Efficient light harvesting by using green Zn-porphyrin-sensitized nanocrystalline TiO2 films

Article abstract of DOI:10.1021/jp052877w

A series of novel intense green porphyrins containing carboxylate anchoring groups have been shown to have potential as light sensitizers in TiO₂ solar cells. The electrochemical, photophysical, and photoelectrochemical properties of a series of novel green Zn porphyrins as well as density functional theory (DFT) and time-dependent DFT (TDDFT) calculations on zinc tetraphenylporphyrin (ZnTPP), cyano-3-(2′-(5′,10′,15′,20′ttet raphenylporphyrinato zinc(II)yl)-acrylic acid (ZN-3), and 3-(trans-2′-(5′,10′15′,20′-tetr aphenylporphyrinato zinc(II)yl)-acrylic acid (Zn-5). DFT and TDDFT calculations showed that the presence of electron-withdrawing moieties on the substituent modulates the electronic absorption spectrum through stabilization of the energies of the LUMO and LUMO + 2. The extension of the LUMO and LUMO + 2 onto the substituent implied that occupation of these molecular orbitals offers the possibility of electron transfer from the substituent to the TiO₂ surface. The ground- and excited-state redox potentials showed that the porphyrins possess suitable potentials to inject electrons into the TiO₂ substrate, yielding 85% incident-to-electron conversion efficiencies (IPCE). Among the reported porphyrins, the green Zn-3 showed the highest power conversion efficiency obtained so far in nanocrystalline films.

Full text of DOI:10.1021/jp052877w

J. Phys. Chem. B 2005, 109, 15397-15409
15397
Efficient Light Harvesting by Using Green Zn-Porphyrin-Sensitized Nanocrystalline TiO2
Films
Qing Wang,† Wayne M. Campbell,‡ Edia E. Bonfantani,‡ Kenneth W. Jolley,‡
David L. Officer,*,‡ Penny J. Walsh,§ Keith Gordon,§ Robin Humphry-Baker,†
Mohammad K. Nazeeruddin,*,† and Michael Gra1tzel*,†
Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Swiss Federal
Institute of Technology, CH-1015 Lausanne, Switzerland, Nanomaterials Research Centre and MacDiarmid
Institute for AdVanced Materials and Nanotechnology, Massey UniVersity, Palmerston North, New Zealand, and
MacDiarmid Institute for AdVanced Materials and Nanotechnology, Chemistry Department,
UniVersity of Otago, Dunedin, New Zealand
ReceiVed: May 31, 2005
A series of novel zinc metalloporphyrins, cyano-3-(2′-(5′,10′,15′,20′-tetraphenylporphyrinato zinc(II))yl)-
acrylic acid (Zn-3), 3-(trans-2′-(5′,10′,15′,20′-tetraphenylporphyrinato zinc(II))yl)-acrylic acid (Zn-5), 2-cyano-
5-(2′-(5′,10′,15′,20′-tetraphenylporphyrinato zinc(II))yl)-penta-2,4-dienoic acid (Zn-8), 4-(trans-2′-(2′′-(5′′,-
10′′,15′′,20′′-tetraphenylporphyrinato zinc(II))yl)ethen-1′-yl))-1,2-benzenedicarboxylic acid (Zn-11), and
2-cyano-3-[4′-(trans-2′′-(2′′′-(5′′′,10′′′,15′′′,20′′′-tetraphenylporphyrinato zinc(II))yl) ethen-1′′-yl)-phenyl]-acrylic
acid (Zn-13) were synthesized and characterized by using various spectroscopic techniques. Density functional
theory (DFT) and time-dependent DFT (TDDFT) calculations show that key molecular orbitals (MOs) of
porphyrins Zn-5 and Zn-3 are stabilized and extended out onto the substituent by π-conjugation, causing
enhancement and red shifts of visible transitions and increasing the possibility of electron transfer from the
substituent. The porphyrins were investigated for conversion of sunlight into electricity by constructing dye-
sensitized TiO2 solar cells using an I-/I3- electrolyte. The cells yield close to 85% incident photon-to-current
efficiencies (IPCEs), and under standard AM 1.5 sunlight, the Zn-3-sensitized solar cell demonstrates a short
circuit photocurrent density of 13.0 ( 0.5 mA/cm2, an open-circuit voltage of 610 ( 50 mV, and a fill factor
of 0.70 ( 0.03. This corresponds to an overall conversion efficiency of 5.6%, making it the most efficient
porphyrin-sensitized solar cell reported to date.
I. Introduction
light over a wide wavelength range, preferably one that
encompasses the visible spectrum. Another essential property
is efficient electron transfer from the excited dye to the TiO2
conduction band; i.e., good electronic coupling between the
lowest unoccupied orbital (LUMO) of the dye and the Ti 3d
orbitals is required. Effective coupling has typically been
accomplished with carboxylate or phosphonate groups, which
bind tightly to the TiO2 surface via a bridging bidentate
mode.12,13 To date, ruthenium polypyridyl complexes have
proven to be the most efficient TiO2 sensitizers, with the N3
dye demonstrating incident photon-to-electron conversion ef-
ficiencies (IPCEs) of up to 85% from 400 to 800 nm and an
overall photovoltaic cell energy conversion efficiency (η) of
11%.4,6
Nature accomplishes enhanced light absorption by stacking
chlorophyll-containing thylakoid membranes of the chloroplast
to form the grana structures that act as light-harvesting antenna.14
These absorb the incident light and then channel the excitation
energy to reaction centers, where light-induced charge separation
takes place.15 Given their primary role in photosynthesis, the
use of porphyrins as light harvesters on semiconductors is
particularly attractive. Owing to the delocalized macrocyclic
structure and very strong absorption in the 400-450 nm region
(Soret band) as well as absorption in the 500-700 nm region
(Q-bands), porphyrins have long been studied as promising
components of molecular electronic and photonic devices,16,17
and numerous artificial photonic assemblies based on multi-
Photoinduced interfacial charge transfer between a discrete
molecular excited state and a continuum of acceptor levels in a
solid is an important photochemical surface reaction1 that
influences the performance of dye-sensitized solar cells.2-4 In
contrast to conventional solar cell systems, where the semicon-
ductor assumes both the task of light absorption and charge
carrier, in dye-sensitized solar cells light is absorbed by the
anchored dye and charge separation takes place at the interface
via photoinduced electron injection from the dye into the
conduction band of the solid. During the last 10 years, with the
development of nanocrystalline films of very high surface area,5
the photosensitization of wide-band-gap semiconductors such
as TiO2 by adsorbed dyes has become more realistic for solar
cell applications.6-10 In a porous film consisting of nanometer-
sized TiO2 particles, the effective surface area for dye adsorption
can be greatly enhanced and efficient light absorption is achieved
from dye monolayers.11
For a molecular/semiconductor junction light-harvesting
system, the first requirement is that the sensitizing dye absorbs
* Authors to whom correspondence should be addressed. Phone:
0041-21-693 6124 (M.K.N.). Fax: 0041-21-693 4111 (M.K.N.).
E-mail:
D.Officer@massey.ac.nz;
MdKhaja.Nazeeruddin@epfl.ch;
michael.graetzel@epfl.ch.
† Swiss Federal Institute of Technology.
‡ Massey University.
§ University of Otago
10.1021/jp052877w CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/21/2005

15398 J. Phys. Chem. B, Vol. 109, No. 32, 2005
Wang et al.
the η value.12,30 Cell efficiency was also found to be susceptible
to porphyrin metalation, type of acid binder, acid position,
binding solvent, and the electrolyte used.30 Replacement of the
porphyrin phenyl substituents with xylyl groups, ZnTXPSCA
(Figure 1), further improved cell efficiency (η ) 4.8%).12 It is
apparent from these studies that the strength of binding of the
dye to the semiconductor has a significant influence on the cell
efficiency, and this presents the opportunity to explore both the
nature of the linker and the type of carboxylic acid in this type
of conjugatively substituted porphyrin.
The modification of meso-tetraphenylporphyrins by substitu-
tion at the â-position with functional groups with extended π
systems, leads to the enhancement of the red-absorbing Q-
bands.33 This is caused by the splitting of the four frontier
molecular orbitals (FMOs) of Gouterman’s four-orbital model.34
Computational chemistry offers an opportunity to gain insight
into the electronic processes, which occur in the FMOs of
porphyrins. Local and nonhybrid methods including LSDA,35
BP86,36 and PW9137 significantly underestimate the oscillator
strengths (f) of the B-bands and give deviations in the energies
of the transitions of 0.5-1.1 eV. However, density functional
theory (DFT) methods such as B3LYP38 have been found to
rival the multireference perturbation theory (PT2) and couple
cluster (SAC-CI, CC) methods for these systems.39 A number
of comparisons between calculated ground-state absorption
spectra using various functionals have been made.40-42 Calcula-
tions using hybrid functionals such as B3LYP are more accurate
over a wider spectral range.
In this paper, we report the electrochemical, photophysical,
and photoelectrochemical properties of a series of novel green
Zn porphyrins as well as density functional theory (DFT) and
time-dependent DFT (TDDFT) calculations on zinc tetraphe-
nylporphyrin (ZnTPP), Zn-3, and Zn-5. It will be shown that
the nature of the carboxylic acid linker to the porphyrin has a
significant influence on the light-harvesting and photovoltaic
properties of the device and that a cell that utilizing the dye
Zn-3 exhibits an efficiency of 5.6%, the best value achieved
so far for a porphyrin dye.
Figure 1. Structures of porphyrin sensitizers.
porphyrin architectures have been designed to mimic photo-
synthetic solar energy transduction by converting excitation
energy to chemical potential in the form of long-lived charge
separation.18-20
Since the earliest reports of the efficient charge injection into
nanocrystalline TiO2 by covalently bound zinc tetrakis(4-
carboxyphenyl)porphyrin (ZnTCPP, Figure 1),21-23 the photo-
sensitization of TiO2 electrodes by porphyrins has been exten-
sively studied. It has been demonstrated that, despite the
differences in oxidation potentials and photophysics between
carboxylated porphyrin sensitizers such as ZnTCPP and its free-
base H2TCPP (Figure 1) and the ruthenium polypyridyl N3 dye,
the efficiency of electron injection into the TiO2 conduction
band and the kinetics of electron injection and recombination
are indistinguishable for all these sensitizers.24 In addition, the
rate of charge recombination between conduction band electrons
and oxidized porphyrins is in the range of several milliseconds,
a time that is sufficiently slow to permit the regeneration of the
ground state of the porphyrin by the iodide in the electrolyte.25
Despite these observations, porphyrins have not proven to be
as efficient sensitizers as ruthenium polypyridyl dyes.
For photovoltaic cells with H2TCPP-sensitized TiO2 elec-
trodes, Cherian and Wamser have reported IPCE values of 25-
55% and a η of 3%,26 although other groups have obtained
somewhat lower IPCE and η values for H2TCPP,25,27-29
ZnTCPP,30 and similar porphyrins.25,29-31 These differences have
been ascribed to aggregation, the effect of coadsorbates, and
poor electronic interaction between the carboxylate linker and
the porphyrin core. This latter issue had been addressed by
Gra¨tzel and co-workers who investigated the sensitization of
TiO2 with a variety of chlorophyll derivatives and related
mesoporphyrins.32 They obtained IPCE values of over 80% and
a η value of 2.6% for a TiO2 electrode sensitized with copper
mesoporphyrin (CuMP, Figure 1), leading to the conclusion that
conjugation of the carboxyl group with the π-electron system
of the chromophore is not essential for efficient electron transfer.
Recently, however, Campbell et al. demonstrated a 40% increase
in cell efficiency (η ) 4.2%) for a tetraphenylporphyrin with a
â-substituted styrylcarboxylic acid (ZnTPPSCA, Figure 1), the
single acid linker being in full conjugation with the porphyrin
π system; removal of the conjugation resulted in a halving of
II. Experimental Section
A. Materials and General Synthetic Procedures. Analytical
reagent grade solvents and reagents were used for synthesis,
and distilled laboratory grade solvents were used for chroma-
tography. Reverse osmosis or Milli-Q water was used for
synthetic and purification purposes. Dry toluene and tetrahy-
drofuran (THF) were prepared by passing argon-degassed
solvent through activated alumina columns. N2 (oxygen-free)
was passed through a KOH drying column to remove moisture.
The intermediate compounds TPP-CHO 1 (Scheme 1) and
TPPps 10 (Scheme 2) were prepared according to Bonfantini
et al.43 TPPdPhCHO 12 (Scheme 2) was synthesized according
to Officer et al.44
Column chromatography employed silica gel (0.032-0.063
mm, Merck Kieselgel 60) or the equivalent. Thin-layer chro-
matography (TLC) was performed using precoated silica gel
plates (Merck Kieselgel 60F254). Either gravity or flash chro-
matography was used for compound purification. Where a dual
solvent system was used, gradient elution was employed, and
the major band was collected. All fractions or solutions
containing a single spot by TLC with the same Rf were
combined and filtered, and the solvent was removed in vacuo
(rotary evaporation followed by high vacuum). All solid
precipitates were separated by filtration or centrifugation, rinsing
with the precipitating solvent, then dried under high vacuum

