Synthesis and properties of anthrylene-substituted phenyleneethynylene dyes having amino/cyano group(s) and their application to dye-sensitized solar cells

Article abstract of DOI:10.1246/bcsj.20110390

A series of anthrylene-substituted phenyleneethynylene dyes having amino/cyano group(s) were prepared by repeating Sonogashira coupling between aryl halides and terminal acetylenes. In UVvis absorption spectra, the 9,10-anthrylene-substituted dyes showed a strong absorption band of which λmax was located from 502 to 521 nm and ε was more than 5.2 × 10⁴L mol-¹ cm-¹ indicating a highly expanded π-system in comparison with 1,5-anthrylene dye (λmax = 431 nm, ε= 2.6 × 10⁴L mol-¹ cm-¹). LippertMataga plots demonstrated that the amino- and cyano-substituted dyes underwent larger charge separation in the excited states. These acetylenic dyes served as sensitizers of dye-sensitized solar cell to achieve 5.0% of photo-to-current energy conversion efficiency.

Full text of DOI:10.1246/bcsj.20110390

© 2012 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 85, No. 6, 687-697 (2012)
687
Selected Papers
Synthesis and Properties of Anthrylene-Substituted Phenyleneethynylene
Dyes Having Amino/Cyano Group(s) and Their Application
to Dye-Sensitized Solar Cells
Xin Yang,1 Shingo Kajiyama,2 Jing-Kun Fang,1 Feng Xu,1 Yu Uemura,2,3
Nagatoshi Koumura,*2,3 Kohjiro Hara,3 Akihiro Orita,*1 and Junzo Otera*1
1Department of Applied Chemistry, Okayama University of Science,
Kita-ku, Okayama 700-0005
2Graduate School of Pure and Applied Sciences, University of Tsukuba,
1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571
3Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology,
1-1-1 Higashi, Tsukuba, Ibaraki 305-8565
Received December 14, 2011; E-mail: orita@high.ous.ac.jp
A series of anthrylene-substituted phenyleneethynylene dyes having amino/cyano group(s) were prepared by
repeating Sonogashira coupling between aryl halides and terminal acetylenes. In UV-vis absorption spectra, the 9,10-
anthrylene-substituted dyes showed a strong absorption band of which -max was located from 502 to 521 nm and ¾ was
¹1
more than 5.2 © 104 L mol¹1 cm indicating a highly expanded ³-system in comparison with 1,5-anthrylene dye
(-max = 431 nm, ¾ = 2.6 © 104 L mol¹1 cm¹1). Lippert-Mataga plots demonstrated that the amino- and cyano-substituted
dyes underwent larger charge separation in the excited states. These acetylenic dyes served as sensitizers of dye-sensitized
solar cell to achieve 5.0% of photo-to-current energy conversion efficiency.
Photovoltaics have attracted great attention as a sustainable
energy resource to overcome increasing energy demand.1 Since
Grätzel achieved high performance in converting solar energy
to electricity by invoking ruthenium bipyridyl complex,2 dye-
Ph2N
COOH
sensitized solar cells (DSSCs) based on Ru(II) complexes have
been extensively investigated to approach 12% solar energy
conversion efficiency (©) so far.3 On the other hand, a number
of metal-free organic dyes have been developed for fabrication
of DSSCs because these dyes have many advantages such as
low cost, high molar extinction coefficient (¾), and facile
modification of their structures. Coumarins,4 merocyanines,5
polyenes,6 oligothiophenes,7 indolines,8 and porphyrins9 have
been synthesized. Recently, it has been reported that the DSSCs
fabricated by acetylenic sensitizers indicate high electron
transfer yield Φ(-)ET giving rise to a higher plateau in incident
photon-to-current conversion efficiency (IPCE) spectra than
those fabricated by the corresponding olefinic sensitizers.10 It
has been revealed that introducing 9,10-anthrylene group to
an acetylenic sensitizer can suppress charge recombination of
NC
TC203
Chart 1.
at 464 nm in CH2Cl2. We envisioned that if -max of ethynyl-
anthracene sensitizer could be red-shifted to more than 500 nm
with high IPCE being kept, higher-performance DSSC would
be achieved. In order to realize such acetylenic sensitizer, we
designed a series of sensitizers 1-6 on the basis of the
following premise: (i) Ph2N and/or CN groups facilitate photo-
induced charge separation, (ii) anthrylene-substituted phenyl-
eneethynylene array achieves absorption maximum (-max) at
more than 500 nm and high molar extinction coefficient (¾)
which enable taking advantage of solar light efficiently, and
(iii) substitution with branched long alkoxy groups ((«)-
3,7-dimethyloctyloxy (DMOO)) improves solubility of the sen-
sitizers. We have been involved in synthesis of functionalized
phenyleneethynylenes by combination of Sonogashira coupling
and double elimination protocol of ¢-substituted sulfone12 and
already reported preliminary results of synthesis of acetylenic
¹
the injected electrons in TiO2 with the I3 ion in electrolyte
resulting in the increase of Voc in their DSSCs.11 For instance, a
DSSC device which was fabricated by use of ethynylanthra-
cene sensitizer TC203 (Chart 1) achieved high performance
(© = 6.78). Although this acetylenic sensitizer TC203 reached
more than 85% plateau in IPCE, the IPCE decreased remarka-
bly in a longer range than 600 nm because TC203 had -max
Published on the web May 30, 2012; doi:10.1246/bcsj.20110390

688
Bull. Chem. Soc. Jpn. Vol. 85, No. 6 (2012)
Synthesis and Properties of Phenyleneethynylene Dyes
sensitizers 1-3 and their application to DSSCs.13 We describe
herein synthetic procedures of 1-6 and their physical properties
such as UV-vis absorption, photoluminescence, and cyclic
voltammetry as well as the performance of DSSCs fabricated
by use of sensitizers 1-5 and substituent effect on their
performance.
