Molecular engineering of thia-bridged triphenylamine heterohelicenes as novel organic dyes for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.jphotochem.2011.09.020

Three organic sensitizers incorporating thia-bridged heterohelicene unit, thiophene-conjugate bridge, and cyanoacrylic acid were synthesized. The power conversion efficiency was quite sensitive to the substituents on heterohelicene donor group. Under stan

Full text of DOI:10.1016/j.jphotochem.2011.09.020

Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 17–25
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Molecular engineering of thia-bridged triphenylamine heterohelicenes
as novel organic dyes for dye-sensitized solar cells
Chulwoo Kima, Hyunbong Choia, Sanghyun Paeka, Jeum-Jong Kimb, Kihyung Songc,
Moon-Sung Kangd, Jaejung Koa,∗
a Department of Advanced Material Chemistry, Korea University, Jochiwon, Chungnam 339-700, Republic of Korea
b Advanced Solar Technology Research Department, Electronics and Telecommunications Research Institute, Daejeon 305-700, Republic of Korea
c Department of Chemical Education, Korea National University of Education, Cheongwon, Chungbuk 363-791, Republic of Korea
d Department of Environmental Engineering, Sangmyung University, 300 Anseo-dong, Dongnam-gu, Cheonan-si, Chungnam Province 330-720, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 April 2011
Received in revised form 7 September 2011
Accepted 22 September 2011
Available online 10 October 2011
Three organic sensitizers incorporating thia-bridged heterohelicene unit, thiophene-conjugate bridge,
and cyanoacrylic acid were synthesized. The power conversion efficiency was quite sensitive to the
substituents on heterohelicene donor group. Under standard global AM 1.5 solar conditions, the JK-206-
sensitized cell gave a short-circuit photocurrent density (Jsc) of 13.44 mA cm−2, an open-circuit voltage
(Voc) of 0.71 V, and a fill factor of 0.74, corresponding to an overall conversion efficiency of 7.08%. The
device based on the JK-206 with an ionic liquid electrolyte showed an excellent long-term stability.
The initial efficiency of 5.6% for JK-206 slightly decreased to only 5.4% after the 1000 h light soaking
at 60 ◦C.
Keywords:
Thia-bridged heterohelicene
Organic dye
Dye-sensitized solar cell
DSSCs
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
on the structural modification of the dye to prevent the aggre-
gation of dye and to diminish the charge recombination between
Photovoltaic solar energy conversion represents a promising
candidate to renewable resource [1]. Despite its advantages and
continuing advances, photovoltaic technology still remains non
cost-effective against traditional fossil fuels. For this purpose, the
development of dye-sensitized solar cells (DSSCs) has received a
great deal of attention from both academic and industrial fields
owing to their low cost and high efficiency [2,3]. In these cells,
dye is one of the key components for high power conversion effi-
ciencies. Although some RuII polypyridyl sensitizers have achieved
impressive efficiencies above 11% [4–7], they have some limita-
tions to be addressed owing to the precious metal and difficult
synthetic procedures. In this regard, organic sensitizers might be
considered as an alternative to ruthenium sensitizers. However,
the efficiency is still low compared with that of the ruthenium
sensitizer. The main factors for the low efficiency of organic dyes
are the formation of dye aggregate on the semiconductor sur-
face [8–14], the interfacial recombination dynamics [15–17], and
the short lifetime [18–20]. Our concept toward the high photo-
voltaic performance of organic-dye-sensitized solar cell is based
the electrons and acceptors as well as to increase the lifetime of
cation species. Organic dyes are composed of three components,
donor, spacer and anchoring group. Triarylamine derivatives are
electron donors in most of organic dyes [21–23]. The unsubsti-
tuted triphenylamine cations formed by the injection of electron
to conduction band of TiO2 are generally unstable. Accordingly,
flattening the structure and introducing the para substituent on
the phenyl unit would increase the stability and induce a red shift
of the absorption spectrum as well as increase the electron life-
times (ꢀe) of the dye by the delocalization of the resulting cation
on a flattened configuration [24,25]. In order to obtain insight
into the structure–photophysical relationships on this system, we
have investigated a bridged triarylamine heterohelicene, 2,2ꢀ:6ꢀ,2ꢀꢀ-
dithiatriphenylamine and its bulky substituents [26,27]. As part of
our continuing efforts to develop novel organic dyes, we set out to
synthesize organic sensitizers, coded as JK-205, JK-206 and JK-207,
which incorporate thia-bridged triarylamines as the electron donor
and a cyano acrylic acid as the electron acceptor bridged by the
oligothiophene.
In this article, as part of our efforts to develop novel organic
dyes, we report new type of organic dyes (JK-205, JK-206, and JK-
207) containing a flattened thia-bridged heterohelicene moiety as
the electron donor and cyanoacrylic acid as the electron acceptor
bridged by a variety of linkers (Structure 1).
∗
Corresponding author. Tel.: +82 41 860 1337; fax: +82 41 867 5396.
E-mail address: jko@korea.ac.kr (J. Ko).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.09.020

18
C. Kim et al. / Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 17–25
2. Experimental
2.1. General methods
2.5. Synthesis
2.5.1. 7-Bromo(DTTPA) (1)
A stirred solution of thia-bridged triarylamine heterohelicene
(1.36 g, 4.45 mmol) in acetic acid (60 mL) was flushed with nitrogen
for 5 min, then bromine (0.25 mL, 4.90 mmol) was added dropwise,
and stirring was continued for 8 h. The solution turned pink imme-
diately, then changed gradually to deep red, next to green, and
finally to black with the evolution of hydrogen bromide. The sol-
vent was evaporated in vacuo to yield a black powdery material
which was redissolved in aqueous sodium bicarbonate (30 mL). The
resulting mixture was extracted with five 20-mL portions of ben-
zene. The orange extracts were combined, dried over anhydrous
Na2SO4, filtered and evaporated to yield an oily material. The pure
product was obtained by silica gel chromatography using a mix-
ture of methylene chloride and hexane (1:3) as an eluent (yield:
60%). Mp: 117 ◦C. 1H NMR (CDCl3): ı 7.33 (s, 2H), 7.24 (m, 4H), 7.02
All reactions were carried out under an argon atmo-
sphere. Solvents were distilled from appropriate reagents.
p-Bromo-N,N-diphenylaniline (1) [42], phthalimide-N-sulfenyl
chloride (4) [32], 4,4,5,5-tetramethyl-2-(5-(5-(5,5-dimethyl-1,3-
dioxan-2-yl)thiophen-2-yl)thiophen-2-yl)-1,3,2-dioxaborolan (7)
[43], 2,2ꢀ:6ꢀ,2ꢀꢀ-dithiatriphenylamine (DTTPA) (10) [26,27], were
synthesized using a modified procedure of previous references.
