Synthesis of multi-donor dyes and influence of molecular design on dye-sensitized solar cells

Article abstract of DOI:10.1039/c6ra08530j

Multi-donor incorporated organic dyes were designed and synthesized and their structure-property relationship was investigated in dye-sensitized solar cells (DSCs). Five N,N-disubstituted aniline donor groups along with carbazole and thiophene secondary donors were combined into the (D)_n-π-A design with cyanoacrylic acid as the acceptor. The dyes showed broad absorption bands with absorption maxima in the range of 470-485 nm and optical band gap around 1.94-2.37 eV. Our results indicate that incorporation of bulky groups and bent-type architecture helps to improve the performance of DSCs.

Full text of DOI:10.1039/c6ra08530j

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Journal Name
ARTICLE
Synthesis of Multi-donor Dyes and Influence of Molecular Design
on Dye-sensitized Solar Cells
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a
a
b,c
b,c
Ashok Keerthi, Ming Hui Chua, Thuang Yuan Timothy Chan, Yeru Liu, Qing Wang, Suresh
a,b
Valiyaveettil*
DOI: 10.1039/x0xx00000x
Multi-donor incorporated organic dyes were designed, synthesized and their structure-property relationship was
investigated in dye-sensitized solar cells (DSCs). Five N,N-disubstituted aniline donor groups along with carbazole and
www.rsc.org/
thiophene secondary donors were combined into the (D)
showed broad absorption bands with absorption maxima in the range of 470 - 485 nm and optical band gap around 1.94 –
.37 eV. Our results indicate that incorporation of bulky groups and bent-type architecture helps to improve the
performance of DSCs.
n
-π-A design with cyanoacrylic acid as the acceptor. The dyes
2
2
2-27
efficiencies.
Several studies have been conducted with
Introduction
triphenylamine and carbazole group as donors in organic
sensitizers due to their strong electron donating and hole
2
8-38
Low-cost and environment friendly technologies that can transporting nature.
efficiently harness the solar energy would provide an attractive shown significant improvements over the traditional D-A
n
Multi-donor configuration (D -A) has
1
,
2
39-42
solution to the challenges for sustainable development.
based molecules.
Dye-sensitized solar cells (DSCs) are still not economically
viable as the commercially available photosensitizers based on
inorganic materials such as ruthenium complexes are
2
-5
expensive and require complicated preparation methods.
6
, 7
8
9
Ruthenium complexes such as N3 , N719 and black dye are
the most commonly used sensitizers in DSCs due to their
1
0
intense and broad absorption of visible light. In contrast to
inorganic ruthenium based sensitizers, metal-free organic dyes
8
, 11-13
offer several advantages,
such as high molar absorption
coefficients, molecular tailoring of the electronic band gap via
1
structural modification, and useful for fabricating flexible
4
1
5-21
DSC devices.
Structural design of organic dyes plays a
crucial role in performance of DSCs for solar energy
conversion. In place of the standard N719 sensitizer, the use of
oligoacene or oligothiophene core dyes with coumarin donor
groups and cyanoacrylic acid acceptors led to high
a.
Dr. Ashok Keerthi, Ming Hui Chua, Thuang Yuan Timothy Chan, Deepa Sriramulu,
Prof. Suresh Valiyaveettil*
Department of Chemistry
National University of Singapore
Figure 1. Molecular structure of synthesized dyes, TC1 – TC5.
3
Science Drive 3, Singapore 117543
E-mail: chmsv@nus.edu.sg
Dr. Ashok Keerthi, Dr. Yeru Liu, Dr. Qing Wang and Prof. Suresh Valiyaveettil
NanoCore-NUSNNI, T-Lab Building,
b.
National University of Singapore
A Engineering Drive 1, Singapore 117411
Dr. Yeru Liu and Dr. Qing Wang
5
c.
Department of Materials Science and Engineering
National University of Singapore
Faculty of Engineering, Singapore 117574
Electronic Supplementary Information (ESI) available:
DOI:
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Scheme 1. Synthesis of multi-donor dyes (TC1
-
TC5
)
.
Synthesis:
The synthetic pathways used for preparing the dyes are
illustrated in Scheme 1. The first two dyes, TC1 and TC2 were
obtained in a two-step reaction scheme, in which 2-bromo-5-
formylthiophene was coupled to boronic ester of donor groups
Owing to enormous efforts by researchers to design better
organic dyes with high solubility, easy accessibility, high open-
circuit voltage and photon-to-electron conversion efficiencies,
detailed investigation of the structure-property relationship of
organic dyes is the need of the hour towards the
establishment of ‘molecular design’ concept for DSCs. Here,
we explore the role of geometry (i.e. linear vs bent), multiple
donor groups attached to a single acceptor (Push-Push-Pull)
and changes in electronic properties of the molecule towards
the performance of DSCs.
(
e.g DHA and PPY) using Suzuki coupling reaction, followed by
Knoevenagel condensation with cyanoacetic acid. Similarly,
TC3 TC4 and TC5 were also synthesized via a two-step
,
sequence using Suzuki coupling reaction followed by
Knoevenagel condensation. The compound, 3b or 3c were
synthesized in a one-pot reaction, which consisted of two
consecutive Suzuki coupling reactions. 3,6-Dibromocarbazole
was reacted with 0.7 equiv. of 1b or 1c and stirred for 8 h,
followed by the addition of 2.0 equiv. of 5-formyl-thiophene-2-
boronic acid to the reaction mixture (Scheme 1). The same
procedure was also used for the synthesis of compound 4.
These reactions resulted in disubstituted side products from
the corresponding boronic acids and bromocompounds;
however, the sequential couplings of boronic acids led to
target compound as the major product. In the next step,
Structural differences are expected to exert significant
2
influence on the packing of dye molecules on TiO surface and
affect the performance of DSCs. In addition, linear vs curved or
branched molecular structures of the dyes lead to reduced
aggregation, followed by potential enhancement in electron
4
3-45
transport.
