D-A-π-A organic dyes for dye-sensitized solar cells: Effect of π-bridge length between two acceptors on photovoltaic properties

Article abstract of DOI:10.1016/j.tet.2015.08.068

We report synthesis, photophysical and photovoltaic properties of organic dyes with D-A-π-A backbone having 1,3,4-oxadiazole moiety as an auxiliary acceptor, triphenylamine as donor with varying π-bridges and acceptors. The effect of increased distance be

Full text of DOI:10.1016/j.tet.2015.08.068

Accepted Manuscript
D-A-π-A organic dyes for dye-sensitized solar cells: Effect of π-bridge length between
two acceptors on photovoltaic properties
Sandeep B. Mane, Chih-Fu Cheng, Albertus Adrian Sutanto, Amitabha Datta,
Amitava Dutta, Chen-Hsiung Hung
PII:
S0040-4020(15)30022-3
10.1016/j.tet.2015.08.068
TET 27090
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 9 July 2015
Revised Date: 24 August 2015
Accepted Date: 26 August 2015
Please cite this article as: Mane SB, Cheng C-F, Sutanto AA, Datta A, Dutta A, Hung C-H, D-A-π-
A organic dyes for dye-sensitized solar cells: Effect of π-bridge length between two acceptors on
photovoltaic properties, Tetrahedron (2015), doi: 10.1016/j.tet.2015.08.068.
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D-A-π-A organic dyes for dye-sensitized
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Sandeep B. Mane, Chih-Fu Cheng, Albertus Adrian Sutanto, Amitabha Datta, Amitava Dutta and Chen-Hsiung
Hung