2.2. Cyclovoltagram
Cyclic voltammetry was carried out with a BAS 100B (Bioanalyti-
cal Systems, Inc.). A three-electrode system was used and consisted
of a gold disk, working electrode and a platinum wire electrode.
Redox potential of dyes on TiO2 was measured in CH3CN with 0.1 M
1
(m, 4H). 13C{ H} NMR (CDCl3): ı 142.2, 139.4, 130.6, 128.1, 127.7,
126.0, 125.1, 121.8, 120.6, 116.9. MS: m/z 384 [M+]. Anal. Calcd. for
C18H10BrNS2: C, 56.25; H, 2.62. Found: C, 56.04; H, 2.48.
(n-C4H9)4N-PF6 with a scan rate of 50 mV s−1
.
2.5.2. 2-(5-(5-(DTTPA-2)thiophen-2yl)thiophen-2-yl)-
5,5-dimethyl-1,3-dioxane (3)
2.3. Electron transport measurements
To a stirred solution of 1 (1.1 g, 1.98 mmol) and Pd(PPh3)4
(0.12 g, 0.099 mmol) in tetrahydrofuran (50 mL) was added
4,4,5,5-tetramethyl-2-(5-(5-(5,5-dimethyl-1,3-dioxan-2-
yl)thiophen-2-yl)thiophen-2-yl)-1,3,2-dioxaborolane (2) (0.7 g,
2.37 mmol), potassium carbonate (1.64 g, 11.9 mmol) in THF
(20 mL) and H2O (5.9 mL). The mixture was refluxed for 18 h. After
cooling the solution, H2O (30 mL) was added to the solution and
extracted by dichloromethane (30 mL × 3). The organic layer was
separated and dried in MgSO4. The solvent was removed in vacuo.
The pure product 3 was obtained by silica gel chromatography
using a mixture of methylene chloride and n-hexane (1:2) as an
eluent (yield: 70%). Mp: 155 ◦C. 1H NMR (CDCl3): ı 7.42 (s, 2H),
7.33 (d, 2H, J = 8.1 Hz), 7.23–6.94 (m, 10H), 5.62 (S, 1H), 3.79 (d,
2H, J = 11.1 Hz), 3.66 (d, 2H, J = 11.1 Hz), 1.29 (S, 3H), 0.81 (S, 3H).
The electron diffusion coefficient (De) and lifetimes (ꢀe) on
TiO2 photoelectrode were measured by the stepped light-induced
transient measurements of photocurrent and voltage (SLIM-PCV)
[44–47]. The transients were induced by a stepwise change in the
laser intensity. A diode laser (ꢁ = 635 nm) as a light source was mod-
ulated using a function generator. The initial laser intensity was
a constant 90 mW cm−2 and was attenuated up to approximately
10 mW cm−2 using a ND filter which was positioned at the front side
of the fabricated samples (TiO2 film thickness = ca. 10 ␮m; active
area = 0.04 cm2). The photocurrent and photovoltage transients
were monitored using a digital oscilloscope through an amplifier.
The De value was obtained by a time constant (ꢀc) determined by
fitting a decay of the photocurrent transient with exp(−t/ꢀc) and the
TiO2 film thickness (ω) using the equation, De = ω2/(2.77ꢀc). The ꢀe
value was also determined by fitting a decay of photovoltage tran-
sient with exp(−t/ꢀe). All experiments were carried out at room
temperature.
1
13C{ H} NMR (CDCl3): ı 142.4, 141.9, 141.6, 140.4, 137.7, 136.6,
132.3, 130.9, 128.1, 127.9, 127.6, 127.3, 127.2, 126.0, 125.5, 124.9,
123.7, 120.9, 98.2, 77.6, 23.1, 21.9. MS: m/z 583 [M+]. Anal. Calcd.
for C32H25NO2S4: C, 65.83; H, 4.32. Found: C, 65.72; H, 4.11.
2.4. Characterization of DSSC
2.5.3. 5-(5-(DTTPA-2-yl)thiophen-2-yl)thiophen-2-carbaldehyde
(4)
The cells were measured using 1000 W xenon light source,
whose power of an AM 1.5 Oriel solar simulator was cal-
ibrated by using KG5 filtered Si reference solar cell. The
incident photon-to-current conversion efficiency (IPCE) spectra
for the cells were measured on an IPCE measuring system (PV
Measurements).
THF (30 mL) and water (10 mL) were added to a flask contain-
ing acetal 3 (0.20 g, 0.37 mmol). Then, TFA (5 mL) was added to the
solution. The resulting reaction mixture was stirred for 6 h at room
temperature, quenched with saturated aqueous sodium bicarbon-
ate, and extracted with dichloromethane. The combined organic
phases were washed with aqueous sodium bicarbonate (2%, w/v),
Structure 1. The structure of JK-205, JK-206, and JK-207.

C. Kim et al. / Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 17–25
19
1
J = 5.1 Hz). 13C{ H} NMR (CDCl3): ı 147.6, 145.1, 138.1, 132.0, 129.4,
124.7, 124.2, 113.8, 35.5, 31.9, 31.6, 29.2, 22.8, 14.2. MS: m/z 491
[M+]. Anal. Calcd. for C30H38BrN: C, 73.16; H, 7.78. Found: C, 73.00;
H, 7.62.
dried (Na2SO4), and evaporated in vacuo. The pure product 4 was
obtained by silica gel chromatography using a mixture of methy-
lene chloride and n-hexane (1:2) as an eluent (yield: 90%). Mp:
178 ◦C. 1H NMR (CDCl3): ı 9.87 (s, 1H), 7.68 (d, 2H, J = 8.1 Hz), 7.45 (s,
1
2H), 7.34 (m, 3H), 7.23 (m, 4H), 7.02 (m, 3H). 13C{ H} NMR (CDCl3):
ı 182.6, 147.1, 145.7, 144.8, 142.7, 142.2, 139.3, 137.5, 135.2, 133.8,
130.5, 130.2, 129.6, 128.1, 127.3, 126.0, 125.4, 124.2, 120.9. MS: m/z
497 [M+]. Anal. Calcd. for C27H15NOS4: C, 65.16; H, 3.04. Found: C,
64.91; H, 2.90.
