high-
frequency path. In studies of the voltammetry of electronically
excited species, the application of the voltage sweep was
triggered by a custom-built silicon diode such that the potential
sweep commenced simultaneously with the arrival of the laser
pulse at the electrode surface. Alternatively, a Stanford
Research Instruments Model DG 535 Four Channel Digital Pulse
Generator/Delay Generator was used as controller to simulta-
neously trigger the laser pulse and the application of the potential
sweep. In this case, the optical path (1.2 m) was sufficiently
short so that it did not introduce any significant difference
between the triggering and arrival of the laser pulse.
In this contribution, we describe ultrafast electrochemical and
transient photochemical measurements on [Ru(bpy)2Qbpy]
Chart 1) monolayers; bpy is 2,2′-bipyridyl and Qbpy is 2,2′:
,4′′:4′4′′-quarterpyridyl.
We have used high-speed chronoamperometry to measure the
rate of heterogeneous electron transfer across the metal/
monolayer interface in the ground state. Despite the rather short
electron-transfer distance, there is weak electronic coupling
between the metal center, or the bipyridyl ligands, and the metal
surface leading to a nonadiabatic reaction in the ground state.
Such weak coupling, together with nonmirror finish microelec-
trodes, appears to impede radiationless deactivation, making the
excited state lifetime on the surface similar to that found in a
frozen glass. We have used fast-scan voltammetry to allow us
probe redox processes within the lifetime of the electronically
excited state and report the first cyclic voltammograms of an
excited-state species.
2
+
(
4
Microelectrodes were fabricated from platinum microwires
(Goodfellow Metals Ltd.) of radii between 1 and 25 µm by
sealing them in soft glass using a procedure described previ-
16
ously. Microdisk electrodes were exposed by removing excess
glass using 600 grit emery paper followed by successive
polishing with 12.5, 5, 1, 0.3 and 0.05 µm alumina. The
polishing material was removed between changes of particle
size by sonicating the electrodes in deionized water for at least
5
min. The polished electrodes were electrochemically cleaned
by cycling in 0.1 M HClO4 between potential limits chosen to
first oxidize and then reduce the surface of the platinum
electrode. Excessive cycling was avoided in order to minimize
the extent of surface roughening. Finally, the electrode was
cycled between -0.300 and 0.900 V in aqueous 0.1 M NaClO4
until hydrogen desorption was complete. Macroscopic platinum
electrodes were polished in an identical manner.
Experimental Section
cis-Ru(bpy)2Cl2 was prepared by standard synthetic meth-
ods.14 All chemicals were purchased from Aldrich and were
used without further purification.
2
,2′:4,4′′:4′4′′-Quaterpyridyl. This ligand was prepared
15
using a modification to the method described by Morgan et al.
The real or microscopic surface area of the electrodes was
found by calculating the charge under the oxide or hydrogen
4,4′-Bipyridyl (5 g, 0.02 M) was placed in a sealed Teflon bomb
over dried Pd/C (1 g, 10% Pd). The bomb was heated in an
oven at 200 °C for 1 week. The product was dissolved in
chloroform, filtered to remove the catalyst, and dried. Then it
was extracted and recrystallized from acetone. Yield, 1.2 g,
1
8
adsorption-desorption peaks. Typically, the surface rough-
ness factor was between 1.3 and 1.6. Obtaining the real, as
opposed to the projected or geometric, surface area of the
electrodes is important if the area occupied per molecule is to
be accurately measured.
15 1
20%, m.p. 235 °C (literature value 235 °C) H NMR data
[(CD3)2SO]: H3, H3′′(d) 8.06, (d) (2H), H5, H6, H5′,H6′, 7.97
For electrochemical measurements in the ground state,
spontaneously adsorbed monolayers were formed in situ using
a 50 µM solution of the metal complex in the electrolyte solu-
tion. A low concentration of the surface active complex in
solution improved the stability of the monolayers when exposed
to organic solvents yet minimized the diffusional contribution
to the overall current in chronoamperometry or cyclic voltam-
metry. In the case of excited-state measurements, monolayers
were formed by placing clean electrodes in a degassed solution
of 50:50 methanol:water containing 1 mM of the complex for
ppm (d) (4H), H2′′′, H3′′′, H2′′, H3′′, 8.81 ppm, (d) (4H)), H5′′′,
H6′′′, H5′′, H6′′, 8.92 ppm, (d) (4H)). Anal. Calcd. for
C20H16N4O: C, 73.3; H, 4.8; N, 17.07. Found: C, 74.2; H,
4
.64; N, 17.18.
Ru(bpy)2(Qbpy)] . 2,2′:4,4′′:4′4′′-Quarterpyridyl (0.12 g,
2+
[
3
0
.38 mM) was dissolved in ethanol/water (20 cm 6/4 v/v) and
heated to reflux. cis-Ru(bpy)2Cl2 (0.276 g, 0.38 mM) dissolved
in ethanol was added to the refluxing solution over 20 min.
Then, the reaction mixture was left refluxing for a further 4 h.
3
The solvent was removed to a volume of 10 cm , and a solution
2
4 h. Subsequent measurements were performed in solutions
of concentrated aqueous NH4PF6 was added. The resulting
orange solid was collected by filtration and recrystallized from
acetonitrile/water (1/1 v/v).
that did not contain any of the dissolved complex.
A nonisothermal cell, where the reference electrode was
isolated from the main compartment by a salt bridge and held
at room temperature, was used for the temperature-resolved
experiments. The high electrolyte concentration and the design
of the bridge minimize any systematic error in the reported
temperature effects on E°′ due to changes in the liquid junction
Apparatus. Electrochemical cells were of conventional
design and were thermostated within (0.2 °C using a Julabo
F10-HC refrigerated circulating bath. All potentials are quoted
with respect to a BAS Ag/AgCl gel-filled reference electrode.
The potential of the ferrocene/ferricenium couple dissolved in
acetonitrile containing 0.1 M TEAP as supporting electrolyte
was +0.322 V. Cyclic voltammetry was performed using a
CH Instruments Model 660 Electrochemical Workstation and
a conventional three-electrode cell. All solutions were degassed
1
9
potential with temperature.
