Carboxyphenyl metalloporphyrins as photosensitizers of semiconductor film electrodes. A study of the effect of different central metals

Article abstract of DOI:10.1021/jp0536596

Free-base (P), Zn(II) (P_zn), Cu(II) (P_Cu), Pd(II) (P_Pd), Ni(II) (P_Ni), and Co(II) (P_Co) 5-(4-carboxyphenyl)-10,15,20-tris(4-methylphenyl) porphyrins were designed and synthesized to be employed as spectral senzitizers in photoelectrochemical cells. The dyes were studied adsorbed on SnO₂ nanocrystalline semiconductor and also in Langmuir - Blodgett film ITO electrodes in order to disclose the effect of molecular packing on the studied properties. Electron injection yields were obtained by fluorescence quenching analysis comparing with the dyes adsorbed on a SiO₂ nanocrystalline insulator. Back electron-transfer kinetics were measured by using laser flash photolysis. The unmetallized and metallized molecules have different singlet state energies, fluorescence quantum yields, and redox properties. The quantum yields of sensitized photocurrent generation are shown to be highly dependent on the identity of the central metal. It is shown that P_Ni and P _CO do not present a photoelectric effect. The other porhyrins present reproducible photocurrent, P_Pd being the one that gives the highest quantum yield even in closely packet ITO/LB films. Photocurrent quantum yields increase as the dye ground-state oxidation potential becomes more anodic, which is in agreement with the observation, obtained by laser flash photolysis, that back electron-transfer kinetics decrease with the increase in the driving force for the recombination process. This effect could be exploited as a design element in the development of new and better sensitizers for high-efficiency solar cells involving porphyrins and related dyes. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp0536596

J. Phys. Chem. B 2005, 109, 20953-20962
20953
Carboxyphenyl Metalloporphyrins as Photosensitizers of Semiconductor Film Electrodes. A
Study of the Effect of Different Central Metals
Miguel Gervaldo, Fernando Fungo, Edgardo N. Durantini, Juana J. Silber,
Leonides Sereno, and Luis Otero*
Departamento de Qu´ımica, UniVersidad Nacional de R´ıo Cuarto, Agencia Postal Nro 3,
X5804BYA R´ıo Cuarto, Argentina
ReceiVed: July 4, 2005; In Final Form: August 24, 2005
Free-base (P), Zn(II) (PZn), Cu(II) (PCu), Pd(II) (PPd), Ni(II) (PNi), and Co(II) (PCo) 5-(4-carboxyphenyl)-
10,15,20-tris(4-methylphenyl) porphyrins were designed and synthesized to be employed as spectral senzitizers
in photoelectrochemical cells. The dyes were studied adsorbed on SnO2 nanocrystalline semiconductor and
also in Langmuir-Blodgett film ITO electrodes in order to disclose the effect of molecular packing on the
studied properties. Electron injection yields were obtained by fluorescence quenching analysis comparing
with the dyes adsorbed on a SiO2 nanocrystalline insulator. Back electron-transfer kinetics were measured by
using laser flash photolysis. The unmetallized and metallized molecules have different singlet state energies,
fluorescence quantum yields, and redox properties. The quantum yields of sensitized photocurrent generation
are shown to be highly dependent on the identity of the central metal. It is shown that PNi and PCo do not
present a photoelectric effect. The other porhyrins present reproducible photocurrent, PPd being the one that
gives the highest quantum yield even in closely packet ITO/LB films. Photocurrent quantum yields increase
as the dye ground-state oxidation potential becomes more anodic, which is in agreement with the observation,
obtained by laser flash photolysis, that back electron-transfer kinetics decrease with the increase in the driving
force for the recombination process. This effect could be exploited as a design element in the development
of new and better sensitizers for high-efficiency solar cells involving porphyrins and related dyes.
Introduction
the light-driven charge separation. The factors affecting the
recombination rates are the object of active investigations. A
study on a series of ruthenium bipyridyl dyes, in addition to
porphyrin and phthalocyanine dyes, shows that the primary
factor controlling the charge recombination dynamics in dye-
sensitized nanocrystalline TiO2 films is the spatial separation
of the dye cation from the electrode surface.18 On the other hand,
Tachibana et al.19 showed that, with favorable energetics,
electron injection proceeds on ultrafast time scales, and charge
recombination rates are nearly independent of the sensitizer and
rather associated with the heterogeneity of the traps in the TiO2.
They suggest that the high photocurrent efficiency obtained with
a Ru(II) complex could be due to the favorable electron transfer
between the iodide redox-active electrolyte (acting as a sacri-
ficial donor) and the photooxidized dye. Nonetheless, a number
of studies that used structurally related dyes showed a strong
observable effect in the photoelectric yields, depending on the
central metal in the organometallic complex. For example,
Kuciauskas et al.20 showed that osmium and ruthenium poly-
pyridyl complexes exhibit very fast (>108 s-1) electron injection
rate constants when they are adsorbed over nanocrystalline TiO2.