2.5.7. 2,2ꢀ-((((4-Bromophenyl)azanediyl)bis(3-hexyl-6,1-
phenylene))bis(sulfanediyl))bis(isoindoline-1,3-dione) (7)
To a solution of 6 (1.15 g, 2.33 mmol) in dry CHCl3 (60 mL) was
added phthalimidesulfenyl chloride (PhtNSCI) (1.14 g, 5.36 mmol)
under a nitrogen atmosphere. After stirring at 60 ◦C for 24 h,
the reaction mixture was diluted with CH2Cl2 (10 mL), and
washed with a saturated NaHCO3 solution (2 × 30 mL) and water
(2 × 30 mL). The organic layer was dried over MgSO4, filtered, and
concentrated under reduced pressure. The crude material was puri-
fied by flash chromatography (CH2Cl2: hexane = 4:1) to provide 7
as a yellow solid (83% yield). 1H NMR (CDCl3): ı 7.92 (m, 4H), 7.75
(m, 4 H), 7.68 (d, 2H, J = 8.1 Hz), 7.28 (d, 2H, J = 7.5 Hz), 7.12 (d, 2H,
J = 8.1 Hz), 6.91 (s, 2H), 6.61 (d, 2H, J = 7.5 Hz), 2.46 (t, 4H, J = 7.2 Hz),
2.5.4. 2-Cyano-3-(-5-(5-(DTTPA-2-yl)thiophen-2-yl)thiophen-
2-yl)acrylic acid (JK-205)
A mixture of 4 (0.17 g, 0.35 mmol) and cyanoacetic acid (0.04 g,
0.45 mmol) was vacuum-dried and added acetonitrile (20 mL),
chloroform (20 mL) and piperidine (0.02 mL). The solution was
refluxed for 6 h. After cooling the solution, the organic layer was
removed in vacuo. The pure product of JK-205 was obtained by
silica gel chromatography using a mixture of methylene chloride
and methanol (9:1) as an eluent (yield: 80%). Mp: 238 ◦C. 1H NMR
(DMSO): ı 8.00 (s, 1H), 7.73 (s, 2H), 7.64 (d, 2H, J = 6.9 Hz), 7.57 (d,
1H, J = 7.5 Hz), 7.49 (d, 1H, J = 6.3 Hz), 7.46 (d, 1H, J = 6.9 Hz), 7.38
1.48 (m, 4H), 1.16 (m, 12H), 0.81 (t, 6H, J = 5.1 Hz); 13C{ H} NMR
1
(CDCl3): ı 168.3, 146.9, 142.4, 136.7, 134.4, 133.8, 132.4, 130.7,
130.4, 129.1, 128.2, 126.8, 124.2, 118.9, 35.5, 32.1, 31.8, 29.7, 23.0,
14.8. MS: m/z 847 [M+]. Anal. Calcd. for C46H44BrN3O4S2: C, 65.24;
H, 5.24. Found: C, 64.88; H, 5.02.
1
(d, 2H, J = 7.5 Hz), 7.19 (m, 5H). 13C{ H} NMR (DMSO): ı 163.1,
145.1, 144.5, 143.5, 141.3, 140.7, 137.8, 136.8, 134.7, 132.9, 129.8,
128.7, 128.0, 127.1, 126.4, 125.2, 124.4, 123.1, 122.1, 118.1. MS: m/z
564 [M+]. Anal. Calcd. for C30H16N2O2S4: C, 63.81; H, 2.86; N, 4.96.
Found: C, 63.57; H, 2.69; N, 4.78.
2.5.8. 4-Bromo(bis(3-hexyl-6,1-phenylene))-DTTPA (8)
To a solution of compound 7 (1 g, 1.2 mmol) in dry CH2Cl2
(50 mL) was added BF3·Et2O (6 mL, 47 mmol) under a nitrogen
atmosphere. After the mixture had been stirred for 3 h at room
temperature, the mixture was diluted with CH2Cl2 (15 mL) and
washed with a saturated Na2CO3 solution (100 mL × 2) and a satu-
rated NaF solution (100 mL × 2). The organic layer was dried over
MgSO4. Evaporation of the solvent gave a crude product that was
purified by flash chromatography using a mixture of petroleum
ether and methylene chloride (1:1) as an eluent. The compound
was further purified by recrystallization from CHCl3 (yield: 60%).
Mp: 169 ◦C. 1H NMR (CDCl3): ı 7.65 (d, 2H, J = 8.4 Hz), 7.25 (s, 2H),
7.21 (d, 2H, J = 8.4 Hz), 6.90 (s, 2H), 2.45 (t, 4H, J = 7.8 Hz), 1.55 (m,
2.5.5. 1,1ꢀ-(4,4ꢀ-(4-Bromophenylazanediyl)bis(4,1-phenylene))
dihexan-1-one (5)
Anhydrous AlCl3 (2.3 g, 17.2 mmol) was suspended in excess
dry methylene chloride (30 mL) and kept at 0 ◦C with stirring. To
this stirred suspension, hexanoyl chloride (1.74 g, 12.4 mmol) was
added slowly for a period of 30 min, keeping the temperature at
0 ◦C during the addition. The reaction mixture was thawed to room
temperature and stirring was continued at room temperature for an
additional 30 min. To a stirred solution of anhydrous AlCl3 and hex-
anoyl chloride in methylene chloride was added 1 (2 g, 6.17 mmol)
in methylene chloride (20 mL). The reaction mixture was stirred at
60 ◦C for an additional 10 h. The AlCl3-complex was then quenched
by pouring the reaction mixture into crushed ice and cold 10% HCl.
