Sensitized electroluminescence on mesoporous oxide Semiconductor films

Article abstract of DOI:10.1021/jp962192j

Forward biasing of a mesoporous TiO₂ (anatase) film whose surface is derivatized with a monolayer of the phosphonated ruthenium bipyridyl complex RuL₂La?2 with L = 4,7-diphenyl-1,10-phenanthroline and La?2 = 2,2a?2-bipyridine-4,4a?2-

Full text of DOI:10.1021/jp962192j

2558
J. Phys. Chem. B 1997, 101, 2558-2563
Sensitized Electroluminescence on Mesoporous Oxide Semiconductor Films†
Yordan Athanassov, Franc¸ois P. Rotzinger, Pe´ter Pe´chy, and Michael Gra1tzel*
Institut de Chimie Physique, Ecole Polytechnique Fe´de´rale de Lausanne, CH-1015 Lausanne, Switzerland
ReceiVed: July 18, 1996; In Final Form: October 7, 1996X
Forward biasing of a mesoporous TiO2 (anatase) film whose surface is derivatized with a monolayer of the
phosphonated ruthenium bipyridyl complex RuL2L′ with L ) 4,7-diphenyl-1,10-phenanthroline and L′ )
2,2′-bipyridine-4,4′-diphosphonic acid produces electroluminescence in the presence of an electrolyte containing
peroxodisulfate in dimethylformamide. Light emission arises from the MLCT excited state of the complex
which is generated directly through interfacial electron transfer from the conduction band of the semiconductor
to the π* orbital of the oxidized Ru(III) state of RuL2L′. The nanocrystalline morphology of the oxide film
used here as electron injection electrode plays an important role in enhancing the electroluminescence process.
Introduction
210 or 230 °C instead of 200 °C. Spectroelectrochemical
measurements employed transparent electrodes prepared by
spreading the nanocrystalline TiO2 autoclaved at 210 °C on a
glass substrate coated with transparent conducting oxide (TCO),
i.e., fluorine doped SnO2. The TCO glass was obtained from
Nippon Sheet Inc. (sheet resistance, 10 Ω/square; optical
transmission, >80% in the visible). After air-drying, the
electrode was fired for 30 min at 450 °C in air. The resulting
film thickness was ca. 5 µm measured by profilometry. X-ray
diffraction established that the colloidal TiO2 film had 100%
anatase crystal structure. SEM studies showed the film to be
mesoporous from the outer layers to the TCO back contact with
a relatively uniform particle size of 15 nm in diameter. Electro-
and photoluminescence experiments employed titanium sub-
strates (plates or rods of 0.5 cm diameter and 1.2 cm height) to
support the nanocrystalline TiO2. In this case the colloid was
autoclaved at 230 °C. Prior to deposition of the TiO2 paste,
the titanium was cleaned by sonication in a detergent (Kieselgur)
and a subsequent rinse with water. Prior to coating of the
electrodes, Scotch tapes were applied in order to delimit the
film area. The paste was then applied to the plate and spread
by means of a glass cylinder. To coat the titanium rod, it was
inserted in a perforated titanium sheet, the top of the rod being
made coplanar with the plate. After spreading the paste, the
electrode was withdrawn from the hole and allowed to dry in
air at room temperature followed by annealing at 500 °C in an
oxygen atmosphere for 30 min. The resulting film was again
approximately 5 µm thick, and it was composed of particles
with an average diameter of 20 nm.
Nanocrystalline semiconductors are the focus of many recent
investigations.1-7 Mesoporous films of these materials are
distinguished by a very high internal surface area, the roughness
factor defined as the ratio between the real and projected surface
of the film being about 1000 for an 8 µm thick layer. Pores in
the nanometer size range are constituted by the voids present
between the semiconductor particles. These are interconnected
and are filled with an electrolyte or a solid conductor or
semiconductor. Due to the very large internal surface area, these
systems offer a number of intriguing features which have been
exploited for the design of improved optoelectronic devices.
Thus, dye sensitized mesoscopic oxide films have shown
strikingly high photovoltaic conversion efficiencies.8-11 The
transparent nature of these films allows for the direct monitoring
of electron transfer processes by spectroscopic means. A new
field for which the term “optical electrochemistry” has been
coined is emerging from these investigations.12-14 TiO2 has
so far been the most studied material for nanocrystalline
electrodes, but other materials have also been investigated, such
as Fe2O3,15 ZnO,5,14 and CdSe/CdS3.