However, charge recombination between photoinjected electrons
and the oxidized dye was observed in the nano-to-millisecond
time range, with rate constants significantly larger for sensitizers
having more negative reduction potentials of the oxidized dye
ground state. The authors concluded that the charge recombina-
tion occurs in the Marcus inverted region, with rates decreasing
with increasing driving force for the recombination process.20,21
The observation of the inverted region in dye-sensitized solar
cell recombination processes implies that the improvement of
Several large band gap metal oxide semiconductors such as
TiO2, ZnO, and SnO2 have been spectrally sensitized by metal-
organic dyes in order to extend their photoelectrochemical
response to the visible region.1-3 The energy difference between
the conduction band edge of an n-type semiconductor film and
the oxidation potential of the excited sensitizer present as an
adsorbate provides a driving force for excited-state heteroge-
neous charge injection.4,5 In a porous film of nanometer-sized
semiconductor particles, the effective surface area can be greatly
enhanced, producing substantial light absorption, even with only
a monolayer of dye on each particle.6 As has been pointed out,
there are several analogies between natural photosynthesis and
dye-sensitized nanocrystalline semiconductor electrodes.7,8 Due
to their primary participation in the photosynthesis process and
properties, many studies on the development of optoelectronic
and energy conversion devices incorporate chlorophyll deriva-
tives, and several related metallized and unmetallized tetrapy-
rrolic compounds, as light receptors and charge storage units.
Porphyrin has been often used in achieving spectral sensitization
of wide band gap semiconductors,9-17 and this point has recently
been thoroughly reviewed by Campbell et al.9 Light harvesting
and charge separation efficiencies in a range of porphyrin dyes
adsorbed on nanostructured TiO2 were found comparable to
those in natural photosynthesis, with a very high (nearly 80%)
incident-photon-to-current efficiency (IPCE).7 However, the
performance of dye-sensitized photoelectrochemical cells can
be severely limited by any recombination reaction which follows
* To whom correspondence should be addressed. E-mail: lotero@
exa.unrc.edu.ar.
10.1021/jp0536596 CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/18/2005

20954 J. Phys. Chem. B, Vol. 109, No. 44, 2005
Gervaldo et al.
Porphyrins. 5-(4-Carboxyphenyl)-10,15,20-tris(4-methylphe-
nyl) porphyrin (P) and its complexes with Zn(II) (PZn), Pd(II)
(PPd), Cu(II) (PCu), and Ni(II) (PNi) were synthesized as
previously described.11,27 Cobalt(II) 5-(4-carboxyphenyl)-
10,15,20-tris(4-methylphenyl) porphyrin (PCo) was synthesized
by refluxing a mixture of P (15 mg, 0.021 mmol) in 30 mL of
chloroform with a saturated solution of cobalt(II) acetate in
methanol (2 mL) for 2 h. Solvents were evaporated under
reduced pressure, and flash chromatography (silica gel, dichlo-
romethane/methanol 10%) yielded 15 mg (94%) of PCo. MS
m/z 757 (M+) (757.2014 calcd for C48H34N4O2Co).
Preparation of Nanocrystalline Films. The SnO2 nano-
structured thin film electrodes were prepared using an already
described method.12 As base contact optically transparent
electrodes of indium tin oxide (ITO, 100 Ω/square, Delta
Technologies) were used. ITO/SnO2 electrodes were prepared
by spin coating using a P6204-A model specialty coating system.
The final films were annealed at 450 °C for 1 h. The SnO2
films were analyzed by X-ray diffractommetry (Siemes D5000,
Cu KR radiation, XRD) and atomic force microscopy (Nano-
scope III, AFM) in order to assess the crystallinity of the
sample.13 SiO2 nanostructured films were formed with the same
procedure as that used for SnO2 photoelectrodes, using a SiO2
colloidal suspension in water (Ludox SM 30).
Figure 1. Structures of the sensitizing dyes.
cell performance could be reached through the use of dyes with
electronic coupling that favor sensitizer regeneration relative
to charge recombination. Dang and Hupp also reported direct
observation of Marcus-type inverted electron transfer in SnO2-
dye systems.22
Because porphyrins are particularly attractive as light harvest-
ers, we designed and synthesized a series of metallo 5-(4-
carboxyphenyl)-10,15,20-tris (4-methylphenyl) porphyrins, with
M ) 2H (P), Zn(II) (PZn), Cu(II) (PCu), Pd(II) (PPd), Ni(II)
(PNi), and Co(II) (PCo) (Figure 1), to analyze the effect of the
metal substitution in the quantum yield generation of photo-
electric effects by the dyes as photosensitizers of SnO2 nano-
crystalline semiconductor film electrodes. Electron injection
yields were measured through fluorescence quenching analysis
and back electron-transfer kinetics by laser flash photolysis. In
addition, to assess the effect of molecular packing of the dyes,
photocurrent generation from Langmuir-Blodgett films of the
dyes over ITO electrodes was studied. The metallized and
unmetallized molecules have different singlet state energies,
fluorescence quantum yields, and redox properties23 and bear a
carboxylic acid group in order to benefit the adsorption and
electronic coupling of the sensitizer with the basic semiconductor
oxide electrode24,25 and the orientation at the air-water interface
in the Langmuir-Blodgett film formation.
ITO/SnO2 electrodes were modified with dyes by soaking
ITO/SnO2 films in a saturated n-hexane/DCM 80/20 solution
of the corresponding dye. The electrodes were then washed with
the same solvent, dried under a N2 stream at room temperature,
and stored in vials. A copper wire was connected to the
electrode’s surface with an indium solder to achieve electrical
contact.
