Synthesis of new N, N-diphenylhydrazone dyes for solar cells: Effects of thiophene-derived π-conjugated bridge

Article abstract of DOI:10.1016/j.dyepig.2011.08.014

Four novel D-π-A hydrazone dyes (HT, HM, HE, and HO) with an N, N-diphenylhydrazone moiety as the electron donor, different thiophene-derived π-conjugated bridges and a cyanoacrylic acid moiety as the electron acceptor have been designed and synthesized f

Full text of DOI:10.1016/j.dyepig.2011.08.014

Dyes and Pigments 92 (2012) 1042e1051
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis of new N, N-diphenylhydrazone dyes for solar cells: Effects
of thiophene-derived
p
-conjugated bridge
,
b
,
a
a
a
a
a
Ping Shena , Xinping Liu , Shenghui Jiang , Ling Wang , Ling Yi , Dandan Ye ,
*
Bin Zhaoa b, Songting Tana
,
,b
a College of Chemistry and Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, PR China
b Key Laboratory of Advanced Functional Polymeric Materials, College of Hunan Province, Xiangtan University, Xiangtan 411105, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four novel De
electron donor, different thiophene-derived
electron acceptor have been designed and synthesized for the application in dye-sensitized solar cells.
The influences of thiophene-derived bridges on the photoelectrochemical and photovoltaic performance
of these hydrazone dyes were investigated. Results demonstrate that the introduction of 3,4-
dialkyloxythiophene could red-shift the dye’s absorption spectrum due to the enhancement of the
p
eA hydrazone dyes (HT, HM, HE, and HO) with an N, N-diphenylhydrazone moiety as the
-conjugated bridges and a cyanoacrylic acid moiety as the
Received 17 June 2011
Received in revised form
21 August 2011
Accepted 22 August 2011
Available online 27 August 2011
p
electron-donating ability of p-conjugated bridges. Importantly, electrochemical impedance spectroscopy
Keywords:
analysis reveal that 3,4-dialkyloxythiophene bridge could change the charge recombination resistance at
the TiO2/dye/electrolyte interface and as a result to improve the open-circuit photovoltage. Among
the four dyes, HO exhibits the maximum power conversion efficiency of 5.83% (Voc ¼ 0.65 V,
Jsc ¼ 12.69 mA/cm2, FF ¼ 0.707) under simulated AM 1.5 irradiation (100 mW/cm2).
Ó 2011 Elsevier Ltd. All rights reserved.
N, N-diphenylhydrazone
DepeA dyes
Dye-sensitized solar cells
Thiophene-derived bridges
Photovoltaic performance
Electrochemical impedance spectroscopy
1. Introduction
found to possess photoinduced intramolecular charge transfer (ICT)
properties [3,4], which makes these compounds as ideal dyes for
The increasing energy demand and the concerns about climate
changes have led to a great focus on environment friendly energy
sources during the last years. Dye-sensitized solar cells (DSSCs)
which emerged as a new generation of photovoltaic devices have
attracted significant attention and have been studied extensively
due to their high efficiency, low cost, and facile fabrication [1].
Although Ru-based complexes hold the record of validated effi-
ciency of over 11% [2], they have encountered problems such as
limited resources and difficult purification. On the other hand,
metal-free organic dyes have also been developed for DSSCs
because they have many advantages such as high molar absorption
coefficient, low cost, simple synthesis, and easier structural modi-
fication [3,4]. A common strategy in the design of highly efficient
metal-free dyes for DSSCs is the linking of electron donor/acceptor
DSSCs application. Using this strategy, many DepeA metal-free
dyes with various donor moieties, such as coumarin [5,6], indo-
line [7,8], triarylamine [9e18], phenothiazine [19], and porphyrin
[20,21], have been designed and synthesized as efficient sensitizers
for DSSCs in recent years. Power conversion efficiencies (PCEs) up
to w9% have been achieved for DSSCs using these metal-free dyes
[5,6]. Moreover, encouraging efficiencies up to 10% have been
reported [22,23] in recent publications. Developing novel and
highly efficient metal-free organic dyes applied in DSSCs is thus an
area of strong current research activity [3,4].
In most dyes, the amine derivatives act as the electron donor
while a 2-cyanoacrylic acid or rhodanine moiety acts as the elec-
tron acceptor. These two parts are connected by
bridges such as the vinyl unit or thiophene chain. The
p
-conjugated
-conjugated
p
(DeA) systems through
p
-conjugated bridges, which is called the
eA dyes has been
bridge influences not only the region of light absorbed by the DSSCs
but also the degree of electron injection from the dye’s excited state
to the TiO2 surface. Consequently, even a small change in the
conjugated bridge of the dye can cause a significant change in the
solar cell performance [24e26]. Moreover, in our previous reports
De eA molecular structure. This kind of De
p
p
* Corresponding author. College of Chemistry and Key Laboratory of Environmen-
tally FriendlyChemistryand ApplicationsofMinistryofEducation, XiangtanUniversity,
Xiangtan 411105, PR China. Tel.: þ86 13789308734; fax: þ86 0731 58292251.
E-mail address: shenping802002@163.com (P. Shen).
[14,15,27], we have proved that
p-conjugated bridge has a great
influence on the molecular structure and photoelectronic proper-
ities of the metal-free organic dyes.