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D-A-π-A organic dyes for dye-sensitized solar cells: Effect of π-bridge
length between two acceptors on photovoltaic properties
Sandeep B. Manea, Chih-Fu Chenga,b, Albertus Adrian Sutantoa,c,d, Amitabha Dattaa, Amitava
Duttae and Chen-Hsiung Hung a,
aInstitute of Chemistry, Academia Sinica, Taipei, Taiwan (ROC)
bDepartment of Chemistry, National Taiwan Normal University, Taipei-116, Taiwan (ROC)
cDepartment of Chemical Engineering, NTUST, Taipei-10607, Taiwan (ROC)
dDepartment of Chemical Engineering, Faculty of Engineering, Diponegoro University, Semarang-50275, Indonesia
eCurrent Address: Department of Chemistry, Bangbasi Morning College, 19, Scott lane, Kolkata-700009. India
Abstract: We report synthesis, photophysical and photovoltaic properties of organic dyes with D-A-π-A backbone
having 1,3,4-oxadiazole moiety as an auxiliary acceptor, triphenylamine as donor with varying π-bridges and
acceptors. The effect of increased distance between the auxiliary acceptor and main acceptor on their photovoltaic
properties is studied with the help of optical and electrochemical spectroscopy, quantum chemical calculations,
current-voltage (I-V) measurements, incident photon-to-current conversion efficiency (IPCE) and electrochemical
impedance spectroscopy (EIS). The length of the π-bridge of the studied dyes has significant effect on the
photophysical and photovoltaic properties. Dye OD3 with the longest π-bridge has superior performance in dye-
sensitized solar cells compared to other dyes.
Keywords: Dye-sensitized solar cells, Organic dyes, Oxadiazoles, Donor-acceptor system, Solar Energy
1. Introduction
The continuous decline of fossil fuels and constantly growing demands for energy stimulates the pursuit
for alternative energy supplies. Henceforth, search for alternative, safe and renewable sources of energy is
particularly essential. Solar energy is undoubtedly the most promising one among all renewable energy
technologies, as it is the most abundant, lavish and renewable source of energy. Since Grätzel’s first
report in 1991, dye-sensitized solar cells (DSSCs) have fascinated titanic attention due to their low cost
and environmentally benign properties.1 As the efficiency deficit between the conventional silicon solar
cells and DSSCs is tumbling, there is a potential chance of DSSCs replacing the conventional silicon solar
cells in commercial market. Amid several crucial components of DSSCs, the sensitizer molecule is of
solemn importance. In the design of efficient sensitizer, properties like chemical stability, step economic
synthesis and low cost materials play a very important role. The sensitizing dye is important in DSSCs as
it governs the light harvesting and charge generation as well as separation. For a long period, Ru-
complexes were the best performer dyes for DSSCs, with the power conversion efficiencies higher than
11%.2 However, its widespread application is hindered by the scarcity of its abundance. In contrast,
Corresponding author. Tel.: +886-2-27898571; fax: ++886-2-27891237; e-mail: chhung@gate.sinica.edu.tw
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organic dyes can be prepared conveniently from a broad range of commercially available raw materials,
making them a cost effective alternative. Recently, organic dyes have surpassed Ru-complexes to become
best performer with 12.8% conversion efficiency.3,4 Several high-performance organic dyes feature a
donor-π bridge-acceptor (D- π -A) structure. In this structure, the electronic interaction between donor
and acceptor is responsible for the strong absorption bands that harvest sunlight in the visible region. Zhu
et al. proposed a new design of highly stable and efficient organic dyes with a D-A-π-A structure.5-7 An
additional electron-withdrawing group has been introduced into the π-bridge as an auxiliary electron
acceptor that provides additional photostability to the sensitizers. Several electron-withdrawing units have
been utilized in DSSCs as the auxiliary acceptor as observed in the literature.8-18 1,3,4-oxadiazole is
another class of electron-deficient motif that not only possess good thermal and chemical stability but also
have high photoluminescence quantum yields.19-21 Owing to these excellent properties it has been applied
as electron transporting material in OLEDs,22-25 signaling components in molecular sensory systems26-29
and molecular logic gates.30 In recent times it has also been evaluated as a π-bridge in D- π -A type
organic sensitizers.31 In this paper we wish to extend the application of these oxadiazole ring as an
auxiliary electron acceptor. Our lab is dedicated to develop efficient sensitizers for the DSSCs.32-36 In this
paper, we have prepared two novel dyes OD1 and OD3 with different distance between the auxiliary
acceptor and the carboxylic acid or cyanoacrylic acid acceptor. The molecular structures of these dyes are
displayed in Fig. 1. Dye OD1 comprised of carboxylic acid anchor, while dye OD2 consist of
cyanoacrylic acid as anchoring group. The dye OD3 was embraced with thiophene group to enhance the
π-conjugation and cyanoacrylic acid as anchor. These oxadiazole dyes demonstrate good absorption
features in visible region, good charge transfer ability upon excitation and suitable redox potentials, which
collectively suggests that these dyes shall work as efficient sensitizers in dye-sensitized solar cells.
Fig. 1. Molecular structures of the OD dyes.
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2. Results and Discussion
4.1. Syntheses
The synthetic pathways for OD dyes are depicted in Scheme 1. The starting compounds (4)-(6) were
prepared following literature procedures. The triphenylamine (TPA) aldehyde (1) was readily converted
in to nitrile (2) in the presence of hydroxylamine hydrochloride and acetic acid, which was further treated
with sodium azide to obtain TPA tetrazole (3). This TPA tetrazole was reacted with corresponding acid
chlorides to yield the compounds (4)-(6). The ester group in compound (4) was converted to carboxylic
acid by saponification in the presence of aq. KOH to get the desired dye OD1. The dye OD2 was
obtained via Knoevenagel condensation of compound (5) with cyanoacetic acid. Compound (6) was
treated with compound (8) under Stille coupling protocol to achieve intermediate compound (7) which
was further condensed with cyanoacetic acid under Knoevenagel condensation conditions to obtain the
OD3 dye. All the final compounds were thoroughly characterized with the help of NMR, HRMS, UV and
CV studies.
Scheme 1. Synthetic pathways for OD dyes. Reagents and Conditions: a) NH2OH•HCl, AcOH, Pyridine;
b) NaN3, NH4Cl; c) RPhCOCl, Pyridine; d) KOH (aq.); e) CNCH2COOH, NH4OAc, AcOH; f) Comp. 8,
Pd(PPh3)4.
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4.2. Optical properties
The UV-Visible absorption spectra of these OD dyes measured in ethanol are displayed in Fig. 2 and the
related photophysical data are summarized in Table 1. The characteristic features of the absorption
spectra are composed of two broad peaks in the range of 260-500 nm. The bands at the wavelength below
400 nm are attributed to the
wavelength region (400-600 nm) are accredited to intramolecular charge transfer (ICT) with
transition character respectively. The spectral coverage of these dyes follow the order of OD3 > OD2 >
π-π* transition of the conjugated molecules, and that in the longer
π-π*
OD1, anticipating the phenomena of more extended π-conjugation displays lower energy absorptions and
larger spectral coverage.
Fig. 2. Absorption spectra measured in EtOH.
Table 1. Optical and Electrochemical data of OD dyes in EtOH.
λabs/nma
/103M−1cm−1)
Dye
λem/nmb
Eox/Vc
E(0,0)/eVd
Eox*/Ve
EHOMO/ELUMO/eVf
(
ε
364 (25.4), 281 (18)
375 (24.3), 299 (23)
406 (32), 289 (13)
473
475
535
1.14
1.31
1.36
3.00
2.91
2.72
-1.86
-1.60
-1.36
-5.33/-2.29
-5.42/-3.04
-5.26/-2.84
OD1
OD2
OD3
aAbsorption maximum of dyes in EtOH, bEmission maximum measured in EtOH by exciting at lower
energy band, cFirst oxidation potentials vs. NHE determined by differential pulse voltammetry in THF
and referenced to a ferrocene redox couple, dE(0,0) values were estimated from the intersection of the
absorption and emission spectra. ereduction potentials approximated from Eox* and E(0,0). fCalculated at
the B3LYP/6-31G level in vacuum.
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The steady-state emission maxima also follows the similar trend as the absorption spectra with more π-
conjugated molecules displaying red-shifted emission. The absorption spectra of all dyes adsorbed on a
transparent TiO2 film are depicted in Fig. 3. Compared to the absorption spectra measured in ethanol,
apparent bathochromic shifts and broadening of the peaks were observed for all the dyes. This
