Ru(II) porphyrins as sensitizers for DSSCs: Axial vs. peripheral carboxylate anchoring group

Article abstract of DOI:10.1142/S108842461950072X

In this study, two porphyrin chromophores metallated with ruthenium, RuTBP and RuTBPPy, were prepared and studied as sensitizers in dye-sensitized solar cells (DSSCs). The difference between the two dyes is the position (axial vs. peripheral) of the carbo

Full text of DOI:10.1142/S108842461950072X

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2019; 23: 1–11
DOI: 10.1142/S108842461950072X
Published at http://www.worldscinet.com/jpp/
Ru(II) porphyrins as sensitizers for DSSCs: Axial vs.
peripheral carboxylate anchoring group
a,b
a
a
Kalliopi Ladomenou , Vasilis Nikolaou , Georgios Charalambidis ,
c
a
Ganesh D. Sharma* and Athanassios G. Coutsolelos*
a
Department of Chemistry, University of Crete, Laboratory of Bioinorganic Chemistry, Voutes Campus, 70013
Heraklion, Crete, Greece
Hellenic Mediterranean University, Department of Agriculture, Estavromenos, 71410, Heraklion, Crete, Greece
b
c
Molecular Electronics and Optoelectronics Device Research Laboratory Department of Physics, LNMIIT,
Jamdoli, Jaipur, Raj, 302031, India
This paper is part of the 2019 Women in Porphyrin Science special issue.
Received 28 May 2019
Accepted 13 June 2019
ABSTRACT: In this study, two porphyrin chromophores metallated with ruthenium, RuTBP and
RuTBPPy, were prepared and studied as sensitizers in dye-sensitized solar cells (DSSCs). The difference
between the two dyes is the position (axial vs. peripheral) of the carboxylic anchoring group. This work
examines the impact of this variation towards the optical, electrochemical and photovoltaic performance
of DSSCs. The thorough photophysical and photovoltaic measurements indicated that the peripherally
substituted sensitizer (RuTBP) presented higher photovoltaic performance compared to RuTBPPy.
More specifically, DSSCs sensitized with RuTBP and RuTBPPy displayed an overall power conversion
efficiency (PCE) of 5.12% and 4.08%, respectively. The higher PCE value of the DSSC sensitized with
RuTBP is mainly attributed to the enhancement of J and FF values. These factors were enhanced due
sc
to the efficient dye regeneration process, the suppression of back-charge recombination reactions and the
longer electron lifetimes as evidenced from the electrochemical impedance spectra.
KEYWORDS: porphyrin, sensitizers, dye-sensitized solar cells, ruthenium, carboxylic anchoring group.
a sensitizer which is anchored onto the surface of a
INTRODUCTION
semiconductor (typically TiO ) absorbs solar radiation
2
Since the first report by O’Regan and Grätzel in 1991
and transfers photoexcited electrons to the conduction
[
1], dye-sensitized solar cells (DSSCs) have attracted the
band of TiO . The photoexcited dye then can return to
2
widespread attention of the scientific community due
to their significant advantages [2, 3]. More specifically,
DSSCs are polychromatic and transparent materials
which have low cost and facile fabrication [4]. In
addition, these type of photovoltaic systems present high
power-conversion efficiencies (PCEs) and are produced
with rich raw materials [2, 5–7]. The most essential
component in DSSCs is the photosensitizer, since it
significantly affects the PCE of the cells. Commonly,
its ground state through an electron transfer process from
the electrolyte of the cell to the dye.
Various sensitizers with donor–p–acceptor (D–p–A)
configuration have been designed to improve the overall
PCE of DSSCs [8–11]. Among them, porphyrins have
been extensively used as sensitizers due to their strongly
absorbing Soret band (400–450 nm) and moderately
intense Q bands (500–700 nm), which cover the visible
to near infrared (NIR) region of the solar spectrum
[12, 13]. Moreover, their optical and electrochemical
*
Correspondence to: Ganesh D. Sharma, email: gdsharma273@
properties can easily be tuned by simple modification
of their chemical structure [14–18]. Notably, porphyrin
gmail.com; Athanassios G. Coutsolelos, email: acoutsol@uoc.gr.
Copyright © 2019 World Scientific Publishing Company

2
K. LADOMENOU ET AL.
dyes that are based on a D–p–A configuration are the
most successful sensitizers for DSSCs reaching PCEs of
respectively, using CDCA as co-adsorbent. RuTBP
demonstrated higher dye loading onto the TiO surface,
2
1
2.75% [19] and 13% [20]. More recently, Kakiage et al.
longer electron lifetime and reduced recombination
between the injected electrons and the electrolyte.
Thus, the axial substitution in the case of RuTBPPy
significantly affected the photovoltaic parameters of the
device, leading to inferior characteristics and lower PCE.
followed a collaborative sensitization approach using
two different metal-free organic dyes [21]. In this report
the silyl- and carboxy-anchor dyes exhibited the highest
photoconversion efficiency ever reported in DSSCs,
specifically 14.3% [21]. However, the most-studied
sensitizers in DSSCs are Ru(II) polypyridyl complexes
[
22, 23]. This is due to their suitable electrochemical
RESULTS AND DISCUSSION
and photophysical features as well as their excellent
photovoltaic behavior [24]. More precisely, Ru dyes
display a broad absorption spectrum and possess great
stability in the oxidized state. Moreover, both their
ground and excited states are ideal in DSSCs using TiO2
as a semiconductor.
The vast majority of the porphyrin derivatives that
have been utilized as sensitizers in DSSCs are metall-
ated with zinc [14, 25–27]. Ruthenium porphyrins
Synthesis
The synthetic procedure that was followed for the
preparation of RuTBP is illustrated in Scheme 1.
