2-(1-acetyl-2-oxopropyl)-5,10,15,20-tetraphenylporphyrin and its transition-metal complexes

Article abstract of DOI:10.1002/ejic.201100179

The 2-(1-acetyl-2-oxopropyl)-5,10,15,20-tetraphenylporphyrin free base (H₂TPP-AOP) and its transition-metal complexes (MTPP-AOP, M = Cu ²⁺, Zn²⁺) have been synthesized. Single-crystal X-ray diffraction analysis revealed th

Full text of DOI:10.1002/ejic.201100179

FULL PAPER
DOI: 10.1002/ejic.201100179
2-(1-Acetyl-2-oxopropyl)-5,10,15,20-tetraphenylporphyrin and Its
Transition-Metal Complexes
Hongshan He,*[a] Mukul Dubey,[a] Yihan Zhong,[a] Maheshwar Shrestha,[a] and
Andrew G. Sykes[b]
Keywords: Porphyrinoids / Dyes / Sensitizers / Solar cells / Nanotubes / Transition metals
The
2-(1-acetyl-2-oxopropyl)-5,10,15,20-tetraphenylpor-
and zinc(II) complex, respectively. Density functional calcu-
lations indicate that these porphyrins are energetically suit-
able for electron injection from their lowest-unoccupied mo-
lecular orbitals to the conduction band of the titanium di-
oxide semiconductor. However, the energy conversion effi-
ciencies of the three porphyrin-sensitized solar cells are low,
that is, 0.14, 0.015 and 0.021% for ZnTPP-AOP, CuTPP-AOP
and H2TPP-AOP, respectively. The poor photovoltaic per-
formance is ascribed to the low dye-loading of these por-
phyrins on the titanium dioxide surface. The neighbouring
phenyl groups exert sufficient steric hindrance on the AOP
to limit its binding to the TiO2 semiconductor.
phyrin free base (H2TPP-AOP) and its transition-metal com-
plexes (MTPP-AOP, M = Cu2+, Zn2+) have been synthesized.
Single-crystal X-ray diffraction analysis revealed that the 1-
acetyl-2-oxopropyl (AOP) group is attached to a β-pyrrolic
position through the methylene group. The mean plane of
the AOP is almost perpendicular to the porphyrin ring. The
zinc(II) complex crystallizes as a five-coordinate species with
one methanol/water solvate in the axial position, whereas the
copper(II) complex is four-coordinate. These compounds ex-
hibit strong absorption in the UV and visible regions. The
fluorescence lifetimes are 1.85 and 8.0 ns for the free base
with an optical onset at 750 nm and TiO2 nanoparticles
with diameters of about 20 nm are used, which have only
around 11.6% efficiency in a single cell.[1] The efficiency
from a module is much lower than this, usually between 4
and 8%.[2] Thus, new light-harvesting materials and nano-
scale semiconductors are needed to advance this technol-
ogy.
Porphyrins are one type of dye that show promise for
cost-effective DSCs. Porphyrins usually exhibit strong ab-
sorption in the UV and visible regions with high absorption
coefficients. They are easy to make and modify and are
ideal candidates for lightweight devices. Numerous por-
phyrin dyes have been synthesized and their photovoltaic
performances have been tested. However, significant
achievements have only been made recently.[3–17] For exam-
ple, Campbell et al.[16] linked a penta-2,4-dienoic acid group
to the β-pyrrolic position of a zinc porphyrin and an energy
conversion efficiency of 7.1% was achieved as a result of
efficient electronic coupling of the porphyrin to TiO2
Introduction
Dye-sensitized solar cells (DSCs) have been recognized
as a key technology for the conversion of solar energy into
electricity.[1] These devices are lightweight and can be
adapted for a variety of indoor and outdoor applications,
for example, as power supplies for cell phones, music play-
ers, laptops and toys.[2] The colourful appearance of DSCs
also makes devices very attractive for installation on win-
dows, walls and ceilings as decorations. However, there is
no mass-produced commercial product available for techni-
cal reasons. For example, they have a low efficiency for the
conversion of solar energy into electricity. As the overall
energy conversion efficiency of a DSC is the product of
light-harvesting efficiency (LHE), electron injection effi-
ciency (ηinj) and electron collection efficiency (ηcoll), high
efficiencies can only be achieved by systematic engineering
of dye molecules, interfaces between dye molecules, semi-
conductors and redox couples. DSCs with dye molecules
with broader absorption ability and TiO2 nanostructures
with reduced electron recombination will lead to high effi-
ciency. In state-of-the-art DSC devices, ruthenium(II) dyes
through
a
conjugated linker. Imahori and co-
workers[4,6,11,18,19] found that bulky mesityl groups can re-
duce aggregation. When one diarylamino group was intro-
duced at a meso position of a porphyrin with 4-benzoic acid
as an anchoring group, the cell exhibited an efficiency of
6.5%. These authors also fused quinoxaline onto the β-pyr-
rolic ring and the resulting cells gave a maximum energy
conversion efficiency of 5.2%. Liu et al.[10] conjugated a
thiophene moiety to a meso position of porphyrin and 5.1%
efficiency was obtained. Diau and co-workers[9,12,13,20] in-
[a] Center for Advanced Photovoltaics, South Dakota State
University,
Brookings, SD 57007, USA
Fax: +1-605-688-4401
E-mail: hongshan.he@sdstate.edu
[b] Department of Chemistry, University of South Dakota,
Vermillion, SD 56069, USA
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100179.