After cooling the solution, H2O (30 mL) was added to the solu-
tion and extracted by methylene chloride (30 mL × 3). The organic
layer was separated and dried in MgSO4. The solvent was removed
in vacuo. The pure product 5 was obtained by silica gel chromatog-
raphy using a mixture of ethyl acetate and n-hexane (1:10) as an
eluent (yield: 87%). Mp: 247 ◦C. 1H NMR (CDCl3): ı 7.88 (d, 4H,
J = 9 Hz), 7.47 (d, 2H, J = 8.4 Hz), 7.10 (d, 4H, J = 9 Hz), 7.04 (d, 2H,
J = 8.4 Hz), 2.89 (t, 4H, J = 7.2 Hz), 1.733 (m, 4H), 1.35 (m, 8H), 0.91
1
4H), 1.27 (m, 12H), 0.87 (t, 6H, J = 5.1 Hz). 13C{ H} NMR (CDCl3): ı
141.4, 137.9, 137.7, 134.3, 133.7, 130.6, 127.6, 126.9, 125.6, 122.2,
34.9, 31.8, 31.5, 29.0, 22.7, 14.2. MS: m/z 553 [M+]. Anal. Calcd. for
C30H38BrN: C, 65.20; H, 6.20. Found: C, 65.03; H, 5.99.
2.5.9. 2-((Bis(3-hexyl-6,1-phenylene)5-(5-(DTTPA-2)thiophen-
2yl)thiophen-2-yl)-5,5-dimethyl-1,3-dioxane) (9)
To
a
stirred solution of
8
(0.2 g, 0.36 mmol) and
in tetrahydrofuran
4,4,5,5-tetramethyl-2-(5-
n-2-yl)thiophen-2-yl)
Pd(PPh3)4
(20 mL)
(5-(5,5-dimethyl-1,3-dioxa
(20 mg,
was
0.02 mmol)
added
1
(t, 6H, J = 5.1 Hz). 13C{ H} NMR (CDCl3): ı 199.2, 150.6, 145.2, 133.1,
132.1, 129.9, 127.9, 122.8, 118.4, 38.5, 31.7, 24.4, 24.3, 22.7, 14.1.
MS: m/z 521 [M+]. Anal. Calcd. for C30H34BrNO2: C, 69.23; H, 6.58.
Found: C, 69.01; H, 6.39.
thiophen-2-yl)-1,3,2-dioxaborolane 2 (0.22 g, 0.54 mmol), potas-
sium carbonate (0.2 g, 1.48 mmol) in THF (20 mL) and H2O
(0.8 mL). The mixture was refluxed for 18 h. After cooling the
solution, H2O (30 mL) was added to the solution and extracted
by dichloromethane (30 mL × 3). The organic layer was separated
and dried in MgSO4. The solvent was removed in vacuo. The pure
product 9 was obtained by silica gel chromatography using a
mixture of methylene chloride and n-hexane (1:3) as an eluent
(yield: 80%). Mp: 163 ◦C. 1H NMR (CDCl3): ı 7.72 (d, 2H, J = 8.4 Hz),
7.34 (d, 1H, J = 8.1 Hz), 7.25 (s, 2H), 7.19 (d, 2H, J = 8.4 Hz), 7.05 (m,
2H), 6.91 (s, 2H), 6.33 (d, 1H, J = 8.1 Hz), 5.62 (S, 1H), 3.78 (d, 2H,
J = 7.5 Hz), 3.66 (d, 2H, J = 7.5 Hz), 2.44 (t, 4H, J = 7.8 Hz), 1.53 (m,
4H), 1.27 (m, 12H), 1.12 (S, 3H), 0.91 (S, 3H), 0.81 (t, 6H, J = 5.1 Hz).
2.5.6. 4-Bromo-N,N-bis(4-hexylphenyl)aniline (6)
The product 5 (2.7 g, 5.23 mmol) was then subjected to reduction
using hydrazine hydrate (1 mL, 31.4 mmol), KOH (1 g, 17.3 mmol) in
ethylene glycol (50 mL) under reflux at 195 ◦C for 6 h. Ethylene gly-
col was then removed by distillation and the reaction mixture was
poured to crushed ice and neutralized with 1 M HCl (10 mL) and
extracted with dichloromethane. The combined dichloromethane
phases were then washed with H2O and dried with MgSO4. The
organic layer was removed in vacuo. The pure product 3 was
obtained by silica gel chromatography using a hexane as an eluent
(yield: 70%). Mp: 240 ◦C. 1H NMR (CDCl3): ı 7.29 (d, 2H, J = 8.7 Hz),
7.08 (d, 4H, J = 8.1 Hz), 7.00 (d, 4H, J = 8.7 Hz), 6.91 (d, 2H, J = 8.1 Hz),
2.57 (t, 4H, J = 7.8 Hz), 1.60 (m, 4H), 1.32 (m, 12H), 0.91 (t, 6H,
1
13C{ H} NMR (CDCl3): ı 147.0, 145.1, 141.2, 141.0, 137.6, 136.9,
134.5, 127.9, 127.1, 126.7, 126.4, 126.1, 125.7, 123.8, 123.7, 123.1,
122.5, 118.1, 98.1, 76.7, 34.9, 31.6, 31.2, 29.0, 22.5, 21.4, 21.1, 14.0.

20
C. Kim et al. / Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 17–25
MS: m/z 751 [M+]. Anal. Calcd. for C44H49NO2S4: C, 70.26; H, 6.57.
Found: C, 70.01; H, 6.45.
The organic layer was separated and dried in MgSO4. The solvent
was removed in vacuo. The pure product 12 was obtained by sil-
ica gel chromatography using a mixture of methylene chloride
and n-hexane (1:1) as an eluent (yield: 70%). Mp: 157 ◦C. 1H NMR
(CDCl3): ı 7.54 (s, 2H), 7.46 (d, 2H, J = 8.1 Hz), 7.36 (d, 1H, J = 7.2 Hz),
7.27 (m, 2H), 7.20–7.01 (m, 3H), 7.00 (s, 2H), 5.62 (s, 1H), 3.78 (d,
2H, J = 11.1 Hz), 3.66 (d, 2H, J = 11.1 Hz), 1.29 (S, 3H), 0.81 (S, 3H).
