Noble metal porphyrin derivatives bearing carboxylic groups: Synthesis, characterization and photophysical study

Article abstract of DOI:10.1016/j.poly.2012.06.080

The synthesis and characterization of meso-carboxyphenylporphyrins metallated with transition metals Pt, Pd, Rh, and Ru are described. The photophysical properties of the dyes were investigated by UV-Vis absorption, emission spectroscopy and cyclic voltam

Full text of DOI:10.1016/j.poly.2012.06.080

Polyhedron 52 (2013) 1016–1023
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Polyhedron
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Noble metal porphyrin derivatives bearing carboxylic groups: Synthesis,
characterization and photophysical study
Christina Stangel a, Dimitra Daphnomili a, Theodore Lazarides a, Marija Drev b, Urša Opara Krašovec b,
Athanassios G. Coutsolelos a,
⇑
a Chemistry Department, University of Crete, Voutes Campus, P.O. Box 2208, 71003 Heraklion, Crete, Greece
b
ˇ
Faculty of Electrical Engineering, University of Ljubljana, Trzaška c. 25, 1000 Ljubljana, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 1 August 2012
The synthesis and characterization of meso-carboxyphenylporphyrins metallated with transition metals
Pt, Pd, Rh, and Ru are described. The photophysical properties of the dyes were investigated by UV–Vis
absorption, emission spectroscopy and cyclic voltammetry. Structures of platinum and palladium deriv-
atives were elucidated by X-ray. The activity of the formed complexes as dyes for DSSCs was studied,
however the performance is rather low compared to the reference dye N719.
Keywords:
Tetracarboxyporphyrins
Noble metals
Ó 2012 Elsevier Ltd. All rights reserved.
X-ray studies
Photophysical studies
DSSCs
1. Introduction
porphyrins with different metals were studied adsorbed on SnO2
nanocrystalline semiconductor [13] with the Pd derivative being
Among the large number of synthetic porphyrin derivatives, the
water-soluble ones are of particular interest due to their wide-
spread applications that include sensitizers for DSSCs [1–5], in
medicine and biology as drugs because of their capability to be en-
riched at the surface of a tumor allowing photodynamic therapy [6],
as building blogs for crystal engineering [7] and supramolecular
assemblies [3] and in catalysis [8,9]. Several porphyrins have been
used as dyes, because of their strong absorption in UV–Vis region
[10]. The most common being the free-base and zinc derivatives
of the meso-benzoic acid substituted porphyrin TCPP. Grätzel
et al. have reported efficient charge injection from the excited state
of Zn-TCPP into the conduction band of TiO2 (IPCESoret = 42%), (IC-
PEQband = 8–10%), however no AM1.5 normalized cell efficiencies
were stated [11]. Photosensitization by Zn-TCPP, giving low solar-
the one that gave the highest quantum yield of sensitized photocur-
rent generation. Meso or b-carboxy-derivatized porphyrins have
been synthesized [14] with the position and nature of the bridge
connecting the porphyrin ring and the carboxylic group having a
significant influence on the photovoltaic properties of the sensitiz-
ers. More recently, Yella et al. introduced long alkoxyl groups into
the porphyrin ring and achieved an unprecedented 12.3% energy
conversion efficiency in combination with a cobalt-based electro-
lyte [15]. The long alkoxyl groups into this porphyrin ring reduce
the dye aggregation. Reduction of
g due to aggregation-induced en-
ergy transfer becomes a severe problem for porphyrin dyes because
of their planar structural characteristics. Various Zn-metallated
porphyrins have been tested as dyes for DSSCs, but few examples
have been reported about metallated porphyrins with other metals
like palladium, platinum for the same application. According to the
literature, platinum porphyrins have not been tested as dyes in
DSSCs but they have extensive applications into organic light emit-
ting diodes (OLEDs). The most representative example is platinum
(II) octaethylporphyrin (PtOEP) based OLEDs [16–18] in which
quantum efficiency of the OLED achieved 6–7% photon/carrier. Shao
energy conversion efficiency (
and Goossens [12]. The best reported value (
g
= 1.1%) was reported from Boschloo
= 3.5%) using TCPP as
g
dye was achieved using deoxycholic acid (DCA) as a co-adsorbate
and reported from Cherian and Wamser [2]. Monocaboxylate
Abbreviations: Porphyrin 1, meso-tetra-[4-(4-ethoxy-4oxobutoxy)-phenyl]-por-
phyrin; N719 dye, di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,20-bipyri-
dyl-4,40-dicarboxylato)ruthenium(II); N3 dye, cis-bis(isothiocyanato)bis(2,20-
bipyridyl-4,40-dicarboxylato ruthenium(II); AM 1.5, air mass = 1.5 (1000 Wmꢀ2);
DSSC, dye-sensitized solar cell; FF, fil factor; Isc, short-circuit current; ICPE, incident
monochromatic photon-to-current conversion efficiency; Voc, open circuit voltage;
and Yang have reported
a new type of organic photovoltaic device
by utilizing materials with long exciton lifetimes [19]. PtOEP was
selected for the electron-donating material and C60 was used as
the electron acceptor. Both possess strong triplet electron states,
excellent thermal stability, and good ability to form thin films
[19]. On the other hand, palladium porphyrins have applications
g
, overall solar light energy conversion efficiency.
