Novel organic sensitizers containing a bulky spirobifluorene unit for solar cell

Article abstract of DOI:10.1016/j.tet.2009.05.028

Three organic sensitizers JK-87, JK-88, and JK-89 containing a bulky spirobifluorene unit in the bridged group are designed and synthesized. Under standard global A.M. 1.5 solar condition, the JK-89 sensitized cell gave a short-circuit photocurrent density (J_sc) of 13.02 mA cm^-2, an open-circuit voltage (V_oc) of 0.75 V, and a fill factor of 0.70, corresponding to an overall conversion efficiency η of 6.83%. The η of JK-89 is higher than those of other two cells due to the larger J_sc. The improved J_sc value is mainly attributed to the broad and red-shifted absorption band.

Full text of DOI:10.1016/j.tet.2009.05.028

Tetrahedron 65 (2009) 6236–6243
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Novel organic sensitizers containing a bulky spirobifluorene unit for solar cell
Nara Cho a, Hyunbong Choi a, Duckhyun Kim a, Kihyung Song b, Moon-sung Kang c,
,
a,
*
Sang Ook Kang a , Jaejung Ko
*
a Department of Advanced Material Chemistry, Korea University, Jochiwon, Chungnam 339-700, Republic of Korea
b Department of Chemical Education, Korea National University of Education, Cheongwon, Chungbuk 363-791, Republic of Korea
c Energy Lab, Samsung SDI Corporate R&D Center, 428-5 Gongse-dong, Giheung-gu, Yongin-si, Gyeonggi-do 449-577, 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 6 April 2009
Received in revised form 8 May 2009
Accepted 12 May 2009
Available online 18 May 2009
Three organic sensitizers JK-87, JK-88, and JK-89 containing a bulky spirobifluorene unit in the bridged
group are designed and synthesized. Under standard global A.M. 1.5 solar condition, the JK-89 sensitized
cell gave a short-circuit photocurrent density (Jsc) of 13.02 mA cmꢀ2, an open-circuit voltage (Voc) of
0.75 V, and a fill factor of 0.70, corresponding to an overall conversion efficiency
h of 6.83%. The h of JK-89
is higher than those of other two cells due to the larger Jsc. The improved Jsc value is mainly attributed to
the broad and red-shifted absorption band.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
surfaces. One of the approaches to prevent aggregation would be to
employ some additives as a co-adsorbant.6 Other methods have
Increasing energy demands and concerns over global warming
urge the development of renewable energy sources in recent years.1
Dye-sensitized solar cells (DSSCs) have attracted significant atten-
tion as low-cost alternatives to conventional solid-state photovol-
taic devices.2 Several Ru polypyridyl complexes have achieved
power conversion efficiencies of 10–11%.3 Recently, several groups
have developed metal free organic sensitizers to overcome the cost
and synthetic difficulty of ruthenium metal complexes and the
solar cell performance of DSSCs based on organic sensitizers has
been remarkably improved.4 Nevertheless, many organic dyes have
still presented the low conversion efficiency and low stability. A
major factor for the low conversion efficiency of organic sensitizers
in the DSSCs is the formation of dye aggregates on the TiO2 surface.5
Therefore, aggregation of organic dyes must be avoided through
appropriate structural modification. Several strategies have been
employed for controlling the aggregation behavior of dyes on TiO2
included the structural modification of the dye molecule.
Recently, we7,19 and others8 have reported the novel organic
dyes containing dimethylfluorenyl unit. The amorphous non-planar
fluorenyl moiety was adapted to prevent aggregation via molecular
stacking and ensure greater resistance to degradation when ex-
posed to light and high temperature. We already introduced the
(9,9-dimethylfluoren-2-yl)amino phenyl,9 benzo[b]furan,10 and
benzo[b]thiophene11 as the donor moiety. As part of our efforts to
investigate the structural modifications of phenylamine derivatives
that can prevent the aggregation, 9,9-spirobifluorenyl amino unit
has been introduced. Changing such electron donor unit would not
only suppress aggregation formation but also affect the absorption
properties. In this paper, we report three new organic sensitizers
(JK-87, JK-88, and JK-89) containing spirobifluorenyl amino unit as
electron donor. The photovoltaic properties, electronic, and optical
properties of the three sensitizers are described.
* Corresponding authors. Tel.: þ82 41 860 1337; fax: þ82 41 867 5396.
E-mail address: jko@korea.ac.kr (S.O. Kang).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.05.028

N. Cho et al. / Tetrahedron 65 (2009) 6236–6243
6237
2. Results and discussion
dimethylfluorene with tert-butyl carbamate13 afforded compound
1 in 60% yield. Although numerous methods have been reported for
the deprotonation of tert-butoxycarbonyl group,14 trifluoroacetic
acid is shown to be an efficient and mild reagent. The arylamination
of the 2-bromo-7-formyl-9,9-spirobifluorene15,23 was readily per-
formed in the presence of a palladium catalyst. The aldehyde 3
reacted with cyanoacetic acid to give the JK-87. The dioxaborolane
derivative 4 was prepared from 2,7-dibromo-9,9-spirobifluorene
with 1.2 equiv n-butyllithium and subsequent quenching with
The three organic dyes JK-87, JK-88, and JK-89 were synthesized
by the stepwise synthetic protocol (Scheme 1). For the synthesis of
a variety of (9,9-dimethylfluoren-2-yl) amino phenyl derivatives,
we first synthesized the (9,9-dimethylfluoren-2-yl) amine 2. Re-
cently, Buchwald12 developed a highly efficient Cu-catalyzed tan-
dem C–N bond formation. Using this methodology, the amine 2 was
conveniently prepared. The Cu-catalyzed reaction of 2-iodo-9,9-
Scheme 1. Schematic diagram for the synthesis of organic dyes JK-87, JK-88, and JK-89.

6238
N. Cho et al. / Tetrahedron 65 (2009) 6236–6243
Table 1
Optical, redox and DSSC performance parameters of dyes
b
c
Dye
labsa/nm
/Mꢀ1 cmꢀ1
Eox
(V)
E0–0
(V)
ELUMOd/V Jsc
Voc
(V)
FF
h
(3
)
(mA cmꢀ2
11.31
)
(%)
JK-87 369 (22,000) 1.14 2.469 ꢀ1.33
427 (19,000)
JK-88 371 (32,000) 1.12 2.474 ꢀ1.36
0.78 0.75 6.58
0.76 0.75 5.28
0.75 0.70 6.83
9.28
410 (28,000)
JK-89 368 (40,000) 1.13 2.431 ꢀ1.31
428 (34,000)
13.02
a
Absorption spectra were measured in ethanol.