silylacetylene (TMSA) followed by desilylation. Subjection
of 19 to coupling with 21 and 9 followed by carboxy group
deprotection provided the desired dye 5. When 1,5-dibromo-
anthracene (22) and 23 were used as starting compounds, dye 6
was obtained after repeating Sonogashira coupling and depro-
tection of terminal acetylenes (Scheme 5).
Results and Discussion
ODMO
Synthesis of Acetylenic Dyes 1-6.
For synthesis of
Ph2N
acetylenic dyes 1-6 (Chart 2), we utilized Sonogashira cou-
pling of aryl halide with terminal ethyne successively
(Schemes 1-5). For synthesis of dyes 1-3, we planned to
employ a convergent synthesis; Sonogashira coupling of 7/8
with 9/10 would afford the target dyes as silylethyl ester-
protected form (Scheme 1). A carboxy group which served
as an anchoring point for TiO2 nanoparticles, was attached at
the 3-position of terminal benzene because 4-substituted com-
pounds showed lower solubility than the 3-substituted deriv-
atives. The bromides 7 and 8 were prepared by using com-
mercially available 4-(diphenylamino)phenyl bromide and
iodobenzene as starting compounds, respectively, while the
terminal ethynes 9 and 10 were from 3-iodobenzoic acid.
Synthesis of 1 is shown representatively in Scheme 2: the
dye 1 was obtained by Sonogashira coupling between 7 and 9
followed by desilylation of carboxylic acid. Dyes 2 and 3 were
synthesized by the same procedure using 7 + 10 and 8 + 9,
respectively. Dye 4 was synthesized from 11 and 12 through
bromide 13 and terminal ethyne 14 as shown in Scheme 3. Dye
4 showed poor solubility in common solvents, and its 13C NMR
spectrum was not recorded with sufficient resolution. We have
already reported that Ph2P(O) serves as a protecting group of
terminal ethyne and its high polarity enables easy separation
of the Ph2P(O)-protected products from by-products.14 This
Ph2P(O)-protection protocol served well for preparation of 15
from 1,3,5-triethynylbenzene (16); successive transformation
such as monoprotection of ethyne in 16, Sonogashira coupling
of 17 with 1-iodo-4-diphenylaminobenzene and t-BuOK-
catalyzed dephosphorylation in 18 afforded 15 in 20% yield
from 16 (Scheme 4). Terminal ethyne 15 was transformed to
19 by successive Sonogashira coupling with 20 and trimethyl-
DMOO
NC
COOH
1
ODMO
Ph2N
DMOO
DMOO
COOH
COOH
2
ODMO
NC
3
ODMO
Ph2N
DMOO
4
NC
COOH
Ph2N
ODMO
DMOO
NC
COOH
5
Ph2N
Ph2N
ODMO
DMOO
6
NC
COOH
DMOO:
O
Chart 2.
ODMO
Ph2N
Br
9
DMOO
DMOO
7
NC
COOCH2CH2TMS
COOCH2CH2TMS
+
ODMO
10
Br
8
1 (from 7+9)
2 (from 7+10)
3 (from 8+9)
Scheme 1. Convergent synthesis of 1-3.

X. Yang et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 6 (2012)
689
ODMO
9
NC
COOCH2CH2TMS
TBAF
[Pd(PPh3)4], CuI
Br
Ph2N
1
THF
rt, 2 h
toluene, i-Pr2NH
80 °C, overnight
DMOO
7
68%
57%
Scheme 2. Syhthesis of 1.
I
Br
12
ODMO
NC
[Pd(PPh3)4], CuI
Ph2N
toluene, i-Pr2NH
rt, 12 h
11
DMOO
39%
ODMO
TMS
[Pd(PPh3)4], CuI K2CO3
Ph2N
Br
toluene, i-Pr2NH THF, MeOH
80 °C, 20 h
91%
rt, 2 h
87%
DMOO
NC
13
I
ODMO
COOH
[Pd(PPh3)4], CuI
Ph2N
4
toluene, i-Pr2NH
50 °C, 20 h
77%
DMOO
NC
14
Scheme 3. Synthesis of 4.