⇑
Corresponding author.
E-mail address: coutsole@chemistry.uoc.gr (A.G. Coutsolelos).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.06.080

C. Stangel et al. / Polyhedron 52 (2013) 1016–1023
1017
in thin films and in solid state solar cells. Kroeze and co-workers
have reported bilayer semiconductor/antenna photovoltaic device
structures, a smooth anatase TiO2 film spin-coated with a carboxyl-
ated palladium porphyrin (PdTPPC) [20,21]. The charge separation
efficiency was large and found to be 12% for the PdTPPC/TiO2 bi-
layer. Ruthenium and rhodium metallated porphyrins are com-
monly used as catalysts in several catalytic reactions. The first use
of metalloporphyrins bearing peripheral carboxylic groups has
been reported in cross-coupling reactions [8] and in the hydrogena-
(TiO2) sol–gel paste was deposited on the conduction glass elec-
trode (TCO) using the ‘‘doctor blading’’ technique [26]. Layers were
sintered at 450 °C for 1 h and later immersed in the ethanol solu-
tion of the porphyrin dyes for 12 h. For the counter electrode,
5 nm thick layer of platinum was sputtered onto a TCO glass sub-
strate. The photoanode and the counter electrode were sealed to-
gether with a 25 lm thick polymer foil (Surlyn, DuPont) and the
ionic liquid electrolyte containing 0.2 M I2 was injected through
the two holes predrilled into the counter electrode [27]. The
average active area of the cells presented in this paper is 0.5 cm2.
The DSSCs characterization was performed with Keithley 238
source measure unit under the Oriel Class A sun simulator [28].
The level of standard irradiance (1000 W/m2) was determined with
a calibrated c-Si reference solar cell. The spectral response of the
assembled DSSCs was analyzed with a Xenon lamp and a mono-
chromator. The measurements were scanned in increments of
5 nm from 300 to 800 nm, while in order to obtain stable reading
the 3 s delay was always applied between setting wavelength
and measurement of the current.
tion of a,b unsaturated aldehydes [9]. As far as we know ruthenium
carboxylate porphyrins have not been tested as dyes in DSSCs. A.
Morandeira and co-workers reported the application in DSSCs of
ruthenium phthalocyanine (Pc) axially coordinated with pyridyl
carboxylic acid [22]. Therefore, since there are not many examples
in the literature of porphyrin dyes metallated with noble metals
studied as dyes in DSSC, we report herein the facile synthesis of var-
ious metallated porphyrins in high yields. Tetracarboxy porphyrins
metaled with Pd, Pt, Rh, Ru and C4 alkoxyl chains were synthesized
for this purpose. Finally, their potential application as dyes for
DSSCs was also studied.
2.4. Synthesis
2. Experimental
Synthesis of porphyrin 1 and of complexes 2c and 3c were
based on previously published procedures [8]. Hydrolysis of esters
afforded the tetracarboxylic acid derivatives that were used for the
DSSCs studies.
