Molecular engineering of organic sensitizers containing p-phenylene vinylene unit for dye-sensitized solar cells

Article abstract of DOI:10.1021/jo8005182

(Chemical Equation Presented) Three organic sensitizers containing bis-dimethylfluorenyl amino donor and a cyanoacrylic acid acceptor bridged by p-phenylene vinylene unit were synthesized. The power conversion efficiency was quite sensitive to the length of bridged phenylene vinylene groups. A nanocrystalline TiO₂ dye-sensitized solar cell was fabricated using three sensitizers. The maximum power conversion efficiency of JK-59 reached 7.02%.

Full text of DOI:10.1021/jo8005182

Molecular Engineering of Organic Sensitizers Containing
p-Phenylene Vinylene Unit for Dye-Sensitized Solar Cells
Chulwoo Kim,† Hyunbong Choi,† Sanghoon Kim,† Chul Baik,† Kihyung Song,‡
Moon-Sung Kang,§ Sang Ook Kang,*,† and Jaejung Ko*,†
Department of Chemistry, Korea UniVersity, Jochiwon, Chungnam 339-700, Korea, Department of
Chemical Education, Korea National UniVersity of Education, Cheongwon, Chungbuk 363-791, Korea, and
Energy & EnVironment Laboratory, Samsung AdVanced Institute of Technology (SAIT),
Yongin, 446-712, Korea
jko@korea.ac.kr
ReceiVed March 6, 2008
Three organic sensitizers containing bis-dimethylfluorenyl amino donor and a cyanoacrylic acid acceptor
bridged by p-phenylene vinylene unit were synthesized. The power conversion efficiency was quite
sensitive to the length of bridged phenylene vinylene groups. A nanocrystalline TiO2 dye-sensitized solar
cell was fabricated using three sensitizers. The maximum power conversion efficiency of JK-59 reached
7.02%.
Introduction
standard soar illumination (global air mass 1.5).2 Although the
ruthenium-based sensitizers exhibited high efficiency and good
stability, they are quite expensive and hard to purify compared
to the organic sensitizers. Some organic dyes show the impres-
sive photovoltaic performance having efficiencies in the range
of 7-9%.3 One of the main drawbacks of organic sensitizers is
the shape and narrow absorption band in the blue region. The
optimal organic sensitizer for solar cell applications should be
broadened and red-shifted. All of the organic dyes have a
structure with donor-to-acceptor moieties bridged by a π-con-
Global warming and depletion of fossil fuels have led to a
greater focus on renewable energy sources in recent years. In
this respect, dye-sensitized solar cells are attracting widespread
interest for a new renewable energy source because of the low
cost and high efficiency.1 Several Ru(II) polypyridyl complexes
have achieved power conversion efficiencies above 10% under
* To whom correspondence should be addressed. Tel: 82 41 860 1337. Fax:
82 41 867 5396.
† Korea University.
‡ Korea National University of Education.
§ Samsung Advanced Institute of Technology (SAIT).
(1) (a) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (b) Grtzel, M. Nature
2001, 414, 338.
(2) (a) Grtzel, M. J.Photochem.Photobiol. C 2003, 4, 145. (b) Nazeeruddin,
M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.;
Takeru, B.; Grtzel, M. J. Am. Chem. Soc. 2005, 127, 16835. (c) Grtzel, M. Prog.
PhotoVolt Res. Appl. 2006, 14, 429.
7072 J. Org. Chem. 2008, 73, 7072–7079
10.1021/jo8005182 CCC: $40.75 2008 American Chemical Society
Published on Web 08/27/2008

Molecular Engineering of Organic Sensitizers
jugation unit. In most organic sensitizers, the amine derivatives
act as the electron donor while a 2-cyanoacrylic acid or
rhodanine moiety acts as the electron acceptor. These two parts
are connected by π-conjugation systems such as the methine
unit or thiophene chain. Recently, a successful approach was
introduced by incorporating a π-conjugation system such as the
chloride.8 The formyl group of diethyl(4-formyl-2,5-bis(iso-
pentoxy)phenyl)methyl phosphonate 4 was protected with
neopentyl glycol to afford acetal 5. Carbaldehyde 7 was prepared
from 6 by means of a Vilsmeier-Haack reaction.9 Coupling
reaction of carbaldehyde 7 with diethyl(2,5-bis(isopentoxy)-4-
(5,5-dimethyl-1,3-dioxan-2-yl)phenyl)methylphosphonate 5 un-
der Horner-Emmons-Wittig reaction10 led to intermediate 8.
The phenylene vinylene derivative 8 was converted into its
corresponding aldehyde 9 by dedioxanylation with trifluoroacetic
acid (TFA). The aldehyde 9, upon reaction with cyanoacetic
acid in the presence of piperidine in acetonitrile, produced the
JK-57 dye. The two sensitizers JK-58 and JK-59 were readily
synthesized by the stepwise addition of the aldehyde 9 with 5
and 12 with 5, respectively.
Figure 1 shows the absorption and emission spectra of the
JK-57, JK-58, and JK-59 sensitizers in THF, and the data are
collected in Table 1. The absorption spectrum of JK-57
exhibited two absorption bands at 457 and 369 nm, which are
due to the π-π* transitions of the conjugated system. Under
the same conditions, the JK-58 sensitizer displays absorption
bands at 466 and 373 nm. The red-shifted band about 10 nm of
JK-58 compared to that of JK-57 is due to the increase of the
π-conjugation systems by phenylene vinylene. A progressive
red-shifted band of JK-59 compared to JK-58 is due to the
introduction of one more phenylene vinylene group to JK-58.
The absorption spectra of JK-57-JK-59 on TiO2 film are
broadened and red-shifted due to the interaction of the anchoring
group with the surface titianium ions (Figure 2). Such a
broadening and red-shift have been reported in several organic
sensitizers on TiO2 electrodes.11 The threshold wavelength of
absorption spectra for sensitizers JK-57, JK-58, and JK-59 are
560, 600, and 620 nm, respectively, indicating that the stepwise
introduction of a phenylene vinylene into the organic framework
expanded the conjugation resulting in a wide absorption in the
visible region. The wide absorption and red shift of absorption
maximum in the visible region is desirable for light harvesting
of solar energy. We observed that the organic dyes JK-57-JK-
59 exhibited strong luminescence maxima of 575-612 nm when
they are excited within their π-π* bands at 298 K.