Initially, 3 equiv. of 3,5-di-tert-butylbenzaldehyde (3),
1 equiv. of 4-formylbenzoate (1) and 4 equiv. of pyrrole
(2) were refluxed in propionic acid for 1.5 h. After
column chromatography, the porphyrin 4 was isolated
in 5% yield as a purple solid [33]. Then the carboxylic
acid 5 derivative was obtained by saponification using
excess of KOH in THF/MeOH mixture followed by
dropwise addition of 2 N HCl until formation of the
precipitate [34]. Insertion of ruthenium was achieved by
(
Ru-porphyrins), on the other hand, have been extensively
studied as catalysts in varied fields and applications [28–
1]. However, they are only few examples in the literature
3
where Ru-porphyrins are tested as dyes in DSSCs [32].
Ruthenium as the central metal offers the advantage of
attaching the chromophore through axial ligands to the
surface of the oxide semiconductor so aggregation issues
can be avoided. Also, to the best of our knowledge, in
Ru-metallated porphyrin derivatives, the effect of the
position of the anchoring group in the performance of
a DSSC has never been studied before. To that end, we
synthesized two Ru-porphyrins, (RuTBP and RuTBPPy)
addition of Ru
under N atmosphere. The corresponding metal complex
(CO)12 to porphyrin 5 in refluxing decalin
3
2
RuTBP was obtained in high yield (95%) and was fully
characterized. The UV-vis absorption spectrum of the
formed metal complex is typical for hexa-coordinated
Ru(III)-porphyrins. Indicative of the formation of the
metal complex is the blue shift for the Soret band related
to the free base. The appearance of two Q bands for the
metal porphyrin instead of four for the free base is also
characteristic of metalation. In addition, the absence of
(
Fig. 1), and investigated their photovoltaic performance
in DSSCs. In addition, the dyes bear bulky tert-butyl
groups in order to diminish porphyrin aggregation. As
illustrated in Scheme 1, RuTBP contains a carboxylic
acid as an anchoring group at its periphery, whereas in
RuTBPPy, thecarboxylategroupisaxiallycoordinatedto
the ruthenium metal center of the porphyrin macrocycle.
These porphyrins were utilized as sensitizers for
the fabrication of DSSCs and displayed overall PCEs
of 5.12% and 4.08% for RuTBP and RuTBPPy,
1
a negative chemical shift in the H NMR spectra (see
Supporting information, Figs S1 and S2) due to the inner
hydrogen of pyrrole ring was indicative of the insertion
of metal.
Tetra(3,5-di-tert-butylphenyl)-porphyrin 6 (Scheme 2)
was synthesized by reaction of the aldehyde with pyrrole
in propionic acid at reflux for 2.5 h. The product was
Fig. 1. The Ru-porphyrins used in this work (RuTBP and RuTBPPy)
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RU(II) PORPHYRINS AS SENSITIZERS FOR DSSC: AXIAL vs. PERIPHERAL CARBOXYLATE ANCHORING GROUP
3
r.t.
MeOH
Scheme 1. Synthesis of RuTBP
Scheme 2. Synthesis of RuTBPPy
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4
K. LADOMENOU ET AL.
obtained as a purple solid after precipitation in 15%
yield. Insertion of ruthenium was achieved by addition
of Ru (CO) to porphyrin 6 in refluxing decalin under
3
12
N atmosphere. The corresponding metal complex 7 was
2
isolated in high yield. In the final step, Ru porphyrin 7
and isonicotinic acid were left stirring at reflux under
nitrogen overnight. The product was isolated after
precipitation using hexane and the solid was washed
with cold hexane and methanol until the washings were
colorless, to obtain RuTBPPy in quantitative yield.
1
Studies of the H NMR spectrum showed that the protons
of the bound pyridine are shielded by the porphyrin ring
current and resonate at 5.78 and 1.76 ppm (Fig. S3).
Optical properties
The UV-vis absorption spectra of RuTBP and
RuTBPPy in DCM solutions are shown in Fig. 2. Both
porphyrin sensitizers exhibit typical porphyrin absorp-
tion characteristics with a strong Soret band appearing
in the 400–450 nm range and moderate Q band appea-
ring in the longer wavelength region of 500–650 nm.
The Soret band for both porphyrins is similar, but the
Q band for RuTBPPy is slightly red shifted relative
to that of RuTBP. Figures 2a and 2b also show the
UV-vis absorption spectra of porphyrins adsorbed onto
TiO films. In comparison with absorption spectra in
2
solution, the trend in the absorption spectra is similar
when porphyrins are adsorbed on TiO films. However,
2
the Soret- and Q-absorption bands are broadened, and
corresponding absorption onsets are clearly red shifted.
The broadening of Soret and Q bands is attributed to the
interaction between dyes and TiO . In addition, the red
2
shift in absorption onsets is most likely attributed to the
formation of J aggregates on the TiO electrode [35].
2
Since the photovoltaic behavior is directly related to the Fig. 2. Optical absorption spectra of (a) RuTBP and (b) RuTBPPy in
absorption spectra of the dye adsorbed on the TiO film, DCM solution (black color) and absorbed on to TiO film (red color)
2
2
the broadening of both Soret and Q band is favorable
for absorbing sun light. The optical energy bandgap
of these porphyrin dyes was estimated from the onset
absorption edge in the Q band, observed in the optical
one reversible reduction. The first oxidation process
corresponds to the highest occupied molecular orbital
(
HOMO) and was measured at 0.93 V and 0.91 V vs.
absorption of these dyes adsorbed on to TiO films at
2
SCE for RuTBP and RuTBPPy, respectively. These
values are sufficiently more positive than the iodine/
iodide redox potential value (0.5 V vs. SCE), ensuing
the efficient regeneration of the oxidized dye through
about 1.99 eV and 1.96 eV for RuTBP and RuTBPPy,
respectively.