2.1. Materials and methods
All chemical were purchased from Aldrich chemical Co. and
used without further purification. The NMR measurements were
recorded on a Bruker AMX-500 or on a Bruker DPX-300. UV–Vis
absorption spectra were measured on a Shimadzu MultiSpec-
1501 spectrometer. HRMS was determined by a Bruker ultrafleX-
treme MALDI–TOF spectrometer. Emission spectra were measured
on a JASCO FP-6500 fluorescence spectrophotometer equipped
with a red-sensitive WRE-343 photomultiplier tube (wavelength
range: 200–850 nm). Quantum yields were determined from cor-
rected emission spectra following the standard methods [23] using
2.4.1. {Meso-tetra-[4-(4-ethoxy-4oxobutoxy)-phenyl]porphyrinato}-
ruthenium(II)carbonyl (2a)
A mixture of porphyrin 1 (150 mg, 0.132 mmol), Ru3(CO)12
(150 mg, 0.235 mmol) and decalin (50 mL) was refluxed for 2 h un-
der nitrogen atmosphere. The mixture was reduced to dryness un-
der vacuum and the crude solid was dissolved in CH2Cl2 (40 mL)
After washing with aqueous NaCl (2 ꢁ 30 mL) the organic layer
was dried over Na2SO4, filtered and concentrated. The residue
was purified by column chromatography on silica gel (CH2Cl2/
EtOH, 100:0.3 v/v) to obtain 2a as a dark red solid (158 mg, 95%).
1H NMR (300 MHz, CDCl3): d = 8.74 (s, 8H, Ar), 8.12 (dd,
J = 8.4 Hz, J = 2.1 Hz, 4H, Ar), 8.01 (dd, J = 8.3 Hz, J = 2.1 Hz, 4H,
Ar), 7.14 (m, 8H, Ar), 4.03 (m, 8H, CH2), 3.85 (m, 8H, CH2), 2.24
(m, 8H, CH2), 1.98 (m, 8H, CH2), 1.15 (t, J = 7.2 Hz, 12H, CH3)
ppm. 13C NMR (75 MHz, CDCl3): d = 181.9 (Ru–CO), 173.2 (ester
CO), 158.2, 144.3, 135.3, 135.1, 134.8, 131.6, 121.4, 112.6 and
112.4 (ArC and porphyrin ring C), 66.8 (CH2), 60.4 (CH2), 30.5
[Ru(bpy)3]Cl2 (U = 0.042 in H2O [24]) as standard. Emission life-
times were determined by the time correlated single-photon
counting technique using an Edinburgh Instruments mini-tau life-
time spectrophotometer equipped with an EPL 405 pulsed diode
laser, at 406.0 nm with a pulse width of 71.52 ps and a pulse period
of 200 ns, and a high speed red-sensitive photomultiplier tube
(H5773-04) as detector.
(CH2), 24.5 (CH2), 14.0 (CH3) ppm. UV–Vis: kabs (CH2Cl2) (e
, mM–
2.2. X-ray crystal structure determination
1 cm–1) 415 (97.7), 531 (10.1), 563 (5.6). HRMS-(MALDI-TOF): calc.
for [M–CO]+ C68H68N4O12Ru 1234.3877, found 1234.3883.
Single crystals for 2c and 2d were obtained by slow evaporation
of solvent of the complexes in 1:1(v/v) mixture of CHCl3 and THF.
The crystals were protected with paratone-N and mounted for data
collection. X-ray diffraction data were recorded on STOE IPDSII dif-
2.4.2. {Meso-tetra-[4-(4-ethoxy-4oxobutoxy)-phenyl]-porphyrinato}-
rhodium(III)chloride (2b)
fractometer using graphite monochromated MoK
(k = 0.71070 Å). The data were collected at 140 K for 2c complex
and at 300 K for 2d. Complete data set was collected using two
a radiation
Porphyrin 1 (150 mg, 0.132 mmol) was dissolved in benzoni-
trile (35 mL) under nitrogen atmosphere and refluxed for 5 min.
RhCl3 (55 mg, 0.26 mmol) was added and the solution was refluxed
for additional 3 h, cooled, and reduced to dryness under vacuum.
