2,7-diaminofluorene-based organic dyes for dye-sensitized solar cells: Effect of auxiliary donor on optical and electrochemical properties

Article abstract of DOI:10.1021/jo200501b

New organic dyes containing a diarylaminofluorene unit as an electron donor and cyanoacrylic acid as acceptor and anchoring group in a donor-π-donor- π-acceptor architecture have been synthesized and characterized as sensitizers for nanocrystalline TiO

Full text of DOI:10.1021/jo200501b

ARTICLE
pubs.acs.org/joc
2,7-Diaminofluorene-Based Organic Dyes for Dye-Sensitized Solar
Cells: Effect of Auxiliary Donor on Optical and Electrochemical
Properties
Abhishek Baheti,† Prachi Singh,† Chuan-Pei Lee,‡ K. R. Justin Thomas,*,† and Kuo-Chuan Ho‡
†Organic Materials Lab, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247 667, India
‡Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
S Supporting Information
b
ABSTRACT: New organic dyes containing a diarylaminofluor-
ene unit as an electron donor and cyanoacrylic acid as acceptor
and anchoring group in a donor-π-donor-π-acceptor architec-
ture have been synthesized and characterized as sensitizers for
nanocrystalline TiO2-based dye-sensitized solar cells. They
have shown three major electronic absorptions originating from
the πꢀπ* and charge-transfer transitions covering the broad
visible range (250ꢀ550 nm) in solution. The charge-transfer
transition of the dyes exhibited negative solvatochromism,
suggesting a polarized ground state. They have also displayed
acidochromism in solution owing to the presence of a proto-
nationꢀdeprotonation equilibrium. On comparison with the triphenylamine and carbazole-based parent dyes (E)-2-cyano-
3-(4-(diphenylamino)phenyl)acrylic acid and (E)-2-cyano-3-(9-ethyl-9H-carbazol-3-yl)acrylic acid they exhibited longer wave-
length absorptions and facile oxidation, indicating the stronger electron-donating ability of the auxiliary chromophores. In addition,
they exhibited nearly two times larger light-to-electron conversion efficiency under simulated AM 1.5 G irradiation (100 mW cmꢀ2
)
with an aperture mask when compared to the parent dyes. Among the new dyes, the one containing the naphthylphenylamine
segment showed better device characteristics attributable to the higher HOMO energy level which probably facilitates the
regeneration of the dye and effective suppression of the back reaction of the injected electrons with the I3ꢀ in the electrolyte. The
optical properties of the dyes were modeled using TDDFT simulations employing different theoretical models (B3LYP, CAM-
B3LYP, and MPW1K), and the best correlations with the observed parameters have been found for CAM-B3LYP and MPW1K
calculations. The electron lifetimes extracted from the electrochemical impedance measurements of the dye-sensitized solar cells
were used to interpret the solar cell efficiency alternations.
’ INTRODUCTION
such as N3 and N719 have remained as the best commercial dyes
showing solar energy-to-electricity conversion efficiency up to 11% at
simulated AM 1.5 G irradiation.8 Thus, research toward the devel-
opment of organic dyes and understanding the structureꢀproperty
relationship in organic dyes has been undertaken with keen interest.
Organic dyes probably suffer from several competing deactivating
processes of the excited state in addition to its shorter lifetime.
Prolongation of the excited state either by the inducement of triplet
character or introduction of energy transferring chromophores may
enhance the light-harvesting capability of the dyes.
In this work, we have studied the changes in the optical and
electrochemical properties of organic dyes due to the incorpora-
tion of an auxiliary electron donor featuring diphenylaminofluor-
ene segment. We have chosen this chromophore for the following
reasons: Fluorene-based organic dyes are attractive because of
their high molar extinction absorption and easy functionalization
Organic dyes capable of absorbing a visible light have received
immense attention since the first report in 1991 by Gr€atzel and
co-workers1 on nanocrystalline anatase TiO2-based dye-sensi-
tized solar cells (DSSCs) as potential alternatives to traditional
silicon-based inorganic photovoltaic devices. These cells are
usually constructed using wide band gap semiconductor oxides
such as TiO2, a molecular sensitizer, and a redox electrolyte.
Recent research has been focused on the methods to optimize
the light-harvesting ability of the sensitizers and to realize a more
robust electrolyte system.2ꢀ6 Also, interest for organic dyes as an
alternative to noble metal complexes has increased because of their
many advantages, such as diversity of molecular structures, high
molar extinction coefficient, simple synthesis, as well as low cost and
environmental friendliness. Metal-free organic dyes based on tri-
phenylamine, fluorene, thiophene, indoline, merocyanine, coumarin,
etc. have been developed and found to exhibit promising device
characteristics.7 Despite the intensive efforts by materials chemists to
achieve an efficient organic dye, the DSSCs based on Ru complexes
Received: March 7, 2011
Published: May 10, 2011
r
2011 American Chemical Society
4910
dx.doi.org/10.1021/jo200501b J. Org. Chem. 2011, 76, 4910–4920
|

The Journal of Organic Chemistry
ARTICLE
Chart 1. Structures of the Dyes
by simple chemical modifications.7 In addition, since fluorene is an
electron-rich moiety, it enhances the redox stability of the organic
dyes. Substitution of the fluorene moiety in place of a phenyl group
in triarylamines has been found to reduce the oxidation potential
because of the electron-richness of the fluorene unit. It is hypothe-
sized that the promising changes expected in the optical and redox
properties due to the introduction of the diphenylaminofluorene
unit will play a beneficial role on the light-harvesting properties of
the dyes. Thus, we demonstrate in this work, by suitable chemical
modification a simple triphenylamine derived organic dye, that
(E)-2-cyano-3-(4-(diphenylamino)phenyl)acrylic acid (D1) show-
ing an efficiency of ∼2.5%9 can be improved significantly. Con-
sequently, we have developed four new dyes (CHART 1) by
substituting one or more of the phenyl groups in D1 with fluorene
or diarylaminofluorene and related chromophores. It is expected that
the replacement of phenyl groups in D1 with fluorene or diarylami-
nofluorene would enhance the donor strength of the amino unit and
alter the optical and electrochemical properties of the resulting dyes.
Modification in the absorption properties or redox propensity of the
dyes is believed to enhance the current density and retard the back
reaction of the injected electrons with the electrolyte or the oxidized
dye, which could lead to efficient DSSCs.
Scheme 1. Synthesis of the Dyes D2, JD1, and JD2
’ RESULTS AND DISCUSSION
Synthesis and Characterization. The dyes (D2, JD1ꢀJD3)
were synthesized in two steps involving the formylation of the
corresponding triarylamines (1aꢀd) followed by the Knoevena-
gel condensation of the resulting aldehydes (2aꢀd) with cyano-
acetic acid (Scheme 1). The carbazole-containing dye JD3 was
synthesized by utilizing the same methodology from 9,90-(9,9-
diethyl-9H-fluorene-2,7-diyl)bis(9H-carbazole) (not shown in
Scheme 1). The dyes were thoroughly characterized by 1H and
13C NMR and mass spectral methods. They are dark red and
moderately soluble in solvents such as dichloromethane, acet-
onitrile, methanol, tetrahydrofuran, toluene, etc.
Optical Properties. The absorption spectra of the dyes recorded
in dichloromethane are displayed in Figure 1, and the related photo-
physical data are summarized in Table 1. All of the dyes exhibited
three absorption peaks, and the prominent red-shifted band appear-
ing in the range of 400ꢀ550 nm is assigned to a charge-transfer
Figure 1. Absorption spectra of the dyes (D2, JD1ꢀJD3) recorded in
dichloromethane.