Redox potential of dyes on TiO2 were measured in CH3CN with 0.1 M
b
(n-C4H9)4NPF6 with a scan rate of 100 mV sꢀ1 (vs Fc/Fcþ).
c
E0–0 was determined from intersection of absorption and emission spectra in
ethanol.
d
ELUMO was calculated by Eox–E0–0
.
Figure 1. Absorption spectra of JK-87 (olive solid line), JK-88 (blue dot line), and JK-89
(red dash line) in 10ꢀ5 M ethanol solution, emission spectra of JK-87 (olive dash–dot–
dot line), JK-88 (blue dash–dot–dot line), and JK-89 (red dash–dot–dot line) in 10ꢀ5 M
ethanol solution, and absorption spectra of JK-87 (olive dash–dot), JK-88 (blue dash–
dot line), and JK-89 (red dash–dot line) adsorbed on nanocrystalline TiO2 film.
We also observed that the sensitizers JK-87–JK-89 exhibited strong
luminescence maxima at 596–642 nm when they are excited with
their
p–
*
p bands in EtOH at 298 K.
To judge the feasibility of electron transfer from the excited dye
molecule to the conduction band of TiO2 electrode, redox potentials
of the three dyes were investigated by cyclic voltammetry. The
redox potentials of JK-87–JK-89 were measured in MeCN with
0.1 M tetra-butylammonium hexafluorophosphate. The three or-
ganic sensitizers absorbed on TiO2 film show quasi-reversible
couples. The oxidation potentials of JK-87, JK-88, and JK-89 were
measured to be 1.14, 1.12, and 1.13 V versus NHE, respectively, en-
ergetically favorable for iodide oxidation (Table 1). The reduction
potentials of the three dyes calculated from the oxidation potentials
and the E0–0 determined from the intersection of absorption and
emission spectra were tabulated (Table 1). The excited state oxi-
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane.16
Suzuki
coupling reaction17 of 4 with 4-bromobenzaldehyde, followed by
N-phenylation of 2 gave 6. The corresponding aldehyde 6 afforded
the dye JK-88 on treatment of cyanoacetic acid. Dye JK-89 was
conveniently synthesized in three steps. The Suzuki coupling of 4
with 5-bromothiophene-2-carbaldehyde, followed by N-phenyl-
ation of 7 with 2 gave 8. The condensation reaction of 8 with cya-
noacetic acid yielded the dye JK-89.
The UV–vis and emission spectra of JK-87, JK-88, and JK-89 in
ethanol are shown in Figure 1, together with the UV–vis spectra of
the corresponding sensitizers absorbed on TiO2 film. The absorp-
tion spectrum of JK-89 exhibits visible bands at 368 and 428 nm,
dation potentials (Eox
*
) of the dyes (JK-87: ꢀ1.33 V vs NHE; JK-88:
ꢀ1.36 V vs NHE; JK-89: ꢀ1.31 V vs NHE) are much negative than the
conduction band of TiO2 at ꢀ0.5 V versus NHE. A slight positive
shift of the reduction potential in JK-89 relative to that of JK-88 is
which is due to the
p–
*
p transition of the conjugate molecule.
Under the same conditions the JK-88 sensitizer that contains the
phenyl group instead of thienyl group causes a blue shift to 410 nm
relative to the JK-89. A blue shift of JK-88 can be readily understood
from molecular modeling studies of the two sensitizers (Fig. 2). The
ground state structure of JK-88 possess a 34ꢁ twist between 9,9-
spirobifluorene and the phenyl unit. For JK-89, the dihedral angle
between 9,9-spirobifluorene and the thiophenyl unit is 22ꢁ.
Accordingly, a red shift of JK-89 relative to JK-88 derives from
more delocalization over an entire conjugated system. Absorption
of sensitizers JK-87–JK-89 absorbed on a TiO2 electrode was ob-
served to broaden the absorption spectrum and to red shift the
absorption threshold up to 660 nm. Such broadening and red shifts
have been observed in other organic sensitizers on TiO2 electrode.18
due to more delocalization of the p-conjugation system.
To gain insight into the geometrical configuration and photo-
physical properties, molecular orbital calculations of JK-87–JK-89
were performed with the TD-DFT on B3LYP/3-21G
calculation illustrates that the HOMO of three sensitizers is
delocalized over the -conjugated system through the phenyl
*
(Fig. 3). The
p
amino group. The LUMO is localized over the twisted fluorine
unit. On the other hand, the LUMOþ1 of JK-87–JK-89 is delo-
calized over the cyanoacrylic unit through the next conduit
channel. As light excitation should be associated with vectorial
electron flow from the HOMO to the LUMO for efficient electron
transfer, the examination of HOMO–LUMO in three sensitizers
indicates that HOMO–LUMO excitation moves the electron dis-
tribution from the phenyl amino unit to the twisted fluorine unit
and the photo-induced electron transfer from the dyes to TiO2
electrode cannot be efficiently occurred by the HOMO–LUMO
transition. A relatively low efficiency in three sensitizers can be
explained as a mismatch of vectorial electron flow from the
HOMO to the LUMO.
Figure 4 shows the photocurrent action spectra for DSSCs based
on JK-87–JK-89 using an acetonitrile based electrolyte (electrolyte:
0.6 M 3-hexyl-1,2-dimethyl imidazolium iodide, 0.05 M I2, 0.1 M LiI,
and 0.5 M 4-tert-butylpyridine). The incident photon-to-current
conversion efficiency (IPCE) of JK-87 and JK-89 shows a plateau of
over 75% from 420 to 530 nm, reaching the maximum of 81% at
464 nm. These two sensitizers show a relatively large photocurrent
due to large IPCEs. The IPCE spectrum of JK-88 is due to blue shift by
about 90 nm compared to those of JK-87 and JK-89 as a result of
distortion of bridging unit, which is consistent with the absorption
spectrum of JK-88. Photovoltaic performances of the three sensi-
tized cells are listed in Table 1.