Results and discussion:
N,N-dihexylaniline (DHA, in TC1) or phenylpyrrolidine (PPY, in appropriate aldehyde was reacted with cyanoacetic acid in the
TC2) is attached to the middle thiophene (electron rich π- presence of ammonium acetate and acetic acid (i.e.
spacer) group acts as donor and cyanoacrylic acid as the Knoevenagel condensation) to produce the target molecules.
1
13
acceptor and anchoring group (Figure 1). In TC3, TC4 and TC5 All synthesized compounds were characterized by H and
C
dyes, functionalization of the central carbazole moiety play a NMR, MS-EI, FT-IR (for final dyes), MALDI-TOF and elemental
key role towards the final geometry of the molecule. In TC3 analyses.
and TC4, the carbazole was linked to other groups via 3,6- Photophysical Properties:
linkage, whereas in TC5, a 2,7- linkage was used. N,N-bis(4-
The absorption and normalized emission spectra of all five
methoxyphenyl)aniline (MTPA) and N-(2-ethylhexyl)carbazole
organic dyes in chloroform solution are shown in Figure 2. TC3
TC4 and TC5 showed two absorption maxima in the region of
10 - 485 nm. The absorption maximum in the shorter
,
attached to the thiophene ring act as donor groups in TC3 and
TC5 In TC4 substituted phenylpyrrolidine and N-(2-
.
,
3
ethylhexyl)carbazole act as donors and a cyanoacrylic acid as
the anchor as well as acceptor group in all five dye molecules.
It is conceivable that changes in geometry of the dye molecule
would influence the packing on the electrode and electronic
conjugation along the molecule, which will affect the
performance of DSCs (Figure 1).
wavelength region is due to π – π* electronic transition of the
conjugated molecule and the peak in the longer wavelength
region is due to intramolecular charge transfer between the
donor and acceptor moieties. Similar absorption maxima were
also observed for TC1 (471 nm) and TC3 (470 nm) (Table 1).
2
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In TC5, the secondary donor and thiophene π-bridge were
moved from 3,6 - positions to less conjugated 2,7 - positions of
the carbazole group. This led to a linear molecular structure
with a threefold decrease in the molar absorptivity of TC5, ε =
-
1
-1
1
0233 M cm compared to TC3.This implies that substitution
at the 3,6 - positions of the carbazole group is more favorable
for electron migration as the substituents are located at para-
positions to the electron rich nitrogen atom. TC3 showed
lowest stokes shift of 97 nm and TC1 with highest value of 138
nm in the present series (Figure 2B). The optical band gap was
calculated using the intersecting point of the normalized
absorption and emission curves of individual dyes (Table 1).
The absorption spectra of dyes on TiO
2
(Supporting
Information, Figure S2) showed broad absorption bands which
are in agreement with solution spectra and the loading is more
or less uniform for all dyes.
-0.5
0.0
0.5
1.0
TC1
TC3
TC5
Figure 2. Absorption (A) and emission spectra (B) of TC1-5 in chloroform at room
temperature.
-
0.5
0.0
0.5
1.0
However, TC3 showed a higher molar absorptivity (ε) of 33901
+
-1
-1
-1
-1
M cm as compared to TC1, ε = 22894 M cm . In TC3, the
donor ability of the carbazole is significantly enhanced by the
incorporation of trimethoxyphenyl amine group and 3,6-
linakge on carbazole that increases the conjugation for the
electron to migrate towards the cyanoacrylic acid moiety. The
carbazole linked through 3- and 6- positions extends
conjugation and improves electron transport through the
organic sensitizer. Consequently, the enhanced donor ability of
TC3 is beneficial to light harvesting and improving efficiency of
DSCs.
Potential (V) vs Fc/Fc
Figure 3. Cyclic voltammetry scans of dyes TC1, TC3 and TC5 at a scan rate of 100
mV/s in THF solution. Potentials reported with respect to ferrocene.
Table 1. Summary of photo physical and electrochemical properties of TC1-5
dyes.
a
λ
a
em
b
g
e
abs / nm
λ
E
/
c
E
d
HOMO
E
LUMO
Dye
-1
-1
E
oxi / V
(ε / M cm )
/ nm
609
533
567
eV
/ eV
/ eV
471(22894),
449 (22101)
TC1
TC2
TC3
2.08
2.17
2.12
0.35
0.49
−5.00
−5.15
-4.85
-2.92
-2.98
-2.73
A
similar
dye,
5-(4-(bis(4-
methoxyphenylamino)styryl)thiophene-2-yl)-2-cyanoacrylic
acid was synthesized and studied by Hagberg et al.,
411 (24529)
3
6
1
4
4
4
70 (33901),
0.14,
0.64
0.27,
0.75
0.13,
0.60
-
displayed an absorption maximum of 462 nm (ε = 33000 M
cm ). In TC3, the ethylene group was substituted with (2-
3
36 (42655)
-1
57 (22238),
TC4
TC5
555
589
2.19
2.05
−4.97
-2.78
-2.79
ethylhexyl)-9H-carbazole, resulting in a red shift of the
-1
3
11 (38425)
-1
absorption maximum to 470 nm (ε = 33901M cm ).
72 (10233),
-4.84
Comparison of electron donating ability of N,N-
dihexylaniline (TC1) with phenylpyrrolidine (TC2) groups,
significant red shift (60 nm) in absorption maximum was
observed for TC1. Similarly, λmax of TC4 was blue shifted by 13
355 (8373)
[
a] recorded in chloroform. [b] optical band gap. [c] vs ferrocene in 0.1 M
-
1
4 6
Bu NPF in THF, platinum disc as working electrode with a scan rate of 100 mVs .
onset
[d] calculated using the relationship EHOMO = -(Eoxi
the relationship ELUMO = -(optical band gap - EHOMO).