Electrochemical properties
-
I present in the electrolyte of DSSC. Accordingly, the
The electron injection from excited state of the dye
first reduction corresponds to the lowest unoccupied
molecular orbital (LUMO) and was -1.47 V and -1.58 V
vs. SCE, for RuTBP and RuTBPPy, respectively. The
excited state or LUMO energy levels for these porphyrin
dyes are more negative than the conduction band edge
into the conduction band of TiO and dye regeneration
2
mechanism plays a critical role in the overall per-
formance of DSSCs. In order to obtain information
about the feasibility of electron injection and dye
regeneration processes in DSSCs, cyclic voltammetry
measurements for these two dyes were performed and
are shown in Fig. 3. A collection of the electrochemical
data is listed in Table 1. In both cases, the ruthenium
porphyrins exhibited two reversible oxidations and
of TiO (-0.7 V vs. SCE), thus providing the necessary
2
driving force for electron injection from the excited state
of the dye into the conduction band of TiO [36]. The
2
HOMO–LUMO gap (H–L) was 2.4 V for RuTBP and
2.5 V for RuTBPPy.
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RU(II) PORPHYRINS AS SENSITIZERS FOR DSSC: AXIAL vs. PERIPHERAL CARBOXYLATE ANCHORING GROUP
5
Fig. 3. Cyclic and square wave voltammograms of RuTBP (left) and RuTBPPy (right)
Table 1. Summary of the electrochemical redox data of
RuTBP and RuTBPPy in Benzonitrile. All potentials are
efficiency(h )andthechargecollectionefficiency(h )[37,
inj
cc
38]. The hLHE mainly depends upon the molar extinction
coefficient and the absorption profile of the sensitizer.
The molar extinction coefficient of RuTBP in both Soret
and Q bands is higher than that of RuTBPPy. The higher
IPCE values of the RuTBP-based DSSC can possibly be
attributed to the higher molar extinction coefficient and
hLHE [39]. Moreover, we have estimated the dye-loading
capability of the working electrode by UV-vis spectro-
scopy after desorption in alkaline solution. In detail, the
+
reported vs. SCE and FcH/FcH was used as internal standard
Ox
(
E1 = 0.50 V)
/2
Ox1
Ox2
1/2
Red1
1/2
Compound
E1
(
E
E
H-L gap
(V)
/2
V)
(V)
(V)
RuTBP
0.93
0.91
1.32
1.28
-1.47
-1.58
2.4
2.5
RuTBPPy
-7
2
dye loading was 1.34 × 10 mol/cm for RuTBP and
-7
2
1
.28 × 10 mol/cm for RuTBPPy. The amount of dye
Photovoltaic studies
loading is slightly higher for RuTBP in comparison to
RuTBPPy, therefore dye loading is another parameter
responsible for the enhanced IPCE and J value. The J
The current–voltage characteristics of the DSSCs sen-
sitized with RuTBP and RuTBPPy under illumination
are shown in Fig. 4a. In addition, the corresponding
photovoltaic parameters are compiled in Table 2. Under
identical conditions, sensitized DSSCs exhibited short-
sc
sc
2
values derived from the IPCE spectra were 12.24 mA/cm
and 10.22 mA/cm for RuTBP and RuTBPPy, respec-
2
tively. This is in good agreement with measured values
from J–V characteristics under illumination. The HOMO
energy level of RuTBP is slightly more positive than
that of RuTBPPy, indicating that dye regeneration is
more effective for the DSSC based on RuTBP leading to
decreased charge recombination and thereby enhanced Jsc
and PCE values.
2
2
circuit current (J ) of 12.32 mA/cm and 10.34 mA/cm ,
sc
open-circuit voltage (V ) of 0.62 V and 0.58 V and fill
oc
factor (FF) of 0.68 and 0.67, respectively. Those three
important parameters correspond to an overall power
conversion efficiency (PCE) of 5.12% and 4.08% for
RuTBP and RuTBPPy, respectively. Compared to
RuTBP, the solar cell sensitized with RuTBPPy showed
lower J and V values. The incident photon-to-current
Compared to the RuTBPPy-based DSSC, the
RuTBP-based device showed higher V , which
oc
sc
oc
could also be the related to the suppression of charge
recombination between the electron injected into the
conversion efficiency (IPCE) for DSSCs sensitized with
these porphyrin dyes is shown in Fig. 4b. As illustrated
in Fig. 4b, two bands are observed in wavelength ranges
-
conduction band of TiO and I in the vicinity of TiO
2
3
2
[40, 41]. To obtain information about interfacial charge
transfer, electrochemical impedance spectroscopy (EIS)
was also performed in darkness under a forward bias
4
00–450 nm and 500–650 nm, respectively, which is in
agreement with the corresponding optical absorption
spectra of these dyes adsorbed on to TiO film (Fig. 2).
voltage equivalent to the V of the DSSCs. The Nyquist
2
oc
Moreover, the DSSC sensitized with RuTBP exhibits
higher IPCE values in both Soret and Q bands as
compared to its RuTBPPy-sensitized counterpart DSSC.
The IPCE for a DSSC is mainly determined by the
light-harvesting efficiency (hLHE), the electron injection
and Bode plots for DSSCs sensitized with RuTBP and
RuTBPPy are presented in Figs 5a and 5b. As illustrated
in the Nyquist plots (Fig. 5a), in the high-frequency
6
region (over 10 Hz), the impedance is dominated by the
serial resistance (R ) owing to the resistance of the FTO
s
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K. LADOMENOU ET AL.