4911
dx.doi.org/10.1021/jo200501b |J. Org. Chem. 2011, 76, 4910–4920

The Journal of Organic Chemistry
ARTICLE
Table 1. Optical and Electrochemical Properties of the Dyes and Related Compounds in Dichloromethane Solution
compd
λmax, nm (εmax, Mꢀ1 cmꢀ1 ꢁ 103)
Eox, mV (ΔEp, mV)a
HOMO,b eV
E0ꢀ0,c eV
LUMO,d eV
1a
347 (25.2), 264 (10.5)
374 (40.2), 309 (27.9)
372 (44.0), 270 (25.6)
334 (39.2), 293 (41.7)
372 (24.6), 331 (18.8)
377 (47.9)
397 (83)
5.197
4.956
4.954
5.596
5.351
5.069
5.072
5.620
5.370
5.063
5.033
5.638
3.263
3.085
2.913
3.415
2.897
2.808
2.793
3.328
2.389
2.288
2.326
2.696
1.934
1.871
2.041
2.181
2.454
2.261
2.279
2.292
2.980
2.775
2.707
2.942
1b
156 (70), 465 (67)
154 (68), 495 (73)
796, 1014
1c
1d
2a
551 (82)
2b
269 (67), 524 (71)
272 (68), 559 (68)
820, 1027
2c
377 (45.9)
2d
340 (42.2), 294 (45.7)
440 (22.9), 320 (14.0), 267 (14.8)
447 (35.8), 370 (38.5), 309 (26.1)
441 (34.1), 370 (38.5), 272 (44.2)
403 (31.3), 327 (47.7), 289 (51.5)
D2
JD1
JD2
JD3
570 (75)
263 (53), 518 (81)
233 (64), 557 (84)
838, 964
a Redox potentials are reported with reference to the ferrocene internal standard. b Deduced from the oxidation potential using the formula HOMO = 4.8þEox.
c Calculated from optical edge. d Obtained from the optical band gap and the electrochemically deduced HOMO value.
transition between the amine and cyanoacrylic segments. The
absorption maximum realized for the dye D2 is bathochromi-
cally shifted when compared to the parent dye (E)-2-cyano-
3-(4-(diphenylamino)phenyl)acrylic acid (D1, 400 nm).9 This
clearly establishes the electron-richness of the fluorene segment,
which enhances the donor strength of the amine segment to result in
a pronounced donorꢀacceptor interaction. Among the dyes, the
compound containing the diphenylamine unit (JD1) displays the
red-shifted absorption profile, while the carbazole-based dye (JD3)
possesses the blue-shifted peak. The relatively shorter wavelength
peak observed for the dye JD3 in the series is indicative of the
reduced donor strength of the carbazole segment when compared to
the other triarylamine units. However, the absorption peak JD3 is
red-shifted when compared to the parent compound (E)-2-cyano-
3-(9-ethyl-9H-carbazol-3-yl)acrylic acid (D3, 385 nm)10 suggesting
an enhancement in the donor strength because of the presence of an
additional electron-rich fluorenylcarbazole unit.
The role of the donorꢀacceptor interactions on the electronic
spectra is clearly evident on comparing the absorption maxima of
the parent amines, 1aꢀd, and the aldehydes, 2aꢀd, with that of the
dyes, D2 and JD1ꢀJD3 (see Table 1). For a similar structure, the
absorption wavelength shifts to the low energy region on intro-
duction of aldehyde or cyanoacrylic acid segment on the amine.
The interesting feature of the absorption spectra of the dyes
(JD1ꢀJD3) is the additional intense bands appearing in the lower
wavelength region (250ꢀ400 nm) when compared to the parent
compounds D1ꢀD3. These lower wavelength absorptions origi-
nate from the πꢀπ* transitions localized within fluorene and
aminofluorene segments.11 The energies of these bands in the dyes
assume the order JD2 < JD1 < JD3 indicating the auxochromic
effect due to the arylamine segments.12
All of the dyes showed negative solvatochromism for the higher
wavelength absorption band when recorded in solvents of different
polarity (Table 2 and Figure 2). Most red-shifted absorption was
observed for dichloromethane (DCM), and the absorption peak
shifted to lower wavelength on increasing the polarity (Figure 3).
This is probably due to the more efficient solvation of the dyes in
the polar solvents. This also confirms the charge-transfer nature for
this electronic transition. In dichloromethane, all of the dyes
exhibited an unexpected red-shift while in tetrahydrofuran a
prominent blue shift was observed. Unusual red-shift in absorption
in dichloromethane has been identified earlier for cationic dyes and
attributed to the instant stabilization due to a fast rearrangement of
polarizable electrons during excitation.13 The unusual blue shift
observed for the dyes in tetrahydrofuran solutions is intriguing. It is
probable that the hydrogen bonding interactions of carboxylic
acid unit with the tetrahydrofuran oxygen may result in a partial
deprotonation. This could in turn reduce the donorꢀacceptor
interaction in the dye and produce a blue shift in absorption.14
Addition of TFA to the THFsolutionscaused the red-shift support-
ing the above hypothesis. Even though such hydrogen bonding
possibilities are present in methanolic solution, because of the
available proton population in the solution, the equilibrium might
be shifted toward the acidic form.
The presence of acidꢀbase equilibria in solution is further
confirmed by the addition of trifluoroacetic acid (TFA) to the dye
solutions in toluene, methanol, and dichloromethane (Table 2 and
Figure 4). Addition of TFA to the dye solutions red-shifted the
higher wavelength absorption because the charge-transfer transi-
tion and no changes were seen on the lower wavelength transi-
tions. This shows that the TFA interacts with the dyes in such a
way to alter the donorꢀacceptor interactions. Oxidation of the
amine moiety may also influence the donorꢀacceptor interaction;
however, it will lead to a blue shift in absorption. So it is concluded
that the TFA supplies protons in the solution which shifts the
equilibrium toward the acidic form. Addition of triethylamine to
the toluene or dichloromethane solutions produced a drastic blue
shift for the charge-transfer transition.
The molar extinction coefficients of the dyes D2 and JD1ꢀJD3,
at the maximum absorption of the charge-transfer transition in di-
chloromethane, range from 22900 to 35800 Mꢀ1 cmꢀ1, which are
higher than those observed for the dyes D1 (15800 Mꢀ1 cmꢀ1 9
)
and D3 (22900 Mꢀ1 cmꢀ1).10 The optical density measured for
these dyes are also significantly higher than those of the Ru dyes N3
(16000 Mꢀ1 cmꢀ1) and N719. It is noteworthy that the incorpora-
tion of additional fluorene or aminofluorene segment not only red
shifts the absorption edge but also increases the molar extinc-
tion coefficient, which is highly desirable for large photocurrent
generation.
Adsorption of the dyes on transparent nanocrystalline anatase
TiO2 films shifts the absorption peak toward higher wavelength
region (Figure 5) when compared to the corresponding spectra
recorded for the dyes in dichloromethane. This is probably due
to the use of thicker TiO2 film.15 On the contrary, dyeꢀTiO2
4912
dx.doi.org/10.1021/jo200501b |J. Org. Chem. 2011, 76, 4910–4920

The Journal of Organic Chemistry
ARTICLE
Table 2. Absorption Spectral Data for the Dyes in Different Solvents
λmax nm (εmax, ꢁ 103 Mꢀ1 cmꢀ1
)
dye
TOL
TOLþTEA TOLþTFA
MeOH
MeOHþNaOH MeOHþTFA DCMþTEA DCMþTFA
ACN
THF
D2
422 (22.2),
326 (15.5)
399 (27.6), 464 (28.7), 405 (32.5),
405 (32.8),
324 (18.6),
262 (23.0)
409 (40.6),
360 (40.6)
430 (34.3),
317 (18.7),
266 (20.8)
434 (22.9),
360 (22.8)
405 (28.1),
331 (16.2)
453 (36.7), 427 (30.2), 411 (20.3),
340 (18.1)
320 (16.6)
324 (18.6),
262 (22.1)
319 (20.2),
269 (20.5)
318 (16.6) 433 (15.0)
JD1
JD2
JD3
446(33.9),
404 (36.9), 458 (36.2), 411 (36.5),
367 (36.9) 360 (36.7)
409 (35.8),
365 (36.6),
308 (23.0)
385 (42.9),
335 (26.7)
463 (32.5), 432 (29.0), 399 (36.0),
367 (35.72) 366 (37.6)
364 (29.0),
308 (21.7)
359 (28.9) 364 (35.0)
440 (31.5),
373 (36.3)
388 (42.2)
344 (34.8)
456 (32.7), 407 (34.8),
372 (36.5) 374 (38.3)
405 (36.2),
377 (39.8)
430 (31.9),
366 (33.6)
457 (38.8), 417 (24.5), 382 (45.9),
368 (28.8),
269 (27.8)
370 (30.0) 334 (27.6)
397 (23.4),
326 (32.7)
409 (22.9), 369 (26.5),
330 (34.4) 327 (36.1)
340 (38.4)
387 (30.0),
323 (46.7)
343 (48.9),
294 (41.9),
284 (42.2)
413 (38.8), 391 (15.2), 329 (41.2)
329 (36.3),
291 (49.9)
324 (25.2)
Figure 2. Absorption spectra of JD1 recorded in different solvents.