4
thiophene chain into the organic framework, which not only
increases the molar extinction coefficient of the organic
sensitizer but also increases the stability. Moreover, the
introduction of thiophene chain led to the broadening of the
absorption spectra and gave a red shift in the absorption
spectrum. As part of our efforts to investigate the structural
modifications of π-conjugated unit that can enhance the ef-
ficiency and stability, phenylene vinylene units instead of
thiophene chain have been synthesized. The π-conjugated
oligomers are intensively studied for use in optoelectronic
devices because their chemical structure facilitates tuning of
electronic properties. In particular, oligo(p-phenylene vinylene)s
are being actively investigated for use in solar cells and light-
emitting diodes due to their stability and high luminescent
efficiency. 5 In this paper, we report three new organic sensitizers
(JK-57, JK-58, and JK-59) containing bis-dimethylfluorenyl
amino group as electron donor and cyanoacrylic acid as electron
acceptor bridged by phenylene vinylene unit. We also describe
the effect of bridged structural modifications on the photovoltaic
properties, electric, and optical properties.
Results and Discussion
Scheme 1 illustrates the stepwise synthetic protocol of organic
dyes JK-57, JK-58, and JK-59. A Williamson etherification
of methylhydroquinone with 1-bromo-3-methy-lbutane gave 1,4-
bis(isopentoxy)-2-methylbenezene 1.6 Compound 3 was ob-
tained by bromination reaction of 1 using N-bromosuccinimide
(NBS) and 2,2′-azobis(2-methylpropionitrile) (AIBN) followed
by a Michaelis-Arbuzor reaction.7 Formylation of 3 was carried
out using R,R-dichloromethyl methyl ether and titanium(IV)
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2004, 126, 12218. (b) Ito, S.; Zakeeruddin, S. M.; Humphry-Baker, R.; Liska,
P.; Charvet, R.; Comte, P.; Nazeeruddin, M. K.; Pe´chy, P.; Takata, M.; Miura,
H.; Uchida, S.; Grtzel, M. AdV. Mater. 2006, 18, 1202. (c) Kitamura, T.; Ikeda,
M.; Shigaki, K.; Inoue, T.; Anderson, N. A.; Ai, X.; Lian, T.; Yanagida, S.
Chem. Mater. 2004, 16, 1806. (d) Wang, Z.-S.; Cui, Y.; Hara, K.; Dan-oh, Y.;
Kasada, C.; Shinpo, A. AdV. Mater. 2007, 19, 1138. (e) Koumura, N.; Wang,
Z.-S.; Mori, S.; Miyashita, M.; Suzuki, E.; Hara, K. J. Am. Chem. Soc. 2006,
128, 14256. (f) Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Ynm, J.-H.; Fantacci,
S.; De Angelis, F.; Di Censo, D.; Nazeeruddin, M. K.; Grtzel, M. J. Am. Chem.
Soc. 2006, 128, 16701. (g) Kim, S.; Choi, H.; Baik, C.; Song, K.; Kang, S. O.;
Ko, J. Tetrahedron 2007, 63, 11436. (h) Choi, H.; Lee, J. K.; Song, K.; Kang,
S. O.; Ko, J. Tetrahedron 2007, 63, 3115.
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M. K.; Grtzel, M. Angew. Chem., Int. Ed. 2008, 47, 327. (b) Hara, K.; Kurashige,
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M. Chem. Commun. 2006, 2792. (d) Hagberg, D. P.; Edvinsson, T.; Marinado,
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R.; Yang, X.; Tian, H.; Wang, X.; Hagfeldt, A.; Sun, L. Chem. Mater. 2007,
19, 4007.
To thermodynamically evaluate the possibility of electron
transfer from the excited-state of the dye to the conduction band
of TiO2 electrode, electrochemical properties of the JK-57, JK-
58, and JK-59 sensitizers were scrutinized by cyclic voltam-
metry in THF solvent containing 0.1 M tetrabutylammonium
hexafluorophosphate (Figure 6 in Supporting Information). The
oxidation potential of JK-57 exhibited a quasi-reversible
behavior at 1.13 V (versus NHE) with a peak separation of 0.12
V, which is attributed to the oxidation of 3-(5-(N,N-bis(9,9-
dimethylfluoren-2-yl)aminophenyl)group. Under similar condi-
tions, dyes JK-58 and JK-59 show a quasi-reversible couple
at 1.13 and 1.09 V, respectively. Analysis of the quasi-reversible
process in three dyes shows that the oxidation potentials are
close, due to the similar donor groups. The reduction potentials
of JK-57-JK-59 calculated from the oxidation potential and
(5) (a) Grell, M.; Bradley, D. D. C. AdV. Mater. 1999, 11, 895. (b) Wienk,
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J. Org. Chem. Vol. 73, No. 18, 2008 7073

Kim et al.