Photoelectrochemistry. Unless other conditions are indicated,
photoelectrochemical experiments were conducted in aqueous
solutions (0.01 M) of hydroquinone (H2Q, recrystallized from
toluene), with phosphate buffer (pH ) 5.2) prepared from 0.05
M NaH2PO4 and NaOH. These solutions were degassed by
bubbling Ar and maintaining a continuous stream on the top of
the cell. A strong effect on the charge collection efficiency in
porphyrin-derivatized TiO2 photoelectrodes with electrolyte pH
was observed, mainly due to oxide surface protonation;28
consequently, special care was taken to control the pH. The
measurements were carried out under potential control in an
already described quartz photoelectrochemical cell13 equipped
with a Ag/AgCl reference electrode, and a Pt foil auxiliary
electrode, using a computer-controlled battery-operated low-
noise potentiostat (Palm Sens). Action spectra were obtained
by illumination of the photoelectrodes with monochromatic light
obtained from a 75 W high-pressure Xe lamp (Photon Technol-
ogy Instrument, PTI) and a computer-controlled PTI high-
intensity grating monochromator (5 nm band wide). The steady-
state photocurrents were obtained in front face configuration
(illuminated area: 1 cm2). The incident light intensities at
different wavelengths were measured with a Coherent Laser-
Mate Q radiometer (sensitivity 1 µW). Transient photocurrents
were generated by laser pulses (vide infra) at different applied
potential bias (M5 LYP Electrnica Argentina potentiostat), and
current output signal of the potentiostat was coupled to a digital
oscilloscope.
Experimental Section
General. The voltammetric characterization of the redox
process for the porphyrin dyes was acquired with a potentiostat-
galvanostat Autolab (Electrochemical Instruments) using a Pt
disk working electrode of 0.204 cm2 and a Pt counter electrode
in a conventional three-compartment Pyrex cell. A freshly
prepared Ag/AgCl quasi-reference electrode was used. The
working electrode was cleaned between each experiment by
polishing with 0.3 µm alumina paste, followed by solvent rinses.
Studies were carried out in N2-degassed dichloromethane (DCM)
with 0.1 M tetrabutylammonium hexaflourophosphate (TBAH-
FP) supporting electrolyte. The voltammograms were corrected
for IR drop by the positive feedback technique. After each
voltammetric experiment, ferrocene was added, and the potential
axis was calibrated against the formal potential for the ferro-
cenium/ferrocene redox couple.26 The oxidation potentials of
the dye-excited state (E*ox) were estimated by subtracting the
excitation energy from the redox potentials of the molecule in
the ground state.
Absorption spectra were recorded on a Shimadzu UV-2401PC
spectrophotometer. Fluorescence spectra were recorded on a
Spex FluoroMax fluorimeter. Mass spectra were taken with a
Varian Matt 312 operating in EI mode at 70 eV. The light-
harvesting efficiency (LHE) of the dye adsorbed over the
photoelectrodes was obtained from LHE ) 1 - 10-A, where A
is the monochromatic absorbance.
Langmuir-Blodgett (LB) Film Preparation. Monolayers
were prepared on a model 610 Nima Langmuir-Blodgett trough
placed in a dark box. The subphase was ultrapure water (from
LABCONCO equipment model 90901-01) with phosphate
buffer (pH ) 5.2). This pH value was selected because it is the
same as that at which the photoelectrochemical experiments

Carboxyphenyl Metalloporphyrin Photosensitizers
J. Phys. Chem. B, Vol. 109, No. 44, 2005 20955
were carried out. The temperature was maintained at 25 ( 0.5
°C. Porphyrins films at the water-air interface were formed
by applying a drop of a suitable dye solution (ca. 1-2 × 10-3
M in chloroform) on the clean subphase by microsyringe. The
chloroform was allowed to evaporate for 15 min, and the
monolayer was then compressed. The substrates for monolayer
depositions were ITO-coated borosilicate glass made hydrophilic
by immersing them in 0.1 M NaOH solution for 30 min and
then thoroughly rinsing them with ultrapure water. The clean
ITO slide was immersed in the subphase, and the monolayer
was formed at the air-water interface and compressed at 50
cm2/min. The monolayers were transferred, at a constant surface
pressure of 25 mN/m, to the slides by the vertical transfer
method at 25 mm/min. Only films having transfer ratios of 1.0
( 0.1 were used in the experiments. A copper wire was
connected to the electrode’s surface with an indium solder to
achieve electrical contact.
Laser Flash Photolysis Experiments. The laser flash pho-
tolysis setup has been described elsewhere.29 A Q-switched Nd:
YAG laser (Spectron SL400) was used as the excitation source
operating at 532 nm (20 ns half-width) in order to excite the
porphyrin compounds adsorbed over ITO/SnO2 electrodes
placed in the already described photoelectrochemical cell. It has
been reported that the electron injection kinetics into the
semiconductor are rather insensitive to experimental condi-
tions.19 By contrast, the recombination kinetics between the dye
cation and the photoinjected electrons are strongly dependent
upon experimental conditions, with variation in the recombina-
tion rate from picoseconds to milliseconds depending upon the
excitation intensity, solvent/electrolyte composition, and applied
electrical bias.19,30 In order to compare between the different
sensitizer dyes, considerable care were taken to minimize such
experimental variation. The applied potential on the ITO/SnO2/
dye electrodes was keep under control in a three-electrode
configuration, in Ar-degassed phosphate buffer solutions without
sacrificial donor hydroquinone. The laser beam was abated by
using neutral filters in order to avoid photodegradation of the
samples. The output of the detector was coupled to a digital
oscilloscope (Hewlett-Packard HP-54504A). About 20 shots
were usually needed for averaging decay times, to get a good
signal-to-noise ratio. The averaged signals were analyzed as
multiple exponential decays by using calculation software.
