Pyrene-based organic dyes with thiophene containing π-linkers for dye-sensitized solar cells: Optical, electrochemical and theoretical investigations

Article abstract of DOI:10.1039/c1cp21714c

A new series of metal-free organic dyes containing pyrene and α-cyanoacrylic acid end groups and thiophene, bithiophene, thienylbenzene or thienylfluorene π-linkers were synthesized and characterized by absorption, emission and electrochemical measurement

Full text of DOI:10.1039/c1cp21714c

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PAPER
Pyrene-based organic dyes with thiophene containing p-linkers for
dye-sensitized solar cells: optical, electrochemical and theoretical
investigationsw
Abhishek Baheti,a Chuan-Pei Lee,b K. R. Justin Thomas*a and Kuo-Chuan Hob
Received 26th May 2011, Accepted 4th August 2011
DOI: 10.1039/c1cp21714c
A new series of metal-free organic dyes containing pyrene and a-cyanoacrylic acid end groups
and thiophene, bithiophene, thienylbenzene or thienylfluorene p-linkers were synthesized and
characterized by absorption, emission and electrochemical measurements. Time-dependent density
functional theoretical calculations were also performed to unravel the nature of the absorption
induced electronic excitations. Extension of conjugation in the p-linker by the incorporation of
phenyl or fluorene was found to enhance the molar extinction coefficient while the use of
thiophene red-shifted the absorption. The longer wavelength absorption peaks found for the dyes
were attributed to p–p* transition with a contribution from the charge transfer transition which
becomes prominent for the bithiophene bridged derivative. The bithiophene containing dye
showed moderate overall light-to-electron conversion efficiency attributable to the favorable
absorption and redox properties originating due to the presence of a bithiophene segment.
The trends observed for the various dyes in the device performance were rationalized by
electron-impedance spectroscopy measurements.
structure–property relationships in donor–acceptor molecules
I
Introduction
gathered over the decades.4 Most of these organic dyes feature
common structural elements such as the terminal amine donor
group, p-conjugation linker and a-cyanoacrylic acid acceptor.5–10
The latter also serves as an anchoring unit to the TiO2
nanoparticles. Alternative acceptor units such as rhodanine-
3-acetic acid have also been explored with limited success.12
The domination of the a-cyanoacrylic acid segment in the
organic dyes suitable for dye-sensitized solar cells is attributed
to its coplanarity with respect to the spacer unit and good
electron coupling with TiO2.13
Polyaryl/heteroaryl p-conjugated segments such as biphenyl,5
terphenyl,6 oligothiophene,7 fluorene,8 anthracene,9 carbazole,10
fluoranthene11 or their combination have been used as a spacer
in organic dyes and found to be beneficial for the light
harvesting capability and charge transfer interaction between
the donor and acceptor units. Similarly, triarylamines have
been used as terminal donors in the organic dyes owing to
many advantages, namely (a) they generally inherit lower
redox potential which is beneficial for the regeneration of
the dyes by the electrolyte system, (b) they may form a denser
hydrophobic film on the surface of the TiO2 and thereby
prevent the interaction of triiodide ions in the electrolyte with
TiO2, and inhibit the aggregation of the dyes at the surface of
the TiO2 due to their nonplanar structure.
There is a growing interest for the renewable energy sources
due to the depletion of fossil fuels and contribution to the
global warming from the conventional energy generation
techniques. Among the alternative energy sources light energy
appears to be more promising as it is available aplenty from
the sun. Many protocols have been developed in the last few
decades to convert the light energy to electrical energy based
on the photovoltaic principle.1 The photoelectrochemical cells
constructed utilizing the organic or organometallic dyes are
attractive due their potential to offer higher conversion efficiency
at a lower cost.2 Though the Ru-based organometallic dyes have
shown better power conversion efficiencies and device stabilities
when compared to the metal-free organic dyes, they are more
expensive and involve cumbersome purification procedures
which inhibit the proliferation for structural diversity.3
Recently, improvements in photovoltaic performance (up to
10%) have been achieved for organic dyes due to extensive
a Organic Materials Lab, Department of Chemistry, Indian Institute of
Technology Roorkee, Roorkee – 247 667, India.
E-mail: krjt8fcy@iitr.ernet.in; Fax: +91-1332-286202;
Tel: +91-1332-285376
b Department of Chemical Engineering, National Taiwan University,
Taipei 10617, Taiwan. E-mail: kcho@ntu.edu.tw
w Electronic supplementary information (ESI) available: Copies of 1H
and 13C NMR spectra, absorption and emission spectra recorded in
different solvents and cartesian coordinates of the optimized
structures. See DOI: 10.1039/c1cp21714c
In this paper, we report the synthesis of a new class of organic
dyes having linkers featuring different aromatic units (phenyl/
thiophene/fluorene) between pyrenyl and a-cyanoacrylic acid
c
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properties. Also, the hydrophobic pyrene units are expected to
retard the recombination of the injected electron with the
aqueous electrolyte and enhance the photocurrent. Thus, we
have designed and synthesized five new organic dyes (Fig. 1)
capable of absorbing light in the visible region and present the
effect of the linkers on the absorption properties such as lmax and
molar absorptivity (emax), and the photovoltaic characteristics.
II Results and discussion
Synthesis and characterization
The pyrene-based organic dyes containing varying lengths of
p-conjugation pathway composed of benzene, thiophene,
and/or fluorene units were synthesized as illustrated in Scheme 1.
The precursor 2-(pyren-1-yl)thiophene was synthesized by
Kumada coupling15 between 1-bromopyrene and thiophen-
2-yl-magnesium bromide in reasonable yield. It was then
converted to the stannyl derivative by treatment with
n-butyllithium followed by quenching with tri-n-butyl-tin
chloride. The Stille coupling16 reactions of this stannane with
appropriate bromo-substituted arylcarboxaldehyde in the
Fig. 1 Chemical structures of the pyrene-based organic dyes.
terminal groups. Even though substituted pyrenes are used as a
sensitizer14 in photochemical studies their use in dyes suitable
for dye-sensitized solar cells remains unexplored. We thought
that the extended p-system present in the pyrene though
considered as a weak acceptor could be exploited as a sensitizing
unit by coupling with electron-rich moieties such as thiophene
and/or fluorene. It would be interesting to evaluate the role of
the p-bridge in enhancing the donor strength of the aromatic
segments and thus modulating the electrochemical and optical
presence of
a palladium catalyst Pd(PPh3)2Cl2 led to
the desired pyrene containing aryl aldehydes (3a–3e). The
aldehydes were subsequently converted in moderate yields to
the target dyes (4a–4e) by a condensation reaction with
Scheme 1 Synthesis of the dyes.
c
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cyanoacetic acid involving ammonium acetate as the catalyst
and glacial acetic acid as the solvent. The dyes are soluble in
tetrahydrofuran (THF) and dimethylformamide (DMF) and
insoluble in hexanes, toluene, and ethyl acetate. The dyes were
characterized by IR, NMR (1H and 13C) and mass spectral
methods and the spectroscopic data are consistent with the
proposed structures.