Zn-Porphyrin-Sensitized TiO2 Films
J. Phys. Chem. B, Vol. 109, No. 32, 2005 15399
SCHEME 1: Synthesis of â-Ethenyl Carboxylic Acids
b
c
a Zn(OAc)2‚2H2O, MeOH/CHCl3, room temperature (20 min). Piperidine (22 equiv), acetonitrile, reflux (2.5 h), N2. H3PO4 (2 M, pH ≈ 2.0).
dToluene, reflux (23.5 h), N2. (i) I2 (1.0 equiv), CHCl3, room temperature (14 h); (ii) saturated Na2S2O3 (excess). Zn(OAc)2‚2H2O (1.2 equiv),
e
f
g
h
CHCl3/MeOH, room temperature (30 min). (i) NaOH (20 equiv), THF/H2O/EtOH (5:1:5), reflux (40 min); (ii) H3PO4 (2 M, pH ≈ 2.5). (i)
DIBAL-H (3.0 equiv), toluene, 0 °C f room temperature f 0 °C, argon; (ii) MeOH. iMnO2 (excess), CH2Cl2, room temperature. jZn(OAc)2‚2H2O
(1.2 equiv), MeOH/CHCl3, room temperature (10 min). (i) Piperidine (24 equiv), MeOH, reflux (50 min), N2; (ii) H+.
k
overnight. All porphyrin reactions were carried out under a
nitrogen or argon atmosphere using dry degassed solvents, and
the apparatus was shielded from ambient light.
Major fragmentations are given as percentages relative to the
base peak intensity.
1H NMR spectra were obtained at 400.132 MHz using a
Bruker 400 Avance NMR spectrometer running X-WIN NMR
software. The chemical shifts are relative to tetramethylsilane
(TMS).
B. Analytical Measurements. UV-vis absorption spectra
were measured on a Cary 5 spectrophotometer, and fluorescence
spectra were recorded on a Spex Fluorolog 112 spectrofluo-
rimeter. Samples were contained in 1-cm path-length quartz
cells. Emission lifetimes were measured by exciting with a pulse
from an active mode-locked Nd:YAG laser, using the frequency-
doubled line at 532 nm. The emission decay was followed on
a Tektronix DSA 640 digitizing signal analyzer, using a
Hamamatsu R928 photomultiplier to convert the light signal to
a voltage waveform.
Voltammetric measurements were performed on a PC-
controlled AutoLab PSTAT10 electrochemical workstation (Eco
Chemie). Cyclic voltammetry experiments were performed on
1 mM Zn porphyrin solutions in dimethylformamide (DMF) at
a scan rate of 0.1 V/s using 0.1 M tetra-n-butylammonium
hexafluorophosphate (TBAPF6) as the supporting electrolyte and
a 2 mm platinum disk as the working electrode. After a cyclic
voltammogram (CV) had been recorded, ferrocene was added,
and a second voltammogram was measured. For measurements
made on Zn porphyrins adsorbed onto TiO2 films, the electrolyte
was 0.2 M TBAPF6 in acetonitrile. Platinum foil and a silver
Fast-atom bombardment (FAB) mass spectra were recorded
on a Varian VG70-SE double-focusing magnetic sector mass
spectrometer. Samples were supported in a m-nitrobenzyl
alcohol matrix. The data were put through a MASSPEC II data
system to give (5 ppm error formulations on molecular ions.

15400 J. Phys. Chem. B, Vol. 109, No. 32, 2005
Wang et al.
SCHEME 2: Synthesis of â-Styryl Carboxylic Acids
a DBU (3.0 equiv), CHCl3, reflux (15 min), N2. b(i) I2 (2.4 equiv), CH2Cl2, room temperature (3 h); (ii) saturated Na2S2O3 (excess). cZn(OAc)2‚2H2O
d
(1.2 equiv), CHCl3/MeOH, room temperature (15 min). (i) NaOH (20 equiv per CO2Me), THF/H2O/MeOH (1:0.2:1), reflux (2 h), N2; (ii) H3PO4
(2 M, pH ≈ 2.0). eDBU (5.5 equiv), CH2Cl2, room temperature (1 min). f(i) I2 (0.7 equiv), CH2Cl2, room temperature (16 h); (ii) saturated Na2S2O3
(excess). gZn(OAc)2‚2H2O (1.2 equiv), MeOH/CHCl3, room temperature (20 min). h(i) Piperidine (55 equiv), MeOH, reflux (15 h), argon; (ii) H+.
disk were employed as the counter and quasi-reference electrode,
respectively.
nitrogen adsorption apparatus (Micromeretics Instrument Co.).
The TiO2 plates (electrodes) were dipped into a 2 × 10-4
M
dye solution in THF and left for 2 h. Finally, the dye-coated
electrodes were rinsed with acetonitrile.
The Fourier transform IR (FTIR) spectra for all the samples
were measured using a FTS Digilab 7000 FTIR spectrometer.
The attenuated total reflectance (ATR) data reported here was
taken with the Golden Gate diamond anvil ATR accessory
(Graseby-Specac), using typically 64 scans at a resolution of 2
cm-1. All samples were placed in contact with the diamond
window using the same mechanical force. The dye-coated TiO2
films were rinsed in acetonitrile and dried at 150 °C before
measuring the spectra. The FTIR spectra of anchored dyes were
obtained by subtracting the IR spectrum of the blank TiO2 films
from the IR spectrum of the dye-coated TiO2 films of the same
thickness.
C. Preparation of Dye-Sensitized Nanocrystalline TiO2
Electrodes. A paste consisting of 20-nm-sized TiO2 colloid and
ethyl cellulose in terpineol was first screen-printed on a fluorine-
doped SnO2 conducting glass (TEC15, 15 Ω/cm) to form a
transparent layer. Subsequently, a second scattering layer made
up of a paste containing 400 nm anatase TiO2 particles (CCIC,
Japan) was further coated on it to form a light-scattering layer.
Last, the screen-printed double-layer film was heated to 500
°C in an oxygen atmosphere and calcinated for 10 min. The
final film thickness was determined by using an Alpha-step 200
surface profilometer (Tencor Instruments). A porosity of 0.63
for the transparent layer was measured with a Gemini 2327
D. Dye-Sensitized Solar Cell Fabrication. A sandwich cell
was prepared using the dye-sensitized electrode as the working
electrode and a platinum-coated conducting glass electrode as
the counter electrode. The latter was prepared by chemical
deposition of platinum from 0.05 M hexachloroplatinic acid.
The two electrodes were placed on top of each other using a
thin transparent film of Surlyn polymer (DuPont) as a spacer
to form the electrolyte space. The empty cell was tightly held,
and the edges were heated to 130 °C to seal the two electrodes
together. A thin layer of electrolyte (0.6 M BMII, 0.05 M I2,
0.1 M LiI, 0.5 M tert-butyl pyridine in 1:1 acetonitrile/
valeronitrile) was introduced into the interelectrode space from
the counter electrode side through a predrilled hole. The hole
was sealed with a microscope cover slide and Surlyn to avoid
leakage of the electrolyte solution.
E. Photoelectrochemical Measurements. The photovoltaic
performances of the Zn-porphyrin-sensitized nanocrystalline
TiO2 cells were determined using two complementary tech-
niques. The spectral response was determined by measuring the
wavelength dependence of the incident photon-to-current con-
version efficiency (IPCE) using light from a 300-W xenon lamp
(ILC Technology) that was focused onto the cell through a