Results and Discussion
Electrochemistry. Cyclic voltammetry was used in order to
evaluate the redox properties of the dyes. Usually, porphyrins
show two well-defined reversible single-electron oxidations in
nonbinding solvent,31 corresponding to ring oxidation and
formation of the radical cation and dication, respectively.23
Figure 2 shows typical oxidation voltammograms for all the
studied porphyrins. In metalloporphyrins the ring or metal-
centered oxidations are determined by various factors, including
the intrinsic redox potentials of the porphyrin ring and metal
center, the number and type of ligands on the metal ion, and
the properties of the solvent. In the case of Zn(II), Cu(II), and
Pd(II) tetraphenyl porphyrins (M(II)TPP) it has been established
that the first anodic process involves porphyrin macrocycle
oxidation and the formation of [M(II)(TTP)]+ • 23,31
.
On the other
Figure 2. Cyclic voltammograms of (a) P, PZn, (b) PPd, PCu, and (c)
PNi, PCo at a platinum electrode in DCM containing 0.1 M TBAHFP
as supporting electrolyte; sweep rate ) 0.100 V/s. The currents have
been normalized to one at the first oxidation wave for comparison.
hand, it is known that in the case of several Co(II) porphyrins
the electrochemistry is dominated by the metal-centered reaction,
the most common of which involves the Co(II)/Co(III) process,
which usually occur at less anodic potentials than macrocycle
oxidation.32 In agreement with this, in our case the anodic scan
of PCo (Figure 2c) shows three voltammetric waves, in which
the one at lower potential corresponds to the central metal, Co-
(II), oxidation and the other two to oxidations of the porphyrin-
ring system. On the other hand, nickel porphyrin electrochem-

20956 J. Phys. Chem. B, Vol. 109, No. 44, 2005
Gervaldo et al.
TABLE 1: Electrochemical, LB, and Spectroscopic
Properties of Dyes
area/
λmax
λmax
λmax
a
dye
Eox
moleculeb
DCMc
SnO2/dyec
LBc
PZn
P
PCu
PPd
PNi
PCo
0.76
0.88
0.87
1.04
0.89
0.75
50
52
63
90
425
421
419
419
419
412
434
431
421
427
425
430
438
427
429
427
a First oxidation potential of dyes (V vs SCE). b LB films (Å2).
c Soret band (nm).
istry has shown some degree of controversy in the past.31 Seth
et al.33 made a careful revision of Ni(II)TPP oxidation, where
the resonance Raman and EPR spectra of the one-electron
oxidation products were examined in detail. They concluded
that the limiting case forms [Ni(III)TPP]+ and [Ni(II)TPP]+ •
do not accurately describe the ground states of the oxidation
products of Ni(II)TPP, and an equilibrium exists between the
electrochemically generated Ni(II) porphyrin’s cation radical and
Ni(III) porphyrins. Additionally, more recently Kadish et al.34
reported the electrochemistry studies of 29 different nickel
porphyrins in dry DCM and confirmed that in all cases it was
the porphyrin macrocycle and not the nickel atom that was
oxidized under their electrochemical experimental conditions
(DCM, 0.1 M tetra n-butylammonium perchlorate or 0.1 M tetra
n-butylammonium hexafluorphosphate). In similar conditions,
we observed two oxidation waves in the anodic scan of PNi,
(Figure 2c) which is consistent with those results.34
Figure 3. Surface pressure-area isotherm of porphyrin dye films at
the air-water interface. Subphase: 0.05 M phosphate buffer, pH )
5.2.
gathered in Table 1. If the dyes lie with the porphyrin ring being
parallel to the air-water interface, the occupied area per
molecule should be at least ≈360 Å2, as can be inferred from
the calculated molecular geometry (AM1) for the P porphyrin.
A similar observation was reported by Ogi et al.,41 who, based
on their single-crystal structure, assumed 220 Å2 as the in-plane
area for TPP. As can be observed in Table 1, the areas are quite
smaller than those calculated, and consequently, it can be
assumed that the molecules assemble perpendicular to the
subphase plane, leaving dye monolayers with typical absorbance,
at the Soret band, of ≈0.06 for all porphyrins.
Table 1 summarizes the redox potentials for the first oxidation
process of each investigated dye, calculated using the expression
(Ep forward - Ep backward)/2. The values are consistent with
the effect of the peripheral substitution of the methyl and
carboxyl groups (Figure 1). It is known that peripheral substitu-
tion in para-substituted tetraphenylporphyrin [H2(p-X)TPP]35 and
metalloporphyrins36-38 ([M(p-X)TPP] where M ) Mn2+, Fe2+
,
Absorption Spectroscopy. The electronic spectra of por-
phyrins were recorded in DCM solution and compared to those
of the porphyrin adsorbed onto thin ITO/SnO2 films and
deposited as ITO/LB monolayers. The absorption spectra of the
dyes in solution show a series of visible bands due to π-π*
transitions of the conjugated macrocycle.42 In the ITO/SnO2/
dye adsorbed films the Soret and Q-bands show similar
electronic transitions to those observed for monomeric absorp-
tion in DCM, but the bands are broader and are shifted to longer
wavelengths in comparison with those in solution (Figure 4 and
Table 1). This may be caused by the interaction of the
porphyrins with the polar surface of the electrode, as well as
the possible formation of porphyrin aggregates on the surface.12
The absorption spectra of ITO/LB films can help to clarify this
point. In fact, the absorption spectra of these porphyrin LB
monolayers are also broadened relative to the solution, and the
Soret bands are red shifted. Choudhury et al.43 found an excellent
linear correlation between the red shift and the area per molecule
for a series of surfactant porphyrin LB monolayers and also
showed that the trend of the spectral properties of the model
porphyrin compounds was not changed substantially by chang-
ing the substrate from quartz to a SnO2-coated glass plate.