+ 4.8). [e] calculated using
nm as compared to TC3 with a lower absorption coefficient (
ε
-
1
-1
22238 M cm ).
=
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Electrochemical Properties:
The calculated HOMO – LUMO levels and trends in energy gaps
of the dye molecules were in good agreement with UV-vis
absorption spectra of the dyes. The frontier molecular orbital
distributions of the dyes connected through the carbazole
moiety were different from non-carbazole dyes. In case of TC1
and TC2, the HOMO levels were fully distributed on the whole
molecule whereas the LUMO levels were localized in between
thiophene and cyanoacrylic acid groups. The HOMO levels of
For efficient electron injection, the LUMO or excited state
oxidation potential of the dye should be above the conduction
band edge of TiO
2
, and for efficient dye regeneration, the
HOMO of the dye should lie below the energy level of iodine
3
1, 40, 46
redox mediator (I¯/I
3
¯) system.
To evaluate the
thermodynamic basis of these electron transfer processes,
cyclic voltammetry was used to identify the energy levels of
the sensitizers. Cyclic voltammograms (CVs) were recorded at
a constant scan rate of 100 mV/s in THF solution (Figure 3 and
S1). Dyes TC1 and TC2 showed the first oxidation potentials at
dyes TC3, TC4 and TC5 were mostly localized on substituted
amine moiety and partially on carbazole unit, which was
observed in the electrochemical analysis. However, the LUMO
levels were localized on thiophene, cyanoacrylic acid acceptor
0.35 V and 0.49 V, respectively. The MTPA substituted dyes
(Figure 4).
TC3 and TC5 showed significant two reversible oxidations and
their HOMO energy levels were found at -4.85 eV. This first
oxidation is assigned to MTPA moiety. In contrast, the PPY
substituted dyes TC2, TC4 had the irreversible oxidation
potentials (Figure 3). However, TC3 and TC5 were found to be
having similar HOMO energy levels as observed from UV-vis
absorption spectra. As the dyes did not show the reduction
potentials in the used voltage scan range with saturated
calomel electrode, LUMO energy levels were calculated using
the optical band gap HOMO energy levels of individual dyes
(Table 1).
Figure 5. Geometry and energy optimized structure of dye molecules were calculated
by density functional theory at the B3LYP/6-31G(d) level.
Table 2. Theoretical calculation results from density functional theory at the B3LYP/6-
3
1G(d) level.
Dye
TC1
TC2
TC3
TC4
TC5
HOMO (eV)
-5.83
LUMO (eV)
-2.68
Eg (eV)
3.15
-5.26
-2.41
2.85
-4.82
-2.56
2.26
-4.82
-2.47
2.35
-4.63
-2.45
2.18
Photovoltaic Performances of DSCs:
The photovoltaic performances of the organic dyes were
evaluated by constructing DSCs using nanocrystalline TiO and
2
Figure 4. Frontier orbital distribution of synthesized dye molecules were
calculated by density functional theory at the B3LYP/6-31G(d) level.
the iodide/triiodide redox mediator system. The J-V and IPCE
curves (Figure 6 and S3) of TC1 to TC5 based devices and the
corresponding results are shown in Table 3. The overall power
conversion efficiency (η) was calculated from the short-circuit
Theoretical calculations:
To further understand the photophysical and electrochemical current (JSC), open-circuit voltage (VOC) and fill factor (FF). The
properties, the Gaussian calculations were carried out on open-circuit photovoltage and overall power conversion
Gaussian 09 at the density functional theory (DFT) level with efficiency of the five dyes are in the order of TC5 > TC3 > TC1 >
the B3LYP function using 6 - 31G* as a basis set. The TC2 > TC4 and TC3 > TC2 > TC5 > TC4 > TC1, respectively. TC3
-2
HOMO/LUMO orbital distributions and the geometry showed a higher short-circuit current (JSC = 9.03 mA cm ) and
optimized structures of carbazole-substituted and non- overall power conversion efficiency (η = 4.08%) than TC4 (η =
2.37%).
carbazole molecules were depicted in Figure 4 and 5.
4
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the diffusion of electron acceptors present in the electrolyte
towards the TiO is hindered by the bulky dyes on the
As shown in Figure 2A, TC3 has the highest
TC1
TC2
TC3
TC4
TC5
1
0
8
6
4
2
0
2
4
7, 48
surface.
extinction coefficient for a broad range of wavelengths, which
can explain its high Jsc owing to better light harvesting
abilities. The peak extinction coefficient of TC2 is lower than
that of TC3, which explains its lower current compared with
TC3. Although TC2 has similar extinction coefficient spectrum
as compared to TC1 and TC4, the observed high Jsc for TC2 is
due to its more negative HOMO energy (Table 1), which should
4
9, 50
facilitate faster dye regeneration, thus improving Jsc.
From these preliminary findings, further improvements to the
organic sensitizer could be done via proper design of multi-
donor and bulky groups on the dye molecule. This would
incorporate both strong donor properties, extended
conjugation and low degree of aggregation.
0.0
0.2
0.4
0.6
0.8
1.0
Voltage V (V)
Figure 6. J-V characteristics of solar cells of TC1-5 dyes under AM 1.5
illumination. The electrodes were immersed into 0.25 mM solution of
sensitizer in tetrahydrofuran solution
a
.
This is due to the addition of a bulky MTPA unit, which
increases the donor ability of the carbazole group and reduces Experimental
2
dye aggregation at the TiO interface. Among the D-π-A dyes,
Materials and Instruments:
TC2 showed significantly higher performance as compared to
TC1 and this could be due to the presence of linear alkyl chains
All chemicals and reagents were purchased from commercial
suppliers (Sigma Aldrich, and Merck) and used without further
purification. Solvents used for spectroscopic measurements
were spectral grade quality. Compounds, 3,6-dibromo-9-(2-
2
in TC1 dye which facilitate more aggregation on TiO surface.