Zn-Porphyrin-Sensitized TiO2 Films
J. Phys. Chem. B, Vol. 109, No. 32, 2005 15401
Gemini-180 double monochromator (Jobin Yvon Ltd., U. K.).
The monochromator was incremented through the visible
spectrum to generate the IPCE(λ) curve as defined below
and 8.793 (ABq, 2H, 3J ) 4.7, 4.7 Hz, Hâ-pyrrolic), 9.298 (d, 1H,
4J ) 1.1 Hz, H3′(â-pyrrolic)). Assignments aided by correlation
spectroscopy (COSY) spectra. UV-vis (THF) λmax (nm) (ꢀ (103
M-1 cm-1)): sh 408 (41.7), 454 (150), 571 (12.6), 620 (11.5).
FAB-LRMS m/z (%, assignment): 86 (100, piperidine+), cluster
at 771-778, 771 (25, M+).
IPCE(λ) ) 12 400(Jsc/λΦ)
(1)
where λ is the wavelength (nm), Jsc is the photocurrent under
short circuit conditions (mA/cm2), and Φ is the incident radiative
flux (mW/cm2). The whole experiment was run automatically
using Wavemetrics Igor Pro software. For I-V measurements,
a 450-W xenon light source (Osram XBO 450) was used as the
irradiation source. The spectral output of the lamp matched the
AM 1.5 solar spectrum in the region of 350-750 nm (mismatch
<2%). Incident light intensities were adjusted with neutral wire
mesh attenuators. The current-voltage characteristics were
determined by applying an external potential bias to the cell
and measuring the photocurrent using a Keithley model 2400
digital source meter. The overall conversion efficiency η of the
photovoltaic cell is calculated from the integral photocurrent
density (Jsc), the open-circuit photovoltage (Voc), the fill factor
of the cell (ff), and the intensity of the incident light (IPh)
3. 2-Cyano-3-(2′-(5′,10′,15′,20′-tetraphenylporphyrinato Zinc-
(II))yl)acrylic Acid (Zn-3). Zn-2 (140 mg, 163 µmol) was
dissolved in DMSO-d6 (1.5 mL). H2O (150 mL) and CHCl3
(50 mL) were added, and the mixture was transferred to a
separating funnel. Next, 2 M H3PO4 (2.0 mL) was added, and
the mixture was shaken vigorously (pH ≈ 2.0). The organic
layer was washed with H2O (150 mL) containing 2 M H3PO4
(1.0 mL) and then carefully separated. The CHCl3 was removed
by azeotrope distillation using acetonitrile in vacuo. Precipitation
from the resulting acetonitrile solution (∼50 mL) using H2O
gave Zn-3 (117.7 mg, 93%) as a green powder. 1H NMR (400
MHz, DMSO-d6, TMS, δ): 7.74-7.85 (m, 12H, Hm,p-Ph), 8.07-
8.11 (m, 3H, 2Ho-Ph + 1H3), 8.15-8.21 (m, 6H, Ho-Ph), 8.759
3
and 8.800 (ABq, 2H, J ) 4.7, 4.7 Hz, Hâ-pyrrolic), 8.740 and
3
8.748 (ABq, 2H, J ) 4.7, 4.7 Hz, Hâ-pyrrolic), 8.881 and 8.900
(ABq, 2H, 3J ) 4.7, 4.7 Hz, Hâ-pyrrolic), 9.492 (d, 1H, 4J ) 1.0
Hz, H3′(â-pyrrolic)), 13.470 (br s, 1H, CO2H). Assignments aided
by COSY spectra. UV-vis (THF) λmax (nm) (ꢀ (103 M-1 cm-1)):
407 (42.5), sh 429 (92.4), 455 (153), 571 (12.7), 620 (11.9).
FAB-LRMS m/z (%, assignment): cluster at 771-778, 771
(100, M+). HRMS for M+ (C48H29N5O2Zn): calcd, 771.1613;
found, 771.1637.
η ) JscVocff/IPh
(2)
F. Computational Methods. The Gaussian 03W package45
was used to perform calculations for ZnTPP, Zn-3, and Zn-5.
Molecular orbitals were visualized using Gaussview. The
geometry optimizations, vibrational frequencies, and time-
dependent calculations were performed using DFT (B3LYP/
3-21G*). The optimized structure was used to calculate vibra-
tional frequencies. Importantly, none of the frequency calcula-
tions generated negative frequencies; this is consistent with an
energy minimum for the optimized geometry.
G. Synthesis. 1. 2-Formyl-5,10,15,20-tetraphenylporphyri-
nato Zinc(II) (Zn-1). A solution of Zn(OAc)2‚2H2O (123 mg,
560 µmol, 1.2 equiv) in MeOH (3.0 mL) was added to a solution
of 143 (300 mg, 467 µmol) in CHCl3 (20 mL) with stirring at
room temperature. After 20 min, TLC analysis indicated that
the reaction was complete with the appearance of a new more
polar green band (Rf ) 0.10, silica, 2:1 (CH2Cl2/hexane).
Precipitation using MeOH gave the crude product. Recrystal-
lization from CH2Cl2/MeOH gave Zn-1 (304.7 mg, 92%) as a
purple microcrystalline solid. 1H NMR (400 MHz, CDCl3, TMS,
δ): 7.71-7.83 (m, 12H, Hm,p-Ph), 8.15-8.23 (m, 8H, Ho-Ph),
8.88-8.94 (m, 6H, Hâ-pyrrolic), 9.224 (s, 1H, H3′′(â-pyrrolic)), 9.515
(s, CHO). UV-vis (CH2Cl2) λmax (nm) (ꢀ (103 M-1 cm-1)):
432 (342), 526 (4.08), 560 (15.6), 602 (11.3), 681 (8.39). Fast-
atom bombardment low-resolution mass spectroscopy (FAB-
LRMS) m/z (%, assignment): cluster at 704-710, 704 (100,
M+). High-resolution mass spectroscopy (HRMS) for M+
(C45H28N4OZn): calcd, 704.1555; found, 704.1542.
2. 2-Cyano-3-(2′-(5′,10′,15′,20′-tetraphenylporphyrinato Zinc-
(II))yl)acrylic Acid Piperidine Salt (Zn-2). A solution of Zn-1
(250 mg, 354 µmol), cyanoacetic acid (90 mg, 1.06 mmol, 3.0
equiv), and piperidine (654 µL, 7.68 mmol, 22 equiv) in
acetonitrile (50 mL) was heated at reflux temperature for 1 h.
After being cooled to room temperature, the resulting green
precipitate was collected by filtration (#4 glass sinter) and rinsed
with acetonitrile (2.0 mL). Recrystallization from CHCl3/5%
MeOH/acetonitrile gave Zn-2 (240.2 mg, 79%) as a purple solid.
1H NMR (400 MHz, deuterated dimethylsufoxide (DMSO-d6),
TMS, δ): 1.510 (br s, 6H, CH2,piperidine), 2.716 (br s, 4H,
CH2,piperidine), 7.67-7.82 (m, 13H, 12Hm,p-Ph + 1H3), 8.01-8.04
(m, 2H, Ho-Ph), 8.16-8.21 (m, 6H, Ho-Ph), 8.635 and 8.717 (ABq,
2H, 3J ) 4.7, 4.7 Hz, Hâ-pyrrolic), 8.749 (s, 2H, Hâ-pyrrolic), 8.769
4. 3-trans-(5′,10′,15′,20′-Tetraphenylporphyrin-2′-yl)acrylic
Acid Ethyl Ester (4). a. Wittig Reaction. A solution of 1 (550
mg, 0.856 mmol) and ethyl (triphenylphosphoranylidene)acetate
(1.18 g, 3.38 mmol, 3.9 equiv) in dry toluene (60 mL) was
heated at reflux temperature under N2. After 23.5 h, TLC
analysis (silica, toluene) indicated that all of starting material 1
had been consumed. After being cooled to room temperature,
the solvent was removed in vacuo. The residue was purified by
column chromatography (silica, 45 mmdia, 100 mm, CH2Cl2/
hexane (2:1)), collecting the major purple fraction to give a cis/
1
trans isomeric mixture of 4 (633 mg, ∼40% cis by H NMR,
1
103%) as a purple solid. H NMR (270 MHz, CDCl3, TMS,
selected data only, δ): -2.725 (s, NHcis), -2.618 (s, NHtrans),
3
3
0.833 (t, J ) 7.3 Hz, CH2-CH3-cis), 3.961 (q, J ) 7.3 Hz,
3
CH2-CH3-cis), 5.628 (d, J ) 11.9 Hz, Hcis-ethenyl), 6.884 (dd,
3J ) 12.4 Hz, J ) 1.2 Hz, Hcis-ethenyl).
4
b. Isomerization. The isomeric mixture was dissolved in
CHCl3 (40 mL) and I2 (255 mg, 0.856 mmol, 1.0 equiv) was
added. After the solution was stirred at room temperature for
14 h in darkness, excess saturated Na2S2O3 (∼20 mL) was
added, and stirring continued for 30 min. The organic layer was
separated and dried (MgSO4), and the product was precipitated
with methanol to give trans-4 (549 mg, 90%) as a dark brown
powder. 1H NMR (400 MHz, CDCl3, TMS, δ): -2.619 (s, 2H,
NH), 1.360 (t, 3H, 3J ) 7.3 Hz, OCH2-CH3), 4.230 (q, 2H, 3J
) 7.1 Hz, OCH2-CH3), 6.574 (d, 1H, 3J ) 15.6 Hz, H2), 7.416
3
4
(dd, 1H, J ) 15.6 Hz, J ) 0.7 Hz, H3), 7.71-7.86 (m, 12H,
H
m,p-Ph), 8.11-8.21 (m, 8H, Ho-Ph), 8.77-8.84 (m, 6H, Hâ-pyrrolic),
8.975 (s, 1H, H3′(â-pyrrolic)). UV-vis (CH2Cl2) λmax (nm) (ꢀ (103
M-1 cm-1)): 429 (256), 523 (17.8), 563 (7.44), 601 (6.03), 659
(4.09). FAB-LRMS m/z (%, assignment): cluster at 712-716,
713 (100, MH+). HRMS for MH+ (C49H37N4O2): calcd,
713.2917; found, 713.2908.
5. 3-(trans-2′-(5′,10′,15′,20′-Tetraphenylporphyrinato Zinc-
(II))yl)acrylic Acid Ethyl Ester (Zn-4). A solution of Zn(OAc)2‚
2H2O (73.9 mg, 337 µmol, 1.2 equiv) in MeOH (2.0 mL) was