bathochromic shift after adsorption on TiO2 might be due to the coordination of the carboxylate group to
the surface of Ti4+, which lowers the
π
* level of the organic dyes.37 The amount of dye loadings on the
TiO2 surface were estimated by measuring the absorbance of the dyes desorbed from the sensitized TiO2
films after being immersed in 0.1M KOH solution in THF. The saturated dye densities for the OD dyes
were found to be 367, 218 and 388 nmol cm-2 for OD1, OD2 and OD3 respectively. It is quite interesting
that in spite of having lower dye loading, the dye OD2 outperforms dye OD1. Although the difference in
the dye loading is not considerable, performance of dye OD3 is far superior to that of dye OD1.
Fig. 3. Absorption spectra measured with dyes adsorbed on TiO2 surface.
2.3. Cyclic voltammetry
The cyclic voltammograms of the dyes were acquired by differential pulse voltammetry and the relevant
electrochemical data are summarized in Table 1. All of the dyes exhibit one oxidation potential, which is
attributed to the oxidation of arylamine and potentials decrease in the order OD3 > OD2 > OD1. The
excited state potential (Eox*) of the sensitizer was estimated from the difference between the first
oxidation potential and the zero–zero excitation energy E(0,0), where the zero–zero excitation energy was
estimated from the intersection of absorption and emission spectra. The systematic energy level diagram
for the studied OD dyes is displayed in the Fig. 4. As evident from the energy level diagram, the increase
in the π-conjugation results in the smaller HOMO and LUMO band gaps and consequently red-shifting
the absorption. The more negative excited state oxidation potential of the OD dyes (Eox*, −1.36 to −1.86
V vs. NHE) than the conduction band-edge level of the TiO2 electrode (−0.5 V vs. NHE), ensures
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effective electron injection into the conduction band of TiO2. The first oxidation potential of the dyes is
more positive than that of I−/I3− redox couple (0.4 V vs. NHE), indicating that regeneration of the dyes
(1.14 to 1.36 V vs. NHE) is energetically favorable.
Fig. 4. Energy level diagram for OD dyes.
2.4. DFT calculations
Theoretical studies based on density functional theory (DFT) calculations using Guassian 09 program at
the B3LYP/6-31G(d) level of theory were carried out for optimization of the OD dyes to gain an insight
into the geometric electronic structures. The frontier molecular orbitals for the three dyes are depicted in
Fig. 5. It is clearly seen that the electron density of the highest occupied molecular orbitals (HOMO) is
predominantly localized on the triphenylamine donor moiety and diffusively distributed over the
conjugation onto the oxadiazole ring.
π-
Fig. 5. Frontier molecular orbital diagram for OD dyes.
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In contrast, the electron density of the lowest unoccupied molecular orbitals (LUMO) is extended over the
oxadiazole and acceptor acid groups. It is noteworthy from the frontier MOs that the dye OD2 and OD3
exhibit more localized electron density distribution than dye OD1 which denotes a prominent charge
separation upon photoexcitation. This effective electron density redistribution should assist effective
electron injection from the excited state of dye to the conduction band of TiO2. This elusive difference,
along with other photophysical parameters, may contribute to the superior performance of dyes OD2 and
OD3 over dye OD1. Time-dependent density functional theory (TD-DFT) calculations were also
performed at B3LYP functional and the 6-31G(d) basis set using Gaussian 9 software and the results are
summarized in Table 2. For all the dyes, the lowest energy transition (S0
→
S1) is about 99% of the
S2 transition for all the OD dyes mainly composed of the
LUMO+1 transition, thus confirming its donor to acceptor charge-transfer character.
HOMO
HOMO
→
LUMO transition. Whereas the S0→
→
Significantly, TD-DFT calculations also revealed a higher molecular dipole moment (11.54 D) for the
OD3 than for OD2 (8.87 D) and OD1 (6.83 D) dyes as shown in Table 2. The higher dipole moments are
beneficial to facilitate efficient intramolecular photoinduced electron transfer. Furthermore, we calculated
the amount of electron density localized on a particular subgroup in HOMO and LUMO with the help of
Mullikan charge analysis. We divided the dyes into three parts, first is the donor (TPA), second the
oxadiazole (Oxa) and the pi-bridge along with the acceptor (CA) as third part. The calculated Mullikan
charge on the particular part is summarized in Table 3. As the carboxylic acid is a weak acceptor, the
charge accumulated on this acceptor in OD1 is comparatively less than the other two dyes with
cyanoacrylic acid acceptor.
Table 2. Calculated TD-DFT composition in terms of frontier molecular orbitals, excitation energy, the
oscillator strengths and the dipole moments of OD dyes.
Dyes
OD1
State
Composition (%)
L (99)
E (eV)
(nm)
f
Dipole Moment (D)
6.83
S1
S2
H
→
2.74
3.61
453.26
343.75
0.4481
0.6135
H−1→L (4)
H
H
H−1
H
H
→
→
L+1 (93)
L (99)
OD2
OD3
S1
S2
2.14
3.29
579.61
376.86
0.331
1.3325
→L (47)
8.87
→
→
L+1 (52)
L (99)
S1
S2
2.18
3.04
569.15
408.35
0.2627
1.6419
H−1
L+1 (27)
H=HOMO, L=LUMO, H−1=HOMO−1, L+1=LUMO+1.
→L (71)
11.54
H
→
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With incorporation of the cyanoacrylic acid acceptor, the charge separation is much higher in OD2
compared to OD1 as seen from the higher charge located on the CA part. With extension of the π-bridge
length between the oxadiazole and the cyanoacrylic acid, the charge located on the CA group is increased
considerably in the HOMO orbital of OD3. While in the LUMO orbital the CA group has accumulated
the highest charge, indicating the charge separation is the biggest in OD3. This is also confirmed by the
large dipole moment observed for OD3.
Table 3. Photovoltaic properties of devices with OD dyes
.
HOMO
Dye
LUMO
TPA Oxa
CA
TPA Oxa
CA
90.68 7.40 1.92 8.67 26.66 64.67
90.17 7.41 2.42 2.64 11.88 85.48
86.69 9.01 4.29 1.08 4.52 94.41
OD1
OD2
OD3
2.5. Photovoltaic studies
The I-V curves for the DSSCs based on the dyes studied in this work under AM 1.5 illumination are
depicted in Fig. 6. The best device performance and the relative data of the assembled DSSC devices are
summarized in Table 4. Chenodeoxycholic acid (CDCA) is used as a co-adsorbent to reduce the
molecular aggregation. The detailed conditions for device preparation are given in the experimental
section. The term Jsc refers to short-circuit current, Voc to open-circuit voltage, FF to fill factor and
η to
the power conversion efficiency. The overall power conversion efficiencies are in the order of OD1
(0.8%) < OD2 (1.76%) < OD3 (2.72%).
Table 4. Photovoltaic properties of devices with OD dyes.
Dye
JSC (mA cm-2)
VOC (V)
FF (%)
η (%)
1.82
3.88
6.87
0.62
0.64
0.58
71
71
68
0.80
1.76
2.72
OD1
OD2
OD3
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The inferior performance of the dye OD1 compared to other dyes is due to the lower short-circuit current
of 1.82 mA cm-2. Although it has higher dye loadings than the other OD dyes, molecular aggregation on
the TiO2 surface might be the reason for its lower photocurrents. It is observed from the absorption
spectrum and the I-V curves that, higher the
π-conjugation, higher is the photocurrent density and
consequently higher is the overall conversion efficiency. Higher Jsc of 3.88 mA cm-2 and Voc of 0.64 V
obtained for device based on dye OD2 compared to the dye OD1 resulted in its superior photovoltaic
performance of 1.76%. The best overall photon-to-current conversion efficiency (η) of 2.72% was
observed for dye OD3, with Jsc of 6.87 mA cm-2, Voc of 0.58 V and fill factor of 68. The Voc of the dye
OD3 is least among all three dyes indicating that the charge recombination might be higher for this dye.
Fig. 6. I-V curve for OD dyes.
Faster charge recombination from the TiO2 film with the oxidized electrolyte corresponds to the lager
dark currents. The I-V curves of the devices under dark conditions are displayed in supporting information
Fig. S1 and the values decrease in the order of OD3 > OD1 > OD2. This trend is consistent with the
obtained Voc values for these dyes. The action spectrum of the incident photon-to-current conversion
efficiency (IPCE) of the studied dyes are presented in Fig. 7. The trend of the IPCE spectra is consistent
with the I-V curves. The broad spectral coverage of the IPCE plots are consistent with the trend of the π-
conjugation of the organic dyes, matching well with the absorption spectra of dyes adsorbed on the TiO2
surface. The device based on dye OD3 produced maximum IPCE values of approximately 45% at 450 nm
and maintained 40% or higher in the range of 400 nm to 460 nm. The IPCE value for dye OD1 is around