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.08.014

P. Shen et al. / Dyes and Pigments 92 (2012) 1042e1051
1043
Changes in the electron-donating nature and structural varia-
tions of amine unit can result in a variation of electronic properties
[28]. Aromatic hydrazones are widely studied and used as organic
nonlinear optical materials [29] and hole-transport materials for
electrophotographic photoreceptors [30e32]. The main advantages
of such compounds are the simple synthesis and high enough
charge mobilities [31]. Many triphenylamine [33], carbazole [34,35]
and thiophene-based hydrazone [36,37] compounds were synthe-
sized and used as glass-forming hole-transport materials. Tran-Van
et al. recently reported on the synthesis and application of biscar-
bazole and terthiophene derivatives containing hydrazone func-
tional groups in solid-state DSSCs as hole-transport materials [38].
However, aromatic hydrazones compounds until now are few
studied as photoactive dyes for DSSCs. Therefore, we became
interested in constructing metal-free organic dyes with hydrazone
moiety to make use of its excellent hole-transport property for the
electron donor.
Perkin-Elmer LS-50 luminescence spectrometer. FT-IR spectra were
obtained on PerkineElmer Spectra One spectrophotometer. The
elemental analysis of all compounds was performed with a Per-
kineElmer 2400 analyzer for C, H, N, and S determination. Elec-
trochemical redox potentials were obtained by cyclic voltammetry
(CV) using a three-electrode cell and an electrochemistry work-
station (CHI830B, Chenhua Shanghai). The working electrode was
a Pt ring electrode; the auxiliary electrode was a Pt wire, and
saturated calomel electrode (SCE) was used as reference electrode.
Tetrabutylammonium hexafluorophosphate (Bu4NPF6) 0.1 M was
used as supporting electrolyte in dry acetonitrile. Ferrocene was
added to each sample solution at the end of the experiments, and
the ferrocenium/ferrocene (Fcþ/Fc) redox couple was used as an
internal potential reference. The potentials of dyes versus NHE
were calibrated by addition of 0.63 V to the potentials versus
Fcþ/Fc. The solutions were purged with argon and stirred for 15 min
before the measurements.
Expanding on the foregoing points and as part of our efforts to
explore novel metal-free dyes, we herein report on the design and
synthesis of a series of DepeA dyes (HT, HM, HE, and HO, as
2.3. General procedure for fabrication and characterization
of DSSCs
shown in Fig. 1) consisting of an N, N-diphenylhydrazone donor,
a 2-cyanoacetic acid acceptor, and a thiophene-derived bridge. The
effects of the p-conjugated bridges on the photophysical, electro-
chemical and photovoltaic properties of these novel dyes were
investigated in detail. We found that the photoelectronic proper-
ties, especially power conversion efficiency is quite sensitive to the
structural variations of the bridging thiophene moiety. Once again,
our results indicate that the slight change of p-conjugated bridge
would have a remarkable impact on structures and photoelectronic
properties of organic dyes.
Fluorine-doped SnO2 conducting glass (FTO) were cleaned and
immersed in aqueous 40 mM TiCl4 solution at 70 ꢀC for 30 min, then
washed with water and ethanol, sintered at 450 ꢀC for 30 min. the
TiO2 colloid was prepared from 12 g P25 (Degussa AG, Germany)
following a literature procedure [2]. First the 20e30 nm particles
sized TiO2 colloid was coated onto the above FTO glass by sliding glass
rod method to obtain a TiO2 film of 14 mm thickness after drying.
Subsequently, the 200 nm particles sized TiO2 colloid was coated on
the electrode by the same method, resulting in a TiO2 light-scattering
layer of 5 mm thickness. The double-layer TiO2-coated FTO glass were
sintered at 450 ꢀC for 30 min, then treated with TiCl4 solution and
calcined at 450 ꢀC for 30 min again. After cooling to 100 ꢀC, the TiO2
electrodes were soaked in a solution with 0.5 mM dyes and kept at
room temperature under dark for 24 h.
2. Experimental section
2.1. Materials and reagents
1,1-Diphenylhydrazine hydrochloride, 3,4-ethylendioxythiophene,
3,4-dimethoxythiophene, and thiophene-2-carbaldehyde (3d) were
purchased from commercial suppliers (Pacific ChemSource and Alfa
Aesar) in analytical grade. THF was dried and distilled over sodium/
benzophenone. DMF, CHCl3, and CH3CN were dried over by accus-
tomed methods and distilled before using. All other solvents and
chemicals used in this work were analytical grade and used without
further purification. All chromatographic separations were carried out
on silica gel (200e300 mesh).
The dye-adsorbed TiO2 electrode washed with ethanol and dried.
A drop of electrolyte solution was deposited onto the surface of the
electrode and a Pt foil counter electrode was clipped onto the top of
the TiO2 electrode to assemble a dye sensitized solar cells for-
photovoltaic performance measurements. The electrolyte consisted
of 0.5 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine (TBP) in
3-methoxypropionitrile and the efficient irradiated area of cell was
0.196 cm2. The photocurrentevoltage (JeV) characteristics of solar
cells were measured using a Keithley 2602 Source meter under the
100 Mw/cm2 irradiation of a 500 W Xe lamp with a global AM 1.5
filter for solar spectrum simulation. Meanwhile, the photoelectric
conversion efficiency was calculated. The measurement of mono-
chromatic incident photon-to-current conversion efficiencies (IPCE)
was performed by a Zolix DCS300PA Data acquisition system and
other optical system.
2.2. Analytical instruments
1H NMR spectra were recorded on a Bruker Avance 400
instrument. UVevis spectra of the dyes were measured on a Perkin-
Elmer Lamada 25 spectrometer. The PL spectra were obtained using
Fig. 1. Molecular structures of the dyes HT, HM, HE, and HO.

1044
P. Shen et al. / Dyes and Pigments 92 (2012) 1042e1051
2.4. Synthesis
column chromatography with petroleum ether/dichloromethane
(4/1, v/v) as eluent to yield compound 3.
