4-Carboxybiphenyl and thiophene substituted porphyrin derivatives for dye-sensitized solar cell

Article abstract of DOI:10.1080/15421406.2016.1184383

Porphyrin sensitizer (Por) containing 4-carboxybiphenyl and 2-thienyl and its metallated Zn(II) derivative (ZnPor) were synthesized and investigated. The porphyrin dyes were characterized by MALDI-MS, ¹H NMR, UV–vis, fluorescence and cyclic vol

Full text of DOI:10.1080/15421406.2016.1184383

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
4-Carboxybiphenyl and thiophene substituted
porphyrin derivatives for dye-sensitized solar cell
Sadik Cogal, Sule Erten-Ela, Kasim Ocakoglu & Aysegul Uygun Oksuz
To cite this article: Sadik Cogal, Sule Erten-Ela, Kasim Ocakoglu & Aysegul Uygun
Oksuz (2016) 4-Carboxybiphenyl and thiophene substituted porphyrin derivatives for
dye-sensitized solar cell, Molecular Crystals and Liquid Crystals, 637:1, 87-95, DOI:
10.1080/15421406.2016.1184383
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Date: 11 November 2016, At: 06:40

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
-Carboxybiphenyl and thiophene substituted porphyrin
derivatives for dye-sensitized solar cell
Sadik Cogala, Sule Erten-Elab, Kasim Ocakogluc,d, and Aysegul Uygun Oksuze
aDepartment of Polymer Engineering, Faculty of Engineering and Architecture, Mehmet Akif Ersoy University,
b
c
˙
Burdur, Turkey; Solar Energy Institute, Ege University, Bornova, Izmir, Turkey; Advanced Technology Research &
Application Center, Mersin University, Yenisehir, Mersin, Turkey; dDepartment of Energy Systems Engineering,
Tarsus Faculty of Technology, Mersin University, Mersin, Turkey; eDepartment of Chemistry, Faculty of Arts and
Science, Suleyman Demirel University, Isparta, Turkey
ABSTRACT
KEYWORDS
Porphyrin;
-carboxybiphenyl;
dye-sensitized solar cell;
thiophene
Porphyrin sensitizer (Por) containing 4-carboxybiphenyl and 2-thienyl
and its metallated Zn(II) derivative (ZnPor) were synthesized and inves-
tigated. The porphyrin dyes were characterized by MALDI-MS, 1H NMR,
UV–vis, fluorescence and cyclic voltammetry methods. It was found that
using of biphenyl bridge between anchoring group and porphyrin core
enhanced the conjugation length of the porphyrin macrocycle. The new
porphyrin dyes were used as sensitizers in dye-sensitized solar cells
(DSSCs). ZnPor-based dye sensitized solar cells yielded a short circuit
photocurrentdensityof3.98mAcm−2, anopencircuitvoltageof500mV,
and a fill factor of 0.67, corresponding to an overall conversion efficiency
(η) of 1.33%.
GRAPHICAL ABSTRACT
CONTACT Sadik Cogal
sadik_cogal@yahoo.com
Department of Polymer Engineering, Mehmet Akif Ersoy University,
Faculty of Engineering and Architecture,  Burdur, Turkey; Sule Erten-Ela
Solar Energy Institute, Ege University, Bornova,  Izmir, Turkey.
suleerten@yahoo.com, sule.erten@ege.edu.tr
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