15402 J. Phys. Chem. B, Vol. 109, No. 32, 2005
Wang et al.
4
3
added to a solution of trans-4 (200 mg, 281 µmol) in CHCl3
(20 mL) with stirring at room temperature under a N2 atmo-
sphere. After 30 min, TLC analysis (silica, toluene) indicated
that all of 4 had been consumed with the appearance of a single
new spot at a lower Rf value. The solvent was removed in vacuo,
and the residue was purified by column chromatography (silica,
37 mmdia × 60 mm, CH2Cl2/hexane (2:1)), collecting the major
red-green band to give Zn-4 (217 mg, 100%) as a purple
15.6 Hz, J ) 0.8 Hz, H3(ethenyl)), 7.478 (dt, 1H, J ) 15.6 Hz,
3J ) 5.5 Hz, H2(ethenyl)), 7.70-7.83 (m, 12H, Hm,p-Ph), 8.06-
8.10 (m, 2H, Ho-Ph), 8.18-8.21 (m, 6H, Ho-Ph), 8.736 (d, 1H, 3J
) 4.8 Hz, Hâ-pyrrolic), 8.783 (d, 2H, 3J ) 4.8 Hz, Hâ-pyrrolic), 8.80-
8.85 (m, 4H, Hâ-pyrrolic). UV-vis (CH2Cl2) λmax (nm) (ꢀ (103
M-1 cm-1)): 423 (240), 519 (17.1), 556 (6.25), 595 (5.27), 650
(2.71). FAB-LRMS m/z (%, assignment): cluster at 670-673,
671 (100, MH+). HRMS for MH+ (C47H35N4O1): calcd,
671.2811; found, 671.2775.
1
powder. H NMR (400 MHz, CDCl3, TMS, δ): 1.326 (t, 3H,
3J ) 7.1 Hz, OCH2-CH3), 4.133 (q, 2H, 3J ) 7.2 Hz, OCH2-
8. 3-(5′,10′,15′,20′-Tetraphenylporphyrin-2′-yl)allyl Aldehyde
(7). Activated MnO2 (367 mg, 4.22 mmol) was added to a
solution of 6 (200 mg, 0.298 mmol) in dry CH2Cl2 (4.0 mL)
and stirred at room temperature. After 19 h, TLC analysis (silica,
CH2Cl2, Rf ) 0.5) indicated all starting material had been
consumed with the appearance of a new less polar band. The
solution was filtered through Celite, and the solvent was
removed in vacuo. The residue was purified by column
chromatography (silica, 37 mmdia × 140 mm, CH2Cl2/hexane
(4:1)) to give 7 (183 mg, 89%) as a purple solid. 1H NMR (400
MHz, CDCl3, TMS, δ): -2.590 (br s, 2H, NH), 6.859 (dd, 1H,
3J ) 15.5 Hz, 3J ) 7.9 Hz, H2(ethenyl)), 7.007 (d, 1H, 3J ) 15.5
Hz, H3(ethenyl)), 7.72-7.85 (m, 12H, Hm,p-Ph), 8.11-8.13 (m, 2H,
3
3
CH3), 6.449 (d, 1H, J ) 15.6 Hz, H2), 7.391 (dd, 1H, J )
15.6 Hz, 4J ) 0.9 Hz, H3), 7.69-7.81 (m, 12H, Hm,p-Ph), 8.08-
8.10 (m, 2H, Ho-Ph), 8.18-8.20 (m, 6H, Ho-Ph), 8.838 and 8.884
(ABq, 2H, 3J ) 4.7, 4.7 Hz, Hâ-pyrrolic), 8.906 and 8.918 (ABq,
2H, 3J ) 4.7, 4.7 Hz, Hâ-pyrrolic), 8.907 (s, 2H, Hâ-pyrrolic), 9.083
(d, 1H, 4J ) 0.9 Hz, H3′(â-pyrrolic)). Assignments aided by COSY
spectra. UV-vis (CH2Cl2) λmax (nm) (ꢀ (103 M-1 cm-1)) 432
(330), 556 (21.8), 596 (8.81). FAB-LRMS m/z (%, assign-
ment): cluster at 774-780, 774 (100, M+). HRMS for M+
(C49H34N4O2Zn): calcd, 774.1973; found, 774.2001.
6. 3-(trans-2′-(5′,10′,15′,20′-Tetraphenylporphyrinato Zinc-
(II))yl)acrylic Acid (Zn-5). A solution of NaOH (156 mg, 20
equiv, 3.9 mmol) in H2O (3.0 mL) was added to a refluxing
solution of Zn-4 (150 mg, 193 µmol) in THF (15 mL) and
ethanol (15 mL) under a N2 atmosphere. After 90 min, TLC
analysis indicated that all of Zn-4 had been consumed. After
the solution was cooled to room temperature, CH2Cl2 (200 mL),
H2O (800 mL), Et2O (600 mL), and aqueous 2 M H3PO4 (7.0
mL) was added, and the solution was transferred to a separating
funnel and shaken vigorously (pH ≈ 2.5). The organic layer
was rinsed with H2O (200 mL, containing 1.0 mL of aqueous
2 M H3PO4), separated, and dried (MgSO4). The solvent was
removed in vacuo, and the residue was dissolved in a mixture
of warm acetone (∼30 mL) and THF (enough to dissolve all
the solid), then filtered (#4 glass sinter). The product was
precipitated from solution with H2O (∼20 mL, containing 4
drops acetic acid) and rinsed with acetone/H2O (1:1) to give
Zn-5 (146 mg, 100%) as a purple powder. 1H NMR (400 MHz,
H
o-Ph), 8.18-8.21 (m, 6H, Ho-Ph), 8.784 and 8.794 (ABq, 2H,
3J ) 4.8, 4.8 Hz, Hâ-pyrrolic), 8.842 (s, 2H, Hâ-pyrrolic), 8.860 (s,
3
2H, Hâ-pyrrolic), 8.996 (s, 1H, H3′(â-pyrrolic)), 9.209 (d, 1H, J )
7.9 Hz, CHO). UV-vis (CH2Cl2) λmax (nm) (ꢀ (103 M-1
cm-1)): 434 (213), 525 (21.4), 567 (8.82), 604 (7.14), 661
(5.19). FAB-LRMS m/z (%, assignment): cluster centered at
669 (100, MH+). HRMS for MH+ (C47H33N4O1): calcd,
669.2654; found, 669.2625.
9. 3-(2′-(5′,10′,15′,20′-Tetraphenylporphyrinato Zinc(II))yl)-
allyl Aldehyde (Zn-7). A solution of Zn(OAc)2‚2H2O (79 mg,
359 µmol, 1.2 equiv) in MeOH (1.5 mL) was added to a solution
of 7 (200 mg, 299 µmol) in CHCl3 (20 mL) with stirring at
room temperature. After 10 min, TLC analysis indicated that
the reaction was complete with the appearance of a new more
polar green band (Rf ) 0.38, silica, CH2Cl2). Precipitation from
methanol gave Zn-7 (219 mg, 100%) as a purple microcrys-
3
1
DMSO-d6, TMS, δ): 6.424 (d, 1H, J ) 15.5 Hz, H2), 7.327
talline powder. H NMR (400 MHz, CDCl3, TMS, δ): 6.632
(d, 1H, 3J ) 15.5 Hz, H3). 7.73-7.85 (m, 12H, Hm,p-Ph), 8.06-
8.08 (m, 2H, Ho-Ph), 8.16-8.21 (m, 6H, Ho-Ph), 8.664 and 8.721
(ABq, 2H, 3J ) 4.7, 4.7 Hz, Hâ-pyrrolic), 8.73-8.77 (m, 4H, Hâ-
pyrrolic), 8.939 (d, 1H, 4J ) 0.7 Hz, H3′(â-pyrrolic)), 12.0 (br s, 1H,
CO2H). Assignments aided by COSY spectra. UV-vis (THF)
(dd, 1H, 3J ) 15.4 Hz, 3J ) 8.0 Hz, H2(ethenyl)), 6.963 (dd, 1H,
3J ) 15.6 Hz, J ) 0.7 Hz, H3(ethenyl)), 7.67-7.82 (m, 12H,
4
H
m,p-Ph), 8.11-8.13 (m, 2H, Ho-Ph), 8.07-8.09 (m, 6H, Ho-Ph),
4
8.85-8.92 (m, 7H, 6Hâ-pyrrolic + 1HCHO), 9.208 (d, 1H, J )
0.7 Hz, H3′(â-pyrrolic)). Assignments aided by COSY spectra. UV-
vis (CH2Cl2) λmax (nm) (ꢀ (103 M-1 cm-1)) 437 (255), sh 522
(4.53), 560 (18.3), 601 (10.9). FAB-LRMS m/z (%, assign-
ment): cluster at 730-737, 730 (95, M+). HRMS for M+
(C47H30N4OZn): calcd, 730.1711; found, 730.1707.
λ
max (nm) (ꢀ (103 M-1 cm-1)): 436 (279), 565 (16.8), 605 (6.73).
FAB-LRMS m/z (%, assignment): cluster at 746-752, 746
(100, M+). HRMS for M+ (C47H30N4O2Zn): calcd, 746.1660;
found, 746.1648.
7. 3-(5′,10′,15′,20′-Tetraphenylporphyrin-2′-yl)allyl Hydrox-
ide (6). DIBAL-H (2.10 mL, 1.5 M in toluene, 3.15 mmol,
3.0 equiv) was added to a solution of 4 (750 mg, 1.05 mmol)
in dry toluene (30 mL) under an argon atmosphere at 0 °C.
After 30 min, the reaction was allowed to warm to room
temperature. After another 30 min, the reaction was cooled to
0 °C, and MeOH (4.5 mL) was added followed by aqueous
potassium sodium L-tartrate tetrahydrate (5 g in 150 mL). EtOAc
(150 mL) was added, and the organic layer was washed with
saturated aqueous NaHCO3 and dried (MgSO4), and the solvent
was removed in vacuo. The residue was purified by column
chromatography (silica, 45 mmdia × 160 mm, CH2Cl2/Et2O (99:
1)), collecting the major red fraction. The product was precipi-
tated with hexane to give 6 (618 mg, 88%) as a purple solid.
1H NMR (400 MHz, CDCl3, TMS, δ): -2.712 (s, 2H, NH),
10. 2-Cyano-5-(2′-(5′,10′,15′,20′-tetraphenylporphyrinato Zinc-
(II))yl)penta-2,4-dienoic Acid (Zn-8). A solution of Zn-7 (150
mg, 205 µmol), cyanoacetic acid (87 mg, 1.02 mmol, 5.0 equiv),
and piperidine (490 µL, 5.0 mmol, 24 equiv) in methanol (15
mL) was heated at reflux temperature for 50 min under N2.
While the solution was cooled to room temperature, CH2Cl2
(50 mL) and H2O (100 mL) were added, and the solution was
shaken vigorously, adjusting the pH of the aqueous layer to pH
) 2 with 2 M H3PO4 (3.0 mL). The organic layer was then
carefully separated, and the CH2Cl2 was removed by azeotrope
distillation using acetonitrile in vacuo. The crude product was
precipitated from the resulting acetonitrile solution using H2O
and collected by filtration (#4 glass sinter). The solid was
dissolved in CHCl3 (aided by a little THF) and filtered (#4 glass
sinter), and the product was precipitated with hexane to give
Zn-8 (137 mg, 84%) as a dark green solid. 1H NMR (400 MHz,
3
3
4.118 (app t, 2H, J ) 5.2 Hz, CH2OH), 6.292 (dd, 1H, J )