Fluorescence. The surfaces of platinum macroelectrodes
modified with spontaneously adsorbed monolayers of [Ru-
2
+
(bpy)2Qbpy] were positioned against an Epiflan Neofluar

10006 J. Phys. Chem. B, Vol. 102, No. 49, 1998
Forster and Keyes
this complex,20 and their redox potentials are in excellent
agreement with those observed here. By comparison with the
2
+
analogous [Ru(bpy)3] complex, the peak observed at 1.15 V
2
+/3+
is attributed to the metal-based Ru
redox reaction. The
processes observed at negative potentials correspond to suc-
2
1-23
cessive ligand-based reductions, respectively.
of this type, if the Ru
In complexes
2
+/1+
redox process occurs, then it does
24
so only at potentials more negative than -2.3 V. Furthermore,
reduction of the metal center would lead to an unstable d7
electronic configuration causing irreversible voltammetric re-
sponses to be observed. In heteroleptic systems of this kind, it
is possible that the first ligand based reduction occurs either on
the bipyridyl or on the quarterpyridyl ligand. The noncom-
1
5
plexed quarterpyridyl ligand has a less negative reduction
potential than that of free 2,2′-bipyridyl. Therefore, the reduc-
tion occurring at -1.09 V in the monolayer is assigned to the
quarterpyridyl. It is perhaps important to note that in solution-
phase experiments of both the ruthenium complex and the free
quarterpyridyl, we find that rapid adsorption occurs causing the
first reduction peak to shift by approximately 250 mV in a
positive potential direction. This sensitivity of the bridge redox
potential to surface immobilization was not observed previously
for complexes adsorbed through linear 4,4′-dipyridyl type
Figure 1. Cyclic voltammogram of a spontaneously adsorbed [Ru-
2
+
(
bpy)
coverage is 1.04 × 10 mol cm . The supporting electrolyte is 0.1
M TBABF in acetonitrile. The radius of the platinum microelectrode
2
Qbpy] monolayer. The scan rate is 1 V/s, and the surface
-
10
-2
4
is 25 µm. The cathodic currents are up, and the anodic currents are
down. The complex is in the 2+ form between approximately (1.0 V.
water-immersion objective (40× magnification) and observed
through a Zeiss Axioskop microscope. The fluorescence was
monitored using a Peltier-cooled TH 7895B CCD that was
operated at -25 °C through a Photometrics NU-200 controller
interfaced to a Macintosh personal computer. The microscope
was equipped with a 200 W mercury-arc lamp for illumination
2
5-28
linkers.
This experimental voltammetric response observed for the
monolayer has all of the characteristics of a redox system
6
confined to an electrode surface. For example, the peak shapes
are symmetrical and independent of scan rate, υ, at least over
(epifluorescence configuration). The intensity of the incident
the range 1 to 50 V/s, and the peak height scales linearly with
light was controlled using neutral density filters between 50
and 1% transmittance. Exposure of the modified surface to the
mercury-arc lamp was minimized to reduce the extent of
photochemical decomposition of the monolayer.
1
/2
the scan rate unlike the υ dependence expected for a freely
diffusing species. The difference between the anodic and
cathodic peak potentials, ∆EP, was ca. 30 mV, and the full width
at half-maximum was approximately 110 mV, i.e., about 20
Luminescent lifetimes were measured using the third har-
monic (355 nm) of a Spectron Q-switched Nd:YAG laser for
excitation. Emission was detected in a right-angled configu-
ration to the laser using an Oriel model IS520 gated intensified
CCD coupled to an Oriel model MS125 spectrograph. With
suitable signal averaging, this configuration allows a complete
emission spectrum (spectral range 250 nm) to be obtained within
times as short as 2 ns. The monolayer emission intensity was
weak, and transients were typically recorded using the average
of 20 laser shots. The gate width, i.e., the exposure time of the
CCD, was never more than 2% of the excited-state lifetime.
The step size, i.e., the time between the acquisition of discrete
spectra, was typically 5% of the excited-state half-life. Where
necessary, monolayer emission spectra were smoothed using
an eight-point Savitsky-Golay algorithm.
2
9,30
mV larger that the ideal value.
These observations suggest
that the Qbpy complex adsorbs onto the surface of a platinum
microelectrode to give an electroactive film. Experiments
2
+
conducted using a complex, [Ru(bpy)3] , that did not contain
any unbound nitrogens did not form stable spontaneously
adsorbed monolayers.
The peak height and area of the wave centered at 1.15 V do
not change by more than 10% when cycled repeatedly over a 5
h period at temperatures up to 40 °C. This behavior indicates
that this redox couple is electrochemically reversible and that
the monolayers are stable for long periods even at relatively
elevated temperatures. Reversible responses for the ligand-
based reductions were obtained provided that the electrolytic
solvent was distilled and dried.
Dilute solutions of complex in acetonitrile (RT) or EtOH/
The Faradaic charge associated with converting the monolayer
from the 2+ to the 3+ oxidation states has been estimated from
the area under the wave centered at 1.15 V after correcting for
double-layer charging. This charge, together with the real
surface area of the electrode, has been used to calculate the
-
4
-5
MeOH 4:1 (77 K) (10 -10 M) were deaerated for 20 min
under nitrogen prior to use. Low-temperature emission lifetime
studies were carried out using an Oxford Instruments gas-
exchange cryostat equipped with a Thor 3030 temperature
controller. Standard iterative techniques were employed to
determine the lifetimes of emission. Absolute error on emission
lifetimes are (6%.
2+
surface coverage, or the number of moles of [Ru(bpy)2Qbpy]
2
per cm . The limiting surface coverage, Γ, was (1.05 ( 0.1)
-10
-2
×
10
mol cm , corresponding to an area occupied per
2
molecule of 158 ( 17 Å . Molecular modeling suggests that
the radius of the complex as dictated by the outermost edge of
the quarterpyridyl bridge is approximately 6.5 Å. This radius
would lead to a theoretical area of occupation that is ap-
proximately 20% smaller than that observed experimentally.
Allowing for charge-compensating counterions and a solvation
sheath, the experimental voltammetric data confirm that close-
packed monolayers are formed in which all of the adsorbates
are electrochemically active.