From the results (Table 3), the D-D-π-A (Push-Push-Pull type)
configuration appears to give better photovoltaic
performance. As seen from the absorption spectra, the
increased propensity for the migration of the electrons
5
1
ethylhexyl)-9H-carbazole , 2,7-dibromo-9-(2-ethylhexyl)-9H-
5
2
carbazole
and N,N-bis(4-methoxyphenyl)aniline-4-boronic
5
3
acid pinacol ester (1c) were synthesized according to
reported literature procedure. Synthesis of compounds 1a and
2
towards anchoring group and TiO surface is significantly
enhanced with increase in number of donor groups and molar
absorptivity, hence improving the light harvesting properties
and short-circuit current.
1b was provided in the supporting information. All reactions
were monitored by thin-layer chromatography (TLC) on silica
gel plates. Preparative separations were performed by column
chromatography on silica gel grade 60 (0.040 - 0.063 mm) from
Merck.
Table 3. DSC performances of TC1 - TC5 under AM 1.5 illumination.
-
2
Dye
TC1
TC2
TC3
TC4
TC5
V
OC (V)
J
SC (mA cm )
FF (%)
63.7
67.2
61.1
69.5
68.4
η (%)
1.99
3.22
4.08
2.37
2.60
1
13
H (300 MHz) and C (75 MHz) NMR spectra were recorded on
0.704
0.651
0.748
0.625
0.793
4.45
a Bruker spectrometer. IR spectra were measured on a Varian
Bio-Rad Excalibur FT-IR spectrophotometer using KBr as
matrix. Mass spectra (Electron ionization) were obtained on a
Finnigan TSQ7000 and MALDI-TOF on Bruker Autoflex III mass
spectrometer. The UV-Vis spectra were measured on a
Shimadzu UV-160lPC spectrophotometer. Photoluminescence
7.35
9.03
5.45
4.80
spectra were recorded with
a Shimadzu RF-5301 PC
Although the VOC obtained for TC5 (793 mV) was higher than
that of TC3, the overall power conversion efficiency produced
was 2.60%, significantly lower than the 4.08% obtained for
TC3. This is due to the lower short-circuit current in the TC5
spectrofluorophotometer in chloroform. Cyclic voltammetry
was recorded with a computer controlled CHI electrochemical
analyzer at a constant scan rate of 100 mV/s. Measurements
were performed in tetrabutylammonium hexafluorophosphate
-2
based device (JSC of 4.80 mA cm ), which implies that
substitution at the 3,6 - positions of the carbazole moiety is
suitable for achieving a better photovoltaic efficiency. On the
other hand, the 2,7 - positions of the carbazole are meta-
directed with respect to the electron rich nitrogen atom,
hence reducing its electron donating properties. It is
conceivable that molecular structures of TC3 and TC5 are
significantly larger than the others and such dyes will block the
(
0.1M) solution prepared in degassed tetrahydrofuran (THF).
The electrochemical cell consists of three electrode systems
with platinum disc as the working electrode, platinum rod as
counter electrode and Standard Calomel Electrode (SCE) as a
reference electrode. The potentials are calibrated using
onset
ferrocene as external standard. The onset of oxidation (Eox
)
was used to calculate the HOMO energy level using the
onset 54
relationship EHOMO = - (4.8 + Eox
). Geometry optimizations
5
5
2
TiO surface, thus reducing recombination and, consequently,
were performed in Gaussian 09 at the density functional
theory (DFT) level with the B3LYP function and a 6-31G* basis
increasing VOC. Owing to the better coverage of the surface,
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1
3
set. The HOMO and LUMO surfaces were generated from the 2.06 (quintet, 4H). C NMR (75.4 MHz, CDCl
3
, ppm):
CHO), 156.55, 148.70, 139.79, 138.14, 127.62, 121.14,
coated 120.12, 111.83, 47.60, 25.47. Elem. Anal. Calcd. for C15
ITO electrodes with a triple layer structure consisting of C, 70.01%; H, 5.87%; N, 5.44%; S, 12.46; found: C, 70.15%; H,
transparent ITO layer (thickness 12 µm), TiO nanoparticle (20 5.89%; N, 5.29%; S, 12.68%.
nm,) layer and a scattering layer with larger TiO nanoparticles Synthesis of 5-[6-(4-pyrrolidin-1yl-phenyl)-9-(2-ethylhexyl)-9H-
400 nm). The electrodes were immersed into a 0.25 mM carbazol-3-yl]thiophene-2-carbaldehyde (3b)
δ = 182.38
5
6
optimized geometries using GaussView 5.
DSCs were fabricated using dyes TC1 to TC5 and TiO
(−
2
H15NOS:
2
2
(
solution of sensitizer in tetrahydrofuran, left overnight,
removed from solution, rinsed in tetrahydrofuran and dried
under the nitrogen gas flow immediately before assembling
the cell. Anodes and platinized counter electrodes were sealed
together in a sandwich configuration and were filled with an
Compound 3b was synthesized by following a similar synthetic
procedure used for 2a. 3,6-Dibromo-9-(2-ethyl-hexyl)-9H-
carbazole (0.846 g, 2 mmol) and 1b (0.546 g, 2 mmol) were
reacted under Suzuki reaction conditions for 8 hours. After
which, 5-formyl-thiophene-2-boronic acid (0.35 g, 2.24 mmol)
electrolyte [0.6M propylmethylimidazolium iodide, 0.03 M I
.1 M guanidinium thiocyanate, and 0.5 M 4-tert-butylpyridine
in a mixture of acetonitrile and valeronitrile (85 : 15 volume
2
,
and Pd(PPh
C for another 12 hours, poured into water (100 mL) and the
mixture was extracted with DCM (2 x 100mL). The collected
organic fractions were dried over anhydrous Na SO , filtered,
3 4
) (0.047 g, 0.04 mmol) were added, stirred at 80
0
°
4
0
ratio)] by vacuum back filling.