The investigation presented here takes advantage of the
unique morphology of nanocrystalline semiconductor junctions
to induce electroluminescence from a light emitting dye that is
anchored as a monolayer through phosphonate groups to the
surface of the oxide. It will be shown that the excited state of
the dye is produced directly by interfacial electron transfer from
the conduction band of the TiO2 into the π* orbital of the
diphenylphenanthroline ligand of RuL2L′, the Ru being pro-
moted to the +III oxidation state through prior oxidation of
the divalent complex by peroxodisulfate. The high surface area
of the films enables this process to take place under mild
accumulation conditions, where energy transfer quenching of
the excited dye by the conduction band electrons present at the
oxide surface is inefficient.
The RuL2L′ was deposited onto the nanocrystalline TiO2 film
by overnight immersion of the electrode in a 2.5 × 10-5
M
dye solution in a mixed solvent of dimethyl sulfoxide (DMSO)
and EtOH (20/80, v/v). The volume of the liquid was adjusted
to 5 mL/cm2 of geometric electrode surface area. The apparent
surface concentration of the dye measured by absorption
spectroscopy was (2.1 ( 0.2) × 10-8 mol/cm2. In order to
ascertain that the dye remained anchored to the TiO2 during
the electrochemical experiments, the nanocrystalline electrode,
after loading with RuL2L′ and rinsing with ethanol, was
immersed overnight in 5 mL of electrolyte consisting of 100
mM tetrabutylammonium tetrafluoroborate ((TBA)BF4) in di-
methylformamide (DMF). At this time the dye concentration
in the electrolyte determined by fluorescence analysis indicated
that less than 1% of the RuL2L′ had desorbed after equilibration.
One infers from this observation that the RuL2L′ is strongly
and irreversibly attached to the mesoporous TiO2 film. Dye
Experimental Section
Nanocrystalline Film Preparation. Nanocrystalline titanium
dioxide was prepared by hydrolysis of titanium tetraisopropoxide
as reported elsewhere.8 The only modification from the original
procedure was that the autoclaving temperature was generally
† In commemoration of the life and scientific achievements of Prof. Heinz
Gerischer.
* Author to whom correspondence should be addressed.
X Abstract published in AdVance ACS Abstracts, March 1, 1997.
S1089-5647(96)02192-X CCC: $14.00 © 1997 American Chemical Society

Mesoporous Oxide Semiconductor Films
J. Phys. Chem. B, Vol. 101, No. 14, 1997 2559
uptake by the film was found to be limited to the amount
corresponding to monolayer coverage, any RuL2L′ in excess
of this quantity remaining in solution. Moreover, the RuL2L′
remained associated with the electrode even when the latter was
held for several hours at -1.5 V, where reduction of the dye
occurs. This distinguishes phosphonated bipyridyl complexes
of ruthenium from carboxylated ones which often desorb from
TiO2 films upon reduction.
allowed to cool, and insoluble impurities were removed by
filtration. After the addition of 7 mL of H2O, the solution was
refluxed for 5 h. Then, 7 mL of concentrated HCl was added,
and the mixture was refluxed for 1 day. The EtOH was
removed, the solution allowed to cool, and the product isolated
by filtration and washed with H2O. Finally, the product was
refluxed for 2-3 h in acetone, filtered, and washed with acetone.