Eur. J. Inorg. Chem. 2011, 3731–3738
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3731

H. He, M. Dubey, Y. Zhong, M. Shrestha, A. G. Sykes
FULL PAPER
vestigated phenylethynyl-substituted porphyrins systemati- c. The Zn2+ lies in the mean plane of N1/N2/N3/N4. It is
cally and found porphyrins with a donor–acceptor structure five-coordinated with the four N atoms of the porphyrin
exhibited very good photovoltaic performance. When the ring and with one O atom from methanol/water binding
cell was co-sensitized with another complementary directly to the central metal ion (Figure 1). The coordina-
indoline-based dye,[21] an energy conversion efficiency of tion geometry of Zn2+ is distorted square-pyramidal. The
around 10% was obtained. These results demonstrate the AOP group is linked to the porphyrin ring through its ter-
potential of porphyrin-based dyes for the development of minal propyl carbon with all of its five C atoms (C1–C5)
cost-effective solar cells. We are very interested in porphyrin and two O atoms (O2 and O3) in one plane. The O2 and
dyes and synthesized three 1-acetyl-2-oxopropyl (AOP)- O3 are hydrogen-bonded. The mean plane of the AOP is
functionalized porphyrins (see Scheme 1). It was hoped that almost parallel to the neighbouring phenyl group P4 and
the AOP group would act as an alternative to the carboxylic perpendicular to the mean plane of N1/N2/N3/N4 with a
group (COOH) as binding group for efficient electron injec- torsion angle of 88.36(8)°. The phenyl groups P1, P2 and
tion. However, the results show that the photovoltaic per- P4 are also almost perpendicular to the porphyrin ring with
formance of these porphyrins is poor. It was found that torsion angles of 89.23(10), 84.80(16) and 86.59(9)°, respec-
steric hindrance around the anchoring group prevents bind- tively. The phenyl group P3 exhibits a torsion angle of
ing of the AOP to the TiO2 semiconductor. This leads to a 61.70(0.10)° to the mean plane of the porphyrin ring. Note
very low dye-loading. Reported herein are the details of the that one phenyl ring (P2) is disordered. In addition, water
synthesis of these porphyrins, their structures, photophysi- and methanol molecules share the axial coordination posi-
cal properties and photovoltaic performance.
tion in a ratio of 0.36:0.64.
Results and Discussion
Snthesis and Structural Characterization
The 1-acetyl-2-oxopropyl (AOP)-functionalized por-
phyrinatocopper(II) complex was prepared by the reaction
of 2-nitroporphyrinatocopper(II) and acetylacetone in the
presence of anhydrous K2CO3. The free base was prepared
by demetallation of the copper(II) complex with concen-
trated H2SO4 and the zinc(II) complex was obtained by the
direct reaction of the free base and zinc acetate (Scheme 1).
All the compounds were purified by silica gel column
chromatography. Their compositions were confirmed by el-
emental analysis and mass spectroscopy. Characteristic vi-
brations of υC–H and υC=O from the AOP group were ob-
served at 3051, 3021 and 1594 cm–1 for H2TPP-AOP, 3051,
3018 and 1596 cm–1 for CuTPP-AOP and 2957, 2927 and
1594 cm–1 for ZnTPP-AOP. An additional peak at
3327 cm–1 from the N–H vibration was also observed for
H2TPP-AOP.
Figure 1. ORTEP diagram of ZnTPP-AOP·0.36CH3OH·0.64H2O
with thermal ellipsoids drawn at the 50% probability level. The
four phenyl groups are labelled as P1, P2, P3 and P4, respectively.
Only O1Ј of methanol is shown. The O1 of water was omitted for
clarity. Selected bond lengths [Å]: N1–Zn1 = 2.071(3), N2–Zn1 =
2.043(3), N3–Zn1 = 2.086(2), N4–Zn1 = 2.049(2), O2–O3 =
2.4398(5), O1Ј–Zn1 = 2.082(11), O1–Zn1 = 2.31(2).
¯
CuTPP-AOP crystallizes in the triclinic space group P1.
The structures of the three compounds were further as-
certained by single-crystal X-ray diffraction analysis. There are two molecules and two CH2Cl2 solvates in each
ZnTPP-AOP crystallizes in the monoclinic space group C2/ asymmetric unit. The Cu2+ ion is coordinated by four N
Scheme 1. Synthesis of 1-acetyl-2-oxopropyl (AOP)-functionalized porphyrins. The structure of a reference porphyrin ZnTTP-COOH is
also shown. Reagents and conditions: (a) Cu(NO2), acetic anhydride, CHCl3, RT, 24 h; (b) acetylacetone, DMSO, anhydrous K2CO3,
50 °C, 1 h; (c) H2SO4 (98%), CH2Cl2, 0.5 h, RT; (d) Zn(OAc)3, MeOH/CHCl3, RT, 12 h.
3732
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 3731–3738

(Acetyloxopropyl)tetraphenylporphyrin and Its Complexes
atoms from the porphyrin ring, as shown in Figure 2. The
AOP group is also located at a β-pyrrolic position. Similar
to the ZnTPP-AOP structure, the five C atoms (C1–C5) and
two O atoms (O1 and O2) of the AOP group also lie in one
plane with a torsion angle of 67.12(12)° to the mean plane
of N1/N2/N3/N4. The torsion angles of the three phenyl
groups P2, P3 and P4 with respect to the mean plane of
N1/N2/N3/N4 are 50.81(17), 51.94(12) and 58.13(15)°,
respectively. The only phenyl group that is almost perpen-
dicular to the porphyrin ring is the P1 group, which has a
torsion angle of 85.08(0.19)°. The H atom on O1 points
away from O2, which indicates there is no hydrogen-bond-
ing interaction between O1 and O2.