Zn-Porphyrin-Sensitized TiO2 Films
J. Phys. Chem. B, Vol. 109, No. 32, 2005 15403
DMSO-d6, TMS, δ): 6.743 (d, 1H, 3J ) 14.7 Hz, H5(pentadienyl)),
7.203 (dd, 1H, 3J ) 14.7, 11.7 Hz, H4(pentadienyl)), 7.388 (d, 1H,
3J ) 11.7 Hz, H3(pentadienyl)), 7.76-7.91 (m, 12H, Hm,p-Ph), 8.07-
8.23 (m, 8H, Ho-Ph), 8.72-8.75 (m, 6H, Hâ-pyrrolic), 9.060 (s,
1H, H3′(â-pyrrolic)). Assignments aided by COSY spectra. UV-
vis (THF) λmax (nm) (ꢀ (103 M-1 cm-1)) 334 (28.4), 414 (73.8),
sh 444 (109), 466 (121), 572 (16.5), 622 (18.5). FAB-LRMS
m/z (%, assignment): cluster at 797-803, 797 (100, M+).
HRMS for M+ (C50H31N5O2Zn): calcd, 797.1769; found,
797.1767.
in H2O (1.2 mL) and MeOH (3.0 mL) was added to a refluxing
solution of porphyrin Zn-10 (100 mg, 112 µmol) in THF (6.0
mL) and MeOH (3.0 mL) under N2. After 2 h, TLC analysis
indicated that all of Zn-10 had been consumed. After the
solution was cooled to room temperature, CH2Cl2 (50 mL), H2O
(100 mL), and aqueous 2 M H3PO4 (3.5 mL) were added, and
the solution was shaken vigorously in a separating funnel (pH
≈ 2.0), observing the porphyrin transferring from the aqueous
layer to the organic layer. The organic layer was washed with
H2O (100 mL, containing 4 drops of aqueous 2 M H3PO4) and
separated carefully. Acetone (50 mL) was added, and the CH2-
Cl2 was removed in vacuo. More acetone (50 mL) was added,
and the volume was reduced to 20 mL. The product was
precipitated from solution with H2O (Milli-Q) and collected on
a glass sinter (#4 glass sinter), washing with acetone/H2O (1:1)
11. Methyl 4-(trans-2′-(5′′,10′′,15′′,20′′-tetraphenyl porphyrin-
2′′-yl)ethen-1′-yl)-1,2-benzenedioate (10). a. Wittig Reaction.
A solution of TPPps 9 (400 mg, 432 µmol) and dimethyl
4-formylphthalate (288 mg, 1.30 mmol, 3.0 equiv) in CHCl3
(50 mL) was heated to reflux under N2. DBU (193 µL, 3.0
equiv) was added. After 15 min, TLC analysis indicated that
all of the starting material had been consumed (Rf ) 0.28, silica,
CH2Cl2). The crude isomeric mixture was precipitated with
methanol to give a cis/trans isomeric mixture of 10 as a purple
1
to give Zn-11 (94.5 mg, 98%) as a purple powder. H NMR
(400 MHz, DMSO-d6, TMS, δ): 7.005 and 7.391 (ABq, 2H,
3
4
3J ) 16.0, 15.9 Hz, H2′,1′), 7.446 (dd, 1H, J ) 7.8 Hz, J )
3
1.2 Hz, ArH5), 7.507 (br s, 1H, ArH3), 7.725 (d, 1H, J ) 8.0
1
1
Hz, ArH6), 7.77-7.89 (m, 12H, Hm,p-Ph), 8.15-8.24 (m, 8H,
powder (17% cis by H NMR). H NMR (400 MHz, CDCl3,
H
o-Ph), 8.672 and 8.727 (ABq, 2H, 3J ) 4.7, 4.6 Hz, Hâ-pyrrolic),
TMS, selected data only, δ): -2.718(s, NHcis), -2.605 (s,
8.727 and 8.763 (ABq, 2H, 3J ) 4.6, 4.7 Hz, Hâ-pyrrolic), 8.7403
(s, 2H, Hâ-pyrrolic), 9.044 (s, 1H, H3′′(â-pyrrolic)). 13.15 (br s, 2H,
CO2H). Assignments aided by COSY spectra. UV-vis (THF)
3
3
NHtrans), 6.306 (d, J ) 11.6 Hz, Hcis-ethenyl), 6.560 (dd, J )
4
11.9 Hz, J ) 1.1 Hz, Hcis-ethenyl).
b. Isomerization. The isomeric mixture was dissolved in CH2-
Cl2 (30 mL), and I2 (267 mg, 1.05 mmol, 2.4 equiv) was added.
After the solution was stirred at room temperature for 3 h in
darkness, excess saturated Na2S2O3 (∼30 mL) was added, and
stirring continued for 15 min. The organic layer was separated
and dried (MgSO4). The solvent was removed in vacuo to give
trans-10 (330 mg, 92%) as a purple solid. 1H NMR (400 MHz,
CDCl3, TMS, δ): -2.602 (s, 2H, NH), 3.946 (s, 3H, CO2CH3),
4.047 (s, 3H, CO2CH3), 7.067 and 7.262 (ABq, 2H, 3J ) 16.2,
15.9 Hz, H2′,1′), 7.361 (dd, 1H, 3J ) 8.2 Hz, 4J ) 2.0 Hz, ArH5),
7.492 (d, 1H, 4J ) 1.6 Hz, ArH3), 7.72-7.84 (m, 13H, 12Hm,p-
Ph + 1ArH6), 8.17-8.25 (m, 8H, Ho-Ph), 8.743 (d, 1H, 3J ) 4.8
Hz, Hâ-pyrrolic), 8.78-8.84 (m, 5H, Hâ-pyrrolic), 8.985 (s, 1H,
λ
max (nm) (ꢀ (103 M-1 cm-1)): 314(22.5), 437 (191), 565 (18.4),
602 (7.89). FAB-LRMS m/z (%, assignment): cluster at 866-
873, 866 (100, M+). HRMS for M+ (C54H34N4O4Zn): calcd,
866.1875; found, 866.1872.
14. 4-(trans-2′-(2′′-(5′′,10′′,15′′,20′′-Tetraphenylporphyrinato
Zinc(II))yl)ethen-1′-yl)-1-benzaldehyde (Zn-12). A solution of
Zn(OAc)2‚2H2O (71 mg, 322 µmol, 1.2 equiv) in MeOH (2.0
mL) was added to a solution of 12 (200 mg, 269 µmol) in CHCl3
(10 mL) with stirring at room temperature. After 20 min, TLC
analysis indicated that the reaction was complete. Precipitation
with methanol gave Zn-12 (218 mg, 100%) as a purple powder.
1H NMR (400 MHz, CDCl3, TMS, δ): 7.177 and 7.256 (ABq,
2H, 3J ) 16.0, 16.0 Hz, H1′,2′), 7.336 (d, 2H, 3J ) 8.0 Hz, H3,5),
7.771 (m, 14H, 12Hm,p-Ph + 2H2,6), 8.217 (m, 8H, Ho-Ph), 8.815
H
3′′(â-pyrrolic)). Assignments aided by COSY spectra. UV-vis
(CH2Cl2) λmax (nm) (ꢀ (103 M-1 cm-1)) 292 (21.3), 428 (187),
524 (18.1), 564 (9.87), 600 (6.76), 663 (4.69). FAB-LRMS m/z
(%, assignment): cluster at 832-836, 832 (75, M+). HRMS
for M+ (C56H40N4O4): calcd, 832.3050; found, 832.3039.
3
and 8.894 (ABq, 2H, J ) 4.8, 4.8 Hz, Hâ-pyrrolic), 8.903 and
3
8.935 (ABq, 2H, J ) 4.8, 4.8 Hz, Hâ-pyrrolic), 8.913 (s, 2H,
Hâ-pyrrolic), 9.134 (d, 1H, 4J ) 0.8 Hz, H3′′), 9.918 (s, 1H, CHO).
UV-vis (CH2Cl2) λmax (nm) (ꢀ (103 M-1 cm-1)): 321 (28.1),
364 (24.3), 433 (231), 525 (5.54), 558 (24.8), 595 (12.7). FAB-
LRMS m/z (%, assignment): cluster at 806-813, 806 (100,
M+). HRMS for M+ (C53H34N4OZn): calcd, 806.2024; found,
806.2002.
12. Methyl 4-(trans-2′-(2′′-(5′′,10′′,15′′,20′′-tetraphenylpor-
phyrinato Zinc(II))yl)ethen-1′-yl))-1,2-benzenedioate (Zn-10). A
solution of Zn(OAc)2‚2H2O (47.0 mg, 216 µmol, 1.2 equiv) in
MeOH (2.0 mL) was added to a solution of ester 10 (150 mg,
180 µmol) in CHCl3 (10 mL) with stirring at room temperature.
After 15 min, TLC analysis indicated that the reaction was
complete with the appearance of a new more polar green band
(Rf ) 0.23, silica, CH2Cl2). Precipitation using MeOH gave Zn-
10 (134.7 mg, 84%) as a purple microcrystalline solid. 1H NMR
(400 MHz, CDCl3, TMS, δ): 3.925 (s, 3H, CO2CH3), 4.036 (s,
15. 2-Cyano-3-[4′-(trans-2′′-(2′′′-(5′′′,10′′′,15′′′,20′′′-tet-
raphenylporphyrinato Zinc(II))yl) ethen-1′′-yl)-phenyl]-acrylic
Acid (Zn-13). A solution of Zn-12 (150 mg, 186 µmol),
cyanoacetic acid (313 mg, 3.68 mmol, 20 equiv), and piperidine
(1.02 mL, 10.3 mmol, 55 equiv) in methanol (15 mL) was heated
at reflux temperature for 15 h under argon. After the solution
was cooled to room temperature, CH2Cl2 (∼100 mL) and H2O
(200 mL) were added, and the solution was shaken vigorously,
adjusting the pH of the aqueous layer to pH ) 2 with 2 M
H3PO4 (7.0 mL). The organic layer was then washed a second
time with H2O (200 mL) containing of 2 M H3PO4 (1.0 mL,
pH ) 2). The organic layer was then carefully separated, and
the product was precipitated with acetonitrile, removing the CH2-
Cl2 by azeotrope distillation in vacuo to give Zn-13 (133.3 mg,
3H, CO2CH3), 7.114 and 7.234 (ABq, 2H, 3J ) 15.8, 14.4 Hz,
3
H
2′,1′), 7.366 (d, 1H, J ) 7.8 Hz, ArH5), 7.494 (s, 1H, ArH3),
7.71-7.84 (m, 13H, 12Hm,p-Ph + 1ArH6), 8.15-8.27 (m, 8H,
o-Ph), 8.833 (d, 1H, J ) 4.5 Hz, Hâ-pyrrolic), 8.88-8.95 (m,
3
H
5H, Hâ-pyrrolic), 9.112 (s, 1H, H3′′(â-pyrrolic)). Assignments aided
by COSY spectra. UV-vis (CH2Cl2) λmax (nm) (ꢀ (103 M-1
cm-1)): 312 (23.9), 432 (228), 557 (22.0), 594 (10.0). FAB-
LRMS m/z (%, assignment): cluster at 894-900, 894 (100,
M+). HRMS for M+ (C56H38N4O4Zn): calcd, 894.2185; found,
894.2200.
1
82%) as a dark purple powder. H NMR (400 MHz, DMSO-
3
13. 4-(trans-2′-(2′′-(5′′,10′′,15′′,20′′-Tetraphenylporphyrinato
Zinc(II))yl)ethen-1′-yl))-1,2-benzenedicarboxylic Acid (Zn-11).
A solution of NaOH (179 mg, 20 equiv per CO2Me, 4.48 mmol)
d6, TMS, δ): 7.143 and 7.354 (ABq, 2H, J ) 15.9, 15.9 Hz,
H
H
1′′,2′′), 7.415 (d, 2H, 3J ) 8.5 Hz, H3′,5′), 7.75-7.89 (m, 12H,
3
m,p-Ph), 8.012 (d, 2H, J ) 8.5 Hz, H2′,6′), 8.15-8.23 (m, 8H,