This product is free (<1%) of RuL3, RuLL′2, and RuL′3 on the
1
basis of H-NMR measurements. 1H-NMR (400 MHz, CD3-
Materials. The ligand 2,2′-bipyridine-4,4′-bis(diethylphos-
phonate) was prepared, using an extension of the pyridine-3-
phosphonic acid diethyl ester synthesis by Hirao et al.16, from
4,4′-dibromo-2,2′-bipyridine (0.47 g, 1.5 mmol, synthesized as
described by Maerker and Case17) with palladium tetrakis-
(triphenylphosphine) (0.18 g, 0.15 mmol), diethylphosphite (0.56
g, 4.1 mmol), and triethylamine (0.42 g, 4.15 mmol) measured
into an argon filled flask, equipped with a cold finger and
magnetic stirrer. Heated on a 98 °C oil bath for 3 h, the reaction
was monitored with TLC on silica gel plates: DCM:MeOH,
10:1; Rf(1) ) 0.64. To the solidified mixture, cooled back to
room temperature, dichloromethane:methanol was added (8 mL,
1:1), stirred for 10 min, and evaporated to dryness under reduced
pressure. The residue was chromatographed on a silica gel
column (gradient elution with DCM:MeOH), the fractions
containing 2,2′-bipyridine-4,4′-bis(diethylphosphonate) were
evaporated to dryness (0.27 g, 53% (corrected for the unreacted
4,4′-dibromo-2,2′-bipyridine)). From earlier fractions unreacted
starting material was recovered (0.1 g). 1H-NMR (200 MHz,
CDCl3): δ 1.36 (12H, t, 7 Hz), 4.20 (8H, m), 7.72 (2H, ddd,
14 Hz, 5 Hz, 1 Hz), 8.77 (2H, dt, 14 Hz, 1 Hz), 8.84 (2H, dt,
OD): δ 7.60-7.76 (24H, m), 7.91 (2H, d, 5 Hz), 7.98 (2H, dd,
5 Hz, 5 Hz), 8.25 (2H, d, 5 Hz), 8.34 (4H, dd, 13.0 Hz, 10.0
Hz), 8.46 (2H, d, 5 Hz), 8.98 (2H, d, 13.0 Hz) ppm. 31P-NMR
(CD3OD): δ 6.01 ppm. Anal. Calcd for C58H49Cl2N6O9.5P2-
Ru for [RuL2L′]Cl2‚3.5H2O: C, 57.29; H, 4.06; N, 6.91%.
Found: C, 57.30; H, 4.15; N, 6.97%.
Methods. 1H-NMR spectra were measured on BRUKER
ACP-200 and BRUKER DPX-400 spectrometers at 200 MHz
and 400 MHz: 31P-and 13C-NMR spectra were measured on
the ACP-200 at 81.0 and 50.2 MHz, respectively. Chemical
shifts are given in δ (ppm) relative to TMS (1H, 13C) and to
85% H3PO4 (31P). Mass spectra (m/z relative percent) were
measured with a Nermag-R-10-10C spectrometer. Electro-
chemical measurements used a three-electrode three-compart-
ment setup. The rod electrodes were incorporated in a Teflon
tube with an external diameter of 1 cm to obtain a rotating disk
electrode. A silver wire quasi-reference electrode was used,
but all potentials are given hereafter with respect to the Ag/
AgCl saturated reference electrode. A platinum wire was used
as counter electrode. The electrolyte consisted in most cases
of a 0.1 M tetrabutylammonium tetrafluoroborate ((TBA)BF4)
solution in water-free N,N-dimethylformamide (DMF). They
were purchased, as sodium peroxodisulfate and iodine, by Fluka,
and used as received, except for (TBA)BF4, which was dried
for 5 h at 120 °C under vacuum (10-2 Torr). 1-Hexyl-3-methyl-
imidazolium iodide was synthesized as reported elsewhere.18
Current voltage curves were recorded using an EG&G Princeton
Applied Research Model 362 or an Eco Chemie Autolab
scanning potentiostat. Luminescence measurements were per-
formed by placing the electrochemical cell in the compartment
of a Spex F 112 fluorimeter equipped with a cooled Hamamatsu
R 2658 photomultiplier tube mounted on the emission mono-
chromator. The photomultiplier was configured for single
photon counting. All spectra were corrected for the spectral
sensitivity of the monochromator and the photomultiplier. A
Balzers B-40 448 nm interference filter was used when the TiO2
was deposited on a titanium plate. Absorption measurements
were performed on a Varian Cary 1E UV-visible spectropho-
tometer. All measurements were performed at room tempera-
ture.