Figure 3. Side-views of the porphyrin cores of (A) CuTPP-AOP,
(B) ZnTPP-AOP, (C) H2TPP-AOP and (D) CuTPP.
Photophysical Properties
The absorption spectra of the three porphyrins are very
similar to each other. All compounds exhibit a strong ab-
sorption at around 420 nm (Soret band) and a weak ab-
sorption at 500–700 nm (Q bands; Figure 4). The spectra
are very similar to those of their parent compounds, which
indicates that the introduction of an AOP group at the β-
pyrrolic position does not alter the absorption properties
significantly. ZnTPP-AOP and H2TPP-AOP exhibit typical
fluorescence upon excitation (Figure S3). ZnTPP-AOP
gives two emission peaks at 599 and 645 nm with a lifetime
of 1.85 ns whereas H2TPP-AOP exhibits two peaks at 655
and 718 nm with a lifetime of 8.0 ns. No fluorescence was
observed for CuTPP-AOP. The quantum yields for ZnTPP-
AOP and H2TPP-AOP in ethanol are 4.5 and 2.8%, respec-
tively. The fluorescence lifetimes are long enough for elec-
tron injection from the excited porphyrins to the conduc-
tion band of TiO2 during the DSC operation, which usually
Figure 2. ORTEP diagram of CuTPP-AOP·CH2Cl2 with thermal
ellipsoids drawn at the 50% probability level. The four phenyl
groups are labelled as P1, P2, P3 and P4. The CH2Cl2 has been
omitted for clarity. Selected bond lengths [Å]: N1–Cu1 2.000(4),
N2–Cu1 1.973(4), N3–Cu1 1.992(4), N4–Cu1 1.984(4), O1–O2 occurs within a time-scale of Ͻ100 fs.[23]
2.468(6).
H2TPP-AOP crystallizes in the monoclinic space group
P21/c. The quality of the refined structure was poor.
Attempts to grow high quality crystals were unsuccessful;
however, it is quite clear from the refined structure that the
AOP group is attached to the porphyrin ring. The structure
is quite similar to those of ZnTPP-AOP and CuTPP-AOP
(an ORTEP diagram of this structure is provided in Fig-
ure S1 of the Supporting Information).
The porphyrin moieties of the three compounds show
different degrees of planarity. The side-views of the core
structures of the three porphyrins are shown in Figure 3.
The side-view of CuTPP, which has a perfect planar struc-
ture, is also presented as a reference.[22] In ZnTPP-AOP, the
Zn2+ is 0.2400(17) Å above the porphyrin ring. The por-
phyrin ring is twisted to a small extent, which becomes Figure 4. Normalized absorption spectra of three porphyrins in
CH2Cl2 at room temperature.
more severe in H2TPP-AOP. The CuTPP-AOP exhibits a
saddled conformation with two pairs of pyrrole groups at
opposite positions bending in opposite directions. The
packing modes of the three porphyrins are also different
(Figure S2). ZnTPP-AOP forms a chain structure as a result
Theoretical Calculations
of an intermolecular hydrogen-bonding interaction between
As the HOMO (highest-occupied molecular orbital) and
O1 and O3. CuTPP-AOP molecules are stacked one above LUMO (lowest-unoccupied molecular orbital) energy levels
another to form a layered structure. In H2TPP-AOP, two of dyes are important for the normal operation of DSCs,
sets of paired molecules align with each other at an angle density functional theory (DFT) studies were carried out to
of around 120° to form a zigzag structure.
ensure the dyes are energetically suitable for electron injec-
Eur. J. Inorg. Chem. 2011, 3731–3738
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3733

H. He, M. Dubey, Y. Zhong, M. Shrestha, A. G. Sykes
FULL PAPER
tion. The calculated HOMO and LUMO energy levels of coupling to the TiO2 conduction band. Note that delocal-
ZnTPP-AOP, CuTPP-AOP, H2TPP-AOP and ZnTTP- ized orbitals are not the only means of electron injection,
COOH in acetonitrile along with the conduction and val- through-space injection is also possible.[4,11,14]
ence bands of TiO2 nanoparticles are depicted in Figure 5.
The LUMO energy levels of the three porphyrins are
Photovoltaic Performance
around –2.80 eV, which is higher than the conduction band
of TiO2 nanoparticles. This ensures electron injection from
the porphyrin to the TiO2 conduction band for the normal
functioning of DSCs. However, the electron density distri-
butions in these orbitals, as shown in Figure 6, are largely
localized on the porphyrin ring. The electron density on the
AOP group is relatively low. As electron delocalization on
to the anchoring group usually facilitates electron injection
and photovoltaic performance, the less delocalized molecu-
lar orbital on the AOP group may weaken their electron
The photovoltaic performances of the synthesized por-
phyrins were evaluated in titanium dioxide nanotube (TiO2
NT) based DSCs. The TiO2 NTs exhibit large specific sur-
face areas, high pore volumes and excellent electron collec-
tion efficiency.[24–27] They were prepared by anodization of
Ti foil under a constant DC voltage (60 V) in ethylene glyc-
erol and detached in the presence of dilute HF, as reported
previously.[28] Figure 7 shows the cross-sectional scanning
electron microscopy (SEM) images of the TiO2 NT film af-
ter sintering. The TiO2 NTs are vertically aligned with
lengths of around 20 μm. The diameters and wall thick-
nesses of the NTs are around 80 and 20 nm, respectively.