2.5.10. Bis(3-hexyl-6,1-phenylene)-5-(5-(DTTPA-2-
yl)thiophen-2-yl)thiophen-2-carbaldehyde (10)
THF (30 mL) and water (10 mL) were added to a flask containing
acetal 9 (0.4 g, 0.53 mmol). TFA (5 mL) was added to the solution.
The resulting reaction mixture was stirred for 6 h at room tem-
perature, quenched with saturated aqueous sodium bicarbonate,
and extracted with dichloromethane. The combined organic phases
were washed with aqueous sodium bicarbonate (2%, w/v), dried
with Na2SO4, and evaporated in vacuo. The pure product 10 was
obtained by silica gel chromatography using a mixture of methy-
lene chloride and n-hexane (1:1) as an eluent (yield: 91%). Mp:
188 ◦C. 1H NMR (CDCl3): ı 9.84 (s, 1H), 7.68 (d, 2H, J = 8.1 Hz), 7.65
(d, 2H, J = 8.1 Hz), 7.33 (m, 5H), 6.99 (s, 1H), 6.76 (d, 1H, J = 8.1 Hz),
6.44 (d, 1H, J = 8.1 Hz), 2.45 (t, 4H, J = 7.8 Hz), 1.52 (m, 4H), 1.26 (m,
1
13C{ H} NMR (CDCl3): ı 142.8, 142.5, 142.1, 141.3, 138.4, 137.5,
132.9, 131.6, 129.1, 128.5, 128.1, 127.8, 127.6, 126.7, 126.1, 125.7,
124.4, 121.5, 98.6, 77.9, 23.3, 22.1. MS: m/z 740 [M+]. Anal. Calcd.
for C32H23Br2NO2S4: C, 51.83; H, 3.13. Found: C, 51.31; H, 2.99.
2.5.14. Bis(4,9-hexylsulfanyl)-5-(5-(DTTPA-2-yl)thiophen-2-
yl)thiophen-2-carbaldehyde
(13)
To the solution of 1-hexanethiol (0.12 g, 0.77 mmol) in HMPA
˚
(30 mL, dried over 4 A molecular sieve) was added sodium hydride
1
12H), 0.86 (t, 6H, J = 5.1 Hz). 13C{ H} NMR (CDCl3): ı 182.5, 147.2,
(60% dispersion in mineral oil, 0.03 g, 0.77 mmol) in one portion.
The mixture was stirred under argon until the hydrogen evolution
ceased, then heated with an oil-bath at 100 ◦C for 5 min. Compound
12 (0.1 g, 0.18 mmol) was added and the mixture was stirred at
100 ◦C under argon atmosphere for 5 h to afford a yellow solu-
tion. Ice-water (50 mL) was added followed by the addition of
hydrochloric acid to adjust the pH to about 1. The mixture was
extracted with dichloromethane (2 × 50 mL) and the DCM phase
was dried over MgSO4. The solvent was removed in vacuo. The
pure product 13 was obtained by silica gel chromatography using
a mixture of methylene chloride and n-hexane (1:3) as an elu-
ent (yield: 55%). Mp: 189 ◦C. 1H NMR (CDCl3): ı 9.87 (s, 1H), 7.69
(d, 2H, J = 6.9 Hz), 7.60 (d, 1H, J = 6.6 Hz), 7.51 (s, 2H), 7.42 (d, 2H,
J = 6.9 Hz), 7.22 (m, 3H), 7.01 (s, 2H), 2.87 (t, 4H, J = 7.5 Hz), 1.64
145.5, 141.8, 138.2, 137.4, 137.3, 135.3, 128.5, 127.9, 127.7, 127.3,
127.2, 126.9, 126.2, 124.4, 124.2, 123.6, 118.5, 35.0, 31.8, 31.4, 29.1,
22.7, 14.2. MS: m/z 665 [M+]. Anal. Calcd. for C39H39NOS4: C, 70.33;
H, 5.90; N, 2.10. Found: C, 70.12; H, 5.78; N, 178.
2.5.11. 2-Cyano-3-((bis(3-hexyl-6,1-phenylene)-5-(5-(DTTPA-2-
yl)thiophen-2-yl)thiop-hen-2-yl)acrylic acid) (JK-206)
A mixture of 9 (0.40 g, 0.60 mmol) and cyanoacetic acid (0.07 g,
0.78 mmol) was vacuum-dried and added acetonitrile (20 mL),
chloroform (15 mL) and piperidine (0.02 mL). The solution was
refluxed for 6 h. After cooling the solution, the organic layer was
removed in vacuo. The pure product was obtained by silica gel chro-
matography using a mixture of methylene chloride and methanol
(9:1) as an eluent (yield: 87%). Mp: 254 ◦C. 1H NMR (CDCl3): ı 8.11
(s, 1H), 7.87 (s, 2H), 7.70 (d, 2H, J = 6.9 Hz), 7.59 (d, 1H, J = 6.9 Hz),
7.53 (d, 2H, J = 8.4 Hz), 7.32 (d, 1H, J = 8.4 Hz), 6.98 (s, 2H), 6.85 (d,
1H, J = 8.1 Hz), 6.40 (d, 1H, J = 8.1 Hz), 2.43 (t, 4H, J = 7.8 Hz), 1.47 (m,
1
(m, 4H), 1.41 (m, 4H), 1.33 (m, 8H), 0.88 (t, 6H, J = 6.9 Hz). 13C{ H}
NMR (CDCl3): ı 182.5, 147.0, 146.4, 144.2, 142.9, 141.8, 137.2, 135.4,
130.7, 130.5, 130.1, 128.9, 128.3, 126.2, 125.5, 124.8, 122.1, 120.4,
117.5, 36.7, 32.4, 32.0, 30.2, 23.4, 14.6. MS: m/z 729 [M+]. Anal.
Calcd. For C39H39NOS6: C, 64.16; H, 5.38. Found: C, 63.89; H, 5.15.