2
4
concentrated under reduced pressure and the crude product
was purified using column chromatography with 5% ethyl
acetate in hexane as eluent to yield a bright yellow solid (3b,
Synthesis and characterization:
General Synthetic precursors for Suzuki coupling reaction:
Compounds 2a 2b 3b 3c and were synthesized from the 1H), 8.28 (d, 1H), 7.75
appropriate bromo compound and boronic ester using Suzuki 7.63 (d, J = 8.7 Hz, 2H), 7.46
coupling reaction. Method described for 2a was also used for 1H), 7.39 7.42 (d, 1H), 6.69
the synthesis of other compounds. In a typical procedure, 4.19 (d, 2H), 3.37 (t, 4H), 2.05 (t, 4H), 1.29
+
1
0
.033 g, 12%). MS (EI) m/z: 534.5 (M ), 534.8 (calcd.). H NMR
300 MHz, CDCl , ppm): = 9.89 (s, CHO, 1H), 8.43 8.44 (d,
7.79 (m, 2H), 7.69 7.73 (dd, 1H), 7.60
7.47 (d, 1H), 7.41 7.44 (d,
6.72 (d, J = 8.55 Hz, 2H), 4.17
1.42 (m, 9H), 0.85
, ppm): = 182.59
CHO), 156.53, 147.02, 141.93, 140.20, 137.89, 133.64,
128.84, 127.92, 125.40, 124.30, 123.89, 123.61, 123.11,
(0.188 g, 0.16 mmol) were added and 122.79, 118.51, 117.83, 112.10, 109.56, 109.51, 47.74, 47.70,
C for 12 hours. The reaction progress was 39.48, 31.01, 29.69, 28.82, 14.02, 25.51, 24.40, 23.040, 10.90.
monitored by TLC. The reaction mixture was extracted with Elem. Anal. Calcd. for C35 OS: C, 78.61%; H, 7.16%; N,
dichloromethane, organic layer was washed with brine, dried 5.24%; S, 6.00; found: C, 78.79%; H, 6.98%; N, 5.31%; S, 6.11%.
over anhydrous Na SO and solvent was removed under Synthesis of 5-[6-(4-[Bis-(4-methoxy-phenyl)-amino]-phenyl)-9-(2-
(
3
δ
−
,
,
,
4
−
−
−
−
−
−
−
−
−
1
3
compound 1a (0.387 g, 1 mmol) and 2-bromo-5-formyl
−
0.96 (m, 6H). C NMR (75.4 MHz, CDCl
3
δ
thiophene (0.191 g, 1 mmol) were dissolved in THF (15 mL)
(−
under inert atmosphere. To the reaction mixture, 2M K
10 mL) and Pd(PPh
stirred at 80
2 3
CO
(
3 4
)
°
38 2
H N
2
4
reduced pressure. The crude compound was purified using ethylhexyl)-9H-carbazol-3-yl]-thiophene-2-carbaldehyde (3c)
silica gel column chromatography with DCM-Hexane (1:1) as
The compound was synthesized by following similar synthetic
eluent.
procedure as 3b. 3,6-Dibromo-9-(2-ethylhexyl)-9H-carbazole
Synthesis of 2-[4-(N,N-dihexylamino)phenyl]-5-formyl-thiophene
(
0.497 g, 1.14 mmol) and 1c (0.35 g, 0.81 mmol) were reacted
under Suzuki reaction conditions for 8 hours. After which, 5-
A bright yellow liquid (0.339 g, yield 91.1%). MS (EI) m/z: 371.3 formyl-thiophene-2-boronic acid (0.35 g, 2.24 mmol) and
(2a)
+
1
(
M ), 371.6 (calcd.). H NMR (300 MHz, CDCl
.79( CHO), 7.64 7.65 (d, 1H, J = 3.93 Hz), 7.51
J = 8.88 Hz), 7.19 – 7.20 (d, 1H, J = 4.11 Hz), 6.61 – 6.64 (d, 2H, compound, 3c was isolated as orange oil (0.102 g, 18% yield).
3
, ppm):
δ
=
3 4
Pd(PPh ) (0.469 g, 0.04 mmol) were added and the mixture
9
−
−
− 7.53 (d, 2H, was stirred at 80 °C for another 12 hours. The purified
+
1
J = 8.88 Hz), 3.27
−
3.32 (t, 4H), 1.59 (s, 4H), 1.33 (s, 12H), 0.92 MS (EI) m/z: 692.5 (M ), 692.31 (Calcd.). H NMR (300 MHz,
s, 6H). C NMR (75.4 MHz, CDCl , ppm): = 182.27 ( CHO), CDCl , ppm): δ = 9.88 (s, 1H, -CHO), 8.42 (b, 1H, -CAR), 8.29 (b,
56.32,14.06, 139.65, 138.21, 127.66, 121.06, 119.68, 111.50, 1H, -CAR), 7.69 – 7.79 (m, 3H), 7.54 (d, J = 8.7 Hz, 2H) 7.40 –
3.47, 51.03, 31.71, 29.72, 27.23, 26.79, 22.70. Elem. Anal. 7.46 (m, 3H), 7.05 – 7.13 (m, 6H), 6.86 (d, J = 8.9 Hz, 4H), 4.19
33NOS: C, 74.34%; H, 8.95%; N, 3.77%; S, 8.63; (d, J = 6.9 Hz, 2H, -CH -), 3.82 (s, 6H, -OMe), 2.09 (m, 1H, -CH-),
1
3
(
3
δ
−
3
1
5
Calcd. for C23
H
2
1
3
found: C, 74.22%; H, 8.71%; N, 3.69%; S, 8.74%.