After fixing the arrays on to the fluorine–tine–oxide (FTO)
glass with the help of a thin layer of TiO2 nanoparticles
(≈3 μm), no clear morphological changes in the TiO2 NTs
were observed. The resulting film is opal. One side of the
TiO2 NTs are embedded inside the NP matrix and makes
very good physical contact. The TiO2 NT arrays are very
easy to remove from the FTO glass if the TiO2 NP layer is
not applied. Note that the TiO2 NTs are open at one end
and closed at the other. The NTs with open ends facing the
TiO2 NP layer bind much more strongly to the FTO glass
than the closed ends and are suitable for cell fabrication.
Therefore only cells with this configuration were tested.
Figure 5. Calculated HOMO and LUMO energy levels of
(A) ZnTPP-AOP,
(B) CuTPP-AOP,
(C) H2TPP-AOP
and
(D) ZnTTP-COOH in acetonitrile from DFT calculations at the
B3LYP/6-31G(D) level of theory.
Figure 7. Scanning electron microscopy (SEM) images of (A) the
open ends and (B) the closed ends of free-standing TiO2 NT arrays.
(C,D) Cross-sectional SEM images of TiO2 NT arrays on FTO af-
ter sintering at 450 °C for 30 min in air. Image D is C with a higher
magnification.
The photovoltaic performances of the three porphyrins
were tested under AM1.5 conditions. The photovoltaic pa-
rameters are listed in Table 1. For comparison, the cell effi-
ciencies of ZnTTP-COOH and a commercial ruthenium dye
N719 under the same conditions are also listed in Table 1.
Of the three synthesized porphyrins, ZnTPP-AOP gives the
Figure 6. HOMO and LUMO profiles of (A) ZnTPP-AOP,
(B) H2TPP-AOP and (C) CuTPP-AOP in acetonitrile from DFT
calculations at the B3LYP/6-31G(D) level of theory.
3734
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 3731–3738

(Acetyloxopropyl)tetraphenylporphyrin and Its Complexes
highest energy conversion efficiency of 0.14%, whereas
CuTPP-AOP and H2TPP-AOP exhibit negligible energy
conversion efficiency. Under the same conditions, ZnTTP-
COOH and N719 show 2.43 and 6.1% energy conversion
efficiency, respectively. Typical J–V curves for these cells are
shown in Figure 8. The open-circuit voltages (VOC), short-
circuit current densities (JSC) and fill factors (FF) of these
cells are all lower than those of ZnTTP-COOH- and N719-
sensitized cells. The IPCE (incident photon to current effi-
ciency) profiles, as shown in Figure 9, are quite similar to
the absorption spectra with high IPCE in the Soret band
range and low IPCE in the Q-band range. The ZnTPP-
AOP-sensitized cell exhibits 7% IPCE in the Soret band
region, whereas the IPCE values of ZnTTP-COOH- and
N719-sensitized cells are around 70 and 90% over a much
wider spectral range.
Figure 9. IPCE spectra of ZnTPP-AOP-, CuTPP-AOP- and
H2TPP-AOP-sensitized DSCs. The insets show the optical images
of TiO2 NT films coated with (A) ZnTTP-COOH, (B) H2TPP-
AOP, (C) ZnTPP-AOP and (D) CuTPP-AOP.
Table 1. Photovoltaic parameters of DSCs with different por-
phyrins and N719 as dyes.
Dye
VOC [mV]
JSC [mA/cm2] FF [%]
η [%]
gle-crystal structure analysis reveals that the AOP group is
close to the adjacent phenyl group and the AOP is too short
to allow effective binding to the titanium atom; this situa-
tion does not occur with the longer ZnTTP-COOH, as
shown schematically in Figure 10. In this regard, removing
the adjacent phenyl group could enhance the binding of
AOP to the TiO2 surface. Currently we are working in this
direction and results will be reported in due course.
ZnTPP-AOP
CuTPP-AOP
H2TPP-AOP
ZnTTP-COOH
N719
530
480
483
612
700
0.41
0.06
0.08
5.70
16.26
65
53
56
70
53
0.14
0.015
0.021
2.43
6.10
Figure 10. Possible alignments of (A) ZnTPP-AOP and
(B) ZnTTP-COOH on the surface of TiO2 nanotubes.
Figure 8. J–V curves of the ZnTPP-AOP-, CuTPP-AOP- and
H2TPP-AOP-sensitized DSCs.
The overall energy conversion efficiency of a DSC is the
product of light-harvesting efficiency (LHE), electron injec-
tion efficiency (ηinj) and electron collection efficiency (ηcoll).