The synthetic routes to the four dyes are shown in Scheme 1. The
detailed synthetic procedures are as follows.
2.4.2.1. 3,4-Dimethoxythiophene-2-carbaldehyde (3a). Yield: 63%
(2.17 g). FT-IR (KBr, cmꢁ1): 1655 (nC]O). 1H NMR (400 MHz, CDCl3,
2.4.1. 3,4-Dioctyloxythiophene (1)
d/ppm): 10.01 (s, 1 H), 6.64 (s, 1 H), 4.12 (s, 3 H), 3.87 (s, 3 H).
3,4-Dimethoxythiophene (3.00 g, 21 mmol), n-octyl alcohol
(11.9 g, 92 mol), and p-toluenesulfonic acid monohydrate (0.15 g)
were added into a round-bottom flask equipped with a water
knockout vessel. The mixture was stirred at 90 ꢀC for 24 h with
vacuum applied occasionally to remove side product methanol to
drive the reaction to completion. Then the mixture was poured
into a separatory funnel. The mixture was neutralized with dilute
HCl and extracted with petroleum ether. The combined organic
layer was washed with distilled water and dried over anhydrous
MgSO4, filtered, condensed by rotary evaporation. The crude
product was purified by silica gel column chromatography
(petroleum ether as eluent) to provide compound 1 (5.35 g, yield:
2.4.2.2. 3,4-Ethylendioxythiophene-2-carbaldehyde (3b). Yield: 74%
(2.52 g). FT-IR (KBr, cmꢁ1): 1649 (nC]O). 1H NMR (400 MHz, CDCl3,
d/ppm): 9.91 (s, 1 H), 6.80 (s, 1 H), 4.38e4.34 (m, 2 H), 4.31e4.27
(m, 2 H).
2.4.2.3. 3,4-Dioctyloxythiophene-2-carbaldehyde (3c). Yield: 69%
(5.09 g). FT-IR (KBr, cmꢁ1): 1664 (nC]O). 1H NMR (400 MHz, CDCl3,
d/ppm): 9.99 (s, 1 H), 6.61 (s, 1 H), 4.32 (t, 2 H), 3.97 (t, 2 H),
1.82e1.72 (m, 4 H), 1.45e1.29 (m, 20 H), 0.88 (t, 6 H).
2.4.3. The general synthetic procedure of (4)
75%). 1H NMR (CDCl3, 400 MHz,
d
/ppm): 6.16 (s, 2 H), 3.97 (t, 4 H),
In a 100 mL three-necked flask, a solution of compound 3
(5 mmol) in 30 mL of THF was added dropwise into the mixture of
N,N-diphenylhydrazine hydrochloride (1.54 g, 7 mmol), sodium
acetate (0.82 g, 10 mmol) and anhydrous MeOH (20 mL) under
nitrogen atmosphere. The mixture was refluxed for 24 h and the
color turned gradually from yellow to dark brown. After cooling, the
resulting mixture was quenched with 40 mL distilled water and
extracted with chloroform. The combined organic layer was
washed with distilled water and brine, dried over anhydrous
MgSO4. The solvent was removed by rotary evaporation. The crude
product was purified by silica gel column chromatography with
petroleum ether/dichloromethane (4/1, v/v) as eluent.
1.85e1.78 (m, 4 H), 1.43e1.28 (m, 20 H), 0.87 (t, 6 H).
2.4.2. The general synthetic procedure of (3)
In a 100 mL three-necked flask, compound 2 (20 mmol), 1,2-
dichloroethane (40 mL), dried DMF (4.65 mL, 60 mmol) and POCl3
(5.54 mL, 60 mmol) were added sequentially under nitrogen
atmosphere. The reaction mixture was stirred at room temperature
for 0.5 h and then heated up to reflux for another 4 h. After cooling
to room temperature, saturated aqueous solution of NaOAc (20 mL)
was added into the reaction mixture and stirred violently for 1 h.
Then the reaction mixture was extracted with chloroform. The
combined organic layer was washed with distilled H2O and brine,
dried over anhydrous MgSO4. Solvents were removed by rotary
evaporation, and the crude product was purified by silica gel
2.4.3.1. 3,4-Dimethoxythiophene-2-carbaldehyde-N,N-diphenylhy-
drazone (4a). Yield: 75% (1.27 g). 1H NMR (400 MHz, CDCl3,
Scheme 1. Synthesis of the HM, HE, HO, and HT dyes.

P. Shen et al. / Dyes and Pigments 92 (2012) 1042e1051
1045
d
/ppm): 7.43e7.39 (m, 4 H), 7.28e7.26 (m, 2 H), 7.25 (s, 1 H),
cooling, most of solvent was removed under vacuum and the
residue was dropped into a mixture of petroleum ether and HCl
(0.1 M) to form a precipitation. The crude product was further
purified by silica gel column chromatograph eluted with
dichloromethane/methanol (15/1, v/v) to obtain the dye.
7.23e720 (m, 4 H), 6.24 (s, 1 H), 4.12 (s, 3 H), 3.75 (s, 3 H). Elem.
Anal. Calcd. for C19H18N2O2S: C, 67.43%; H, 5.36%; N, 8.28%; S,
9.47%. Found: C, 67.40%; H, 5.17%; N, 8.20%; S, 9.56%.