Figure 2. The optimized structure calculated with TD-DFT on B3LYP/3-21G
JK-88 and (b) JK-89.
*
of (a)

N. Cho et al. / Tetrahedron 65 (2009) 6236–6243
6239
Figure 3. Isodensity surface plots of the HOMO, HOMO-1, LUMO, and LUMOþ1 of (a) JK-87, (b) JK-88, and (c) JK-89.
Under standard global A.M. 1.5 solar condition, the JK-89 sen-
sitized cell gave a short-circuit photocurrent density (Jsc) of
13.02 mA cmꢀ2, as an open-circuit voltage (Voc) of 0.75 V, and a fill
power conversion efficiency of JK-88 is relatively low compared to
those of JK-87 and JK-89. To clarify the above result, we have
measured the amount of dyes adsorbed TiO2 film to explain the
higher efficiency of JK-87 and JK-89 sensitized cells compared to
the JK-88 sensitized cell. The adsorbed amounts of
2.26ꢂ10ꢀ6 mmol cmꢀ2 for JK-87, 2.58ꢂ10ꢀ6 mmol cmꢀ2 for JK-88,
factor of 0.70, corresponding to
condition, the JK-88 sensitized cell gave a Jsc of 9.28 mA cmꢀ2, Voc of
0.76 V, and a fill factor of 0.75, corresponding to of 5.28%. The
h of 6.83% (Fig. 5). Under the same
h
Figure 4. Spectra of monochromatic incident photon-to-current conversion efficien-
cies (IPCEs) for DSSC based on JK-87 (dashed line), JK-88 (dotted line), and JK-89 (solid
line).
Figure 5. A photocurrent–voltage curve obtained with a DSSC based on JK-87 (dashed
line), JK-88 (dotted line), and JK-89 (solid line) under A.M. 1.5 radiation
(100 mW cmꢀ2).

6240
N. Cho et al. / Tetrahedron 65 (2009) 6236–6243
1E-3
1E-4
1E-5
1E-6
(a)
(b)
JK-2
JK-2
JK-89
JK-89
1000
100
10
0.1
1
Short-circuit current (mA cm-2)
10
0.4
0.5
0.6
0.7
Open-circuit voltage (V)
0.8
0.9
Figure 6. (a) Electron diffusion coefficients and (b) lifetimes of the photovoltaic cells employing JK2 and JK-89.
and 2.71ꢂ10ꢀ6 mmol cmꢀ2 for JK-89 are observed. Therefore, the
higher efficiencies of JK-87 and JK-89 compared to that of JK-88
may be attributable to the broad and red-shifted absorption bands
rather than the absorbed quantity, resulting in the large IPCE.
Figure 6 shows the electron diffusion coefficients and lifetimes
of the DSSCs (employing JK2 and JK-89, respectively) displayed as
a function of the Jsc and Voc, respectively. Note that we selected JK-2
dye for the comparison because this dye has not only the backbone
structure similar to that of JK-89 but also the reliable performances
as we reported previously.19 There is no significant difference
among the De values at the identical short-circuit current condi-
tions. This result indicates that the De values are hardly affected by
the kind of dye molecules, showing the similar trend to those of
coumarin dyes.20 On the other hand, the se values show a signifi-
(25.67 U)>JK-89 (21.61 U), indicating the improved charge genera-
tion and transport, which correspond to the overall device efficiency.
In summary, we have designed and synthesized three organic
sensitizers containing N,N-bis(9,9-dimethylfluoren-2-yl) amine
unit bridged by sterically bulky spirobifluorenyl unit. The power
conversion efficiency of the DSSCs based on the JK-89 sensitizer
reached 6.83%. The power conversion efficiency was demonstrated
to be quite sensitive to the bridging group and its molecular con-
figuration. This work also proved that the development of highly
efficient organic sensitizers is possible through molecular engi-
neering, and these works are now in progress.
3. Experimental section
3.1. General methods
cant gap between the dyes. The se values of JK-89 were somewhat
smaller than those of JK-2 due to the relatively poor dye adsorption
onto the TiO2 surface.21 The amounts of the dye molecules adsor-
All reactions were carried out under an argon atmosphere.
Solvents were distilled from appropriate reagents. All reagents
were purchased from Sigma–Aldrich, TCI, and Acros Organics.
2-Iodo-9,9-dimethylfluorene22 and 2-bromo-7-formyl-9,9-spi-
robifluorene15,23 were synthesized using a modified procedure of
previous references.
bed on the TiO2 electrodes (film thickness¼ca. 6
mm) were shown
to be 1.27ꢂ10ꢀ7 and 0.98ꢂ10ꢀ7 mol cmꢀ2 for JK-2 and JK-89, re-
spectively. The electron recombination can be facilitated along with
the insufficient coverage on the TiO2 surface, resulting in the de-
crease in the s
e values.20 This result might be owing to the relatively
bulky structure of JK-89 compared to that of JK-2.
The ac impedances of the cells were measured under the illumi-
nation conditions. Figure 7 shows the ac impedance spectra mea-
sured under open-circuit conditions and under illumination of
100 mW cmꢀ2. The radius of the intermediate-frequency semicircle
in the Nyquist plot decreased in the order of JK-88 (30.64 U)>JK-87
3.1.1. Spectroscopic measurements
1H NMR and 13C NMR spectra were recorded on a Varian Mer-
cury 300 spectrometer. Elemental analyses were performed with
a Carlo Elba Instruments CHNS-O EA 1108 analyzer. Mass spectra
were recorded on a JEOL JMS-SX102A instrument. The absorption
and photoluminescence spectra were recorded on a Perkin–Elmer
Lambda 2S UV–visible spectrometer and a Perkin LS fluorescence
spectrometer, respectively.
3.1.2. Electrochemical measurements
Cyclic voltammogram was carried out with a BAS 100B (Bio-
analytical System, 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 (n-C4H9)4NPF6 with a scan rate between 100 mV sꢀ1 (vs
Fc/Fcþ).