Table 2. Photovoltaic properties of DSSCs sensitized with RuTBP and RuTBPPy
2
a
2
Sensitizer
J (mA/cm )
V (V)
oc
FF
PCE (%)
Rrec (W cm )
te (ms)
sc
RuTBP
12.32
10.34
0.62
0.58
0.68
0.67
5.12
4.08
44
32
27.3
18.6
RuTBPPy
aThe reported PCE value is the average of three independent experiments with maximum deviation between the measurements
0.05
±
Fig. 4. (a) Current–voltage (J–V) characteristics under
illumination and (b) IPCE spectra of DSSC sensitized with
RuTBP and RuTBPPy porphyrin dyes
Fig. 5. Electrochemical impedance spectra (EIS) for DSSCs
sensitized with RuTBP and RuTBPPy (a) Nyquist plots and
(
b) Bode phase plots in the dark under a forward bias of -0.6 V
glass substrate, where the phase is zero [42]. The first
semicircle observed in the Nyquist plot is assigned to
impedances related to charge transport at the electrolyte/
counter electrode interface. In the middle-frequency
range, the large semicircle is attributed to electron
the former than the latter. Moreover, the V is basically
influenced by the degree of back electron recombination
reaction [45]. Based on the above assumption, the
oc
higher value of V for the RuTBP-based DSSC may be
oc
attributed to the suppression of electron recombination.
In the Bode phase plots (Fig. 5b), the middle-frequency
peak represents the charge transfer process of the injected
recombination resistance (R ) and chemical capacitance
rec
at the TiO /dye-electrolyte interface [43, 44]. It can be
2
seen from Fig. 5a that the semicircle which corresponds
electrons inTiO and corresponds to the second semicircle
2
to the RuTBP-sensitized DSSC is higher than that for
in the Nyquist plots. The electron lifetime was calculated
RuTBPPy. This finding indicates that the R is higher for
according to the relationship t = 1/pfpeak, where fpeak is
rec
e
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RU(II) PORPHYRINS AS SENSITIZERS FOR DSSC: AXIAL vs. PERIPHERAL CARBOXYLATE ANCHORING GROUP
7
the peak regarding the frequency observed in the middle
range concerning the Bode phase plot [46]. The electron
lifetime was determined to be 27.3 ms and 18.6 ms for
RuTBP and RuTBPPy, respectively. The longer electron
lifetime of the RuTBP-sensitized DSSC indicates an
effective suppression of the charge recombination back
reaction; this can be reflected in the improvement of Voc
and thus leads in improvement of the overall PCE. In
other words, the RuTBP acts as a blocking layer through
the suppression of the recombination reaction between
since the central ruthenium porphyrin acts as donor
group (D), the carboxylic acid groups in both positions
(axial or meso) as acceptor units (A) and the phenyl ring
that bears the carboxylic acid unit as p system. Upon
photoirradiation, intramolecular electron transfer is
expected to take place from the ruthenium porphyrin to
the carboxylate groups, which would facilitate efficient
electron injection into the conduction band of TiO2.
The dipole moments (m), the HOMO and LUMO
energies along with the HOMO–LUMO gap (HL) for
compounds RuTBP and RuTBPPy are listed in Table 3.
The calculated dipole moments of RuTBP and RuTBPPy
were found to be 4.84 and 2.46 D, respectively. The larger
m value implies enhanced intramolecular transfer ability
for RuTBP, which is in agreement both with the data
from the electrochemical impedance spectroscopy and
with the photovoltaic measurement results. The HOMO–
LUMO gaps of both compounds are found to be similar to
the corresponding values obtained from electrochemistry
measurements presented in Table 3.
the electron injected into the conduction band of TiO
2
-
and I in the vicinity of TiO .
3
2
DFT calculations
Theoretical calculations (B3LYP/6-31G*) were
performed in an effort to explore the molecular structure
along with the electronic properties of the porphyrins
used in this study. The gas-phase optimized structures
of RuTBP and RuTBPPy are presented in Fig. 6, while
the corresponding coordinates are listed in Tables S1 and
S2 respectively (see Supporting information). In both
compounds the central Ru atoms are coplanar with the
four pyrrole rings, resulting in planar central scaffolds.
The meso-substituents of RuTBP and RuTBPPy,
namely 3,5-tert-butyl-phenyl and 4-carboxy-phenyl, are
almost perpendicular to the porphyrin ring.
EXPERIMENTAL SECTION
Materials
1
H NMR spectra were recorded on Bruker AMX-
The electron-density distributions of the frontier
molecular orbitals and the corresponding energy levels
of both compounds are depicted in Fig. 7. We observe
that the LUMO energy levels either in RuTBPPy or
in RuTBP are very similar, since their corresponding
electron densities are predominantly located on the
carboxylic acid groups with some additional contributions
on the porphyrin units. In addition, the HOMO energy
levels in both compounds (RuTBP and RuTBPPy) are
mainly spread over the central ruthenium-porphyrin unit
and partially on their meso-substituents. Both porphyrin
dyes can be described as “push–pull” D–p–A systems,
500 MHz spectrometer as solutions in deuterated solvents
by using the solvent peak as the internal standard. High-
resolution mass spectra were performed on a Bruker
ultrafleXtreme MALDI-TOF/TOF spectrometer. UV-vis
absorption spectra were measured on a Shimadzu
UV-1700 spectrophotometer.
Cyclic and square-wave voltammetry experiments
were carried out at room temperature using an AutoLab
PGSTAT20 potentiostat and appropriate routines avail-
able in the operating software (GPES version 4.9).