Figure 4. Absorption spectra of the dye JD2 recorded in toluene before
and after the addition of TEA or TFA.
Figure 3. Variation of absorption peak positions observed for JD1 in
different solvents with ET(30) parameter.
Figure 5. Absorption spectra of the dyes anchored on nanocrystalline
TiO2.
4913
dx.doi.org/10.1021/jo200501b |J. Org. Chem. 2011, 76, 4910–4920

The Journal of Organic Chemistry
ARTICLE
Table 3. Computed Vertical Transition Energies and Their Oscillator Strengths and Configurations for the Dyes Using Different
Theories
theory
parameter
λmax
D2
JD1
JD2
JD3
B3LYP (gas)
446.5
0.65
519.8
0.22
501.4
0.23
421.0
0.21
f
configuration HOMOfLUMO (100%) HOMOfLUMO (100%)
HOMOfLUMO (99%)
HOMOfLUMO (95%)
CAM-B3LYP (gas)
λmax
363.8
0.95
369.8
359.1
1.10
330.9
1.10
f
1.02
configuration HOMOfLUMO (88%)
HOMO-1fLUMO (6%)
HOMO-1fLUMO (45%)
HOMOfLUMO (43%)
HOMOfLUMOþ1 (5%)
385.3
HOMO-1fLUMO (44%) HOMO-1fLUMO (70%)
HOMOfLUMO (40%)
HOMOfLUMO (17%)
CAM-B3LYP (THF) λmax
382.1
1.13
372.2
347.4
f
1.18
1.24
1.14
configuration HOMOfLUMO (93%)
HOMOfLUMO (53%)
HOMO-1fLUMO (21%)
387.2
HOMOfLUMO (61%)
HOMO-1fLUMO (68%)
HOMO-1fLUMO (32%) HOMOfLUMO (25%)
MPW1K (gas)
λmax
373.1
0.89
372.0
336.0
f
0.82
0.88
0.99
configuration HOMOfLUMO (94%)
HOMOfLUMO (68%)
HOMO-1fLUMO (26%)
404.5
HOMOfLUMO (65%)
HOMO-1fLUMO (69%)
HOMO-1fLUMO (26%) HOMOfLUMO (23%)
MPW1K (THF)
λmax
391.4
1.08
387.3
352.4
f
0.99
1.02
1.02
configuration HOMOfLUMO (93%)
HOMOfLUMO (63%)
HOMO-1fLUMO (31%)
HOMOfLUMO (61%)
HOMO-1fLUMO (68%)
HOMO-1fLUMO (32%) HOMOfLUMO (25%)
Figure 6. Energies and the electronic distribution of the frontier molecular orbitals of the dyes JD1ꢀJD3 and D2 computed in THF using TDDFT/
MPW1K/6-31G(D, P).
interactions or deprotonation of the dyes at the surface of the
TiO2 films would lead to a blue-shift.2d
and LUMO of the dyes along with their energies obtained using
MPW1K. The HOMOs in the dyes are delocalized over the
triarylamine unit constituted by the phenyl, naphthyl, or carbazole
and fluorene segments with larger electron density located at the two
nitrogen atoms of the amines, while the LUMOs are located in
the cyanoacrylic acid unit through the phenyl group. Thus, the
HOMOꢀLUMO excitation induced by light irradiation could move
the electron distribution from the triarylamine part to the cyano-
acrylic acid segment. This will ensure an efficient electron injection in
the TiO2 layer. The HOMO energies of the dyes increase in the
To understand the electronic structure of the dyes, the geometry
of the dyes was optimized by density functional theory16 (DFT)
calculations at the 6-31þG(D,P) level using three different correla-
tion functionals (B3LYP,17 CAM-B3LYP,19 and MPW1K19), and
the vertical excitation energies were computed using time-dependent
density functional theory in the same level. The computed energies
of the vertical excitations and their assignments are collected in
Table 3. Figure 6 shows the electron distribution of the HOMO
4914
dx.doi.org/10.1021/jo200501b |J. Org. Chem. 2011, 76, 4910–4920

The Journal of Organic Chemistry
ARTICLE
Figure 8. Cyclic voltammograms of the dyes JD1, JD2, and D2 and their
parent amines (1a, 1b, and 1d) recorded in dichloromethane solutions.
Figure 7. Comparison of observed (THF) and calculated electronic
excitations for the dyes.
order, JD1 < JD2 < D2 < JD3, while the LUMO energies remain
unaltered. It has been suggested in a recent work that the HOMO
energies have to match the redox potential of the redox electrolyte to
achieve effective regeneration of the dyes in DSSC. It appears that by
altering the donor strength of the additional donor groups in a D-π-
D-π-A architecture, the HOMO levels can be adjusted while
retaining the LUMO levels.
The vertical excitations, corresponding to the HOMO to LUMO
transitions obtained for the dyes in different theory levels, are plotted
against the experimental absorption maxima observed in THF in
Figure 7. The longer wavelength transition calculated using B3LYP
for the dyes are red-shifted from the observed values in tetrahydro-
furan, whereas the gas phase results obtained from CAM-B3LYP and
MPW1K methods are showing slight blue shift (Table 3). However,
the vertical excitations calculated in the presence of tetrahydrofuran
using PCM and CAM-B3LYP or MPW1K show close agreement to
the experimentally observed values for tetrahydrofuran. Though the
higher wavelength absorption in the dyes is predicted to originate
from the HOMO/LUMO transition in B3LYP calculations, CAM-
B3LYP and MPW1K results suggested contributions from HOMO/
LUMO and HOMO-1/LUMO transitions for the dyes JD1ꢀJD3.
On this basis, it may be argued that the longer wavelength absorption
present in these dyes possesses a contribution from the πꢀπ*
transition as well.
Electrochemical Properties. Cyclic voltammetry measure-
ments on the dyes were performed to determine the oxidation
potentials (Eox) of the dyes and the excited-state oxidation
potential (Eox*). The later quantity can be obtained by subtract-
ing the 0ꢀ0 transition energy (E0ꢀ0) from Eox, whereas E0ꢀ0 is
derived from the absorption edge of the dye.20 The data obtained
from the electrochemical measurements are listed in Table 1.
The dyes JD1 and JD2 displayed two quasi-reversible oxidation
couples (Figure 8), excluding the redox couple arising due to the
internal ferrocene standard. The first oxidation occurring at ca.
250 mV corresponds to the removal of electron from the amine
furthest away from the electron-withdrawing segment. The second
oxidation occurs at more positive potentials (>520 mV) and
originates from the removal of an electron from the amino segment
in direct conjugation with the cyanoacrylic acid unit. This amine
unit is difficult to oxidize when compared to the peripheral amine as
Figure 9. Comparison of the oxidation potentials of the dyes JD1ꢀJD3
and D2 dissolved in dichloromethane in the ground and excited states.
the electron density on the nitrogen is depleted by the electron-
accepting cyanoacrylic unit. The oxidation potential observed
for the control dye D2 is close to the second oxidation potential
observed for the dyes JD1 and JD2. This also confirms the
assignment. The first oxidation potentials of the dyes (JD1, JD2,
and D2) are positively shifted when compared to the parent amines
1a, 1b, and1d (Figure 8), whichconfirms the electron-withdrawing
effect of the acceptor segment. Similarly, there is a slight negative
shift for the second oxidation potentials of the dyes when compared
to the oxidation potential of the control dye D2, which reflects the
electron-donating effect of the additional amine unit on facilitating
the oxidation of the amine adjacent to the acceptor unit.
The variation of oxidation potentials in the ground and excited
states for the dyes are illustrated in Figure 9. For comparison, the
conduction band of TiO2 layer and the redox potential of the
redox electrolyte (Iꢀ/I3ꢀ) used in the DSSC is also included.21
The incorporation of additional diarylamine segment in the dye
structure is beneficial to raise both the Eox and Eox* but narrows
the band gap. Stronger donors obviously result in larger donorꢀ
acceptor interaction, which leads to the reduction in the E0ꢀ0
.