3.1.3. Fabrication of DSSC
For the preparation of DSSC, a washed FTO (Pilkington, 8
glass plate was immerged in 40 mM TiCl4 aqueous solution as
reported by the Gra¨tzel group.19 The first TiO2 layer of 12
U )
sqꢀ1
mm
thickness was prepared by screen printing with transparent mes-
oporous TiO2 paste (13 nm anatase, solaronix), and the second
opaque layer of 4
purpose of light scattering. The TiO2 electrodes were immersed into
mm thickness (400 nm, CCIC) was coated for the
Figure 7. Electrochemical impedance spectra measured under the illumination
(100 mW cmꢀ2) for the devices employing different dyes (C JK-87, ; JK-88, - JK-89).

N. Cho et al. / Tetrahedron 65 (2009) 6236–6243
6241
the dyes (JK-87, JK-88, and JK-89) solution (0.3 mM in ethanol
containing 10 mM 3a,7a-dihydroxy-5b-cholic acid) and kept at
room temperature for 18 h. Counter electrodes were prepared by
coating with a drop of H2PtCl6 solution (2 mg Pt in 1 ml ethanol) on
an FTO plate. 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 0.5 M 4-tert-butylpyridine in
acetonitrile.
(m, 4H), 7.48 (d, J¼6.9 Hz), 7.36 (s, 2H), 7.31–7.22 (m, 4H), 7.18 (d,
2H, J¼7.2 Hz), 1.46 (s, 12H). 13C NMR (75 MHz, (CD3)2CO):
d 155.97,
153.83, 144.45, 140.29, 132.48, 127.79, 126.73, 123.27, 121.67, 119.69,
117.32, 112.46, 47.28, 27.53. Anal. Calcd for C30H27N: C, 89.73; H,
6.78. Found: C, 89.57; H, 6.68.
3.1.8. 7-(Bis(9,9-dimethyl-9H-fluoren-7-yl)amino)-9,9-
spirobifluorene-2-carbaldehyde (3)
Under nitrogen atmosphere, a mixture of 2-bromo-7-formyl-9,9-
spirobifluorene (0.3 g, 0.7 mmol), 2 (0.44 g, 1.05 mmol), Pd(OAc)2
(0.023 g, 0.1 mmol), P(tBu)3 (0.04 g, 0.2 mmol), and Cs2CO3 (1.4 g,
4.3 mmol) in dry toluene (15 ml) was stirred and heated at 130 ꢁC for
overnight. After cooling to room temperature, saturated ammonium
chloride solution was added to the reaction solution. The solution
was extracted with dichloromethane, dried over MgSO4, and sub-
jected to column chromatography (silica gel, dichloromethane/
hexanes¼3:1). A yellow solid was obtained. Yield: 50% (0.26 g). Mp:
3.1.4. Characterization of DSSC
The cells were measured using 1000 W xenon light source,
whose power of an AM 1.5 Oriel 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 (PV Measurements).
3.1.5. Electron transport measurements
The electron diffusion coefficient (De) and lifetimes (
s
e) in TiO2
180 ꢁC. 1H NMR (300 MHz, (CD3)2CO):
d 9.85 (s, 1H), 8.12 (d, 1H,
photoelectrode were measured by the stepped light-induced
transient measurements of photocurrent and voltage (SLIM-
PCV).24–27 The transients were induced by a stepwise change in the
J¼8.4 Hz), 8.03 (d, 1H, J¼8.4 Hz), 7.97 (d, 1H, J¼7.8 Hz), 7.86 (d, 2H,
J¼7.2 Hz), 7.70 (d, 2H, J¼7.2 Hz), 7.64 (d, 2H, J¼8.1 Hz), 7.45 (d, 2H,
J¼7.2 Hz), 7.35 (m, 12H), 6.96 (dd, 2H, J¼8.1, 1.2 Hz), 6.84 (d, 2H,
J¼7.5 Hz), 6.56 (s, 1H), 1.28 (s, 12H). 13C NMR (75 MHz, (CD3)2CO):
laser intensity. A diode laser (
l
¼635 nm) as a light source was
modulated using a function generator. The initial laser intensity
was a constant 90 mW cmꢀ2 and less than 10% of the light intensity
was turned down using an ND filter. The laser beam was positioned
at the front side of the fabricated samples (TiO2 film thickness¼ca.
d 191.91, 155.95, 154.39, 152.44, 150.41, 148.7, 148.53, 147.5, 142.55,
139.48, 136.41, 135.76, 135, 131.49, 129.01, 127.9, 127.66, 124.74,
124.65, 124.49, 123.41, 123.29, 121.75, 121.25, 120.74, 120.43, 119.99,
117.87, 66.52, 54.93, 47.42, 27.56. Anal. Calcd for C56H41NO: C, 90.41;
H, 5.56. Found: C, 90.24; H, 5.48.
6
m
m; active area¼0.04 cm2). The photocurrent and photovoltage
transients were monitored using a digital oscilloscope through an
amplifier. The De value was obtained by a time constant (
termined by fitting a decay of the photocurrent transient with
exp(ꢀt/ c) and the TiO2 film thickness ( ) using the equation,
De¼ e value was also determined by fitting a de-
2/(2.77 c).24 The
s
c) de-
3.1.9. 3-(2-(Bis(9,9-dimethyl-9H-fluoren-7-yl)amino)-9,9-
spirobifluoren-7-yl)-2-cyanoacrylic acid (JK-87)
s
u
A mixture of 3 (0.14 g, 0.188 mmol) and cyanoacetic acid
(0.025 g, 0.3 mmol) in dry toluene (15 ml) and piperidine (0.028 ml,
0.29 mmol) were added. The solution was refluxed for 10 h. After
cooling the solution was extracted with dichloromethane and 0.1 M
aqueous solution of HCl. Then, the organic layer was dried over
MgSO4. The pure product JK-87 was obtained by column chroma-
tography (silica gel, ethyl acetate, Rf¼0.1). Yield: 45% (0.068 g). Mp:
u
s
s
cay of photovoltage transient with exp(ꢀt/
s
e).24 All experiments
were conducted at room temperature.