3
5 Hz, 1 Hz) ppm. 13C-NMR (CDCl3): δ 16.34 (d, JC-P ) 5
Hz), 62.79 (d, 2JC-P ) 5 Hz), 122.84 (d, 2JC-P ) 9 Hz), 125.65
(d, 3JC-P ) 9 Hz), 138.72 (d, 1JC-P ) 186 Hz), 149.58 (d, 2JC-P
3
) 13 Hz), 155.71 (d, JC-P ) 14 Hz) ppm. 31P-NMR
(CDCl3): 15.06 ppm. MS (IC; m/z (rel intens): 429 (M + 1,
44.82), 428 (M+, 5.7), 386 (19.8), 355 (5.0), 335 (5.9), 321
(3.2), 320 (24.8), 314 (9.0), 305 (4.9), 292 (100), 291 (4.9),
281 (9.3), 264 (4.5), 248 (5.3), 236 (13.9), 218 (8.1), 217 (3.2),
216 (8.5), 211 (2.6), 188 (14.7), 176 (9.5), 157 (10.7), 151 (3.6),
147 (4.3), 109 (8.3), 98 (15.5). Anal. Calcd for C18H26N2O6P2
(428.36): C, 50.47; H, 6.12; N, 6.54; P, 14.46%. Found: C,
50.67; H, 6.25; N, 6.69; P, 14.50%
The heteroleptic ruthenium complex RuL2L′ (L ) 4,7-
diphenyl-1,10-phenanthroline and L′ ) 2,2′-bipyrid-4,4′-yl-
diphosphonic acid) was produced in two steps: first the
bisphenanthroline complex RuL2Cl2 was prepared followed by
replacement of the chloride by the bipyridyldiphosphonic ester
ligand. A 260 mg amount of commercial RuCl3‚xH2O (1.0
mmol) and 340 mg of LiCl (8 mmol) were dissolved in 20 mL
of DMF. Then, 664 mg (2 mmol) of commercial 4,7-diphenyl-
1,10-phenanthroline were added, and the mixture was refluxed
under N2 for 3 h. The solution was allowed to cool and stand
for 18 h. Most of the DMF was then removed on the rotary
evaporator, and the crude product was dissolved in 100 mL of
hot EtOH to which was added slowly 100 mL of hot water
containing 1-1.5 mL of concentrated HCl. The mixture was
refluxed (10 min) and allowed to cool. The precipitate was
collected by filtration and washed with 50 mL of EtOH:H2O
(1:1) containing 0.5 mL of concentrated HCl, and then with
H2O. This product was contaminated with [RuL3]Cl2. A [RuL3]-
Cl2-free product was obtained by repeating the above procedure
three times. This crude material is sufficiently pure for use in
the subsequent step. A 167 mg amount of RuL2Cl2 (0.2 mmol),
90 mg of 2,2′-bipyrid-4,4′-yldiphosphonic acid diethyl ester
(0.21 mmol), and ca. 50 mg of triethylamine were refluxed for
7 h under N2 in 10 mL of EtOH. The reaction mixture was
Results and Discussion
Figure 1 shows the luminescence and luminescence excitation
spectrum of RuL2L′ in DMF. The emission peaks at 650 nm,
its quantum yield being about 8%. The feature of the excitation
spectrum is very similar to that of the MLCT absorption of the
dye, its maximum being located around 455 nm. The lowest
excited state of the RuL2L′ is likely to correspond to an
electronic configuration, where the electron is predominantly
present on one of the two phenanthroline ligands. This is due
to the fact that the energy level of the π* orbital of the
diphenylphenanthroline is lower than that of the bipyridine-4,4′-
diphosphonate.
Figure 2a shows a current voltage curve obtained with a
nanocrystalline TiO2 film deposited on the titanium rod and
coated with a monolayer of RuL2L′. The electrolyte was DMF

2560 J. Phys. Chem. B, Vol. 101, No. 14, 1997
Athanassov et al.