UV-vis Absorption and Cyclic Voltammetry of 1-6 in
Solution. We recorded UV-vis absorption of the dyes 1-6
instead of DMOO in 1 were subjected to structural optimiza-
tion by using B3LYP/6-31G(d) basis set (Scheme 6), and
distribution ratio of the four conformers 1a-1d were deter-
mined based on difference of heat of formation (¦¦H) as
shown in Table 2. This indicates that planar-planar conformer
1a would be the most favorable in this series of phenyl-
eneethynylene conjugated systems. TDDFT calculations (LC-
BLYP/6-31G(d)) for planar-planar conformers of 1-6 provid-
ed their plausible excitation wavelengths and the oscillator
strengths which correspond to 1st excitation energies (Table 1).
These calculated wavelengths of -max which correspond to
HOMO-LUMO transition are consistent with those observed
in toluene.
¹1
(1.0 © 10¹4 mol L in toluene, Figure 1) in order to inves-
tigate their optical properties as summarized in Table 1. Dyes
¹1
1-5 show -max (¾) at 502-521 nm (>5.2 © 104 L mol¹1 cm
)
indicating that introducing 9,10-anthrylene in a phenylene-
ethynylene array gives rise to red shift of absorption maximum
(-max) with high molar extinction coefficient (¾).15 In sharp
contrast to this result, dye 6 with 1,5-anthrylene array shows
inefficient expansion of ³-system providing -max at 431 nm
with a rather small ¾ value (2.6 © 104 L mol¹1 cm¹1).
Substitution with cyano group provides bathochromic shift
of -max in 1 and 3 in comparison with 2, and amino- and cyano-
substituted dye 1 exhibits a large molar extinction coefficient
(7.2 © 104 L mol¹1 cm¹1). Dye 4 with an expanded ³-system
demonstrated a hypochromic shift in -max and decrement of
¾ in comparison with 1 showing that Ph2N and CN groups
which were located at a distance displayed no cooperative
electronic effect such as bathochromic shift and increment of
¾ which were observed in 1. UV-vis absorption profile of
dye 5 is similar to that of dye 3, and this result is diagnostic
of inefficient conjugation in the meta-substituted phenylene-
ethynylene ³-system.
Cyclic voltammograms were recorded for dyes 1-6 in order
to investigate their electrochemical properties in CH2Cl2 by
using Ag/AgNO3 as a reference electrode. Half-wave poten-
tials of reversible oxidation and reduction for 1-6 are shown
in Table 3. Substitution with Ph2N enables facile oxidation of
the dyes 1, 2, and 4-6, and the dyes 1, 2, 4, and 6 undergo
oxidation at less than 0.73 V. Reduction potentials are ob-
served in all the dyes, and 1, 3, and 5 undergo facile reduction
red
(E1/2 ½ ¹1.46 V). The ionization potentials (EHOMO) and
electron affinity (ELUMO) of 1-6 are estimated to be 4.4 V
higher than their oxidation and reduction potentials, respec-
tively, which were referred to a saturated calomel electrode
(SCE).16 Since the potential of Ag/AgNO3, which is used as
Ab initio calculations were carried out for 1-6 in order to
get further insight in their conformation in the ground states.
First, four conformers 1a-1d which had methoxy groups

690
Bull. Chem. Soc. Jpn. Vol. 85, No. 6 (2012)
Synthesis and Properties of Phenyleneethynylene Dyes
Ph2N
Ph2N
I
[Pd(PPh3)4], CuI
Ph2PCl, CuI
H2O2
P(O)Ph2
P(O)Ph2
toluene, i-Pr2NH
rt, 12 h
toluene, Et3N
80 °C, 12 h
THF,
rt, 4 h
17
16
57% (2 steps)
18 77%
Ph2N
Ph2N
Ph2N
ODMO
I
I
DMOO
ODMO
I
20 (3.2 equiv)
[Pd(PPh3)4], CuI
t-BuOK
toluene, i-Pr2NH
rt, 12 h
THF,
rt, 2 h
DMOO
15 46%
63%
Ph2N
Ph2N
Ph2N
Br
Br
ODMO
21 (1.5 equiv)
TMS
[Pd(PPh3)4], CuI
[Pd(PPh3)4], CuI K2CO3
toluene, i-Pr2NH
80 °C, 24 h
toluene, i-Pr2NH THF, MeOH
DMOO
19
rt, 12 h
rt, 2 h
89%
92%
Ph2N
Ph2N
ODMO
51%
9
NC
COOCH2CH2TMS
TBAF
[Pd(PPh3)4], CuI
Br
5
THF
rt, 2 h
52%
toluene, i-Pr2NH
80 °C, overnight
DMOO
46%
Ph2N
Scheme 4. Synthesis of 5.