Conclusions
Three asymmetric porphyrins have been synthesized and
However, given their similar photophysical properties and their structures characterized by single-crystal X-ray dif-
the orthogonal arrangement of the anchoring group relative fraction analysis. The 1-acetyl-2-oxopropyl (AOP) group is
to ZnTTP-COOH, the poor photovoltaic performances of linked to a β-pyrrolic position. The mean plane of the AOP
the three porphyrins can only be ascribed to their low dye- is almost perpendicular to the mean plane of the porphyrin
loading on the TiO2 film. It was found that the TiO2 films ring. Theoretical calculations and photophysical properties
after porphyrin impregnation for 12 h were slightly yellow, indicate that these porphyrins are suitable for the normal
whereas the films of ZnTTP-COOH were deep purple, as operation of DSCs. However, the neighbouring phenyl
shown in the inset of Figure 9. The addition of pyridine did groups of the AOP prevent their binding to TiO2 nano-
not improve the loading density. It is unlikely that the poor tubes. As a result, the photovoltaic performances of these
dye-loading is a result of the intrinsic binding ability of porphyrins are quite poor. The zinc compound exhibits
AOP to TiO2. Instead, the steric hindrance of the phenyl 0.14% energy conversion efficiency, whereas the reference
group adjacent to the AOP plays a significant role. The sin- dyes ZnTTP-COOH- and N719-sensitized solar cells show
Eur. J. Inorg. Chem. 2011, 3731–3738
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3735

H. He, M. Dubey, Y. Zhong, M. Shrestha, A. G. Sykes
FULL PAPER
2-(1-Acetyl-2-oxopropyl)-5,10,15,20-tetraphenylporphyrinatozinc(II)
(ZnTPP-AOP): H2TPP-AOP (0.11 g) was dissolved in CHCl3/
CH3OH (1:1, v/v, 25 mL) and excess zinc acetate (0.5 g) was added.
The mixture was stirred at room temperature overnight and the
organic solvent was removed. The solid was dissolved in a small
amount of chloroform and loaded onto a column of silica gel and
eluted with chloroform. The main band was collected. The final
product was obtained after recrystallization from CHCl3/CH3OH
(5:100, v/v); yield 0.10 g, 83%. C49H34N4O2Zn·CH3OH: calcd. C
74.30, H 4.74, N 6.93; found C 74.56, H 4.63, N 6.86. UV/Vis
(CHCl3, 25 °C): λ [log(ε/m–1 cm–1)] = 421 [5.68], 549 [4.35] nm. MS:
2.4 and 6.1% energy conversion efficiency, respectively. The
results clearly indicate that the effective binding of the dye
to the TiO2 surface is critical for achieving high efficiency
in dye-sensitized solar cells. Reducing the steric hindrance
by removing one or two neighbouring phenyl groups might
produce dyes with better photovoltaic performance.
Experimental Section
All solvents were treated by standard methods prior to use. Rea-
gent-grade benzaldehyde, 4-methylbenzaldehyde and acetylacetone
were obtained commercially and used without further purification.
Pyrrole was distilled twice prior to use. Other chemicals were ana-
lytical-grade and used as received. The N719 dye was obtained
from Solaronix. The FTO glass with a sheet resistance of 8 Ω/cm2
from Hartfort Glass (USA) was used for cell fabrication. Elemental
compositions were determined by using a commercial CHN ana-
lyser. The scanning electron microscopy (SEM) images were taken
with a Hitachi S3400 microscope. The FTIR spectra were recorded
with a Nicolet 6700 spectrometer in ATR mode.
calcd. for C H N O Zn 775.2; found 775.2. FTIR: ν = 2957 (w),
˜
49 35
4
2
2927 (w), 1594 (s) cm–1.
X-ray Crystallographic Analysis: Single crystals of ZnTPP-AOP,
CuTPP-AOP and H2TPP-AOP were obtained by slow evaporation
of solvent of the complexes in 1:1 (v/v) mixtures of methanol and
dichloromethane at room temperature for 2 weeks. The crystals
were mounted on glass fibres for data collection. Diffraction mea-
surements were made with a CCD-based commercial X-ray dif-
fractometer using Mo-Kα radiation (λ = 0.71073 Å). The frames
were collected at 125 K with a scan width of 0.3° in ω and inte-
grated with the Bruker SAINT software package[30] using a narrow-
frame integration algorithm. The unit cell was determined and re-
fined by least-squares methods upon the refinement of XYZ
centroids of reflections above 20σ(I). The data were corrected for
absorption by using the SADABS program.[31] The structures were
refined on F2 by using the Bruker SHELXTM (version 5.1) soft-
ware package.[30]
Synthesis of Porphyrins: 5,10,15,20-Tetraphenylporphyrin (H2TPP)
and the 5-(p-carboxyphenyl)-10,15,20-tris(4-methylphenyl)por-
phyrin-zinc complex (ZnTTP-COOH) were prepared according to
literature methods.[29]
2-(1-Acetyl-2-oxopropyl)-5,10,15,20-tetraphenylporphyrinatocopper-
(II) (CuTPP-AOP): Cu(NO3)2·3H2O (0.28 g) and acetic anhydride
(18 mL) were added to a solution of CuTPP (0.25 g) in CHCl3
(375 mL). The mixture was magnetically stirred for 48 h at room
temperature. The mixture was then washed with water. Then the
organic solvent was reduced to around 5 mL, the mixture was
loaded on to a silica gel column and eluted with chloroform. The
second band, 2-nitro-5,10,15,20-tetraphenylporphyrinatocopper(II)
(CuTPP-NO2), was collected and recrystallized from CHCl3/
CH3OH; yield 0.21 g. The dry CuTPP-NO2 (0.17 g) was dissolved
in DMSO (10 mL). Acetylacetone (257 μL) and anhydrous K2CO3
(0.25 g) were added to this solution. The mixture was heated at
50 °C for 1 h and then chloroform (50 mL) and water (20 mL) were
added. The organic phase was washed with water several times.
After removing all the solvent, the crude product was loaded on to
a column of silica gel and eluted with chloroform for purification.