Hence, they concluded that, except for monolayer disorder, no
additional effect is experienced by porphyrin compounds when
they are deposited onto a SnO2 surface.43 On the basis of these
antecedents and our experimental results, we can conclude that
the red shift observed in the ITO/SnO2/dye adsorbed films is
due, at least in part, to the close packing of the porphyrin onto
the electrode. Since this can drive to self-quenching of the
Co2+, Ni2+, Cu2+, and Zn2+) has an effect in modulating their
electron donor-acceptor capabilities. In our case, we observed
that the dye oxidation potentials are less anodic than the values
reported for the tetraphenylporphyrins without a substituent,31
which is evidence that the electron donor effect characteristic
of the three methyl groups prevails over the single carboxylic
group electron acceptor effect. Aside from this effect for P, PZn,
PPd, PNi, and PCu, where ring oxidation produces the porphyrin
radical cation, the change in the redox potential with different
metal substitution follows the same order as that already reported
for M(II)TPP,31 that is, that PZn has the lowest and PPd the
highest.
LB Film Properties. It is known that unsubstituted meso-
tetraphenylporphyrins do not form stable monolayer films in
pure form at gas-liquid interfaces.39 However, 5-(4-carboxy-
phenyl)-10,15,20-tris(4-methylphenyl) porphyrins exhibit well-
behaved monomolecular films at an air-water interface, in
which the carboxylic group of the dye is assumed to be oriented
into the water.40 The photoelectrochemical response of the LB
monolayer over ITO was analyzed only for the dyes that showed
photocurrent generation on ITO/SnO2/dye electrodes (vide infra).
In the compressed monolayers the molecules appear to be
oriented so that the plane of the ring is perpendicular to the
surface, as can be deduced from the average area per molecule
of porphyrins at the air-water interface, which was determined
by extrapolating the linear region of the surface pressure versus
area isotherms to zero pressure (Figure 3). The observed
molecular areas for the monolayers of the compounds are

Carboxyphenyl Metalloporphyrin Photosensitizers
J. Phys. Chem. B, Vol. 109, No. 44, 2005 20957
Figure 4. Absorption spectra of porphyrins: DCM solution (dotted line), in the adsorbed state over ITO/SnO2/electrodes (dashed line), and Langmuir-
Blodgett films (solid line). The absorbance has been normalized to one at the Soret band for comparison.
porphyrin excited state,44 in the photoelectrochemical studies
we kept similar and low dye coverage for the different studied
porphyrins. For ITO/SnO2 electrodes constructed as described
in the Experimental Section, we found that the maximum
porphyrin dye coverage (Θ ) 1) produces absorbance around
1.0-1.2 in the Soret band. Therefore, electrodes with absorbance
of ≈0.2 were used.
In contrast, when the dye is adsorbed on ITO/SiO2 (band gap
) 6.9 eV),45 the oxidation potential of the dye-excited state
(E*ox) lies at much lower energy than the conduction band edge
of the insulator. Consequently, electron transfer is not possible,
and the fluorescence is not affected by this additional mecha-
nism. For a dye adsorbed on ITO/SnO2 there is a driving force
for photoinduced electron transfer from the dye to the semi-
conductor, defined as ∆E ) -e(Eox*- EFB),46 where EFB is
the potential of the semiconductor flat band (∼ -0.02 V vs
NHE)47 and e is the electronic charge. Since electron transfer
is in competition with radiative and nonradiative processes, one
can express the charge injection yield (Φinj) from the excited
dye to the semiconductor as shown in eq 1:13,48
Fluorescence Spectroscopy. P and PZn show observable
fluorescence, both in solution and adsorbed on ITO/SnO2/dye
electrodes; however, the emission intensities for the other
porphyrins are much lower in solution, and no emission was
detected in adsorbed state. Thus, electron injection yields
measured through fluorescence quenching in ITO/SnO2/dye
electrodes were analyzed for P and PZn. Figure 5a shows the
absorption-corrected fluorescence spectra of the dyes in toluene
solution. The PZn has two maxima at 604 and 652 nm, whereas
for P they are at 653 and 719 nm. On the other hand,
fluorescence intensity of the dyes adsorbed over the ITO/SnO2
film is much smaller than the same over the insulator ITO/SiO2
(Figure 5b). In other words, an additional quenching pathway
seems to be effective when the dyes are adsorbed on the
semiconductor surface. This quenching can be interpreted in
terms of electron transfer from the excited dye to the semicon-
ductor. After production of the excited state by photon absorp-
tion, several deactivation pathways are possible: radiative
relaxation, thermal (nonradiative) relaxation, or interfacial charge
injection. When the latter occurs an external circuit could collect
the injected electrons, producing an observable photocurrent.
If(0) - If(e)
Φinj
)
(1)
If(0)
where If(0) and If(e) are the fluorescence intensity in the absence
and in the presence of charge-transfer respectively, obtained
from the area below the corrected emission spectra of the
photoelectrodes (Figure 5b). Under our experimental conditions
the electron injection efficiency was found to be similar for both
PZn and P porphyrins with Φinj ≈ 0.9.