2.4.3.2. 3,4-Ethylendioxythiophene-2-carbaldehyde-N,N-diphenylhy-
drazone (4b). Yield: 71% (1.19 g). 1H NMR (400 MHz, CDCl3,
d
/ppm):
2.4.5.1. 5-(2-Cyanoacrylic
acid-3-yl)-3,4-dimethyoxythiophene-2-
7.42e7.39 (m, 4 H), 7.25 (s, 1 H), 7.19e7.15 (m, 6 H), 6.25 (s, 1 H), 4.14
(m, 4 H). Elem. Anal. Calcd. for C19H16N2O2S: C, 67.84%; H, 4.79%; N,
8.33%; S, 9.53%. Found: C, 67.80%; H, 4.77%; N, 8.50%; S, 9.73%.
carbaldehyde-N,N-diphenylhydrazone (HM). Yield: 75% (0.39 g).
FT-IR (KBr, cmꢁ1): 2216 (nC^N), 1678 (nC]O). 1H NMR (400 MHz,
CDCl3, d/ppm): 8.44 (s, 1 H), 7.54 (s, 1 H), 7.50e7.42 (m, 6 H),
7.23e7.16 (m, 4 H), 4.05 (s, 3 H), 3.72 (s, 3 H). Elem. Anal. Calcd. for
C23H19N3O4S: C, 63.73%; H, 4.42%; N, 9.69%; S, 7.40%. Found: C,
64.00%; H, 4.37%; N, 9.70%; S, 7.53%.
2.4.3.3. 3,4-Dioctyloxythiophene-2-carbaldehyde-N,N-diphenylhy-
drazone (4c). Yield: 70% (1.87 g). 1H NMR (400 MHz, CDCl3,
d/ppm): 7.42e7.38 (m, 6 H), 7.34 (s,1 H), 7.17e7.15 (m, 4 H), 6.06 (s,1
H), 3.94e3.87 (m, 4 H), 1.77e1.74 (m, 4 H), 1.56e1.28 (m, 20 H), 0.88
(t, 6 H). Elem. Anal. Calcd. for C33H46N2O2S: C, 74.11%; H, 8.67%; N,
5.24%; S, 6.00%. Found: C, 74.10%; H, 8.37%; N, 5.27%; S, 6.15%.
2.4.5.2. 5-(2-Cyanoacrylic acid-3-yl)-3,4-ethylendioxythiophene-2-
carbaldehyde-N,N-diphenylhydrazone (HE). Yield: 70% (0.36 g).
FT-IR (KBr, cmꢁ1): 2214 (nC^N), 1682 (nC]O). 1H NMR (400 MHz,
CDCl3, d/ppm): 8.39 (s, 1 H), 7.54 (s, 1 H), 7.51e7.47 (m, 6 H),
2.4.3.4. Thiophene-2-carbaldehyde-N,N-diphenylhydrazone
7.23e7.17 (m, 4 H), 4.36e4.35 (m, 2 H), 4.23e4.21 (m, 2 H). Elem.
Anal. Calcd. for C23H17N3O4S: C, 64.03%; H, 3.97%; N, 9.74%; S, 7.43%.
Found: C, 64.10%; H, 4.03%; N, 9.70%; S, 7.48%.
(4d). Yield: 72% (1.00 g). 1H NMR (400 MHz, CDCl3,
d/ppm):
7.44e7.42 (m, 4 H), 7.31 (s, 1 H), 7.27 (s, 1 H), 7.22e7.20 (m, 6 H),
7.97e7.93 (m, 2 H). Elem. Anal. Calcd. for C17H14N2S: C, 73.35%; H,
5.07%; N, 10.06%; S, 11.52%. Found: C, 73.50%; H, 5.07%; N, 10.50%; S,
11.58%.
2.4.5.3. 5-(2-Cyanoacrylic
acid-3-yl)-3,4-dioctyloxythiophene-2-
carbaldehyde-N,N- diphenylhydrazone (HO). Yield: 72% (0.54 g).
FT-IR (KBr, cmꢁ1): 2215 (nC^N), 1680 (nC]O). 1H NMR (400 MHz,
2.4.4. The general synthetic procedure of (5)
CDCl3,
7.21e7.19 (d, 4 H), 4.18 (t, 2 H), 3.80 (t, 2 H), 1.74e1.71 (m, 4 H),
1.63e1.24 (m, 20 H), 0.90 (t, H). Elem. Anal. Calcd. for
C37H47N3O4S: C, 70.56%; H, 7.52%; N, 6.67%; S, 5.09%. Found: C,
70.80%; H, 7.57%; N, 6.70%; S, 5.23%.
d/ppm): 8.43 (s, 1 H), 7.47e7.43 (m, 6 H), 7.24 (s, 1 H),
Compound 5 was synthesized according to the same procedure
as that of compound 3, except that compound 4 (0.68 g, 2 mmol)
was used instead of compound 2.
6
2.4.4.1. 5-Formyl-3,4-dimethyoxythiophene-2-carbaldehyde-N,N-
diphenylhydrazone (5a). Yield: 84% (0.62 g). FT-IR (KBr, cmꢁ1):
2.4.5.4. 5-(2-Cyanoacrylicacid-3-yl)-thiophene-2-carbaldehyde-N,N-
1664 (nC¼O). 1H NMR (400 MHz, CDCl3,
d
/ppm): 10.00 (s, 1 H),
diphenylhydrazone (HT). Yield: 74% (0.33 g). FT-IR (KBr, cmꢁ1):
7.48e7.44 (m, 4 H), 7.28e7.26 (m, 2 H), 7.25 (s, 1 H), 7.21e7.19 (m, 4
H), 4.10 (s, 3 H), 3.73 (s, 3 H). Elem. Anal. Calcd. for C20H18N2O3S: C,
65.55%; H, 4.95%; N, 7.64%; S, 8.75%. Found: C, 65.80%; H, 4.97%; N,
7.60%; S, 8.83%.