88
S. COGAL ET AL.
Introduction
Dye-sensitized solar cells (DSSCs) are emerging as one of the most promising low-cost alter-
native technologies to conventional inorganic photovoltaic devices [1]. Significant research
efforts have been devoted to the development of new and efficient sensitizers suitable for prac-
tical use. In TiO2-based DSSCs, ruthenium–polypyridyl complexes have been known as the
most efficient sensitizers and solar cell efficiency of up to 11.4% has been achieved under
simulated sunlight (AM 1.5) [2, 3]. However, the main drawback of ruthenium complexes is
the lack of absorption in the red region of the visible light and the high cost. Among various
organic sensitizers have been used in DSSCs, porphyrins have been extensively studied and
used in DSSC due to their light-harvesting potential and suitable physicochemical properties
[4]. These dyes exhibited an intense absorption around 420 nm (called Soret band) and mod-
erate Q absorption bands between 550–600 nm, as well as their many reaction sites, i.e., four
meso positions and eight β positions which led to obtain variety of porphyrin derivatives
[5, 6]. Therefore, porphyrins are excellent alternative materials for DSSC applications and
considerable research efforts have been made on the synthesis of various porphyrins
and metalloporphyrins. Recently, the porphyrin dyes YD2-o-C8 and SM315 showed the
highest power conversion efficiencies of 12.3% and 13%, respectively, which broke the
ruthenium based DSSCs record [7, 8].
The electronic structure of porphyrin dyes, anchoring groups of dyes, the bridging dis-
tance between dye and TiO2 particles and type of central metal are the key factors which
affect the overall solar energy conversion [9–11]. To enhance the performance of porphyrin
in DSSCs, the properties of porphyrin macrocycle can be tuned by introducing various sub-
stituents on the periphery of the ring, and by changing the metal center [12]. The cova-
lent attachment of the thiophene moiety to the porphyrin macrocycle is an effective way
to enhance the photophysical and electrochemical properties [13]. Ambre et al. studied the
effects of number and position of p-carboxyphenyl and thienyl groups on zinc porphyrins
based DSSCs and it was found that these changes significantly affected the performance of the
device [14].
In order to move the Q-band absorption of porphyrin to longer wavelength, the π-electron
conjugation system can be extended by using various substituents. In this study, we have
designed new porphyrin dyes containing 4-carboxybiphenyl and 2-thienyl substituent and
investigated their performance in DSSC. 4-Carboxybiphenyl group was selected as an anchor-
ing moiety to the TiO2 surface. Porphyrins were characterized using 1H NMR, MALDI-MS,
UV–vis, PL and CV analysis. The synthesized porphyrin dyes were applied in DSSC. ZnPor
based dye sensitized solar cells gave a short circuit photocurrent density of 3.98 mA cm−2, an
open circuit voltage of 500 mV, and a fill factor of 0.67, corresponding to an overall conversion
efficiency (η) of 1.33%.
Experimental
Materials
All materials were obtained from commercial sources and used without further purifi-
cation. Pyrrole, 4-bromobenzaldehyde, 2-thienylcarboxyaldehyde, 4-methoxycarbonyl-
phenylboronic acid, trifluoroacetic acid (TFA), tetrakis(triphenylphosphine)palladium(0)
[Pd(PPh3)4], 1,2-dimethoxyethane (DME), zinc (II) acetate [Zn(OAc)2], sodium carbon-
ate (Na2CO3), dichloromethane, and hexane were purchased from Sigma–Aldrich. Silica