1.30 – 1.42 (m, 8H, -CH
(75.4 MHz, CDCl , ppm): δ = 182.5, 156.3, 155.7, 147.5, 141.9,
41.1, 141.0, 140.5, 137.8, 133.9, 132.8, 127.6, 126.3, 125.4,
2 3
-), 0.90 – 0.96 (m, 6H, -CH ). C NMR
Synthesis of 2-[4-(pyrrolidin-1-yl) phenyl]-5-formyl thiophene (2b)
3
1
1
1
Compound 1b (0.546 g, 2 mmol) and 2-bromo-5-formyl
thiophene (0.31 g, 1.62 mmol) were reacted under Suzuki
coupling conditions to yield a yellow solid (2b, 0.40 g, 95.7%).
24.4, 124.0, 123.5, 123.0, 122.8, 121.2, 118.4, 118.2, 114.6,
09.6, 109.5, 55.4, 39.4, 30.9, 29.6, 28.7, 24.3, 23.0, 13.9, 10.8.
+
1
45 44 2 3
Elem. Anal. Calcd. for C H N O S: C, 78.00%; H, 6.40%; N,
MS (EI) m/z: 257.1 (M ), 257.4 (calcd.). H NMR (300 MHz,
4.04%; S, 4.63; found: C, 78.19%; H, 6.09%; N, 4.12%; S, 4.53%.
CDCl
3
, ppm):
δ
= 9.81 (s, 1H,
7.57 (d, J = 8.7 Hz, 2H), 7.22
6.58 (d, J = 8.88 Hz, 2H), 3.32
−
CHO), 7.67
−
7.68 (d, J = 3.39 Hz,
Synthesis of 5-[7-(4-[Bis-(4-methoxy-phenyl)-amino]-phenyl)-9-(2-
ethylhexyl)-9H-carbazol-2-yl]-thiophene-2-carbaldehyde (4)
1H), 7.54
H), 6.55
−
−
− 7.23 (d, J = 4.11 Hz,
−
1
3.34 (t, 4H), 2.02 −
6
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6RA08530J
ARTICLE
2
,7-Dibromo-9-(2-ethylhexyl)-9H-carbazole (0.5 g, 1.14 mmol) 47.75, 25.42. FTIR (KBr): 3441.26 (OH Stretch), 2210.59 (C
and 1c (0.345 g, 0.80 mmol) were reacted under Suzuki Stretch), 1631.63 (C=O Stretch), 1118.67 (C O Stretch). Elem.
reaction conditions for 8 hours. After which, 5-formyl- Anal. Calcd. for C18 S: C, 66.64%; H, 4.97%; N, 8.64%; S,
thiophene-2-boronic acid (0.35 g, 2.24 mmol) and Pd(PPh 9.88; found: C, 66.78%; H, 4.86%; N, 8.48%; S, 9.70%.
0.07 g, 0.06 mmol) were added and the mixture was stirred at Synthesis of 3-(5-[6-(4-[bis(4-methoxy-phenyl)-amino]-phenyl)-9-
0 °C for another 12 hours. The crude product was purified (2-ethylhexyl)-9H-carbazol-3-yl]-thiophene-2-yl)-2-cyano-acrylic
using column chromatography and compound was isolated acid (TC3)
as orange oil (0.114 g, 14% yield). MS (EI) m/z: 692.4 (M ),
≡N
−
16 2 2
H N O
3 4
)
(
8
4
+
The compound 3c (0.088 g, 0.13 mmol), and cyanoacetic acid
0.022 g, 0.25 mmol) were under general Knoevenagel
condensation procedure gave a maroon solid (0.038 mg, 40%
1
6
92.31 (Calcd.). H NMR (300 MHz, DMSO-d6, ppm): δ = 9.91
s, 1H, -CHO), 8.09 (d, J = 8.1 Hz, 2H), 7.78 (d, J = 3.9 Hz, 1H),
.65 (b, 1H), 7.45 – 7.57 (m, 6H), 7.03 – 7.14 (m, 6H), 6.88 (d, J
8.7 Hz, 4H), 4.22 (d, J = 6.9 Hz, -CH ), 3.82 (s, 6H, -OMe), 2.10
2.14 (t, 1H, -CH), 1.26 – 1.43 (m, 8H, -CH ), 0.85 – 0.98 (m,
). C NMR (75 MHz, DMSO-d6, ppm): δ = 182.6, 167.7,
(
(
7
=
–
6
1
1
1
2
7
6
+
1
yield). MS (MALDI-TOF) m/z: 759.5 (M ), 759.31 (Calcd.). H
NMR (300 MHz, DMSO-d6, ppm): δ = 8.76 (b, 1H, -CAR), 8.60
2
2
(
b, 1H, -CAR), 8.48 (s, 1H, =CH), 8.05 (b, 1H, -CAR), 7.90 – 7.94
dd, 1H), 7.78 – 7.83 (m, 2H), 7.65 – 7.72 (m, 4H), 7.10 (d, J = 9
1
3
H, -CH
3
(
55.9, 155.9, 148.2, 148.0, 142.5, 141.9, 141.5, 140.8, 139.6,
37.5, 133.7, 130.0, 127.8, 127.7, 126.6, 123.9, 123.7, 120.9,
20.7, 118.6, 117.6, 114.7, 106.7, 55.4, 47.3, 39.4, 30.9, 28.7,
Hz, 4H), 6.94 – 6.99 (m, 6H), 4.35 (d, Hz = 7.4, 2H, -CH
2
), 3.80
), 0.80
3
0.93 (m, 6H, -CH ). C NMR (75.4 MHz, DMSO-d6, ppm): δ =
(
s, 6H, -OMe), 2.05 (m, 1H, -CH), 1.25 – 1.39 (m, 8H, -CH
2
13
–
1
1
1
5
44 2 3
4.4, 23.0, 13.9, 10.9. Elem. Anal. Calcd. for C45H N O S: C,
64.1, 156.0, 154.5, 147.5, 141.9, 141.8, 140.7, 140.6, 140.4,
34.0, 133.9, 133.2, 133.2, 132.1, 132.0, 127.8, 127.7, 126.7,
25.3, 123.8, 123.3, 123.3, 123.0, 122.9, 120.7, 120.6, 115.3,
5.6, 47.2, 30.5, 28.3, 24.0, 22.8, 14.1, 11.0. FT-IR (KBr): 3442.3
8.00%; H, 6.40%; N, 4.04%; S, 4.63; found: C, 78.13%; H,
.31%; N, 4.16%; S, 4.71%.