TiO2 films employed here the definition of Vfb is problematic
because the band bending in the depletion range is small for
weakly doped solids.12 Therefore we shall identify in the
following Vfb with the potential at which accumulation of
majority carriers starts at the interface. In other words at V <
Vfb the electrode is under forward bias. An alternative terminol-
ogy employed for nanocrystalline films is the potential of the
conduction band edge at the solid liquid interface Vcb. The two
potentials are related by the equation19
[e-
]
cb
Vcb ) Vfb + kt ln
[ncb]
where [e-cb] is the concentration of electrons in the conduction
band estimated to be between 1016 and 1017 cm-3 and [ncb] is
the concentration of accessible electronic states in the conduction
band, about 4 × 1021 for TiO2. Therefore Vcb is about 300 mV
more negative than Vfb). The value of Vfb ≈ -0.5 V is estimated
from the onset of the electroluminescence which coincides with
the maximum of the photoluminescence, as will be shown below
in Figures 3a and 4a. Under forward bias, a small hump appears
first at ca -0.65 V, which is attributed to capacitive charging
of the film including filling of surface states and to the reduction
of small amounts of titanium peroxides formed inadvertently
by band gap excitation of TiO2.20 This feature is also observed
with blank TiO2 films in the absence of adsorbed dye. The
current voltage curve obtained after deduction of the background
current due to the TiO2 is shown in the inset of Figure 2a. Two
pronounced waves are clearly discernible. For the first reduc-
tion, the anodic and cathodic peaks are at the same potential,
i.e., at -1.22 V, indicating a reversible redox process involving
a surface-adsorbed depolarizer. For the second wave the
cathodic and anodic peaks are separated by ca. 20 mV, the
midpoint potential being -1.49 V. The ratio of areas under
the two waves was determined as 1.05, indicating reduction of
the surface-anchored RuL2L′ by two successive one-electron
transfer reactions, i.e.,
Figure 1. Excitation and emission spectra of a 10-6 M solution of
RuL2L′ in DMF: excitation wavelength, 450 nm; emission wavelength,
642 nm. The absorption matches the excitation spectrum: the maximum
is located at 455 nm, the decadic molar extinction coefficient being
ꢀ(455) ) 27 400 l mol-1 cm-1
.
RuL2L′ + e-cb(TiO2) f RuLL-L′
RuLL-L′ + e-cb(TiO2) f RuL22-L′
(1)
(2)
Because the two LUMOs of RuL2L′ correspond to the diphen-
ylphenanthroline π* orbitals, the product of the reduction steps
are best described as Ru(II) complexes having one or two
unpaired electrons located on the phenanthroline ligands.
This assignment was confirmed by spectro-electrochemical
experiments. The absorption spectrum of the nanocrystalline
TiO2 film, loaded with a monolayer of RuL2L′ and deposited
on a conducting glass electrode, was monitored as a function
of electrode potential. Results are shown in Figure 2b, which
illustrates the effect of electrode potential on the absorption
spectrum of the film. The spectra displayed have been corrected
for the extinction of the electrolyte. The absorption of the
electrode remained practically unchanged, corresponding to the
MLCT transition of RuL2L′ in the voltage range positive of the
flat-band potential (≈-0.5 V). Applying a forward bias
increases at first slightly the absorption in the red due to the
accumulation of conduction band electrons.12-14 At more
negative potentials, a pronounced red shift in the absorption
appears which is clearly associated with the two electrochemical
reduction waves shown in Figure 2a. At -1.5 V the absorption
maximum is at 520 nm, the spectral features turning into those
characteristic for reduced ruthenium-polypyridyl complexes,21
Figure 2. (a) Cyclic voltammetry of a TiO2 electrode coated with
RuL2L′, the electrolyte being 0.1 M (TBA)BF4 in DMF (scan rate, 20
mV/s). (b) Effect of applied potential on the absorption in a nano-
crystalline TiO2 layer coated with a monolayer of RuL2L′, electrolyte:
0.1 M (TBA)BF4 in DMF. The data presented are difference spectra,
the background absorption of the electrolyte being subtracted from the
measured absorption values.
containing 0.1 M (TBA)BF4. At potentials positive of the flat
band potential (Vfb), which is estimated to be ≈-0.5 V where
the electrode is under reverse bias, the current is practically zero.
(For conventional semiconductors, the flat band potential is
defined as the potential at which there is no band bending
present in the solid. Negative polarization with respect to Vfb
results in majority carrier accumulation, while positive polariza-
tion leads to their depletion. In the case of the nanocrystalline

Mesoporous Oxide Semiconductor Films
J. Phys. Chem. B, Vol. 101, No. 14, 1997 2561
which is red-shifted by about 10 nm upon changing the
polarization from 0 to -1 V as shown in Figure 3b. The
appearance of this red shift is attributed to the effect of the local
electrostatic field on the MLCT electron transition, which
changes upon varying the electrode polarization.