CH2Cl2 (40 mL) was added and the mixture was washed with aque-
ous NaCl (2 ꢁ 30 mL). The organic layer was dried over Na2SO4, fil-
tered, concentrated, and the residue was purified by column
chromatography on a silica gel (CH2Cl2/EtOH, 100:1 v/v) to obtain
2b as a dark red solid (164 mg, 98%). 1H NMR (500 MHz, CDCl3):
d = 8.94 (s, 8H, Ar), 8.17 (d, J = 6.5 Hz, 4H, Ar), 8.03 (d, J = 7.0 Hz,
4H, Ar), 7.27 (d, J = 6.0 Hz, 8H, Ar), 4.29 (m, 16H, CH2), 2.72 (t,
J = 7.5 Hz, 8H, CH2), 2.33 (q, J = 6.5 Hz, 8H, CH2), 1.36 (t, J = 7.0 Hz,
12H, CH3) ppm. 13C NMR (75 MHz, CDCl3): d = 173.3 (CO), 158.6,
142.1, 135.7, 135.0, 132.2, 120.5 and 112.6 (ArC and porphyrin ring
C), 67.0 (CH2), 60.5 (CH2), 29.9 (CH2), 24.8(CH2), 14.2 (CH3) ppm.
x
scans with an increment of 1°, an exposure time of 2 min. for each
oscillation and distance of 100 mm. Data reduction including
absorption correction and cell dimension post refinement were per-
formed using X-Area package software. The structure was solved
using direct methods (SHELXS-97) and all non hydrogen atoms were
refined with anisotropic displacement parameters using SHELXL-97
[25] by full matrix least squares on F2 values. All the hydrogen atoms
were introduced at calculated positions as riding on bonded atoms.
2.3. Dye sensitized solar cells
All the chemicals for the TiO2 paste preparation and other com-
ponents of DSSC were used as received. Unique titanium oxide
UV–Vis: kabs (CH2Cl2) (e
, mM–1 cm–1) 427 (71.2), 537 (9.1), 574

1018
C. Stangel et al. / Polyhedron 52 (2013) 1016–1023
(5.6). HRMS (MALDI-TOF): m/z calc. for [M–Cl]+ C68H68N4O12Rh
1236.3889, found 1236.3882.
3. Results and discussion
3.1. Synthesis and characterization
2.4.3. {Meso-tetra-[4-(4-ethoxy-4oxobutoxy)-phenyl]-porphyrinato}-
platinum(II) (2d)
The preparation of rhodium, ruthenium, palladium and plati-
num porphyrins with four anchoring groups is shown in Scheme 1.
The free-base porphyrin 1 was prepared as previously described
[8]. Insertion of rhodium, palladium and platinum was achieved
by addition of RhCl3, PdCl2, PtCl2 to porphyrin 1 in refluxing
benzonitrile. Insertion of ruthenium was achieved by addition of
Ru3(CO)12 to porphyrin 1 in refluxing decalin under N2 atmo-
sphere. The corresponding metal complexes 2a, 2b, 2c, 2d were
obtained in high yields and were characterized by various spectro-
scopic techniques. UV–Vis absorption spectra of the formed
metal-complexes are of normal type for five and four coordinated
porphyrins (Table 1S). Indicative for the formation of the metal
complexes is the blue or red shift for the Soret band related to
the free base. In the case of Pd(II), Pt(II) and Ru(III) coordinated
complexes the Soret band is blue shifted related to the free base
(422 nm). On the contrary the Soret band of Rh(III) porphyrin is
red-shifted (427 nm). Also the appearance of two Q bands for the
metal porphyrins instead of four for the free base is characteristic
for the metallation. In addition, the absence of negative chemical
shift in the 1H NMR spectra due to the hydrogen of pyrrole ring
was indicative for the insertion of metal. The carboxylic acids
derivatives were obtained by saponification using excess of KOH
in THF/MeOH mixture followed by dropwise addition of 2 N HCl
until formation of the precipitate. The UV–Vis absorption spectra
of the carboxylic acids had the typical shift in the Soret band due
to the solvent effect. In the carboxylic acid 3d, the Soret band
shifted at 403 nm and the Q-band at 514 nm.