In general, the peak position of a p–p* transition band depends
on the effective conjugation length while the charge-transfer
band will be affected by the nature of electron donor, acceptor,
and the bridge connecting them. A closer inspection of the
absorption parameters reveals the following important features.
Compound 4b inherits the most blue-shifted absorption profile
and exhibits a significant hypsochromism when compared to
the analogous derivative 4c. It appears that the disposition of
the cyanoacrylic acid unit in the meta position not only
restricts the conjugation but also disfavors the electronic
interaction between the donor and acceptor units.18 Among
the dyes, 4d possesses the elongated conjugation; however the
absorption peak of 4d is positioned close to that of 4c. From
this it may be deduced that the longer wavelength absorption
in these dyes is not purely p–p* in nature. The bithiophene
bridged compound 4e shows the most bathochromically
shifted absorption profile while the thiophene linked derivative
4a displays the least intense peak in the series suggestive of a
role attributable to the electron donation ability of the
p-linker. It is reasonable to assume that the increment in
the electron richness (4e 4 4c 4 4a) of the p-bridge facilitates
the charge transfer transition. The larger extinction coefficient
observed for the dye 4d when compared to 4c is assigned to the
extension of conjugation in 4d and the electron richness of
the thienylfluorene segment. Extension of conjugation favors the
transition probability of the p–p* transition while the increment
in the donor strength of the p-linker facilitates the charge transfer
transition. In view of the above facts, the longer wavelength broad
absorption profile is assigned as a charge-transfer transition with
contribution from the p–p* transition.
Optical properties
The light absorbing properties of the new dyes were examined
by measuring the absorption spectra in THF solutions. The
absorption spectra of the dyes measured for the THF solutions
are displayed in Fig. 2 and the relevant parameters are listed in
Table 1. All the dyes display a broad absorption envelope with
a peak in the range 363–433 nm and a higher energy shoulder.
In addition all the dyes show a weak band below 300 nm. The
peak at the longer wavelength is attributed to a transition
originating from the entire molecule with a prominent charge
transfer character, while the shorter wavelength peak stems from
the pyrene localized p–p* excitation. The latter assignment was
based on the resemblance of this peak to that observed for
parent pyrene.17
The deprotonation of the carboxylic acid will decrease the
electron-accepting strength of the cyanoacrylic acid moiety; on
the other hand protonation will restore the acceptor strength.5
Thus, a blue shift for the lower energy absorption band in the
presence of triethylamine and a slight red shift on addition of
trifluoroacetic acid (Fig. 3) observed for the dyes 4c–4e suggest the
occurrence of acid–base equilibria for the dyes in solutions.5,19
However, addition of TEA/TFA did not affect the absorption
profile of the dyes 4a and 4b (Fig. S1 and S2, ESIw). Since the
pyrene moiety is not an effective donor, it appears that the donor
strength needs to be augmented by the p-bridge to ensure an
Fig. 2 Absorption spectra of the dyes recorded in tetrahydrofuran.
Table 1 Electro-optical data of the dyes 4a–4e
l
max/nm (emax/Mꢀ1 cmꢀ1 ꢁ 103)
lema/nm Stokes shift/cmꢀ1 Eoxb/mV HOMOc/eV LUMOd/eV E0–0e/eV Eox*f/eV
THF
THF + TEA THF + TFA
Dyes
4a
388 (10.5), 386 (15.6),
329 (7.9),
277 (10.2)
412 (24.9),
337 (22.9)
509
6127
906
5.71
3.01
2.69
ꢀ1.02
329 (10.9)
4b
363 (33.6), 364 (32.9),
302 (34.8), 280 (37.6)
280 (34.2)
399 (39.1), 385 (42.5),
278 (24.3) 278 (24.2)
407 (63.8), 394 (69.4),
278 (28.1) 278 (27.9)
433 (37.2), 412 (40.0),
278 (24.9) 278 (23.2)
363 (33.4),
310 (34.5),
280 (32.6)
401 (40.2),
278 (25.4)
410 (53.1),
278 (23.2)
439 (37.6),
278 (24.7)
459
5762
785, 1071 5.59
2.74
2.85
ꢀ1.30
4c
4d
4e
520
504
541
5832
4729
4610
800, 1072 5.50
747, 1064 5.55
785, 1107 5.58
2.97
3.01
3.23
2.53
2.54
2.36
ꢀ0.96
ꢀ1.02
ꢀ0.80
a
b
c
Recorded for tetrahydrofuran solutions. Potentials quoted with reference to internal ferrocene standard. HOMO = 4.8 + Eox
d
.
LUMO =
e
. Obtained from optical edge. Excited state oxidation potential.
f
HOMO ꢀ E0–0
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Fig. 4 Computed interplanar angles between the different aryl
groups in the dyes.
Fig. 3 Absorption spectra of the dye 4e in tetrahydrofuran recorded
in the presence of TFA or TEA.
pyrenylthiophene segment while the lowest unoccupied mole-
cular orbital (LUMO) is delocalized over the aryl acrylic acid
unit. The presence of such well separated frontier molecular
orbitals at the two different ends featuring donor and acceptor
characters in the molecular materials is expected to produce a
charge migration on electronic excitation from the HOMO to
the LUMO.22 According to the B3LYP23 computations, the
longer wavelength vertical transitions in the dyes originate from
the HOMO to LUMO electronic excitations with reasonable
oscillator strengths. However, the CAM-B3LYP24 model
suggested this prominent absorption to result from the electronic
promotion from the HOMO to LUMO/LUMO + 1 and
HOMO ꢀ 1 to LUMO. Since the trend observed in the
absorption peak positions of the dyes and the values are within
5% deviation limit of the CAM-B3LYP results (Fig. 6), we
presume that the composition of the transitions predicted by the
CAM-B3LYP is more reliable. From the electronic distributions
observed for the HOMO, HOMO ꢀ 1, LUMO and LUMO + 1
orbitals (Fig. 5), the longer wavelength electronic excitation for
the dyes is predicted to contain both charge transfer and p–p*
characters.
effective charge transfer. This further confirms the charge
transfer nature of this prominent peak particularly for the dyes
with electron rich p-linkers (4c, 4d and 4e).