15404 J. Phys. Chem. B, Vol. 109, No. 32, 2005
Wang et al.
H
o-Ph), 8.285 (s, 1H, H3,acrylic), 8.656 and 8.723 (ABq, 2H, 3J )
afforded the building block 12.44 Quantitative metalation of 12
to give Zn-12 and the subsequent condensation with cyanoacetic
acid gave the desired R-cyanoacetic acid product Zn-13.
Generally all the final acid products were purified via precipita-
tion from appropriate solvents. Chromatography of the acids
was avoided since this generally resulted in irretrievable loss
3
4.7, 4.6 Hz, Hâ-pyrrolic), 8.722 and 8.760 (ABq, 2H, J ) 4.7,
4.7 Hz, Hâ-pyrrolic), 8.738 (s, 2H, Hâ-pyrrolic), 9.036 (s, 1H, H3′′′).
Assignments aided by COSY and LR-COSY spectra. UV-vis
(THF) λmax (nm) (ꢀ (103 M-1 cm-1)): 362 (27.4), 437 (136),
568 (21.9), 608 (14.8). FAB-LRMS m/z (%, assignment):
cluster at 871-881, 873 (100, M+). HRMS for M+ (C56H35N5O2-
Zn): calcd, 873.2082; found, 873.2063.
1
of the compounds. All compounds were characterized by H
NMR spectroscopy, UV-vis spectroscopy, and FAB HRMS
and afforded 1H NMR spectra typical of substituted tetra-
phenylporphyrins with no unusual features. Where necessary,
1H NMR assignments were aided by two-dimensional NMR
Results and Discussion
A. Synthetic Studies. The synthesis of these â-substituted
porphyrin carboxylic acids was readily achieved from the free-
base (FB) â-formylporphyrin TPP-CHO 1 using established
procedures.43 The R-cyanoacetic acid derivative Zn-3 was
obtained in three steps by metalation of 1 using the acetate
method
4
spectra. Often a small long-range J coupling (∼0.8 Hz) was
observed between the closest vinylic proton at the nearest
â-pyrrolic proton on the porphyrin ring. Also, in the majority
of cases, the broadened acid proton resonance was readily visible
(and integrateable) in DMSO-d6. The HRMS data was as
expected for the desired compounds.
46
to give the metallo derivative Zn-1 (Scheme 1),
followed by Knoevenagel condensation with cyanoacetic acid,
affording the piperidine salt Zn-2 in good yield. Subsequent
treatment of Zn-2 with phosphoric acid (pH ≈ 2) gave the
desired carboxylic acid Zn-3 without demetalation. Both the
piperidine salt proton resonances/integrals of Zn-2 and the acid
proton of Zn-3 were observable in the 1H NMR spectra. In the
piperidine salt Zn-2, a broadened baseline was observed under
the â-pyrrolic resonances; this was presumed to be due to the
NH+ resonance.
B. Electronic Spectroscopy. The UV-vis spectra of the
metalloporphyrins ZnTTP, Zn-3, Zn-5, Zn-8, Zn-11, and Zn-
13 in THF solvent are shown in Figure 2, and the absorption
maxima and extinction coefficients for the Soret- and Q-bands
of the derivatized porphyrins are listed in Table 1. The
metalloporphyrins show a series of bands between 400 and 650
55
nm due to π-π* absorptions of the conjugated macrocycle
that are red-shifted with respect to the Soret- and Q-bands of
ZnTPP, and the R-cyanoacetic acids, Zn-3, Zn-8, and Zn-13,
are red-shifted with respect to those of Zn-5 and Zn-11. The
Q-bands of Zn-3 and Zn-13 also show a significant red shift,
and the cyanoporphyrins generally have higher molar extinction
coefficients in this region. The absorption spectra of the
porphyrins adsorbed onto 9-µm-thick TiO2 electrodes are similar
to those of the corresponding solution spectra but exhibit a small
red shift due to the interaction of the anchoring groups with
the surface.
The synthesis of the acrylic acid derivative Zn-5 has
previously been reported to proceed via the Wittig reaction of
nickel â-formylporphyrin47 with ethyl (triphenylphosphoran-
ylidene)acetate to give Ni-4,48 and subsequent demetalation of
this with sulfuric acid yielded 4.49 Metalation using zinc acetate,
followed by hydrolysis with potassium hydroxide, gave Zn-5
(no characterization of Zn-5 was reported).50 A more direct
approach to Zn-5 has been achieved here with the conversion
of the FB â-formylporphyrin 1 directly to the FB porphyrin
ethyl acrylate 4. FB 4 has also been previously synthesized by
Effenberger and Strobel using a phosphonium salt but not fully
characterized.51 The Wittig reaction of aldehyde 1 with the ylide
ethyl (triphenylphosphoranylidene)acetate in refluxing toluene
gave a cis/trans mixture (2:3) of 4 in excellent yield. Isomer-
ization of this mixture with I2 then gave 4 in 90% overall yield.
The metalation of 4 to give Zn-4, followed by ester hydrolysis,
The emission spectra of metalloporphyrins Zn-3, Zn-5,
Zn-8, Zn-11, and Zn-13, shown in Figure 3, were obtained at
room temperature in THF solution. The emission maxima (Table
1) are independent of the excitation wavelength between 400
and 600 nm, and the spectra show characteristic vibronic bands
for Zn-5, Zn-11, and Zn-13 at around 630 and 675 nm similar
to those reported in other Zn porphyrins.56 Porphyrins Zn-3 and
Zn-8 display only a broad single band at 670 and 688 nm,
respectively. The excitation spectra, monitored at 675 nm,
exhibit intense Soret and Q-bands that correspond with the
ground-state absorption spectra, indicating the presence of a
single emitting species in each case. No emission spectra are
observed for the porphyrins adsorbed onto 9-µm-thick TiO2
layers as a consequence of electron injection from the excited
singlet state of the porphyrin into the conduction band of the
TiO2.57 The emission from the argon-degassed solution of
Zn-3, Zn-5, Zn-8, Zn-11, and Zn-13 at 298 K excited at 532
nm in the π-π* absorption band decays as a single exponential
with a time constants τ given in Table 1. The emission time
constants are several orders of magnitude greater than the
electron injection rate into the conduction band of the TiO2.24
1
quantitatively gave Zn-5. The H NMR spectra of 4 and Zn-4
are consistent with those reported,49-51 and the resulting acid
Zn-5 has now been fully characterized by 1H NMR, FAB
HRMS, and UV-vis spectroscopy.
Synthesis of the R-cyanopentadienoic acid Zn-8 was achieved
via the DIBAL-H reduction of ester 4 to alcohol 6 using a
similar procedure to that of Effenberger and Strobel.51 MnO2
oxidation of 6 to aldehyde 7, followed by metalation, gave Zn-
7. An alternative synthesis of aldehyde 7 has also been
previously reported by Ishkov et al. via the Wittig reaction of
â-formylporphyrin TPP-CHO 1 with the protected ylide of
2-bromoethanal, followed by hydrolysis to an inseparable cis/
trans mixture of aldehyde 7.52 The 1H NMR data of 6 and 7 is
consistent with those published.51,52 The Knoevenagel condensa-
tion of Zn-7 with cyanoacetic acid then gave the pentadienoic
acid Zn-8 in good yield (84%).
The conversion of â-formyl porphyrin TPP-CHO 1 into the
phosphonium salt 943 allowed the synthesis of Zn-11 and Zn-
13 (Scheme 2). Using the Wittig chemistry of phosphonium
salt 9 with an excess of dimethyl 4-formylphthalate53,54 allowed
the efficient synthesis of 10, after I2 isomerization. Metalation
then hydrolysis gave Zn-11 in excellent yield. In an analogous
fashion, the Wittig reaction of 9 with terephthaldialdehyde
The changes in the electronic absorption spectra with the
nature of the linker can be understood by examining the
calculated orbital energies for ZnTPP, Zn-3, and Zn-5. DFT
calculations (B3LYP) using the basis set 3-21G* show that for
ZnTPP the highest occupied molecular orbital (HOMO) and the
HOMO - 1 are split by 0.14 eV, and the unoccupied MOs are
doubly degenerate. The HOMO-LUMO gap is calculated at
2.97 eV. When porphyrin ring substituents increase the splitting
between the key filled orbitals or lift the degeneracy present in

Zn-Porphyrin-Sensitized TiO2 Films
J. Phys. Chem. B, Vol. 109, No. 32, 2005 15405
Figure 2. UV-vis absorption spectra of zinc tetraphenylporphyrin (ZnTPP), Zn-3, Zn-5, Zn-8, Zn-11, and Zn-13 in THF.
TABLE 1: Electronic Absorption/Emission and Electrochemical Data for Zinc Porphyrins
absorptiona λmax, nm (ꢀ, 103 M-1 cm-1
)
emissionb λmax, nm
τ, ns
Eox,c V
Eox*,d V
Zn-3
Zn-5
Zn-8
Zn-11
Zn-13
sh 429 (92.4), 455 (153), 571 (12.7) 620 (11.9)
436 (279), 565 (16.8), 605 (6.73)
414 (73.8), sh 444 (109), 466 (121), 572 (16.5), 622 (18.5)
437 (191), 656 (18.4), 602 (7.89)
670
628, 671
688
625, 674
629, 678
4
3
4
3
4
0.45
0.39
0.43
0.38
0.39
-1.40
-1.46
-1.43
-1.48
-1.46
437 (136), 568 (21.9), 608 (14.8)
a Absorption data were obtained in THF solution at 298 K. b Emission spectra were obtained at 298 K in THF by exciting at 570 nm.
c Electrochemical measurements were performed at 298 K using square wave voltammetry on solutions in DMF containing 0.1 M TBAPF6 as the
supporting electrolyte. Potentials are quoted with respect to ferrocene (Fc+/Fc). d Approximate values for the excited-state redox potentials extracted
from the potentials of the ground-state couples and the zero excitation energy (E0-0).
the unoccupied orbitals, the Q-transition gains in intensity.34,58-60
The electron-withdrawing substituent has a stabilizing effect on
the energies of all of the frontier molecular orbitals (FMOs),
relative to their energies in ZnTPP (Table 2). The degeneracy
of the LUMO and LUMO + 1 is broken, with the LUMO
extending out onto the substituent (Figure 4) and being further
stabilized by the stronger electron-withdrawing cyanocarboxy-
late group in Zn-3 than in Zn-5; the LUMO + 2 is also
stabilized in this manner.
0.05. Two strong transitions are predicted at 3.44 eV (360 nm);
these correspond to the Soret band. The largest coefficient of
these bands belongs to the HOMO - 2 f LUMO (40%) and
HOMO - 2 f LUMO + 1 (41%) transitions, respectively.
Transitions in the near-UV are largely limited to oscillator
strengths of below 0.05. At energies of less than 4 eV (310
nm), these transitions comprise lower orbitals up to the HOMO
- 5; at energies above 4 eV, orbitals to HOMO - 10 and the
LUMO + 2 are involved.
DFT and TDDFT studies on the optical absorptions of meso-
substituted porphyrins have shown that the Soret- and Q-
absorptions of these porphyrins each correspond to two over-
lapping one-electron transitions involving the four FMOs.61
TDDFT singlet calculations have been performed on ZnTPP,
Zn-3, and Zn-5 at the 3-21G* level. The ZnTPP calculation
shows two pairs of transitions in the visible region. The two at
2.18 eV (569 nm) are comprised of the four FMOs, and the
two at 2.45 eV (506 nm) are comprised of the four FMOs and
the HOMO - 2. They all have oscillator strengths of below
In general, upon substitution there is a red shift and an
increase in the calculated oscillator strengths of the visible
transitions. This reflects the higher absorptions in this region,
in the experimental UV-vis spectra of the substituted porphy-
rins. The sum of the oscillator strengths for these four transitions
increases from 0.014 in ZnTPP, to 0.040 for Zn-5, and to 0.108
for Zn-3.
The TDDFT calculation on Zn-5 shows splitting of the two
pairs of visible transitions. The lowest energy transition is
slightly red-shifted (0.08 eV) from that of ZnTPP (Table 3).