30% at 400 nm and that for dye OD2 is 25% at 400 nm but it covers broader area up to 550 nm. The
higher IPCE of the DSSC based on dye OD3 is probably due to the combined effects of a particularly
high surface dye loading, a large driving force from the LUMO level of the dye to the conduction band
edge of TiO2, a high molar extinction coefficient, and the efficient charge separation that led to a greater
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electron injection efficiency and higher light-harvesting efficiency (LHE) than DSSCs based on the other
OD dyes.
Fig. 7. IPCE spectra for OD dyes.
2.6. Electrochemical Impedance Spectroscopy results
Electrochemical impedance spectra (EIS) was measured in the dark as well as under the illumination
conditions to explicate the information about electron injection and the recombination at the
TiO2/dye/electrolyte interface. The Nyquist plot recorded in the dark under a forward bias of -0.55V are
depicted in Fig. 8. For all the dyes one semicircle is observed which is related to the recombination
resistance between the TiO2 surface and the oxidized electrolyte. The radius of the semicircle is directly
proportional to the recombination resistance ie. Larger the radius of semicircle, higher is the
recombination resistance. From Fig.8, it is observed that the recombination resistance decreases in the
order of OD1 > OD3 > OD2.
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Fig. 8. EIS spectra for OD dyes under dark condition.
As presented in Fig. 9, under illumination condition with applied open-circuit voltage, the intermediate
frequency semicircle in the Nyquist plot reflects charge-transfer resistance at the semiconductor-dye-
electrolyte interface. The radius of the semicircle decrease in the order of OD1 > OD2 > OD3 indicating
that the dye OD3 has least electron transport resistance which implies higher electron transport than the
dyes OD2 and OD1. This trend is consistent with the DSSC performance of the OD dyes.
Fig. 9. EIS spectra for OD dyes under illumination condition.
3. Conclusion
In summary, three metal-free dipolar organic dyes (OD1-OD3) with D-A-
oxadiazole moiety as an auxiliary acceptor incorporated within the -framework, with triphenylamine
donor and carboxylic acid and cyanoacrylic acid as acceptor were designed and synthesized for DSSC
application. The absorption spectra is red-shifted with the increase in the length of the -bridge. DFT and
π-A structure, having
π
π
TD-DFT studies confirm the bipolar nature of these dyes suggesting feasibility of these as efficient
sensitizers in DSSCs. The Mullikan charge analysis data and the higher dipole moment indicated the
largest charge separation for OD3, suggesting the length of the π-bridge is an important factor for getting
higher photovoltaic performance. The electrochemical studies shown that the HOMO-LUMO potentials
for these dyes were well matched with TiO2 conduction band and the electrolyte redox potential. The
photovoltaic experiments revealed that the dye with the longer
π-bridge between the auxiliary acceptor
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and the main acceptor exhibit superior photon-to-current conversion efficiencies. Dye OD3 achieved the
best overall conversion efficiency among the three studied dyes. This study expands the scope to use
oxadizole ring as an auxiliary acceptor in DSSCs which will be beneficial for future design of efficient
organic sensitizers.
4. Experimental section
4.1. General Techniques and Materials
All chemicals were obtained from commercial sources and used as received without further purification.
All the reactions were carried out under nitrogen atmosphere. Solvents used in reactions were dried by
PureSolv MD 5 system (Innovative Technology, Inc). Flash chromatography was carried out by using
silica gel (40-63
spectra were recorded on a Bruker Avance 400 FT spectrometer. NMR samples were prepared in CDCl3
and DMSO-d6 as d-solvents and chemical shifts were reported in scale. The standard abbreviations s, d,
µ
m, Merck). Analytical TLC was performed on Merck silica gel plates. 1H and 13C NMR
δ
t, q, m and bs refer to singlet, doublet, triplet, quartet, multiplet and broad singlet respectively. Coupling
constant (J) values are reported in Hertz. The ESI ion trap mass spectra were measured by a Finnigan
MAT LCQ mass spectrometer. The HR-FAB spectra were conducted on a JMS-700 double focusing mass
spectrometer. Transmittance and reflection UV visible absorption spectra of the OD dyes in EtOH and
adsorbed on TiO2 electrodes, respectively, were recorded on a JASCO V-670 UV-vis/NIR
spectrophotometer. Steady-state fluorescence spectra were acquired by using a Varian Cary Eclipse
fluorescence spectrophotometer. The cyclic voltammetry measurements of all dyes were carried out on
CHI 621B electrochemical analyzer (CH Instruments, Austin, TX, USA) in degassed THF containing 0.1
M tetrabutylammonium hexafluorophosphate (Bu4NPF6) as the supporting electrolyte. The cell assembly
consists of a glossy carbon as the working electrode, a silver wire as the pseudo-reference electrode, and a
platinum wire as the auxiliary electrode. The scan rate for all measurements was fixed at 50 mV/sec. A
ferrocene+1/0 redox couple was used as the internal standard and the potential values obtained in reference
to the silver electrode were converted to NHE scale. The density functional theory (DFT) and time-
dependent density functional (TD-DFT) calculations were performed with Gaussian 09 package to study
the electron distribution of the frontier molecular orbitals and the photoexcitation transitions. All ground
state geometries of OD dyes were optimized in the gas phase by the hybrid B3LYP functional and the 6-
31G(d) basis set, and the TD-DFT calculation were based on the same functional and basis set.38 The
molecular orbitals were visualized by the Chemoffice software.
4.2. Syntheses
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The intermediate oxadiazole compounds (4)-(6) were synthesized by following literature procedures with
slight variations.
4-Cyanotriphenylamine (2): A mixture of N,N-diphenylaminobenzaldehyde (1.5 g, 5.49 mmol),
hydroxylamine hydrochloride (0.46 g, 6.62 mmol), acetic acid (1.13 g, 18.83 mmol), pyridine (0.65 g,
8.21 mmol) and DMF (4 ml) was stirred and heated at 150 °C for 2.5 h. The product was purified by
column chromatography (developing solvent: toluene) and recrystallized from n-hexane. Yield 1.28 g
1
(86%). H NMR (400 MHz, CDCl3)
δ: 6.96 (d, 2H, J = 8.76 Hz; Ph), 7.16 (m, 6H, Ph), 7.33 (t, 4H, J =
7.80 Hz; Ph), 7.41 (d, 2H, J = 8.80 Hz; Ph) ppm; 13C NMR (100 MHz, CDCl3)
δ: 102.50, 119.70, 125.13,
126.17, 129.78, 133.17, 145.97, 151.61 ppm; HRMS-FAB+: m/z calcd for C19H14N2: 270.1157, found
270.1161 [M]+.
4-Tetrazolyltriphenylamine (3): A mixture of 4-cyanotriphenylamine (2, 0.7 g, 2.59 mmol), sodium
azide (2.7 g, 41.5 mmol), ammonium chloride (2.22 g, 41.5 mmol), and dry DMF (20 ml) was refluxed
for 24 h. The cooled solution was then added to aqueous hydrochloric acid. The precipitate was collected
and washed with water. Recrystallization from toluene gave the desired product as creamy solid (0.49 g,
60%). 1H NMR (400 MHz, CDCl3)
4H, J = 7.50 Hz; Ph), 7.97 (d, 2H, J = 8.16 Hz; Ph) ppm; 13C NMR (100 MHz, CDCl3)
δ
: 7.10 (d, 2H, J = 8.20 Hz; Ph), 7.17 (d, 6H, J = 7.40 Hz; Ph), 7.38 (t,
δ: 117.92, 122.27,
125.34, 126.53, 129.11, 130.66, 147.86, 151.53, 206.21 ppm; HRMS-ESI: m/z calcd for C19H14N5:
312.1254, found 312.1249 [M-H]−.
Methyl-4-(5-(4-(diphenylamino)phenyl)-1,3,4-oxadiazol-2-yl)benzoate (4): 4-tetrazolyl triphenyl
amine (3, 500 mg, 1.6 mmol) and methyl-4-(chlorocarbonyl) benzoate (476 mg, 2.4 mmol) were mixed
with pyridine (10 ml). The resulting mixture was refluxed under N2 for 12 h. After the completion of
reaction the solvent was evaporated and the crude product was purified by column chromatography to get
1
the desired compound as faint yellow solid. (536 mg, 75%). H NMR (400 MHz, CDCl3)
δ: 3.96 (s, 3H,
OCH3), 7.14 (m, 8H, Ph), 7.33 (t, 4H, J = 7.78 Hz; Ph), 7.94 (d, 2H, J = 8.68 Hz; Ph), 8.18 (s, 4H, Ph)
ppm; 13C NMR (100 MHz, CDCl3)
: 52.40, 115.51, 120.85, 124.50, 125.75, 126.62, 127.89, 128.11,
δ
129.60, 130.20, 132.50, 146.51, 151.17, 163.17, 165.08, 166.13 ppm.
4-(5-(4-(diphenylamino)phenyl)-1,3,4-oxadiazol-2-yl)benzaldehyde (5):
4-tetrazolyltriphenyl amine (3, 500 mg, 1.6 mmol) and 4-formylbenzoyl chloride (1 g, 5.9 mmol) in
pyridine (15 ml) were refluxed under N2 for 18 h. After completion of reaction, the solvent was removed
under vacuum and the crude product was extracted with DCM, dried over MgSO4 and concentrated. The
crude product was purified by flash chromatography using DCM as eluent to get the titled compound as
1
yellow solid (420 mg, 62%). H NMR (400 MHz, CDCl3)
δ: 7.14 (m, 8H, Ph), 7.33 (t, 4H, J = 7.78 Hz;
13