These observations can be rationalized in terms of electron
and energy transfer quenching of the excited state of RuL2L′ at
the TiO2 surface. Sensitized electron injection from RuL2L′ in
the conduction band of TiO2 occurs in the potential domain
positive of -0.52 V where the film is under reverse bias. A
nanocrystalline photovoltaic cell constructed from a working
electrode constituted by a TiO2 film deposited on a conductive
SnO2 glass and loaded with RuL2L′, a conducting glass counter
electrode activated with a 5 µg/cm2 Pt catalyst, and a solution
of 0.5 M 1-hexyl-3-methylimidazolium iodide and 40 mM
iodine in DMF as redox electrolyte yielded a current action
spectrum whose features closely resembled those of the RuL2L′
absorption spectrum. The injection was highest at 460 nm,
where the monochromatic efficiency for photon to current
conversion reached 9% under short circuit conditions. The
enhancement of the luminescence intensity upon changing the
potential from -0.3 to -0.5 V in Figure 3a is due to the
decrease of the efficiency of electron injection which competes
with photoluminescence. As the electrode potential approaches
the flat band value, the electron injection is impaired, decreasing
the injection yield and hence enhancing the luminescence. A
similar effect was observed earlier1 with ruthenium tris(2,2′-
bipyridine-4,4′-dicarboxylate), whose luminescence was turned
on upon approaching the flatband potential of the nanocrystalline
TiO2 film. However, the decline of the emission intensity under
forward bias, which is apparent in Figure 3a, was not observed
in the earlier study, probably due to desorption of the sensitizer
from the electrode under accumulation condition. Such a
desorption does not occur in the case of RuL2L′, which due to
the phosphonate groups remains strongly anchored to the film
even after reduction. The intimate association of RuL2L′ with
the TiO2 favors energy transfer from the excited sensitizer to
conduction band electrons. Because the concentration of the
latter is increasing exponentially with applied voltage under
forward bias, energy transfer is competing more and more
effectively with other deactivation pathways until, at potentials
below -0.9 V, it becomes the dominant channel of the RuL2L′
excited state reaction.
Figure 3. Photoluminescence of RuL2L′ deposited on the TiO2 layer.
Electrolyte:0.1 M (TBA)BF4 in DMF: excitation wavelength, 448 nm
with interference filter. (a) Photoluminescence versus potential: emis-
sion wavelength, 632 nm. The measurements have been performed in
steady state. (b) Spectra taken at different potentials. The spectrum of
a 10-6 M solution of the dye in DMF is reported for comparison.
confirming the occurrence of reactions 1 and 2 at the nano-
crystalline TiO2 surface.
Cyclic voltammetry was also performed with the mesoporous
TiO2 films deposited on conducting glass which due to their
larger surface area allow for a more precise determination of
the adsorbed quantity of RuL2L′ than the titanium rod based
electrodes employed in Figure 2a. The adsorbed amount of dye
on this particular electrode was calculated as 2.3 × 10-8 mol/
cm2 from the decrease of the dye concentration in solution
measured by transmission spectroscopy. Integrating of the first
wave in the cyclic voltammogram and taking the average of
the charge passed during the cathodic and anodic part of the
redox process gave 2 × 10-8 mol/cm2. Since the graphical
integration is not very precise this agreement is satisfactory,
indicating that all of the RuL2L′ adsorbed on the mesoporous
surface is electroactive.
Figure 3a illustrates the effect of the electrode potential on
the intensity of the photoluminescence of RuL2L′ measured at
632 nm. These experiments employed a 1 cm2 sized mesopo-
rous TiO2 film deposited on a titanium plate substrate as a
working electrode together with an electrolyte consisting of 0.1
M (TBA)BF4 in dry DMF. At a potential of 0 V, a significant
emission is observed yielding a photon count of 6.8 × 106 s-1
at the detector. Applying a negative potential to the electrode
enhances first the emission until it reaches a maximum of 9.8
× 106 photons s-1 at -0.52 V. Further decreasing the potential
leads to a decline of the luminescence intensity, which becomes
negligibly small at potentials below -1 V. The electrode
potential exerts also a small influence on the emission maximum,
It should be noted that, apart from energy transfer, there is
another pathway for excited state deactivation. It involves the
reductive quenching of the excited state of RuL2L′ by conduction
band electrons to form the reduced state of the complex:
[RuL2L′]* + e-cb(TiO2) f RuLL-L′
(3)
In order to check whether such a mechanism is operative in the
present case, we have examined films loaded with RuL2L′ by
laser photolysis. The RuL2L′ was electronically excited by a
532 nm pulse of 10 ns half-width from a frequency-doubled
Nd Laser and the time course of the visible absorption monitored
by transient optical absorption spectroscopy. It was placed in
contact with an electrolyte consisting of 0.1 M (TBA)BF4 in
dry DMF and its potential adjusted to -0.9 V. The changes in
the absorbance in the 500 nm region where RuLL-L′ has an
absorption maximum were monitored following laser excitation.