reference electrode in our experiments, has been reported to be
¹0.02 V in reference to SCE, energy levels of ELUMO and
EHOMO of 1-6 can be determined by the following eqs 1
and 2.16 Table 3 shows the energy levels of EHOMO and ELUMO
which are estimated by invoking eqs 1 and 2. These results are
by ca. 0.35 V deeper than the HOMO and LUMO levels which
have been obtained from calculations for planar-planar con-
formers of 1-6 at B3LYP/6-31G(d) level: for instance, HOMO;

X. Yang et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 6 (2012)
691
Br
ODMO
22
ODMO
Br
Ph2N
[Pd(PPh3)4], CuI
Ph2N
toluene, i-Pr2NH
80 °C, overnight
DMOO
23
DMOO
52%
Br
I
Br
ODMO
NC
TMS
[Pd(PPh3)4], CuI K2CO3
toluene, i-Pr2NH THF, MeOH
Ph2N
[Pd(PPh3)4], CuI
toluene, i-Pr2NH
rt, overnight
85%
DMOO
80 °C, overnight
rt, 2 h
95%
89%
ODMO
I
TMS
COOH
[Pd(PPh3)4], CuI
Ph2N
[Pd(PPh3)4], CuI K2CO3
6
DMOO
toluene, i-Pr2NH THF, MeOH toluene, i-Pr2NH
rt, overnight
91%
rt, 2 h
96%
80 °C, overnight
70%
Br
NC
Scheme 5. Synthesis of 6.
80000
60000
40000
20000
0
Table 1. UV-vis Absorption Spectra of 1-6 in Toluene
(1.0 © 10¹4 M) and on TiO2 Nanoparticles and the
Simulated Wavelength of -max and Oscillator Strength f a)
1
2
3
4
5
6
Calculated wavelength
of -max/nm
(Oscillator strength f )
-
max/nm
-
on
max
Dye
¹1
(¾max/104 L mol¹1 cm
)
TiO2/nm
1
2
3
4
5
6
521 (7.2)
502 (5.6)
516 (5.6)
506b) (5.2)
517 (5.6)
431 (2.6)
507 (2.28)
494 (2.30)
504 (2.03)
500 (2.76)
505 (2.24)
431 (2.50)
477, 540 (s)
469, 517 (s)
471, 536 (s)
476, 526 (s)
478, 540 (s)
400
450
500
wavelength/nm
550
600
a) LC-BLPY/6-31G(d) level of calculations were carried out
for simulation of the wavelength of -max and oscillator strength
f. b) Recorded in CH2Cl2 (1.0 © 10¹4 M).
Figure 1. UV-vis absorption spectra of 1-6 in toluene
(1.0 © 10¹4 mol L¹1).
¹5.10 V (Calcd for 1 by eq 2) vs. ¹4.75 V (DFT-Calcd for 1),
LUMO; ¹2.94 V (Calcd for 1 by eq 1) vs. ¹2.59 V (DFT-
which is ascribable to increase of the dipole moment in the
excited state (®e) in comparison with that in the ground state
(®g). In order to access the effect of amino- and/or cyano
substituents of phenylene(anthrylene)ethynylenes 1-6 on their
change of dipole moments (®e ¹ ®g), we invoked a Lippert-
Mataga plot17 which enabled quantitative evaluation of ¦®
Calcd for 1).
red
ELUMO ¼ 4:38 V þ E1=2 ðvs: Ag=AgNO3Þ
EHOMO ¼ 4:38 V þ E1=2
ð1Þ
ð2Þ
oxid
ðvs: Ag=AgNO3Þ
Photoluminescence and Lippert-Mataga Plot of 1-6 in
Solution. When photoluminescence was recorded for 1-6
in toluene, all the dyes exhibited strong emission with high
quantum yields (Table 4). All the dyes emitted strong fluores-
cence in other solvents such as CHCl3, THF, and CH2Cl2, and
in the polar solvents, the wavelengths of emission maximum
by eq 3.
2
ð®e ꢀ ®gÞ
2
ꢀ¯ ¼
ꢀf þ Constant
hc
a3
¼ μꢀf þ C
¾ ꢀ 1
ð3Þ
ð4Þ
n2 ꢀ 1
ꢀf ¼
ꢀ
2¾ þ 1 2n2 þ 1
pffiffiffiffiffiffiffi
-
underwent a bathochromic shift. The large Stokes shift
em
®e ꢀ ®g ꢁ μa3
ð5Þ
indicates a large change of the structure in the excited state

692
Bull. Chem. Soc. Jpn. Vol. 85, No. 6 (2012)
Synthesis and Properties of Phenyleneethynylene Dyes
OMe
Ph2N
MeO
NC
1a (planar-planar)
COOH
OMe
Ph2N
MeO
NC
1b (twisted-planar)
COOH
OMe
Ph2N
NC
COOH
MeO
1c (planar-twisted)
OMe
Ph2N
MeO
NC
COOH
1d (twisted-twisted)
Scheme 6. Structures of conformers 1a-1d.