All measurements were carried out in freshly distilled
and deoxygenated benzonitrile in the presence of
Fig. 6. Gas phase geometry optimized structure of RuTBP (left) and RuTBPPy (right). Carbon, nitrogen, hydrogen, oxygen, and
ruthenium atoms correspond to grey, blue, white, red and light green spheres, respectively
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8
K. LADOMENOU ET AL.
Fig. 7. Frontier molecular orbitals of RuTBP (upper part) and RuTBPPy (lower part) and the corresponding energy levels
Table 3. DFT calculated properties of RuTBP and RuTBPPy:
HOMO and LUMO energies, the theoretical HOMO–LUMO
gap and dipole moment (m)
tetrabutylammonium hexafluorophosphate (0.1 M) as
supporting electrolyte. A three-electrode cell setup was
used with a glassy carbon working electrode, a saturated
calomel (SCE) reference electrode and a platinum wire
as counter electrode. All potentials are reported vs. SCE.
The ferrocene/ferrocenium couple (0.50 V vs. SCE under
the above conditions) was used as internal standard.
Compound HOMO (eV) LUMO (eV) HL (eV) m (D)
RuTBP
-4.868
-4.813
-1.973
-2.183
2.90
2.63
4.84
2.46
RuTBPPy
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RU(II) PORPHYRINS AS SENSITIZERS FOR DSSC: AXIAL vs. PERIPHERAL CARBOXYLATE ANCHORING GROUP
9
Synthesis of the compounds
pyrrole (0.35 mL) was added and the mixture was
refluxed at 141°C for 2.5 h. Then the solution was cooled
to room temperature, methanol was added and the flask
was put in an ice bath. A purple solid was precipitated
and filtered. The solid was washed with methanol and
Synthesis of 5-(4-methoxycarbonylphenyl)-10,15,
0-tris(3,5-di-tert-butylphenyl)-porphyrin (4). Pyrrole
2
(
10.2 mL, 0.147 mol, 3.9 equiv.) was added dropwise at
1
0
3
30°C to a solution of methyl 4-formylbenzoate (6.23 g,
.038 mol, 1 equiv.) and 3,5-di-tert-butylbenzaldehyde
(24.11 g, 0.110 mol, 3.1 equiv.) in propionic acid
1
product 6 obtained in 15% yield (190 mg). H NMR (500
MHz, CDCl ): 8.89 (s, 8H), 8.09 (d, J = 1.2 Hz, 8H),
3
7
2
5
.89 (t, J = 1.3 Hz, 4H), 1.52 (s, 72H, H
) -2.66 (s,
tert-butyl
(
640 mL). The reaction mixture was refluxed for 1.5 h.
-1
-1
H, NH). UV-vis (CH Cl ) l , nm (e, mM cm ) 421,
2
2
max
The propionic acid was then evaporated and the resulting
precipitate was purified by chromatography on silica
gel (CH Cl /Hex 6:4) affording porphyrin 4 (1.57 g,
18, 554, 593, 648; HRMS (MALDI-TOF): m/z calcd for
+
C H N : 1063.7557 [M + H] . Found: 1063.7562 [47].
7
6
95
4
2
2
Synthesis of porphyrin 7. A mixture of porphyrin 6
150 mg, 0.132 mmol), Ru (CO) (150 mg, 0.235 mmol)
1
5
%) as a purple solid. H NMR (500 MHz, CDCl ):
3
(
3
12
d 8.91 (s, 6H, H ), 8.78 (d, J = 5 Hz, 2H, H ), 8.43
b
b
and decalin (50 mL) was refluxed for 2 h under nitrogen
atmosphere. The mixture was reduced to dryness under
vacuum and the crude solid was dissolved in CH Cl
(
(
d, J = 8 Hz, 2H, Ar), 8.32 (d, J = 8 Hz, 2H, Ar), 8.09
d, J = 2 Hz, 4H, Ar), 8.08 (d, J = 2 Hz, 2H, Ar), 7.80 (t,
2
2
J = 2 Hz, 3H, Ar), 4.12 (s, 3H, -COOCH ), 1.53 (s,
5
3
(
40 mL). After washing with aqueous NaCl (2 × 30 mL)
4H, t-Bu), -2.69 (s, 2H, NH). HRMS (MALDI-TOF):
the organic layer was dried over Na SO , filtered and
+
2
4
m/z calcd for C H N O : 1009.6360 [M + H] . Found:
7
0
81
4
2
concentrated. The residue was purified by column
chromatography on silica gel (CH Cl /EtOH, 100:0.5
1
009.6367 [33].
Synthesis of 5-(4-carboxylphenyl)-10,15,20-tris(3,5-
2
2
1
v/v) to obtain 7 as a dark red solid (160 mg, 95%). H
di-tert-butylphenyl)-porphyrin (5). To a solution of
porphyrin 4 (76 mg, 0.0752 mmol) in THF/MeOH
NMR (500 MHz, CDCl ): 8.75 (s, 8H, H ), 8.04 (d,
3
pyr
J = 14.0 Hz, 8H, Hortho), 7.75 (s, 4H, Hpara), 1.52 (s, 72H,
Htert-butyl). UV-vis (CH Cl ) lmax, nm 415, 531, 565.
(
10:6 v/v), aqueous 0.5 M KOH (73 mL) was added and
2
2
stirred at room temperature for 24 h. The course of the
reaction was monitored by TLC (CH Cl /MeOH 9:1
HRMS (MALDI-TOF): m/z calcd for C H N ORu:
7
7
94
4
2
2
+
1
192.6471 [M – EtOH + H] . Found: 1192.6476.