15406 J. Phys. Chem. B, Vol. 109, No. 32, 2005
Wang et al.
Figure 3. Emission spectra of porphyrins Zn-3, Zn-5, Zn-8, Zn-11, and Zn-13 in THF solution. The excitation wavelength was 570 nm.
and 0.40 eV. This compares with an experimental red shift of
0.24 eV (Table 3). The HOMO - 2 and LUMO + 2 are
involved in both transitions, and the lower-energy transition
involves the HOMO - 3. The largest coefficient of the more
intense, higher-energy transition involves the stabilized LUMO
+ 2. Calculated transitions in the near-UV are also red-shifted
and enhanced in intensity.
C. ATR-FTIR Spectroscopy. ATR-FTIR spectroscopy has
been shown to be a powerful tool to extract structural informa-
tion for the metal complexes adsorbed onto the TiO2 surface.62,63
In particular, by comparison of the characteristic stretching
modes of the -COOH and -CN groups of a solid-state
spectrum with those for molecules adsorbed on a surface, the
mode of attachment of porphyrins to the surface can be
determined. ATR-FTIR spectra of metalloporphyrins Zn-3,
Zn-5, Zn-8, Zn-11, and Zn-13 were measured both as a solid
and when adsorbed on TiO2 films, and the spectra for Zn-3 are
shown in Figure 5. The spectra of the solid samples all show a
strong and intense absorption at around 1700 cm-1 due to the
υ(CdO) of the carboxylic acid group.64 The strong bands at
1520, 1490, 1484, 1439, 1334, and 997 cm-1 are due to the
ring stretching modes of the porphyrin. The IR band at around
1595 cm-1 is due to characteristic υ(CdC) stretching frequency.
The R-cyanoacetic acids porphyrins Zn-3, Zn-8, and Zn-13
display a medium intensity band at 2223 cm-1 due to the
υ(CN) group stretching frequency.
Figure 4. LUMOs of (a) ZnTPP (doubly degenerate) and (b) Zn-3.
TABLE 2: Calculated Energies of Frontier Molecular
Orbitals for ZnTPP, Zn-5, and Zn-3
ZnTPP
Zn-5
Zn-3
E(LUMO + 2)/eV
E(LUMO + 1)/eV
E(LUMO)/eV
E(HOMO)/eV
E(HOMO - 1)/eV
-0.52
-2.10
-2.10
-5.06
-5.20
-1.17
-2.20
-2.37
-5.20
-5.31
-1.61
-2.29
-2.67
-5.28
-5.47
TABLE 3: Experimental and Calculated Red Shifts for
Zn-5 and Zn-3 Relative to ZnTPP
Zn-5 red shifts, eV
experimental calculated
Zn-3 red shifts, eV
experimental calculated
Q(0,0)
B(0,0)
-0.05
-0.12
-0.08
-0.28
-0.21
-0.10
-0.24
-0.20
-0.48
-0.40
ATR-FTIR spectra of the adsorbed complex on TiO2 film
show the presence of strong carboxylate asymmetric 1597 cm-1
υ(-COO-as) and symmetric 1383 cm-1 υ(-COO- ) bands.
All four of the visible transitions now involve the HOMO - 2.
The calculated Soret transitions are also split and red-shifted;
these transitions now involve MOs from the HOMO - 3 to the
LUMO + 2. A third strong calculated transition (f ) 0.49)
appears to the blue of the two Soret transitions at 3.31 eV (375
nm). In the near-UV region, other calculated transitions are red-
shifted and enhanced in intensity (Supporting information).
The TDDFT calculation on Zn-3 shows that the increase in
the oscillator strengths of the visible transitions is more
pronounced, and the lowest calculated transition has a larger
red shift relative to ZnTPP of 0.20 eV. The splitting of the
calculated visible transitions is larger for Zn-3 than that for
Zn-5. The calculated Soret transitions are red-shifted by 0.48
s
Porphyrins, Zn-3, Zn-8, and Zn-13, i.e., those that contain an
R-cyano group, also display a very weak υ(NC) band at 2214
cm-1. The presence of carboxylate bands in the IR spectra of
adsorbed complexes on TiO2 show that the carboxylic acid
groups are adsorbed on the TiO2 surface in a dissociated form.
Indeed, on the basis of this IR data and the recent theoretical
studies on the interaction of formic acid and sodium formate
on the anatase (101) surface,65 it is likely that the binding occurs
through a bidentate bridging mode to the TiO2 surface.12
D. Electrochemical Properties. Cyclic voltammetry was
used to determine the redox potentials of the porphyrins. All
porphyrins show chemically reversible waves for a one-electron

Zn-Porphyrin-Sensitized TiO2 Films
J. Phys. Chem. B, Vol. 109, No. 32, 2005 15407
Figure 5. ATR-FTIR spectra of Zn-3 measured as a pure solid (top trace) and that of the dye adsorbed onto a 3-µm-thick TiO2 film (bottom trace).
The latter spectrum was obtained by subtracting the IR spectrum of the untreated film from that of the dye-coated film.
oxidation and reduction, and the measured redox potentials are
listed in Table 1. The oxidation potentials of Zn-3 and Zn-8
are positively shifted by about 60 mV compared with the other
porphyrins as a result of the strong electron-withdrawing nature
of the CN group. Zn-13, the other cyano porphyrin, does not
show this shift, probably as a result of the phenyl vinylene spacer
substituent, which attenuates the effect. Additional one-electron
oxidations are present at a potential of about 0.8 V (not included
in Table 1), corresponding to the formation of the dication from
the radical cation.66 There are also reversible one-electron
reductions at around -1.7 V due to the reduction of the
porphyrin ring and a second one-electron reduction at about
-2.0 V as a result of the reduction of the porphyrin anion. The
CVs obtained for the dyes adsorbed onto TiO2 film show little
change from those observed in THF solution despite the fact
that nanocrystalline TiO2 films are conductive only in the
accumulation regime and are electronically blocking under
reverse bias due to the electronic band gap. The phenomenon
Figure 6. Photocurrent action spectra obtained with Zn-3 (1), Zn-8
(2), Zn-13 (3), Zn-5 (4), and Zn-11 (5) anchored on nanocrystalline
has been observed previously for fullerene and triphenylamine
on nanocrystalline oxides 67,68 and is attributed to lateral charge
hopping between neighboring adsorbed molecules, in this case
between the adsorbed porphyrin molecules. This is not surprising
in view of the highly delocalized porphyrin structures and the
fact that electronic communication between porphyrin molecules
in self-assembled multilayers has been previously observed.69
E. Light-Induced Charge Separation at the Porphyrin/
TiO2 Interface. Excitation of the porphyrin molecules with
visible light leads to an electronically excited state that
undergoes electron-transfer quenching as a consequence of
electrons being injected into the conduction band of the
semiconductor. The oxidized porphyrin is subsequently reduced
back to the ground state (S) by the electron donor (I-) present
in the electrolyte. The electrons in the conduction band collect
at the back-current collector and subsequently pass through the
external circuit to arrive at the counter electrode. The photo-
currents generated as a result of this charge separation were
measured as a function of wavelength in the 400-800 nm
region, and the resulting photocurrent action spectra are shown
in Figure 6. The shapes of these spectra are slightly broader
but clearly follow the shape of the corresponding absorption
TiO2 films.
spectra shown in Figure 2. In particular, the red shifts seen in
the Q-band absorptions of Zn-3 and Zn-8 are also seen in the
photoaction spectra of these molecules. In addition, the cyano-
porphyrins exhibit enhanced IPCE values over most of the
spectral range. For example, the maximum IPCE value achieved
at around 463 nm (Soret band) for Zn-3 of 89% corresponds to
almost unity quantum yield (electrons per absorbed photon)
when light losses are taken into account. An IPCE value of more
than 70% is also obtained for the Q-band for this compound.
The photovoltaic data for the porphyrin-sensitized cells are given
in Table 4, where it is seen that the better IPCE values associated
with the cyanoporphyrins are reflected in the higher short circuit
currents of these cells.
The most efficient sensitizer, Zn-3, yields an overall ef-
ficiency of 5.2%, a value that is the best thus far obtained for
porphyrin-sensitized cells. For macrocyclic organic dyes such
as phthalocyanines or porphyrins, solution aggregation as a result
of π-stacking might lead to an adsorption of aggregates onto
the TiO2. One way to prevent this is to add a bulky molecule
such as chenodeoxycholic acid to the Zn-3 solution during the