The ground-state oxidation potentials of the dyes occupy a wide
range covering 1.03 to 1.60 V versus NHE and are more positive
4915
dx.doi.org/10.1021/jo200501b |J. Org. Chem. 2011, 76, 4910–4920

The Journal of Organic Chemistry
ARTICLE
Table 4. Performance Parameters of the DSSCs Fabricated
Using the Dyes JD1ꢀ3 and D2
dye JSC (mA cmꢀ2
)
VOC (mV)
ff
η (%) RCt2 (Ω) τe (ms)
JD1
JD2
JD3
D2
10.4
10.5
571
595
605
603
0.69 4.11
0.70 4.36
0.70 2.30
0.69 3.60
17.30
15.36
28.79
19.28
15.27
22.69
27.68
22.69
5.46
8.73
Figure 11. IPCE plots of the DSSCs fabricated using the dyes
JD1ꢀJD3 and D2.
Figure 12. Equivalent circuit for the DSSCs.
Figure 10. IꢀV characteristics of the DSSCs fabricated using the dyes
JD1ꢀJD3 and D2.
to the conduction band of TiO2; recombination of the injected
electrons on TiO2 with theꢀoxidized dye; dark current generated
due to the reduction of I3 by the injected electrons; and the
regeneration of the dye by the electrolyte which is dependent on
the HOMO level of the dye and the redox potential of the
electrolyte. As the electron injection and regeneration of the dye
are largely dependent on the electronic properties of the dyes,
we will first examine the electron injection propensity and the
regeneration feasibility for the dyes. For a facile electron injec-
tion, the excited-state oxidation potential of the dyes must be
more negative than that of the TiO2 conduction band. The
excited state oxidation potentials of the dyes assume the follow-
ing order: JD2 < JD1 < JD3 < D2. More negative excited state
oxidation potential observed for JD2 among the series gives
larger driving force the electron injection from this dye. How-
ever, this trend is not matching with the photovoltaic efficiency
trend which indicates that other factors also influence the
photovoltaic performance. The HOMO energies of the dyes
deduced in dichloromethane solution are in the order, JD2 > JD1 >
D2 > JD3. This suggests that the regeneration will be more facile for
JD2 when compared to the other dyes.
The effects of the different dyes on the electron transport at the
interfaces in the DSSCs can be estimated with the aid of electro-
chemical impedance spectroscopy (EIS) measurements. The
electrochemical impedance analysis of the DSSCs were performed
over the frequency range of 10 mHz to 65 kHz under illumination
at the applied bias voltage set at the open-circuit voltage of the
DSSCs with an ac amplitude of 10 mV. In general, the EIS
spectrum of a dye-sensitized solar cell having a configuration
FTO/TiO2/dye/electrolyte/Pt/ITO displays three semicircles in
the frequency range of 10 mHz to 65 kHz. The equivalent circuit is
shown in Figure 12. The ohmic serial resistance (RS) corresponds
to the overall series resistance. The first and second semicircles
than the redox potential of iodine/iodide couple.21 The excited
state oxidation potentials of the dyes are in between ꢀ1.04 and
ꢀ1.32 V versus NHE and more negative than the conduction
band of TiO2, which is located at ca. ꢀ0.5 V versus NHE.22 These
observations indicate that the redox potentials of the dyes in the
ground and excited states are favorable for efficient electron injection
to TiO2 and the dye regeneration by iodine/iodide redox system.
DSSC Characteristics. The photovoltaic performances (Table 4)
of the newly synthesized dyes (D2, JD1ꢀJD3) were evaluated
using the Gr€atzel photoelectrochemical cells. Figure 10 shows IꢀV
characteristics, while Figure 11 displays the IPCE action spectra for
DSSCs based on these four dyes using a 18.5 μm TiO2 (14 μm
transparent layer þ4.5 μm scattering layer) electrode. IPCE is
defined as the number of electrons generated by light in the external
circuit divided by the number of incident photons. The dyes exhibit
maximum IPCE (>75%) between 400 and 500 nm and the IPCE
spectra broadens and extends to 600 nm on incorporation of
additional diarylamine unit (for example, compare D2 with JD1
or JD2) which is consistent with the bathochromic shift observed
for these dyes in the UVꢀvis absorption spectra of the dye loaded
TiO2 films (Figure 5). The broadening of the IPCE spectra is
desired for a larger photocurrent23 which explains the higher
efficiency observed for the dyes D2, JD1, and JD2 when compared
to the carbazole dye JD3. JSC increased in the order of JD3 < D2 <
JD1 < JD2 as a result of the broadening of the IPCE spectra and is
similar to the trend observed for the absorption peak in dichlor-
omethane solution.24 DSSC based on JD1 showed lower VOC when
compared to the DSSCs fabricated using the other dyes (JD2, JD3,
and D2). In general, the lower VOC of a DSSC is attributed to the
shorter electron lifetime, τe.25
There are four critical processes that determine the photo-
voltaic performance of the DSSC: electron injection from the dye
4916
dx.doi.org/10.1021/jo200501b |J. Org. Chem. 2011, 76, 4910–4920

The Journal of Organic Chemistry
ARTICLE
dye and the electrolyte constituents. The presence of naphthyl
group in JD2 may facilitate the electronic coupling when compared
to the phenyl unit in JD1.
In the Bode phase plot (Figure 14) the lower frequency peak of
the dye JD1 isshifted to higher frequency when compared tothose
of other dyes. This corresponds to a decrease in the electron
lifetime for the JD1 sensitized DSSC. The electron lifetime can be
extracted from the angular frequency (ωmin) at the midfrequency
26
peak in the Bode phase plot using τe = 1/ωmin
.
The electron
lifetime follows the trend, JD1 < JD2 ∼ D2 < JD3. The increase in
electron lifetime supports more effective suppression of the back
reaction of the injected electrons with the I3ꢀ in the electrolyte by
alternation of HOMO of the sensitizer, which leads to the
improvement in the photocurrent and photovoltage and to the
substantial enhancement of the device efficiency for the dye JD2.
Despite the enhancement in the electron lifetime, the lower device
efficiency observed for the DSSC based on JD3 is attributed to the
lower number of photoinduced electron generation and lesser
electron injection efficiency of JD3. The high-frequency peak
observed in the Bode plot corresponds to charge transfer at the
counter electrode. Slight changes observed for this peak indicate a
probable reaction at the counter electrode/electrolyte interface.
Figure 13. Nyquist plots of the DSSCs fabricated using the dyes
JD1ꢀJD3 and D2.
’ CONCLUSIONS
In summary, we have synthesized four new dyes based on
triarylamine donors featuring additional diarylaminofluorene chro-
mophores and cyanoacrylic acid acceptors. They were characterized
by optical and electrochemical studies. They display red-shifted
absorption and negatively shifted oxidation potentials when com-
pared to the simple triphenylamine-based dyes. Subsequently, they
exhibit improved light-harvesting performance in nanocrystalline
anatase TiO2-based dye-sensitized solar cells when compared to the
parent dyes derived from the monoamines. The enhanced light-to-
electron conversion efficiency is attributed to the broader and
intense absorption profile observed for the fluorenediamine-based
dyes and their high lying HOMO. The enhanced optical property
contributes to the increase in the photocurrent density and the
comparatively larger HOMO value facilitates the regeneration of the
dyes by the electrolyte and suppression of the back reaction of the
injected electrons with the electrolyte constituents. The success of
the design strategy used in this work to improve the device perfor-
mance indicates that the fluoreneamine additional donors may
be incorporated in the demonstrated dye architectures to obtain
efficient organic dyes for dye-sensitized solar cells.
Figure 14. Bode-phase plots of the DSSCs fabricated using the dyes
JD1ꢀJD3 and D2.
correspond to the charge-transfer resistances at the counter
electrode (Rct1) and the TiO2/dye/electrolyte interface (Rct2),
ꢀ
respectively. The Warburg diffusion process of Iꢀ/I3 in the
electrolyte (Zw) is associated with the third semicircle. However,
in this work the conventional diffusion resistance of the redox
couple is apparently greatly overlapped by Rct2 because of a short
length for Iꢀ ion diffusion available with the thin spacer used
(25 μm thick) and owing to the low viscosity of the solvents used
in our electrolyte (viscosities of ACN and MPN are 0.37 and
1.60 cP, respectively). The Nyquist plot displayed in Figure 13
shows two semicircles which correspond to the charge-transfer
resistances at the counter electrode and TiO2/dye/electrolyte
interface, respectively. The radius of the bigger semicircle of the
dye JD3 is significantly larger than the other dyes, indicating the
enlarged charge-transfer resistance for this dye. The charge-transfer
resistance assumes the order JD2 < JD1 < D2 < JD3. Though the
charge-transfer resistance ofJD2 is smaller than that of JD1, the Voc
is larger. The larger Voc observed for JD2 may originate from the
enhanced electronic coupling between the amino segment of the
’ EXPERIMENTAL SECTION
General Methods. All reactions and manipulations were carried
out under N2 with the use of standard inert atmosphere and Schlenk
techniques. Solvents were dried by standard procedures. All column
chromatography was performed with the use of 100ꢀ200 mesh silica gel
as the stationary phase in a column of 30 cm long and 2.0 cm diameter.
