Organic dyes containing fluorene decorated with imidazole units for dye-sensitized solar cells

Article abstract of DOI:10.1021/jo500330r

New organic dyes containing fluorene functionalized with two imidazole chromophores as donors and cyanoacrylic acid acceptors have been synthesized and successfully demonstrated as sensitizers in nanocrystalline TiO ₂-based dye-sensitized solar

Full text of DOI:10.1021/jo500330r

Article
pubs.acs.org/joc
Organic Dyes Containing Fluorene Decorated with Imidazole Units
for Dye-Sensitized Solar Cells
Dhirendra Kumar,† K. R. Justin Thomas,*,† Chuan-Pei Lee,‡ and Kuo-Chuan Ho‡
†Organic Materials Laboratory, 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
ABSTRACT: New organic dyes containing fluorene functionalized with
two imidazole chromophores as donors and cyanoacrylic acid acceptors
have been synthesized and successfully demonstrated as sensitizers in
nanocrystalline TiO2-based dye-sensitized solar cells (DSSCs). The
monoimidazole analogues were also synthesized for comparison. The
Sommelet reaction of bromomethylated 2-bromo-9,9-diethyl-9H-fluorene
produced the key precursor 7-bromo-9,9-diethyl-9H-fluorene-2,4-dicar-
baldehyde required for the preparation of imidazole-functionalized
fluorenes. Since the dyes possess weak donor segment, the electron-
richness of the conjugation pathway dictated the optical, electrochemical,
and photovoltaic properties of the dyes. The dyes served as sensitizers in
DSSC and exhibited moderate efficiency up to 3.44%. The additional
imidazole present on the fluorene has been found to retard the electron
recombination due to the bulkier hydrophobic environment and led to high open-circuit voltage in the devices.
striking photophysical and electrochemical properties as well as
their admirable hole-transporting abilities.26 Due to their high
molar extinction coefficients and efficient light harvesting
properties, fluorene-based compounds, especially thienylfluor-
ene-bridged dyes, exhibited significant power conversion
efficiencies in DSSCs.27 Though the 2,7-disubstituted fluorene
derivatives have been explored widely as constituents for
electronic applications, the use of 2,4,7-trisubstituted fluorene
derivatives is limited.28 Also, to the best of our knowledge,
fluorenes tethered to multiple imidazole chromophores have
not been utilized as donors in the design of organic dyes
suitable for DSSCs. In this paper, we present new organic dyes
developed on a fluorene core featuring two imidazole units
(Figure 1). For comparison, we have also synthesized dyes
containing one imidazole unit (IF1 and IF2). The substituents
on the imidazole nucleus were varied from phenyl to fluorene.
Additional imidazole on the C4 of fluorene drastically affects
the absorption and, consequently, the photovoltaic perform-
ance. It is presumed that the slight nonplanarity induced in the
fluorene unit due to crowded functionalization is responsible
for the blue-shifted absorption. However, the presence of
bithiophene unit in the conjugation pathway has been found to
supplement the donor−acceptor interactions with its electron
richness.
INTRODUCTION
■
Organic dyes suitable for application as sensitizers in dye-
sensitized solar cells (DSSCs)1 and as donors or acceptors in
bulk heterojunction solar cells2 have been intensively searched
in recent years owing to the demand for renewable energy
technologies. The molecular composition of the organic dyes
plays an important role in the performance of DSSCs. The
structural components of organic sensitizers suitable for DSSCs
can be divided into three categories: donor (D), π-linker, and
acceptor with an anchoring (A) group. A large number of
organic dyes containing nitrogenous electron donors such as
triphenylamine,3 carbazole,4 phenothiazine,5 amine-substituted
coumarin,6 indoline,7 thienoindole,8 and fluoreneamine9 have
been used with great success in DSSCs. The π-conjugated
linker in a D−π-linker−A system besides being a part of the
light absorbing chromophore functions as a molecular wire for
electron transfer from the donor to acceptor. Various aromatic/
heteroaromatic units such as naphthalene,10 dihydrophenan-
threne,11 fluorene,12 spirobifluorene,13 anthracene,14 oligothio-
phene,15 thienothiophene,16 dithienothiophene,17 carbazole,18
phenoxazine,19 phenothiazine,20 benzothiadiazole,21 and benzo-
triazole22 have been explored as π-linkers between the electron
donor and electron acceptor for tuning the absorption,
electrochemical, and other characteristics required for DSSC
performance. Triarylamine-containing imidazoles23 or fused
aromatic imidazoles such as phenanthromidazole24 and
pyrenoimidazole25 have also been found to serve as promising
donors when connected with oligothiophene units.
Fluorene-derived compounds are well documented in the
literature as promising electro-optical materials due to their
Received: February 11, 2014
Published: March 14, 2014
© 2014 American Chemical Society
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Figure 1. Structures of the organic sensitizers.
Scheme 1. Synthesis of the Precursors
consistent with the proposed structures. The dyes are
reasonably soluble in common organic solvents such as
dichloromethane, tetrahydrofuran, acetonitrile, dimethylforma-
mide, etc.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. For the synthesis of the
dyes we required the aldehyde 2a and the diketone 4 (Scheme
1). The diketone 4 was synthesized by a two-step synthetic
sequence involving Sonogashira coupling29 and iodine-medi-
ated oxidation in dimethyl sulfoxide.30 The dialdehyde 2a was
obtained by the exhaustive bromomethylation followed by
Sommelet reaction.31 In addition to the required dialdehyde 2a,
the monoaldehyde 2b and the trialdehyde 2c were also
obtained in trace amounts from this reaction sequence. The
target dyes IF1−IF6 were synthesized by a four-step synthetic
protocol shown in Scheme 2. First, the imidazole-tethered
fluorene fragment was constructed from 7-bromo-9,9-diethyl-
9H-fluorene-2,4-dicarbaldehyde or 7-bromo-9,9-diethyl-9H-flu-
orene-2-carbaldehyde and the corresponding diketone in a
multicomponent reaction in the presence of ammonium
acetate. The −NH imidazoles (5 and 8) were then alkylated
to increase the solubility either by using phase-transfer catalysis
or using K2CO3 in DMF. The aldehyde functionality was
introduced by a Stille coupling32 reaction of the bromides 6 and
9 with the tin derivative of the protected thiophene and
bithiophene aldehydes followed by acidic hydrolysis. Finally,
these aldehydes were converted to the desired cyanoacrylic acid
derivatives IF1−IF6 by Knoevenagel reaction.33 All the new
compounds were well characterized by IR, NMR, and high-
resolution mass spectral analysis. The spectral data were
Photophysical Properties. The absorption spectra
recorded for the dyes in tetrahydrofuran are displayed in
Figure 2 and relevant data summarized in Table 1. All the dyes
exhibited two or three prominent absorptions. The absorption
bands at the higher energy side of the profile are attributed to
the localized π−π* transitions. The longer wavelength
absorption peak appearing in the range 420−460 nm for the
dyes is primarily arising from the charge transfer (CT) from the
imidazole donor to the cyanoacrylic acid acceptor (vide infra).