PtCl2 (175.7 mg, 0.660 mmol) was dissolved in benzonitle
(30 mL) and refluxed for 1 h. Porphyrin 1 (150 mg, 0.132 mmol)
was added and the solution was further refluxed for 1 h, cooled,
and evaporated to dryness under reduced pressure. CH2Cl2
(40 mL) was added, and the mixture was washed with aqueous
NaCl (2 ꢁ 30 mL). The organic layer was dried over Na2SO4, filtered,
concentrated, and the residue was purified by column chromatog-
raphy on a silica gel (CH2Cl2/EtOH, 100:0.3 v/v) to obtain 2d as an
orange solid (170 mg, 96%). 1H NMR (300 MHz, CDCl3): d 8.78
(s,8H, Ar), 8.02 (d, J = 8.4 Hz, 8H, Ar), 7.20 (d, J = 8.57 Hz, 8H, Ar),
4.24 (m, 16H, CH2), 2.67 (t, J = 7.2 Hz, 8H, CH2), 2.28 (q, J = 6.9 Hz,
8H, CH2), 1.35 (t, J = 7.2 Hz, 12H, CH3). 13C NMR (75 MHz,CDCl3):
d 173.3 (CO), 158.6 (C), 141.1 (C), 134.8 (Ar), 134.7 (C), 133.8(Ar),
121.9 (C), 112.7 (Ar), 66.9 (CH2), 60.5 (CH2), 31.1 (CH2), 24.8
(CH2), 14.3 (CH3). UV–Vis: kabs (CH2Cl2) (
(290.4), 512(25,4), 575 (4,9). HRMS (MALDI-TOF): m/z calc. for
68H68N4O12Pt 1327.4481, found 1327.4475.
e
, mMꢀ1 cmꢀ1) 406
C
2.4.4. General procedure for synthesis of porphyrins 3a, 3b, 3d
To a solution of ester complex (2a, 2b or 2c, respectively)
(0.0752 mmol) in THF/MeOH (10:6.4 v/v), aqueous 0.5 M KOH
(73 mL) was added and stirred at room temperature for 24 h. The
course of the reaction was monitored by T.L.C (CH2Cl2/MeOH 9:1
v/v). The solution was evaporated to dryness to afford the potas-
sium salt as orange solid. The solid was dissolved in distilled water
(100 mL), aqueous 2 N HCl was added dropwise until pH reached 3
and a precipitate was formed. It was filtered, washed several times
with distilled water to afford a purple solid (yield 95%).
3.2. Photophysical properties
2.4.4.1.
{Meso-tetra-[4-(3-carboxypropoxy)-phenyl]-porphyrina-
to}ruthenium(II)carbonyl (3a). 1H NMR (300 MHz,DMSO-d6):
d = 12.19 (br s, 4H, COOH), 8.60 (s, 8H, Ar), 8.08 (d, J = 7.8 Hz, 4H,
Ar), 7.96 (d, J = 8.1 Hz, 4H, Ar), 7.32 (m, 8H, Ar), 4.28 (t, J = 6.0 Hz,
8H, CH2), 2.56 (m, 8H, CH2), 2.13 (t, J = 6.6 Hz, 8H, CH2) ppm.
13CNMR (75 MHz, DMSO-d6): d = 180.2 (Ru–CO), 174.2 (ester CO),
158.1, 143.4, 134.8, 133.9, 131.3, 121.1 and 112.6 (ArC and porphy-
rin ring C), 66.8 (CH2), 30.3 (CH2), 24.5 (CH2) ppm. UV–Vis: kabs
It is known that Pt(II), Pd(II), Rh(III) complexes phosphoresce in
the red region of the spectrum at both room temperature and 77 K
and on passing from Pd(II) to Rh(III) and Pt(II) the effieciency of
intersystem crossing to the first triplet excited state, following
photoexcitation, increases due to enhancement of the heavy atom
effect [29]. In our case the emission spectra of the metal tetraester
porphyrinic complexes were measured at RT in air equilibrated
dichloromethane solution and at 77 K in EtOH/MeOH glass. A
collection of emission maxima, quantum yields and lifetimes are
summarized in Table 1. Representative emission spectra are shown
in Figs. 1 and 2. Compounds 2c and 2d were also studied by time
resolved emission spectroscopy in dichloromethane at room tem-
perature. The derivative 2c shows the characteristic emission of
palladium porphyrins with kmax at 710 nm due to the phosphores-
cence and an emission at 510 and 616 nm due to the fluorescence
at 298 K. The lifetime of the phosphorescence is 752 ns and that of
the fluorescence is shorter than 1 ns. In a frozen glass at 77 K the
emission spectrum shows small blue shift of the phosphorescence
while no fluorescence was observed. The emission appears at 679
and 775 nm. The derivative 2d exhibits only the characteristic
phosphorescence of the platinum porphyrins with kmax at
679 nm at 298 K. The lifetime of the phosphorescence is 585 ns.