Photoelectrochemistry. Upon excitation of ITO/SnO2/dye
electrodes anodic photocurrents and negative open-circuit pho-
topotentials are generated in the case of PPd, PCu, PZn, and P,
indicating that the electrons flow from the solution to ITO base
contact through the illuminated electrode. On the other hand,

20958 J. Phys. Chem. B, Vol. 109, No. 44, 2005
Gervaldo et al.
(II)TPP that excitation at 400 nm converts the porphyrin system
into the S2(π,π*) state, which produced the S1(π,π*) state in a
few femtoseconds, and the S1(π,π*) state decays to the metal-
centered excited state (d,d*) with τ ) 0.6 ps, which decays to
the S0 ground state with τ ) 4-10 ps. On the other hand, Yu
et al.56 have shown that deactivation of the Co(II)TPP S1 state
occurs through the formation of intramolecular electron transfer
from the porphyrin ring to the metal center, with charge
separation and recombination processes of 1.8 and 16 ps,
respectively. However, it has been reported for several dyes,
including prophyrins,57 that the primary step of electron injection
across the sensitizer-semiconductor interface is extremely fast
in the case of a TiO2 semiconductor (<100 fs for tetracarboxy-
phenyl porphyrins).19 In addition, Benko¨ et al.58 demonstrated
that the time constant and overall quantum yield of electron
injection from RuN3 dye are similar for both TiO2 and SnO2
semiconductors, and the authors concluded that the electron
injection process is almost independent of the semiconductor
used. Moreover, the oxidation potentials of the dye-excited states
(E*ox) used in this work lie at higher energy than the conduction
band edge of the SnO2. Thus, it is possible that, although having
a short lifetime, PNi and PCo excited states undergo heteroge-
neous photoinduced electron transfer to the semiconductor. If
this is the case, back electron transfer must compete quite
efficiently with regeneration of the reduced dye ground state
by the sacrificial donor.
For the PPd, PCu, PZn, and P porphyrins photocurrents are
reproducible under several (hundreds) repeated on-off il-
lumination cycles. Action spectra of ITO/SnO2/dye and ITO/
dye LB electrodes (measured under potentiostatic control)
closely match the absorption spectrum of the adsorbed organic
molecules in every case, as is shown in Figure 6a-d.
Monochromatic photocurrent quantum yield, expressed as the
charge injection yield (Φinj) from the excited dye to the
semiconductor times the charge collection efficiency (ηc) of the
system, was evaluated from eq 2.59
Φinjηc ) IPCE/LHE
(2)
where IPCE is the incident-photon-to-photocurrent efficiency
evaluated from:59
IPCE ) (isc1240)/(Iincλ)
(3)
isc is the short-circuit photocurrent (A cm-2), Iinc is the incident
light intensity (W cm-2), λ is the excitation wavelength (nm),
and LHE is the light-harvesting efficiency (see the Experimental
Section). The dependence of the photocurrent quantum yield
(Φinjηc) on the applied potential has shown that recombination
rates between photoinjected electrons and a dye cation increase
as more negative potentials are applied to the semiconduc-
tor.20,30,50 Lower cell efficiencies at negative bias correlate with
this increase in the charge recombination rate, which involves
the level of the steady-state trap occupancy in the nanostructured
semiconductor.50 Thus, the effect of the bias was analyzed by
recording the transient photocurrents obtained after a 532 nm
laser pulse at different applied potentials (Figure 7a). As can
be observed, the transient photocurrents generated by an ITO/
SnO2/PZn electrode increase as the applied potential becomes
more anodic. This effect was observed with all the porphyrins
and is mainly due to the increase in the collection efficiency of
the injected electrons. Consequently, for the comparative
analysis on the efficiency of the different dyes used in this work,
the measurements were carried out under potential control.
Figure 5. (a) Corrected fluorescence emission spectrum in toluene
solution of PZn and P; λex ) 550 nm. (b) Fluorescence emission
spectrum of PZn and P adsorbed on SiO2 and SnO2; λex ) 420 nm. All
the emission spectra were corrected by the absorbance and instrumental
response.
PNi and PCo show none or extremely low photoelectric effect.
The lack of dye-sensitized photocurrent on large band gap
semiconductor electrodes could be due to several reasons, such
as inefficient electron injection, fast back electron transfer, and/
or inefficient regeneration of the reduced dye ground state by
the sacrificial donor.49-52
It is has been reported that Ni(II) porphyrin’s excited state
has a very short lifetime due to the π and d orbital interactions.53
Soret and Q-band photoexcitation rapidly form a metal-centered
(d,d)* state. Rodriguez et al.54,55 showed that common Ni(II)
porphyrins undergo π* f d* crossing within 1 ps, generating
a porphyrin molecule with a ground-state system and an excited
metal center. The d* state repopulates the ground state with a
lifetime of around 300 ps. Zamyatin et al.53 reported for Ni-

Carboxyphenyl Metalloporphyrin Photosensitizers
J. Phys. Chem. B, Vol. 109, No. 44, 2005 20959
Figure 6. Photocurrent action spectra, IPCE of ITO/SnO2/dye electrodes (- - -) and Langmuir-Blodgett films (s) modified with P, PPd, PCu, and
Zn. H2Q 0.01 M in phosphate buffer. Applied bias potential ) 0.05 V vs Ag/AgCl.
P
The effect of the applied potential on the observed stationary analyzed with the same procedure as that used in nanostructured
photocurrent quantum yield expressed as the Φinjηc value of
the systems at the Soret bands is shown in Figure 7b for different
ITO/SnO2/dye electrodes. This experiment allows a comparative
analysis of photoelectric generation efficiency of the different
dyes under similar experimental conditions. Φinjηc increases with
the anodic bias in all cases, and Figure 7b also shows that the
photocurrent quantum yield is higher in the PPd molecule than
for the other dyes in the range of applied potential studied. Table
2 summarizes the results of the product Φinjηc for the four dyes
at the Soret band.