Increasing the conjugation pathway by the insertion of
additional thiophene has been found to be beneficial for
shifting the CT absorption to the longer wavelength region. For
the 4,5-diphenylimidazole-functionalized dyes (IF1, IF2, IF3,
and IF4), the intensity of the CT transition diminished on
elongation of conjugation. A similar effect was observed on
increasing the number of imidazole units on a fluorene nucleus
(compare IF2 and IF4). However, for the dyes containing a
4,5-difluorenylimidazole unit, elongation of conjugation in the
bridge increased the molar extinction coefficient for the CT
transition. We have recently observed that the polysubstitution
on the fluorene core affects the planarity of the fluorene ring.34
It is probable that incorporation of the additional imidazole on
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Scheme 2. Synthesis of the Dyes
acceptor fragments. Such an effect is more severely manifested
for the much bulkier 4,5-difluorenylimidazole group. However,
on elongation of conjugation by the insertion of bithiophene
between the fluorene and cyanoacrylic acid units in IF6 a
reversal in the trend is observed. It is more likely due to the
increased electron-richness in the conjugation bridge which
facilitates the donor−acceptor interactions. Additionally, the
dyes containing 4,5-difluorenylimidazole possessed intense
absorption peaks in the shorter wavelength region due to the
fluorene localized π−π* electronic transitions, but the CT
transition did not show wavelength variations attributable to
the changes in the imidazole chromophore (compare, for
instance, IF2, IF4, and IF6).
The charge-transfer character of the longer wavelength
absorption was established by the solvatochromism and
theoretical calculations (vide infra). All the dyes showed red-
shifted CT peak for the nonpolar solvents such as toluene,
DCM, etc. and blue-shifted peak for the solvents such as ACN,
DMF, and methanol (Figure 3, Table 2). The blue-shift may be
explained by considering a relatively effective solvation by the
Figure 2. Absorption spectra of the dyes recorded in THF.
the fluorene core disturbs the planarity of the fluorene ring and
affects the electronic interaction between the donor and
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Table 1. Optical and Electrochemical Properties of the Dyes in THF
λabs, nm (ε × 103, M−1 cm−1
)
a
b
c
λem
(nm)
Eox
Eox*
EHOMO/ ELUMO
dye
THF
THF + TFA
THF + TEA
TiO2
471
(V)
(V)
(eV)
IF1 425 (78.3),
415 (82.8), 293 (32.6),
276 (32.3)
410 (73.6), 287 (37.8)
558
562
543
558
530
562
0.99
−0.81
5.79/3.22
5.82/3.42
5.74/3.12
5.77/3.35
5.75/3.05
5.76/3.35
291 (40.5)
IF2 458 (56.8),
452 (59.4), 354 (35.0),
279 (29.2)
436 (55.1), 363 (33.7),
274 (29.9)
533
466
509
461
512
1.02
0.94
0.97
0.95
0.96
−0.61
−0.91
−0.68
−0.98
−0.68
356 (35.6)
IF3 424 (45.3),
413 (48.9)
408 (43.9), 376 (45.3)
431 (32.9), 275 (26.2)
334 (55.4), 308 (50.1)
442 (36.4), 333 (56.9)
366 (38.9)
IF4 457 (32.1),
451 (34.2), 358 (15.7)
411 (19.2), 320 (56.3)
451 (40.3)
362 (18.5)
IF5 425 (15.5),
334 (56.1)
IF6 459 (37.9),
333 (58.0)
a
b
Oxidation potentials with reference to the ferrocene which was used as an internal standard. Computed from the formula Eox* = Eox − E0−0, where
c
E0−0 (band gap) is derived from the optical edge and reported vs NHE. Deduced from the equation EHOMO = Eox + 4.8 and ELUMO = EHOMO−E0−0
.
Figure 3. Absorption spectra of (a) IF1 and (b) IF3 recorded in different solvents.
Table 2. Absorption and Emission Spectral Data for the Dyes Recorded in Different Solvents
λabs (nm)
λem (nm)
Tol DCM CHCl3 THF DMF ACN MeOH
dye
Tol
DCM
CHCl3
THF
DMF
ACN
MeOH
IF1 426, 337 428, 292
432, 294
425, 291
395, 281
419
391, 282
397, 282
536
544
545
568
562
569
558
562
502
521
505
515
492
543
IF2 459, 368 464, 355,
470, 356,
277
458, 356
427, 355,
272
427, 348,
271
277
IF3 410, 380 410, 364,
413, 364,
284
424, 366,
287
380, 275
427
373
395, 374
495
532
489
509
564
518
512
566
510
543
558
530
492
505
497
498
507
491
487
523
492
283, 261
IF4 445
IF5 421,
458, 366,
464, 363,
273
a
457, 362
425, 264
426, 260
273
a
a
a
a
a
a
428, 333, 429, 332,
425, 334,
395, 335,
380, 331,
308, 279
409, 323,
333,
309
310
310, 281
309, 280
309, 280
309, 279
IF6 453, 329, 449, 329,
309 309, 280
463, 329
310, 281
459, 333,
308, 280
428, 330,
309, 280
422, 327,
308, 279
430, 321,
309, 278
545
553
566
562
506
497
536
a
Shoulder.
polar solvents or a zwitterionic character. It has been well
established that the D−π−A organic sensitizers exist in
deprotonated form in solution, particularly in polar solvents.35
In the present dyes, since the imidazole nitrogen can be easily
protonated, the dyes may assume a zwitterionic structure with a
protonated imidazole and deprotonated carboxylate sites. Such
a configuration may also result in a blue shift in the absorption
(vide supra). Unusual red-shifted absorption observed in
dichloromethane and chloroform when compared to toluene
may be originating from the stabilization due to the fast
rearrangement of polarizable electrons during electronic
excitation.36 Interestingly, the dyes containing one imidazole
chromophore (IF1 and IF2) displayed similar absorption
profiles in toluene and THF, but the dyes containing two
imidazole chromophores (IF3−IF6) displayed hypsochromic
absorption in toluene when compared to that of THF. The
ineffective solvation of the bulkier bis-imidazoles by the toluene
may be the reason for such deviation. Additionally, the donor−
acceptor interactions could be attenuated by the addition of
trifluoroacetic acid (TFA) or triethylamine (TEA) to the dye
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solutions. Addition of TFA or TEA resulted in a blue-shifted
absorption, but the magnitude of shift is larger for TEA (Figure
4). Triethylamine deprotonates the carboxylic acid unit while
requirement of twisted π-bridge to deter the aggregation at the
surface of the TiO2.
All the compounds displayed a weak emission with
characteristic colors ranging from green to orange (Figure 6).
Figure 4. Absorption spectra of IF4 recorded in THF on addition of
TFA/TEA.
Figure 6. Emission spectra of the dyes recorded in THF.
the TFA protonates the imidazole unit. Deprotonation of the
carboxylic acid will reduce the acceptor strength while the
protonation of imidazole will diminish the donor strength. In
both cases, donor−acceptor interaction in the dye is decreased
substantially which manifests as a blue shift in the absorption
spectrum.