Synthesis of RuTBPPy. Ru porphyrin 7 (50 mg, 0.042
v/v). The solution was evaporated to dryness to afford the
potassium salt as orange solid. The solid was dissolved
in distilled water (100 mL), aqueous 2 N HCl was
added dropwise until the pH reached 3 and a precipitate
was formed. It was filtered, washed several times with
distilled water to afford a purple solid 5 (71 mg, 95%).
mmol), and isonicotinic acid (5.2 mg, 0.042 mmol) were left
stirring in THF (10 mL) at reflux under nitrogen overnight.
The product was isolated with column chromatography
on silica gel (CH Cl /CH OH, 100:20 v/v), to obtain
2
2
3
1
RuTBPPy in quantitative yield. H NMR (500 MHz,
1
H NMR (500 MHz, CDCl ): d 8.94 (s, 6H, H ), 8.74 (d,
3
b
CDCl ): 8.65 (s, 8H, H ), 8.02 (s, 4H, H ), 7.88 (s, 4H,
3
pyr
aryl
J = 5 Hz, 2H, H ), 8.47 (d, J = 8 Hz, 2H, Ar), 8.36 (d, J =
b
H ), 7.72 (s, 4H, H ), 5.78 (brs, 2H, Hpyrid), 1.76 (brs,
aryl
aryl
8
2
Hz, 2H,Ar), 8.03 (d, J = 2 Hz, 4H,Ar), 8.05 (d, J = 2 Hz,
H, Ar), 7.84 (t, J = 2 Hz, 3H, Ar), 1.51 (s, 54H, t-Bu),
2
H, Hpyrid), 1.48 (m, 72H, Htert-butyl). HRMS (MALDI-TOF):
+
m/z calcd for C H N O Ru: 1315.6791 [M + H] . Found:
83
99
5
3
-
2.65 (s, 2H, NH). HRMS (MALDI-TOF): m/z calcd for
1315.6796. UV/vis (CH Cl ) lmax, nm 415, 532.
+
2
2
C H N O : 995.6203 [M + H] . Found: 995.6210 [34].
6
9
79
4
2
Synthesis of RuTBP. A mixture of porphyrin 5
150 mg, 0.132 mmol), Ru (CO) (150 mg, 0.235 mmol)
and decalin (50 mL) was refluxed for 2 h under nitrogen
atmosphere. The mixture was reduced to dryness under
Preparation of dye-sensitized nanocrystalline TiO2
thin films
(
3
12
DSSC fabrication and characterization were performed
based on the same procedures followed in our previous
publications [48–50]. It is worth mentioning that the
working electrode was prepared by immersing the TiO2
anode in a 0.3 mM dye solution (in DCM/toluene 1/1
v/v) containing CDCA (0.4 mM) overnight.
vacuumandthecrudesolidwasdissolvedinCH Cl (40mL).
2
2
After washing with aqueous NaCl (2 × 30 mL) the organic
layer was dried over Na SO , filtered and concentrated. The
2
4
residue was purified by column chromatography on silica
gel (CH Cl /EtOH, 100:0.3 v/v) to obtain RuTBP as a dark
2
2
1
red solid (158 mg, 95%). H NMR (500 MHz, CDCl ):
3
8
.92 (s, 6H), 8.81 (d, J = 4.8 Hz, 2H), 8.53 (brd, 2H),
Computational details
8.38 (brd, 2H), 8.08 (m, 6H), 7.80 (brs, 3H), 1.53 (s, 54H,
Htert-butyl). UV-vis (CH Cl ) lmax, nm 415, 531, 565. HRMS
Density functional theory calculations (DFT) [51]
were performed at the B3LYP/6-31G(d) [51] level of
theory using the Gaussian 03 program suite [52]. For
the geometry optimizations, the LANL2DZ basis set
was used for Ru atoms and the 6-31G(d) basis sets for
lighter atoms. The computed structures and molecular
orbitals were modeled using ChemCraft software [53].
2
2
(
MALDI-TOF): m/z calcd for C H N O Ru: 1124.5117
70 78 4 3
+
[
M – EtOH + H] . Found: 1124.5122.
Synthesis of 5,10,15,20-tetra(3,5-di-tert-butylphenyl)-
porphyrin (6). In a two neck, round-bottomed flask
,5-di-tert-butylbenzaldehyde (1 g, 4.58 mmol) and
3
propionic acid (16 mL) were stirred for 10 min. Then
Copyright © 2019 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2019; 23: 9–11

1
0
K. LADOMENOU ET AL.
The optimized minimum-energy structures were verified
as stationary points on the potential energy surface by
vibrational frequency analysis calculation. Solvent
effects were evaluated by using the polarizable continuum
model (PCM) implemented on Gaussian 03, in which the
cavity is generated via overlapping spheres, previously
developed by Tomasi [54, 55].
RESEARCH — CREATE — INNOVATE (project
code:T1EDK-01504).
Supporting information
The NMR spectra and the theoretical coordinates
of RuTBP and RuTBPPy are presented in the supple-
mentary material. This material is available free of
charge via the Internet at http://www.worldscinet.com/
jpp/jpp.shtml.
CONCLUSION
In summary, we have designed and prepared two
ruthenium-metallated porphyrins and investigated their
optical and electrochemical properties as sensitizers
in DSSCs. The distinctive feature between the two
porphyrin derivatives is the location of the anchoring
group. In the case of RuTBP, the carboxylic acid
is located in the periphery of the porphyrin macro-
cycle while in RuTBPPy the anchoring group is axially
coordinated to the ruthenium metal center. Under
identical device conditions, the photovoltaic device
sensitized with RuTBP exhibited a PCE of 5.12%,
REFERENCES
1
. O’Regan B and Grätzel M. Nature 1991; 353:
37–740.
7
2
. Upadhyaya HM, Senthilarasu S, Hsu M-H and
Kumar DK. Sol. Energy Mater. Sol. Cells 2013;
1
19: 291–295.