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
89
Gel 60 (0.04–0.063) was purchased from Merck. 5-(4-Bromophenyl)dipyrromethane was
synthesized using a procedure by Lindsey and coworkers [15].
Synthesis of 5,15-Bis(4ꢀ-bromophenyl)-10,20-bis(2-thienyl)porphyrin (1)
5-(4-Bromophenyl)dipyrromethane (723 mg, 2.4 mmol) and 2-thienylcarboxyaldehyde
(300 mg, 3.2 mmol) were dissolved in 80 mL of propionic acid. The obtained reaction mix-
ture was heated at 110 °C under argon atmosphere for 2 hr and then the reaction mixture was
allowed to cool to room temperature. The reaction mixture was neutralized with dilute NaOH
and filtered. The obtained solid was dissolved in DCM and washed twice with water. DCM
was evaporated under vacuum and the residue was purified by column chromatography on
silica gel by using hexane:DCM (3:1) as an eluent. Compound 1 was obtained as a purple solid
(200 mg, 21%). 1H NMR (CDCl3, 400 MHz): δ (ppm) = −2.76 (s, 2 H), 7.49–7.51 (dd, 4 H,
J = 5.2 Hz), 7.85–7.92 (m, 6 H), 8.05–8.07 (d, 4 H, J = 8.0 Hz), 8.80–8.83 (m, 6 H), 9.05–9.07
(dd, 4 H, J = 8.2 Hz). MALDI TOF MS: calcd. for C40H24Br2N4S2 ([M+H]+) 784.580; found
785.096.
Synthesis of 5,15-Bis(4ꢀ-methoxycarbonyl-1,1ꢀ-biphenyl)-10,20-bis(2-thienyl)
porphyrin (2)
Synthesis of 5,15-Bis(4ꢀ-bromophenyl)-10,20-bis(2-thienyl)porphyrin (1) (157 mg,
0.2 mmol), 4-methoxycarbonylphenylboronic acid (100 mg, 0.56 mmol) and Pd(PPh3)4
(23.12 mg, 0.02 mmol) were dissolved in 8 mL of DME, and stirred under argon atmosphere
for 10 min. Then, 2 mL of aqueous Na2CO3 (21.2 mg, 0.2 mmol) was added slowly in a
few portions, and the mixture was heated at 80°C for 24 hr. The reaction mixture was then
allowed to cool to room temperature and the solvent was removed under vacuum. The
residue was dissolved in DCM and washed twice with distilled water. DCM was evaporated
under vacuum and the residue was purified by column chromatography on silica gel by using
DCM:hexane (2:1) as an eluent. Compound 2 was obtained as a purple solid (35 mg, 19.5%).
1H NMR (CDCl3, 400 MHz): δ (ppm) = −2.66 (s, 2 H), 3.95–4.02 (m, 6 H), 7.50–7.52 (dd,
2 H, J = 5.4 Hz), 7.68–7.70 (d, 2 H, J = 8.4 Hz), 7.85–7.87 (dd, 4 H, J = 5.2 Hz), 7.93–7.94
(t, 4 H, J = 1.8 Hz), 7.99–8.05 (m, 4 H), 8.12–8.14 (d, 2 H, J = 8.4 Hz), 8.26–8.32 (m, 4 H),
8.67–8.70 (t, 4 H, J = 7.2 Hz), 8.91 (s, 4 H), 9.06–9.09 (t, 2 H, J = 4.8 Hz). MALDI TOF MS:
calcd. for C56H38N4O4S2 ([M+H]+) 895.06; found 895.597.
Synthesis of 5,15-Bis(4ꢀ-carboxy-1,1ꢀ-biphenyl)-10,20-bis(2-thienyl) porphyrin (Por)
5,15-Bis(4ꢀ-methoxycarbonyl-1,1ꢀ-biphenyl)-10,20-bis(2-thienyl)porphyrin (2) (12 mg,
0.013 mmol) was dissolved in 20 mL of THF:MeOH (3:1) mixture and was hydrolyzed by 1 M
NaOH at 60–70 C for 1 hr under argon atmosphere. The reaction mixture was then allowed
to cool to room temperature and the organic solvent was removed under vacuum. The pH
of the reaction mixture was set to pH 3–4 by adding 1 M HCl solution, and precipitate was
formed. The precipitate was collected and washed several times with dilute water and dried. If
necessary further purification can be carried out with column chromatography on silica gel by
using DCM:hexane (2:1) as an eluent. Compound Por was obtained as a purple solid (11 mg,
95%). 1 H NMR (DMSO, 400 MHz): δ (ppm) = −2.79 (s, 2 H), 6.58 (s, 2 H), 6.63–6.65 (d, 2 H,
J = 8.4 Hz), 6.87–6.90 (d, 4 H, J = 10.08 Hz), 7.02 (s, 4 H), 7.61–7.63 (dd, 2 H, J = 5.2 Hz),
7.86–7.88 (d, 4 H, J = 8.4 Hz), 8.04–8.07 (m, 4 H), 8.17–8.22 (m, 2 H), 8.34–8.36 (d, 2 H,