Results and Discussion
General Electrochemical Properties. Figure 1 shows a
2
+
representative cyclic voltammogram for a [Ru(bpy)2Qbpy]
monolayer, where the supporting electrolyte is 0.1 M tetrabu-
tylammonium tetrafluoroborate (TBABF4) in acetonitrile that
contains 50 µM of the complex. The formal potentials, E°′,
are 1.15, -1.09, -1.38, and -1.55 V. Strekas, Baker, and co-
workers previously reported the solution-phase voltammetry of

Ru(bpy)2Qbpy]2 Monolayers
+
J. Phys. Chem. B, Vol. 102, No. 49, 1998 10007
[
Figure 2. Room-temperature emission spectra for [Ru(bpy)
2
Qbpy]2
+
dissolved in 4:1 ethanol:methanol (s) and as a spontaneously adsorbed
monolayer (---). In the case of the monolayer, the contacting solvent
is 4:1 ethanol:methanol. The y-axes are normalized relative to each
peak’s maximum emission intensity.
Figure 3. Emission transients for an ethanolic solution of [Ru-
2+
2
(bpy) Qbpy] (fine upper line of transient) and for a spontaneously
adsorbed monolayer (thick lower line of transient) in contact with 0.1
Monolayer Fluorescence. Figure 2 illustrates fluorescence
spectra obtained at 298 K for a platinum macroelectrode
modified with a [Ru(bpy)2Qbpy]2 monolayer where the solvent
is blank degassed 4:1 v/v ethanol:methanol and for a solution
of the same complex. In both cases the excitation wavelength
was 355 nm. The emission signal observed for the monolayer
is significantly lower in intensity than that observed for the
solution-phase species. Therefore, to facilitate comparison of
the spectral features of the solution phase vs bound species,
the y-axis has been normalized relative to their respective peak
intensities at λmax. The monolayer surface coverage, as
M LiClO dissolved in ethanol. The inset shows semilog plots of the
4
responses.
+
lower energy by approximately 25 nm. The peak broadening
observed suggests slow dipolar relaxation around the excited
state. The shift of the emission to lower energy is somewhat
unexpected as the lower dielectric constant and less efficient
solvation of the polar excited state within the monolayer would
be anticipated to shift the emission spectrum to higher energy.
However, reaction entropies (vide infra) suggest that the more
highly charged states are stabilized on the electrode surface.
This shift is similar to that described previously by Bard and
Zhang for Langmuir-Blodgett monolayers incorporating func-
-
10
-2
measured by voltammetry, is 1.05 × 10
mol cm . It is
important to confirm that the monolayer fluorescence arises from
ruthenium centers that are intimately bound to the electrode
surface. For example, luminescence from solution-phase, or
physisorbed, species must be excluded. As discussed above,
in the absence of dissolved complex, the half-life of the
monolayer is approximately 12 h. However, even when
electrochemical measurements indicate that the monolayer has
fully desorbed, significant fluorescence cannot be detected from
the solution. This result suggests that the signal shown in Figure
2
+
33
tionalized [Ru(bpy)3] centers. These authors attributed the
shift to interactions between the adsorbates and between the
adsorbates and the electrode surface.
Figure 3 shows the emission transients obtained for the
complex dissolved in degassed ethanol and for an adsorbed
monolayer. Repeated application of the laser pulse onto the
modified surface causes the cyclic voltammetry response
2
+/3+
corresponding to the Ru
redox reaction to change, causing
2
does not come from diffusive species. A second possibility
the peak to become less well-defined and the peak current to
decrease. Thus, while 5 laser pulses typically cause the peak
current to decrease by less than 5% and the peak shape to remain
essentially unchanged, 50 pulses increase the fwhm to 150 (
is that the luminescence arises from physically adsorbed
material. We find that if the electrode is not carefully rinsed
with the electrochemical solvent after removal from the deposi-
tion solution then a different voltammetric response to that
illustrated in Figure 1 is observed. Specifically, a diffusion-
2
0 mV and decrease the peak current by 30%. Both thermal
desorption and photolysis of the adsorbed monolayer contribute
to these changes. To minimize the effect of laser-induced
changes on the measurements, data were collected only from
films exposed to less than 20 pulses. That photochemical
decomposition is observed for these monolayers suggests that
rapid quenching by electron transfer to the metal surface does
not occur efficiently in these systems. Where rapid charge
2
+/3+
like tail appears, and the charge associated with the Ru
redox reaction is between 5 and 10 times that expected for
monolayer coverage. This behavior suggests that, beyond a
chemisorbed monolayer, material remains physically adsorbed
to the surface. That physisorbed material can be detected
electrochemically makes it unlikely that the monolayers com-
prise an intimately bound electrochemically active, but ef-
ficiently quenched inner layer, and an electrochemically inactive,
but fluorescent, outer layer.
34
injection does occur, e.g., in the Graetzel cell, compounds that
are normally photosensitive become highly photostable upon
3
immobilization because the MC state that leads to decomposi-
Although it is difficult to estimate the quantum yield for
emission for the adsorbate, that luminescence is observed for
the monolayers indicates that the excited states are not com-
tion is not efficiently populated.
Figure 3 shows that the complex exhibits a relatively long-
lived excited-state both in solution and as an adsorbed species.
The inset of Figure 3 shows semilog intensity vs time plots.
The emission observed for the solution-phase reactant exhibits
the expected single-exponential decay behavior with a lifetime
31,32
pletely quenched by energy transfer to the electrode surface.
The two spectra are qualitatively similar, although the emission
spectrum for the monolayer shows a shoulder on the low-energy
side, and the wavelength of maximum emission is shifted to

10008 J. Phys. Chem. B, Vol. 102, No. 49, 1998
Forster and Keyes
of 1420 ( 60 ns. This excited-state lifetime is in excellent
agreement with the value of 1418 ns reported by Strekas and
2
0
Baker.