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Ph), 7.94 (d, 2H, J = 8.68 Hz; Ph), 8.03 (d, 2H, J = 8.12 Hz; Ph), 8.29 (d, 2H, J = 8.12 Hz; Ph), 10.09 (s,
1H, CHO) ppm; 13C NMR (100 MHz, CDCl3)
δ: 115.32, 120.78, 124.57, 125.80, 127.23, 128.17, 129.12,
129.63, 130.21, 137.95, 146.48, 151.29, 162.96, 165.30, 191.24 ppm; HRMS-FAB+: m/z calcd for
C27H20O2N3: 418.1556, found 418.1548 [M+H]+.
4-(5-(4-bromophenyl)-1,3,4-oxadiazol-2-yl)-N,N-diphenylaniline
(6):
A
mixture
of
4-
tetrazolyltriphenylamine (500 mg, 1.6 mmol), 4-bromobenzoyl chloride (421 mg, 1.92 mmol) and dry
toluene (20 ml) was stirred and heated under reflux for 8 h. After the mixture was cooled, the solvent was
evaporated. The crude was subjected to column chromatography on silica gel with DCM as eluent to yield
the title compound as yellow solid (458 mg, 61%). 1H NMR (400 MHz, CDCl3)
(t, 4H, J = 7. 86 Hz; Ph), 7.66 (d, 2H, J = 8.64 Hz; Ph), 7.93 (d, 2H, J = 8.80 Hz; Ph), 7.98 (d, 2H, J =
8.64 Hz; Ph) ppm; 13C NMR (100 MHz, CDCl3)
: 115.75, 120.98, 123.09, 124.47, 125.73, 126.73,
δ: 7.14 (m, 8H, Ph), 7.33
δ
128.04, 128.16, 129.60, 132.35, 146.60, 151.11, 163.26, 164.80 ppm; HRMS-ESI: m/z calcd for
C26H19N3O79Br: 468.0711, found 468.0701 [M+H]+.
5-(4-(5-(4-(diphenylamino)phenyl)-1,3,4-oxadiazol-2-yl)phenyl)thiophene-2-carbaldehyde (7):
A
mixture of compound (6) (400 mg, 0.85 mmol) compound (8) (511 mg, 1.27 mmol) and Pd(PPh3)4 (104
mg, 0.09 mmol) in dry DMF was heated at 100 oC for 24 h. After the reaction was complete, the reaction
mixture was extracted with water and NH4Cl and then subjected to silica gel chromatography using DCM
1
as eluent to get the titled compound as yellow solid (224 mg, 53%). H NMR (400 MHz, CDCl3)
δ: 7.14
(m, 8H, Ph), 7.33 (t, 4H, J = 7.82 Hz; Ph), 7.52 (d, 1H, J = 4.00 Hz;
β-thiophene), 7.78 (d, 1H, J = 3.96
Hz;
β
-thiophene), 7.82 (d, 2H, J = 8.44 Hz; Ph), 7.94 (d, 2H, J = 8.76 Hz; Ph), 8.18 (d, 2H, J = 8.44 Hz;
13
Ph), 9.92 (s, 1H, CHO) ppm; C NMR (100 MHz, CDCl3)
δ: 115.71, 120.95, 124.49, 124.67, 125.06,
125.75, 126.88, 127.51, 128.08, 129.62, 135.76, 137.19, 143.36, 146.58, 151.12, 152.37, 163.33, 164.87,
182.72 ppm; HRMS-ESI: m/z calcd for C31H22O2N3S: 500.1433, found 500.1441 [M+H]+.
OD1: Methyl-4-(5-(4-(diphenylamino)phenyl)-1,3,4-oxadiazol-2-yl)benzoate (4, 150 mg, 0.34 mmol)
was dissolved in THF/MeOH (3:1, v/v). To this aqueous KOH solution was added and the reaction
mixture was refluxed under N2 overnight. The solvent was removed under vacuum and crude is acidified
with 1 N HCl. The precipitates were washed with water and collected by filtration to afford the desired
1
product as pale yellow solid (135 mg, 92%). H NMR (400 MHz, DMSO-d6)
δ: 7.14 (m, 8H, Ph), 7.33
13
(m, 4H, Ph), 7.95 (d, 2H, J = 8. 84 Hz; Ph), 8.24 (m, 4H, Ph) ppm; C NMR (100 MHz, DMSO-d6)
δ:
115.42, 120.86, 124.58, 125.83, 126.77, 128.22, 128.57, 129.65, 130.86, 131.75, 146.54, 151.31, 163.15,
165.24, 169.78 ppm; HRMS-FAB+: m/z calcd for C27H20 O3N3: 434.1505, found 434.1512 [M+H]+.
OD2: 4-(5-(4-(diphenylamino)phenyl)-1,3,4-oxadiazol-2-yl)benzaldehyde (300 mg, 0.72 mmol) and
cyanoacetic acid (306 mg, 3.6 mmol) dissolved in acetic acid (10 ml) were condensed in the presence of
14