Table 2. Summary of Simulated Heats of Formation,
Excitation Wavelengths, and Their Oscillator Strengths
for Conformers 1a-1da)
The dye 6 with 1,5-anthrylene shows largest μ and (μa3)1/2
values; μ = 9761 and (μa3)1/2 = 2672. In a series of 9,10-
anthrylene dyes, dually substituted dyes 1 and 4 with Ph2N
and CN demonstrate rather large (μa3)1/2 values 1569 and
1649, respectively, exhibiting these dyes undergo effective
intramolecular charge transfer in the excited states.
¦H
/kcal mol
¦¦H
/kcal mol
Distribution
/%
¹1
¹1
1a (planar-planar)
¹1754323.97
¹5.56
¹3.02
¹2.35
0
98.2
1.3
0.4
UV-vis Absorption and CV Characterization of Phenyl-
eneethynylene Dyes on TiO2 Film. Because 9,10-anthrylene
dyes 1-5 recorded -max at longer than 500 nm and enabled
taking advantage of both UVand visible light, these compounds
are sensitizers of choice for further evaluation of dye-sensitized
solar cells. In order to evaluate the photochemical and electro-
chemical properties of these acetylenic dyes 1-5 adsorbed on
TiO2 nanoparticles, we fabricated the dye-loaded TiO2 films
which were prepared by immersing a TiO2 glass plate (1.5 ¯m in
thickness) into a toluene solution of each dye for 18 h. After
having been rinsed with acetonitrile, these films were subjected
to measurement of UV-vis absorption. Normalized profiles are
shown in Figure 2, and the selected wavelengths of -max are
summarized in Table 1. Absorption of all the dyes broadens
and undergoes ca. 20 nm-red shift on TiO2 in comparison with
the wavelength of the corresponding absorption band in toluene;
1b (twisted-planar) ¹1754321.43
1c (planar-twisted) ¹1754320.76
1d (twisted-twisted) ¹1754318.41
<0.1
a) Optimized structures for 1a-1d and their heat of formation
were determined by calculation on B3LYP/6-31G(d).
In eq 3, ¦¯ represents Stokes shift, h the Planck constant, c the
light velocity, and a Onsager radius. The solvent orientation
polarizability ¦f is calculated in eq 4 by using solvent
dielectric constant ¾ and refractive index n. Onsager radius
a is calculated at the PM3// B3LYP/6-31G(d) level for all
the solutes 1-6, respectively. Because plotting ¦¯ versus ¦f
provides 2(¦(®e ¹ ®g))2/hca3 as the slope μ of linear fit,
the dipole difference between the excited and ground states
¦(®e ¹ ®g) is proportional to (μa3)1/2 (eq 5). The Onsager
radius a, the slope μ, and (μa3)1/2 are shown in Table 5.

X. Yang et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 6 (2012)
693
Table 3. Results of Cyclic Voltammogram of 1-6 in CH2Cl2 (1.0 © 10¹4 M)a) and the Calculated Energy
Potentials of 1-6
EHOMO, ELUMO
Calcd by eqs 1 and 2c)
/V
EHOMO, ELUMO
Calcd by DFTd)
/V
oxid b)
red b)
E1/2
E1/2
Dye
/V vs. Ag/Ag+
/V vs. Ag/Ag+
1
2
3
4
5
6
0.72
0.70
0.90
0.64
0.83
0.73
¹1.44
¹1.58
¹1.46
¹1.56
¹1.44
¹1.74
¹5.10, ¹2.94
¹5.08, ¹2.80
¹5.28, ¹2.92
¹5.02, ¹2.82
¹5.21, ¹2.94
¹5.11, ¹2.64
¹4.75, ¹2.59
¹4.71, ¹2.41
¹4.89, ¹2.63
¹4.69, ¹2.54
¹4.88, ¹2.62
¹4.77, ¹2.41
a) Measured by CV with 0.1 M n-Bu4NPF6 in spectroscopic grade CH2Cl2. b) Half-wave redox potential
(Ag/AgNO3, in CH2Cl2 (1.0 © 10¹4 M)). c) Calcd by eqs 1 and 2. d) B3LYP/6-31G(d) level of
calculations were carried out.
Table 4. Photoluminescence Spectra of 1-6 in Toluene
(1.0 © 10¹7 M)
Table 6. Electrochemical Properties of Acetylenic Dyes on
TiO2 Filmsa)
1
2
3
4a)
5
6
Dye
EHOMO/V
Gap/V
ELUMOb)/V
Emaxb)/nm
ΦF
550
0.79
537
0.80
534
0.70
536
0.69
535
0.68
481
0.76
1
2
3
4
5
¹4.93
¹4.95
¹4.91
¹4.90
¹4.92
2.04
2.17
2.10
2.07
2.05
¹2.88
¹2.78
¹2.82
¹2.83
¹2.88
c)
a) Concentration was not determined. b) Emission maximum.
c) Absolute quantum yield.