In contrast, the response observed for the monolayer is more
complex, and simple first-order kinetics are not followed. The
lifetimes were fit to a biexponential decay eq 1, using standard
nonlinear optimization procedures
-k1t
-k2t
I(t) ) Ae
+ Be
(1)
where A and B are preexponential factors and k1 and k2 are the
rates of each first-order decay. Such nonidealities are not
unusual for luminescent species immobilized within polymer
films or sol-gels and may reflect different local microenvi-
35
ronments around discrete sites within the monolayer. Excited-
state lifetimes of approximately 1.2 (63%) and 6.2 µs (37%)
are obtained at short and long times, respectively. Therefore,
beyond the nonidealities observed for the monolayers, Figure
2
+/3+
Figure 4. Current response for the Ru
redox reaction occurring
within an [Ru(bpy)
Qbpy]2 monolayer formed on a 5 µm platinum
microelectrode. From top to bottom on the left-hand side, the
overpotentials η are 100, 75, and 50 mV, respectively. The supporting
4 F
electrolyte is 0.1 M TBABF in acetonitrile. The inset shows ln i (t)
+
2
3
indicates that at least some of the immobilized complexes
exhibit increased excited-state lifetimes compared to their
lifetime in solution. It is likely that this behavior arises because
of decreased freedom of rotation when the complex is ad-
sorbed.36 This interpretation is supported by excited-state
lifetimes measured for the complex in frozen ethanol:methanol
glass at cryogenic temperatures where the lifetime is ap-
proximately 6 µs. Moreover, that the excited-state lifetimes of
solution-phase and monolayer reactants are different discounts
an emission mechanism in which the impinging laser pulse
causes rapid thermal desorption of the adsorbates, which then
undergo emission in solution before readsorbing onto the
electrode surface.
Heterogeneous Electron-Transfer Dynamics. As discussed
in the Introduction, in trying to probe the redox properties of
electronically excited states, weak electronic coupling between
the adsorbate and the electrode is desirable in order to prevent
quenching of the excited state by the metal surface. However,
as revealed by the emission lifetimes, the redox properties of
the excited states within the monolayer must be probed on a
microsecond time scale. This short time scale demands the use
of microelectrodes and ultrafast electrochemical techniques. An
even more important consideration is whether the rate of
heterogeneous electron transfer is sufficient to switch the
oxidation state of the monolayer within a few microseconds.
vs t plots for the Faradaic reactions. From top to bottom on the
right-hand side, the overpotentials η are 50, 75, and 100 mV,
respectively.
domain two current decays can be separated. These responses
are time-resolved owing to the shorter time constant of double-
layer charging compared to that of the Faradaic reaction. In
these investigations, the electron-transfer rate constant has been
determined only when the time constant of double-layer charging
is at least five times shorter than the time constant of the
Faradaic reaction. This condition has been satisfied by carefully
selecting the radius of the microelectrode employed.
Beyond the issue of fast double-layer charging, it is important
to consider the effect of ohmic drop on the observed current-
6
time transients. When Faradaic and charging currents flow
through a solution, they generate a potential that acts to weaken
the applied potential by an amount iR, where i is the total current
flow through the solution and R is the total cell resistance. This
ohmic drop can lead to severe distortions of experimental
responses resulting in inaccurate measurements of the hetero-
geneous electron-transfer rate. As illustrated in Figure 4, the
Faradaic currents that flow in these high-speed chronoampero-
metric experiments are typically in the low microampere range
even for 5 µm radius electrodes. Given that the cell resistance
in this experiment is approximately 15 000 Ω, the average iR
drop is less than 70 mV for this system. We have used three
strategies to minimize the effects of uncompensated resistance
in the large potential step experiments. First, we use a relatively
high supporting electrolyte concentration (g0.1 M). Second,
we use smaller electrodes to measure k at large overpotential.
This approach is useful since the resistance increases with
decreasing electrode radius, but the current decreases as the
square of the radius leading to reduced ohmic effects for smaller
electrodes, e.g., the maximum ohmic loss observed at the highest
overpotential employed is reduced from approximately 3000
mV at a 12.5 µm microelectrode to 240 mV at a 1 µm
microelectrode. Third, and perhaps most importantly, we extract
rate constants only from data obtained relatively late in the
lifetime of the current decay transient, i.e., when the anticipated
iR drop is less than 15 mV.
Therefore, we have probed the dynamics of heterogeneous
electron transfer for the Ru2
ligand-based couples.
+/3+
-and, more importantly, the
Chronoamperometry. For an ideal electrochemical reaction
involving a surface-bound species, the Faradaic current follow-
ing a potential step that changes the redox composition of the
monolayer exhibits a single-exponential decay in time according
2
6,28,37
to
iF(t) ) kQ exp(-kt)
(2)
where k is the apparent rate constant for the overall reaction
and Q is the total charge passed in the redox transformation.
Figure 4 illustrates typical examples of the chronoampero-
2
+
-
3+
metric response observed for the Ru - e f Ru redox
reaction of an [Ru(bpy)2Qbpy]2 monolayer formed on a 5 µm
radius platinum microelectrode, where the electrolyte is 0.1 M
TBABF4 in acetonitrile. In these experiments, the potential was
stepped from 0.600 V to potentials, E, that were 50, 75, and
+
The linearity of the semilog plots shown in the inset of Figure
4 indicates that heterogeneous electron-transfer associated with
2
+
oxidizing an [Ru(bpy)2Qbpy] monolayer is a first-order
process. Since uncompensated resistance causes the applied
potential, and hence the apparent rate, to evolve with time,
nonlinear responses would be expected if substantial ohmic drop
1
00 mV more positive than the formal potential E°′. These
transients are structured, and the response observed for η ) 50
mV (η ≡ E - E°′) shows that on a low-microsecond time

Ru(bpy)2Qbpy]2 Monolayers
+
J. Phys. Chem. B, Vol. 102, No. 49, 1998 10009
[
Our interest in the dynamics of the oxidation of the reduced
ligand arises because there is a considerable body of work
available that indicates that the same orbital is populated by
2
4,38,39
ligand-based reduction and photoexcitation.
Therefore,
•-
the rate at which the reduced bpy ligand, bpy , can be
reoxidized is pivotal to probing the dynamics of electronically
excited states since photochemical excitation initially promotes
an electron into the same singlet orbital. Thus, while recogniz-
ing that the electronic charge on the two reactants differ by a
whole unit and that emission occurs from a triplet state, one
•
-
might expect the oxidation kinetics for bpy and the electroni-
cally excited state to be similar. Strekas and Baker20 report
excited-state lifetimes of 1418 and 63 ns for the [Ru-
2
+
4+
(
bpy)2Qbpy] and [Ru(bpy)2Qbpyme2] complexes, respec-
tively, where Qbpyme2 is the methylated derivative of the
quarterpyridyl ligand. Photoinduced electron transfer from the
bipyridyl-based excited state to the viologen electron acceptor
site is a likely mechanism for this significant reduction in
excited-state lifetime. The rate of this excited-state electron
transfer can be calculated according to eq 3
Figure 5. Tafel plots where 9 and 2 denote experimental data for the
+/2+ and 2+/1+ redox couples, respectively. The supporting
electrolyte is 0.2 M TBABF in acetonitrile. The errors on the rate
3
4
constants are approximately equal to the size of the symbols.