Similar to the absorption, insertion of a thiophene unit led to
red-shifted emission. However, the results are more pro-
nounced for bis-imidazoles (IF3−IF6) than that of mono-
imidazoles (IF1 and IF2). Further, the negative solvatochrom-
ism observed in the fluorescence also confirmed the charge-
transfer nature of the excited state (Table 2).
All the dyes showed red-shifted absorption profile (Figure 5)
when adsorbed on TiO2 as compared to the corresponding
Electrochemical Properties. In the present dyes, there are
no easily oxidizable chromophores. Thus, the electron removal
must be difficult and highly dependent on the nature of the
conjugation backbone which altered the donor−acceptor
interactions and the oxidation potentials. As expected, all the
dyes displayed an irreversible oxidation wave (Table 1). The
bithiophene derivatives (IF2, IF4, and IF6) possessed a slightly
more positive oxidation waves when compared to the
corresponding thiophene derivatives (IF1, IF3, and IF5)
(Table 1). If the electron-richness alone is considered then
the bithiophene derivatives are expected to display more facile
oxidation. However, the contradictory observation suggests a
strong donor−acceptor interaction in the bithiophene deriva-
tives which in turn could make the molecule difficult to
undergo oxidation. This behavior is consistent with the optical
properties of the dyes where the bithiophene derivatives
showed pronounced donor−acceptor interactions. In addition,
the oxidation potentials of the dyes containing two imidazoles
(IF1−IF6) were found to be cathodically shifted compared to
those observed for the comparable monoimidazoles (IF1 and
IF2). This could be attributed to the slight conjugation
extension arising from the additional imidazole unit. The
excited-state oxidation potentials (Eox*) of the dyes are more
negative (−0.61 to −0.91 V versus NHE) than the conduction
band edge of TiO2 (−0.5 V versus NHE),39 which ensures an
energetically favorable electron injection from the excited dye
to TiO2 (Figure 7). The ground-state oxidation potentials of
the dyes are more positive (1.71−1.79 V versus NHE) than
Figure 5. Absorption spectra for the dyes anchored on the TiO2 film.
spectra recorded in solution. The red-shifted absorption may be
originating from the aggregation of the dyes at the TiO2 surface
or due to the use of thicker TiO2 layer,37 which is opposite to
the blue-shift observed for most of the organic dyes because of
partial deprotonation of the carboxylic acid unit due to the
dye−TiO2 interaction.38 The fluorenylbithiophene-bridged
dyes (IF2, IF4 ,and IF6) showed more stacking as evidenced
by broader and more red-shifted absorption on TiO2 film than
the fluorenyl-thiophene-bridged dyes (IF1, IF3, and IF6).
Despite the presence of nonplanar moieties in the donor side,
the existence of intermolecular dye interactions suggest the
that of the I−/I3 redox couple (0.4 V versus NHE),40 which
−
suggests a thermodynamically favorable dye regeneration by the
electrolyte. Moreover, the bithiophene-containing dyes (IF2,
IF4, and IF6), having closer excited-state oxidation potential
with the TiO2 conduction band, would ensure better electron
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on the anchoring group and the π-bridge. Thus, the absorption
leading to the electronic excitation from the HOMO or
HOMO-1 to the LUMO orbital can constitute to the charge
transfer from the imidazole-donor unit to the cyanoacrylic acid
acceptor. This favorable condition may ensure the charge
separation in the molecule on light absorption and subsequent
electron injection into the conduction band of TiO2 for the
monoimidazole derivatives IF1 and IF2. However, for the
diimidazole dyes (IF3 and IF4) the major contribution for the
charge-transfer transition arises from the HOMO-1 to LUMO
excitation. Since the HOMO-1 is located on the nonconjugated
imidazole segment in these dyes, the corresponding excitation
may not produce the desirable charge migration. In addition,
the tilting between the fluorene and thiophene units explains
the blue-shifted absorption observed for the dyes IF1, IF3, and
IF5, but the bithiophene unit assumes near-planar arrangement.
The theoretical predictions about the absorption peaks and
their orbital contributions are listed in Table 3. From the
computational results, the longer wavelength absorption
originates from the HOMO to LUMO electronic excitation
which involves the charge migration from the imidazole unit to
the cyanoacrylic acid acceptor. The trends observed in the
solution absorption wavelengths are consistent with the
theoretical predictions. The bithiophene dyes (IF2 and IF4)
possessed the red-shifted absorption when compared to their
thiophene analogues (IF1 and IF3). However, the molar
extinction coefficients observed for the bithiophene derivatives
(IF2 and IF4) are not complying with the theoretical
predictions. The relatively low extinction coefficients observed
for the bithiophene derivatives (IF2 and IF4) when compared
to their corresponding thiophene analogues (IF1 and IF3) may
originate from their poor solubility. It is interesting to note that
the fluorene derivatives IF5 and IF6 show molar extinction
coefficients in keeping with the expectations. The fluorene dyes
are highly soluble due to the presence of solubilizing alkyl
groups on the fluorene nucleus.
Figure 7. Comparison of the ground- and excited-state oxidation
potentials for the dyes.
collection, higher photocurrent, and better efficiency (vide
infra).
Theoretical Calculations. To gain more insight into the
electronic properties of the compounds, density functional
theoretical (DFT) calculations were performed for the dyes
containing phenyl substituents (IF1−IF4) on the imidazole
nucleus with the Gaussian 09 program package.41 The
structures were optimized using the B3LYP functional and 6-
31G(d,p) basis set. Since the B3LYP functional was not suitable
for the charge-transfer excitations in the donor−acceptor
compounds, we have used the range-separated hybrid func-
tional LC-BLYP to compute the vertical excitation energies and
their orbital contributions (Table 3). The electron density
Table 3. Computed Vertical Excitation Wavelengths and
Their Orbital Contribution for the Dyes IF1−IF4 Using LC-
BLYP/6-31G(d,p) for THF with IEFPCM Model
Photovoltaic Properties. The performance of DSSCs with
the new dyes (IF1−IF6) as sensitizers was evaluated under
standard conditions and the relevant parameters listed in Table
4. In general, the bithiophene-containing dyes (IF2, IF4, and
IF6) exhibited higher device efficiencies than their thiophene
analogues (IF1, IF3, and IF5) (Figure 10). This difference
originated mainly from the variation in the photocurrent
density. The higher JSC and IPCE values observed for the
bithiophene dyes are consistent with their absorption proper-
ties.42 The overall power conversion efficiency of the devices
fall in the range 1.39−3.44%. In addition, the additional
imidazole on the C4 position of the fluorene led to reduced
photocurrent density (for instance compare IF1 with IF3).
This is probably attributable to the larger size of the dyes due to
the additional imidazole unit which may reduce the dye-loading
on the surface of TiO2 and consequently the light-harvesting
propensity. The low photocurrent density may arise due to
poor interaction between the dye and TiO2 because of the
larger bulkiness of the dyes containing two imidazole units.