SCHEME 1. Schematic Diagram for the Synthesis of Organic Sensitizers JK-57, JK-58, and JK-59
7074 J. Org. Chem. Vol. 73, No. 18, 2008

Molecular Engineering of Organic Sensitizers
Figure 5 shows the electron-diffusion coefficient (De) and
lifetime (τe) of the DSSCs employing different dyes (i.e., N719,
JK-57, JK-58, and JK-59) as a function of the Jsc. The De and
τe values were determined by the photocurrent and photovoltage
transients induced by a stepwise change in the laser light
intensity controlled with a function generator.12 The De value
was obtained by a time constant (τc) determined by fitting a
decay of the photocurrent transient with exp(-t/τc) and the TiO2
film thickness (ω) using the equation De ) ω2/(2.77τc). The τ
value was also determined by fitting a decay of photovoltage
transient with exp(-t/τ). The Jsc values in the x-axis increased
with an increase in the initial laser intensity controlled by ND
filters with different optical densities. The De values of the
photoanodes adsorbing the organic dyes are shown to be very
similar to those of N719 at the identical short-circuit current
conditions. Meanwhile, the significant difference in the τe values
was observed among the cells employing different dyes. The
different τe values might be caused by the different molecular
size and structures of the dyes.13 In particular, the τe values of
JK-59 were smaller than those of JK-58, demonstrating the
decreased amount of dyes on TiO2 due to the steric hinderence
caused by the bulky structure. We have measured the amounts
of dyes adsorbed on a 10 µm TiO2 film. The adsorbed amounts
of 2.27 × 10-7 mol/cm2 for JK-57, 7.81 × 10-8 mol/cm2 for
JK-58, and 3.45 × 10-8 mol/cm2 for JK-59 are observed. It
seems that the electron recombination rates were dependent upon
the molecular structures and the intermolecular π-π stacking
interactions of the dyes due to the different coverages on the
TiO2 surface. The results of the electron lifetime are well
consistent with those of the Voc shown in Table 1. The
conflicting contribution of diffusion coefficient and lifetime can
be combined and expressed by means of the diffusion length
(Ld). Diffusion length of electrons is denoted as Ld ) (De ·τe)0.5.14
The diffusion length was shown to increase in the order of N719
(17.02 µm) > JK-58 (15.29 µm) > JK-59 (14.27 µm) > JK-
57 (12.76 µm).
FIGURE 1. Absorption and emission spectra of JK-57 (dash dot line),
JK-58 (dash line), and JK-59 (solid line) in THF.
the E0-0 determined from the intersection of absorption and
emission spectra are listed in Table 1.22 The excited-state
oxidation potential (E*OX) of the sensitizers (JK-57, -1.28 V;
JK-58, -1.25 V; JK-59, -1.27 V versus NHE) is much
negative than the conduction band level of TiO2 at ap-
proximately -0.5 V versus NHE, ensuring that there is enough
driving force for the electron injection into the conduction band
of TiO2.
In order to determine the photophysical properties, molecular
orbital calculations of JK-57-JK-59 sensitizers were performed
with the TD-DFT on B3LYP/3-21G* (Figure 3). The calculation
illustrates that the HOMO of JK-57 is delocalized over the
π-conjugated system centered on the amino phenyl unit, and
LUMO is delocalized over the cyanoacrylic unit through the
phenylene vinylene group. From these results, we could induce
that the photoinduced electron transfer from JK-57, JK-58, and
JK-59 dyes to TiO2 electrode can efficiently occur by the
HOMO f LUMO transition.
Figure 4 shows the incident monochromatic photon-to-current
conversion efficiency (IPCE) for the DSSCs based on the three
dyes. The IPCE data of JK-57 sensitizer plotted as a function
of excitation wavelength exhibits a high plateau at 68%. The
decline of the IPCE above 570 nm toward the red is caused by
the decrease in the extinction coefficient of JK-57. The JK-58
and JK-59 sensitizers’ IPCE spectra are broadened and red-
shifted by about 40 nm compared to JK-57 as a result of
extended π-conjugation, which is consistent with the absorption
spectra of the JK-58 and JK-59 sensitizers. The onset of the
IPCE spectra of the JK-58 and JK-59 sensitized cells are 701
and 720 nm. Figure 4 also shows the photocurrent-voltage
(J-V) curves of three DSSCs fabricated with TiO2 films, which
are anchored with JK-57-JK-59. The photovoltaic performance
of the device is listed in Table 1. Under standard global AM
1.5 solar condition, the JK-57-sensitized cell gave a short circuit
photocurrent density (Jsc) of 10.50 mA cm-2, open circuit
voltage (Voc) of 0.69 V, and a fill factor of 0.73, corresponding
to an overall conversion efficiency η of 5.34%. Under the same
conditions, an overall conversion efficiency (η) of 6.89% for
JK-58 and 7.02% for JK-59 sensitized cells (for JK-58: short-
circuit photocurrent density, Jsc ) 13.26 mA cm-2; open-circuit
photovoltage, Voc ) 0.70 V; fill factor, ff ) 0.71. For JK-59:
Jsc ) 14.26 mA cm-2; Voc ) 0.70 V; ff ) 0.70) was obtained.
The Jsc enhancement of JK-58 and JK-59 relative to JK-57
can be related to the expansion of the π-conjugated system of
the dyes.
In summary, we have designed and synthesized three organic
sensitizers containing amorphous N,N-bis(9,9-dimethylfluoren-
2-yl)aniline unit bridged by phenylene vinylene units. The power
conversion efficiency of the DSSCs based on the three sensitiz-
ers was shown to be sensitive to the conjugation length of
bridged phenylene vinylene groups. The maximum power
conversion efficiency of the DSSCs using the JK-59 sensitizer
reaches 7.02%.
Experimental Section
Preparation of DSSC. For the preparation of DSSC, fluorine-
doped tin oxide (FTO) glass plates (2.3 mm thickness) were cleaned
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109, 23776.
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O.; Drillon, M.; Rabu, P. Chem. Mater. 2004, 16, 2933.
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Lysenko, A. B.; Mathur, G.; Li, Q.; Bocian, D. F.; Misra, V.; Lindsey, J. S. J.
Org. Chem. 2004, 69, 1453.
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Kim et al.
TABLE 1. Optical, Redox, and DSSC Performance Parameters of Dyes
a
b
c
d
dye
λabs /nm(ꢀ/M-1cm-1
)
Eox (V)
E0-0 (V)
ELUMO (V)
Jsc (mA cm-2
)
Voc (V)
FF
ηe (%)
JK-57
JK-58
JK-59
N719
369 (35710), 457 (38650)
373 (49670), 466 (70056)
375 (45590), 472 (68390)
535 (14700)
1.13
1.13
1.09
2.41
2.38
2.38
-1.28
-1.25
-1.27
10.50
13.26
14.26
17.33
0.69
0.73
0.70
0.73
0.73
0.71
0.70
0.72
5.34
6.89
7.02
9.02
a Absorption spectra were measured in THF solution. b Oxidation potentials of dyes on TiO2 were measured in THF with 0.1 M (n-C4H9)4NPF6 with a
c
d
scan rate of 50 mVs-1 (vs NHE). E0-0 was determined from intersection of absorption and emission spectra in THF. ELUMO was calculated by Eox
-
E0-0
.
e Performances of DSSCs were measured with 0.18 cm2 working area. Key: ꢀ, 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; η, total
power conversion efficiency.