3.1.6. tert-Butyl bis(9,9-dimethyl-9H-fluoren-7-yl)carbamate (1)
An oven-dried flask was charged with CuI (0.014 g, 0.074 mmol),
2-iodo-9,9-dimethylfluorene (1 g, 3.12 mmol), tert-butyl carbamate
(0.174 g, 1.48 mmol), and Cs2CO3 (1.45 g, 4.45 mmol). The flask was
evacuated and backfilled with nitrogen gas. Under a positive
pressure of nitrogen gas, N,N0-dimethylethylenediamine (0.031 ml,
0.29 mmol) was added via syringe, followed by the addition of dry
tetrahydrofuran (THF) (5 ml). The flask was refluxed at 80 ꢁC in oil
bath for 36 h. The reaction mixture was cooled to room tempera-
ture. The solution was extracted with dichloromethane, dried over
MgSO4, and subjected to column chromatography (silica gel, ethyl
acetate/hexanes¼1:10, Rf¼0.4). A white sponge solid was obtained.
243 ꢁC. 1H NMR (300 MHz, (CD3)2SO):
d 8.02(m, 3H), 7.83 (d, 2H,
J¼7.5 Hz), 7.75 (s, 1H), 7.67 (m, 4H), 7.46 (d, 2H, J¼7.5 Hz), 7.33 (m,
6H), 7.16 (m, 4H), 7.02 (m, 2H), 6.85 (dd, 2H, J¼8.1, 1.5 Hz), 6.76 (d,
2H, J¼7.8 Hz), 6.3 (s, 1H), 1.18 (s, 12H). 13C NMR (75 MHz, (CD3)2SO):
d
163.32, 154.76, 153.21, 150.68, 148.83, 148.28, 147.65, 147.07, 146.1,
143.82, 141.1, 138.08, 134.31, 131.87, 129.76, 128.24, 128.11, 127.14,
126.89, 124.26, 123.6, 123.48, 122.72, 122.37, 122, 121.22, 120.6,
120.25, 119.71, 119.27, 118.8, 116.21, 111.99, 110.99, 65.31, 46.41,
26.72. MS: m/z 810.53 [Mþ]. Anal. Calcd for C59H42N2O2: C, 87.38; H,
5.22. Found: C, 87.30; H, 5.17.
Yield: 60% (0.45 g). Mp: 156 ꢁC. 1H NMR (300 MHz, CDCl3):
d
7.69–
7.62 (m, 4H), 7.42–7.40 (m, 2H), 7.32 (m, 6H), 7.2–7.17 (m, 2H), 1.48
(s, 12H), 1.46 (s, 9H). 13C NMR (75 MHz, CDCl3):
154.27, 154.07,
3.1.10. 2-(2-Bromo-9,9-spirobifluoren-7-yl)-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (4)
d
153.9, 142.62, 138.79, 136.61, 127.13, 125.75, 122.65, 121.42, 120.06,
119.98, 81.174, 46.99, 28.44, 27.19. Anal. Calcd for C35H35NO2: C,
83.80; H, 7.03. Found: C, 83.52; H, 6.96.
Under nitrogen atmosphere and at ꢀ78 ꢁC, n-BuLi (5.1 ml, 1.6 M
in hexane) was added dropwise to a dry THF solution containing
2,7-dibromo-9,9-spirobifluorene (3.50 g, 7.38 mmol). After 30 min
stirring at ꢀ78 ꢁC, 1.7 ml (8.33 mmol) of 2-isopropoxy-4,4,5,5-tet-
ramethyl-1,3,2-dioxaborolane was added slowly to the reaction
solution at ꢀ78 ꢁC. The solution was warmed to room temperature
and the reaction mixture was stirred for 8 h. Then, the reaction was
quenched with water. The solution was extracted with dichloro-
methane, dried over MgSO4, and subjected to column chromatog-
raphy (silica gel, ethyl acetate/hexane¼1:10, Rf¼0.45). A white solid
was obtained. Yield: 85% (3.26 g). Mp: 305 ꢁC. 1H NMR (300 MHz,
3.1.7. Bis(9,9-dimethyl-9H-fluoren-7-yl)amine (2)
tert-Butoxycarbonyl (BOC)-protected arylamine
1
(0.4 g,
0.8 mmol) was dissolved in tetrahydrofuran(THF) (1 ml) and tri-
fluoroacetic acid (TFA) (8 ml). And then, the reaction mixture was
stirred at room temperature for 10 min. The resulting mixture was
changed into an intense deep-green color. After evaporation of TFA,
dichloromethane was added to the residue, then the mixture was
neutralized with saturated aqueous solution of NaOH. The organic
layer was extracted with dichloromethane, dried over MgSO4, and
subjected to flash column chromatography (silica gel, ethyl acetate/
hexanes¼1:10, Rf¼0.3). A white-yellow solid was obtained. Yield:
CDCl3):
d
7.85 (d, 4H, J¼7.2 Hz), 7.73 (d, 1H, J¼7.2 Hz), 7.48 (d, 1H,
J¼7.2 Hz), 7.38 (dd, 2H, J¼7.2, 1.2 Hz), 7.16 (s, 1H), 7.11 (dd, 2H, J¼7.2,
1.2 Hz), 6.79 (s, 1H), 6.71 (d, 2H, J¼7.2 Hz), 1.25 (s, 12H). 13C NMR
(75 MHz, CDCl3):
d 151.8, 147.8, 147.61, 143.8, 142, 140.4, 135.08,
95% (0.3 g). Mp: 178 ꢁC. 1H NMR (300 MHz, (CD3)2CO):
d
7.70–7.67
134.93, 131.01, 130.41, 128.08, 128.04, 127.34, 124.29, 122.11, 121.84,

6242
N. Cho et al. / Tetrahedron 65 (2009) 6236–6243
120.29, 119.51, 83.9, 65.9, 48.2, 24.9. Anal. Calcd for C31H26BBrO2: C,
71.43; H, 5.03. Found: C, 71.24; H, 4.98.
chromatography (silica gel, dichloromethane/hexane¼3:1, Rf¼0.45).