As the experimental evidences indicated the presence of
both p–p* and charge transfer characteristics for the electronic
transitions in these dyes, we have performed density functional
theoretical (DFT) calculations20 to understand the electronic
structure of the new dyes and the nature of the electronic
excitations. Firstly, the geometries of the dyes were optimized
using tight SCF convergence conditions implemented in the
Gaussian 09 program21 suite without imposing any symmetry
constraints. The optimized geometries were confirmed to be free
from imaginary IR frequencies and correspond to the global
minimum in the potential energy surface (Fig. S9–S11, ESIw).
Then the optimized structures were used in the TDDFT
computations for the identification of probable vertical transitions
in these molecules.
The computed CRN stretching frequencies of the dyes
4a–4e are 2336, 2343, 2339, 2336 and 2333 cmꢀ1, respectively,
and agree well with the observed values (2213, 2227, 2220,
2229 and 2217 cmꢀ1). The co-planarity of the aryl groups
involved in the p-bridge is essential for the efficient electronic
migration through the linker from the donor to the acceptor.
The optimized geometries exhibited wider inter-planar angles
between the aryl groups for the phenylene or fluorene containing
derivatives (4b–4d) and the deviation from planarity increased in
the order 4e o 4c o 4d o 4b (Fig. 4). This suggests that the
usage of an oligothiophene segment as a p-linker is highly
beneficial for charge transfer as it tends to assume planar
arrangement besides being electron rich. For all the dyes, the
cyanoacrylic acid group is found to be coplanar with the
adjacent spacer unit (phenyl/fluorene/thiophene), indicating
a facile extension of conjugation between the aryl spacer and
cyanoacrylic acid groups.
All the compounds display a weak emission with characteristic
colors ranging from blue to orange in THF solutions (Fig. 7 and
Table 1). The emission wavelengths of the dyes assume the
following increasing order: 4b o 4d o 4a o 4c o 4e, which is
slightly different from that observed for the absorption peaks
(cf. 4b o 4a o 4c o 4d o 4e). Though the fluorene containing
dye 4d contains elongated conjugation, due to the enhanced
rigidity and reduced charge transfer interaction it displays a
blue shifted emission profile when compared to the dyes 4a, 4c
and 4e. It is probable that due to the less polar hydrophobic
excited state structure, the dye 4d is not effectively solvated by
THF while the other dyes (4a, 4c and 4e) are stabilized in the
excited state by the solvent–solute interactions.25
In order to probe the effect of solvent environments on
the ground and excited state structures of the dyes, the solvato-
chromism was investigated by measuring the absorption and
emission spectra in selected solvents with different dielectric
constants. In general, the dyes exhibited red-shifted absorption
and emission profiles in toluene and dichloromethane and
shorter wavelength absorptions in dimethylformamide,
The electronic distributions observed for the frontier molecular
orbitals of the dyes are shown in Fig. 5 and the computed
excitation energies for the vertical transitions and their oscillator
strengths and compositions in terms of molecular orbitals are
compiled in Table 2. The highest occupied molecular orbital
(HOMO) of the dyes is mainly contributed by the
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Fig. 5 Electronic distributions in the frontier molecular orbitals of the dyes 4c–4e.
Table 2 Computed vertical transition energies, oscillator strength and their configurations for the dyes 4a–4eusing B3LYP and CAM-B3LYP.
B3LYP (air)
max/nm f
CAM-B3LYP (air)
CAM-B3LYP (THF)
l
Configuration
lmax/nm f
Configuration
lmax/nm f
Configuration
Dyes
4a
483.4
394.6
516.2
0.51 HOMO - LUMO (88%) 371.9
0.84 HOMO - LUMO (99%) 336.4
0.64 HOMO - LUMO (100%) 366.8
1.00 HOMO - LUMO (84%)
HOMO ꢀ 1 - LUMO (6%)
389.1
1.17 HOMO - LUMO (81%)
HOMO ꢀ 1 - LUMO (7%)
1.12 HOMO - LUMO + 1 (67%)
HOMO - LUMO (19%)
1.84 HOMO - LUMO (57%)
HOMO ꢀ 1 - LUMO (22%)
HOMO - LUMO + 1 (12%)
2.23 HOMO - LUMO (40%)
HOMO ꢀ 1 - LUMO (32%)
HOMO - LUMO + 1 (14%)
1.57 HOMO - LUMO (60%)
HOMO ꢀ 1 - LUMO (30%)
4b
4c
0.93 HOMO - LUMO + 1 (65%) 344.4
HOMO - LUMO (21%)
1.61 HOMO - LUMO (59%)
HOMO ꢀ 1 - LUMO (19%)
HOMO - LUMO + 1 (13%)
2.01 HOMO - LUMO (40%)
HOMO ꢀ 1 - LUMO (31%)
HOMO - LUMO + 1 (17%)
1.43 HOMO - LUMO (63%)
HOMO ꢀ 1 - LUMO (27%)
380.5
378.5
415.6
4d
4e
508.1
524.0
0.67 HOMO - LUMO (99%) 367.1
0.73 HOMO - LUMO (99%) 395.7
acetonitrile and tetrahydrofuran (for instance, see Fig. 8).
Unusual red-shift in the absorption and emission spectra of
the dyes may originate from the stabilization due to the fast
rearrangement of polarizable electrons during excitation which is
induced by the relatively large polarizability of dichloromethane.26
Lippert–Mataga analysis of the solvatochromic data showed
non-linear relationships for the dyes with the exception of 4b
(Fig. 9). This indicates that in the dyes which possess strong
push–pull nature, besides the general effects due to the
interaction of the dye dipole with its environment, the specific
interaction between the dye and solvent such as hydrogen
bonding or the formation and stabilization of intramolecular
charge transfer states in particular solvent surroundings are
also playing a role. Hydrogen bonding of the solvent with the
carboxylic acid group of the dye will reduce the acceptor
strength of the cyanoacrylic acid unit and decrement the
absorption maximum. Reasonable linearity observed for 4b
in the Lippert–Mataga analysis is attributed to the absence of
donor–acceptor interaction in this dye due to the meta
disposition of the acceptor unit.