Though the bithiophene dyes exhibited comparatively large
photocurrent density when compared to their thiophene
analogues, the trends in the VOC values were found to be
dependent on the nature of the substituents on the imidazole
unit. For the dyes containing 4,5-diphenylimidazole units
(IF1−IF4), the VOC values for the thiophene derivatives (IF1
and IF3) are larger than those of the corresponding
bithiophene derivatives (IF2 and IF4), but the dyes containing
dye
λabs (nm)
oscillator strength
1.4711
assignment
IF1
368.5
HOMO → LUMO (32%)
HOMO-1 → LUMO (47%)
HOMO → LUMO+1 (44%)
HOMO-2 → LUMO (16%)
HOMO → LUMO (44%)
HOMO-1 → LUMO (34%)
HOMO → LUMO+1 (49%)
HOMO-2 → LUMO (17%)
HOMO → LUMO (34%)
HOMO-1 → LUMO (46%)
HOMO → LUMO+1 (45%)
HOMO-3 → LUMO (16%)
HOMO → LUMO (48%)
HOMO-1 → LUMO (22%)
HOMO → LUMO+1 (50%)
HOMO-3 → LUMO (17%)
279.7
405.1
299.9
369.6
286.6
405.1
300.4
0.4041
1.8017
0.4750
1.5152
0.5626
1.8136
0.4300
IF2
IF3
IF4
distribution in HOMO is primarily located at the imidazole at
C2 and spread over the fluorene and thiophene linker (Figure
8). In the case of IF3 and IF4, the auxiliary imidazole unit at C4
is considerably tilted from the fluorene core (65−70°) (Figure
9), and it contributed to the construction of HOMO-1. In the
case of monoimidazole derivatives, the HOMO-1 is also
contributed by the C2 imidazole and the conjugation
framework. The electron density of the LUMO is delocalized
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Figure 8. Electronic distribution in the selected molecular orbitals of the dyes IF1and IF3.
Figure 9. Calculated interplanar angles (deg) between various aromatic segments and cyanoacrylic acid plane in the optimized geometries of the
dyes IF1, IF2, IF3, and IF4.
TiO2/dye/electrolyte interfacial electron recombination ki-
netics. The prominent semicircle observed at the midfrequency
is attributed to the electron recombination resistance at the
TiO2/electrolyte interface. The larger the radius of the
semicircle, the smaller is the rate for electron recombination
at the interface. The resistance of the recombination (Rrec) at
the TiO2/electrolyte assumed the order for the similar dyes as
IF1 > IF2, IF3 < IF4, and IF5 < IF6. The enhanced electron
recombination rate is attributed to the dye characteristics such
as aggregation propensity and complex formation with iodide in
the electrolyte. Factors that retard aggregation between the
dyes and complex formation with iodide will reduce the
electron recombination rate. Smaller recombination resistance
observed for IF2, IF3, and IF5 indicated a pronounced electron
recombination rate for them when compared to the structurally
similar dyes IF1, IF4, and IF6 respectively. On this basis, one
would expect comparatively high VOC for the dyes IF1, IF4, and
IF6. This is in agreement with the dye pairs IF1/IF2 and IF5/
IF6, but the order is reversed for the dye pair IF3/IF4. The
relatively large amount of electrons in the conduction band of
TiO2 could also give rise to high VOC. The electron density in
the conduction band of TiO2 may be a result of better charge
collection efficiency in the corresponding device. In order to
probe the relative charge collection rate in the devices,
electrochemical impedance spectra were measured under
illumination conditions (Figure 11b). The charge transport
Table 4. Photovoltaic Parameters of the DSSCs Fabricated
Using the Dyes
a
b,
c
c
c
VOC
(V)
JSC
Rrec
(Ω)
Rct2
τe
dye η (%)
(mA cm−2
)
ff
(Ω)
(ms)
IF1
IF2
IF3
IF4
IF5
IF6
2.42
3.44
1.67
2.75
1.39
3.27
0.61
0.57
0.60
0.58
0.57
0.63
5.78
9.25
4.16
7.13
3.76
7.88
0.68
0.66
0.68
0.67
0.65
0.66
16.00
10.20
9.49
34.35
23.13
46.02
28.13
55.03
27.81
8.41
1.16
4.63
2.09
2.55
4.63
14.00
5.55
21.60
a
Measured under an irradiation of 100 mW cm−2 (AM 1.5 G
b
simulated sunlight) at room temperature. Measured in the dark.
c
Estimated with the aid of electrochemical impedance spectroscopy
(EIS) measurements.
4,5-difluoreneylimidazole units exhibited a reverse trend.
Among them, the bithiophene dye (IF6) displayed larger VOC
in the device than the thiophene analogue (IF5). In order to
probe these differences, the electrochemical impedance spectral
measurements were performed for the dyes. The Nyquist plots
observed for the devices under illumination and dark conditions
are shown in Figure 11. Under dark conditions, as there is no
photocurrent generation, the resistance of the electron
recombination can be estimated from the interfacial kinetic
parameters. The Nyquist plot measured under dark condition
(Figure 11a) exhibited three semicircles corresponding to the
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Figure 10. (a) I−V and (b) IPCE plots for the DSSCs using the dyes 7a−d, 11a, and 11b.
Figure 11. Nyquist plots for the DSSCs under (a) dark and (b) illumination conditions.
resistance (Rct2) is small for the bithiophene dyes (IF2, IF4,
and IF6) relative to the thiophene analogues (IF1, IF3, and
IF5) (Figure 11b). This suggests an effective electron collection
for the bithiophene dyes. This observation is in agreement with
the larger JSC values observed for the bithiophene dyes.
Interestingly, the bithiophene dyes possessed comparatively low
lying Eox*, which probably led to an effective electronic
coupling between the excited dyes and the TiO2 and facilitated
electron injection.
Additionally, the electron lifetimes (τe) of the DSSCs were
large for the dyes IF1, IF3, and IF6 when compared to the
analogous dyes IF2, IF4, and IF5, respectively (Figure 12). The
larger electron lifetime observed for the former dyes is
consistent with their superior VOC values. On the basis of the
EIS studies, the better performance of the bithiophene
derivatives IF2 and IF6 is attributed to their comparatively
good light-harvesting properties, the favorable electron
Figure 12. Bode phase plots of the DSSCs under the illumination of
injection, and the effective retardation of electron recombina-
tion.
100 mW cm−2.