In a frozen glass at 77 K the phosphorescence has also been blue-
shifted with emission maximum at 667 nm. In both 2c and 2d
complexes the introduction of alkoxy chains is seen to slightly
red shift the emission (5–10 nm) relative to their tetraphenyl ana-
logs [30,31]. The observed lifetimes are considerably shorter than
those observed in dearated solution of tetraphenyl poprhyrins.
For the Rh derivative 2b very weak fluorescence was observed
[32] while phosphorescence is heavily quenched due to the
presence of oxygen. As far as concerns Ru derivative 2a, emission
(0.4% Et3N in H2O) (e
, mM–1 cm–1) 413 (143.4), 533 (11.9), 569
(6.7) HRMS (MALDI-TOF): m/z calc.
C
60H52N4O12Ru [M–CO]+
1122.2620, found 1122.2611.
2.4.4.2.
{Meso-tetra-[4-(3-carboxypropoxy)-phenyl]-porphyrina-
NMR(300 MHz, DMSO-d6):
to}rhodium(III)chloride(3b). 1H
d = 12.21 (br s, 4H, COOH;, 8.93 (m, 8H, Ar), 8.14 (m, 8H, Ar),
7.41 (m,8H, Ar), 4.30 (m, 8H, CH2), 2.56 (m, 8H, CH2), 2.14 (m,
8H, CH2) ppm. 13C NMR (75 MHz, DMSO-d6): d = 174.3 (CO),
158.4, 141.6, 134.9, 131.9121.1 and 112.9 (ArC and porphyrin ring
C), 66.5 (CH2), 30.4 (CH2), 24.5 (CH2) ppm. UV–Vis: kabs (0.4% Et3N
in H2O) (e
, mM–1 cm–1) 422 (130.4), 535 (22.4), 572 (9.7). HRMS
(MALDI-TOF): m/z calc. for [M–Cl]+ C60H52N4O12Rh 1123.2631,
found 1123.2649.
2.4.4.3.
{Meso-tetra-[4-(3-carboxypropoxy)-phenyl]-porphyrina-
to}platinum(II) (3d). 1H NMR (300 MHz, CD3SOCD3): d 8.76 (s, 8H,
Ar), 8.02 (d, J = 7.2 Hz, 8H, Ar), 7.31 (d, J = 7.2 Hz, 8H, Ar), 4.24 (s
br, 8H, CH2), 2.50 (s br, 8H, CH2), 2.12 (s br 8H, CH2). 13C NMR
(75 MHz, CD3SOCD3): d 174.6 (CO), 158.9 (C), 140.8 (C), 135.1
(Ar), 133.0 (C), 131.3 (Ar), 122.4 (C), 113.5 (Ar), 67.3 (CH2), 30.7
(CH2), 24.9 (CH2) ppm. UV–Vis: kabs (0.4% Et3N in H2O) (e,
mMꢀ1 cmꢀ1) 403 (141.4), 514 (15.8). HRMS (MALDI-TOF): m/z calc.
for C60H52N4O12Pt [M]+: 1215.3229, found 1215.3235.

C. Stangel et al. / Polyhedron 52 (2013) 1016–1023
1019
Scheme 1. Reaction scheme for complexes 2a–d and 3a–d.
Table 1
Summary of spectroscopic data for derivatives 2c and 2d.
Compound
Absorption
kmax/nm (
/Mꢀ1 cmꢀ1
Emission
e
)
kmax/nm
Ulum ꢁ 102
s
/ns
kmax/nm
77 K
a
298 K
298 K
298 K
2c
420(402.3), 525(29.2), 559(3.5)
406(290.4), 512(25.4), 475(4.9)
570, 616, 710b
1.110ꢀ3,d
1.9 10ꢀ2e
3.5
<1, 752f
679, 775g
2d
679, 746c
585
667,734
a
b
c
d
e
f
Quantum yields were measured using (Ru(bpy)3)Cl2 in H2O as the standard, as described in the experimental section.