Electrochemical properties
In order to ascertain the feasibility of electron transfer from
the excited dye molecule to the conduction band of TiO2 and
the dye regeneration the electrochemical characteristics of the
dyes were investigated by using cyclic voltammetry (CV) and
differential pulse voltammetry (DPV). All the dyes with the
exception of 4a exhibited two irreversible oxidation peaks
(Table 1). The first irreversible oxidation wave detected for the
dyes 4a–4e in the range 0.75–0.91 V versus ferrocene was attributed
to the removal of an electron from the pyrenylthiophene segment
while the second irreversible oxidation occurring at the higher
potential values belongs to the oxidation of the pyrene moiety.
c
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Fig. 6 Comparison of observed (THF + TFA) and calculated electronic
absorption maxima for the dyes using different theoretical models.
Fig. 8 Absorption (a) and emission (b) spectra of 4c recorded in
different solvents.
Fig. 7 Emission spectra of the dyes (4a–4e) recorded in tetrahydrofuran.
The magnitude of the first oxidation potential observed for the
dyes was highly dependent on the nature of the p-bridge which
either modulates the donor–acceptor interaction or increments
the oxidation propensity of the pyrenylthiophene unit.
Stronger communication between the donor and acceptor
moieties will retard the electron removal from the donor
fragment while the electron-rich p-bridge will facilitate it.
Consequently the first oxidation potential for the dyes assumes
the trend: 4d o 4e E 4b o 4c o 4a. Facile oxidation observed
for the dye 4d points to a weaker donor–acceptor interaction
between the pyrenylthiophene and cyanoacrylic acid segments
owing to the longer p-spacer.27 Similarly, the meta disposition
of the cyanoacrylic acid unit in the dye 4b is responsible for the
comparatively smaller oxidation potential. More positive
oxidation potential observed for 4a is due to the close
association of the cyanoacrylic acid unit with the pyrenylthiophene
unit which depletes the electron density successfully from the
donor part and the relatively shorter p-segment could not facilitate
the removal of electrons.
Fig. 9 Lippert–Mataga plots of the dyes 4a–4e.
from the first oxidation potential (Eox), whereas E0–0 is derived
from the absorption edge of the dyes since no reduction peak
was observed for the dyes. The trends observed in the ground
The excited state oxidation potential (Eox*) of the sensitizer
is estimated by subtracting the 0–0 transition energy (E0–0
)
c
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Fig. 10 The comparison of the ground and excited state oxidation
potentials for the dyes.
and excited state oxidation potentials for the dyes are depicted
in Fig. 10. The ground state oxidation potentials of the dyes
occur in the range of 1.51 to 1.67 V versus NHE. These values
are more positive than that of the Iꢀ/I3ꢀ redox couple (B0.4 V
vs. NHE)28 and suggest that the dye regeneration by the
electrolyte is thermodynamically allowed. The excited state
oxidation potentials of the dyes are in the range of ꢀ0.80 to
ꢀ1.30 V versus NHE, and more negative than the conduction
band of TiO2 (ꢀ0.5 V vs. NHE).29 This ensures that the
electron injection from the excited dye to TiO2 should be
energetically favorable.
Fig. 11 I–V characteristics of the DSSCs fabricated using the dyes
4a–4e.
DSSC characteristics
From the above discussions it is evident that the dyes
developed in this work possess promising light absorption
properties and suitable ground/excited state oxidation
potentials to use them as potential sensitizers in the conventional
dye-sensitized solar cells. We have fabricated nanocrystalline
TiO2-based dye-sensitized solar cells with a liquid electrolyte
system and tested the performance of the dyes as sensitizers. The
pertinent parameters observed under AM 1.5 global one sun
light intensity of 100 mW cmꢀ2 are listed in Table 3. The
current–voltage characteristics and the IPCE spectra of the
devices are shown in Fig. 11 and 12 respectively.
Fig. 12 IPCE plots observed for the DSSCs fabricated using the dyes
4a–4e.
conditions, Voc of the photoelectrochemical is the potential
difference between the quasi-Fermi level of the electrons in the
nanocrystalline TiO2 film and redox potential of the
electrolyte. The pronounced rate of electron recombination
between the injected electron from the conduction band of
TiO2 and the triiodide or oxidized dye could reduce the Voc.
Additionally the band-edge shift occurring due to the
interaction of the dye with TiO2 will also alter the Voc.
However, a strong dipole present at the TiO2/dye interface
will contribute to a more negative shift in the Fermi level of the
TiO2 conduction band. The highest Voc observed for the
devices fabricated using the dyes 4d and 4e may be attributed
to the stronger interaction between the TiO2 and the dye while
the lowest Voc exhibited by 4a may be probably due to poor
interaction between the dye and TiO2 and aggregation of the
dye.31 Aggregation of the dyes is reported to facilitate the
recombination of the injected electrons and reduce the Voc.32
Since the ground state oxidation potentials of the dyes are
falling in a narrow range (1.51–1.67 eV) we presume that the
dye regeneration may remain as a common factor. However,
the excited state oxidation potentials of the dyes range
The light to electrical energy conversion efficiencies of the
DSSCs are largely in agreement with the optical properties of
the dyes. The higher overall efficiency observed for the dyes
4c–4e is attributable to the red-shifted high molar extinction
coefficient absorption realized for these dyes on comparison to
the dyes 4a and 4b, which led to a significant hike in the
short-circuit photocurrent density.30 Under illumination
Table 3 Performance parameters of the DSSCs fabricated using the
dyes 4a–4ea
Dyes Jsc/mA cmꢀ2 Voc/mV
ff
Z (%) Rct2b/ohm teb/ms
4a
4b
4c
4d
4e
2.19
3.37
7.63
7.01
9.51
438
510
500
526
520
0.63 0.60
0.70 1.21
0.68 2.09
0.70 2.58
0.67 3.31
49.69
35.57
19.93
18.72
15.58
0.64
2.09
1.16
2.09
0.95
a
Measured under an irradiation of 100 mW cmꢀ2 (AM 1.5 G
b
simulated sunlight) at room temperature. Estimated with the aid of
electrochemical impedance spectroscopy (EIS) measurements.
c
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from ꢀ0.8 to ꢀ1.3 eV which may lead to differences in the electron
injection propensity of the dyes and alter the photocurrent
generation. Consequently high IPCE values (Fig. 10) were realized
for the dyes 4c–4e attributable to their intense red-shifted
absorption and favorable electron injection.