CONCLUSIONS
linear dyes to branched dyes featuring bis-donors. The
synthesis of branched dyes could be achieved from novel 7-
bromo-9,9-diethyl-9H-fluorene-2,4-dicarbaldehyde, which was
synthesized by convenient Sommelet reaction along with 7-
bromo-9,9-diethyl-9H-fluorene-2,4,5-tricarbaldehyde. These in-
termediates have vast synthetic utility and can become
■
In summary, we have synthesized new organic dyes containing
imidazole and fluorene units. The D−π−A configuration of the
dyes was fulfilled by the thienylfluorene π-linker and
cyanoacrylic acid acceptor. The photophysical and photovoltaic
properties were investigated after structural variation from
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was cooled down to room temperature, the resultant colloid was
transferred to an autoclave and then heated in 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 PEG 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
incorporated with 50 wt % of light scattering TiO2 particles (PT-501A,
15 m2 g−1, 100 nm, 99.74%) (with respect to the 20 nm TiO2) for
reducing the light loss by back scattering.
benchmarks to construct a variety of electronic materials. All
the dyes displayed moderate absorption properties with molar
extinction coefficients higher than the most successful
ruthenium dyes. The performance of DSSCs with these dyes
as the sensitizers exhibited efficiencies ranging from 1.39% to
3.44% under one sun illumination. Although bisimidazoles
exhibited slightly lower efficiencies than monoimidazoles,
because of poor dye-loading, further introduction of more
electron-rich segments led to improvement. The maximum
power conversion efficiency of 3.44% was achieved from IF2, a
monoimidazole dye featuring 4,5-diphenyl and bithiophene
conjugation segment, which was dropped to 2.75% for bis-
imidazole analogue (IF4). When the phenyl units were
replaced by fluorene, the efficiency improved to 3.27% with
the highest open-circuit voltage in the series. The results
concluded the importance of the bulkier bis-structure which is
able to control the electrolyte interactions and thus charge
recombination. The present design is promising, and further
structural optimization can be customized to attain the
improved photovoltaic properties. Encouraged by these studies,
we are further exploring the possibility of developing electron-
rich thiophene fused imidazoles as donors for DSSC dyes in
our laboratory.
A fluorine-doped SnO2 conducting glass (FTO, 7 Ω sq.−1
,
transmittance ∼80%) was first cleaned with a neutral cleaner and
then washed with DI water, acetone, and IPA, sequentially. The
conducting surface of the FTO was treated with a solution of TTIP (1
g) in 2-methoxyethanol (99.5%, 3 g) for obtaining 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. For each coating 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 × 10−4 M solution of dye at room temperature for 24
h. N719 was dissolved in ACN and tert-butylamine (volume ratio of
1:1) to obtain a standard dye solution. Various organic dye solutions
were prepared in a mixing solvent containing ACN, tert-butylamine,
and 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 sq.−1), keeping the two electrodes separated by
a 25 μm-thick Surlyn (SX1170-25). The two electrodes were then
sealed by heating. A mixture of 0.1 M LiI, 0.6 M DMPII, 0.05 M I2,
and 0.5 M TBP in 3-methoxypropionitrile (MPN)/ACN (volume
ratio of 1:1) was used as the electrolyte. 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 (AM1.5G). Incident light intensity (100 mW cm−2)
was calibrated with a standard Si Cell (PECSI01). Photocurrent−
voltage curves of the DSSCs were obtained with a potentiostat/
galvanostat. Cross-sectional image of the TiO2 film was observed in
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 UV−vis spectropho-
tometer equipped with an equipment of total integrating sphere.
Electrochemical impedance spectra (EIS) were obtained by the above-
mentioned potentiostat/galvanostat, equipped with an FRA2 module,
under a constant light illumination of 100 mW cm−2 or dark at a
potential of −0.55 V at room temperature. 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 counter electrode
and the FTO-TiO2-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 photo-to-current conversion efficiency (IPCE) curves were
obtained at short-circuit condition. The light source was a class A
quality solar simulator (AM1.5G); 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/λφ), where λ is the wavelength, JSC is
short-circuit photocurrent (mA cm−2) recorded with a potentiostat/
galvanostat, and φ is the incident radiative flux (W m−2) measured
with an optical detector and power meter.
EXPERIMENTAL SECTION
■
Material and Methods. Chemicals were purchased from
commercial sources and used as received. Solvents were dried and
distilled immediately prior to use by standard procedures. Column
chromatography purification was performed with the use of silica gel
(230−400 mesh) as the stationary phase in a column with 40 cm long
and 3.0 cm diameter. 1H NMR and 13C NMR were recorded in
acetone-d6, CDCl3, or DMSO-d6 on an FT-NMR spectrometer
operating at 500.13 and 125.77 MHz, respectively. High-resolution
mass spectrometric measurements were carried out using a FAB mass
spectrometer. Absorption spectra were recorded at room temperature
in quartz cuvettes using a UV−vis spectrophotometer. The cyclic
voltammetry (CV) and differential pulse voltammetry (DPV)
measurements were carried out on an electrochemical analyzer in
tetrahydrofuran by using 0.1 M tetrabutylammonium perchlorate as
supporting electrolyte. The experiments were performed at room
temperature in nitrogen atmosphere with a three-electrode cell
consisting of a platinum wire as auxiliary electrode, a nonaqueous
Ag/AgNO3 reference electrode, and a glassy carbon working electrode.
The highest occupied molecular orbital (HOMO) energy levels of
organic materials were measured from the oxidation potential obtained
from cyclic voltammetry, and lowest unoccupied molecular orbital
(LUMO) energy levels were determined from the HOMO energy
levels and optical band gap estimated from the intersection of
absorption and emission. The IR spectra were recorded on FT-IR
spectrometer.
Computational Methods. All the computations were performed
with the Gaussian 09 program package41 on a computer workstation.
The ground-state geometries were fully optimized without symmetry
constraints with Becke’s43 three parameters hybrid functional and Lee,
Yang, and Parr’s correlational functional (B3LYP)44 using 6-31G(d, p)
basis set on all atoms. The default parameters for the convergence
criteria were used. Vibrational analyses on the optimized structures
were performed to confirm the structure. The excitation energies and
oscillator strengths for the lowest 10 singlet−singlet transitions at the
optimized geometry in the ground state were obtained by TD-DFT
calculations using the same basis set as for the geometry minimization
but with the range separated hybrid functional LC-BLYP.
DSSC Fabrication and Characterization. For the TiO2 colloid
solution, the TiO2 precursor was prepared by the sol−gel 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
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Synthesis. The precursor compounds 2-bromo-9,9-diethyl-9H-
fluorene (1),45 9,9-diethyl-2-ethynyl-9H-fluorene,46 2,2′-bithiophene-
5-carbaldehyde,47 (5-(1,3-dioxolan-2-yl)thiophene-2-yl)-
tributylstannane,48 and (5′-(1,3-dioxolan-2-yl)-2,2′-bithiophene-5-yl)-
tributylstannane49 were synthesized by following reported procedures.
Synthesis of 2a−2c. A mixture of 2-bromo-9,9-diethyl-9H-
fluorene (3 g, 10 mmol) and paraformaldehyde (1.5 g, 50 mmol) in
20 mL of HBr (33% solution in acetic acid) was taken in a sealed tube.
The tube was closed and heated at 110 °C for 48 h. After completion
of the reaction, the reaction mixture was cooled, poured into water,
and extracted with dichloromethane. The organic layer was thoroughly
washed with brine solution, collected, and dried over anhydrous
Na2SO4. The solvent was evaporated and residue dissolved in 50 mL
of chloroform. Hexamethylenetetramine (4.2 g, 30 mmol) was added,
and the resultant mixture was refluxed for 12 h. The solvent was
removed and residual salt washed with chloroform two to three times.
It was dried and dissolved in 20 mL of acetic acid/water mixture (1:1).
The mixture was again refluxed for 4 h. It was cooled and neutralized
with aq sodium bicarbonate and extracted with dichloromethane. The
organic layer was washed with brine solution liberally, collected, and
dried over anhydrous Na2SO4. The solvent was evaporated and the
residue purified by column chromatography on silica gel by using
dichloromethane/ethyl acetate mixture as eluent. The compounds 2b,
2a, and 2c were obtained, in that order, from the column in pure form.