Compounds TC1 to TC5 were synthesized using similar
procedures and the synthesis of TC1 was described in detail.
Synthesis of 3-(5-[4-(N,N-dihexylamino)phenyl]thiophene-2-yl)-2-
cyano-acrylic acid (TC1)
(
OH stretch), 3035.8 (=CH stretch), 2927.4 (-CH stretch),
216.1 (CN stretch), 1685.9 (CO stretch), 1577.7 (C=C alkene
stretch), 1506.1 (C=C aromatic stretch) and 1240.0 (-C-O
S: C, 75.86%; H,
2
45 3 4
Compound 2a (0.268 g, 0.72 mmol) was dissolved in 8 mL stretch). Elem. Anal. Calcd. for C48H N O
glacial acetic acid. To this, cyanoacetic acid (0.122 g, 1.44 5.97%; N, 5.53%; S, 4.22; found: C, 75.68%; H, 6.11%; N, 5.63%;
mmol) and ammonium acetate (0.111 g, 1.44 mmol) were S, 4.37%.
added. The mixture was stirred and refluxed for 8h. After Synthesis of 3-(5-[6-(4-pyrrolidin-1yl-phenyl)-9-(2-ethylhexyl)-9H-
cooling to room temperature, the mixture was poured into carbazol-3-yl]-thiophene-2-yl)-2-cyano-acrylic acid (TC4)
cold water (20 mL), filtered and washed with water (2 x 20 mL)
The compound 3b (0.1 g, 0.19 mmol) and cyanoacetic acid
followed by methanol (2 x 20 mL). The crude product was
(
0.033 g, 0.40 mmol) were reacted under general Knoevenagel
purified by column chromatography using DCM and methanol
condensation procedure gave product TC4 as red solid (0.032
(
8:2) mixture as eluent to give a red solid (0.264 g, yield
g, yield 85%). MS (MALDI-TOF) m/z: 601.2777(M+), 601.8
1
calc.). H NMR (300 MHz, DMSO-d6, ppm): δ = 8.52 (s, 1H),
+
1
8
3.4%). MS (MALDI-TOF) m/z: 439.3137 (M ), 438.6 (calcd.). H
CHO), 7.60 –
.63 (dd, 1H), 7.42 – 7.50 (2 d, 2H), 7.32 – 7.35 (dd, 1H), 6.58 –
.67 (2 d, 2H), 3.26 – 3.30 (t, 4H), 1.51 (broad, 4H),1.28 (s,
(
NMR (300 MHz, DMSO-d6, ppm): δ = 8.09 (s, 1H,
−
8
1
8
4
9
.74 (s, 1H), 8.00
H), 7.79 7.80 (d, 1H), 7.72
.37Hz, 2H), 7.58 7.61 (d, 2H), 6.65
.29 4.31 (d, 2H),3.17 (overlapped),1.99 (b, 2H), 1.23 (m,
H),0.79
−
8.01 (d, 1H), 8.44 (s, 1H), 7.85
7.75 (d, 1H), 7.64 7.66 (d, J =
6.68 (d, J = 8.55 Hz),
− 7.88 (d,
7
6
1
−
−
−
−
−
1
3
2H), 0.85 – 0.86 (2 s, 6H). C NMR (75.4 MHz, DMSO-d6,
−
ppm): δ = 164.67, 150.19, 150.23, 148.61, 141.36, 136.97,
13
−
0.87 (m, 6H). C NMR (75.4 MHz, DMSO-d6, ppm):
1
4
3
33.72, 127.42, 121.54, 120.35, 119.78, 112.37, 111.93, 50.46,
δ = 148.39, 147.06, 141.90, 141.70, 134.00, 127.96, 127.62,
2.90, 31.51, 28.94, 27.16, 26.44, 22.49, 14.23. FT-IR (KBr):
3
422 (OH Stretch), 2926, 2855 (sp CH Stretch), 2210 (C
1
2
24.62, 123.63, 123.38, 112.47, 110.85, 47.78, 30.53, 29.38,
8.44, 25.35, 24.01, 22.87, 14.20, 11.05. FTIR (KBr): 3443.25 (-
≡
N
Stretch), 1608 (C=O Stretch). Elem. Anal. Calcd. for
S: C, 71.19%; H, 7.81%; N, 6.39%; S, 7.31; found: C,
1.24%; H, 7.49%; N, 6.19%; S, 7.51%.
OH Stretch), 2958.03, 2923.32 and 2852.80 (sp3 -CH stretch),
214.97 (C N Stretch), 1684.88 (C=O Stretch), 1610.03 (C=C
Stretch), 1063.72 (C O Stretch). Elem. Anal. Calcd. for
S: C, 75.84%; H, 6.53%; N, 6.98%; S, 5.33; found: C,
5.71%; H, 6.63%; N, 6.73%; S, 5.44%.