The diameter of the semicircle in the Nyquist plots (Fig. 13)
obtained under illumination conditions increased in the order
of 4e o 4d o 4c o 4b o 4a, which indicates that the
photoelectron generation and transport are enhanced in the
order of 4a o 4b o 4c o 4d o 4e. This is in accordance with
the progressive increase in the intramolecular charge transfer
for the dyes on moving from 4a to 4e. The electron rich
conjugation bridge enriches the intramolecular charge transfer
and increases the charge-injection rate. Retardation of charge
recombination on extension of the conjugation pathway may
also act in concert with the above factor. The Bode phase plots
observed for the devices fabricated using the dyes are shown in
Fig. 14 Bode phase plots of the DSSCs fabricated using the dyes
4a–4e.
Fig. 14. The electron lifetime can be extracted from the
33
angular frequency (omin
)
at the mid-frequency peak in the
absorption and reduction in the oxidation potentials which
manifested in the photovoltaic performance of the devices.
Enhanced light absorption properties observed for the
bithiophene and thienylfluorene containing derivatives were
responsible for the higher overall efficiency realized for them.
The present study clearly reveals that harmony of structural
elements in the organic dyes is essential to achieve desired
positive outcome in the device performance.
Bode phase plot using te = 1/omin and falls in the range of
0.64–2.09 ms. The electron lifetime follows the trend 4a o 4e o
4c o 4b = 4d. The shorter electron lifetime of 4a is
self-explanatory and is in accordance with the shorter
conjugation, however the relatively shorter electron lifetime
for 4e is noteworthy as it exhibited larger overall conversion
efficiency in the DSSC. Electron recombination is suppressed
in molecules containing a semi-rigid conjugation pathway with
nonplanar arrangement as it inhibits aggregation.34 The more
planar structural arrangement (vide supra) observed for 4e
may be responsible for the shorter electron lifetime.
IV Experimental
The general experimental methods and instruments used in the
present study were described in our earlier publication.5
III Conclusions
Synthesis of 5-(pyren-1-yl)thiophene-2-carbaldehyde (3a)
We have developed new organic dyes based on pyrene and
a-cyanoacrylic acid end groups and p-linkers derived from
rigid aromatic/heteroaromatic segments of differing electron
richness. We have observed a significant role for the bridging
units in the present dyes which lack trivial donor groups.
Elongation of conjugation and/or increment in the electron
A round bottom flask was charged with 1-bromopyrene
(0.843 g, 3.0 mmol), (5-(1,3-dioxolan-2-yl)thiophen-2-yl)tributyl-
stannane (3.3 mmol) and 7 ml DMF. Nitrogen was purged
into this, followed by the addition of Pd(PPh3)2Cl2 (21.0 mg,
1 mol%). This was kept on heating at 80 1C for 16 h under a
nitrogen atmosphere. After completion of reaction time, the
mixture was poured into cold water and extracted with
dichloromethane followed by washing with brine solution
and dried over anhydrous sodium sulfate. Then the solvent
was evaporated by applying vacuum. The resultant liquid was
dissolved in 5 ml acetic acid and heated to 60 1C for 30 min.
Then 10 ml of water was added to it and heating was
continued for 6 h. The resulting solution was extracted with
dichloromethane and washed with brine solution and dried
over anhydrous sodium sulfate. Further purification was
performed by silica gel column chromatography using
hexane/chloroform. Yellow solid. Yield: 0.58 g (62%). Mp
120–122 1C; IR (KBr, cmꢀ1): 1663. 1H NMR (CDCl3,
500.13 MHz) d ppm: 10.00 (s, 1 H), 8.43 (d, J = 9.0 Hz,
1 H), 8.19–8.24 (m, 3 H), 8.11–8.14 (m, 2 H), 8.03–8.09 (m, 3 H),
7.90 (d, J = 4.0 Hz, 1 H), 7.48 (d, J = 4.0 Hz, 1 H). 13C NMR
(CDCl3, 125.77 MHz) d ppm: 183.0, 153.1, 143.9, 136.8, 131.9,
131.3, 130.7, 129.1, 128.9, 128.7, 128.5, 128.05, 128.03, 127.2,
126.4, 126.0, 125.5, 124.9, 124.7, 124.5, 124.1. Anal. calcd for
C21H12OS: C, 80.74; H, 3.87%; found: C, 80.56; H, 3.74%.
richness for the p-linkers produced
a red-shift in the
Fig. 13 Nyquist plots of the DSSCs fabricated using the dyes 4a–4e.
c
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Syntheses of the aldehyde derivatives (3b–3e)
124.7, 123.2, 121.6, 120.1, 120.0, 56.5, 32.7, 8.6. Anal. calcd for
C38H28OS: C, 85.68; H, 5.30%; found: C, 85.47; H, 5.18%.
The syntheses of the aldehyde derivatives 3b–3e were carried
out by following the general method described below for 3b.
Synthesis of 50-(pyren-1-yl)-[2,20-bithiophene]-5-carbaldehyde (3e)
Synthesis of 3-(5-(pyren-1-yl)thiophen-2-yl)benzaldehyde (3b)
This was synthesized by a procedure similar to that of 3b
except that 5-bromothiophene-2-carbaldehyde was used in
place of 3-bromobenzaldehyde. Orange solid. Yield: 0.52 g
(66%). Mp 160–163 1C; IR (KBr, cmꢀ1): 1660. 1H NMR
(CDCl3, 500.13 MHz) d ppm: 9.90 (s, 1 H), 8.52 (d, J =
9.0 Hz, 1H), 8.20–8.24 (m, 4 H), 8.08–8.14 (m, 3 H), 8.05
(t, J = 7.5 Hz, 1 H), 7.25 (d, J = 3.5 Hz, 1 H), 7.52 (d, J =
4.0 Hz, 1 H), 7.34–7.62 (m, 2 H). 13C NMR (CDCl3,
125.77 MHz) d ppm: 184.6, 147.2, 144.7, 141.7, 137.5, 136.6,
131.4, 130.9, 129.2, 129.0, 128.6, 128.4, 128.2, 128.1, 127.3,
126.7, 126.3, 125.7, 125.3, 125.1, 124.7, 124.6, 124.2. Anal. calcd
for C25H14OS2: C, 76.11; H, 3.58%; found: C, 75.93; H, 3.41%.