90
S. COGAL ET AL.
J = 8.0 Hz), 8.93 (s, 2 H), 9.08 (s, 2 H), 13.08 (s, 2 H). UV–vis (THF): λmax, nm (log ε, M−1
cm−1) = 423(5.62), 518(4.32), 554(4.08), 597(3.85), 656(3.74). MALDI TOF MS: calcd. for
C54H34N4O4S2 ([M+H]+) 867.0; found 868.618.
Synthesis of 5,15-Bis(4ꢀ-carboxy-1,1ꢀ-bipheny)-10,20-bis(2-thienyl)porphyrin zinc
(ZnPor)
5,15-Bis(4ꢀ-methoxycarbonyl-1,1ꢀ-biphenyl)-10,20-bis(2-thienyl)porphyrin (2) (22 mg,
0.025 mmol) was dissolved in 10 mL of CHCl3. To this solution 1.5 equivalent of
Zn(OAc)2ꢀ2 H2O prepared in 5 mL of MeOH was added. The reaction mixture was
refluxed at 60–70 °C for 1 hr under Ar atmosphere. Then, the reaction mixture was allowed
to cool to room temperature and the solvent was removed under vacuum. The obtained
solid was dissolved in CHCl3, washed twice with dilute water and dried over MgSO4. CHCl3
was evaporated under vacuum and the residue was purified by column chromatography on
silica gel by using DCM as an eluent. The obtained porphyrin was hydrolyzed as described
in Section 2.4 to give compound ZnPor as a purple solid (20 mg, 86%). 1H NMR (DMSO,
400 MHz): δ (ppm) = 6.58 (s, 2 H), 6.63–6.65 (d, 2 H, J = 8 Hz), 6.87 (s, 4 H), 7.57–7.59
(m, 4 H), 7.96 (d, 2 H, J = 2.8 Hz), 8.11–8.19 (m, 4 H), 8.28–8.30 (d, 6 H, J = 7.2 Hz),
8.86–8.88 (m, 2 H), 9.01–9.03 (t, 4 H, J = 4.6 Hz), 13.07 (s, 2 H). UV–vis (THF): λmax, nm
(log ε, M−1 cm−1) = 429(5.58), 524(3.31), 559(4.17), 601(3.73). MALDI TOF MS: calcd. for
C54H32N4O4S2Zn ([M+H]+) 930.37; found 931.789.
DSSC preparation
DSSC preparation started with cleaning FTO counter electrodes (TEC 8 & Hartford Glass)
using Helmanex solution for 20 minutes, then rinsed and cleaned with distilled water. And
then, glasses were cleaned by acetone and ethanol solvents using ultrasonic bath for 10 min-
utes. The TiO2 coated FTO substrates were immersed in a 0.5 mM solution of porphyrin
complexes in acetonitrile:methanol:chlorobenzene (2:2:1) at room temperature overnight and
dried over MgSO4 in a desicator. The active area was arranged to 0.16 cm2 using a spe-
cial mask. The cells were prepared in a sandwich geometry. The 0.6 M N-methyl-N-butyl-
imidazolium iodide (BMII) + 0.1 M LiI + 0.05 M I2 + 0.5 M 4-tert-butylpyridine (TBP)
in acetonitrile as redox electrolyte solution was introduced through pre-drilled holes in the
counter electrode. The filling holes were sealed using Surlyn and a microscope cover glass.
Characterization
The NMR (400 MHz) spectra of the obtained compounds were recorded using a Bruker Ultra-
shield Plus Biospin GmbH spectrometer. The absorption spectra of dyes were detected on
PG UV–vis spectrometer. The fluorescence spectra were measured on Varian Cary Eclipse
fluorescence spectrophotometer. Mass spectra were measured using Bruker Microflex LT
MALDI-TOF mass spectrometer. The electrochemical measurements were performed at
room temperature on IVIUMSTAT potentiostat.
Dye-sensitized solar cells were characterized by current–voltage measurement. All
current–voltage characteristics were done under 100 mW/cm2 light intensity and AM 1.5