The 1H and 13C NMR spectra were measured by using 500 MHz
spectrometer. Cyclic voltammetric experiments were carried out at
room temperature with a conventional three-electrode configuration
consisting of a glassy carbon working electrode, a platinum wire auxiliary
electrode, and a nonaqueous Ag/AgNO3creference electrode. The E1/2
1
a
a
c
values were determined as /2(Ep þ Ep ), where Ep and Ep are the
anodic and cathodic peak potentials, respectively. The potentials are
quoted against the ferrocene internal standard. The solvent in all
experiments was dichloromethane and the supporting electrolyte was
0.1 M tetrabutylammonium perchlorate. Electronic absorption spectra
4917
dx.doi.org/10.1021/jo200501b |J. Org. Chem. 2011, 76, 4910–4920

The Journal of Organic Chemistry
ARTICLE
were obtained on a UVꢀvisible spectrophotometer using freshly pre-
pared solutions. The precursor compounds 1aꢀd were obtained
according to the literature procedures.27ꢀ29
organic extracts were concentrated and adsorbed on silica gel. It was purified
by column chromatography by using hexane/ethyl acetate mixture (5:1) as
eluant: cream-white solid; yield 0.27 g (30%); mp 233 °C; IR (KBr, cmꢀ1
)
1686 (νCdO); 1H NMR (CDCl3, 500.13 MHz) δ 0.55ꢀ0.58 (m, 6 H),
2.10ꢀ2.14 (dd, J=15Hz, 7.5Hz, 4H), 7.31ꢀ7.34 (m, 2 H),7.40ꢀ7.64 (m,
12 H), 7.99ꢀ8.04 (m, 3 H), 8.19 (d, J = 7.5 Hz, 2 H), 8.25 (d, J = 8.0 Hz, 1
H), 8.71 (d, J = 1.0 Hz, 1 H); 13C NMR (CDCl3, 125.75 MHz) δ 191.7,
152.4, 152.1, 144.6, 142.0, 141.0, 140.9, 139.6, 137.1, 135.6, 129.5, 127.6,
127.1, 126.2, 126.0, 123.9, 123.6, 123.4, 123.3, 121.9, 121.2, 121.2, 121.2,
120.8, 120.4, 120.0, 110.4, 110.1, 109.7, 56.8, 32.7, 8.8; HRMS calcd for
C42H32N2O m/z 580.2515, found 580.2527.
4-((9,9-Diethyl-9H-fluoren-2-yl)(phenyl)amino)benzaldehyde
(2a): yellow solid; yield 60%; mp 86 °C; IR (KBr, cmꢀ1) 1676 (νCdO); 1H
NMR (CDCl3, 500.13 MHz) δ 0.34ꢀ0.37 (m, 6 H), 1.91ꢀ2.17 (m, 4 H),
7.07 (d, J = 8.5 Hz, 2 H), 7.11 (dd, J = 8.0, 2.0 Hz, 1 H), 7.15ꢀ7.22 (m, 4 H),
7.30ꢀ7.36 (m, 4 H), 7.65ꢀ7.71 (m, 4 H), 9.82 (s, 1 H); 13C NMR (CDCl3,
125.75 MHz) δ 190.5, 153.5, 151.8, 150.1, 146.4, 145.3, 140.8, 138.9, 131.4,
129.8, 129.7, 128.1, 125.0, 122.9, 121.3, 120.8, 119.7, 119.6, 119.4, 56.2,
32.7, 8.6.
(E)-2-Cyano-3-(4-((7-(diphenylamino)-9,9-diethyl-9H-fluo-
ren-2-yl)(phenyl)amino)phenyl)acrylic Acid (JD1). A mixture of
the aldehyde 2b (1.0 g, 1.71 mmol), 2-cyanoacetic acid (0.18 g, 1.90 mmol),
ammonium acetate (0.58 g, 0.75 mmol), and acetic acid (5 mL) was
refluxed for 12 h. The resulting dark red solution was poured into water. The
separated solid was filtered and thoroughly washed with water and dried:
orange powder; yield 0.90 g (80%); mp 190 °C; IR (KBr, cmꢀ1) 2216
4-((7-(Diphenylamino)-9,9-diethyl-9H-fluoren-2-yl)(phenyl)
amino)benzaldehyde (2b). A two-neck flask was charged with
9,9-diethyl-N2,N2,N7,N7-tetraphenyl-9H-fluorene-2,7-diamine (3.33 g, 6.0
mmol). This was dissolved in 15 mL of chlorobenzene and 10 mL of DMF
with stirring. POCl3 (0.6 mL, 6.6 mmol) was then added dropwise into the
mixture at room temperature. The mixture was heated at 90 °Cfor10hwith
stirring. It was then poured into iceꢀwater and neutralized with sodium
bicarbonate. The organic products formed were extracted with dichlor-
omethane (3 ꢁ 50 mL). The combined organic extracts were concentrated
and adsorbed on silica gel and purified by column chromatography by using
hexane/ethyl acetate mixture (5:1) as eluant: yellow solid; yield 2.27 g
1
(νCꢂN); H NMR (DMSO-d6, 500.13 MHz) δ 0.26ꢀ0.29 (m, 6 H),
1.79ꢀ1.84 (m, 4 H), 6.89 (d, J = 9.0 Hz, 2 H), 6.93ꢀ6.95 (dd, J = 8.5 Hz,
2.0 Hz, 1 H), 7.00ꢀ7.02 (m, 6 H), 7.06 (d, J = 1.5 Hz, 1 H), 7.11ꢀ7.13 (dd,
J = 8.5 Hz, 2.0 Hz, 1 H), 7.20ꢀ7.30 (m, 8 H), 7.39ꢀ7.43 (m, 2 H), 7.69 (d,
J = 8.0 Hz, 1 H), 7.74 (d, J = 8.0 Hz, 1 H), 7.91(d, J = 9.0 Hz, 2 H), 8.11 (s, 1
H); 13C NMR (DMSO-d6, 125.75 MHz) δ 164.6 152.9, 151.3, 148.60,
147.8, 144.1, 143.5, 141.5, 138.5, 135.4, 135.3, 134.9, 133.3, 130.7, 130.4,
129.7, 129.2, 129.03, 128.6, 128.2, 127.6, 127.4, 127.1, 127.0, 126.9, 126.8,
126.2, 124.7, 124.0, 123.5, 122.9, 122.1, 121.7, 121.5, 121.0, 120.7, 120.4,
117.3, 117.8, 116.4, 56.1, 32.1, 8.9; HRMS calcd for C45H38N3O2 [M þ H]
m/z 652.2959, found 652.2961. Anal. Calcd for C45H37N3O2: C, 82.92; H,
5.72; N, 6.45. Found: C, 82.81; H, 5.64; N, 6.53.
1
(65%); mp 170 °C; IR (KBr, cmꢀ1) 1685 (νCdO); H NMR (CDCl3,
500.13 MHz) δ0.39ꢀ0.42 (m, 6 H), 1.84ꢀ1.88 (m, 4 H), 7.01ꢀ7.10 (m, 7
H), 7.13ꢀ7.15 (m, 5 H), 7.15ꢀ7.18 (m, 1 H), 7.22 (d, J = 7.5 Hz, 2 H),
7.26ꢀ7.29 (m, 4 H), 7.35ꢀ7.38 (m, 2 H), 7.54 (d, J = 8.5 Hz, 1 H),7.58 (d,
J = 8.0 Hz, 1 H), 7.71 (d, J = 9.0 Hz, 2 H), 9.83 (s, 1 H); 13C NMR (CDCl3,
125.75 MHz) δ 190.4, 153.5, 151.7, 151.4, 147.9, 147.1, 146.3,144.5, 138.7,
135.8, 131.3, 129.6, 129.2, 128.9, 125.8, 125.5, 124.8, 123.9, 123.6, 122.5,
121.2, 120.2, 120.1, 119.3, 119.2, 56.1, 32.5, 8.6; HRMS calcd for
C42H37N2O [M þ H] m/z 585.2906, found 585.2918.