A round bottom flask was charged with 3-bromobenzaldehyde
(0.0.37 g, 2.0 mmol) and tributyl(5-(pyren-1-yl)thiophen-2-yl)-
stannane (2.2 mmol) and 5 ml DMF. Nitrogen was purged
into this, followed by the addition of Pd(PPh3)2Cl2 (14.0 mg,
1 mol%). This was kept on heating at 80 1C for 16 h under a
nitrogen atmosphere. After completion of time the reaction
mixture was poured into cold water and extracted with
dichloromethane and washed with brine solution. This was
dried over anhydrous sodium sulfate. Further purification was
performed by silica gel column chromatography using hexane/
chloroform. Yellow solid. Yield: 45%. Mp 117–119 1C; IR
1
(KBr, cmꢀ1): 1703. H NMR (CDCl3, 500.13 MHz) d ppm:
10.10 (s, 1 H), 8.58 (d, J = 9.0 Hz, 1 H), 8.20–8.23 (m, 4 H),
8.08–8.15 (m, 4 H), 8.02–8.05 (m, 1 H), 7.95–7.97 (m, 1 H),
7.81–7.83 (m, 1 H), 7.60 (t, J = 7.5 Hz, 1 H), 7.56 (d, J =
3.5 Hz, 1 H), 7.39 (d, J = 4.0 Hz, 1 H). 13C NMR (CDCl3,
125.77 MHz) d ppm: 192.2, 143.3, 143.1, 137.0, 135.4, 131.5,
131.4, 131.2, 130.9, 129.7, 129.2, 129.1, 129.0, 128.8, 128.23,
128.17, 128.0, 127.4, 126.4, 126.3, 125.5, 125.2, 125.1, 124.8,
124.8, 124.7, 124.6. Anal. calcd for C27H16OS: C, 83.48; H,
4.15%; found: C, 83.22; H, 4.03%.
Synthesis of the dyes (4a–4e)
The synthesis of the dyes was carried out by following the
general method described below for 4a.
Synthesis of (E)-2-cyano-3-(5-(pyren-1-yl)thiophen-2-yl)acrylic
acid (4a)
5-(Pyren-1-yl)thiophene-2-carbaldehyde (0.30 g, 0.96 mmol)
(3a), cyanoacetic acid (0.12 g, 1.44 mmol), acetic acid (5 ml)
and ammonium acetate (5 mg) were mixed together and kept
on reflux at 120 1C for 12 h. The resulting orange solution was
poured into ice-cold water to produce an orange precipitate.
This was filtered and washed thoroughly with water and dried.
The solid was further recrystallized with chloroform. Orange
solid. Yield: (0.27 g, 74%). Mp 236–238 1C; IR (KBr, cmꢀ1):
2213, 1698. 1H NMR (DMSO-d6, 500.13 MHz) d ppm:
8.45–8.47 (m, 1 H), 8.45–8.47 (m, 1 H), 8.45–8.47 (m, 1 H),
8.35–8.41 (m, 4 H), 8.24–8.31 (m, 3 H), 8.23 (d, J = 11.0 Hz,
1 H), 8.15 (t, J = 8.0 Hz, 1 H), 8.06 (d, J = 3.5 Hz, 1 H), 7.67
(d, J = 4.0 Hz, 1 H). 13C NMR (DMSO-d6, 125.77 MHz)
d ppm: 162.0, 152.2, 137.7, 131.7, 131.4, 130.8, 130.0, 129.3,
128.9, 128.7, 128.5, 128.3, 127.8, 127.3, 126.5, 126.1, 125.6,
124.6, 124.4, 124.2. HRMS (GC/TOF EI) m/z 380.0738
[M + H+] calcd for C30H17NO2S (M + H+) 380.0741.
Synthesis of 4-(5-(pyren-1-yl)thiophen-2-yl)benzaldehyde (3c)
This was synthesized by a procedure similar to that of 3b
except that 4-bromobenzaldehyde was used in place of
3-bromobenzaldehyde. Yellow solid. Yield: 60%. Mp
168–170 1C; IR (KBr, cmꢀ1): 1629. 1H NMR (CDCl3,
500.13 MHz) d ppm: 10.03 (s, 1 H), 8.56 (d, J = 9.5 Hz,
1 H), 8.21–8.23 (m, 3 H), 8.09–8.14 (m, 4 H), 8.03–8.06 (m, 1 H),
7.93–7.95 (m, 2 H), 7.85–7.87 (dd, J = 6.5 Hz, 0.15 Hz, 2 H),
7.62 (d, J = 3.5 Hz, 1 H), 7.41 (d, J = 4.0 Hz, 1 H). 13C NMR
(CDCl3, 125.77 MHz) d ppm: 191.5, 144.4, 143.8, 143.3, 142.1,
140.1, 140.1, 139.9, 137.4, 135.2, 131.5, 131.3, 130.6, 129.4,
129.1, 129.0, 128.3, 128.2, 128.1, 127.4, 126.3, 125.8, 125.6,
125.3, 125.1, 124.7. Anal. calcd for C27H16OS: C, 83.48; H,
4.15%; found: C, 83.30; H, 3.99%.
Synthesis of 9,9-diethyl-7-(5-(pyren-1-yl)thiophen-2-yl)-9H-
Synthesis of (E)-2-cyano-3-(3-(5-(pyren-1-yl)thiophen-2-yl)-
fluorene-2-carbaldehyde (3d)
phenyl)acrylic acid (4b)
This was synthesized by a procedure similar to that of 3b
except that 7-bromo-9,9-diethyl-9H-fluorene-2-carbaldehyde
was used in place of 3-bromobenzaldehyde. Yellow solid.
Yield: 47%. Mp 178–180 1C; IR (KBr, cmꢀ1): 1692.