A light yellow solid was obtained. Yield: 80% (0.34 g). Mp: 225 ꢁC. 1H
NMR (300 MHz, CDCl3):
d
9.79(s,1H), 7.89(m, 3H), 7.74 (dd,2H, J¼8.1,
3.1.11. 4-(2-Bromo-9,9-spirobifluoren-7-yl)benzaldehyde (5)
1.5 Hz), 7.60 (d,1H, J¼4.8 Hz), 7.53 (dd,1H, J¼8.1,1.4 Hz), 7.42 (m, 2H),
Under nitrogen atmosphere, a mixture of 4 (1.52 g, 2.92 mmol), 4-
bromobenzaldehyde (0.54 g, 2.92 mmol), Pd(PPh3)4 (0.23 g,
0.2 mmol), K2CO3 (4.04 g, 29.2 mmol), and degassed water (2 M) in
dry THF (60 ml) was refluxed overnight. After cooling to room
temperature, the solution was extracted with dichloromethane,
dried over MgSO4, and subjected to column chromatography (silica
gel, dichloromethane/hexane¼3:1, Rf¼0.37). A light yellow solid was
obtained. Yield: 85% (1.23 g). Mp: 186 ꢁC. 1H NMR (300 MHz, CDCl3):
7.19 (d,1H, J¼4.2 Hz), 7.14 (m, 2H), 6.99 (dd,1H, J¼7.4,1.3 Hz), 6.86 (s,
1H), 6.76 (dd, 2H, J¼7.5, 1.3 Hz). 13C NMR (75 MHz, CDCl3):
d 182.64,
153.8, 151.31, 149.74, 148.13, 147.31, 142.23, 142.02, 141.79, 139.73,
137.29, 136.65, 134.68, 133.04, 131.52, 131.25, 130.48, 128.36, 127.38,
126.66, 124.07, 122.3, 121.8, 121.79, 120.87, 120.4, 83.29, 48.24. Anal.
Calcd for C30H17BrOS: C, 71.29; H, 3.39. Found: C, 71.08; H, 3.31.
3.1.15. 5-(2-(Bis(9,9-dimethyl-9H-fluoren-7-yl)amino)-9,9-
d
9.97 (s, 1H), 7.93 (m, 3H), 7.83 (d, 2H, J¼8.4 Hz), 7.77 (d, 1H,
spirobifluoren-7-yl)thiophene-2-carbaldehyde (8)
J¼8.7 Hz), 7.69 (dd, 1H, J¼8.4,1.5 Hz), 7.58 (d, 2H, J¼8.7 Hz), 7.54 (dd,
1H, J¼8.1,1.3 Hz), 7.44 (m, 2H), 7.18 (m, 2H), 7.1 (s,1H), 6.89 (s,1H), 6.8
Under nitrogen atmosphere, a mixture of 2 (1.05 g, 2.61 mmol), 7
(0.88 g, 1.74 mmol), Pd(OAc)2 (0.016 g, 0.071 mmol), P(t-Bu)3 (0.03 g,
0.148 mmol), and Cs2CO3 (1.25 g, 3.84 mmol) in dry toluene (30 ml)
was stirred and heated at 130 ꢁC overnight. After cooling to room
temperature, saturated ammonium chloride solution was added to
the reaction solution. The solution was extracted with dichloro-
methane, dried over MgSO4, and subjected to column chromatogra-
phy (silica gel, dichloromethane/hexanes¼3:1). An orange-yellow
solid was obtained. Yield: 60% (1.15 g). Mp: 197 ꢁC. 1H NMR
(d, 2H, J¼7.8 Hz). 13C NMR (75 MHz, CDCl3):
d 191.87, 151.41, 149.67,
147.72, 146.76, 141.9, 141.25, 140.07, 139.87, 135.21, 131.24, 130.2,
128.31, 127.68, 124.22, 123, 122.09, 121.71, 120.73, 120.4, 66, 48.2.
Anal. Calcd for C32H19BrO: C, 76.96; H, 3.83. Found: C, 76.74; H, 3.72.
3.1.12. 4-(2-(Bis(9,9-dimethyl-9H-fluoren-7-yl)amino)-9,9-
spirobifluoren-7-yl)benzaldehyde (6)
Under nitrogen atmosphere, a mixture of 2 (0.5 g, 1.25 mmol), 5
(0.4 g, 0.8 mmol), Pd(OAc)2 (0.023 g, 0.1 mmol), P(t-Bu)3 (0.04 g,
0.2 mmol), and Cs2CO3 (1.4 g, 4.3 mmol) in dry toluene (15 ml) was
stirred and heated at 130 ꢁC overnight. After cooling to room
temperature, saturated ammonium chloride solution was added to
the reaction solution. The solution was extracted with dichloro-
methane, dried over MgSO4, and subjected to column chromatog-
raphy (silica gel, dichloromethane/hexanes¼3:1). A yellow solid
was obtained. Yield: 50% (0.33 g). Mp: 185 ꢁC. 1H NMR (300 MHz,
(300 MHz, CDCl3):
d
9.96 (s, 1H), 7.98 (m, 6H), 7.77 (dd, 3H, J¼8.1,
1.4 Hz), 7.68 (d, 2H, J¼7.5 Hz), 7.55 (m, 8H), 7.31 (dd, 6H, J¼7.8,1.5 Hz),
7.11 (m, 4H), 6.85 (s, 1H), 1.46 (s, 12H). 13C NMR (75 MHz, CDCl3):
d
182.69,155.06, 154.64, 154.64, 150.74,150.12,148.28, 146.97, 143.22,
141.89, 141.8, 138.97, 137.35, 135.08, 134.46, 131.63, 128.07, 127.08,
126.64, 124.16, 124.01, 123.91, 123.55, 123.28, 122.57, 121.68, 121.14,
120.65, 120.36, 119.97, 119.52, 118.74, 118.47, 65.99, 46.86, 27.14. Anal.
Calcd for C60H43NOS: C, 87.24; H, 5.25. Found: C, 87.02; H, 5.18.