Table 5. Results of Lippert-Mataga Plot for 1-6
a) The formal oxidation potentials of the dyes were measured
under the following conditions: dye-coated TiO2 electrode
(thickness: ca. 1.5 ¯m) as the working electrode, a Pt counter
electrode, and an Ag/Ag+ reference electrode (0.01 M AgNO3
and 0.1 M tetrabutylammonium perchlorate in acetonitrile) in
0.1 M LiClO4/acetonitrile. Potentials measured vs. Ag/Ag+
were converted to EHOMO and ELUMO by eqs 1 and 2. b) The
LUMO energy levels were calculated with the expression of
Dye
Slope μ
a/¡a)
a3/¡3
(μa3)1/2
1
2
3
4
5
6
3490
3158
2197
3170
1758
9761
8.90
8.85
8.33
9.50
9.85
9.01
705.0
693.2
578.0
857.4
955.7
731.4
1569
1480
1127
1649
1296
2672
E
LUMO = EHOMO + gap, where the gaps were derived from the
a) a: Onsager radii. Calculated by use of Gaussian0918
(PM3// B3LYP/6-31G(d)).
absorption onset wavelength of the dye-loaded film.
TiO2 surface.19 Table 6 exhibits HOMO and LUMO energy
levels of these dyes, which were determined by oxidation
potentials in CV measurements and the onset wavelengths of
these dyes on TiO2 films in UV-vis absorption. All the dyes
1.2
1
¹
show the HOMO potentials which are lower than I oxidation
1
0.8
potential (0.44 V vs. NHE) while the LUMO potentials are
higher than that of TiO2 conduction band edge (¹0.46 V vs.
NHE).20 These results indicate that in the DSSC, electron
2
3
0.6
4
¹
injection could occur smoothly both from I in the electrolyte
5
0.4
to the dye and from the photoexcited dye to TiO2.
Fabrications of DSSCs with Phenyleneethynylene Dyes
and Photovoltaic Performance. F-SnO2 (FTO)-coated glass
substrates were cleaned in a detergent solution by an ultrasonic
bath, rinsed with water and ethanol, and then dried using a
N2 stream. The TiO2 paste (transparent layer, SOLARONIX
(T20/SP)) was printed on an FTO glass substrate and sub-
sequently sintered at 500 °C in air for 1 h, and on it was
deposited aqueous TiCl4 (50 mM), followed by heat treatment
(450 °C, 30 min). The thickness of the TiO2 thin films, mea-
sured with a Dektak 8 (Veeco), was ca. 6 ¯m. After treatment
with TiCl4, the TiO2 films were immersed into toluene solution
of the dyes (0.3 mM) for 18 h. Each dye-adsorbed TiO2 film
electrode and Pt-counter electrode were assembled into a sealed
sandwich solar cell with a hot-melt Surlyn film (30 ¯m in
0.2
0
350
400
450
500
550
600
650
700
wavelength/nm
Figure 2. UV-vis absorption spectra of dyes on TiO2
films (transparent layer, µ20 nm nanoparticles, 1.5 ¯m
in thickness).
for instance, 521 nm (in toluene) vs. 540 nm (on TiO2) for 3.
Onsets of these absorption are observed at longer wavelengths
(>600 nm) as well. These results may be explained by the
interaction between the anchoring groups of the dyes and the

694
Bull. Chem. Soc. Jpn. Vol. 85, No. 6 (2012)
Synthesis and Properties of Phenyleneethynylene Dyes
1 (light)
2 (light)
3 (light)
4 (light)
5 (light)
1 (dark)
2 (dark)
3 (dark)
4 (dark)
5 (dark)
(a)
(b)
0.7
9
7
0.6
0.5
0.4
0.3
0.2
0.1
0
1
2
3
4
5
5
3
1
0
0.2
0.4
0.6
0.8
1
-1
-3
-5
300
400
500
600
700
800
wavelength/nm
voltage/V
Figure 3. (a) IPCE spectra of DSSCs with 1-5 and (b) J-V curves of DSSCs with acetylenic dyes 1-5 both in light and
dark conditions.
Table 7. Solar-Cell Performance for DSSCsa)
b)
½
Jsc
/mA cm
Voc
/V
©
/%
Γc)
Dye
FF
¹2
¹3
/¯m
/10¹4 mol cm
1
1
2
3
4
5
5.8
14d)
6.1
6.0
6.0
6.0
8.0
9.4
5.4
7.5
7.0
5.7
0.75
0.71
0.71
0.74
0.74
0.71
0.70
0.75
0.68
0.70
0.69
0.70
4.2
5.0
2.6
3.9
3.5
2.8
1.4
®
1.8
2.0
1.5
1.5
a) Incident light: AM 1.5G (100 mW cm¹2) with a mask (0.2399 cm2) and without an antireflection film.
Electrolyte: 0.6 M DMPImI + 0.1 M LiI + 0.05 M I2 + 0.5 M TBP in acetonitrile. b) TiO2 thickness. c) Γ is the
adsorption amount of the dye on the TiO2 film. d) 7 ¯m thickness of T20/SP + 7 ¯m thickness of D37.
thickness) as a spacer. A drop of the electrolyte solution
(0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPImI) +
0.1 M LiI + 0.05 M I2 + 0.5 M 4-tert-butylpyridine (TBP) in
acetonitrile) was driven into the cell through the hole in the
counter electrode. Finally, the two holes were sealed using
additional hot-melt Surlyn film covered with a thin glass slide.