1H NMR (CDCl3, 500.13 MHz) d ppm: 10.07 (s, 1 H), 8.62
(d, J = 9.5 Hz, 1 H), 8.15–8.23 (m, 3 H), 8.09–8.17 (m, 4 H),
8.04 (t, J = 8.0 Hz, 1 H), 7.82–7.89 (m, 4 H), 7.60–7.77 (dd,
J = 8.0 Hz, 0.15 Hz, 1 H), 7.06 (d, J = 1.5 Hz, 1 H), 7.86 (d,
J = 3.5 Hz, 1 H), 7.40 (d, J = 3.5 Hz, 1 H), 2.13–2.17 (m,
4 H), 0.36–0.39 (m, 6 H). 13C NMR (CDCl3, 125.77 MHz)
d ppm: 192.4, 152.4, 150.9, 147.5, 145.2, 142.4, 139.6, 135.3,
134.9, 131.5, 131.1, 130.8, 129.5, 129.1, 129.0, 128.2, 128.1,
127.9, 127.3, 126.3, 125.5, 125.2, 125.13, 125.07, 124.9, 124.8,
This was synthesized by a procedure similar to that of 4a
except that 3b was used in place of 3a. Orange solid. Yield:
80%. Mp 258–259 1C; IR (KBr, cmꢀ1): 2227, 1693. 1H NMR
(DMSO-d6, 500.13 MHz) d ppm: 8.55 (d, J = 9.5 Hz, 1 H),
8.46 (d, J = 4.0 Hz, 2 H), 8.34–8.39 (m, 3 H), 8.25–8.30
(m, 3 H), 8.20 (d, J = 8.0 Hz, 1 H), 8.11–8.14 (m, 1 H),
8.01–8.06 (m, 2 H), 7.81 (d, J = 3.5 Hz, 1 H), 7.67–7.70
(m, 1 H), 7.57 (d, J = 4.0 Hz, 1 H). 13C NMR (DMSO-d6,
125.77 MHz) d ppm: 163.6, 154.5, 143.4, 142.2, 134.9, 132.9,
131.4, 131.2, 130.8, 130.3, 129.9, 129.8, 129.1, 128.9, 128.6,
128.5, 127.8, 127.2, 126.3, 126.0, 125.9, 125.5, 124.7, 124.6,
124.3, 116.6, 105.2. HRMS (GC/TOF EI) m/z [M + H+]
456.1055 calcd for C30H17NO2S (M + H+) 456.1054.
c
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Synthesis of (E)-2-cyano-3-(4-(5-(pyren-1-yl)thiophen-2-yl)-
The time-dependent DFT (TD-DFT) at the level of B3LYP or
CAM-B3LYP/6-31G** was further carried out to determine
atleast 10 vertical excitations to the excited state of the
molecules. The absorption spectra were simulated by using
the 10 lowest spin-allowed singlet transitions, mixed Lorent-
zian–Gaussian lineshape (0.5) and an average full-width at
half maximum (3000 cmꢀ1) for all peaks. The computations in
THF were performed by applying the polarizable continuum
model (PCM)35 using the integral equation formalism variant
(IEFPCM) to describe the electrostatic solute–solvent interactions
by the creation of solute cavity via a set of overlapping spheres.
phenyl)acrylic acid (4c)
This was synthesized by a procedure similar to that of 4a
except that 3c was used in place of 3a. Orange solid. Yield:
1
84%. Mp 268–270 1C; IR (KBr, cmꢀ1): 2220, 1690. H NMR
(DMSO-d6, 500.13 MHz) d ppm: 8.54 (d, J = 9.0 Hz, 1 H),
8.31–8.38 (m, 4 H), 8.23–8.29 (m, 3 H), 8.21 (d, J = 7.5 Hz,
1 H), 8.11–8.15 (m, 3 H), 7.99 (d, J = 8.5 Hz, 2 H), 7.96
(d, J = 4.0 Hz, 1 H), 7.58 (d, J = 4.5 Hz, 1 H). 13C NMR
(DMSO-d6, 125.77 MHz) d ppm: 163.9, 153.8, 143.5, 143.2,
138.1, 132.2, 131.4, 131.3, 130.9, 129.0, 128.9, 128.6, 128.4,
127.8, 127.3, 127.2, 126.3, 126.1, 125.5, 124.7, 124.6, 124.3,
116.9. HRMS (GC/TOF EI) m/z [M + H+] 456.1054 calcd
for C30H17NO2S (M + H+) 456.1054.
DSSC fabrication and characterization
A fluorine-doped SnO2 conducting glass (FTO, 7 O sq.ꢀ1
,
transmittance B80%, NSG America, Inc., New Jersey, USA)
was first cleaned with a neutral cleaner, and then washed with
DI-water, acetone, and IPA, sequentially. The conducting
surface of the FTO was treated with a solution of TTIP
(1 g) in 2-methoxyethanol (99.5%, Sigma-Aldrich, 3 g) for
obtaining a good mechanical contact between the conducting
glass and TiO2 film, as well as to isolate the conducting glass
surface from the electrolyte. TiO2 paste36 was coated onto the
treated conducting glass by using the doctor blade technique.
For coating each TiO2 layer, the dried TiO2 film was gradually
heated to 450 1C in an oxygen atmosphere, and subsequently
sintered at that temperature for 30 min. The TiO2 photoanodes
of the DSSCs employed in the experiments were composed of
a 14 mm thick transparent TiO2 layer with a scattering layer of
4.5 mm thickness. After sintering at 450 1C and cooling to
80 1C, the TiO2 film was immersed in a 3 ꢁ 10ꢀ4 M solution of
dye at room temperature for 24 h. N719 (Solaronix S.A.,
Aubonne, Switzerland) was dissolved in acetonitrile (ACN)
and tert-butyl alcohol (volume ratio of 1 : 1) as a standard dye
solution. Various organic dye solutions were prepared in a
mixing solvent containing ACN, tert-butyl alcohol and
dimethyl sulfoxide (DMSO) (volume ratio of 1 : 1 : 3). The
thus prepared TiO2/dye electrode was placed on a platinum-
sputtered conducting glass electrode (ITO, 7 O sq.ꢀ1, Ritek
Corporation, Hsinchu, Taiwan), keeping the two electrodes
separated by a 25 mm-thick Surlyns (SX1170-25, Solaronix
S.A., Aubonne, Switzerland). The two electrodes were then
sealed by heating. A mixture of 0.1 M LiI, 0.6 M DMPII
(Solaronix S.A., Aubonne, Switzerland), 0.05 M I2, and 0.5 M
TBP in 3-methoxypropionitrile (MPN, Fluka)/ACN (volume
ratio of 1 : 1) was used as the electrolyte. The electrolyte was
injected into the gap between the electrodes by capillarity; the
electrolyte-injecting hole was previously made in the counter
electrode with a drilling machine, and the hole was sealed with
hot-melt glue after the injection of the electrolyte.