The second band was collected. The final product was recrys-
tallized from CHCl3/CH3OH (5:100, v/v); yield 0.14 g, 78%.
C49H34CuN4O2 (774.38): calcd. C 76.00, H 4.43, N 7.24; found C
75.73, H 4.17, N 7.20. UV/Vis (CH2Cl2, 25 °C): λ [log(ε/m–1 cm–1)]
= 417 [5.61], 541 [4.36] nm. MS: calcd. for C49H35CuN4O2 774.2;
Crystal Data for ZnTPP-AOP: C49.35H35.39N4O3Zn, Mr = 797.77,
monoclinic, space group = C2/c, a = 41.352(4), b = 10.4384(10), c
= 22.390(2) Å, β = 119.6230(10)°, V = 8401.4(14) Å3, Z = 8, ρcalcd.
= 1.254 Mg/m3, μ(Mo-Kα) = 0.630 mm–1, F(000) = 3288, T =
125(2) K. 40076 reflections were measured, of which 7708 were
unique (Rint = 0.0571). Final R1 = 0.0506 and wR2 = 0.1278 values
were obtained for 6192 observed reflections with IϾ2σ(I), 555 pa-
rameters and GOF = 1.102.
Crystal data for CuTPP-AOP: C50H35N4O2Cl2Cu, Mr = 858.26,
¯
triclinic, space group = P1, a = 8.771(2), b = 15.038(4), c =
16.556(4) Å, α = 114.571(3), β = 94.977(3), γ = 99.032(3)°, V =
1933.0(8) Å3,
Z = 2, ρcalcd. = =
1.475 Mg/m3, μ(Mo-Kα)
0.753 mm–1, F(000) = 884, T = 125(2) K. 18655 reflections were
measured, of which 7007 were unique (Rint = 0.0541). Final R1 =
0.0685 and wR2 = 0.2410 values were obtained for 4727 observed
reflections with IϾ2σ(I), 532 parameters and GOF = 1.032.
Crystal data for H2TPP-AOP: C49H35N4O2, Mr = 709.76, mono-
clinic, space group = P21/c, a = 15.103(4), b = 10.535(3), c =
found 774.2. FTIR: ν = 3051 (w), 3018 (w), 1596 (s) cm–1.
˜
23.665(7) Å, β = 100.823(18)°, V = 3698.5(18) Å3, Z = 4, ρcalcd.
=
1.275 Mg/m3, μ(Mo-Kα) = 0.079 mm–1, F(000) = 1484, T =
125(2) K. 6826 reflections were measured, of which 6342 were
unique (Rint = 0.0000). Final R1 = 0.1227 and wR2 = 0.3715 values
were obtained for 2973 observed reflections with IϾ2σ(I), 497 pa-
rameters and GOF = 1.169. Several attempts were carried out to
grow high quality crystals of H2TPP-AOP for X-ray diffraction
analysis, but they were not successful.
2-(1-Acetyl-2-oxopropyl)-5,10,15,20-tetraphenylporphyrin (H2TPP-
AOP): CuTPP-AOP (0.28 g) was dissolved in CH2Cl2 (20 mL) and
concentrated H2SO4 (5 mL) was added. The resulting mixture was
stirred at room temperature for 0.5 h. Then water was added very
slowly under vigorous stirring of the reaction mixture. After wash-
ing the mixture with water several times, the organic solvent was
reduced to around 5 mL with a rotary evaporator and was loaded
on to a silica gel column for purification. Chloroform was used to
elute the column. The second band was collected. The product was
obtained after recrystallization from CHCl3/CH3OH (5:100, v/v);
yield 0.21 g, 84%. C49H36N4O2 (712.8): C 82.56, H 5.09, N 7.86;
found C 82.10, H 4.43, N 7.74. UV/Vis (CHCl3, 25 °C): λ [log(ε/
CCDC-78491 (for ZnTPP-AOP), -78492 (for CuTPP-AOP) and
-78493 (for H2TPP-AOP) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
m
–1 cm–1)] = 420 [5.52], 517 [4.29], 551 [3.97], 592 [3.91], 649
[3.82] nm. MS: calcd. for C49H37N4O2 713.3; found 713.3. FTIR:
˜
Fabrication of DSCs: Free-standing TiO2 nanotube membranes
ν = 3051 (w), 3021 (w), 1594 (s) cm–1.
were prepared as described previously.[28] The FTO glass was first
3736
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 3731–3738

(Acetyloxopropyl)tetraphenylporphyrin and Its Complexes
cleaned successively with toluene, acetone, 2-propanol and deion-
ized water. Then TiO2 nanoparticles with diameters of around 5 nm
prepared by a microwave-assisted sol–gel process was spin-coated
on the FTO glass as a compact layer. The film was sintered at
450 °C for 30 min. Then commercial TiO2 nanoparticle (NP) paste
(diameter 9 nm) was screen-printed on to the top of the compact
layer. The free-standing TiO2 NT membrane with a length of
around 22 μm was put on to the top of the TiO2 NP layer. After
air-drying for about 0.5 h, a small piece of Parafilm® was placed
on top of the film and around 100 g of metal was put on top. The
film was kept at –20 °C for 12 h and then air-dried, sintered at
450 °C for 0.5 h in oxygen and cooled to room temperature slowly.