Upon adsorption of dyes on TiO2, no coadsorption compounds
were used.
The solar energy conversion efficiency (©) of 1-5-DSSCs
was calculated from the short current density (Jsc), the open
circuit voltage (Voc), the fill factor (FF) of the cell, and the
intensity of the incident light (Pin).21 Figure 3a shows the
action spectra of incident photon-to-current conversion effi-
ciency (IPCE) of DSSCs which are sensitized by 1-5. The
onset wavelengths in IPCE spectra are by 50 nm longer than
those in their UV-vis absorption spectra on TiO2. IPCE values
of DSSCs with 1, 3, and 4 exceed 60% in 400-600 nm, and
1 shows the highest IPCE, 64% at 480 nm, while IPCE maxima
of 2 and 5 are rather lower; 54% for 2 and 47% for 5 at 460 nm.
In Figure 3b are shown J-V curves of DSSCs both in
light and dark conditions, and in Table 7 are summarized the
detailed photovoltaic parameters of these DSSCs. Under one-
sun conditions, 1-DSSC achieved the highest photo-to-current
energy conversion (4.2%) in this series of DSSCs. When 14 ¯m
thick TiO2 electrode was used, the 1-DSSC produced 5.0%
efficiency under one-sun conditions (Jsc = 9.4 mA cm¹2, Voc =
0.71 V, FF = 0.75).
Although all the dyes showed comparable Voc values (0.71-
0.75 V) in the DSSCs, 2 and 5 did remarkably lower Jsc values
(5.4 and 5.7 mA cm¹2, respectively) giving rise to lower photo-
voltaic performance (© < 2.8). These inefficient photovoltaic
performance of 2 and 5 was consistent with low IPCEs of
these cells. In order to get further insight into relationship of
the dye structures with photovoltaic performance, electron
lifetime in TiO2 electrodes was recorded by electrochemical
impedance spectroscopy (EIS), which is a powerful technique
for characterization of interfacial charge-transfer processes in
DSSC.22 The Bode and Nyquist plots of DSSCs at open-circuit
under one-sun irradiation are described in Figures 4a and 4b,
respectively. Small signals located at high-frequency (103-
105 Hz) in Bode plot correspond to small semicircles at small
Z¤ (8-15 ³) in Nyquist plot, and only a little difference in these
profiles indicates that the electron transfer proceeds smoothly
from the Pt-counter electrode to the oxidized species in the
electrolyte. Large signals at middle-frequency (10-103 Hz) in
the Bode plot correspond to large semicircles at Z¤ (15-60 ³)
in the Nyquist plot representing the charge recombination of
¹
the injected electrons in TiO2 with the I3 ion in electrolyte.
Observation that there is no significant change in the position
of the frequency maxima in the Bode plot for these sensitizers
reveals that the electron lifetimes in these DSSCs are com-
parable to each others. This is consistent with the similar
magnitude of Voc values (0.71-0.75 V) which are recorded in
1-5-fabricated DSSCs (Table 7). In contrast to this, the electron

X. Yang et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 6 (2012)
695
-40
-35
-30
-25
-20
-15
-10
-5
(a)
30
25
20
15
10
5
(b)
1
2
3
4
5
1
2
3
4
5
0
0
0
20
40
60
0.1
10
1000
100000
Frequency/Hz
Z'/Ω
Figure 4. (a) EIS Bode plots and (b) EIS Nyquist plots for DSSCs based on phenyleneethynylene dyes under one sun condition.