7-Bromo-9,9-diethyl-9H-fluorene-2,4-dicarbaldehyde (2a): white
solid; yield 1.14 g, 32%; mp 48−50 °C; IR (KBr, cm−1) 1696 (νCO);
1H NMR (CDCl3, 500.13 MHz) δ 10.57 (s, 1 H), 10.15 (s, 1 H), 8.57
(d, J = 8.5 Hz, 1 H), 8.36 (d, J = 1.0 Hz, 1 H), 8.07 (d, J = 1.5 Hz, 1
H), 7.56−7.58 (m, 2 H), 2.06−2.17 (m, 4 H), 0.26 (t, J = 7.5 Hz, 6
H); 13C NMR (CDCl3, 125.77 MHz) δ 191.2, 191.0, 154.7, 153.1,
146.9, 138.0, 135.4, 135.0, 132.3, 131.0, 128.3, 126.6, 126.3, 125.1,
56.3, 32.8, 8.3; HRMS (ESI) m/z calcd for C19H18BrO2 (M + H)
357.0485, found 357.0494.
then treated with 10% aq sodium thiosulfate solution followed by
extraction with ethyl acetate. The extract was dried over anhydrous
Na2SO4. The solvent was evaporated and residue chromatographed on
a silica gel column (hexane/dichloromethane) to obtain the pure
product: light orange solid; yield 2.29 g, 92%; mp 130−132 °C; IR
(KBr, cm−1) 1663 (νCO); 1H NMR (CDCl3, 500.13 MHz) δ 8.08 (s,
2 H), 7.96 (dd, J = 8 Hz, 1 Hz, 2 H), 7.98 (t, J = 7.5 Hz, 4 H), 7.37−
7.43 (m, 6 H), 2.09 (m, 8 H), 0.32 (t, J = 7.5 Hz, 12 H); 13C NMR
(CDCl3, 125.77 MHz) δ 195.1, 151.5, 150.8, 148.4, 139.8, 131.9,
130.6, 129.1, 127.3, 123.7, 123.3, 121.1, 119.9, 56.5, 32.6, 8.5; HRMS
(ESI) m/z calcd for C36H35O2 (M + H) 499.2632, found 499.2635.
General Procedure for the Synthesis of 5a and 5b. A mixture
of 7-bromo-9,9-diethyl-9H-fluorene-2,4-dicarbaldehyde (2a) (0.71 g, 2
mmol), diketone benzil or 4 (4 mmol), and ammonium acetate (1.54
g, 20 mmol) in 10 mL of acetic acid was refluxed for 24 h. The
reaction mixture was cooled and poured into water to obtain a solid
precipitate. The solid was washed thoroughly with water, dried, and
recrystallized from DCM/methanol mixture
2,2′-(7-Bromo-9,9-diethyl-9H-fluorene-2,4-diyl)bis(4,5-diphenyl-
1H-imidazole) (5a): cream-colored solid; yield 2.12 g, 72%; mp 256−
1
258 °C; H NMR (acetone-d6, 500.13 MHz) δ 12.12 (s, 1 H), 12.03
(s, 1 H), 8.41 (d, J = 1.5 Hz, 1 H), 8.22 (s, 1 H), 7.77 (d, J = 7.0 Hz, 2
H), 7.58−7.65 (m, 5 H), 7.36−7.43 (m, 10 H), 7.29−7.32 (m, 4 H),
7.24 (t, J = 7.5 Hz, 2 H), 2.17 (q, J = 7.5 Hz, 4 H), 0.33 (t, J = 7.5 Hz,
6 H); 13C NMR (CDCl3 + TFA, 125.77 MHz) δ 153.8, 153.5, 143.5,
141.8, 136.0, 131.1, 130.5, 130.2, 130.1, 130.0, 129.6, 129.5, 129.3,
129.2, 127.2, 126.2, 126.0, 125.9, 125.6, 124.9, 122.7, 121.3, 57.1, 32.4,
7.8; HRMS (FAB) m/z calcd for C47H38BrN4 (M + H) 737.2274,
found 737.2282.
2,2′-(7-Bromo-9,9-diethyl-9H-fluorene-2,4-diyl)bis(4,5-bis(9,9-di-
ethyl-9H-fluoren-2-yl)-1H-imidazole) (5b): yellow solid; yield 3.99 g,
1
76%; mp 190−192 °C; H NMR (acetone-d6, 500.13 MHz) δ 12.74
(s, 1 H), 12.30 (s, 1 H), 8.53 (s, 1 H), 8.32 (s, 1 H), 7.63−7.87 (m, 17
H), 7.32−7.43 (m, 14 H), 1.90−2.01 (m, 20 H), 0.27−0.34 (m, 30 H),
13C NMR (CDCl3 + TFA, 125.77 MHz) δ 153.7, 153.6, 151.5, 151.3,
7-Bromo-9,9-diethyl-9H-fluorene-2-carbaldehyde (2b):50 white
solid; yield 0.23 g, 7%; mp 122−124 °C; IR (KBr, cm−1) 1690
(νCO); 1H NMR (CDCl3, 500.13 MHz) δ 10.06 (s, 1 H), 7.86−7.88
(m, 2 H), 7.82 (d, J = 7.5 Hz, 1 H), 7.64−7.65 (m, 1 H), 7.51−7.53
(m, 2 H), 2.03−2.13 (m, 4 H), 0.30 (t, J = 7.5 Hz, 6 H). The
compound can also be synthesized by the reaction of 2,7-dibromo-9,9-
diethyl-9H-fluorene with n-BuLi/DMF in THF as reported in
literature.50
150.4, 144.2, 144.0, 143.9, 143.0, 141.2, 140.3, 140.2, 135.9, 131.3,
130.9, 130.5, 129.4, 128.3, 128.2, 127.3, 127.23, 127.19, 127.16, 127.0,
125.5, 125.2, 124.4, 124.1, 123.14, 123.10, 122.7, 122.6, 122.5, 121.5,
120.5, 120.4, 120.3, 116.8, 57.3, 56.6, 56.5, 32.5, 8.21, 8.19, 7.7; HRMS
(ESI) m/z calcd for C91H86BrN4 (M + H) 1313.6030, found
1313.6041.
7-Bromo-9,9-diethyl-9H-fluorene-2,4,5-tricarbaldehyde (2c):
white solid; yield 77 mg, 2%; mp 55−58 °C; IR (KBr, cm−1) 1702
(νCO); 1H NMR (CDCl3, 500.13 MHz) δ 10.66 (s, 1 H), 10.45 (s, 1
H), 10.17 (m, 1 H), 9.18 (s, 1 H), 8.41 (d, J = 1.5 Hz, 1 H), 8.09 (d, J
= 1.5 Hz, 1 H), 7.70 (s, 1 H), 2.11 (m, 4 H), 0.28 (t, J = 7.5 Hz, 6 H);
13C NMR (CDCl3, 125.77 MHz) δ 191.3, 190.8, 190.5, 159.9, 152.9,
145.4, 139.1, 135.8, 135.0, 133.2, 132.9, 128.9, 128.5, 128.2, 126.6,
56.8, 32.9, 8.4; HRMS (ESI) m/z calcd for C20H18BrO3 (M + H)
385.0434, found 385.0441.