3
4
. Mohiuddin O, Obaidullah M and Sabah C. Opt.
Quantum Electron. 2018; 50: 377.
. Carella A, Borbone F and Centore R. Front. Chem.
2
018; 6: 481.
while RuTBPPy showed a PCE of 4.08%. The J and
sc
5. Hagfeldt A, Boschloo G, Sun LC, Kloo L and Pet-
Voc values for the RuTBP-sensitized cell were higher
than those for RuTBPPy, proving that the carboxylic
acid as peripheral and not as axial substituent leads to
enhanced photovoltaic parameters. In addition, RuTBP
presented suspended charge recombination back reaction
and longer electron lifetime compared to RuTBPPy.
To sum up, the axial substitution was less effective
than the peripheral substitution in terms of photovoltaic
efficiency. This is in accordance with many examples in
the literature in which the covalent derivative was more
efficient in electron transfer processes in comparison to
the non-covalent analogue. What is more, in the case of
RuTBP, the tilted orientation that the porphyrin adopts
onto the semiconductor surface is possibly a positive
parameter that enhances the charge separation between
tersson H. Chem. Rev. 2010; 110: 6595–6663.
. Yang XD,Yanagida M and Han LY. Energ. Environ.
Sci. 2013; 6: 54–66.
. Sharifi N, Tajabadi F and Taghavinia N. ChemPhys
Chem 2014; 15: 3902–3927.
. Urbani M, Ragoussi M-E, Nazeeruddin MK and
Torres T. Coord. Chem. Rev. 2019; 381: 1–64.
. Duvva N, Prasanthkumar S and Giribabu L. Sol.
Energy 2019; 184: 620–627.
0. Chen J, Gao Y, Xu Y, Xu F, Zhang Q and Lu X.
Mater. Chem. Phys. 2019; 225: 417–425.
1. Lu J, Liu S and Wang M. Front. Chem. 2018; 6: 541.
2. Urbani M, Grätzel M, Nazeeruddin MK and Torres
T. Chem. Rev. 2014; 114: 12330–12396.
3. Yella A, Lee HW, Tsao HN, Yi CY, Chandiran AK,
Nazeeruddin MK, Diau EWG, Yeh CY, Zakeerud-
din SM and Gratzel M. Science 2011; 334: 629–634.
6
7
8
9
1
1
1
1
the porphyrin and the TiO . This orientation is due to the
2
peripheral substitution, while in RuTBPY the porphyrin
possibly adopts a parallel orientation onto TiO with
2
14. Chaudhri N, Sawhney N, Madhusudhan B, Raghav
A, Sankara M and Satapathi S. J. Porphyrins Phtha-
locyanines 2018; 2: 1–9.
respect to the porphyrin macrocycle.
1
5. Sánchez-Bojorge NA, Flores-Armendáriz S, Fuentes-
Montero ME, Ramos-Sánchez VH, Zaragoza-Galán
G and Rodríguez-Valdez LM. J. Porphyrins Phtha-
locyanines 2017; 21: 1–15.
Acknowledgments
General Secretariat for Research and Technology
(
GSRT) and Hellenic Foundation for Research and
Innovation (HFRI) (project code: 508) are gratefully
acknowledged for the financial support of this research.
Also, the European Commission’s Seventh Framework
Program (FP7/2007–2013) under grant agreement
no. 229927 (FP7-REGPOT-2008-1, Project BIO-
SOLENUTI) and the Special Research Account of
the University of Crete. This research has also been
co-financed by the European Union and Greek national
funds through the Operational Program Competitive-
ness, Entrepreneurship and Innovation, under the call
16. Martínez-Díaz MV, de la Torre G and Torres T.
Chem. Commun. 2010; 46: 7090–7108.
17. Nikolaou V, Charisiadis A, Charalambidis G, Cout-
solelos AG and Odobel F. J. Mater. Chem. A 2017;
5: 21077–21113.
18. Ladomenou K, Nikolaou V, Charalambidis G and
CoutsolelosAG. DaltonTrans. 2016; 45: 1111–1126.
19. Yella A, Mai C-L, Zakeeruddin SM, Chang S-N,
Hsieh C-H,Yeh C-Y and Grätzel M. Angew. Chem.,
Int. Ed. 2014; 53: 2973–2977.
Copyright © 2019 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2019; 23: 10–11

RU(II) PORPHYRINS AS SENSITIZERS FOR DSSC: AXIAL vs. PERIPHERAL CARBOXYLATE ANCHORING GROUP
11
2
0. Mathew S, Yella A, Gao P, Humphry-Baker R,
CurchodBasile FE, Ashari-Astani N, Tavernelli I,
Rothlisberger U, Nazeeruddin Md K and Grätzel
M. Nat. Chem. 2014; 6: 242–247.
42. Liu GH, Wang H, Li X, Rong YG, Ku ZL, Xu M,
Liu LF, Hu M,YangY, Xiang P, Shu T and Han HW.
Electrochim. Acta 2012; 69: 334–339.
43. Han LY, Koide N, ChibaY and Mitate T. Appl. Phys.
Lett. 2004; 84: 2433–2435.
44. Itagaki M, Hoshino K, Nakano Y, Shitanda I
and Watanabe K. J. Power Sources 2010; 195:
6905–6923.
2
2
1. Kakiage K,AoyamaY,YanoT, Oya K, Fujisawa J-I and
Hanaya M. Chem. Commun. 2015; 51: 15894–15897.
2. Bomben PG, Robson KCD, Koivisto BD and
Berlinguette CP. Coord. Chem. Rev. 2012; 256:
1
438–1450.
45. Cao K, Lu JF, Cui J, Shen Y, Chen W, Alemu G,
Wang Z, Yuan HL, Xu J, Wang MK and Cheng YB.