The resulting film was dipped into a 2 mm TiCl4 aqueous solution
for 1 h and sintered again at 450 °C for 0.5 h. The films were then
immersed in 2 mm methanolic solution of the dye for 12 h. The
counter-electrode was prepared by sputtering a 10-nm-thick layer
of Pt on to the FTO glass. Two electrodes were assembled in a
Grätzel-type cell using Surlyn as sealant. The electrolyte solution
for porphyrins was 0.10 m LiI, 0.60 m tert-butylmethylimidazolium
iodide, 0.05 m I2 and 0.05 m 4-tert-butylpyridine in acetonitrile. The
electrolyte for N719 was 0.10 m LiI, 0.60 m tert-butylmethylimid-
azolium iodide, 0.05 m I2 and 0.05 m 4-tert-butylpyridine in acetoni-
trile/valeronitrile (1:1, v/v).
the conductor-like polarizable continuum model (CPCM) solvation
model, as implemented in the Gaussian 09 program package.[33]
Acetonitrile was used to mimic the solvents in electrolyte. Molecu-
lar orbitals were visualized by GaussView 3.0 software.[33] The
method has been widely used for geometry optimizations and elec-
tronic calculations of porphyrin derivatives due to its accu-
racy,[4,5,6,34,35] which was further validated by the very similar bond
angles and lengths in the optimized structures of MTPP-AOP (M
= 2H+, Zn2+, Cu2+) and single-crystal structures.
Supporting Information (see footnote on the first page of this arti-
cle): ORTEP diagram of H2TPP-AOP, fluorescence spectra of
ZnTPP-AOP and H2TPP-AOP, and packing diagrams of the three
porphyrins.
Acknowledgments
This paper is based on work supported by the National Science
Foundation/EPSCoR (grant number 0903804), the State of South
Dakota, the Ph. D. program of the Department of Electrical Engi-
neering of the South Dakota State University, the Northern Plain
Undergraduate Research Center, and the South Dakota State Uni-
versity Research and Scholar Fund (334578)
Photocurrent–Voltage Measurements: Photoelectrochemical data
were measured using a 450 W xenon light source. The light inten-
sity at the surface of the cell was calibrated to 100 mW/cm2, equiva-
lent to one sun at air mass 1.5G conditions. The applied potential
and cell current were measured by using an Agilent 4155C semicon-
ductor parameter analyser. The efficiency (η) and fill factor (FF)
[1] M. Grätzel, Acc. Chem. Res. 2009, 42, 1788–1798.
[2] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson,
Chem. Rev. 2010, 110, 6595–6663.
[3] N. K. S. Davis, A. L. Thompson, H. L. Anderson, Org. Lett.
2010, 12, 2124–2127.
[4] H. Imahori, Y. Matsubara, H. Iijima, T. Umeyama, Y. Matano,
S. Ito, M. Niemi, N. V. Tkachenko, H. Lemmetyinen, J. Phys.
Chem. C 2010, 114, 10656–10665.
[5] C.-F. Lo, S.-J. Hsu, C.-L. Wang, Y.-H. Cheng, H.-P. Lu, E. W.-
G. Diau, C.-Y. Lin, J. Phys. Chem. C 2010, 114, 12018–12023.
[6] A. Kira, Y. Matsubara, H. Iijima, T. Umeyama, Y. Matano, S.
Ito, M. Niemi, N. V. Tkachenko, H. Lemmetyinen, H. Imahori,
J. Phys. Chem. C 2010, 114, 11293–11304.
[7] W. W. H. Wong, T. Khoury, D. Vak, C. Yan, D. J. Jones, M. J.
Crossley, A. B. Holmes, J. Mater. Chem. 2010, 20, 7005–7014.
[8] N. R. de Tacconi, W. Chanmanee, K. Rajeshwar, J. Rochford,
E. Galoppini, J. Phys. Chem. C 2009, 113, 2996–3006.
[9] C.-Y. Lin, C.-F. Lo, L. Luo, H.-P. Lu, C.-S. Hung, E. W.-G.
Diau, J. Phys. Chem. C 2009, 113, 755–764.
were calculated from η (%) = 100Pmax/(PinA) and FF = Pmax
/
(ISCVOC), in which Pmax is the maximum output power of cells
(mW), Pin is the power density of the light source (mW/cm2), ISC
is the short-circuit current (mA), VOC is the open-circuit voltage
(V) and A is the active area (0.16 cm2) of the cell.
Photophysical Measurements: Absorption spectra were recorded
with an HP Agilent 8543 UV/Vis spectrophotometer in CH2Cl2
at room temperature. The steady-state fluorescence spectra were
obtained with a fluorimeter (FS920, Edinburg Instrument, Inc.,
UK) with a Xenon arc lamp as the light source. The decay curves
of fluorescence were measured with a fluorimeter by using a time-
correlated single-photon-counting technique (LifeSpec II, Edin-
burg Instrument, Inc.). A diode laser (EPL 375, Edinburgh Instru-
ment, Inc., UK) with a wavelength of 375 nm was used as the light
source. The pulse repetition frequency was 20 MHz. The lifetimes
were determined by exponential fitting of deconvoluted decay
curves. Quantum yields were determined according to the literature
method by using ZnTPP as the reference standard (Φ = 0.033 in
air-equilibrated toluene).[32] Equation (1) was used to calculate the
quantum yields in which ns and nr are the refractive indices of the
solvents of the sample and reference, Gs and Gr are the gradients
of the plots of integrated fluorescence intensity versus absorbance
for the samples and reference at different concentrations.
[10] Y. Liu, N. Xiang, X. Feng, P. Shen, W. Zhou, C. Weng, B.