26 34 2 2
C H N O
2
≡
7
−
Synthesis of 3-(5-[4-(pyrrolidin-1-yl) phenyl] thiophene-2-yl)-2-
cyano-acrylic acid (TC2)
C
7
38 39 3 2
H N O
Compound 2b (0.192 g, 0.75 mmol) and cyanoacetic acid Synthesis of 3-(5-[7-(4-[bis(4-methoxy-phenyl)-amino]-phenyl)-9-
0.132 g, 1.55 mmol) were reacted according to general (2-ethylhexyl)-9H-carbazol-2-yl]-thiophene-2-yl)-2-cyano-acrylic
(
Knoevenagel condensation procedure to yield product (TC2), acid (TC5)
red solid (0.145 g, yield 60%). MS (MALDI-TOF) m/z: 322.7128
1
M+), 324.4 (calcd.). H NMR (300 MHz, DMSO-d6, ppm): δ =
The compound
0.024 g, 0.29 mmol) were reacted under general Knoevenagel
condensation procedure gave an orange solid (0.062 mg, 57%
4 (0.1 g, 0.14 mmol) and cyanoacetic acid
(
(
7
8
8
.94 (s, 1H), 7.55
−
−
7.57 (d, J = 3.63 Hz, 1H), 7.51
7.35 (d, J = 3.93 Hz, 1H), 6.58
−
−
7.54 (d, J =
6.61 (d, J =
.7 Hz, 2H), 7.33
+
1
yield). MS (MALDI-TOF) m/z: 759.5 (M ), 759.31 (Calcd.). H
NMR (300 MHz, DMSO-d6, ppm): δ = 8.16 – 8.21 (m, 2H), 8.03
1
3
.73 Hz, 2H), 1.97 (broad m, 4H). C NMR (75.4 MHz, DMSO-
d6, ppm): δ = 148.40, 136.65, 134.16, 127.33, 121.56, 112.44,
(s, 1H), 7.86 (s, 1H), 7.73 (d, 3H), 7.65 (d, J = 8.7 Hz, 2H), 7.55
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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DOI: 10.1039/C6RA08530J
ARTICLE
Journal Name
(
d, J = 8.2 Hz, 1H), 7.46 (d, J = 8.2 Hz, 1H), 7.08 (d, J = 8.9 Hz,
H), 6.88 – 6.96 (m, 6H), 4.22 (d, J = 7.1 Hz, -CH ), 3.76 (s, 6H, -
OMe), 2.04 (t, 1H, -CH), 1.18 – 1.33 (m, 8H), 0.76 – 0.92 (m, 6H,
Fischer and M. Grätzel, Inorg. Chem., 1999, 38, 6298-
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6
4
2
7
8
.
.
1
3
-CH
3
). C NMR (75.4 MHz, DMSO-d6, ppm): δ = 156.2, 149.4,
1
1
1
3
48.2, 142.4, 141.6, 141.6, 140.5, 139.8, 138.6, 136.7, 135.9,
33.0, 130.7, 128.1, 127.1, 127.0, 124.6, 122.7, 121.3, 121.2,
20.8, 120.1, 120.0, 118.2, 117.6, 115.3, 107.1, 106.7, 55.6,
0.5, 28.3, 24.1, 22.9, 14.1, 11.2. FT-IR (KBr): 3437.0 (OH
9
1
.
stretch), 3034.1 (=CH stretch), 2927.9 (-CH stretch), 2211.4 (CN
stretch), 1602.9 (CO stretch), 1506.4 (C=C aromatic stretch)
0.
and 1240.9 (-C-O stretch). Elem. Anal. Calcd. for C48
C, 75.86%; H, 5.97%; N, 5.53%; S, 4.22; found: C, 75.61%; H,
.91%; N, 5.75%; S, 4.33%.
45 3 4
H N O S:
1
1.
5
1
1
2.
3.
T. Saga, NPG Asia Materials, 2010, 2, 96-102.
H.-H. Chou, Y.-C. Chen, H.-J. Huang, T.-H. Lee, J. T. Lin, C.
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2015, 5, 7711.
Conclusions
A series of organic dyes with multi-donor ‒ acceptor (Push-
Push-Pull) configuration and different architecture were
synthesized, characterized and primarily tested for improving
1
4.
the photovoltaic efficiency. The results showed that the Push- 15.
M. Grätzel, Inorg. Chem., 2005, 44, 6841-6851.
M. Yanagida, T. Yamaguchi, M. Kurashige, K. Hara, R.
Katoh, H. Sugihara and H. Arakawa, Inorg. Chem., 2003,
1
6.
Push-Pull system exhibited interesting performance due to the
enhanced electron donor ability of the N,N-
di(methoxyphenyl)phenylamine group at different positions
2,6- vs 3,7-) of the carbazole. TC3 showed high values for
4
2, 7921-7931.
1
1
1
7.
8.
9.
A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson,
Chem. Rev., 2010, 110, 6595-6663.
N. Koumura, Z.-S. Wang, S. Mori, M. Miyashita, E. Suzuki
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(
-
1
-1
molar absorptivity (ε = 33718 M cm ) and open circuit voltage
(V
OC = 748 mV) with an overall power conversion efficiency of
4.08%. The results reported here indicate that further
improvements of the organic sensitizers could be achieved
through systematic changes in molecular design via judicious 20.
choice of the number of donor groups, their chemical nature,
and fine-tuning the overall geometry of the molecular dyes to 21.
M. Kimura, H. Nomoto, N. Masaki and S. Mori,
Angewandte Chemie, 2012, 124, 4447-4450.
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prevent aggregation and reduce the dark current.
2
2
2.
3.
K. Hara, K. Sayama, Y. Ohga, A. Shinpo, S. Suga and H.
Arakawa, Chem. Commun., 2001, 0, 569-570.
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Suga, K. Sayama, H. Sugihara and H. Arakawa, J. Phys.
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Acknowledgements
We greatly acknowledge the support of department of
chemistry, National University of Singapore for providing 24.
funding (R-143-000-570-112) and technical support and NUS
computer centre for high performance computation. Also, the
K. Hara, M. Kurashige, Y. Dan-oh, C. Kasada, A. Shinpo, S.
Suga, K. Sayama and H. Arakawa, New J. Chem., 2003, 27,
783-785.
2
5.
K. Hara, Y. Tachibana, Y. Ohga, A. Shinpo, S. Suga, K.
Sayama, H. Sugihara and H. Arakawa, Sol. Energy Mater.
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authors thank Ms Sriramulu Deepa for helping to revise the
paper.
2
2
6.
7.
K. Hara, K. Miyamoto, Y. Abe and M. Yanagida, J. Phys.
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