1
4H), 1.23 (m, 12H), 0.83 (t, 6H, J = 5.1 Hz). 13C{ H} NMR (CDCl3): ı
163.3, 145.6, 145.1, 140.2, 138.1, 136.4, 136.1, 134.2, 132.9, 127.9,
127.6, 127.4, 127.1, 126.8, 126.4, 125.9, 123.8, 123.5, 122.9, 118.3,
34.8, 31.6, 31.3, 29.0, 22.4, 14.1. MS: m/z 732 [M+]. Anal. Calcd. for
2.5.15. 2-Cyano-3-(bis(4,9-hexylsulfanyl)-5-(5-(DTTPA-2
-yl)thiophen-2-yl)thiophen-2-yl)acrylic acid (JK-207)
A mixture of 13 (0.07 g, 0.10 mmol) and cyanoacetic acid
(0.012 g, 0.14 mmol) was vacuum-dried and added acetonitrile
(20 mL), chloroform (20 mL) and piperidine (0.02 mL). The solution
was refluxed for 6 h. After cooling the solution, the organic layer
was removed in vacuo. The pure product of JK-207 was obtained
by silica gel chromatography using a mixture of methylene chlo-
ride and methanol (9:1) as an eluent (yield: 70%). Mp: 241 ◦C. 1H
NMR (CDCl3): ı 8.08 (s, 1H), 7.65 (d, 2H, J = 6.9 Hz), 7.62 (s, 2H), 7.50
(m, 3H), 7.15 (m, 5H), 2.84 (t, 4H, J = 7.5 Hz), 1.61 (m, 4H), 1.38 (m,
C
42H40N2O2S4: C, 68.82; H, 5.50; N, 3.82. Found: C, 68.71; H, 5.37;
N, 3.72.
2.5.12. 5-(5-(4,9-Dibromo-DTTPA-2-yl)thiophen-2-yl)thiophen-
2-carbaldehyde
(11)
To a solution of 3 (0.17 g, 0.35 mmol) in dichloromethane
(20 mL) and freshly distilled acetonitrile (20 mL) was added pyri-
dinium bromide perbromide (0.248 g, 0.78 mmol) in portions over
a period of 20 min. The pure product was obtained by silica gel
chromatography using a mixture of methylene chloride and hexane
(1:2) as an eluent (yield: 30%). Mp: 181 ◦C. 1H NMR (CDCl3): ı 9.89
(s, 1H), 7.68 (s, 2H), 7.55 (s, 2H), 7.48 (d, 2H, J = 7.8 Hz), 7.37 (d, 1H,
1
4H), 1.19 (m, 8H), 0.86 (t, 6H, J = 6.9 Hz). 13C{ H} NMR (DMSO): ı
163.1, 146.0, 145.1, 143.8, 142.1, 141.2, 136.7, 134.1, 132.5, 130.1,
129.6, 129.3, 128.4, 128.1, 125.9, 125.1, 124.2, 121.7, 119.8, 117.1,
36.3, 32.1, 31.6, 29.8, 22.9, 14.3. MS: m/z 796 [M+]. Anal. Calcd. For
C42H40N2O2S6: C, 63.28; H, 5.06; N, 3.51. Found: C, 63.02; H, 4.89;
N, 3.38.
J = 7.8 Hz), 7.28 (m, 2H), 7.14 (d, 1H, J = 8.1 Hz), 7.02 (m, 2H). 13C{ H}
1
NMR (CDCl3): ı 182.1, 147.9, 146.2, 145.4, 143.2, 142.7, 139.9, 138.0,
135.8, 134.1, 130.9, 130.7, 130.1, 128.7, 127.9, 126.4, 125.9, 124.8,
121.3. MS: m/z 654 [M+]. Anal. Calcd. for C27H13Br2NOS4: C, 49.47;
H, 2.00. Found: C, 49.31; H, 1.88.
3. Results and discussion
3.1. Synthesis
2.5.13. 2-(5-(5-(4,9-Dibromo-DTTPA-2)thiophen-2yl)
thiophen-2-yl)-5,5-dimethyl-1,3-dioxane (12)
Scheme
JK-205–207. Compound
tion of thia-bridged triarylamine heterohelicene with NBS
in CHCl3 [28]. The Suzuki coupling reaction [29–31] of
with 4,4,5,5-tetramethyl-2-(5-(5-(5,5-dimethyl-1,3-dioxan-2-
1
outlines the stepwise synthetic protocol of
To
a
stirred solution of 11 (0.4 g, 0.60 mmol) and p-
1
was synthesized by bromina-
toluenesulfonic acid monohydrate (0.012 g, 0.06 mmol) in benzene
(50 mL) was added neophentylglycol (0.08 g, 0.72 mmol) in ben-
zene. The mixture was refluxed at 90 ◦C for 8 h. After cooling the
solution, aqueous sodium bicarbonate (40 mL) was added to the
solution and extracted by H2O and dichloromethane (40 mL × 3).
a
1
yl)thiophen-2-yl)thiophen-2-yl)-1,3,2-dioxaborolane, 2, followed
by subsequent cleavage of 1,3-dioxalane protecting group in TFA

C. Kim et al. / Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 17–25
21
Scheme 1. Synthetic route of dyes JK-205–207.

22
C. Kim et al. / Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 17–25
The introduction of hexylsulfanyl unit in JK-205 decreases the
JK205
JK206
JK207
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
dihedral angles between phenyl amine and thienyl unit and the
two thienyl units from 25.6◦ and 6.3◦ in JK-205 to 23.3◦ and 5.9◦ in
JK-207, giving a more planar configuration (Fig. 2). A slight blue
shift relative to JK-206 can be interpreted as the H-aggregation
due to the intermolecular S· · ·S interaction. Such an intermolecular
S· · ·S contact has been observed in thiadiazole units incorporated
into polythiophene chains [33]. When the JK-205, JK-206, and
JK-207 sensitizers are excited within their ␲–␲* bands in an air-
equilibrated solution at 298 K, the photoluminescence spectrum
of JK-205 gives a broad absorption with a maximum centered at
643 nm. However, the PL spectra of JK-206 and JK-207 exhibit
strong luminescence maxima of 559 and 592 nm, respectively. The
blue shift of 84 nm in JK-206 and 51 nm in JK-207 from the JK-205
PL maximum at 643 nm is related to the narrow energy gap between
the ground and excited state due to the rigidity of both organic dyes,
which are difficult to rotate. Therefore, the large Stokes shift of JK-
205 relative to that of JK-206 and JK-207 may be attributable to the
easy rotation of JK-205 relative to JK-206 and JK-207 having long
alkyl chains.