0.5 M tert-butylpyridine in acetonitrile. For photovoltaic measure-
ment of the DSSCs, the cells were measured using 1000 W xenon
light source.
Photoelectrochemical Measurement. The cells were measured
using 1000 W xenon light source, whose power of an AM 1.5 solar
simulator was calibrated 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.
1,4-Bis(isopentoxy)-2-methylbenzene (1). Yield: 59%. 1H NMR
(CDCl3): δ 6.77 (d, J ) 7.5 Hz, 1H), 6.76 (s, 1H), 6.69 (d, J ) 7.5
Hz, 1H), 3.95 (t, J ) 6.3 Hz, 2H), 3.94 (t, J ) 6.6 Hz, 2H), 2.23
(s, 3H), 1.86 (m, 2H), 1.68 (m, 4H), 1.00 (d, J ) 6.9 Hz, 6H), 0.99
(d, J ) 6.9 Hz, 6H). 13C{1H} NMR (CDCl3): δ 152.9, 151.6, 128.2,
117.8, 112.2, 111.7, 67.2, 66.9, 38.4, 38.3, 25.3, 25.2, 22.8, 22.7,
16.5. Anal. Calcd for C17H28O2: C, 77.22; H, 10.67. Found: C,
77.01; H, 10.51.
2-(Bromomethyl)-1,4-bis(isopentoxy)benzene (2). Yield: 40%. 1H
NMR (CDCl3): δ 6.90 (s, 1H), 6.80 (d, J ) 1.2 Hz, 2H), 4.53 (s,
2H), 3.98 (t, J ) 6.5 Hz, 2H), 3.95 (t, J ) 6.5 Hz, 2H), 1.85 (m,
FIGURE 2. Absorption spectra of JK-57 (dash dot line), JK-58 (dash
2H), 1.68 (q, J ) 5.0 Hz, 2H), 1.63 (q, J ) 5.0 Hz, 2H), 0.99 (d,
line), and JK-59 (solid line) adsorbed on TiO2 film.
J ) 6.9 Hz, 6H), 0.97(d, J ) 6.9 Hz, 6H). 13C{1H} NMR (CDCl3):
δ 152.9, 151.6, 127.2, 117.1, 115.9, 113.2, 67.5, 67.2, 38.3, 38.2,
in a detergent solution using an ultrasonic bath for 30 min and
then rinsed with water and ethanol. Then, the plates were immersed
in 40 mM TiCl4 (aqueous) at 70 °C for 30 min and washed with
water and ethanol. A transparent nanocrystalline layer was prepared
on the FTO glass plates by using a doctor blade printing TiO2 paste,
which was then dried for 2 h at 25 °C. The TiO2 electrodes were
gradually heated under an air flow at 325 °C for 5 min, at 375 °C
for 5 min, at 450 °C for 15 min, and at 500 °C for 15 min. The
thickness of the transparent layer was measured by using an Alpha-
step 250 surface profilometer. A paste containing 400 nm sized
anatase particles was deposited by means of doctor blade printing
to obtain the scattering layer and then dried for 2 h at 25 °C. The
TiO2 electrodes were gradually heated under an air flow at 325 °C
for 5 min, at 375 °C for 5 min, at 450 °C for 15 min, and at 500
°C for 15 min. The resulting film was composed of a 10 µm thick
transparent layer and a 4 µm thick scattering layer. The TiO2
electrodes were treated again with TiCl4 at 70 °C for 30 min and
sintered at 500 °C for 30 min. Then, The TiO2 electrodes were
immersed into the dyes (JK-57, JK-58, and JK-59) solution (0.3
mM in THF) and kept at room temperature for 18 h. FTO plates
for the counter electrodes were cleaned in an ultrasonic bath in
H2O, acetone, and 0.1 M aqueous HCl, subsequently. The counter
electrodes were prepared by placing a drop of an H2PtCl6 solution
(2 mg Pt in 1 mL ethanol) on an FTO plate and heating it (at 400
°C) for 15 min. The dye-adsorbed TiO2 electrodes and the Pt counter
electrodes were assembled into a sealed sandwich-type cell by
heating with a Surlyn film as a spacer between the electrodes. A
drop of the electrolyte solution was placed in the drilled hole of
the counter electrode and was driven into the cell via vacuum
backfilling. Finally, the hole was sealed using additional Surlyn
and a cover glass (0.1 mm thickness). The electrolyte was then
introduced into the cell, which was composed of 0.6 M 3-hexyl-
1,2-dimethyl imidazolium iodide, 0.05 M iodine, 0.1 M LiI and
29.2, 25.3, 25.2, 22.8, 22.7. Anal. Calcd for C17H27BrO2: C, 59.48;
H, 7.93. Found: C, 59.21; H, 7.81.
Diethyl (2,5-Bis(isopentoxy)phenyl)methylphosphonate (3). Yield:
75%. 1H NMR (CDCl3): δ 6.94 (s, 1H), 6.78 (d, J ) 9.0 Hz, 1H),
6.75 (d, J ) 9.0 Hz, 1H), 4.02 (t, J ) 6.0 Hz, 4H), 3.94 (t, J ) 5.0
Hz, 4H), 3.22 (d, J ) 21.3 Hz, 2H), 1.81 (m, 2H), 1.65 (m, 4H),
1.26 (q, J ) 6.9 Hz, 6H), 0.97 (d, J ) 6.3 Hz, 6H), 0.96 (d, J )
6.3 Hz, 6H). 13C{1H} NMR (CDCl3): δ 152.9, 151.1, 121.5, 117.6,
114.0, 112.7, 67.2 (Jc-p ) 39.2 Hz), 62.0, 61.9, 38.4, 38.3, 27.6,
25.2, 25.1, 22.8, 22.7, 16.5, 16.4. 31P NMR (CDCl3): δ 21.6. Anal.