On the other hand, as was already mentioned, dye aggregation
onto the electrode can drive to self-quenching of the excited
state of the porphyrin in the photocurrent generation process.
For example, an enhancement of photocurrent quantum yield
on diluting monolayers of surfactant porphyrins with dio-
leoylphosphatidylcholine has been observed.43,60 Thus the
photoelectric properties of Langmuir-Blodgett films of P and
PM used in this work, deposited on ITO electrodes, were
films, to study the possible effects of the closer molecular
packing and intermolecular interactions in the ultrathin layer.
As can be observed in Table 2, the photocurrent quantum yield
of the LB film electrodes are lower than those obtained with
the nanostructured material for the same porphyrin. This could
be interpreted as resulting from the aggregate mediated quench-
ing, which diminishes interfacial isoenergetic electron transfer,
and hence makes lower the photocurrent quantum yield.
However, deposition of dyes over the highly doped oxide
semiconductor in the ITO/LB films electrodes instead of
nanostructured SnO2 also affects the photocurrent generation
process, through changes in the kinetics of the involved stages.
Nevertheless, as can be seen in Table 2, the PPd porphyrin was
shown to be the most effective sensitizer in both systems (ITO/
LB films and ITO/SnO2 electrodes). The reason for this
efficiency could be related to the kinetics of the different steps
in the spectral sensitization process. As shown before, the
electron injection efficiency is high and similar for P and PZn

20960 J. Phys. Chem. B, Vol. 109, No. 44, 2005
Gervaldo et al.
Figure 8. Kinetics of the dye ground-state recovery on ITO/SnO2/PZn
followed at 430 nm after a laser pulse in phosphate buffer solution
without H2Q. The arrow indicates the bias potential vs Ag/AgCl.
generation through back electron transfer (i.e., the charge
recombination between the photoinjected electron and the
porphyrin cation).
Laser Flash Photolysis. To analyze the back electron-transfer
kinetics, we examined the changes in the absorbance, measured
at the dye’s Soret band, following excitation after a 3.0 mJ cm-2
laser pulse at 532 nm for ITO/SnO2/dye electrodes. Figure 8
shows the transients in an ITO/SnO2/PZn electrode at different
applied potentials. Electron injection results in fast bleaching
followed by regeneration of the ground-state absorbance of the
porphyrin compounds. As has been mentioned, in several dyes,
including prophyrins,19,57 the primary step of electron injection
across the sensitizer-semiconductor interface is fast and almost
independent of the semiconductor used.58 The recovery pro-
cesses, in the absence of sacrificial donor hydroquinone, are
associated with the reduction of the oxidized species formed
after excitation and electron injection and are dependent on
applied potential. In agreement with the observed effect on
stationary and transients photocurrents, back electron transfer
increases as the applied potential becomes more cathodic and
reaches a nearly constant value at an anodic applied potential
higher than 0.2 V vs Ag/AgCl. The regeneration process, which
occurs by recombination between a photoinjected electron in
the semiconductor and the oxidized dye, presents a compara-
tively fast step followed by a slower one, showing a complex
kinetics49,50 attributed to multiple electrons trapping in surface
defects, which affect the recovery process.51,52 In the kinetic
analysis (Table 2) we assumed pseudo-first-order kinetics, fitted
by a biexponential function, but it is known that the back
electron-transfer process is a very complex phenomenon,50 and
an analysis of the relation between kb and the photocurrent
quantum yield should be taken as semiquantitative. Moreover,
it has been reported30 that a proportion of charge recombination
occurs on time scales less than the time resolution of our laser
flash photolysis setup. On the other hand, as has already been
pointed out by Boschloo and Hagfeldt,61 in practice rather high
laser pulse intensities are needed in order to obtain sufficient
signal in laser flash photolysis experiments on dye-sensitized
semiconductor electrodes, making a direct comparison with
conditions that exist under normal illumination slightly difficult.
Figure 7. (a) Transient photocurrent generated by PZn dye on ITO/
SnO2/electrodes after a laser pulse (λexc ) 532 nm) at different applied
bias potentials. H2Q 0.01 M in phosphate buffer. The arrow indicates
the increase of anodic bias. (b) Dependence of Φinjηc at the maximum
of the Soret bands of ITO/SnO2/dye electrodes on applied bias. H2Q
0.01 M in phosphate buffer.
TABLE 2: Photoelectrochemical Properties of Dyes
Φinjηc
Φinjηc
dye
ITO/SnO2/dyea
ITO/LB/dyea
kb × 106 s-1
b
PZn
0.10
0.25
0.35
0.49
0.007
0.040
0.028
0.126
8.13 (0.72)
0.25 (0.27)
4.79 (0.79)
0.17 (0.21)
2.23 (0.65)
0.11 (0.34)
1.66 (0.66)
0.14 (0.32)
P
PCu
PPd
a Maximum Φinjηc at Soret bands under bias voltage (V ) 0.05 V
vs Ag/AgCl; H2Q 0.01 M). The values are an average for more than
20 electrodes for each dye. b Back electron-transfer kinetics fitted by a
biexponential function; in parentheses, relative amplitudes (bias voltage
) 0.05 V vs Ag/AgCl).