(E)-2-Cyano-3-(4-((9,9-diethyl-7-(naphthalen-1-yl(phe-
nyl)amino)-9H-fluoren-2-yl)(naphthalen-1-yl)amino)phe-
nyl)acrylic Acid (JD2). Compound JD2 was prepared in 86% yield
from 2c by following a procedure similar to that described above for
4-((9,9-Diethyl-7-(naphthalen-1-yl(phenyl)amino)-9H-fluo-
ren-2-yl)(naphthalen-1-yl)amino)benzaldehyde. (2c). A two-
neck flask was charged with 9,9-diethyl-N2,N7-di(naphthalen-1-yl)-N2,
N7-diphenyl-9H-fluorene-2,7-diamine (1.0 g, 1.52 mmol). This was dis-
solved in chlorobenzene (5 mL) and N-methylformanilide (0.35 mL).
POCl3 (0.35 mL, 3.85 mmol) was then added to slowly at room temper-
ature. The resultant mixture was allowed to heat at 90 °C for 15 h and
then poured into iceꢀwater and neutralized with sodium bicarbonate. The
organic products formed were extracted with dichloromethane (3 ꢁ
50 mL). The combined organic extracts were concentrated and adsorbed
on silica gel. It was purified by column chromatography by using hexane/
ethyl acetate mixture (5:1) as eluant: yellow solid; yield 0.46 g (44%); mp
175 °C; IR (KBr, cmꢀ1): 1685 (νCdO);1HNMR(CDCl3, 500.13 MHz) δ
0.32 (t, J = 7.5 Hz, 6H), 6.88ꢀ6.94 (m, 4H), 7.02ꢀ7.07 (m, 4H),
7.18ꢀ7.22 (m, 3H), 7.31ꢀ7.33 (m, 2H), 7.38ꢀ7.51 (m, 8H), 7.64 (d,
J= 9.0 Hz, 2H), 7.75 (d, J= 8.5 Hz, 1H), 7.83 (d, J= 8.0 Hz, 1H), 7.86ꢀ7.92
(m, 4H), 9.77(s, 1H); 13C NMR (CDCl3, 125.75 MHz) δ 190.4, 154.2,
151.4, 151.3, 148.8, 144.6, 143.9, 142.1, 138.5, 135.3, 134.9, 131.3, 130.9,
130.6, 129.1, 128.6, 128.4, 127.51, 127.1, 126.7, 126.7, 126.4, 126.3, 126.3,
126.2, 126.6, 126.7, 124.2, 124.1, 123.8, 121.8, 121.60, 121.4, 120.0, 119.9,
119.6, 117.7, 116.9, 56.1, 32.5, 8.4; HRMS calcd for C50H41N2O [M þ H]
m/z 685.3219, found 685.3242.
9-(7-(9H-Carbazol-9-yl)-9,9-diethyl-9H-fluoren-2-yl)-9H-car-
bazole-3-carbaldehyde. (2d). In a round-bottom flask, 9,90-(9,9-
diethyl-9H-fluorene-2,7-diyl)bis(9H-carbazole) (1.0 g, 1.81 mmol) was
dissolved in chlorobenzene (5 mL) and N,N-dimethylformamide (3 mL)
under nitrogen atmosphere. In a separate reaction flask, POCl3 (0.40 mL,
4.0 mmol) was added slowly to DMF (2 mL) at 0 °C under nitrogen
atmosphere and the mixture kept at room temperature for 1 h. This
Vilsmeier reagent was then added to the reaction mixture. The resultant
mixture was allowed to heat at 100 °C for 15 h and then poured into
iceꢀwater and neutralized with sodium bicarbonate. The organic products
formed were extracted with dichloromethane (3 ꢁ 50 mL). The combined
1
JD1: dark red solid; mp 202 °C; IR (KBr, cmꢀ1) 2218 (νCꢂN); H
NMR (DMSO-d6, 500.13 MHz) δ 0.21 (t, J = 7.5 Hz, 6 H),
1.65ꢀ1.80 (m, 4 H), 6.72 (d, J = 8.5 Hz, 2 H), 6.80ꢀ6.82 (m, 1
H), 6.90- 6.98 (m, 4 H), 7.10ꢀ7.12 (m, 1 H),7.22 (t, J = 8.0 Hz, 2 H),
7.32ꢀ7.37 (m, 3 H), 7.46ꢀ7.48 (m, 2 H), 7.52ꢀ7.57 (m, 4 H),
7.53ꢀ7.57 (m, 3 H), 7.81ꢀ7.88 (m, 5 H), 7.96ꢀ8.05 (m, 4 H); 13C
NMR (DMSO-d6, 125.75 MHz) δ 164.6 152.9, 151.3, 148.60, 147.8,
144.1, 143.5, 141.5, 138.5, 135.4, 135.3, 134.9, 133.3, 130.7, 130.4,
129.7, 129.2, 129.03, 128.6, 128.2, 127.6, 127.4, 127.1, 127.0, 126.9,
126.8, 126.2, 124.7, 124.0, 123.5, 122.9, 122.1, 121.7, 121.5, 121.0,
120.7, 120.4, 117.3, 117.8, 116.4, 56.1, 32.1, 8.9; HRMS calcd for
C53H42N3O2 [M þ H] m/z 752.3273, found 752.3273. Anal. Calcd
for C53H41N3O2: C, 84.66; H, 5.50; N, 5.59. Found: C, 84.78; H,
5.61; N, 5.47.
(E)-3-(9-(7-(9H-Carbazol-9-yl)-9,9-diethyl-9H-fluoren-2-yl)-
9H-carbazol-3-yl)-2-cyanoacrylic Acid (JD3). Compound JD3
was obtained in 82% yield from 2d by following the general procedure
described for JD1: yellow solid; mp 282 °C; IR (KBr, cmꢀ1) 2221
(νCꢂN); 1H NMR (DMSO-d6, 500.13 MHz) δ 0.45 (t, J = 7.5 Hz, 1H),
2.16ꢀ2.20 (m, 4H), 7.32 (t, J = 7.5 Hz, 2 H), 7.45ꢀ7.40 (m, 4 H), 7.49 (t,
J = 7.5 Hz, 2 H), 7.53 (d, J = 8.5 Hz, 1 H), 7.58 (t, J = 7.5 Hz, 1 H),
7.71ꢀ7.65 (m, 2 H), 7.84ꢀ7.78 (m, 2 H), 8.51ꢀ8.25 (m, 6 H), 9.00 (s, 1
H); 13C NMR (DMSO-d6, 125.75 MHz) δ 164.4, 152.5, 152.4, 143.3,
141.8, 140.8, 136.7, 135.4, 128.8, 126.8, 126.5, 124.0, 123.2, 122.8, 122.2,
121.9, 121.0, 120.5, 110.0, 57.1, 32.0, 9.1; HRMS calcd for C45H34N3O2
[M þ H] m/z 648.2646, found 648.2648. Anal. Calcd for C45H33N3O2:
C, 83.44; H, 5.13; N, 6.49. Found: C, 83.32; H, 5.07; N, 6.45.
(E)-2-Cyano-3-(4-((9,9-diethyl-9H-fluoren-2-yl)(phenyl)-
amino)phenyl)acrylic Acid (D2). Compound D2 was obtained in
64% yield from 2a by following the general procedure described for
4918
dx.doi.org/10.1021/jo200501b |J. Org. Chem. 2011, 76, 4910–4920

The Journal of Organic Chemistry
ARTICLE
JD1: red solid; IR (KBr, cmꢀ1) 2219 (νCꢂN); 1H NMR (CDCl3, 500.13
MHz) δ 0.24 (t, J = 7.5 Hz, 6 H), 1.90ꢀ1.97 (m, 4 H), 6.89ꢀ6.92 (m, 2
H), 7.14ꢀ7.23 (m, 1 H), 7.24ꢀ7.31 (m, 7 H), 7.33ꢀ7.35 (m, 3 H),
7.41ꢀ7.44 (m, 3 H), 7.78ꢀ7.80 (m, 1 H), 7.85 (d, J = 8.0 Hz, 1 H), 7.92
(d, J = 9.0 Hz, 2 H), 8.13 (s, 1 H); 13C NMR (DMSO-d6, 125.75 MHz) δ
164.6, 153.7, 152.3, 151.8, 151.5, 150.1, 145.8, 145.4, 144.9, 140.7, 140.0,
139.2, 138.0, 133.4, 130.4, 127.7, 127.6, 126.5, 126.3, 126.2, 125.9, 123.4,
123.3, 122.0, 121.7, 120.9, 120.2, 118.9, 118.7, 117.5, 56.2, 32.2, 8.9;
HRMS calcd for C33H29N2O2 [M þ H] m/z 485.2225, found 485.2226.