15408 J. Phys. Chem. B, Vol. 109, No. 32, 2005
Wang et al.
TABLE 4: Photovoltaic Performance of Dye-Sensitized Cells
Using Different Zn Porphyrins as Sensitizersa
These findings open up new avenues for further improving the
efficiency of nanocrystalline solar cells for practical utility, by
engineering porphyrins with a slightly decreased gap between
the HOMO and the LUMO that absorb in the visible and the
near-IR regions of the solar spectrum.
metalloporphyrin
Voc, mV
Jsc, mA/cm2
η, %
Zn-3
Zn-5
Zn-8
Zn-11
Zn-13
566
555
535
509
530
13.5
10.9
11.4
7.2
5.2
4.0
4.0
2.4
3.7
Acknowledgment. We acknowledge financial support of this
work by the Swiss Federal Office for Energy (OFEN), the U.
S. Air Force Research Office under contract number F61775-
00-C0003, and the New Zealand Foundation for Research,
Science and Technology New Economy Research Fund con-
tracts MAUX0014 and MAUX0202. We thank Dr. Marie
Jirousek and P. Comte for their kind assistance in obtaining
photovoltaic data and TiO2 electrodes.
11.0
a The illumination is 100 mW/cm2. The electrolyte is 0.6 M BMII,
0.1 M LiI, 0.5 M tBuPy, and 0.05 M I2 in a mixture of acetonitrile and
valeronitrile (1:1 in volume ratio).
Supporting Information Available: Electrochemical data
and the DFT calculation showing calculated transitions and
intensities of these sensitizers. This material is available free
of charge via the Internet at http://pubs.acs.org.
References and Notes
(1) Kavarnos, G. J. Fundamentals of Photoinduced Electron Transfer;
VCH: Weinheim, New York, 1993.
(2) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737.
(3) Gra¨tzel, M. Nature 2001, 414, 338.
(4) Gra¨tzel, M. J. Photochem. Photobiol., A 2004, 164, 3.
(5) Gra¨tzel, M. J. Sol-Gel Sci. Technol. 2001, 22, 7.
(6) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.;
Muller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc.
1993, 115, 6382.
(7) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissortel, F.; Salbeck,
J.; Spreitzer, H.; Gra¨tzel, M. Nature 1998, 395, 583.
(8) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.;
Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover,
V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gra¨tzel, M. J. Am. Chem.
Soc. 2001, 123, 1613.
Figure 7. Photocurrent-voltage characteristics of the nanocrystalline
photoelectrochemical cell sensitized with Zn-3 (A) with an overall
efficiency of 5.2%. The response shown in curve B was achieved from
a cell in which the dye sensitization of the TiO2 was carried out in the
presence of 2 mM chenodeoxycholic acid in THF solvent. The overall
cell efficiency for this cell is 5.6% (Voc ) 610 mV, Jsc ) 13 mA/cm2,
ff ) 0.70). A light source simulating global AM 1.5 solar radiation
was employed, the incident intensity being 100 mW/cm2. The insert
shows a Zn-3-coated TiO2 film.
(9) Kruger, J.; Plass, R.; Gra¨tzel, M.; Matthieu, H. J. Appl. Phys. Lett.
2002, 81, 367.
(10) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.;
Sekiguchi, T.; Gra¨tzel, M. Nat. Mater. 2003, 2, 402.
(11) Gra¨tzel, M.; Moser, J.-E. In Electron Transfer in Chemistry; Balzani,
V., Gould, I., Eds.; Wiley-VCH: Weinheim, 2001; Vol. V, pp 589-644.
(12) Nazeeruddin, M. K.; Humphry-Baker, R.; Officer, D. L.; Campbell,
W. M.; Burrell, A. K.; Gra¨tzel, M. Langmuir 2004, 20, 6514.
(13) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Gra¨tzel, M. J. Phys.
Chem. B 2000, 104, 1300.
(14) Anderson, J. M. Aust. J. Plant Physiol. 1999, 26, 625.
(15) Ben Shem, A.; Frolow, F.; Nelson, N. Photosynth. Res. 2004, 81,
239.
(16) Lammi, R. K.; Wagner, R. W.; Ambroise, A.; Diers, J. R.; Bocian,
D. F.; Holten, D.; Lindsey, J. S. J. Phys. Chem. B 2001, 105, 5341.
(17) Liu, Z. M.; Yasseri, A. A.; Lindsey, J. S.; Bocian, D. F. Science
2003, 302, 1543.
sensitization process.32 Indeed, a chenodeoxycholic acid con-
centration of 0.2 mM in the dye solution does result in an
increase in cell performance (η ) 5.6%), as shown in Figure 7,
but only by a small amount, suggesting that aggregation is not
a significant factor in influencing the efficiency of the cell.
Nevertheless, to the best of our knowledge, this is the highest
value reported until now for a porphyrin/TiO2 light-harvesting
system. Attempts to further improve the overall efficiency of
these functionalized porphyrin-sensitized cells will concentrate
on tuning the dye deposition methods and the composition of
the electrolytes.
(18) Imahori, H.; Mori, Y.; Matano, Y. J. Photochem. Photobiol., C
2003, 4, 51.
(19) Choi, M. S.; Yamazaki, T.; Yamazaki, I.; Aida, T. Angew. Chem.,
Int. Ed. 2004, 43, 150.
(20) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34,
Conclusions
40.
A series of novel intense green porphyrins containing
carboxylate anchoring groups have been shown to have potential
as light sensitizers in TiO2 solar cells. DFT and TDDFT
calculations have shown that the presence of electron-withdraw-
ing moieties on the substituent modulates the electronic absorp-
tion spectrum through stabilization of the energies of the LUMO
and LUMO + 2. The extension of the LUMO and LUMO + 2
onto the substituent implies that occupation of these MOs offers
the possibility of electron transfer from the substituent to the
TiO2 surface. The ground- and excited-state redox potentials
show that the porphyrins possess suitable potentials to inject
electrons into the TiO2 substrate, yielding 85% IPCE. Among
the reported porphyrins, the green Zn-3 shows the highest power
conversion efficiency obtained so far in nanocrystalline films.
(21) Dabestani, R.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T.
E.; Webber, S. E.; White, J. M. J. Phys. Chem. 1988, 92, 1872.
(22) Vlachopoulos, N.; Liska, P.; Mcevoy, A. J.; Gra¨tzel, M. Surf. Sci.
1987, 189, 823.
(23) Kalyanasundaram, K.; Vlachopoulos, N.; Krishnan, V.; Monnier,
A.; Gra¨tzel, M. J. Phys. Chem. 1987, 91, 2342.
(24) Tachibana, Y.; Haque, S. A.; Mercer, I. P.; Durrant, J. R.; Klug,
D. R. J. Phys. Chem. B 2000, 104, 1198.
(25) Odobel, F.; Blart, E.; Lagree, M.; Villieras, M.; Boujtita, H.; El
Murr, N.; Caramori, S.; Bignozzi, C. A. J. Mater. Chem. 2003, 13, 502.
(26) Cherian, S.; Wamser, C. C. J. Phys. Chem. B 2000, 104, 3624.
(27) Jasieniak, J.; Johnston, M.; Waclawik, E. R. J. Phys. Chem. B 2004,
108, 12962.
(28) Ma, T. L.; Inoue, K.; Yao, K.; Noma, H.; Shuji, T.; Abe, E.; Yu,
J. H.; Wang, X. S.; Zhang, B. W. J. Electroanal. Chem. 2002, 537, 31.
(29) Ma, T. L.; Inoue, K.; Noma, H.; Yao, K.; Abe, E. J. Photochem.
Photobiol., A 2002, 152, 207.

Zn-Porphyrin-Sensitized TiO2 Films
J. Phys. Chem. B, Vol. 109, No. 32, 2005 15409
(30) Campbell, W. M.; Burrell, A. K.; Officer, D. L.; Jolley, K. W.
Coord. Chem. ReV. 2004, 248, 1363.
(46) Smith, K. M. Porphyrins and Metalloporphyrins; Elsevier: Am-
sterdam: 1975.
(47) Callot, H. J. Tetrahedron 1973, 29, 899.
(48) Callot, H. J.; Castro, B.; Selve, C. Tetrahedron Lett. 1978, 19, 2877.
(49) Selve, C.; Niedercorn, F.; Nacro, M.; Castro, B.; Gabriel, M.
Tetrahedron 1981, 37, 1893.
(50) Selve, C.; Niedercorn, F.; Nacro, M.; Castro, B.; Gabriel, M.
Tetrahedron 1981, 37, 1903.
(51) Effenberger, F.; Stobel, H. Chem. Ber. 1993, 126, 1683.
(52) Ishkov, Y. V.; Zhilina, Z. I.; Grushevaya, Z. V. Zh. Org.Khim.
1993, 29, 2270.
(53) Baker, W. R.; Rosenberg, S. H.; Fung, K. L. A.; Rockway, T. W.;
Fakhoury, S. A.; Garvey, D. S.; Donner, B. G.; O’Connor, S. J.; Prasad, R.
N.; Shen, W.; Stout, D. M.; Sullivan, G. M.; Abbott Laboratories.
International Patent Application 241, 1996.
(54) Ainsworth, A. T.; Smith, D. G.; Beecham Group, Ltd., U. K.
European Patent Application 89, 1980.
(55) Kalyanasundaram, K. Photochemistry of Polypyridine and Por-
phyrin Complexes; Academic Press: London: 1992.
(56) Tait, C. D.; Holten, D.; Barley, M. H.; Dolphin, D.; James, B. R.
J. Am. Chem. Soc. 1985, 107, 1930.
(31) Fungo, F.; Otero, L. A.; Sereno, L.; Silber, J. J.; Durantini, E. N.
J. Mater. Chem. 2000, 10, 645.
(32) Kay, A.; Gra¨tzel, M. J. Phys. Chem. 1993, 97, 6272.
(33) Chen, C. T.; Yeh, H. C.; Zhang, X. Q.; Yu, J. W. Org. Lett. 1999,
1, 1767.
(34) Gouterman, M. J. Chem. Phys. 1959, 30, 1139.
(35) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
(36) Becke, A. D. Phys. ReV. A 1988, 38, 3098.
(37) Burke, K.; Perdew, J. P.; Wang, Y. In Electronic Density Functional
Theory: Recent Progress and New Directions, Proceedings of the Inter-
national Workshop on Electronic Density Functional Theory: Recent
Progress and New Directions, Nathan, Australia, July 14-19, 1996; Dobson,
J. F., Vignale, G., Das, M. P., Eds.; Plenum Press: New York, 1998, pp
81-111
(38) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(39) Nguyen, K. A.; Day, P. N.; Pachter, R. J. Phys. Chem. A 2000,
104, 4748.
(40) Nguyen, K. A.; Day, P. N.; Pachter, R. J. Chem. Phys. 1999, 110,
9135.
(41) van Gisbergen, S. J. A.; Rosa, A.; Ricciardi, G.; Baerends, E. J. J.
Chem. Phys. 1999, 111, 2499.
(42) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys.
1998, 109, 8218.
(43) Bonfantini, E. E.; Burrell, A. K.; Campbell, W. M.; Crossley, M.
J.; Gosper, J. J.; Harding, M. M.; Officer, D. L.; Reid, D. C. W. J.
Porphyrins Phthalocyanines 2002, 6, 708.
(57) Nazeeruddin, M. K.; Humphry-Baker, R.; Gra¨tzel, M.; Murrer, B.
A. Chem. Commun. 1998, 719.
(58) LeCours, S. M.; DiMagno, S. G.; Therien, M. J. J. Am. Chem. Soc.
1996, 118, 11854.
(59) Spellane, P. J.; Gouterman, M.; Antipas, A.; Kim, S.; Liu, Y. C.
Inorg. Chem. 1980, 19, 386.
(44) Belcher, W. J.; Burrell, A. K.; Officer, D. L.; Reid, D. C. W.; Scott,
S. M. Journal of Porphyrins and Phthalocyanines 2002, 6, 720.
(45) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian
03; Gaussian, Inc.: Wallingford, CT, 2004.
(60) Gouterman, M. Porphyrins 1978, 3, 1.
(61) Cramariuc, O.; Hukka, T. I.; Rantala, T. T. Chem. Phys. 2004, 305,
13.
(62) Duffy, N. W.; Dobson, K. D.; Gordon, K. C.; Robinson, B. H.;
McQuillan, A. J. Chem. Phys. Lett. 1997, 266, 451.
(63) Finnie, K. S.; Bartlett, J. R.; Woolfrey, J. L. Langmuir 1998, 14,
2744.
(64) Nazeeruddin, M. K.; Humphry-Baker, R.; Liska, P.; Gra¨tzel, M. J.
Phys. Chem. B 2003, 107, 8981.
(65) Persson, P.; Lunell, S.; Ojamae, L. Int. J. Quantum Chem. 2002,
89, 172.
(66) Burrell, A. K.; Officer, D. L.; Reid, D. C. W.; Scott, S. M.; Gordon,
K. C. J. Porphyrins Phthalocyanines 2000, 4, 626.
(67) Bonho´te, P.; Gogniat, E.; Tingry, S.; Barbe, C.; Vlachopoulos, N.;
Lenzmann, F.; Comte, P.; Gra¨tzel, M. J. Phys. Chem. B 1998, 102, 1498.
(68) Papageorgiou, N.; Gra¨tzel, M.; Enger, O.; Bonifazi, D.; Diederich,
F. J. Phys. Chem. B 2002, 106, 3813.
(69) Savenije, T. J.; Goossens, A. Phys. ReV. B 2001, 64, 115323.