Calcd for C21H37O5P: C, 62.98; H, 9.31. Found: C, 62.77; H, 9.10.
Diethyl (4-Formyl-2,5-bis(isopentoxy)phenyl)methylphosphonate
1
(4). Yield: 75%. H NMR (CDCl3): δ 10.48 (s, 1H), 7.34 (s, 1H),
7.14 (s, 1H), 4.10 (m, 8H), 3.38 (d, J ) 22.2 Hz, 2H), 1.89 (m,
2H), 1.78 (m, 4H), 1.30 (t, J ) 7.2 Hz, 6H), 1.04 (d, J ) 6.6 Hz,
12H). 13C{1H} NMR (CDCl3): δ 189.6, 155.9, 150.9, 129.6, 124.0,
116.3, 109.2, 67.4 (JC-P ) 28.1 Hz), 62.3, 62.2, 38.2, 38.0, 28.2,
25.3, 25.2, 22.7, 22.6, 16.5, 16.4. 31P NMR (CDCl3): δ 20.3. Anal.
Calcd for C22H37O6P: C, 61.67; H, 8.70. Found: C, 61.37; H, 8.56.
Diethyl (2,5-Bis(isopentoxy)-4-(5,5-dimethyl-1,3-dioxan-2-yl)phe-
nyl)methylphosphonate (5). Yield: 96%. 1H NMR (CDCl3): δ 7.09
(s, 1H), 6.90 (s, 1H), 5.66 (s, 1H), 3.95 (m, 8H), 3.69 (s, 2H), 3.58
(s, 2H), 3.17 (d, J ) 21.6 Hz, 2H), 1.77 (m, 2H), 1.61 (m, 4H),
(20) Yang, F.; Xu, X.-L.; Gong, Y.-H.; Qiu, W.-W.; Sun, Z.-R.; Zhou, J.-
W.; Audebert, P.; Tang, J. Tetrahedron 2007, 63, 9188.
(21) (a) Kim, S.; Choi, H.; Kim, D.; Song, K.; Kang, S. O.; Ko, J. Tetrahedron
2007, 63, 9206. (b) Kim, D.; Lee, J. K.; Kang, S. O.; Ko, J. Tetrahedron 2007,
63, 1913. (c) Choi, H.; Lee, J. K.; Song, K. H.; Song, K.; Kang, S. O.; Ko, J.
Tetrahedron 2007, 63, 1553.
(22) (a) Wang, Z.-S.; Li, F.-T.; Huang, C.-H. J. Phys. Chem. B 2001, 105,
9210. (b) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo, A.; Suga,
S.; Sayama, K.; Sugihara, H.; Arakawa, H. J. Phys. Chem. B 2003, 107, 597.
(c) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Yoshihara, T.; Murai, M.;
Kurashige, M.; Ito, S.; Shinpo, A.; Suga, S.; Arakawa, H. AdV. Funct. Mater.
2005, 15, 246.
7076 J. Org. Chem. Vol. 73, No. 18, 2008

Molecular Engineering of Organic Sensitizers
FIGURE 3. Molecular structures and the frontier molecular orbitals of the HOMO and LUMO calculated with B3LYP/3-21G* of (a) JK-57, (b)
JK-58, and (c) JK-59.
120.1, 119.9, 47.1, 27.1. Anal. Calcd for C37H31NO: C, 87.89; H,
6.18. Found: C, 87.67; H, 6.09.
N,N-Bis(9,9-dimethylfluoren-2-yl)-2-(2,5-bis(isopentoxy)-4-
styrylphenyl)-5,5-dimethyl-1,3-dioxane (8). A stirred mixture of 7
(0.3 g, 0.60 mmol), 5 (0.34 g, 0.65 mmol), and potassium tert-
butoxide (0.08 g, 0.72 mmol) was vacuum dried, and THF (25 mL)
was added. The solution was refluxed for 6 h. THF was removed
under reduced pressure. The crude mixture was redissolved in
dichloromethane, washed with water, and dried with MgSO4. The
organic layer was removed in vacuo. The pure product 8 was
obtained by silica gel chromatography (eluent MC/Hx ) 1:2, Rf )
1
0.3) to afford 8 (0.24 g) in 80% yield. Mp: 197 °C. H NMR
(CDCl3): δ 7.70-7.71 (m, 22H), 5.80 (s, 1H), 4.12 (t, J ) 6.3 Hz,
2H), 4.10 (t, J ) 6.3 Hz, 2H), 3.83 (d, J ) 10.2 Hz, 2H), 3.73 (d,
J ) 10.2 Hz, 2H), 1.94 (m, 2H), 1.77 (m, 4H), 1.46 (s, 12H), 1.38
(s, 3H), 1.05 (d, J ) 5.1 Hz, 6H), 1.04 (d, J ) 5.1 Hz, 6H), 0.84
(s, 3H). 13C{1H} NMR (CDCl3): δ 155.2, 153.7, 151.1, 150.0, 147.4,
139.1, 134.3, 129.8, 129.2, 128.3, 127.5, 127.1, 126.6, 124.0, 123.3,
122.6, 122.3, 120.8, 119.6, 118.8, 117.2, 111.3, 110.6, 97.2, 72.0,
68.2, 47.0, 38.4, 30.4, 27.2, 25.5, 23.4, 22.9, 22.2. Anal. Calcd for
C60H67NO4: C, 83.20; H, 7.80. Found: C, 83.02; H, 7.71.
FIGURE 4. Photocurrent-voltage characteristics of representative TiO2
electrodes sensitized with dye: JK-57 (dash dot line), JK-58 (dash line),
JK-59 (solid line), and N719(short dot). The inset shows the IPCE
spectra of the DSSCs as a function of the wavelength of the light.