2213 (nC^N), 1673 (nC]O). 1H NMR (400 MHz, DMSO-d6,
d/ppm):
8.36 (s, 1 H), 7.82 (d, 1 H), 7.52 (s, 1 H), 7.50 (d, 1 H), 7.37e7.26 (m, 6
H), 7.19e7.17 (m, 4 H). Elem. Anal. Calcd. for C21H15N3O2S: C, 67.54%;
H, 4.05%; N, 11.25%; S, 8.59%. Found: C, 67.58%; H, 4.07%; N, 11.60%;
S, 8.63%.
2.4.4.2. 5-Formyl-3,4-ethylendioxythiophene-2-carbaldehyde-N,N-
diphenylhydrazone (5b). Yield: 82% (0.60 g). FT-IR (KBr, cmꢁ1):
3. Results and discussion
1666 (nC]O). 1H NMR (400 MHz, CDCl3,
d/ppm): 9.89 (s, 1 H),
7.47e7.43 (m, 4 H), 7.28e7.26 (m, 2 H), 7.25 (s, 1 H), 7.23e7.19 (m, 4
H), 4.34e4.33 (m, 2 H), 4.23e4.22 (m, 2 H). Elem. Anal. Calcd. for
C20H16N2O3S: C, 65.92%; H, 4.43%; N, 7.69%; S, 8.80%. Found: C,
65.86%; H, 4.47%; N, 7.70%; S, 8.85%.
3.1. Synthesis and characterization of sensitizers
The synthesis of the four hydrazone-based organic dyes HM, HE,
HO, and HT is outlined in Scheme 1. The four dyes were synthesized
by the similar stepwise synthetic protocol. First, 2-thienaldehyde or
its derivatives (3aec) reacted with N, N-diphenylhydrazine
hydrochloride to afford thiophene-based hydrazone 4aed accord-
ing to the previous reports [20] in high yields. Then Vilsmeier for-
mylation of 4aed afforded the key intermediates 5aed with good
yields (82e85%). Finally, Knoevenagel condensation reactions of
aldehyde derivatives 5aed with two-fold excess of cyanoacetic acid
afforded the four target dyes in acetonitrile using piperidine as
catalyst. The structures of all important new compounds were
characterized unambiguously with FT-IR, 1H NMR spectroscopy and
elemental analyze.
2.4.4.3. 5-Formyl-3,4-dioctyloxythiophene-2-carbaldehyde-N,N-
diphenylhydrazone (5c). Yield: 83% (0.93 g). FT-IR (KBr, cmꢁ1): 1665
(
nC]O). 1H NMR (400 MHz, CDCl3,
d/ppm): 9.95 (s, 1 H), 7.45e7.41
(m, 6 H), 7.24 (s, 1 H), 7.19 (d, 4 H), 4.24e4.20 (t, 2 H), 3.84e3.81
(t, 2 H), 1.76e1.73 (m, 4 H), 1.50e1.24 (m, 20 H), 0.90 (t, 6 H). Elem.
Anal. Calcd. for C34H46N2O3S: C, 72.56%; H, 8.24%; N, 4.98%; S, 5.70%.
Found: C, 72.60%; H, 8.26%; N, 4.80%; S, 5.90%.
2.4.4.4. 5-Formyl-thiophene-2-carbaldehyde-N,N-diphenylhydrazone
(5d). Yield: 85% (0.52 g). FT-IR (KBr, cmꢁ1): 1664 (nC]O). 1H NMR
(400 MHz, CDCl3, d/ppm): 9.86 (s, 1 H), 7.63 (d, 1 H), 7.62 (s, 1H), 7.48
(d, 1 H), 7.47e7.45 (m, 4 H), 7.27e7.21 (m, 2 H), 6.99e6.98 (m, 4 H).
Elem. Anal. Calcd. for C18H14N2OS: C, 70.56%; H, 4.61%; N, 9.14%;
S, 10.47%. Found: C, 70.50%; H, 4.67%; N, 9.16%; S, 10.53%.
3.2. Photophysical properties
The ultravioletevisible (UVevis) absorption spectra of the four
dyes in diluted THF solutions are shown in Fig. 2, and the corre-
sponding data are collected in Table 1. All the dyes exhibit a strong
absorption maximum (lmax) in the visible region (460e490 nm)
corresponding to the intramolecular charge transfer (ICT) between
the N, N-diphenylhydrazone donating unit and the cyanoacrylic
2.4.5. The general synthetic procedure of sensitizers
Under nitrogen atmosphere,
a mixture of compound 5
(1.2 mmol), 2-cyanoacetic acid (0.21 g, 2.4 mmol), piperidine
(0.5 mL), and CH3CN (30 mL) was refluxed at 80 ꢀC for 24 h. After

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P. Shen et al. / Dyes and Pigments 92 (2012) 1042e1051
As depicted in Table 1, the molar extinction coefficient ( ) of the
3
ICT bands of the four dyes (18.3e29.7 ꢂ 103 Mꢁ1 cmꢁ1) are higher
than that of the standard N719 sensitizer (14.0 ꢂ 103 Mꢁ1 cmꢁ1) [2],
indicating a better ability of light harvesting of these new N,
N-diphenylhydrazone-based metal-free organic dyes than that of
classical organometallic dye. Moreover, the
increases in the order of HT < HM < HE < HO, which suggests that
the enhancing of electron-donating ability of the -conjuagted
3
of the ICT bands
p
bridge in a dye significantly increases the light harvesting efficiency.