400
500
600
700
800
Wavelength (nm)
Fig. 1. Absorption and emission spectra of JK-205 (solid line), JK-206 (dash line),
and JK-207 (dot line) in THF.
produced the aldehyde 4. The aldehyde 4 reacted with cyanoacetic
acid in the presence of a catalytic amount of piperidine to yield
the JK-205. For the synthesis of the dyes JK-206 and JK-207
the compounds 7 and 11 are key intermediates. The first key
intermediate 7 was synthesized by disulfenylation of phenyl unit
ortho to the amine nitrogen atom using the phthalimidesulfenyl
chloride (PhtNSCl) [32]. The second key material is the compound
11, which is synthesized by bromination of 3 using pyridinium
bromide perbromide. The three organic dyes are obtained in
moderate yields and soluble in common organic solvents.
3.3. Redox behavior of dyes
Cyclic voltammetry measurements were used to study the redox
behavior of the three dyes in THF (Table 1). The three dyes adsorbed
on TiO2 films show quasi-reversible couples. The oxidation poten-
tials of three dyes (1.15–1.41 V vs NHE) are more positive than the
Nernst potential of I−/I3− redox couple. The excited state oxidation
potentials (Eo∗x) of the dyes (−1.16 to −1.32 V vs NHE) are suffi-
ciently negative than the conduction band of TiO2 (−0.5 V vs NHE),
ensuring the thermodynamic driving force for electron injection
[34–36].
3.2. Spectroscopic studies and theoretical calculations of
geometrical properties
Fig. 1 shows the electronic absorption spectrum of JK-205–207
in THF. The absorption spectrum of JK-206 exhibits one strong
absorption peak at 432 nm, which is due to the ␲–␲* transition
of the conjugate molecule. Under similar conditions the JK-205
sensitizer shows an absorption peak at 422 nm that is slightly blue-
shifted relative to that of JK-206. A slight blue shift of JK-205
relative to JK-206 can be readily interpreted by the molecular mod-
eling study of the two dyes. The ground state structure of JK-205
possesses at 25.6◦ twist between phenylamine and thienyl unit.
The dihedral angle of the two thienyl units in JK-205 is 6.3◦ (Fig. 2).
For JK-206, the dihedral angles of the corresponding units are 22.6◦
and 6.1◦, respectively, giving a more planar configuration. Accord-
ingly, a red shift of JK-206 compared to JK-205 derives from more
delocalization over an entire conjugated system in JK-206.
3.4. Theoretical calculations of electronic properties
We performed DFT/TDDFT calculations to gain insight into the
nature of the electronic absorption origin. The calculation shows
that the HOMO−1 and HOMO of JK-205 are delocalized over the
␲-conjugated system throughout the dye (Fig. 3). The LUMO is a ␲*
orbital delocalized over ␲-conjugated system, with a sizable elec-
tron density distribution over the cyanoacrylic unit. Theoretical
excited-state TDDFT calculations illustrate that the lowest tran-
sition of JK-205 at 439 nm (f = 0.90) is mainly characterized by a
mixture of HOMO−1 → LUMO (64%) and HOMO → LUMO excita-
tion, affording a charge flow from the amino core to the anchoring
unit.
Fig. 2. The optimized structure calculated with TDDFT on B3LYP/3-21G* of JK-205, JK-206, and JK-207.

C. Kim et al. / Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 17–25
23
Table 1
Optical, redox, and DSSC performance parameters of dyes.
a
b
c
d
Dye
ꢁabs (nm) (ε, M−1 cm−1
)
Eox (V)
E0–0 (V)
ELUMO (V)
Jsc (mA cm−2
)
Voc (V)
ff
Áe (%)
JK-205
JK-206
JK-207
422(17,370)
432(13,090)
427(12,900)
1.15
1.24
1.41
2.47
2.4
2.43
−1.32
−1.16
−1.02
12.19
13.44
10.88
0.67
0.71
0.63
0.73
0.74
0.75
5.96
7.08
5.24
a
Absorption spectra were measured in THF solution.
b
c
Oxidation potentials of dyes on TiO2 were measured in CH3CN with 0.1 M (n-C4H9)4NPF6 with a scan rate of 50 mV s−1 (vs NHE).
E0–0 was determined from intersection of absorption and emission spectra in THF.
d
e
ELUMO was calculated by Eox − E0–0
.
Performances of DSSCs were measured with 0.18 cm2 working area. ε: absorption coefficient; Eox: oxidation potential; E0–0: voltage of intersection point between
absorption and emission spectra; Jsc: short circuit photocurrent density; Voc: open circuit photovoltage; ff: fill factor; Á: total power conversion efficiency.
Fig. 3. The molecular structures and the frontier molecular orbitals of the HOMO−1, HOMO and LUMO, LUMO+1 calculated with B3LYP/3-21G* of JK-205.
3.5. Photovoltaic performance
For reference, the photovoltaic performance of JK-2 is included.
The incident monochromatic photon-to-current conversion effi-
ciency (IPCE) of JK-206 exceeds 70% over the spectral region from
380 to 590 nm, reaching its maximum of 77% at 525 nm (inset of
Fig. 4). The bands tail off toward 710 nm, under standard global air
mass (AM) 1.5 solar conditions, the JK-206 sensitized cell gave a
short-circuit photo current density (Jsc) of 13.44 mA cm−2, an open-
circuit voltage (Voc) of 0.71 V, and a fill factor (ff) of 0.74, affording
an overall conversion efficiency (Á) of 7.08%. Under the same con-
The photovoltaic performances of the three dye-sensitized cells
are shown in Table 1.
JK-2
JK-205
JK-206
JK-207
17
16
15
14
13
12
11
ditions, the JK-205 sensitized cell gave a Jsc value of 12.19 mA cm−2
,
a Voc of 0.67 V and a ff of 0.73, corresponding to an Á value of 5.96%.