CDCl3):
4H), 7.03 (m, 4H), 6.75 (s, 1H), 1.33 (s, 12H). 13C NMR (75 MHz,
CDCl3): 191.85, 155.03, 153.6, 150.7, 149.96, 148.59, 148.41, 147.04,
142.26, 141.8, 138.97, 138.29, 135.42, 134.95, 134.35, 130.15, 128.03,
127.33, 127.07, 126.6, 124.29, 123.45, 122.7, 122.54, 121.05, 120.63,
120.29, 119.87, 119.5, 118.65, 66.09, 46.83, 27.13. Anal. Calcd for
C62H45NO: C, 90.81; H, 5.53. Found: C, 90.68; H, 5.42.
d
9.97 (s, 1H), 7.9 (m, 6H), 7.69 (m, 8H), 7.41 (m, 9H), 7.2 (m,
3.1.16. 3-(5-(2-(Bis(9,9-dimethyl-9H-fluoren-7-yl)amino)-9,9-
spirobifluoren-7-yl)thiophen-2-yl)-2-cyanoacrylic acid (JK-89)
A mixture of 8 (0.19 g, 0.23 mmol) and cyanoacetic acid (0.04 g,
0.43 mmol) in dry chloroform (12 ml) and piperidine (0.047 ml,
0.47 mmol) were added. The solution was refluxed for 10 h. After
cooling the solution was extracted with dichloromethane and 0.1 M
aqueous solution of HCl. Then, the organic layer was dried over
MgSO4. The pure product JK-89 was obtained by column chroma-
tography (silica gel, ethyl acetate, Rf¼0.1). Yield: 50% (0.1 g). Mp:
d
3.1.13. 3-(4-(2-(Bis(9,9-dimethyl-9H-fluoren-7-yl)amino)-9,9-
spirobifluoren-7-yl)phenyl)-2-cyanoacrylic acid (JK-88)
285 ꢁC.1H NMR (300 MHz, (CD3)2SO):
d 8.06 (s,1H), 7.98 (m, 2H), 7.86
A mixture of 6 (0.27 g, 0.33 mmol) and cyanoacetic acid (0.056 g
0.66 mmol) in dry chloroform (8 ml) and piperidine (0.0652 ml,
0.66 mmol) were added. The solution was refluxed for 10 h. After
cooling the solution was extracted with dichloromethane and 0.1 M
aqueous solution of HCl. Then, the organic layer was dried over
MgSO4. The pure product JK-88 was obtained by column chroma-
tography (silica gel, ethyl acetate, Rf¼0.1). Yield: 55% (0.32 g). Mp:
(d, 2H, J¼7.2 Hz), 7.79 (d, 1H, J¼8.1 Hz), 7.67 (d, 2H, J¼7.2 Hz), 7.62 (d,
2H,, J¼8.4 Hz), 7.45 (m, 3H), 7.34 (d, 1H, J¼7.8 Hz), 7.28 (m, 7H), 7.14
(m, 4H), 7.04 (d,1H, J¼8.4 Hz), 6.84(m, 4H), 6.3 (s,1H),1.18 (s,12H).13C
NMR (75 MHz, (CD3)2SO):
d 189.11, 163.51, 154.67, 153.12, 150.24,
149.19, 148.46, 147.7, 147.84, 146.13, 142.08, 141.29, 141.01, 138.04,
135.62, 134.43, 134.11, 131.41, 128.24, 128.11, 127.04, 126.76, 126.24,
124.48, 123.47, 123.36, 122.62, 122.2, 121.92, 121.1, 120.67, 119.91,
119.91, 119.6, 118.57, 118.82, 65.34, 46.33, 26.64. MS: m/z 892.6 [Mþ].
Anal. CalcdforC63H44N2O2S:C, 84.72;H, 4.97. Found:C, 84.58;H, 4.92.
263 ꢁC.1HNMR(300 MHz,(CD3)2SO):
d7.99(m, 3H), 7.82(m, 4H), 7.65
(d, 2H, J¼6.9 Hz), 7.59 (d, 2H, J¼8.1 Hz), 7.51 (d, 2H, J¼7.2 Hz), 7.43 (d,
2H, J¼6.3 Hz), 7.28 (m, 9H), 7.12 (m, 4H), 6.82 (m, 4H), 6.31 (s,1H),1.15
(s,12H).13C NMR (75 MHz, (CD3)2SO):
d 163.69,154.77,153.21,150.29,
Acknowledgements
149.27,148.06,147.75,147.4,146.3,141.89,141.42,141.16,138.15,137.73,
134.93, 134.15, 131.97, 130.17, 128.27, 128.11, 127.18, 126.84, 123.56,
123.42, 122.75, 122.34, 121.93, 121.22, 120.63, 119.71, 119.03, 118.61,
116.75, 112.43, 46.42, 26.75. MS: m/z 866.62 [Mþ]. Anal. Calcd for
C65H46N2O2: C, 88.01; H, 5.23. Found: C, 87.88; H, 5.17.
This work was supported by the Korea Science and Engineering
Foundation through the ERC program (no. R11-2008-088-02001-0),
the Ministry of Information & Communication, Korea, under the
Information Technology Research Center (ITRC, no. IITA 2008 C1090
0904 0013) program, NRL program (No. R0A-2005-000-10034-0)
and BK21.
3.1.14. 5-(2-Bromo-9,9-spirobifluoren-7-yl)thiophene-2-
carbaldehyde (7)
Under nitrogen atmosphere, a mixture of 4 (0.45 g, 0.86 mmol),
5-bromothiophene-2-carbaldehyde (0.15 ml, 1.3 mmol), Pd(PPh3)4
(0.07 g, 0.06 mmol), K2CO3 (1.19 g, 8.61 mmol), and degassed water
(4.3 ml) in dry THF (20 ml) was refluxed overnight. After cooling to
room temperature, the solution was extracted with dichloro-
methane, dried over MgSO4, and subjected to column
References and notes
1. Robertson, N. Angew. Chem., Int. Ed. 2006, 45, 2338.
2. (a) Asbury, J. B.; Ellingson, R. J.; Gosh, H. N.; Ferrere, S.; Notzig, A. J.; Lian, T.
J. Phys. Chem. B 1999, 103, 3110; (b) Heimer, T. A.; Heilweil, E. J.; Bifnozzi, C. A.;
Meyer, G. J. J. Phys. Chem. A 2000, 104, 4256; (c) Lee, J.-J.; Coia, G. M.; Lewis, N. S.

N. Cho et al. / Tetrahedron 65 (2009) 6236–6243
6243
J. Phys. Chem. B 2004, 108, 5269; (d) Saito, Y.; Fukuri, N.; Senadeera, R.; Kita- 12. Martin, R.; Larsen, C. H.; Cuenca, A.; Buchwald, S. L. Org. Lett. 2007, 9, 3379.
mura, T.; Wada, Y.; Yanagida, S. Electrochem. Commun. 2004, 6, 71.