When the dyes were attached on TiO2 films, the absorption
spectra may shift more or less as compared with those in solutions
because of strong interactions between the dyes and the semi-
conductor surface, which can lead to form aggregates of the dyes on
semiconductor surface, such as H-aggregation for the blue shift or
J-aggregation for the red shift. Fig. 4 shows the absorption spectra
of the four dyes adsorbed on the TiO2 films, and the corresponding
data are collected in Table 1. We can see that absorption spectra of
the four dyes on TiO2 films display a slight blue shift of 3 nm (HM)
and 15 nm (HE), or a small red shift of 4 nm (HT) and 16 nm (HO),
compared to those in THF solutions. The slight red or blue shift may
Fig. 2. Absorption spectra of the four dyes in THF solutions (10ꢁ5 M) at 25 ꢀC.
be due to the reduction of intermolecular
be concluded that the stack was broken up completely, but it is
reasonable to presume that the intermolecular interaction is
pep interaction. It cannot
acid acceptor moiety [39] and a relatively weak peak in the near-UV
region (290e315 nm) due to the * electron transitions of the
pep
pep
pep
conjugated molecules. It can be seen that the lmax in the visible
region red-shifts from 465 nm to 479 nm, 483 nm, and 490 nm for
HT, HM, HO and HE, respectively. This red-shifted phenomenon of
lmax in the visible region can be ascribed to the different electron-
donating ability of the thiophene-derived bridges. Obviously, 3,4-
dialkoxyl substituted thiophene bridges (HM, HE, and HO) have
more strong electron-donating ability than non-substituted thio-
phene bridge (HT), which enhances the ICT between the N,
N-diphenylhydrazone donating unit and the cyanoacrylic acid
acceptor moiety, and as a result to shift the lmax to the long
wavelength. For three 3,4-dialkoxyl substituted thiophene bridged
dyes (HM, HE, and HO), we intend to understand the change
regularity of their lmax from the molecular modeling studies (Fig. 3)
which is a common method to be widely used in other dyes
[15,22,23]. The ground-state structures of HT, HM, HE, and HO
have almost planar geometry with small twist angles of 0.012ꢀ,
1.32ꢀ, e1.60ꢀ and 1.62ꢀ, respectively, between N]C double bond
and the adjacent thiophene-derived bridge. This result shows that
the twist angles for these dyes are very similar which indicates that
it is impossible to use the plananity of dye molecules to explain the
change regularity of their lmax. Moreover, the result also shows that
hydrazone-based dyes have more coplanar geometry compared
with tripheneylamine-based analogues [14] which benefits ICT
between the donor and the acceptor moiety, thus red-shifting the
lmax in the visible region even though the influence of difference
solvents is taken into consideration. Therefore, the red-shifts of
lmax for HO and HE compared to HM may attribute to more
strong electron-donating ability of 3,4-ethylendioxythiophene and
3,4-dioctyloxythiophene bridges than 3,4-dimethyloxylthiophene
bridge.
weakened and the dye aggregation is inhibit effectively. This
implies that these N, N-diphenylhydrazone-based dyes have the
relatively strong ability to inhibit possible intermolecular aggre-
gation and retard the electron transfer from TiO2 to the oxidized
dye or electrolyte, thus possibly enhancing open-circuit voltage
(Voc) [40,41]. It is noteworthy that the absorption spectra of those
adsorbed on TiO2 films show a markedly broad profile, which is
beneficial to light-harvesting.
3.3. Electrochemical properties
To evaluate the possibility of electron transfer from the excited
dye molecule to the conductive band of TiO2 and the dye regener-
ation, cyclic voltammetry (CV) method was employed in acetoni-
trile solution, using 0.1 M Bu4NPF6 as a supporting electrolyte
(Fig. 5), and the data are summarized in Table 2.
The first oxidation potential vs. NHE (Eox) corresponding to the
highest occupied molecular orbital (HOMO vs. NHE), was calibrated
by addition of 0.63 V to the potential (vs. SCE) vs. Fc/Fcþ by CV
(Table 2). The Eox of the four dyes HT, HM, HE, and HO were
measured as around 1.30 V, indicating no obvious effect on the Eox
for different the thiophene-derivatived bridges. We also can see
that all the HOMO levels of four dyes are more positive than the
iodide/triiodide redox couple (w0.42 V vs. NHE) [41], ensuring that
there is enough driving force for efficient regeneration of the dye
through the recapture of the injected electrons by the dye cation
radical.
By neglecting any entropy change during light absorption, the
reduction potential vs. NHE (Ered), which corresponds to the lowest
unoccupied molecular orbital (LUMO vs. NHE), can be obtained
from Eox and E0e0 value determined from the intersection of
absorption and emission spectra, namely, EoxeE0e0. The calculated
LUMO levels of HT, HM, HE, and HO were ꢁ1.09, ꢁ1.06, ꢁ1.05,
and ꢁ1.05 V vs. NHE (Table 2), respectively, which all are suffi-
ciently higher than that of the conduction band (CB) level of TiO2
(ꢁ0.5 V vs. NHE). Provided that energy gap (Egap) between LUMO
levels and CB level of TiO2 of 0.2 eV is necessary for efficient elec-
tron injection [28], these driving forces are sufficiently large for
effective electron injection. Noticeably, the slight positive shift of
the reduction potentials in HM, HE, and HO compared to that of HT
implies that the electron-donating 3,4-dialkyoxlythiophene
bridges can heighten the LUMO levels of the dyes.
Table 1
Maximum absorption and emission data of the four dyes.