1
1
kET
)
-
(3)
effects were present. Beyond the issue of iR drop, the linear
responses illustrated in Figure 4 indicate that the local microen-
vironments and electron-transfer distances are similar for
individual adsorbates within the monolayer. Given that the
absolute value of the slope of the semilog plot represents the
rate constant, it is apparent that heterogeneous electron transfer
occurs very rapidly in this system. For example, even for a
modest overpotential of 50 mV, the heterogeneous electron-
transfer rate constant, k, for oxidation of the ruthenium centers
is (1.3 ( 0.2) × 10 s . As given by eq 2, the intercept of
these semilog plots equals ln(kQ). The total charge passed, as
calculated using these intercepts, agrees with the slow scan
voltammetric data to within 10% confirming that all of the
adsorbates are electrochemically active on a microsecond time
scale.
τ[Ru(bpy)
4+
τ[Ru(bpy)
(Qbpy)]
2+
(Qbpyme )]
2
2
2
where τ[Ru(bpy)2(Qbpyme2)]4 and τ[Ru(bpy)2Qbpy]2+ are the
lifetimes for the quaternized and free pyridine complexes. This
analysis yields an electron-transfer rate of 1.5 × 10 s where
the driving force is approximately 0.32 V. When the data in
Figure 5 are extrapolated to this driving force, a rate constant
+
7
-1
8
-1
of 6.6 × 10 s is obtained for the ground-state oxidation of
6
-1
•-
the bpy species. Given the limitations of these calculations
and the complexity of the reaction in question, this analysis
suggests that the oxidation of the ground-state bpy anion radical
is a reasonable model for oxidative excited-state electrochem-
istry. However, a more reliable conclusion is that the rate of
heterogeneous electron transfer from the excited-state to the
electrode ought to be sufficiently fast to allow an excited state
voltammetric response be observed; i.e., the time constant for
electron transfer from the excited state is approximately 300 ns
under conditions of zero driving force compared to an emission
lifetime of approximately 6 µs.
This monolayer is somewhat unusual in that the electronic
coupling between the emitting state and the electrode surface
is sufficiently weak so that complete quenching of the electroni-
cally excited states by energy transfer to the metal does not
occur, yet the coupling is sufficiently strong to support
heterogeneous electron transfer on a submicrosecond time scale.
An insight into this balance can be achieved by determining
Potential Dependence of k. One of the great advantages of
using redox-active monolayers as model systems for under-
standing those factors that control electron-transfer rates is the
6
ability to tune the driving force for the reaction. We have
probed how k depends on driving force for both metal- and
ligand-based redox reactions. Figure 5 illustrates Tafel plots
of ln k vs overpotential, η, for the 3+/2+ and 2+/1+ redox
reactions, where the supporting electrolyte is 0.2 M TBABF4
in acetonitrile. This figure shows that ln k depends ap-
proximately linearly on η for the range of overpotentials
investigated. This behavior is consistent with the Butler-
6
Volmer formulation of electrode kinetics with the slopes being
equal to -RcnF/RT and (1 - Ra)nF/RT, for the reduction and
oxidation processes, where Rc and Ra are the cathodic and anodic
transfer coefficients, respectively. The Tafel slopes yield Rc
and Ra values of 0.49 ( 0.05 and 0.51 ( 0.05, respectively.
This behavior would not be expected if the determination of
the heterogeneous electron-transfer rate constant were adversely
affected by ohmic losses. That both transfer coefficients equal
the value of 0.5 expected for a reversible reaction indicates that
the energy barrier to electron transfer is highly symmetrical.
The standard heterogeneous electron-transfer rate constant,
k°, has been determined by linearly extrapolating ln k to zero
overpotential. The standard heterogeneous rate constant depends
on the redox process being probed, and while a k° of (5.1 (
the electronic transmission coefficient, κ , describing the
probability of electron transfer from the transition state. One
cannot assume that differences in the electronic transmission
coefficients alone cause the observed differences in the standard
rate constants since both preexponential and free energies of
el
3
7
activation influence k°. As discussed previously by Chidsey,
40
41
Creager, and Murray, the reorganization energy λ and hence
κel can be determined by modeling Tafel plots. However, the
upper limit on the heterogeneous electron-transfer rate that is
7
-1
measurable with our current system is of the order of 10 s .
Therefore, as illustrated in Figure 5, it is possible to determine
k only over a limited range of overpotentials. The accessible
range of η is approximately (75 mV for the bpy-based
reactions, which is insufficient for an accurate determination
5
-1
0
.8) × 10 s is observed for the metal-based 3+/2+ reaction,
a significantly higher standard rate constant of (3.0 ( 0.5) ×
of λ. An alternate approach that we have employed previ-
6
-1
26,28
1
0 s is observed for the ligand-based reduction.
ously
is to explicitly measure the enthalpy and entropy of

10010 J. Phys. Chem. B, Vol. 102, No. 49, 1998
Forster and Keyes
TABLE 1: Standard Rate Constants, Activation Parameters,
and Preexponential Factors for Metal- and Ligand-Based
n+
Electron-Transfer Reaction within [Ru(bpy)
2
Qbpy]
Monolayersa
q
q b
-6
et
cs-1
∆
H ,
∆G
c
,
10
A ,
charge
10-5k°, s-1
kJ mol
-1
kJ mol
-1
3+/2+
2+/1+
1+/0
5.1(0.3)
30.2(1.3)
34.2(2.2)
12.1(1.7)
7.5(0.8)
8.9(1.1)
6.8(0.4)
6.3(0.5)
7.6(0.9)
7.9(1.1)
38.2(1.9)
73.6(3.6)
a
Numbers in parentheses represent the standard deviations for at
least three individual monolayers. Supporting electrolyte is 0.2 M
b
4
TBABF in acetonitrile. Free energy of activation determined from
the cathodic ideal electrochemical enthalpies as measured from the
temperature dependence of the heterogeneous rate constant and the
c
reaction entropy, ∆Src°. Preexponential factor extracted from the
q
standard heterogeneous electron-transfer rate constant using ∆G
c
.
activation by investigating the temperature dependence of k and
E°′, respectively.