N,N-Bis(9,9-dimethylfluoren-2-yl)-2-(2,5-bis(isopentoxy))-4-styryl-
benzaldehyde (9). THF (30 mL) and water (10 mL) were added to
a flask containing 8 (0.3 0.35 mmol). Then, TFA (3 mL) was added
to the solution. The resulting reaction mixture was stirred for 2 h
at room temperature. The solution was quenched with saturated
aqueous sodium bicarbonate and extracted with dichloromethane.
The combined dichloromethane phases were then washed with
aqueous sodium bicarbonate and dried with MgSO4. The organic
layer was removed in vacuo. The pure product 9 was obtained by
silica chromatography (eluent MC/Hx ) 1:1, Rf ) 0.3) to afford 9
(0.24 g) in 80% yield. Mp: 220 °C. 1H NMR (CDCl3): δ 10.47 (s,
1H), 7.69 ∼ 7.14 (m, 22H), 4.19 (t, J ) 6.6 Hz, 2H), 4.11 (t, J )
6.6 Hz, 2H), 1.92 (m, 2H), 1.78 (m, 4H), 1.45 (s, 12H), 1.03 (d, J
1.26 (s, 3H), 1.20 (t, J ) 7.0 Hz, 6H), 0.90 (d, J ) 6.6 Hz, 12H),
0.73 (s, 3H). 13C{1H} NMR (CDCl3): δ 150.8, 149.7, 126.3, 121.5,
115.8, 110.1, 97.0, 67.8, 67.1, 61.8, 61.7, 38.2, 38.1, 30.2, 27.2,
25.4, 25.1, 25.0, 23.2, 22.6, 21.8, 16.4, 16.3. 31P NMR (CDCl3): δ
20.3. Anal. Calcd for C27H47O7P: C, 63.01; H, 9.21. Found: C,
62.87; H, 9.10.
4-(N,N-Bis(9,9-dimethylfluoren-2-yl))benzaldehyde (7). Yield:
1
91%. H NMR (CDCl3): δ 9.84 (s, 1H), 7.73 ∼ 7.18 (m, 18H),
1.43 (s, 12H). 13C{1H} NMR (CDCl3): δ 190.6, 155.6, 153.8, 145.8,
138.7, 136.3, 131.5, 129.4, 127.3, 127.2, 125.0, 122.7, 121.1, 120.5,
J. Org. Chem. Vol. 73, No. 18, 2008 7077

Kim et al.
5.76 (s, 1H), 4.09 (m, 8H), 3.80 (d, J ) 10.2 Hz, 2H), 3.70 (d, J
) 10.2 Hz, 2H), 1.93 (m, 4H), 1.76 (m, 8H), 1.43 (s, 12H), 1.35
(s, 3H), 1.02 (d, J ) 6.3 Hz, 24H), 0.82 (s, 3H). 13C{1H} NMR
(CDCl3): δ 155.2, 154.5, 153.7, 151.1, 150.8, 150.5, 147.4, 147.1,
139.1, 134.4, 133.6, 131.3, 128.6, 128.3, 127.5, 127.1, 126.9, 126.3,
124.0, 123.7, 123.4, 122.6, 122.3, 121.9, 120.7, 119.6, 118.8, 111.5,
111.3, 110.6, 110.4, 97.3, 68.0, 67.9, 67.6, 47.0, 38.5, 38.4, 38.3,
30.5, 27.2, 25.6, 25.5, 25.3, 23.4, 22.9, 22.8, 22.7, 22.1. Anal. Calcd
for C78H93NO6: C, 82.14; H, 8.22. Found: C, 81.92; H, 8.07.
N,N-Bis(9,9-dimethylfluoren-2-yl)-2-(4-(2,5-bis(isopentoxy)-4-
styrylstyryl)-2,5-bis(isopentoxy)phenyl)benzaldehyde (12). THF (30
mL) and water (10 mL) were added to a flask containing 11 (0.36
g, 0.32 mmol). Then TFA (5 mL) was added to the solution. The
resulting reaction mixture was stirred for 2 h at room temperature.
The solution was quenched with saturated aqueous sodium bicar-
bonate and extracted with dichloromethane. The combined dichlo-
romethane phases were then washed with aqueous sodium bicar-
bonate and dried with MgSO4. The organic layer was removed in
vacuo. The pure product 12 was obtained by silica gel chromatog-
raphy (eluent MC/Hx ) 1:1, Rf ) 0.3) to afford 12 (0.3 g) in 90%
1
yield. Mp: 254 °C. H NMR (CDCl3): δ 10.47 (s, 1H), 7.68 ∼
7.12 (m, 26H), 7.67 (t, J ) 5.7 Hz, 2H), 7.62 (s, 2H), 7.55 (s, 2H),
7.49 (d, J ) 8.4 Hz, 4H), 7.43 (d, J ) 19.5 Hz, 1H), 7.40 (d, J )
18.6 Hz, 1H), 7.34 (d, J ) 7.5 Hz, 2H), 7.31 (m, 4H), 7.30 (d, J
) 8.4 Hz, 2H), 7.28 (s, 2H), 7.22 (d, J ) 18 Hz, 1H), 7.20 (d, J )
18.9 Hz, 1H), 4.15 (m, 8H), 1.95 (m, 4H), 1.79 (m, 8H), 1.44 (s,
12H), 1.05 (d, J ) 5.4 Hz, 12H), 1.04 (d, J ) 5.7 Hz, 12H). 13C{1H}
NMR (CDCl3): δ 189.3, 156.4, 155.2, 153.7, 151.5, 151.1, 150.8,
147.6, 147.3, 139.1, 135.3, 134.5, 132.2, 128.9, 128.2, 127.6, 127.1,
126.9, 126.7, 126.3, 124.1, 123.9, 123.4, 122.6, 122.3, 121.9, 120.8,
119.6, 118.9, 110.5, 110.3, 110.2, 110.1, 67.9, 67.7, 67.6, 47.0,
38.4, 38.2, 38.1, 27.2, 25.6, 25.5, 25.3, 22.9, 22.8, 22.7. Anal. Calcd
for C73H83NO5: C, 83.15; H, 7.93. Found: C, 82.89; H, 7.81.