A band pass filter of 540 nm was used.
A band pass filter of 600 nm was used.
Fluorescence quantum yield.
Phosphorescence quantum yield.
A band pass filter of 600 nm was used for the detection of fluorescence with
= 752 ns.
s
< 1 ns, a band pass filter of 750 nm was used for the detection of phosphorescence with
s
g
A band pass filter of 540 nm was used.
could not be detected, due to the presence of traces of unmetallat-
ed porphyrin [33,34] not detectable by the others spectroscopic
methods.
Usually, porphyrins show two-well defined reversible single-elec-
tron oxidations corresponding to ring oxidation and formation of
the radical cation and dication [35]. In metalloporphyrins the ring
or metal centered oxidations are determined by various factors
such as the intrinsic redox potential of the porphyrinic ring, the
metal center, the type of ligands on the metal and the properties
of the solvent [36–38]. In tetraphenyl analog complexes it has been
established that the first oxidation process involves porphyrin
macrocycle oxidation. As has been reported electron withdrawing
3.3. Electrochemistry
The electrochemical properties of the metallated porphyrins
were investigated by cyclic and square wave voltammetry in
dichloromethane. The oxidation data are collected in Table 2.

1020
C. Stangel et al. / Polyhedron 52 (2013) 1016–1023
Fig. 1. Emission spectra in dichloromethane at room temperature of (a) 2c and (b) 2d.
Fig. 2. Emission spectra in EtOH/MeOH (4:1, v/v) at 77 K of (a) 2c and (b) 2d.
Table 2
reported [35] and for 2c and 2d are in the range 0.55–0.64 V and
0.95–1.01 versus (Fc/Fc+). In CH2Cl2 only the first reduction poten-
tial was observed in the case of Pt and Pd complexes.
Oxidation and reduction data reported vs. Fc/Fc+ in dichloromethane using tetrabu-
tylammonium hexafluorophosphate (TPAPF6) as supporting electrolyte. All data are
reported vs the ferrocene/ferrocenium couple (Fc/Fc+).
Compound
E1/2Ox1/V
E1/2Ox2/V
E1/2Red1/V
3.4. X-ray structure determination
2a
2b
2c
2d
0.29
0.75
0.55
0.64
0.72
1.09
0.95
1.01
–
–
Compound 2c and 2d were further ascertained by single crystal
X-ray diffraction analysis (selected bond distances and angles are
presented in Tables 2S and 3S). 2c crystallizes in monoclinic sys-
tem (Fig. 4). The structure is centrosymmetric and the asymmetric
unit contains half of the porphyrin. The carbon atoms (C17–C22)
lie in a plane P1 with an angle of 51.85 (0.19)° relative to the N4
plane and P2 (C11–C16) in a rather perpendicular position of an an-
gle of 87.68 (0.30)°. The aliphatic chains were disordered and re-
fined over two positions with occupation factors summing one.
The isostructural Pt porphyrin crystallizes in orthorhombic system
(Fig. 5). The data were collected at low temperature and the asym-
metric unit contains half of the porphyrin ring. The plane of one
phenyl ring forms an angle of 88.36 (0.30)° and the other of
48.18 (0.22)°. One of the ethoxy groups of the aliphatic chain were
found disordered and refined over two positions with occupation
factors summing one. In both structures the metals are on the
ꢀ3.19
ꢀ3.18
groups on para-position shifts the oxidations potentials to more
positive values and electron donating groups shift the reduction
potentials to more negative values with a linear Hammett relation-
ship between the E1/2 and the substituent constant
r [36]. Fig. 3
shows some representative cyclic voltammograms. All data are re-
ported versus the ferrocene/ferrocenium couple (Fc/Fc+). In our
case, the oxidations are one-electron reversible. The complexes
are more easily oxidized correlated to TPP analogs, which is
evident of electron donating properties of the C4-alkoxy chains to
the porphyrin ring. The change in oxidation potential with
different metal substitution follows the same order as that already
Fig. 3. Cyclic voltammograms of (a) 2c and (b) 2d.