1,2-Bis(9,9-diethyl-9H-fluoren-2-yl)ethyne (3). A mixture of 1 (3 g,
10 mmol), 9,9-diethyl-2-ethynyl-9H-fluorene (2.71 g, 11 mmol),
Pd(PPh3)2Cl2 (70 mg, 0.1 mmol), PPh3 (52 mg, 0.2 mmol), and CuI
(20 mg, 0.1 mmol) in 50 mL of triethylamine was degassed with
nitrogen. The mixture was stirred and refluxed under nitrogen for 12 h
and then cooled to room temperature. It was poured into water and
extracted with ethyl acetate. The organic layer was washed with brine
for several times, separated, and dried over anhydrous Na2SO4. The
solvent was removed, and crude product purified by column
chromatography on silica gel (hexane/dichloromethane): white
solid; yield 3.03 g, 65%; mp 120 °C; IR (KBr, cm−1) 2202 (νCC);
1H NMR (CDCl3, 500.13 MHz) δ 7.70−7.73 (m, 4 H), 7.56−7.58 (m,
4 H), 7.33−7.36 (m, 6 H), 2.06 (q, J = 7.5 Hz, 8 H), 0.33 (t, J = 7.5
Hz, 12 H); 13C NMR (CDCl3, 125.77 MHz) δ 150.2, 150.0, 141.8,
140.9, 130.6, 127.5, 127.0, 126.1, 123.0, 121.6, 120.0, 119.7, 90.5, 56.2,
32.8, 8.5; HRMS (ESI) m/z calcd for C36H35 (M + H) 467.2733,
found 467.2742.
2,2′-(7-Bromo-9,9-diethyl-9H-fluorene-2,4-diyl)bis(1-ethyl-4,5-di-
phenyl-1H-imidazole) (6a). A mixture of 5a (0.74 g, 1 mmol) and
K2CO3 (0.55 g, 4 mmol) in 10 mL of dry DMF was degassed with
nitrogen and heated at 50 °C. After 1 h, bromoethane (0.32 g, 3
mmol) was added to the reaction mixture slowly and heating
continued for additional 48 h. The reaction mixture was poured into
water and extracted with dichloromethane. The organic layer was
washed with brine solution several times, collected, and dried over
anhydrous Na2SO4. The solvent was evaporated and crude product
purified by column chromatography on silica gel by using dichloro-
methane/ethyl acetate mixture as eluent: light-yellow solid; yield 0.56
g, 70%; mp 100−102 °C; 1H NMR (CDCl3, 500.13 MHz) δ 8.02 (s, 1
H), 7.85−7.87 (m, 2 H), 7.58−7.60 (m, 2 H), 7.43−7.54 (m, 12 H),
7.35 (dd, J = 8.5 Hz, 2.0 Hz, 1 H), 7.20−7.25 (m, 4 H), 7.13−7.18 (m,
2 H), 6.69 (d, J = 8.0 Hz, 1 H), 4.04−4.06 (m, 2 H), 3.62 (s, 2 H),
2.07−2.20 (m, 4 H), 1.02 (t, J = 7.0 Hz, 3 H), 0.76 (t, J = 7.0 Hz, 3 H),
0.38 (t, J = 7.5 Hz, 6 H); 13C NMR (CDCl3, 125.77 MHz) δ 152.92,
150.86, 147.0, 144.9, 141.0, 139.2, 138.2, 137.7, 134.6, 131.4, 131.3,
131.1, 131.0, 130.6, 130.5, 130.4, 130.0, 129.31, 129.26, 129.2, 128.9,
128.8, 128.21, 128.15, 126.9, 126.8, 126.43, 126.37, 126.2, 125.4,
124.9, 123.3, 122.3, 56.7, 40.1, 39.6, 33.0, 16.3, 16.1, 8.6; HRMS (ESI)
m/z calcd for C51H46BrN4 (M + H) 793.2900, found 793.2911.
2,2′-(7-Bromo-9,9-diethyl-9H-fluorene-2,4-diyl)bis(4,5-bis(9,9-di-
ethyl-9H-fluoren-2-yl)-1-ethyl-1H-imidazole) (6b). A mixture of 5b
(1.31 g, 1 mmol), bromoethane (0.32 g, 3 mmol), BTEAC (0.10 g),
10 mL of 50% aq NaOH, and 5 mL of benzene was refluxed for 12 h.
After a clear organic layer formed, the reaction mixture was poured
into hot water and allowed to sit overnight in the hood. The residual
mass was extracted with dichloromethane and the extract dried over
1,2-Bis(9,9-diethyl-9H-fluoren-2-yl)ethane-1,2-dione (4). A mix-
ture of 3 (2.33 g, 5 mmol) and iodine (1.27 g, 5 mmol) was dissolved
in DMSO. The resulting solution was heated at 140 °C for 12 h and
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anhydrous Na2SO4. The compound was purified by column
above for 7a using 6b in place of 6a: yellow solid; yield 0.29 g, 41%;
1
chromatography on silica gel by using dichloromethane/ethyl acetate
mp 138−140 °C; IR (KBr, cm−1) 1655 (νCO); H NMR (CDCl3,
1
as eluent: light-yellow solid; yield 1.05 g, 77%; mp 140−142 °C; H
500.13 MHz) δ 9.91 (s, 1 H), 7.98 (dd, J = 10.0 Hz, 1.5 Hz, 2 H), 7.85
(d, J = 7.5 Hz, 1 H), 7.77−7.82 (m, 4 H), 7.57−7.70 (m, 9 H), 7.48−
7.53 (m, 6 H), 7.38−7.43 (m, 7 H), 7.27−7.28 (m, 1 H), 7.23−7.25
(m, 4 H), 7.09 (d, J = 8.0 Hz, 1 H), 4.18−4.19 (m, 2 H), 3.74 (s, 2 H),
2.21−2.28 (m, 4 H), 2.06−2.09 (m, 8 H), 1.83−1.91 (m, 8 H), 1.11 (t,
J = 7.0 Hz, 3 H), 0.85 (t, J = 7.0 Hz, 3 H), 0.48 (t, J = 7.0 Hz, 6 H),
0.40 (t, J = 7.5 Hz, 12 H), 0.21−0.28 (m, 12 H); 13C NMR (CDCl3,
125.77 MHz) δ 182.8, 154.6, 151.9, 151.8, 150.9, 150.7, 150.2, 150.1,
150.0, 149.9, 149.7, 147.1, 145.0, 142.3, 142.2, 141.7, 141.64, 141.59,
140.1, 140.8, 139.93, 139.89, 138.9, 138.4, 137.5, 133.5, 132.6, 130.9,
130.8, 130.7, 130.04, 129.97, 129.9, 127.73, 127.67, 127.1, 126.7,
126.6, 126.0, 125.8, 125.7, 125.4, 125.0, 124.2, 123.0, 122.80, 122.76,
121.5, 121.3, 120.6, 120.5, 120.4, 120.01, 119.97, 119.4, 119.3, 56.6,
56.37, 56.35, 56.1, 56.0, 40.3, 39.7, 32.8, 32.7, 32.6, 16.4, 16.1, 8.8, 8.6,
8.5; HRMS (ESI) m/z calcd for C100H97N4OS (M + H) 1402.7410,
found 1402.7419.