Dye
lmaxa/nm (
3
c/Mꢁ1 cmꢁ1
)
lmaxb/nm
HT
465(18 300)
479(23 300)
490(28 000)
483(29 700)
295(8500)
469
476
475
496
HM
HE
HO
309(16 000)
313(16 600)
312(19 300)
a
Maximum absorption in THF solution (10ꢁ5 M).
Maximum absorption on TiO2 film.
b
c
3
is the molar extinction coefficient at lmax of maximum absorption.

P. Shen et al. / Dyes and Pigments 92 (2012) 1042e1051
1047
Fig. 3. Optimized structures calculated by TD-DFT using the B3LYP functional and the 3-21G* bases set for the four dyes.
These above-discussed results indicate that the photo-
electrochemical properties of this series dyes could easily be tune
by changing the thiophene-derived bridges.
photon-to-electron conversion efficiency (IPCE) as a function of the
wavelength for the sandwiched DSSCs based on the four dyes as
sensitizers. The IPCEs action spectra for DSSCs based on HO and HE
are broader than those of HT and HM, which are in accordance with
their absorption spectra on the transparent TiO2 films (Fig. 3). As
shown in Fig. 6, the IPCEs of HO and HT sensitizers exceed 50% in
the spectral region from 390 to 520 nm, reaching their maximum
over 60%. The relatively higher maximum IPCE for HO compared
with other three dyes is well in agreement with the tendency of
UVevis absorption spectra on TiO2 films (Fig. 3). These results
imply that HO and HT would show a relatively large photocurrent
in DSSCs. Additionally, it should be noted that the four dyes still
show weak IPCE response at the range of long wavelength region
(650e750 nm), as shown in Fig. 6. We think this feature should be
ascribed two reasons: one is the weak absorption band of the dyes
on TiO2 films at the range of 650e750 nm (see Fig. 4), which is
consistent with the previous reports [42,43]; the other is the effect
of light scattering by TiO2 nanoparticles [44], which increases the
photocurrents for the weak absorption in that region.
3.4. Molecular orbital calculations
To gain an insight into the molecular structure and electron
distribution, density functional theory (DFT) calculations were
performed on the four dyes using the Gaussian 03 program package.
In particular we used B3LYP as exchange-correlation functional and
3-21G* as basis set. The electron distribution of the HOMO and
LUMO of the dyes is shown in Table 3. It is obvious that the electron
density of HOMO is localized mainly on the N, N-diphenylhydrazone
moiety and is extended along the p-conjugated bridge to the central
region of the molecule. On the other hand, the LUMO is localized
over the cyanoacrylic unit through aromatic bridges. We notice that
the HOMOeLUMO excitation induced by light irradiation could
move the electron distribution from the N, N-diphenylhydrazone
unit to the cyanoacrylic acid moiety via p-conjugated thiophene-
Fig. 7 shows current densityevoltage (JeV) characteristics of
devices based on the four dyes. The detailed photovoltaic param-
eters of the short-circuit photocurrent density (Jsc), open-circuit
photovoltage (Voc), fill factor (FF), and power conversion effi-
ciency (PCE) are listed in Table 4. According to Fig. 7, it is clear that
the photovoltaic performances of the DSSCs can be evidently
affected by the conjugated thiophene bridges in the dye molecules.
The PCE increases in the order of HE (4.20%) < HM (4.70%) < HT
(5.29%) < HO (5.83%). HO exhibits the best PCE due to both the
derived bridges and the photo-induced electron transfer from the
dyes to the TiO2 electrode can occur efficiently by the HOMOeLUMO
transition.
3.5. Photovoltaic performance of DSSCs based on the four dyes
DSSCs were fabricated using these dyes as the sensitizers, with
an effective area of 0.196 cm2, TiO2 particles on FTO, and the elec-
trolyte composed of 0.5 M LiI, 0.05 M I2, and 0.5 M 4-tert-butyl-
pyridine (TBP) in 3-methoxy-propionitrile. Fig. 6 shows incident
Fig. 5. Cyclic voltammograms of the three dyes: working electrode, Pt ring; auxiliary
Fig. 4. Normalized absorption spectra of the four dyes adsorbed on TiO2 surface.
electrode, Pt wire; reference electrode, Hg/Hg2Cl2; scanning rate is 100 mV/s.

1048
P. Shen et al. / Dyes and Pigments 92 (2012) 1042e1051
Table 2
better IPCE spectrum. In addition, consistent with the increased
Electrochemical data of the four dyes.a
tendency of IPCE, the measured Jsc increases in the order of HM
(10.20 mA/cm2) < HE (10.44 mA/cm2) < HT (11.04 mA/cm2) < HO
(12.69 mA/cm2). The improved Voc values of HM (0.66 V) and HO
(0.65 V) compared with HT (0.64 V) and HE (0.62 V) could be
attributed to the effective retardation of charge recombination
between the injected electrons and Ie3 ions in the electrolyte, which
resulted from the hydrophobic dialkyloxyl-substitutes in the dyes
HM and HO [14,15,27,45]. We will discuss it in detail as the
following electrochemical impedance spectroscopy studies.
Dye
lint/nm
E0e0/eV
Eox/V vs. NHE
Ered/V vs. NHE
Egap/V
HT
520
525
529
526
2.38
2.36
2.34
2.36
1.29
1.30
1.29
1.31
ꢁ1.09
ꢁ1.06
ꢁ1.05
ꢁ1.05
0.59
0.56
0.55
0.55
HM
HE
HO
a
E0e0 values were calculated from intersection of the normalized absorption and
the emission spectra (lint): E0e0 ¼ 1240/lint. The first oxidation potential (vs. NHE),
Eox was measured in acetonitrile and calibrated by addition of 0.63 V to the potential
versus Fc/Fcþ. The reduction potential, Ered, was calculated from EoxeE0e0. Egap is the
energy gap between the Ered of dye and the CB level of TiO2 (ꢁ0.5 V vs. NHE).