The signal observed was very weak, typically absorbance
changes were below 10-4 , even though the energy of the
exciting laser pulse was as high as 53 mJ. This shows that if
any RuLL-L′ formed via reaction 3, it remained below detection
level and that its yield is therefore very small. One might argue
that reaction 3 could be followed by charge injection from the

2562 J. Phys. Chem. B, Vol. 101, No. 14, 1997
Athanassov et al.
reduced state of the complex in the TiO2 conduction band, eq
4, and that this reaction could be so fast that the RuLL-L′ would
RuLL-L′ f e-cb(TiO2) + RuL2L′
(4)
become undetectable on the nanosecond time scale of the laser
experiment. While this possibility cannot be totally discarded
on the basis of the available evidence, it is unlikely for reaction
4 to happen on a subnanosecond time scale as the accumulation
layer field present in the TiO2 at the applied forward bias impairs
charge injection. This effect is clearly visible in Figure 3 and
was already discussed in this context.
The efficient quenching of the RuL2L′ luminescence observed
under forward bias confirms that the phosphonated ruthenium
polypyridyl-type complexes remain strongly anchored to the
TiO2 surface under accumulation conditions, in agreement with
the electrochemical results shown above. Only if during
accumulation the ruthenium complex remains in close proximity
to the semiconductor solid can the excited state deactivation
take place so efficiently. Similar observations have been made
very recently by Yan and Hupp22 with the phosphonated
bipyridyl complex RuL3, where L stands for 4,4′-H2O3PCH2-
2,2′-bipyridine which was also anchored to the surface of a
mesoporous TiO2 film. Under forward bias a very rapid excited
state deactivation was observed which was tentatively attributed
to energy transfer quenching by conduction band electrons. The
existence of such a quenching mode has important consequences
for reactions involving excited states at semiconductor elec-
trodes. In general, it constitutes a loss channel, reducing the
yield of the competing desirable excited state reaction.
Figure 4a shows the electroluminescence obtained with
RuL2L′ as a sensitizer using a nanocrystalline TiO2 film for
electron injection. The corresponding current voltage curve is
also shown on this picture. The working electrode was a
titanium rod covered with the mesoporous TiO2 together with
an electrolyte consisting of 0.1 M (TBA)BF4 and 5 mM Na2S2O8
in dry DMF. A cathodic current due to the reduction of
peroxodisulfate appears at potentials negative of the flat band
potential (≈-0.5 V). Electroluminescence is produced con-
comitantly with the flow of cathodic current across the interface.
The emission intensity is strongly enhanced by increasing the
forward bias, and this is associated with a steep rise in the
cathodic current due to the reduction of peroxodisulfate. The
spectral distribution of the emission shown in Figure 4b matches
well the features of the photoemission of RuL2L′ presented by
the solid line. This confirms that the electroluminescence arises
from the excited state of RuL2L′ and not from the TiO2. The
emission originates from electron injection from the conduction
band of the TiO2 film into the π* orbital of the phenanthroline
ligand of [RuL2L′]+, generating the lowest excited state of the
ruthenium complex which is the light emitting species.
Figure 4. Electroluminescence of RuL2L′ deposited on the TiO2 layer
(electrolyte:5 mM Na2S2O8, 0.1 M (TBA)BF4 in DMF): rotation rate,
1000 rpm. (a) Dependence of the EL vs potential: scan rate, 2 mV/s;
emission wavelength, 640 nm. (b) Spectrum taken at a pulsed potential:
-0.17 V/900 ms, -0.87 V/100 ms. The photoluminescence spectrum
in the same electrolyte is reported for comparison (excitation wave-
length, 448 nm with interference filter, TiO2 layer deposited on a Ti
plate).
of an electron from the Ru(II) metal center, yielding the trivalent
ruthenium complex followed by the addition of an electron to
one of the phenanthroline ligands. The resulting electronic
configuration where one electron is missing in the HOMO while
the LUMO is occupied by one electron corresponds to that of
the MLCT excited state of RuL2L′.