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
91
conditions. 450 W Xenon light source (Oriel) was used to give an irradiance of various inten-
sities. J−V data collection was made by using Keithley 2400 Source-Meter and LabView data
acquisition software.
Results and discussion
The synthetic procedures of Por and ZnPor are represented in Scheme 1. First, 5-(4-
bromophenyl)dipyrromethane was synthesized via the Lindsey method. Porphyrin 1 com-
pound was synthesized by using Adler–Longo method which includes the condensation of
dipyrromethane and pyrrole in propionic acid [16]. The key step is palladium-catalyzed
Suzuki-Miyaura cross-coupling reaction for introducing the 4-methoxycarbonylphenyl group
at the porphyrin meso position (Compound 2). The subsequent metalation by using
Zn(OAc)2.2 H2O and base hydrolysis (NaOH) gave the zinc porphyrin (ZnPc) dye. In addi-
tion, the base hydrolysis of Compound 2 afforded the free base porphyrin (Por).
The absorption spectra of porphyrin dyes considered in this study are recorded in THF
solution and shown in Fig. 1. The UV-Vis spectra of Por show characteristic Soret band at
423 nm and four Q-bands at 518, 554, 597, and 656 nm. On the other hand, the UV–vis spectra
of ZnPor show Soret band at 429 nm and two Q-bands at 559 and 601 nm which are the
characteristic absorption bands of metalloporphyrins. Metal center zinc (II) shifts the Soret
absorption band to red region. The fluorescence spectra of porphyrins are presented in Fig. 2.
The fluorescence maxima of Por and ZnPor were observed at 625 and 659 nm, respectively,
Scheme . Synthesis of Por and ZnPor.

92
S. COGAL ET AL.
Figure . UV–vis absorption spectra of Por and ZnPor in THF solution.
which measured in THF solution by excitation at the Soret band. Zero–zero excitation energy
(E0–0) was obtained from the intersection of absorption and emission spectra (λint).
1240 eV
λint
E0−0
=
The calculated E0–0 energies of porphyrins were given in Table 1.
Cyclic voltammetry (CV) was employed to determine the electrochemical behaviors of the
porphyrins. The electrochemical experiments were carried out in a typical three-electrode
cell in which a glass sheet with deposited indium–tin-oxide (ITO) was used as the working
electrode, a platinum wire was used as the counter electrode, an Ag/AgCl electrode was used
Figure . Fluorescence spectra of Por and ZnPor in THF solution.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
93
Table . Optical and electrochemical data of Por and ZnPor in THF.
Dye
B-band (nm)
Q-band (nm)
λem (nm)
Eox (V)a
E– (eV)b
HOMO (eV)c
LUMO (eV)d
Por
ZnPor