Anal. Calcd for C33H28N2O2: C, 81.79; H, 5.82; N, 5.78. Found: C,
81.92; H, 5.73; N, 5.71.
Computational Methods. All the computations were performed
with the Gaussian 09 program package.30 The ground-state geometries
were fully optimized without any symmetry constrains at the DFT level
with Becke’s31 three-parameter hybrid functional and Lee, Yang, and Parr’s
correlational functional B3LYP17 using the 6-31g(d,p) basis set on all
atoms. Vibrational analysis on the optimized structures was performed to
confirm the structure. The excitation energies and oscillator strengths for
the lowest 10 singletꢀsinglet transitions at the optimized geometry in the
ground state were obtained by TD-DFT calculations using the same basis
set as for the ground state and three kinds of hybrid functional B3LYP,
CAM-B3LYP,18 and MPW1K,19 respectively. Solvation effects were
modeled using the polarizable continuum model (PCM)32 implemented
in the Gaussian program, for both geometry optimizations and TD-DFT
calculations.
DSSC Fabrication and Characterization. For the TiO2 colloid
solution, the TiO2 precursor was prepared by the solꢀgel method as
described below. Titanium(IV) tetraisopropoxide (72 mL) was added to
430 mL of a 0.1 M nitric acid aqueous solution with constant stirring and
heated to 85 °C simultaneously for 8 h. When the mixture was cooled to
room temperature, the resultant colloid was transferred to an autoclave
and then heated at 240 °C for 12 h in order to allow the TiO2 particles to
grow uniformly (ca. 20 nm). Consequently, the TiO2 colloid was
concentrated to 10 wt % (with respect to the TiO2). The first type
of TiO2 paste (for transparent layer) was prepared by the addition
25 wt % (with respect to the TiO2) of poly(ethylene glycol) (PEG,
MW ∼20000) to the above solution in order to control the pore
diameters and to prevent the film from cracking during drying. For the
second one (for scattering layer), the first type of TiO2 paste was
incorporated with 50 wt % of light scattering TiO2 particles for reducing
the light loss by back scattering.
0.1 M LiI, 0.6 M DMPII, 0.05 M I2, and 0.5 M TBP in 3-methoxypro-
pionitrile (MPN)/ACN (volume ratio of 1:1) was used as the electro-
lyte. The electrolyte was injected into the gap between the electrodes by
capillarity; the electrolyte-injecting hole was previously made in the
counter electrode with a drilling machine, and the hole was sealed with
hot-melt glue after the injection of the electrolyte.
The surface of the DSSC was covered by a mask with a light-
illuminated area of 0.16 cm2 and then illuminated by a class A quality
solar simulator. Incident light intensity (100 mW cmꢀ2) was calibrated
with a standard silicon cell. Photocurrentꢀvoltage curves of the DSSCs
were obtained with a potentiostat/galvanostat. The thickness of the
TiO2 film was judged by scanning electron microscopic images (SEM).
For UV absorption spectra, dye molecules were coated on the TiO2
films, and the corresponding spectra were obtained using an UVꢀvis
spectrophotometer equipped with an integrating sphere. Electrochemi-
cal impedance spectra (EIS) were obtained from the potentiostat/
galvanostat, equipped with an FRA2 module, under a constant light
illumination of 100 mW cmꢀ2. The frequency range explored was 10
mHz to 65 kHz. The applied bias voltage was set at the open-circuit
voltage of the DSSC between the ITOꢀPt counter electrode and the
FTOꢀTiO2 dye working electrode, starting from the short-circuit
condition; the corresponding AC amplitude was 10 mV. The impedance
spectra were analyzed using an equivalent circuit model. Incident
phototo-current conversion efficiency (IPCE) curves were obtained
under short-circuit conditions. The light source was a class A quality
solar simulator (PEC-L11, AM 1.5 G); light was focused through a
monochromator onto the photovoltaic cell. The monochromator was
incremented through the visible spectrum to generate the IPCE (λ) as
defined by IPCE (λ) = 1240 (JSC/λj), where λ is the wavelength, JSC is
the short-circuit photocurrent density (mA cmꢀ2) recorded with a
potentiostat/galvanostat, and j is the incident radiative flux (W mꢀ2
)
measured with an optical detector and a power meter.
’ ASSOCIATED CONTENT
Supporting Information. 1H and 13C NMR spectra of the
S
b
newly synthesized compounds and the optical spectra of the com-
pounds recorded in different solvents. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
A fluorine-doped SnO2 conducting glass (FTO, 7 Ω sqꢀ1, transmit-
tance ∼80%) was first cleaned with a neutral cleaner and then washed
with deionized water, acetone, and isopropyl alcohol, sequentially. The
conducting surface of the FTO was treated with a solution of titanium
tetraisopropoxide (1 g) in 2-methoxyethanol (3 g) to obtain a good
mechanical contact between the conducting glass and TiO2 film, as well
as to isolate the conducting glass surface from the electrolyte. TiO2
pastes were coated onto the treated conducting glass by using the doctor
blade technique. To coat each TiO2 layer, the dried TiO2 film was
gradually heated to 450 °C in an oxygen atmosphere and subsequently
sintered at that temperature for 30 min. The TiO2 photoanodes of the
DSSCs employed in the experiments were composed of a 14 μm thick
transparent TiO2 layer and with a scattering layer of 4.5 μm thickness.
After sintering at 450 °C and cooling to 80 °C, the TiO2 film was
immersed in a 3 ꢁ 10ꢀ4 M solution of dye at room temperature for 24 h.
The standard ruthenium complex, N719, was dissolved in acetonitrile
(ACN) and tert-butyl alcohol (volume ratio of 1:1) to make a reference
dye solution. Various organic dye solutions were prepared in a mixing
solvent containing ACN, tert-butyl alcohol, and dimethyl sulfoxide
(DMSO) (volume ratio of 1:1:3). The thus prepared TiO2/dye
electrode was placed on a platinum-sputtered conducting glass electrode
(ITO, 7 Ω sqꢀ1), keeping the two electrodes separated by a 25 μm thick
Surlyn. The two electrodes were then sealed by heating. A mixture of
Corresponding Author
*Tel: þ91-1332-285376. Fax: þ91-1332-286202. E-mail:
krjt8fcy@iitr.ernet.in.
’ ACKNOWLEDGMENT
K.R.J.T. is thankful to Department of Science and Technology
(ref DST/TSG/ME/2010/27), New Delhi, India, for financial
support. K.C.H. acknowledges a grant-in-aid from the National
Science Council, Taiwan.
’ REFERENCES
(1) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737–740.
(2) (a) Unger, E. L.; Ripaud, E.; Leriche, P.; Cravino, A.; Roncali,
J.; Johansson, E. M. J.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. C 2010,
114, 11659–11667. (b) Liang, Y. L.; Peng, B.; Liang, J.; Tao, Z. L.; Chen,
J. Org. Lett. 2010, 12, 1204–1207. (c) Marinado, T.; Nonomura, K.;
Nissfolk, J.; Karlsson, M. K.; Hagberg, D. P.; Sun, L. C.; Mori, S.;
Hagfeldt, A. Langmuir 2010, 26, 2592–2598. (d) Baheti, A.; Tyagi, P.;
Thomas, K. R. J.; Hsu, Y. C.; Lin, J. T. J. Phys. Chem. C 2009,
113, 8541–8547. (e) Li, R. Z.; Lv, X. J.; Shi, D.; Zhou, D. F.; Cheng,
Y. M.; Zhang, G. L.; Wang, P. J. Phys. Chem. C 2009, 113, 7469–7479.
4919
dx.doi.org/10.1021/jo200501b |J. Org. Chem. 2011, 76, 4910–4920

The Journal of Organic Chemistry
ARTICLE
(f) Preat, J.; Michaux, C.; Jacquemin, D.; Perpete, E. A. J. Phys. Chem.