Zhao, S. Tan, Chem. Commun. 2009, 2499–2501.
[11] H. Imahori, S. Hayashi, H. Hayashi, A. Oguro, S. Eu, T. Ume-
yama, Y. Matano, J. Phys. Chem. C 2009, 113, 18406–18413.
[12] L. Cheng-Wei, L. Hsueh-Pei, L. Chi-Ming, H. Yi-Lin, L. You-
Ren, Y. Wei-Nan, L. Yen-Chun, L. You-Shiang, D. E. Wei-Gu-
ang, Y. Chen-Yu, Chem. Eur. J. 2009, 15, 1403–1412.
[13] H.-P. Lu, C.-Y. Tsai, W.-N. Yen, C.-P. Hsieh, C.-W. Lee, C.-Y.
Yeh, E. W.-G. Diau, J. Phys. Chem. C 2009, 113, 20990–20997.
[14] S. Eu, S. Hayashi, T. Umeyama, A. Oguro, M. Kawasaki, N.
Kadota, Y. Matano, H. Imahori, J. Phys. Chem. C 2007, 111,
3528–3537.
[15] A. J. Mozer, P. Wagner, D. L. Officer, G. G. Wallace, W. M.
Campbell, M. Miyashita, K. Sunahara, S. Mori, Chem. Com-
mun. 2008, 4741–4743.
[16] W. M. Campbell, K. W. Jolley, P. Wagner, K. Wagner, P. J.
Walsh, K. C. Gordon, L. Schmidt-Mende, M. K. Nazeeruddin,
Q. Wang, M. Grätzel, D. L. Officer, J. Phys. Chem. C 2007,
111, 11760–11762.
[17] L. Giribabu, C. V. Kumar, P. Y. Reddy, J. Porphyrins Phthalocy-
anines 2006, 10, 1007–1016.
[18] H. Imahori, T. Umeyama, S. Ito, Acc. Chem. Res. 2009, 42,
1809–1818.
[19] S. Eu, S. Hayashi, T. Umeyama, Y. Matano, Y. Araki, H. Im-
ahori, J. Phys. Chem. C 2008, 112, 4396–4405.
(1)
Theoretical Calculations: Theoretical calculations were performed
at the density functional theory level. The single-crystal structures
were used as frameworks for the construction of initial input struc-
tures for the calculations. Geometry optimizations and electronic
structure calculations were performed by using the B3LYP func-
tional and the 6-31G(D) basis set in acetonitrile solution by using
Eur. J. Inorg. Chem. 2011, 3731–3738
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3737

H. He, M. Dubey, Y. Zhong, M. Shrestha, A. G. Sykes
FULL PAPER
[20]
[21]
[22]
[23]
[24]
[25]
[32] R. Katoh, A. Furube, T. Yoshihara, K. Hara, G. Fujihashi, S.
Takano, S. Murata, H. Arakawa, M. Tachiya, J. Phys. Chem.
B 2004, 108, 4818–4822.
C.-Y. Lin, Y.-C. Wang, S.-J. Hsu, C.-F. Lo, E. W.-G. Diau, J.
Phys. Chem. C 2009, 113, 755–764.
T. Bessho, S. Zakeeruddin, C. Y. Yeh, E. G. Diau, M. Grätzel,
Angew. Chem. Int. Ed. 2010, 49, 6646–6649.
Raed Rahal, S. Daniele, L. G. Hubert-Pfalzgraf, V. Guyot-
Ferréol, J.-F. Tranchant, Eur. J. Inorg. Chem. 2008, 980–987.
M. Grätzel, J. Photochem. Photobiol. C: Photochem. Rev. 2003,
4, 145–153.
S. Yoriya, M. Paulose, O. K. Varghese, G. K. Mor, C. A.
Grimes, J. Phys. Chem. C 2007, 111, 13770–13776.
[33] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hase-
gawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M.
Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Starov-
erov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell,
J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M.
Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Ad-
amo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Mar-
tin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B.
Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09,
Revision A.1, Gaussian, Inc., Wallingford, CT, 2009.
[34] M. P. Balanay, D. H. Kim, Phys. Chem. Chem. Phys. 2008, 10,
5121–5127.
G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese, C. A.
Grimes, Nano Lett. 2006, 6, 215–218.
[26] L. Luo, C.-J. Lin, C.-S. Hung, C.-F. Lo, C.-Y. Lin, E. W.-G.
Diau, Phys. Chem. Chem. Phys. 2010, 12, 12973–12977.
[27] J. R. Jennings, F. Li, Q. Wang, J. Phys. Chem. C 2010, 114,
14665–14674.
[28] M. Dubey, H. He, Proceedings of 34th IEEE Photovoltaic Spe-
cialist Conference, Philadelphia, PA, 2009.
[29] A. D. Adler, F. R. Longo, J. D. Finarelli, J. Goldmacher, J. Ass-
our, L. Korsakoff, J. Org. Chem. 1967, 32, 476–476.
[30] G. M. Sheldrick, SHELXTLTM, reference manual, version
5.1, Madison, WI, 1997.
[31] G. M. Sheldrick, SADABS, Empirical Absorption Correction
Program, University of Göttingen, Göttingen, Germany, 1997.
[35] C.-Y. Lin, Y.-C. Wang, S.-J. Shu, C.-F. Lo, E. W.-G. Diau, J.
Phys. Chem. C 2010, 114, 687–693.
Received: February 22, 2011
Published Online: July 28, 2011
3738
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 3731–3738