Synthesis of (E)-2-cyano-3-(9,9-diethyl-7-(5-(pyren-1-yl)-
thiophen-2-yl)-9H-fluoren-2-yl)acrylic acid (4d)
This was synthesized by a procedure similar to that of 4a
except that 3d was used in place of 3a. Orange solid. Yield:
1
52%. Mp 256–258 1C; IR (KBr, cmꢀ1): 2229, 1698. H NMR
(DMSO-d6, 500.13 MHz) d ppm: 8.58 (d, J = 9.5 Hz, 1 H),
8.42 (s, 1 H), 8.33–8.38 (m, 3 H), 8.18–8.30 (m, 4 H), 8.06–8.14
(m, 4 H), 8.03 (d, J = 7.5 Hz, 1 H), 7.94 (d, J = 0.10 Hz, 1 H),
7.88 (d, J = 3.5 Hz, 1 H), 7.83–7.85 (dd, J = 8.0 Hz, 0.15 Hz,
1 H), 7.55 (d, J = 3.5 Hz, 1 H), 2.08-2.19 (m, 4 H), 0.31
(t, J = 7.0 Hz, 6 H). 13C NMR (DMSO-d6, 125.77 MHz)
d ppm: 164.1, 155.1, 152.5, 150.9, 146.0, 145.0, 141.6, 139.8,
134.6, 131.5, 130.9, 130.3, 129.4, 128.6, 127.8, 127.3, 126.3,
125.7, 125.3, 124.8, 123.4, 122.6, 121.3, 120.3, 118.8, 117.2,
102.9, 56.6, 32.2, 8.9. HRMS (GC/TOF EI) m/z [M + H+]
600.1994 calcd for C41H29NO2S (M + H+) 600.1993.
Synthesis of (E)-2-cyano-3-(50-(pyren-1-yl)-[2,20-bithiophen]-5-yl)-
acrylic acid (4e)
This was synthesized by a procedure similar to that of 4a
except that 3e was used in place of 3a. Red solid. Yield: 80%.
Mp 270–272 1C; IR (KBr, cmꢀ1): 2217, 1680. 1H NMR
(DMSO-d6, 500.13 MHz) d ppm: 8.54 (d, J = 8.0 Hz, 2 H),
8.35–8.39 (m, 3 H), 8.28–8.32 (m, 1 H), 8.26 (d, J = 6.0 Hz,
2 H), 8.21–8.24 (m, 1 H), 8.12–8.15 (m, 1 H), 8.04 (d, J =
4.0 Hz, 1 H), 7.82 (d, J = 4.0 Hz, 1 H), 7.71 (d, J = 4.0 Hz,
1 H), 7.59 (d, J = 3.5 Hz, 1 H). 13C NMR (DMSO-d6, 125.77
MHz) d ppm: 164.1, 146.9, 145.8, 144.0, 136.4, 134.6, 131.4,
130.8, 130.6, 129.2, 128.7, 128.6, 128.4, 128.3, 127.8, 127.2,
125.0, 125.8, 125.6, 124.7, 124.5, 124.3. HRMS (GC/TOF EI)
m/z [M + H+] 462.0616 calcd for C28H15NO2S2 (M + H+)
462.0618.
Computational methods
Surface of the DSSC was covered by a mask with a
light-illuminated area of 0.16 cm2 and then illuminated by a
class-A quality solar simulator (XES-301S, AM 1.5 G, San-Ei
Electric Co., Ltd.). Incident light intensity (100 mW cmꢀ2) was
calibrated with a standard Si Cell (PECSI01, Peccell Technologies,
Inc.). Photocurrent–voltage curves of the DSSCs were
We have performed theoretical calculations to understand the
electronic structure of the fuctional materials by using the
Gaussian 09W (Revision-A.02)21 program suite. The geome-
tries of the dyes in vacuo were optimized using density
functional theory (DFT)20 with the hybrid B3LYP23 or
CAM-B3LYP24 density functional and the 6-31G** basis set.
All optimized geometries were subjected to vibrational analysis
and potential energy scan and characterized as global minima.
obtained with
a potentiostat/galvanostat (PGSTAT 30,
Autolab, Eco-Chemie, the Netherlands). The thickness of
the TiO2 film was judged by scanning electron microscopic
c
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images (SEM, NanoSEM 230, Novat). For UV-absorption
spectra, dye molecules were coated on the TiO2 films and the
corresponding spectra were obtained using an UV-visible
spectrophotometer (V-570, Jasco, Japan) equipped with a total
integrating sphere. Electrochemical impedance spectra (EIS) were
obtained by the above-mentioned potentiostat/galvanostat,
equipped with an FRA2 module, under a constant light
illumination of 100 mW cmꢀ2. The frequency range explored
was 10 mHz to 65 kHz. The applied bias voltage was set at the
open-circuit voltage of the DSSC, between the ITO-Pt counter
electrode and the FTO-TiO2-dye working electrode, starting
from the short-circuit conditions; the corresponding AC
amplitude was 10 mV. The impedance spectra were analyzed
using an equivalent circuit model. Incident photo-to-current
conversion efficiency (IPCE) curves were obtained under
short-circuit conditions. The light source was a class A quality
solar simulator (PEC-L11, AM 1.5 G, Peccell Technologies,
Inc.); light was focused through a monochromator (Oriel
Instrument, model 74100) onto the photovoltaic cell. The
monochromator was incremented through the visible
spectrum to generate the IPCE (l) as defined by IPCE (l) =
1240 (JSC/lj), where l is the wavelength, JSC is short-circuit
photocurrent density (mA cmꢀ2) recorded with a potentiostat/
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galvanostat, and j is the incident radiative flux (W mꢀ2
)
measured with an optical detector (Oriel Instrument, model
71580) and a power meter (Oriel Instrument, model 70310).
Acknowledgements
KRJT is thankful to Council of Scientific and Industrial
Research (01(2480)/11/EMR-II), New Delhi, India, for
financial support. KCH acknowledges a grant-in-aid from
National Science Council, Taiwan.
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