Figure 6. Cyclic voltammetry of a 5 µm radius platinum microelec-
Enthalpy and Entropy of Activation. The electrochemical
enthalpy of activation has been determined from an Arrhenius
2+
2
trode modified with an [Ru(bpy) Qbpy] monolayer following laser
excitation at 355 nm. The scan rate is 4300 V/s, the surface coverage
-
1
-10
-2
plot of ln k vs T . The rate constant was measured using an
overpotential of 50 mV as determined at 298 K. Also, the
entropy of activation was measured from the temperature
dependence of the formal potential. In both cases, the experi-
is 1.1 × 10
mol cm , and the supporting electrolyte is 0.1 M
TBABF in acetonitrile. The initial potential is -1.2 V.
4
emission spectra obtained at cryogenic temperatures. The
excited-state oxidation potential obtained in this way using the
data presented in Figures 1 and 2 is approximately -0.67 V.
However, this value is only an estimate of the true excited-
state redox potential and typically contains uncertainties of 100
mV or more. For example, in the case of ruthenium and
osmium polypyridyl complexes, the vibrational fine structure
is never well-resolved, making it difficult to accurately deter-
25,26,28
ments and analysis was performed as described previously,
and the results are presented in Table 1. This table clearly show
that the enthalpic and entropic barriers to electron transfer
depend on the identity of the redox couple. However, a
significant observation is that there is mutual compensation
between these two contributions such that experimentally
indistinguishable free energies of activation are observed for
the ruthenium- and ligand-based redox processes.
0
-0
mine E
.
Preexponential Factor. That indistinguishable values of
G are observed for both metal- and ligand-based processes
is significant given the differences in their charge states. Table
The inherent inaccuracies in using ground-state parameters
to approximate excited-state characteristics makes a direct
measurement of the excited-state redox properties invaluable.
Moreover, direct measurement of E* offers a means of
unequivocally confirming that the redox and spectroscopic
orbitals are the same, thereby confirming the validity of the
Rehm-Weller equation for determining excited-state potentials.
q
∆
1
contains values of Aet (≡κelυn) that have been determined using
q
the experimental values of ∆G and k°. Previous studies on
related systems2
6,28
indicate that the frequency factor for
1
2
-1
heterogeneous electron transfer is of the order of 10 s .
43
Therefore, for both metal- and ligand-based processes, κel is
Measurements of this type were first reported by Bock et al.,
-
5
-6
2+
considerably less than unity (of the order of 10 -10 )
indicating weak electronic coupling between the redox centers
and the electrode. Under these circumstances, the electron
transfer is considered to be nonadiabatic. This weak coupling
is likely to be the reason energy transfer to the surface from
the electronically excited state does not completely quench the
monolayer emission.
where the excited-state oxidation potential of [Ru(bpy)3] was
indirectly estimated from kinetic studies of quenching by a series
5
of nitrobenzene complexes. However, Jones and Fox have
described how phase-modulated voltammetry can be used to
directly measure the excited-state redox potential. This tech-
nique is essentially a steady-state method and is applicable only
when the excited-state lifetime exceeds 500 ns.
Excited-State Electrochemistry. Since the time constant
for heterogeneous electron transfer from the reduced ligand is
considerably shorter than the fluorescence lifetime, it ought to
be possible to probe the dynamics of electron transfer to, and
from, the excited-state species. However, having established
the time scale demands, it is important to consider where on
the potential axis a voltammetric response corresponding to the
electronically excited state would occur. The most commonly
used method to estimate the excited-state oxidation potential is
the Rehm-Weller equation,42 which is given by eq 4 for
oxidative quenching of an excited state.
Figure 6 shows the 4300 V/s voltammetric response of an
2+
[Ru(bpy)2Qbpy] monolayer formed on a 5 µm radius platinum
microelectrode. The voltammetric scan was optically triggered
so that it commenced simultaneously with the arrival of the
incident laser pulse on the electrode surface. The potential limits
2
+
are chosen so that the monolayer exists in the Ru state prior
to photoexcitation and allows the excited-state response expected
at approximately -0.67 V to be captured. Given that this initial
potential is negative of the excited-state oxidation potential,
quenching of the excited state by electron transfer to the
electrode surface is not expected. Beyond a small current
oscillation at approximately -0.62 V, the voltammetry is
entirely consistent with that observed in the absence of laser
excitation indicating that pulsing the laser does not cause
artifacts in the cyclic voltammetry, at least not at this scan rate.
This behavior is entirely expected since the voltammetric scan
rate is too slow to capture the excited state response prior to its
return to the ground state by emission or quenching. However,
+
+
0-0
E(A /A*) ) E(A /A) - E
(4)
+
-
0-0
where E(A /A ) is the potential for the first reduction and E
is the energy difference between the lowest vibrational levels
+
-
of the ground and excited states. E(A /A ) can be obtained
0
-0
from ground-state solution-phase voltammetry and E
from

Ru(bpy)2Qbpy]2 Monolayers
+
J. Phys. Chem. B, Vol. 102, No. 49, 1998 10011
[
Figure 7. Conditions as in Figure 6 except that the scan rate is 3 ×
Figure 8. Conditions as in Figure 6 except that the scan rate is 22 000
V/s.
5
1
0 V/s. The dashed line illustrates the voltammetric response obtained
for a bare electrode under identical conditions.
tion. Taking into account the time taken to scan the potential
from its initial value to the excited-state peak potential, this
result suggests that close to 100% of the adsorbed complexes
were excited during the laser pulse.
this response confirms that impinging a laser pulse onto the
monolayer surface does not significantly alter the ideality of
the voltammetry.