3. (a) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska,
P.; Ito, S.; Bessho, T.; Gra¨tzel, M. J. Am. Chem. Soc. 2005, 127, 16835; (b) Gra¨tzel,
M. J. Photochem. Photobiol., C 2003, 4, 145; (c) Gra¨tzel, M. Prog. Photovoltaics Res.
Appl. 2006, 14, 429.
13. Jiang, L.; Job, G. E.; Klapars, A.; Buchwald, S. L. Org. Lett. 2003, 5, 3667.
14. Li, B.; Bemish, R.; Buzon, R. A.; Chiu, C. K.-F.; Colgan, S. T.; Kissel, W.; Le, Tung;
Leeman, K. R.; Newell, L.; Roth, J. Tetrahedron Lett. 2003, 44, 8113.
15. Chiang, C.-L.; Wu, M.-F.; Dai, D.-C.; Wen, Y.-S.; Wang, J. K.; Chen, C.-T. Adv. Funct.
Mater. 2005, 15, 231.
4. (a) Ito, S.; Miura, H.; Uchida, S.; Takata, M.; Sumioka, K.; Liska, P.; Comte, P.;
Pe´chy, P.; Gra¨tzel, M. Chem. Commun. 2008, 5194; (b) Wang, Z. S.; Cui, Y.; Hara,
K.; Dan-oh, Y.; Kasada, C.; Shinpo, A. Adv. Mater. 2007, 19, 1138; (c) Koumura, N.;
Wang, Z. S.; Mori, S.; Miyashita, M.; Suzuki, E.; Hara, K. J. Am. Chem. Soc. 2006,
128, 14256; (d) Kitamura, T.; Ikeda, M.; Shigaki, K.; Inoue, T.; Anderson, N. A.; Ai,
X.; Lian, T.; Yanagida, S. Chem. Mater. 2004, 16, 1806.
16. Park, Y.-I.; Son, J.-H.; Kang, J.-S.; Kim, S.-K.; Lee, J.-H.; Park, J.-W. Chem. Commun.
2008, 2143.
17. Zhang, X.; Gorohmaru, H.; Kadowaki, M.; Kobayashi, T.; Ishi-I, T.; Thiemann, T.;
Mataka, S. J. Mater. Chem. 2004, 14, 1901.
18. (a) Nuesch, F.; Moser, J. E.; Shklover, V.; Gra¨tzel, M. J. Am. Chem. Soc. 1996, 118,
5420; (b) Iriyama, K.; Mizutani, F.; Yoshiura, M. Chem. Lett. 1980, 1399; (c)
Sayama, K.; Hara, K.; Mori, N.; Satsuki, M.; Suga, S.; Tsukagoshi, S.; Abe, Y.;
Sugihara, H.; Arakawa, H. Chem. Commun. 2000, 1173.
5. (a) Ehret, A.; Stuhl, L.; Spitler, M. T. Electrochim. Acta 2000, 45, 4553; (b) Ehret,
A.; Stuhl, L.; Spiltler, M. T. J. Phys. Chem. B 2001, 105, 9960.
6. (a) Khazraji, A. C.; Hotchandani, S.; Das, S.; Kamat, P. V. J. Phys. Chem. B 1999,
103, 4693; (b) Das, S.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 209.
7. Kim, S.; Kim, D.; Choi, H.; Kang, M.-S.; Kang, S. O.; Ko, J. Chem. Commun. 2008,
4951.
8. Qin, H.; Wenger, S.; Xu, M.; Gao, F.; Jing, X.; Wang, P.; Zakeeruddin, S. M.;
Gra¨tzel, M. J. Am. Chem. Soc. 2008, 130, 9202.
9. (a) Chio, H.; Baik, C.; Kang, S. O.; Ko, J.; Kang, M. S.; Nazeeruddin, M. K.; Gra¨tzel, M.
Angew. Chem., Int. Ed. 2008, 47, 327; (b) Kim, J.-J.; Choi, H.; Lee, J.-W.; Kang, M.-S.;
Song, K.; Kang, S. O.; Ko, J. J. Mater. Chem. 2008,18, 5223; (c) Kim, C.; Choi, H.; Kim,
S.; Baik, C.; Song, K.; Kang, M.-S.; Kang, S. O.; Ko, J. J. Org. Chem. 2008, 73, 7072; (d)
Kim, S.; Choi, H.; Kim, D.; Song, K.; Kang, S. O.; Ko, J. Tetrahedron 2007, 63, 9206.
10. Jung, I.; Lee, J. K.; Song, K. H.; Song, K.; Kang, S. O.; Ko, J. J. Org. Chem. 2007, 72,
3652.
19. Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J.-H.; Fantacci, S.; De Angelis, F.; Di
Censo, D.; Nazeeruddin, M. K.; Gra¨tzel, M. J. Am. Chem. Soc. 2006, 128, 16701.
20. Hara, K.; Miyamoto, K.; Abe, Y.; Yanagida, M. J. Phys. Chem. B 2005, 109, 23776.
21. Yum, J.-H.; Jang, S.-R.; Walter, P.; Geiger, T.; Nu¨esch, F.; Kim, S.; Ko, J.; Gra¨tzel,
M.; Nazeeruddin, M. K. Chem. Commun. 2007, 4680.
22. Shirota, Y.; Kinoshita, M.; Noda, T.; Okumoto; Ohara, T. J. Am. Chem. Soc. 2000,
122, 11.
23. Faming Zhuanli Shenqing Gongkai Shuomingshu, CN1634839, 2005.
24. Nakade, S.; Kanzaki, T.; Wada, Y.; Yanagida, S. Langmuir 2005, 21, 10803.
25. Kang, M.-S.; Ahn, K.-S.; Lee, J.-W.; Kang, Y. S. J. Photochem. Photobiol., A 2008,
195, 198.
26. Ahn, K.-S.; Kang, M.-S.; Lee, J.-K.; Shin, B.-C.; Lee, J.-W. Appl. Phys. Lett. 2006, 89,
013103.
11. Choi, H.; Lee, J. K.; Song, K.; Kang, S. O.; Ko, J. Tetrahedron 2007, 63, 3115.
27. Ahn, K.-S.; Kang, M.-S.; Lee, J.-W.; Kang, Y. S. J. Appl. Phys. 2007, 101, 084312.