J. Mater. Chem. A 2014; 2: 4945–4953.
46. Wan ZQ, Jia CY, DuanYD, Zhou LL, LinY and Shi
Y. J. Mater. Chem. 2012; 22: 25140–25147.
47. Webb MJ and Bampos N. Chem. Sci. 2012; 3:
2351–2366.
48. Gros CP, Michelin C, Bucher L, Desbois N, Dev-
illers CH, Coutsolelos AG, Biswas S and Sharma
GD. New J. Chem. 2018; 42: 8151–8159.
2
3. Shalini S, Balasundaraprabhu R, Kumar TS,
Prabavathy N, Senthilarasu S and Prasanna S. Int.
J. Energy Res. 2016; 40: 1303–1320.
4. Hagfeldt A, Boschloo G, Sun L, Kloo L and Petters-
son H. Chem. Rev. 2010; 110: 6595–6663.
5. Di Carlo G, Biroli AO, Tessore F, Caramori S and
Pizzotti M. Coord. Chem. Rev. 2018; 358: 153–177.
6. Sehgal P and Narula AK. J. Photochem. Photobiol.
A 2019; 375: 91–99.
2
2
2
2
2
7. Iqbal MZ, Ali SR and Khan S. Sol. Energy 2019;
49. Panagiotakis S, Giannoudis E, Charisiadis A,
Paravatou R, Lazaridi M-E, Kandyli M, Ladome-
nou K, Angaridis PA, Bertrand HC, Sharma GD and
Coutsolelos AG. Eur. J. Inorg. Chem. 2018; 2018:
2369–2379.
1
81: 490–509.
8. Stangel C, Charalambidis G, Varda V, Coutsolelos
AG and Kostas ID. Eur. J. Inorg. Chem. 2011; 2011:
4
709–4716.
2
3
9. Busch M, FabrizioA, Luber S, Hutter J and Cormin-
boeuf C. J. Phys. Chem. C 2018; 122: 12404–12412.
0. Carbone M, Micheli L, Limosani F, Possanza F,
Abdallah Y and Tagliatesta P. J. Porphyrins Phtha-
locyanines 2018; 22: 491–500.
50. Zervaki GE, Tsaka V, Vatikioti A, Georgakaki I,
Nikolaou V, Sharma GD and Coutsolelos AG. Dal-
ton Trans. 2015; 44: 13550–13564.
51. W. Kohn LS, J. Phys. Rev. 1965; 140: A1133.
52. Gaussian 03; M. J. Frisch GWT, H. B. Schlegel,
G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A.
Montgomery, T. Vreven, K. N. Kudin, J. C. Burant,
J. M. Millam, S. S. Lyengar, J. Tomasi, V. Barone,
B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara,
K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene,
X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V.
Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O.Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Moro-
kuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels,
M. C. Strain, O. Farkas, D. K. Malick,A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q.
Cui, A. G. Baboul, S. Clifford, J. Cioslowski,
B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A.
Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challa-
combe, P. M. W. Gill, B. Johnson, W. Chen, M. W.
Wong, C. Gonzalez, J. A. Pople, Gaussian 03, rev.
D.01, Gaussian, Inc., Wallingford CT, 2004.
53. Zhurko DA and Zhurko GA. Chemcraft 1.6, Plimus,
San Diego, CA, Available at http://www.chemcraft-
prog.com.
3
3
1. Wu K, Zhou C-Y and Che C-M. Org. Lett. 2019; 21:
8
5–89.
2. Stangel C, Daphnomili D, Lazarides T, Drev M,
Krasovec UO and Coutsolelos AG. Polyhedron
2
013; 52: 1016–1023.
3
3. Nakano A, Yasuda Y, Yamazaki T, Akimoto S,
Yamazaki I, Miyasaka H, Itaya A, Murakami M and
Osuka A. J. Phys. Chem. A 2001; 105: 4822–4833.
4. Tanasova M, Vasileiou C, Olumolade OO and
Borhan B. Chirality 2009; 21: 374–382.
5. Kim J-J, Choi H, Lee J-W, Kang M-S, Song K, Kang
SO and Ko J. J. Mater. Chem. 2008; 18: 5223–5229.
6. Daeneke T, Mozer AJ, UemuraY, Makuta S, Fekete
M, TachibanaY, Koumura N, Bach U and Spiccia L.
J. Am. Chem. Soc. 2012; 134: 16925–16928.
7. Zhang SF, Yang XD, Numata YH and Han LY.
Energ. Environ. Sci. 2013; 6: 1443–1464.
8. Nazeeruddin MK, KayA, Rodicio I, Humphrybaker
R, Muller E, Liska P, Vlachopoulos N and Gratzel
M. J. Am. Chem. Soc. 1993; 115: 6382–6390.
3
3
3
3
3
3
4
4
9. Kang SH, Choi IT, Kang MS, Eom YK, Ju MJ,
Hong JY, Kang HS and Kim HK. J. Mater. Chem. A
2
013; 1: 3977–3982.
0. Koumura N, Wang ZS, Mori S, Miyashita M,
Suzuki E and Hara K. J. Am. Chem. Soc. 2006; 128:
1
4256–14257.
54. Cossi M, Barone V, Cammi R and Tomasi J. Chem.
Phys. Lett. 1996; 255: 327–335.
55. Tomasi J, Mennucci B and Cammi R. Chem. Rev.
2005; 105: 2999–3093.
1. Wang ZS, Koumura N, Cui Y, Takahashi M, Seki-
guchi H, Mori A, Kubo T, Furube A and Hara K.
Chem. Mater. 2008; 20: 3993–4003.
Copyright © 2019 World Scientific Publishing Company
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