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ammonium acetate (5 mol%) for 12 h under N2. After cooling to room temperature, the reaction mixture
was poured into a crushed ice and solid obtained was washed thoroughly with water to remove excess of
acetic acid and cyanoacetic acid. Finally, washed with hexane and 3% ethyl acetate/hexane to afford a
1
fine yellow colored solid (280 mg, 80%). H NMR (400 MHz, DMSO-d6)
δ: 7.03 (d, 2H, J = 8.28 Hz;
Ph), 7.19 (m, 6H, Ph), 7.41 (t, 4H, J = 7.54 Hz; Ph), 7.99 (m, 3H, Ph), 8.07 (d, 2H, J = 8.00 Hz; Ph), 8.18
(d, 2H, J = 8.12 Hz; Ph) ppm; 13C NMR (100 MHz, DMSO-d6)
: 115.50, 116.35, 119.34, 120.62, 125.30,
δ
126.20, 127.36, 128.66, 130.41, 130.51, 136.89, 146.21, 146.49, 151.14, 162.69, 163.41, 164.72 ppm;
HRMS-ESI: m/z calcd for C30H19O3N4: 483.1457, found 483.1457 [M−H]+.
OD3: 5-(4-(5-(4-(diphenylamino)phenyl)-1,3,4-oxadiazol-2-yl)phenyl)thiophene-2-carbaldehyde (7) (50
mg, 0.1 mmol), cyanoacetic acid (43 mg, 0.5 mmol), ammonium acetate (12 mg) and acetic acid (10 ml)
o
was heated at 100 C for 12 h. The solution was cooled by pouring into crushed ice. The resulting solid
was filtered, washed thoroughly with water, dried and further re-precipitated from dichloromethane by
1
pouring into methanol to afford the titled compound as a reddish yellow solid (47 mg, 82%). H NMR
(400 MHz, DMSO-d6)
7.94 (d, 1H, J = 4.08 Hz;
(d, 1H, J = 4.08 Hz;
MHz, DMSO-d6)
δ: 7.03 (d, 2H, , J = 8.88 Hz; Ph), 7.19 (m, 6H, Ph), 7.41 (t, 4H, J = 7.84 Hz; Ph),
β-thiophene), 7.99 (d, 2H, J = 8.80 Hz; Ph), 8.04 (d, 2H, J = 8.48 Hz; Ph), 8.07
13
β
-thiophene), 8.18 (d, 2H, J = 8.44 Hz; Ph), 8.54 (s, 1H, CH) ppm; C NMR (100
δ
: 115.00, 116.34, 120.12, 123.88, 124.82, 125.72, 126.48, 126.98, 127.45, 128.17,
129.93, 135.09, 135.69, 141.19, 145.99, 146.43, 150.61, 162.92, 163.38, 164.18 ppm; HRMS-ESI: m/z
calcd for C34H21O3N4S: 565.1334, found 565.1339 [M−H]+.
4.3. Photovoltaic Measurements
TiO2 photoanode films and Pt counter electrodes were purchased from Yingkou Opvtech New Energy Co.
Ltd. Liaoning, China. The films, which were prepared by using the screen-printing method, were
composed of a transparent layer (thickness ≈12 µm), a scattering layer (thickness ≈ 4 µm), and a working
area of 0.4×0.4 cm2 and were used as received. The films were pretreated according to the following
activation procedures before use: heating at 100 oC for 22 min, at 110 oC for 60 min, at 450 oC for 68 min,
at 500 oC 60 min, at 250 oC for 60 min, cooling at 80 oC and keeping at 80 oC before immersion. The TiO2
o
films were immersed in a 1×10−4 M solution of the dye OD1 and OD2 in EtOH for 6 h at 30 C and dye
o
OD3 in EtOH for 1 h at 50 C. A co-adsorbent (3 eq.) chenodeoxycholic acid (CDCA) was used in the
same solution to reduce the molecular dye aggregation. The dye-sensitized TiO2 films were washed with
THF, dried in hot air, and used as the working electrode. To fabricate the DSSC device, the two
electrodes were tightly clipped together into a sandwich-type cell that was spaced by a 40 µm film spacer.
A thin layer of electrolyte, which contained 0.05 M I2, 0.1 M lithium iodide (LiI), 0.6 M dimethyl-propyl-
benzimidiazole iodide (DMPII), and 0.6 M 4-tert-butylpyridine (TBP) in dry CH3CN, was introduced into
15