Figure 7 shows the response observed at a scan rate of 3 ×
It is significant that the area under the anodic peak at -0.42
V is larger than the cathodic peak observed at approximately
0.7 V. This observation is consistent with the heterogeneous
electron-transfer rate constants discussed previously. These data
5
1
0 V/s immediately after photoexcitation. Now the voltam-
metric time scale is shorter than the excited-state lifetime, and
voltammetry characteristics of the electronically excited state
can be obtained. This figure shows that on the first scan an
oxidative current response is observed at approximately -0.42
V. The peak potential of this process is in approximate
agreement with the calculated excited-state redox potential of
0.67 V. That this current response is not observed for the
second or subsequent scans confirms the transient nature of the
phenomenon.
This figure also illustrates the voltammetric response obtained
for a bare microelectrode. That a flat featureless voltammogram
is obtained confirms that currents induced by thermal, photo-
electric, or photoacoustic effects, e.g., through double-layer
restructuring, are absent at this time scale. However, chrono-
amperometry experiments in which the current is measured on
a submicrosecond time scale indicate that for both bare and
monolayer-modified microelectrodes, a significant photoinduced
current is observed. This current decays approximately expo-
nentially over time but with a significant damped oscillation
superimposed on the decay. The time scale of this decay
depends on the applied potential and is more rapid for the
monolayer-coated electrodes; e.g., at a potential of -1.0 V, the
decay is complete within 100 ns for the monolayer-coated
electrode but takes 300 ns for the unmodified surface.
•
-
indicate that k° for the bpy oxidation was approximately a
2
+/3+
factor of 6 larger than that found for the Ru
reaction.
Therefore, we anticipate more facile kinetics for the oxidation
of the excited state and a consequently larger charge than for
2
+/3+
-
the Ru
reaction. Indeed, the large peak-to-peak separation
2
+/3+
observed for the Ru
redox reaction confirms that the
voltammetric response is significantly influenced by the dynam-
ics of heterogeneous electron transfer. The area under the
cathodic peak located at approximately 0.7 V increases by less
than 10% between the first and successive scans indicating that
3
+
the number of Ru centers on the electrode surface does not
change significantly between scans.
A complication in the voltammetry shown in Figure 7 is that
the product obtained by oxidizing the electronically excited state
3+
is the ground-state oxidized product [Ru(bpy)2Qbpy] . The
complication arises because this ruthenium 3+ species is created
at a potential that is approximately 1.5 V negatiVe of the ground-
state formal potential. Under these circumstances, one would
expect that the ground-state product would be rapidly reduced.
Thus, as shown in the following scheme, the overall reaction
would consist of removal of an electron from the electronically
excited species followed by electron injection into the ground-
state oxidized product, i.e., two opposing electron transfers.
An important test of internal self-consistency is to correlate
the amount of excited-state reactant with the value obtained from
ground-state cyclic voltammetry such as that illustrated in Figure
2+*
-
Qbpy]3+
[Ru(bpy)
irreversible excited-state oxidation Epeak ≈ -0.42 V)
Qbpy] - e f [Ru(bpy)
2
2
1
. The oxidative charge under the peak centered at -0.42 V
(
-
17
in Figure 7 corresponds to approximately 5.8 × 10
mol of
reactant on the electrode surface. Voltammetric measurements
conducted both before and after the excited-state voltammetry
indicate that there are 8.7 × 10 mol of reactant on the surface.
This value is consistent with that expected for a 5 µm radius
microelectrode after taking a surface roughness of 1.5 and a
surface coverage of 1.1 × 10 mol cm into account. While
recognizing the difficulties caused by other deactivation path-
ways and the influence of heterogeneous kinetics, that ap-
proximately 66% of the total surface coverage appears to
undergo an excited-state redox reaction is an important observa-
3+
-
2+
[
Ru(bpy) Qbpy] + e f [Ru(bpy) Qbpy]
2
2
-17
(irreversible reduction of ground-state oxidized product)
An insight into the reason this mechanism is not followed at
megavolt per second scan rates can be obtained by systemati-
cally varying the scan rate. Figure 8 shows the voltammetric
response observed at the slower scan rate of 22 000 V/s. In
this figure, a current oscillation of the type expected for opposing
oxidation-reduction reactions that are proceeding on compa-
-10
-2

10012 J. Phys. Chem. B, Vol. 102, No. 49, 1998
Forster and Keyes
rable time scales is observed. Although the exact rate constants
have not yet been determined, this response is consistent with
the rate constant for oxidation of the electronically excited state
being only about an order of magnitude smaller than reduction
of the ground-state product. This result is not expected on the
basis of the kinetic analysis presented earlier. The standard
the Irish Science and Technology Agency, under Basic Research
Grant SC/96/424, and the European Union Joule Program is
gratefully acknowledged. The generous loan of ruthenium
trichloride and platinum microwires by Johnson Matthey under
the loan scheme is deeply appreciated. We appreciate the many
insightful comments provided by the referee. This paper is
dedicated to Professor Allen J. Bard whose outstanding experi-
mental and theoretical contributions underpin this work.
2+/3+
heterogeneous rate constant for the Ru
reaction is 5.1 ×
0 s . Although too fast for us to determine experimentally,
given that the reorganization energy is approximately 0.3 eV
5
-1
1
q
(
Table 1, λ ) 4∆Gc ), one would expect a limiting rate constant
References and Notes
8
-1
of approximately 10
s
to be observed for very large
(
1) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.;
overpotentials. Given this large rate constant, we do not
understand why a significant Ru3 population remains on the
surface to generate the unusual current oscillation observed at
approximately -0.5 V.
M u¨ ller, E.; Liska, P.; Vlachopoulos, N.; Gr a¨ tzel, M. J. Am. Chem. Soc.
1993, 115, 6382.
+
(2) Knox, R. S. Photosynth. Res. 1996, 48, 35.
(3) Ward, M. D. Chem. Soc. ReV. 1997, 26, 365.
(4) Juris, A.; Balzani, V.; Barigelletti, Campagna, S.; Belser, P.; Von
Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85.
Conclusions
(5) Jones, W. E.; Foxe, M. A. J. Phys. Chem. 1994, 98, 5095.
(
6) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamen-
Ruthenium polypyridyl complexes that include a quarterpy-
ridyl ligand within their coordination shell are surface active
and yield highly stable monolayers. Five distinct oxidation
states are electrochemically accessible, and the electrochemical
responses are nearly ideal as the potential, temperature, and
experimental time scale are varied over a wide range. Chro-
noamperometry has been used to probe the rate of heterogeneous
electron transfer across the monolayer/microelectrode interface.