N,N-Bis(9,9-dimethylfluoren-2-yl)-2-(4-(2,5-bis(isopentoxy)-4-
styrylstyryl)-2,5-bis-(isopentoxy)phenyl)-2-cyanoacrylic Acid (13,
JK-58). A mixture of 12 (0.245 g, 0.23 mmol) and cyanoacetic
acid (0.026 g, 0.30 mmol) was vacuum-dried, and MeCN (60 mL)
and piperidine (0.023 mL, 0.23mmol) were added. The solution
was refluxed for 6 h. After the solution was cooled, the organic
layer was removed in vacuo. The pure product 13 was obtained by
silica gel chromatography (eluent EA, Rf ) 0.3) to afford 13 (0.12
FIGURE 5. Electron diffusion coefficients (a) and lifetimes (b) in the
photoelectrodes adsorbing different dyes (i.e., N719, JK-57, JK-58,
and JK-59).
) 6.0 Hz, 6H), 1.02 (d, J ) 6.6 Hz, 6H). 13C{1H} NMR (CDCl3):
δ 189.2, 156.4, 155.3, 153.7, 150.7, 148.3, 147.1, 139.0, 134.9,
134.7, 132.0, 131.3, 128.0, 127.2, 126.7, 124.0, 123.6, 123.5, 122.6,
121.1, 120.8, 119.6, 119.1, 110.3, 110.1, 67.7, 67.6, 47.0, 38.2,
38.1, 27.2, 25.5, 25.3, 22.8, 22.7. Anal. Calcd for C55H57NO3: C,
84.69; H, 7.37. Found: C, 84.46; H, 7.31.
1
g) in 50% yield. Mp: 272 °C. H NMR (CDCl3): δ 8.65 (s, 1H),
N,N-Bis(9,9-dimethylfluoren-2-yl)-2-(2,5-bis(isopentoxy)-4-
styrylphenyl)-2-cyanoacrylic Acid (10, JK-57). A mixture of 9
(0.224 g, 0.29 mmol) and cyanoacetic acid (0.03 g, 0.37 mmol)
was vacuum-dried. MeCN (60 mL) and piperidine (0.026 mL, 0.29
mmol) were added to the mixture. The solution was refluxed for
6 h. After the solution was cooled, the organic layer was removed
in vacuo. The pure product 10 was obtained by silica gel
chrolmatography (eluent EA, Rf ) 0.3) to afford 10 (0.12 g) in
50% yield. Mp: 238 °C. 1H NMR (CDCl3): δ 8.56 (s, 1H), 7.78 (s,
1H), 7.60-7.05 (m, 22H), 3.97 (t, J ) 6.2 Hz, 2H), 3.89 (t, J )
6.2 Hz, 2H), 1.92 (m, 2H), 1.67 (m, 6H), 1.38 (s, 12H), 0.93 (d, J
) 6.3 Hz, 12H). 13C{1H} NMR (CDCl3): δ 169.4, 155.2, 153.6,
153.2, 150.4, 147.9, 147.1, 146.8, 146.7, 139.0, 134.5, 132.5, 131.7,
130.9, 127.9, 127.1, 126.7, 123.7, 123.5, 122.6, 121.6, 120.8, 119.6,
118.9, 111.7, 109.4, 68.1, 67.6, 47.0, 38.2, 37.9, 27.2, 25.5, 25.4,
22.9, 22.8. Anal. Calcd for C58H58N2O4: C, 82.24; H, 6.90. Found:
C, 82.02; H, 6.76.
N,N-Bis(9,9-dimethylfluoren-2-yl)-2-(4-(2,5-bis(isopentoxy)-4-
styrylstyryl)-2,5-bis(isopentoxy)phenyl)-5,5-dimethyl-1,3-dioxane
(11). A stirred mixture of 9 (0.645 g, 0.83 mmol), 5 (0.43 g, 0.83
mmol), and potassium tert-butoxide (0.12 g, 1.00 mmol) was
vacuum-dried, and THF (35 mL) was added. The solution was
refluxed for 6 h. THF was removed under reduced pressure. The
crude mixture was redissolved in dichloromethane, washed with
water, and dried with MgSO4. The organic layer was removed in
vacuo. The pure product 11 was obtained by silica gel chromatog-
raphy (eluent MC/Hx ) 1:1, Rf ) 0.4) to afford 11 (0.66 g) in
70% yield. Mp: 200 °C. 1H NMR (CDCl3): δ 7.67-7.11 (m, 26H),
7.84-7.06 (m, 26H), 4.02 (m, 8H), 1.89 (m, 4H), 1.74 (m, 8H),
1.41 (s, 12H), 0.98 (br s, 24H). 13C{1H} NMR (CDCl3): δ 169.4,
155.2, 153.7, 153.2, 151.3, 150.3, 147.5, 147.3, 139.1, 134.4, 132.9,
132.4, 128.6, 127.7, 127.6, 127.1, 126.6, 125.4, 124.0, 123.4, 122.6,
122.2, 122.1, 121.0, 120.7, 119.6, 118.8, 111.8, 110.1, 108.9, 105.0,
67.9, 67.7, 67.6, 47.0, 38.4, 38.2, 37.8, 27.2, 25.7, 25.6, 25.4, 23.0,
22.9, 22.8. Anal. Calcd for C76H84N2O6: C, 81.39; H, 7.55. Found:
C, 81.11; H, 7.41.
N,N-Bis(9,9-dimethylfluoren-2-yl)-2-(4-(4-(2,5-bis(isopentoxy)-4-
styrylstyryl)-2,5-bis(isopentoxy)styryl)-2,5-bis(isopentoxy)phenyl)-
5,5-dimethyl-1,3-dioxane (14). A stirred mixture of 12 (0.5 g, 0.47
mmol), 5 (0.27 g, 0.52 mmol), and potassium tert-butoxide (0.064
g, 0.57 mmol) was vacuum-dried and THF (35 mL) was added.