The presence of a maximum in the electroluminescence
intensity in Figure 4a is expected since the conduction band
electrons necessary for the generation of excited states act also
as quenchers for the same states. Sweeping the voltage negative
of the flat-band potential enhances the rate of excited state
production via reaction 5-7. At the same time the quenching
rate of the excited state by energy transfer is enhanced since
the concentration of the conduction band electrons augments
with increasing forward bias. Hence, there is an optimum
potential situated around -0.88 V. The position of this
maximum remained approximately constant under repeated
scanning. However, a marked reduction in both the cathodic
current and the electroluminescence was observed when the
potential of the electrode was cycled between 0 and -1 V. This
effect presumably arises from a side reaction of the persulfate
leading to oxidation of DMF under formation of a passivating
layer at the TiO2 surface.
[RuL2L′]+ + e-cb(TiO2) f [RuL2L′]* f RuL2L′ + hυ (5)
[RuL2L′]+ is generated via reaction of the sensitizer with sulfate
radical ion which is a strong oxidant (E0 ) +2.43 (vs SCE) in
water)23
2-
RuL2L′ + SO4- f [RuL2L′ ]+ + SO4
(6)
the latter being generated through the reduction of peroxodi-
sulfate with conduction band electrons:
The present study shows that electroluminescence can be
produced via electron injection from the conduction band of a
semiconducting oxide into the oxidized state of a surface
adsorbed sensitizer, producing directly its light emitting elec-
2-
e-cb(TiO2) + S2O82- f SO4- + SO4
(7)
This excited state production can be visualized as the removal

Mesoporous Oxide Semiconductor Films
J. Phys. Chem. B, Vol. 101, No. 14, 1997 2563
tronic state. An alternative mechanism for which the term
electrogenerated chemiluminescence has been coined24,25 is often
operative in the case of metallic electrodes. Here, the oxidized
and reduced form of sensitizer is produced electrochemically
and light is generated when the two species react to give back
two sensitizer molecules in their original oxidation state. It
involves reduction of the RuL2L′ by conduction band electrons
generating RuLL-L′ followed by the reaction of the latter with
the oxidized complex [RuL2L′]+ to produce the excited state
of the complex. In the present case, direct electron injection
from the nanocrystalline oxide to the oxidized sensitizer is the
dominant pathway by which electroluminescence is generated.
This is clearly apparent from the fact that the onset of the
luminescence is at a voltage sufficiently negative to produce
the excited state directly via reaction 5 but insufficient to reduce
the ground state complex according to eq 1. Recall that the
electroluminescence rises at potentials more negative than -0.5
V, whereas the thermodynamical potential of the first reduction
for RuL2L′ is at -1.22 V. Moreover, it is well-established26
that the redox potential for the oxidation of the excited state of
polypyridyl-ruthenium(II) complexes is about 0.5 V more
positive than the ground state reduction potential of the same
complexes. Thus, while the production of an excited state
through electron transfer into the LUMO of the Ru(III) complex
at potential of -0.5 V is possible, the reduction of the ground
state complex does not occur at this potential.
overpotential necessary to drive the electron transfer but also
allows a dramatic decline of the quenching rate of the excited
state by energy transfer to free electrons in the conduction band.
Therefore, one can be confident that these films will play an
important role in the design of future electroluminescent devices.
Acknowledgment. Support of this work by a grant from
the Swiss National Science Foundation and by the Optics
Priority Program of the Swiss Confederation is gratefully
acknowledged. The authors are grateful to Dr. Robin Humphry-
Baker for assistance in the laser experiment and for helpful
discussions.
References and Notes
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1993, 115, 6382.
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Conclusion
For the first time, the production of an excited state by
heterogeneous electron transfer from the conduction band of a
nanocrystalline TiO2 electrode into the π* level of an oxidized
polypyridyl-ruthenium(II) complex which is grafted on the
surface of the oxide has been demonstrated. The morphology
of the electrode plays an important role in favoring the
electroluminescent process. The high surface area of such an
electrode allows a decrease of the electron density in the
conduction band compared to a flat electrode, maintaining the
high current intensity based on the projected surface area. The
high internal surface area not only permits a decrease in the