3.6. Electrochemical impedance spectroscopy studies
highest Jsc (12.69 mA/cm2) and Voc (0.65 V) values. The increased Jsc
value of HO compared with other three dyes mainly derived from
its relatively better light harvesting ability (broad absorption
spectrum and high molar extinction coefficient), reflecting in its
Electrochemical impedance spectroscopy (EIS) is a powerful
technique of characterizing the important interfacial charge
transfer processes in a DSSC. The EIS was measured in the dark to
Table 3
Frontier molecular orbitals of the HOMO and LUMO calculated with DFT on a B3LYP/3-21G* level of the four dyes.
Dye
HOMO
LUMO
HT
HM
HE
HO

P. Shen et al. / Dyes and Pigments 92 (2012) 1042e1051
1049
Table 4
Photovoltaic performance of DSSCs based on the four dyes.
Dye
Jsc (mA/cm2)
Voc (V)
FF
PCE (%)
HT
11.04
10.20
10.44
12.69
0.64
0.66
0.62
0.65
0.749
0.698
0.649
0.707
5.29
4.70
4.20
5.83
HM
HE
HO
Fig. 6. IPCE plots for the DSSCs based on the four dyes.
elucidate correlation of Voc with those dyes. The Nyquist plots of
DSSCs with four dyes are shown in Fig. 8. Three semicircles are
observed in the Nyquist plots. The small and large semicircles
respectively located in the high- and middle-frequency regions, are
assigned to the charge transfer at Pt/electrolyte and TiO2/dye/
electrolyte interface, respectively [45e48]. Another small semi-
circle, which should have appeared at the low-frequency region, is
overlapped by the middle-frequency large semicircle. The charge
recombination resistance (Rrec) at the TiO2 surface can be deduced
by fitting curves from the range of the middle-frequency using
a Z-view software. Rrec is related to the charge recombination rate
between injected electron and electron acceptor (Iꢁ3 ) in the elec-
trolyte, estimated by the large semicircle width. A large Rrec means
the small charge recombination rate and vice versa. The Rrec values
for HM and HO were estimated to be 133 and 115 ohm, respectively.
In contrast, HE and HT were found to have relatively smaller Rrec
values at 66 and 57 ohm, respectively. The result appears to be
consistent with the larger Voc values for the dyes HM (0.66 V) and
Fig. 8. EIS Nyquist plots for DSSCs based on the four dyes measured in the dark
under ꢁ0.65 V bias.
HO (0.65 V). The significantly increased Rrec values of HM and HO
imply the retardation of the charge recombination between injec-
ted electron and Iꢁ3 ions in the electrolyte, with a consequent
increase of Voc
.
At the same time, DSSCs based on HM and HO produced higher
Voc than HE and HT, which can be explained by the electron life-
time. Fig. 9 shows the Bode plot for DSSCs based on these four dyes.
Two peaks in Fig. 9 located at the high-frequency (right) and
middle-frequency (left) respectively correspond to the small
semicircle (left) and large semicircle (right) in the Nyquist plots
Fig. 9. EIS Bode plots for DSSCs based on the four dyes measured in the dark
under ꢁ0.65 V bias.
Fig. 7. The JeV characteristics for DSSCs based on the four dyes.

1050
P. Shen et al. / Dyes and Pigments 92 (2012) 1042e1051
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4. Conclusions
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In summary, we have successfully designed and synthesized
a series of novel De
the same electron donor and acceptor groups and different
-conjugated thiophene bridges to apply in DSSCs. The influences
peA hydrazone dyes (HT, HM, HE, and HO) with
p
of the various thiophene bridges on the photophysical, electro-
chemical, and photovoltaic properties of these hydrazone sensi-
tizers were investigated. The results of calculation and experiments
clearly demonstrate that photoelectrochemical properties can be
regulated by introducing different p-conjugated thiophene bridges.
Among the four synthesized dye molecules, hydrazone dye HO
exhibits the maximum power conversion efficiency of 5.83%.
Therefore, we have proved that N, N-diphenylhydrazone and
3,4-dialkyloxyl substituted thiophene units can be employed as the
[22] Zhang G, Bala H, Cheng Y, Shi D, Lv X, Yu Q, et al. High efficiency and stable
dye-sensitized solar cells with an organic chromophore featuring a binary
effective and promising donor and
p-conjugated bridge, respec-
p-conjugated spacer. Chem Commun; 2009:2198e200.
tively in De eA dyes, which not only expands the selection scope
of building blocks for further dye design but also provides a reve-
lation to exploit highly efficient hydrazone sensitizers for DSSCs.
p
[23] Zeng W, Cao Y, Bai Y, Wang Y, Shi Y, Zhang M, et al. Efficient dye-sensitized
solar cells with an organic photosensitizer featuring orderly conjugated
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Acknowledgements
[25] Choi H, Lee JK, Song KH, Song K, Kang SO, Ko J. Synthesis of new julolidine
dyes having bithiophene derivatives for solar cell. Tetrahedron 2007;63:
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This work was supported by the General Program of the
Education Department of Hunan Province (No. 09C946), National
Natural Science Foundation of China (No. 50973092, 21004050,
51003089), and National Natural Science Foundation of Hunan
Province (No. 10JJ4007).
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2011;21:10666e71.
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ylthiophene moiety for high performance dye-sensitized solar cells. Dyes
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