The solution was refluxed for 6 h. THF was removed under reduced
pressure. The crude mixture was redissolved in dichloromethane,
washed with water, and dried with MgSO4. The organic layer was
removed in Vacuo. The pure product 14 was obtained by silica gel
chromatography (eluent MC/Hx ) 1: 1, Rf ) 0.4) to afford 14 (0.47
g) in 70% yield. Mp: 220 °C. 1H NMR (CDCl3): δ 7.67-7.11 (m,
28H), 5.75 (s, 1H), 4.10 (m, 12H), 3.80 (d, J ) 10.2 Hz, 2H), 3.69
(d, J ) 10.2 Hz, 2H), 1.94 (m, 6H), 1.79 (m, 12H), 1.43 (s, 12H),
1.35 (s, 3H), 1.01 (m, 36H), 0.81 (s, 3H). 13C{1H} NMR (CDCl3):
δ 155.2, 153.7, 152.1, 151.5, 151.2, 151.1, 150.4, 150.1, 147.4,
147.2, 139.1, 136.2, 135.2, 134.4, 132.4, 128.6, 128.2, 127.5, 127.1,
127.0, 126.6, 126.3, 126.1, 124.0, 123.4, 123.3, 123.1, 122.6, 122.4,
122.1, 120.7, 119.6, 118.8, 118.1, 117.2, 111.4, 110.5, 110.3, 110.2,
97.2, 78.0, 68.0, 67.9, 67.7, 67.6, 67.5, 47.0, 38.5, 38.2, 38.1, 30.5,
7078 J. Org. Chem. Vol. 73, No. 18, 2008

Molecular Engineering of Organic Sensitizers
27.2, 25.6, 25.5, 25.4, 23.4, 22.9, 22.8, 22.7, 22.1. Anal. Calcd for
C96H119NO8: C, 81.49; H, 8.48. Found: C, 81.19; H, 8.32.
mmol) and cyanoacetic acid (0.03 g, 0.34 mmol) was vacuum-
dried, and MeCN (60 mL) and piperidine (0.027 mL, 0.26 mmol)
were added. The solution was refluxed for 6 h. After the solution
was cooled, the organic layer was removed in vacuo. The pure
product 16 was obtained by silica gel chromatography (eluent EA,
Rf ) 0.3) to afford 16 (0.18 g) in 50% yield. Mp: 274 °C. 1H NMR
(CDCl3): δ 8.66 (s, 1H), 7.85-7.10 (m, 31H), 4.05 (m, 12H), 1.92
(m, 6H), 1.75 (m, 12H), 1.41 (s, 12H), 0.96 (m, 36H). 13C{1H}
NMR (CDCl3): δ 186.2, 173.8, 169.9, 168.9, 166.1, 163.4, 155.2,
153.7, 153.3, 151.4, 151.1, 150.4, 147.3, 142.3, 141.7, 139.1, 138.6,
137.9, 134.3, 133.2, 132.5, 128.2, 127.5, 127.2, 126.6, 125.7, 125.1,
124.0, 123.3, 122.9, 122.6, 122.1, 120.8, 119.6, 118.8, 111.7, 110.3,
110.0, 109.2, 101.7, 68.1, 68.0, 67.8, 67.7, 67.6, 47.0, 38.4, 38.2,
38.0, 27.2, 25.6, 25.5, 25.4, 22.9, 22.8, 22.7. Anal. Calcd for
C94H110N2O8: C, 80.88; H, 7.94. Found: C, 80.59; H, 7.83.
N,N-Bis(9,9-dimethylfluoren-2-yl)-4-(4-(2,5-bis(isopentoxy)-4-
styrylstyryl)-2,5-bis-(isopentoxy)styryl)-2,5-bis(isopentoxy)benzal-
dehyde (15). THF (30 mL) and water (10 mL) were added to a
flask containing 14 (0.4 g, 0.28 mmol). Then TFA (5 mL) was
added to the solution. The resulting reaction mixture was stirred
for 2 h at room temperature. The solution was quenched with
saturated aqueous sodium bicarbonate and extracted with dichlo-
romethane. The combined dichloromethane phase was then washed
with aqueous sodium bicarbonate and dried with MgSO4. The
organic layer was removed in vacuo. The pure product 15 was
obtained by silica gel chromatography (eluent MC/Hx ) 1:1, Rf )
1
0.3) to afford 15 (0.34 g) in 91% yield. Mp: 257 °C. H NMR
(CDCl3): δ 10.46 (s, 1H), 7.68-7.12 (m, 30H), 4.11 (m, 12H),
1.94 (m, 6H), 1.79 (m, 12H), 1.43 (s, 12H), 1.03 (m, 36H). 13C{1H}
NMR (CDCl3): δ 189.3, 156.4, 155.2, 153.7, 151.5, 151.2, 151.2,
151.1, 150.8, 147.5, 147.3, 139.1, 135.3, 134.4, 132.4, 128.9, 128.6,
128.4, 127.5, 127.4, 127.2, 127.0, 126.7, 126.5, 124.2, 124.0, 123.7,
123.4, 122.9, 122.6, 122.4, 122.1, 120.8, 119.6, 118.8, 110.6, 110.5,
110.3, 110.2, 68.1, 68.0, 67.8, 67.7, 67.6, 47.0, 38.4, 38.2, 38.2,
27.2, 25.6, 25.5, 25.3, 23.0, 22.9, 22.8, 14.3. Anal. Calcd for
C91H109NO7: C, 82.25; H, 8.27. Found: C, 82.02; H, 8.09.
Acknowledgment. We are grateful to the KOSEF for support
through the National Research Laboratory (No. R0A-2005-000-
10034-0) program and BK21 (2006).
1
Supporting Information Available: H and 13C NMR for
all compounds and 31P NMR spectra for compounds 4-6. This
material is available free of charge via the Internet at
http://pubs.acs.org.
N,N-Bis(9,9-dimethylfluoren-2-yl)-2-(4-(4-(2,5-bis(isopentoxy)-4-
styrylstyryl)-2,5-bis(isopentoxy)styryl)-2,5-bis(isopentoxy)phenyl)-
2-cyanoacrylic Acid (16, JK-59). A mixture of 15 (0.35 g, 0.26
JO8005182
J. Org. Chem. Vol. 73, No. 18, 2008 7079