This process can be characterized by a single rate constant at
high electrolyte concentrations suggesting that heterogeneous
electron transfer across these metal/monolayer interfaces is
mechanistically uncomplicated.
tals and Applications; Wiley: New York, 1980.
(7) Forster, R. J. Ultrafast Electrochemical Techniques; Encyclopedia
of Analytical Chemistry; Wiley and Sons: New York, 1998.
(
(
8) Pflug, J. S.; Faulkner, L. R. J. Am. Chem. Soc. 1980, 102, 6144.
9) Pflug, J. S.; Faulkner, L. R.; Seitz, W. R. J. Am. Chem. Soc. 1983,
1
05, 4890.
(
10) Kuhn, H. J. Chem. Phys. 1970, 53, 101.
(11) Chance, R. R.; Prock, A. Sibley, R. AdV. Chem. Phys. 1978, 37, 1.
(12) Weitz, D. A.; Garoff, S.; Gersten, J. I.; Nitzan, A. J. Chem. Phys.
983, 78, 5324.
13) Wokuan, A.; Lutz, H.-P.; King, A. P.; Wild, U. P.; Ernst, R. R. J.
Chem. Phys. 1983, 79, 509.
14) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem., 1978,
17, 3334.
15) Morgan R. J.; Baker, A. D. J. Org. Chem, 1990, 55, 1986.
16) Xu, C. Ph.D. Thesis, University of Illinois at Urbana-Champaign,
992.
17) Faulkner, L. R.; Walsh, M. R.; Xu, C. Contemporary Electroana-
lytical Chemistry; Plenum Press: New York, 1990.
(18) Trasatti, S.; Petrii, O. A. J. Electroanal. Chem. 1992, 327, 354.
19) Yee, E. L.; Cave, R. J.; Guyer, K. L.; Tyma, P. D.; Weaver, M. J.
J. Am. Chem. Soc. 1979, 101, 1131.
20) Bierig, K.; Morgan, R. J.; Tysoe, S.; Gafney, H. D.; Strekas, T.
C.; Baker, A. D. Inorg. Chem. 1991, 30, 4898.
(21) Griffith, W. P. In ComprehensiVe Coordination Chemistry; Wilkin-
son, G., Ed.; Pergamon: Oxford, England, 1987; Vol. 4, Chapter 46.
22) Sullivan, B. P.; Conrad, D.; Meyer, T. J. Inorg. Chem. 1985, 24,
640.
23) Tokel-Takvoryan, N. E.; Hemmingway, R. W.; Bard, A. J. J. Am.
Chem. Soc. 1973, 95, 6582.
(24) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser P.;
Von Zelewsky, A., Coord. Chem. ReV., 1988, 84, 85.
25) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5444,
26) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5453.
27) Forster, R. J.; Faulkner, L. R. Langmuir 1995, 11, 1014.
28) Forster, R. J.; O′Kelly, J. P. J. Phys. Chem. 1996, 100, 3695.
(29) Laviron, E. J. Electroanal. Chem. 1974, 52, 395.
(30) Brown, A. P.; Anson, F. C. Anal. Chem. 1977, 49, 1589.
31) Kalyanasundaram, K. Coord. Chem. ReV. 1982, 46, 218.
(32) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 5007.
33) Zhang, X, Bard, A. J. J. Phys. Chem. 1988, 92, 5566.
34) O’Regan, B.; Graetzel, M. Nature 1991, 353, 737.
1
(
(
(
(
Significantly, upon laser excitation at 355 nm the monolayers
fluoresce; i.e., despite having the excited state located close to
a metal surface, energy transfer does not completely quench
the excited state. Moreover, the excited-state lifetime of the
adsorbate is approximately three times longer than that found
for the complex in solution and is indistinguishable from that
observed for the complex in an ethanol:methanol glass at
cryogenic temperatures. In the first report of its kind, we have
attempted to probe the energetics and dynamics of excited-state
oxidation using voltammetry conducted at scan rates on the
megavolts per second range. The voltammetric data reported
are qualitatively consistent with the response anticipated for an
electronically excited state in terms of the behavior of bare
electrodes, as well as the peak potential and voltammetric time
scale over which the transient response is observed for mono-
layer-modified electrodes. Specifically, although influenced by
the dynamics of heterogeneous electron transfer, the measured
excited-state redox potential agrees with that predicted by the
Rehm-Weller equation to within 180 mV. Fast-scan voltam-
metry reveals that the heterogeneous electron-transfer rate
constant is larger for the excited-state than for the ground-state
1
(
(
(
(
3
(
(
(
(
(
(
•
-
bpy anion radical, bpy , suggesting that the redox and
spectroscopic orbitals may not be identical. We expect that
further studies including electrochemical Stern-Volmer plots
and the effects of separating the excited states using diluents
will provide further insight into this challenging and exciting
area. Beyond the useful insight into the energetics and dynamics
of excited-state quenching, we are exploring the use of
luminescent monolayers as fast-responding active displays and
in trace analysis.
(
(
(
35) Kalyanasundaram, K. In Photochemistry in Organized and Con-
strained Media; Ramamurthy, V., Ed.; VCH Publishers: New York, 1991.
(36) Reisfeld, R. J. Non-Cryst. Solids 1990, 121, 254.
37) Chidsey, C. E. D. Science 1991, 251, 919.
38) Dodsworth, E. S.; Lever, A. B. P. Chem. Phys. Lett. 1986, 124,
52.
39) Vlcek, A. A.; Dodsworth, E. S.; Pietro, W. J.; Lever, A. B. P. Inorg.
Chem. 1995, 34, 1906.
(
(
1
(
(
40) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307.
(
41) Tender, L.; Carter, M. T.; Murray, R. W. Anal. Chem. 1994, 66,
Acknowledgment. R.J.F. gratefully acknowledges Professor
Larry R. Faulkner of the University of Texas at Austin for the
generous loan of the high-speed potentiostat that made part of
this work possible. Financial support from Enterprise Ireland,
3
173.
(
(
42) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259.
43) Bock, C. R.; Meyer, T. J.; Whitten, D. J. J. Am. Chem. Soc., 1975,
97, 2909.