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the space between the two electrodes. The Photoelectrochemical characterizations on the solar cells were
performed by using an Oriel Class AAA solar simulator (Oriel 94043A, Newport Corp.). Photocurrent–
voltage characteristics of the DSSCs were recorded with a potentiostat/galvanostat (CHI650B, CH
Instruments, Inc.) at a light intensity of 100 mWcm–2 calibrated by an Oriel reference solar cell (Oriel
91150, Newport Corp.). The monochromatic quantum efficiency was recorded through a monochromator
(Oriel 74100, Newport Corp.) under short-circuit condition. The intensity of each wavelength was in the
range of 1 to 3 mW cm–2. Electrochemical impedance spectra were recorded for DSSCs under
illumination at an open-circuit voltage (Voc) or in the dark at −0.55 V at room temperature. The
frequencies explored ranged from 10 mHz to 100 kHz. The data was analyzed using Zview software.
Acknowledgements
Authors gratefully acknowledge financial support from Ministry of Science and Technology (MOST),
Taiwan and the Academia Sinica. Mass spectroscopy analysis was performed by the mass spectrometry
facility of the Institute of Chemistry, Academia Sinica. Help form Dr. Jiann-T’suen Lin with the
measurement and instrumentation is greatly appreciated.
Supplementary data
Supplementary data (1H, 13C NMR and HRMS spectra of the dyes, Bode phase plots) related to this article
can be found at http://dx.doi.org.
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Supporting Information
D-A-π-A organic dyes for dye-sensitized solar cells: Effect of π-bridge length
between two acceptors on photovoltaic properties
Sandeep B. Mane, Chih-Fu Cheng, Albertus Adrian Sutanto, Amitabha Datta, Amitava Dutta
and Chen-Hsiung Hung