, , , 
, 
, 
, 
.
.
.
.
−.
−.
−.
−.
aFirst oxidation onset potential vs. Ag/AgCl reference electrode.
bMeasured from the intersection of the normalized absorption and emission spectra.
on
cHOMO = −[(E −Eferrocene)+.]eV, where . V is the value for ferrocene vs. Ag/AgCl and . eV is the energy level of
ox
ferrocene below the vacuum.
dLUMO = HOMO+E–
.
as the reference electrode and THF containing 0.1 M tetrabuthylammonium hexaflourophos-
phate (TBAPF6) was used as electrolyte. Figure 3 shows the cyclic voltammograms of Por
and ZnPor. The first oxidation onset potentials of Por and ZnPor were given in Table 1. The
first oxidation of ZnPor shows the reversible redox process under a scan rate of 25 mV s−1.
However, Por shows irreversible first oxidation wave under the same condition. The high-
est occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
of the dyes were also determined due to their importance in DSSC application. The HOMO
energy level can be calculated from the onset oxidation potential, and values were calculated
to be −5.25 and −5.11 eV for Por and ZnPor, respectively. The HOMO levels of the dyes are
lower than that I−/I3− couple (−4.8 eV) [17]. This result indicated that both dyes can be effi-
ciently regenerated by electron transfer from the electrolyte after photo oxidation. The LUMO
levels were calculated from the differences between the HOMO energy levels and E0–0 [18].
The LUMO levels of the Por and ZnPor were estimated to be −3.23 and −3.04 eV, respec-
tively. The LUMO levels of the dyes were higher than that of the conducting band energy of
the TiO2, suggesting that there should be enough thermodynamic driving force for electron
injection from the excited state of the dyes into the conduction band of TiO2.
In comparison with trans-2S2A [(5,15-bis(4-carboxyphenyl)-10,20-bis(2-thienyl)
porphyrinato zinc(II)] reported by Ambre et al. [14], ZnPor exhibited a redshift absorp-
tion by 3 nm and lower oxidation potential. This result indicated that the use of biphenyl
bridge between anchoring group and porphyrin core enhanced the conjugation length of the
porphyrin macrocycle.
Figure . Cyclic voltammograms of Por and ZnPor recorded in . M TBAPF/THF at a scan rate of  mV/s.

94
S. COGAL ET AL.
Figure . J-V curves of dye-sensitized solar cells.
The photocurrent–photovoltage (J–V) curves of porphyrin based dye sensitized solar cells
were shown in Fig. 4. The detailed performance parameters including the short circuit cur-
rent (Jsc), open-circuit photovoltage (Voc), fill factor (FF), and photo to electricity conversion
efficiency (η) were summarized in Table 2. According to DSSC performances, the ZnPor sen-
sitized solar cell gave a short circuit photocurrent density, Jsc of 3.98 mA cm−2, the open
circuit voltage, Voc of 500 mV, and filling factor, FF of 0.67, corresponding to an overall con-
version efficiency of 1.33% under the standard global AM 1.5 solar conditions. Zinc metal free
complex, Por-based dye sensitized solar cell gave an overall conversion efficiency (η) of 0.58,
and a short circuit photocurrent density, Jsc of 1.98 mA cm−2, the open circuit voltage, Voc of
Table . Photovoltaic performance of dye sensitized solar cells.
Jsc (mA cm−
)
Voc (mV)
FF
η (%)
Por
ZnPor
.
.


.
.
.
.
Figure . Incident photon to charge carrier efficiency of dye-sensitized solar cells.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
95
500 mV and filling factor, FF of 0.60. In this study, we investigated the central metal effect in
dye sensitized solar cells. Our results show that central zinc metal increased the efficiency in
dye sensitized solar cells.
Overall conversion efficiencies were obtained as 1.33% and 0.58%, respectively. These per-
formances are actually in accord with those obtained in IPCE spectrum. IPCE spectrum of
porphyrin based dye sensitized solar cells was presented in Fig. 5. ZnPor and Por showed
IPCE efficiencies of 8.79% and 2.39% at 429 nm, respectively.
Conclusion
In summary, new porphyrin dyes Por and ZnPor were synthesized and applied in DSSC. 0.58%
and 1.33% of light to electricity conversion efficiencies were achieved for Por and ZnPor,
respectively. It was found that Zn(II) shifted the Soret absorption band to the red region and
decreased the band gap energy of porphyrin. Based on these results, ZnPor exhibited better
conversion efficiency in DSSC. In addition to thienyl group, biphenyl group also enhanced
the conjugation length. Our studies will go on through a rational molecular design of new
porphyrin dyes for higher efficiencies in dye sensitized solar cells.
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