From these results (Table 1), we have observed that the Á value
of JK-206 based cell is higher than that of JK-205 based cell due
to a large photocurrent and voltage. Of particular importance is
the 40 mV increase of open-circuit voltage in JK-206 relative to
the JK-205 based cell. It has well been documented that alkyl
substitution of the sensitizer improved the Voc due to the blocking
10
100
90
JK-205
9
JK-206
8
80
70
60
50
40
30
20
10
JK-207
7
6
5
4
3
2
1
0
−
effect of the charge recombination between I3 and electrons
injected in the nanocrystalline electrode [37–41]. Similarly, the
power conversion efficiency of JK-207 is relatively low compared
to that of JK-205. To clarify the above result, we have measured the
amount of dyes adsorbed on the TiO2 film. The adsorbed amounts
of 2.86 × 10−6 mmol cm−2 for JK-205, 2.68 × 10−6 mmol cm−2
for JK-206, and 2.32 × 10−6 mmol cm−2 for JK-207 are observed.
Therefore, the large adsorbed amount of JK-205 relative to JK-207
contributes a high Voc due to the decrease of dark current.
0
300
400
500
600
)
700
Wavelength (nm
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Photovoltage (V)
Fig. 4. Photocurrent–voltage characteristics of representative TiO2 electrodes sen-
sitized with dye: JK-205 (solid line), JK-206 (dash line), and JK-207 (dot line). The
inset shows the IPCE spectra of the DSSCs as a function of the wavelength of the
light.
Another explanation is that the bent hexylsulfanyl units with a
large sweeping angle in JK-207 lead to a loose packing, leaving more

24
C. Kim et al. / Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 17–25
sc (mA cm-2)
13.5
J
13.0
12.5
12.0
11.5
11.0
10.5
0
0
200
200
400
400
600
600
800
800
1000
1000
Voc (mV)
700
600
500
0.8
0.7
0.6
FF
0
200
400
600
800
1000
7
6
5
4
3
E / %
0
200
400
600
800
1000
T / hr
Fig. 5. Evolution of solar-cell parameters with JK-205 (᭹), JK-206 (ꢀ) and JK-2 (ꢁ) during visible-light soaking (AM 1.5 G, 100 mW cm−2) at 60 ◦C. A 420 nm cut-off filter was
placed on the cell surface during illumination. Ionic liquid electrolyte 2: 0.2 M iodine, 0.5 M NMBI, 0.1 M GuNCS in PMII/EMINCS (13:7).
vacant sites between dye molecules. This may lead to facilitate the
the sulfur linkage in the alkyl group were much smaller than that
of JK-206 because the S· · ·S interaction of hexylsulfanyl units may
lead to the dye aggregation.
−
charge recombination between the electrons and I3
.
3.6. Stability of dyes
3.8. Electrochemical impedance study
Fig. 5 shows the photovoltaic performance during long-term
accelerated aging test of JK-205 and JK-206 using an ionic-liquid
electrolyte composed of 0.2 M I2, 0.5 M N-methyl benzimidazole
(NMBI), and 0.1 M guanidiniun thiocyanate (GuNCS) in 1-meth yl-
3-propylimidazolium iodide (PMII)/1-methyl-3-ethylimidazolium
thiocyanate (EMI NCS) (13:7). The device of JK-206 showed an
excellent long-term stability. The initial efficiency of 5.6% slightly
decreased to 5.4% during the 1000 h light-soaking test at 60 ◦C.
After 1000 h of light soaking test, the Voc of JK-206 decreased by
66 mV, but the loss was compensated by a slight increase in the
short-circuit current density.
Fig. 7 shows the ac impedance spectra of the DSSCs mea-
sured under illumination. A smaller radius of the semicircle
in this intermediate frequency regime implies a lower rate of
On the other hand, the initial efficiency of 4.8% in JK-205 sharply
decreased to 4.2% after 1000 h of light soaking at 60 ◦C because the
Voc of JK-205 sharply decreased by 97 mV. The enhanced long sta-
bility of JK-206 relative to JK-205 can be related t−o the introduction
of hexyl units by suppressing the approach of I3 to the TiO2 film,
resulting in preventing the dark current.
3.7. Laser-induced transient photocurrent and photovoltage
measurements
Fig. 6 shows the lifetimes of the DSSCs employing three dyes
(JK-205, JK-206, and JK-207) displayed as a function of the Voc. The
order of magnitude of the ꢀe values was well consistent with that
of the Voc shown in Table 1. The JK-206 dye having hexyl groups
exhibited approximately 3.7 times longer electron lifetime com-
pared with that of JK-205. However, the ꢀe values of JK-207 with
Fig. 6. Electrochemical impedance spectra measured under the illumination
(100 mW cm−2) for the cells employing different dyes (i.e. JK-205, JK-206, and JK-
207).

C. Kim et al. / Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 17–25
25
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electron recombination at the TiO2/dye/electrolyte interface. Upon
illumination under open-circuit conditions (100 mW cm−2), the
response of intermediate semicircle is attributable to the deposited
dye on the TiO2/electrolyte. A capacitance (689.3 ␮F/cm2) of JK-
206 derived from (Y0 × R)(1/n)/R, where Y0 [sn cm−2] is the symbol
for the constant phase element, n is freq power, and R is resis-
tance, is higher than that of JK-205 (523 ␮F/cm2), and JK-207
(428.3 ␮F/cm2) and the resistance of JK-206 (13.42 ꢂ) is low
compared with that of JK-205 (14.36 ꢂ) and JK-207 (15.44 ꢂ).
As a result, the photocurrent of JK-206 (13.44 mA cm−2), JK-205
(12.19 mA cm−2), and JK-207 (10.88 mA cm−2) is in good agree-
ment with the trends observed in the above data.
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We have meticulously designed and synthesized a new class of
thia-bridged heterohelicene dyes. A solar-cell device based on the
sensitizer JK-206 in conjunction with a volatile electrolyte gave an
overall conversion efficiency of 7.08%. The power conversion effi-
ciency of the DSSCs based on the three sensitizers was shown to be
sensitive to the substituents. The JK-206 device showed an excel-
lent stability under light soaking at 60 ◦C for 1000 h. We believe that
development of highly efficient DS SCs devices with robustness is
possible through the structural modifications, and work on these is
now in progress.
Acknowledgment
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(2008) 4951.
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We are supported by a Korea University Grant (2011).
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