porphyrins. Thus, low charge collection efficiency of the ITO/
SnO2/PZn system could be the limiting factor in photocurrent

Carboxyphenyl Metalloporphyrin Photosensitizers
J. Phys. Chem. B, Vol. 109, No. 44, 2005 20961
P
Zn in comparison with that of P. Thus, it is possible that, as in
the case of ruthenium and osmium polypyridyl complexes,20-22
the charge recombination rates in ITO/SnO2/PM photoelectrodes
occur in the Marcus inverted region. However, Clifford et al.18
also analyzed the molecular control of recombination dynamics
in dye-sensitized nanocrystalline TiO2 films and found an
excellent correlation between ket and the spatial separation for
a series of ruthenium bipyridyl dyes, showing the importance
of this factor and its implications in the design of the sensitizer
for dye-sensitized solar cells. In the present case, the studied
porphyrins that produce an observable photoelectric effect are
expected to be bound to the electrode surface through the
peripheral carboxylate ligand, with the dye cation orbital
delocalized over the conjugated macrocycle. The change of the
central metal affects the orbital distribution in the porphyrins62
and probably the distance to the surface, and this cannot be
ruled out as a possible factor that influences the electron-transfer
rate. The data we present here show a variation of kb that is
less than one decade over 250 mV of variation in Eox, and this
seems to be low for accounting for it only in terms of the match
of semiconductor and dye energy levels in the Marcus model.
Figure 9. Dependence of Φinjηc at the maximum of the Soret bands
of ITO/SnO2/dye electrodes (H2Q 0.01 M in phosphate buffer, voltage
) 0.05 V vs Ag/AgCl) on the component with the larger relative
amplitude of the back electron rate constant.
Conclusions
The quantum yields of sensitized photocurrent generation in
nanocrystallineSnO2 thin films with metalloporphyrins have
been shown to be highly dependent on the identity of the central
metal. Free-base porphyrin and its metal complexes with Zn-
(II), Cu(II), Pd(II), Ni(II), and Co(II) have been compared in
the spectral sensitization of wide band gap semiconductor
particles. The SnO2 films modified only with PZn, P, PCu, and
PPd porphyrin dyes exhibit a photoresponse in the visible region,
and the close match between the photocurrent action spectrum
and the absorption spectrum show that the photosensitization
mechanism is operative in extending the photocurrent response
of ITO/SnO2/dye electrode to the visible region.
In the case of PZn and P, electron injection yields were
observed to be high and nearly equal, as has been already
reported for similar molecular systems.13 This fact would
indicate that, in the present case, electron injection yield is not
the main cause of the difference observed in the Φinjηc product
for the different porphyrins.
Photocurrent quantum yields increase as the dye ground-state
oxidation potential (Eox) becomes more anodic in both the ITO/
SnO2/dye and ITO/LB electrodes. Laser flash photolysis experi-
ments showed that the back electron-transfer rates decrease with
the increase in the driving force for the recombination process,
indicating that electron recombination to the oxidized dye
probably occurs in the Marcus inverted region, affecting the
observed photocurrent quantum yield. This effect could be
exploited as a design element in the development of new and
better sensitizers for high-efficiency solar cells involving
porphyrins and related dyes. Under the present experimental
condition, the formation of a metal complex with palladium
produces a considerable increase in the photocurrent quantum
yield with respect to the widely used free-base and Zn(II)
porphyrins.
Figure 10. Dependence of Φinjηc at the same conditions that those in
Figure 9 of ITO/SnO2/dye electrodes on the ground-state dye’s oxidation
potential.
Nevertheless, as can be observed in Figure 9, back electron
transfer is faster in the case of the ITO/SnO2/PZn electrode and
decreases as the dye ground-state oxidation potential becomes
more anodic, with rates decreasing with increasing driving force
for the recombination process.
We found that, under our experimental conditions, photo-
current quantum yields obtained with porphyrin sensitizers
increase as the dye ground-state oxidation potential becomes
more anodic, as shown in Figure 10; meanwhile, in the case of
PZn and P, electron injection yields are high and nearly equal.
Likewise, Tachibana et al.19 measured similar and very fast
electron injection into TiO2 from free-base and Zn(II)-tetracar-
boxyphenil porphyrin, but they found that the recombination
kinetics for the free-base dye is approximately 8-fold slower
than that of the metallized one, which would be in agreement
with our observation of lower photocurrent quantum yield in
On the other hand, in the case of porphyrins where the
oxidation process is dominated by the metal-centered reaction
(Co(II)) or presents valence tautomerism (Ni(II)) it could be
possible that there exists a very fast charge recombination
between the photoinjected electrons and the M(III)-complex
cation that precludes photocurrent generation. However, inef-
ficient electron injection due to the very short lifetime originated

20962 J. Phys. Chem. B, Vol. 109, No. 44, 2005
Gervaldo et al.
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electrodes.
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Acknowledgment. We are grateful to Consejo Nacional de
Investigaciones Cient´ıficas y Te´cnicas (CONICET-Argentina),
Agencia Nacional de Promocio´n Cient´ıfica y Tecnole´gica
(ANPCYT-Argentina), Secretar´ıa de Ciencia y Te´cnica de la
Universidad Nacional de R´ıo Cuarto (SECYT-UNRC), and
Fundacio´n Antorchas for financial support. L.O., E.N.D., F.F.,
and J.J.S. are scientific members of CONICET. M.G. thanks
CONICET for a research fellowship. Thanks are also due to
Professor Previtali for his help in the laser flash photolysis
experiments.
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