C. Stangel et al. / Polyhedron 52 (2013) 1016–1023
1021
Fig. 4. X-ray structure of complex 2c. Hydrogen atoms have been omitted for clarity.
mean plane defined by the four nitrogen of the porphyrin core and
have planar structure. The packing of the two porphyrins is differ-
ent (shown in the Supporting info).
dyes (N3, N719)[39]. For comparison the photovoltaic parameters
of the DSSC sensitized with N719 are also presented. The results
confirm the activity of the porphyrin complexes in the DSSCs,
however the performance of the cells sensitized with porphyrin
complexes is low compared to the N719 dye. Furthermore, when
compared to the free porphyrin acting as a dye, an addition of
the Rh or Pd as a metal complex ion improves the performance
of the solar cell. The maximum short circuit current (Jsc) reaching
0.49 mA/cm2 and a maximum open circuit voltage of 0.473 V was
3.5. Solar cell measurements
The results of the I/V measurements of the DSSCs sensitized
with different complexes are presented in Fig. 6 and summarized
in Table 3. The most successful DSSCs are based on Ru-polypyridyl
Fig. 5. X-ray structure of complex 2d. Hydrogen atoms have been omitted for clarity.

1022
C. Stangel et al. / Polyhedron 52 (2013) 1016–1023
porphyrin complexes could be expected if the cell composition
would be changed i.e., by choosing a semiconductor with lower po-
tential of the conduction band (e.g. SnO2) and a redox couple with
lower potential (e.g., Co2+/Co3+ complexes) [40]. The second op-
tion is the development of a novel porphyrin complex with a high-
er potential of its LUMO level.
4. Conclusions
The synthesis of metallated tetracarboxyporphyrins and their
characterization has been presented. The complexes were charac-
terized by UV–Vis, N.M.R. and MALDI-TOF mass spectroscopy.
Emmision studies in air-equillibrated solutions indicated that the
Pd derivative 2c shows both phosphorescence (kmax = 710 nm,
s
= 752 ns) and fluorescence (kmax = 510 and 616 nm,
298 K. The Pt derivative 2d exhibits only the characteristic phos-
phorescence of platinum porphyrins (kmax = 679 nm, = 585 ns)
s < 1 ns) at
s
at 298 K. The cyclic voltammograms of complexes 2a–d were mea-
sured in CH2Cl2 and reversible one-electron oxidations were found
in all cases. Pd and Pt derivatives 2c and 2d show two oxidations in
the range 0.55–0.64 V and 0.95–1.01 versus (Fc/Fc+). The tetracarb-
oxy derivatives 3a–d was also absorbed on TiO2 and was tested as
Fig. 6. Current to voltage characteristics of DSSCs sensitized with different
porphyrin based dyes.
Table 3
The short circuit current (Isc), fill factor (FF), open circuit voltage (Voc) and conversion
sensitizers in DSSCs. The maximum conversion efficiency
g = 0.13%
efficiency (
different porphyrin dyes and N719.
g
) of the DSSCs evaluated under STC (100 mW/cm2, AM 1.5, 25 °C) for
gave the Rh-porphyrin complex 3b. The surface orientation, elec-
tronic nature of the porphyrin and electronic communication, all
these parameters affects porphyrin’s light harvesting ability. In
our case, the C4-alkoxyl chains into the porphyrin ring prevent
aggregations but the lack of conjugation at the chains reduces
the efficiency of these type of dyes. The use of these noble metals
with a different porphyrin structural design (ex. conjugated long
chains) might improve their efficiency as dyes in DSSC.
Dye
Isc mA/cm2
Voc
V
FF %
g %
Porphyrina
3a
3b
3c
3d
N719
0.19
0.05
0.49
0.28
0.11
13.80
0.402
0.149
0.473
0.435
0.392
0.736
58
36
56
61
58
64
0.045
0.002
0.129
0.075
0.030
6.480
a
Tetra-meso-[4-(3-carboxypropoxy)-phenyl]-porphyrin.
Acknowledgments
Financial support from the European Community’s Seventh
Framework Programme (FP7) under Grant Agreement No.
229927, project BIOSOLENUTI and Greek Secretariat of Research
and Technology (Heraklitos) are gratefully acknowledged.
Appendix A. Supplementary data
CCDC AI631510 contains the supplementary crystallographic
data for Pt-tetraester and Pd-tetraester. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated
with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.poly.2012.06.080.
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