C 2009, 113, 16821–16833. (g) Shen, P.; Liu, Y. J.; Huang, X. W.; Zhao,
B.; Xiang, N.; Fei, J. J.; Liu, L. M.; Wang, X. Y.; Huang, H.; Tan, S. T. Dyes
Pigm. 2009, 83, 187–197. (h) Tian, H. N.; Yang, X. C.; Pan, J. X.; Chen,
R. K.; Liu, M.; Zhang, Q. Y.; Hagfeldt, A.; Sun, L. C. Adv. Funct. Mater.
2008, 18, 3461–3468.
(3) (a) Chen, C. H.; Hsu, Y. C.; Chou, H. H.; Thomas, K. R. J.; Lin,
J. T.; Hsu, C. P. Chem.—Eur. J. 2010, 16, 3184–3193. (b) Karim, M. A.;
Cho, Y. R.; Park, J. S.; Kim, S. C.; Kim, H. J.; Lee, J. W.; Gal, Y. S.; Jin,
S. H. Chem. Commun. 2008, 1929–1931.
(4) (a) Sakuragi, Y.; Wang, X. F.; Miura, H.; Matsui, M.; Yoshida, T.
J. Photochem. Photobiol. A. Chem. 2010, 216, 1–7. (b) Dentani, T.;
Kubota, Y.; Funabiki, K.; Jin, J.; Yoshida, T.; Minoura, H.; Miura, H.;
Matsui, M. New J. Chem. 2009, 33, 93–101. (c) Kim, D.; Song, K.; Kang,
M. S.; Lee, J. W.; Kang, S. O.; Ko, J. J. Photochem. Photobiol. A: Chem.
2009, 201, 102–110. (d) Liu, B.; Zhu, W. H.; Zhang, Q.; Wu, W. J.; Xu,
M.; Ning, Z. J.; Xie, Y. S.; Tian, H. Chem. Commun. 2009, 1766–1768.
(5) (a) Imoto, K.; Takahashi, K.; Yamaguchi, T.; Komura, T.;
Nakamura, J. I.; Murata, K. Bull. Chem. Soc. Jpn. 2003, 76, 2277–2283.
(b) Sayama, K.; Tsukagoshi, S.; Hara, K.; Ohga, Y.; Shinpou, A.; Abe, Y.;
Suga, S.; Arakawa, H. J. Phys. Chem. B 2002, 106, 1363–1371. (c) Sayama,
K.; Tsukagoshi, S.; Mori, T.; Hara, K.; Ohga, Y.; Shinpou, A.; Abe, Y.; Suga,
S.; Arakawa, H. Sol. Energy Mater. Sol. Cells 2003, 80, 47–71.
(6) (a) Daeneke, T.; Kwon, T.-H.; Holmes, A. B.; Duffy, N. W.;
Bach, U.; Spiccia, L. Nature Chem. 2010, 3, 211–215. (b) Wang, Z. S.;
Cui, Y.; Dan-Oh, Y.; Kasada, C.; Shinpo, A.; Hara, K. J. Phys. Chem.
C 2008, 112, 17011–17017. (c) Wang, Z. S.; Cui, Y.; Dan-Oh, Y.;
Kasada, C.; Shinpo, A.; Hara, K. J. Phys. Chem. C 2007, 111, 7224–7230.
(d) Wang, Z. S.; Cui, Y.; Hara, K.; Dan-Oh, Y.; Kasada, C.; Shinpo, A.
Adv. Mater. 2007, 19, 1138–1141.
(24) Hardin, B. E.; Hoke1, E. T.; Armstrong, P. B.; Yum, J.-H.;
Comte, P.; Torres, T.; Frꢀechet, J. M. J.; Nazeeruddin, M. K.; Gr€atzel, M.;
McGehee, M. D. Nat. Photonics 2009, 3, 406–411.
(25) Zhang, X.-H.; Cui, Y.; Katoh, R.; Koumura, N.; Hara, K. J. Phys.
Chem. C 2010, 114, 18283–13290.
(26) Lagemaat, J.; van de; Park, N.-G.; Frank, A. J. J. Phys. Chem. B
2000, 104, 2044–2052.
(27) Jeona, S.-H.; Anandakathira, R.; Chianga, J.; Chiang, L. Y.
J. Macromol. Sci., Part A: Pure Appl. Chem. 2007, 44, 1275–1282.
(28) Low, P. J.; Paterson, M. A. J.; Yufit, D. S.; Howard, J. A. K.;
Cherryman, J. C.; Tackley, D. R.; Brookc, R.; Brown, B. J. Mater. Chem.
2005, 15, 2304–2315.
(29) Liu, Q.-D.; Lu, J.; Ding, J.; Tao, Y. Macromol. Chem. Phys. 2008,
209, 1931–1941.
(30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.;
Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.;
Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.;
Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.;
Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich,
€
S.; Daniels, A. D.; Farkas, O .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.;
Fox, D. J. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT,
2009.
(7) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H.
Chem. Rev. 2010, 110, 6595–6663.
(8) (a) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.;
Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gr€atzel, M. J. Am. Chem. Soc.
2005, 127, 16835–16847. (b) Gr€atzel, M. J. Photochem. Photobiol. A
2004, 164, 3–14.
(31) Becke, A. D. J. Chem. Phys. 1993, 98, 1372–1377.
(32) (a) Cossi, M.; Barone, V.; Cammi, R.; Tomasi, J. Chem. Phys.
Lett. 1996, 255, 327–335. (b) Tomasi, J.; Mennucci, B.; Cammi, R.
Chem. Rev. 2005, 105, 2999–3093.
(9) Xu, W.; Peng, B.; Chen, J.; Liang, M.; Cai, F. J. Phys. Chem. C
2008, 112, 874–880.
(10) Wu, T.-Y.; Tsao, M.-H.; Chen, F.-L.; Su, S.-G.; Chang, C.-W.;
Wang, H.-P.; Lin, Y.-C.; Ou-Yang, W.-C.; Sun, I.-W. Int. J. Mol. Sci. 2010,
11, 329–353.
(11) Belfield, K. D.; Bondar, M. V.; Hernandez, F. E.; Przhonska,
O. V.; Yao, S. J. Phys. Chem. B 2007, 111, 12723–12729.
(12) Thomas, K. R. J.; Tyagi, P. J. Org. Chem. 2010, 75, 8100–8111.
(13) (a) Granzhan, A.; Ihmels, H.; Viola, G. J. Am. Chem. Soc. 2007,
129, 1254–1267. (b) van den Berg, O.; Jager, W. F.; Picken, S. J. J. Org.
Chem. 2006, 71, 2666–2676.
(14) Nazeeruddin, M. K.; Pꢀechy, P.; Renouard, T.; Zakeeruddin,
S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.;
Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gr€atzel, M.
J. Am. Chem. Soc. 2001, 123, 1613–1624.
(15) Cheng, H. M.; Hsieh, W. F. Energy Environ. Sci. 2010, 3,
442–447.
(16) Parr, R. G.; Yang, W. T. Annu. Rev. Phys. Chem. 1995, 46,
701–728.
(17) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789.
(18) Yanai, T.; Tew, D. P.; Handy, N. C. Chem. Phys. Lett. 2004,
393, 51–57.
(19) Lynch, B. J.; Fast, P. L.; Harris, M.; Truhlar, D. G. J. Phys. Chem.
A 2000, 104, 4811–4815.
(20) Lin, S.-H.; Hsu, Y.-C.; Lin, J. T.; Lin, C.-K.; Yang, J.-S. J. Org.
Chem. 2010, 75, 7877–7886.
(21) Gr€atzel, M. Nature 2001, 414, 338–344.
(22) Hagfeldt, A.; Gr€atzel, M. Chem. Rev. 1995, 95, 49–68. (b)
Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Gr€atzel, M. J. Phys. Chem.
B 2003, 107, 13280–13285.
(23) Paek, S.; Choi, H.; Choi, H.; Lee, C.-W.; Kang, M.-S.; Song, K.;
Nazeeruddin, M. K.; Ko, J. J. Phys. Chem. C 2010, 114, 14646–14653.
4920
dx.doi.org/10.1021/jo200501b |J. Org. Chem. 2011, 76, 4910–4920