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NMR and HRMS spectra
1H NMR spectrum of 4-Cyanotriphenylamine (2)
13C NMR spectrum of 4-Cyanotriphenylamine (2)

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1H NMR spectrum of 4-Tetrazolyltriphenylamine (3)
13C NMR spectrum of 4-Tetrazolyltriphenylamine (3)

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1H NMR spectrum of Methyl 4-(5-(4-(diphenylamino)phenyl)-1,3,4-oxadiazol-2-yl)benzoate (4)
13C NMR spectrum of Methyl 4-(5-(4-(diphenylamino)phenyl)-1,3,4-oxadiazol-2-yl)benzoate (4)

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1H NMR spectrum of 4-(5-(4-(diphenylamino)phenyl)-1,3,4-oxadiazol-2-yl)benzaldehyde (5)
13C NMR spectrum of 4-(5-(4-(diphenylamino)phenyl)-1,3,4-oxadiazol-2-yl)benzaldehyde (5)

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1H NMR spectrum of 4-(5-(4-bromophenyl)-1,3,4-oxadiazol-2-yl)-N,N-diphenylaniline (6)
13C NMR spectrum of 4-(5-(4-bromophenyl)-1,3,4-oxadiazol-2-yl)-N,N-diphenylaniline (6)

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1H spectrum of 5-(4-(5-(4-(diphenylamino)phenyl)-1,3,4-oxadiazol-2-yl)phenyl)thiophene-2-carbaldehyde (7)
13C spectrum of 5-(4-(5-(4-(diphenylamino)phenyl)-1,3,4-oxadiazol-2-yl)phenyl)thiophene-2-carbaldehyde (7)

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1H NMR spectrum of OD1
13C NMR spectrum of OD1

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1H NMR spectrum of OD2
13C NMR spectrum of OD2