¹
recombination resistance from TiO2 to I3 in the electrolyte
is increased in 2- and 5-DSSCs because the larger radii of
semicircles are recorded for 2- and 5-DSSCs in the Nyquist
plots (Figure 3b). Remarkably low performance of the 2- and
5-DSSCs despite high recombination resistances could be
explained as follows. The DFT calculation revealed that the
LUMO orbital which was located at the terminal benzene
having COOH in the dye 2 was smaller than that in the dye 1
(See Supporting Information). Therefore, the electron injection
from the LUMO to TiO2 in 2 would be inefficient resulting in
the lower Jsc value as well as the lower Voc value. This indicates
a pivotal role of electron-withdrawing CN substituent in the
sensitizer which locates the LUMO close to the anchoring
carboxy group and tunes the energy potential of LUMO to
enhance the charge injection from the excited sensitizer to
TiO2. Although the sensitizer 5 shows oxidation and reduction
potentials on TiO2 nanoparticles which are comparable to those
of other dyes in Table 5, DFT calculations for 5 indicate that no
contribution of Ph2N group to the HOMO and both HOMO and
LUMO are located at the same place, namely the phenyl-
eneethynylene moiety which expands from the junction
benzene to carboxy group (See Supporting Information). This
calculation reveals that the HOMO and LUMO of 5 should be
consistent with those of 3, and, in fact, the same profiles are
observed in the UV-vis absorption of 3 and 5 (Figure 1). In
sharp contrast, the IPCE of 5-DSSC was decreased drastically
in comparison with that of 3-DSSC presenting the slower
reduction rate of the cation of 5 by the electrolyte. When
1-5 in toluene exhibited strong absorption bands (-max =
502-521 nm, ¾ > 52000 L mol¹1 cm¹1) in UV-vis absorption
spectra. Lippert-Mataga plots for 1-5 indicated that Ph2N-
and CN-substituted sensitizer underwent larger change in
dipole moment upon photoinduced excitation. It was revealed
by recording cyclic voltammogram of 1-5 in CH2Cl2 that
HOMO and LUMO potentials could be tuned by introducing
Ph2N and/or CN groups, respectively: a HOMO potential was
increased by substitution with electron-donating Ph2N group
while a LUMO potential was decreased by electron-withdraw-
ing CN group. DSSCs fabricated by use of 1-5 showed photo-
voltaic performance (Voc > 0.71 V, © = 2.6-4.2%) achieving
the highest photo-to-current energy conversion efficiency
(© = 5.0%) under the optimized conditions. The Bode plot of
EIS indicated the comparable electron lifetime for the charge
¹
recombination of the injected electrons in TiO2 with the I3 ion
in electrolyte in all these DSSCs. Although the Nyquist plot
showed that 2- and 5-DSSCs had large electron recombination
resistance, these two DSSCs exhibited rather low solar energy
conversion efficiencies (©). The low performance of these
DSSCs could be explained by inefficient electron injections
¹
from the sensitizer 2 to TiO2 induction band and from I3 to
dye 5, respectively. In this study, acetylenic expanded ³-sys-
tems were used as DSSC sensitizer in order to take advantage
of visible light at 500-550 nm efficiently to achieve 5.0%
solar energy conversion efficiency (©). Because phenylene-
ethynylene dye with an expanded ³-system has demonstrated
facile synthesis and easy tuning of the physical properties such
as -max, ¾ and energy potentials of HOMO and LUMO by
introducing anthrylene and electron-donating Ph2N and elec-
tron-withdrawing CN as well as the high potential as sensitizer
of DSSC, further design and synthesis of acetylenic dye are
still under investigation in order to realize higher performance
of DSSC.
UV-vis light is irradiated to 5-DSSC, charge injection from
¹
I
would proceed through DMOO substituents because the
DMOO group contributed to the HOMO but Ph2N groups
¹
do not. The approach of I to DMOO groups which would be
required in this charge injection, would be disturbed by bulky
double-headed amino groups giving rise to the lower Jsc of
5-DSSC than that of 3.
A.O. thanks Prof. Kan Wakamatsu (Okayama University
of Science) for helpful discussion. This work was supported by
a Grant-in-Aid for Scientific Research on Innovative Areas
“Organic Synthesis based on Reaction Integration. Develop-
ment of New Methods and Creation of New Substances”
(No. 2105), matching fund subsidy for private universities
from the Ministry of Education, Culture, Sports, Science and
Conclusion
We designed the expanded ³-system sensitizers 1-6 which
were composed by substitution of anthrylene-containing
phenyleneethynylene array with Ph2N- and/or CN group(s),
and these sensitizers were prepared successfully by repeating
Sonogashira coupling. Sensitizers having 9,10-anthrylene

696
Bull. Chem. Soc. Jpn. Vol. 85, No. 6 (2012)
Synthesis and Properties of Phenyleneethynylene Dyes
Koumura, A. Furube, K. Hara, J. Phys. Chem. C 2009, 113, 13409.
f ) Z.-S. Wang, N. Koumura, Y. Cui, M. Takahashi, H. Sekiguchi,
A. Mori, T. Kubo, A. Furube, K. Hara, Chem. Mater. 2008, 20,
3993. g) N. Koumura, Z.-S. Wang, S. Mori, M. Miyashita, E.
Suzuki, K. Hara, J. Am. Chem. Soc. 2006, 128, 14256; Addition
and Correction: N. Koumura, Z.-S. Wang, S. Mori, M. Miyashita,
E. Suzuki, K. Hara, J. Am. Chem. Soc. 2008, 130, 4202.
Technology, Japan, the Japan Society for the Promotion of
Science (JSPS) through its “Funding Program for World-
Leading Innovative R&D on Science and Technology (FIRST
Program)” and Okayama Prefecture Industrial Promotion
Foundation.
Supporting Information
8
a) S. Higashijima, H. Miura, T. Fujita, Y. Kubota, K.
Full experiments for synthesis of dyes 1-6, spectral data of
UV-vis absorption, photoluminescence and Lippert-Mataga
plot, CV profiles, results of the DFT calculations, procedures of
determination of amount of adsorbed dye, and photovoltaic
measurement, and all the profiles of EIS. This material is
available free of charge on the web at http://www.csj.jp/
journals/bcsj/.
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c) T. Horiuchi, H. Miura, K. Sumioka, S. Uchida, J. Am. Chem.
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