NMR (CDCl3, 500.13 MHz) δ 7.94 (dd, J = 13.0 Hz, 1.5 Hz, 2 H),
7.78−7.84 (m, 4 H), 7.67 (d, J = 1.5 Hz, 1 H), 7.60−7.62 (m, 3 H),
7.56−7.58 (m, 2 H), 7.50−7.54 (m, 3 H), 7.47−7.48 (m, 2 H), 7.43−
7.46 (m, 2 H), 7.38−7.41 (m, 8 H), 7.23−7.25 (m, 5 H), 6.88 (d, J =
8.5 Hz, 1 H), 4.14−4.19 (m, 2 H), 3.70 (s, 2 H), 2.11−2.24 (m, 4 H),
2.04−2.09 (m, 8 H), 1.82−1.89 (m, 8 H), 1.09 (t, J = 7.0 Hz, 3 H),
0.82 (t, J = 7.0 Hz, 3 H), 0.38−0.45 (m, 18 H), 0.20−0.26 (m, 12 H);
13C NMR (CDCl3, 125.77 MHz) δ 153.0, 151.0, 150.9, 150.7, 150.2,
150.12, 150.07, 149.9, 149.8, 147.1, 145.0, 142.3, 142.2, 141.7, 141.6,
140.92, 140.87, 140.0, 139.9, 139.3, 138.9, 138.4, 133.6, 130.8, 130.7,
130.6, 130.1, 130.03, 129.98, 129.9, 127.8, 127.7, 127.2, 126.8, 126.7,
126.3, 125.9, 125.7, 125.6, 125.5, 125.0, 123.5, 123.0, 122.82, 122.79,
122.4, 121.6, 121.3, 120.5, 120.4, 120.1, 120.0, 119. 5, 119.3, 56.7,
56.40, 56.37, 56.12, 56.07, 40.3, 39.7, 32.9, 32.70, 32.65, 16.4, 16.1, 8.8,
8.63, 8.58; HRMS (ESI) m/z calcd for C95H94BrN4 (M + H)
1370.6689, found 1370.6697.
5′-(5,7-Bis(4,5-bis(9,9-diethyl-9H-fluoren-2-yl)-1-ethyl-1H-imida-
zol-2-yl)-9,9-diethyl-9H-fluoren-2-yl)-2,2′-bithiophene-5-carbalde-
hyde (7d). Compound 7d was obtained by following the procedure
described above for 7a using 6b and (5′-(1,3-dioxolan-2-yl)-2,2′-
bithiophene-5-yl)tributylstannane: dark-yellow solid; yield 0.22 g,
30%; mp 130−132 °C; IR (KBr, cm−1) 1653 (νCO); 1H NMR
(CDCl3, 500.13 MHz) δ 9.88 (s, 1 H), 7.97 (dd, J = 8.5 Hz, 1.5 Hz, 2
H), 7.85 (d, J = 7.5 Hz, 1 H), 7.77−7.82 (m, 3 H), 7.70−7.71 (m, 2
H), 7.57−7.65 (m, 7 H), 7.48−7.53 (m, 5 H), 7.38−7.43 (m, 9 H),
7.28−7.30 (m, 2 H), 7.23−7.25 (m, 5 H), 7.05 (d, J = 8.0 Hz, 1 H),
4.18−4.19 (m, 2 H), 3.74 (s, 2 H), 2.20−2.29 (m, 4 H), 2.06−2.09
(m, 8 H), 1.83−1.90 (m, 8 H), 1.11 (t, J = 7.0 Hz, 3 H), 0.85 (t, J =
7.0 Hz, 3 H), 0.48 (t, J = 7.5 Hz, 6 H), 0.40 (t, J = 7.5 Hz, 12 H),
0.21−0.28 (m, 12 H); 13C NMR (CDCl3, 125.77 MHz) δ 182.3,
151.6, 151.4, 150.7, 150.5, 150.0, 149.93, 149.87, 149.7, 149.5, 147.0,
146.3, 145.0, 142.1, 142.0, 141.5, 141.44, 141.41, 141.0, 140.74,
140.68, 140.4, 139.72, 139.69, 138.7, 138.2, 137.4, 135.0, 133.39,
133.35, 132.9, 130.6, 130.43, 130.35, 129.9, 129.81, 129.76, 129.7,
127.6, 127.5, 127.1, 127.0, 126.5, 126.4, 125.7, 125.5, 125.4, 125.30,
125.25, 125.1, 124.8, 124.2, 123.9, 122.8, 122.63, 122.59, 121.4, 121.1,
120.3, 120.2, 119.8, 119.7, 119.2, 119.1, 56.4, 56.20, 56.17, 55.92,
55.87, 40.1, 39.6, 32.7, 32.49, 32.45, 16.2, 15.9, 8.7, 8.42, 8.37; HRMS
(ESI) m/z calcd for C104H99N4OS2 (M + H) 1484.7287, found
1484.7298.
5-(9,9-Diethyl-5,7-bis(1-ethyl-4,5-diphenyl-1H-imidazol-2-yl)-9H-
fluoren-2-yl)thiophene-2-carbaldehyde (7a). A mixture of 6a (0.397
g, 0.5 mmol) and (5-(1,3-dioxolan-2-yl)thiophene-2-yl)-
tributylstannane (0.245 g, 0.55 mmol) in 5 mL of DMF was degassed
with nitrogen followed by the addition of Pd(PPh3)2Cl2 (3.5 mg, 5 ×
10−3 mmol). The reaction mixture was heated at 80 °C for 36 h under
nitrogen. It was cooled, poured into water, and extracted with
dichloromethane. The solvent was evaporated and the solid residue
suspended in glacial acetic acid (5 mL). The acetic acid solution was
warmed to dissolve the solids and water (1 mL) added. It was stirred
for 1 h at 50 °C and then extracted with dichloromethane. The organic
layer was washed liberally with water and dried over anhydrous
Na2SO4. After removal of solvent, the residue obtained was purified by
column chromatography on silica gel (dichloromethane/ethyl
acetate): yellow solid; yield 0.23 g, 55%; mp 112−114 °C; IR (KBr,
cm−1) 1658 (νCO); 1H NMR (CDCl3, 500.13 MHz) δ 9.90 (s, 1 H),
7.90 (dd, J = 13.0 Hz, 1.5 Hz, 2 H), 7.76 (d, J = 4.0 Hz, 1 H), 7.65 (d, J
= 1.5 Hz, 1 H), 7.60−7.62 (m, 2 H), 7.57 (dd, J = 8.5 Hz, 1.5 Hz, 1
H), 7.55 (d, J = 1.5 Hz, 1 H), 7.49−7.53 (m, 9 H), 7.47 (d, J = 4.0 Hz,
1 H), 7.45 (dd, J = 7.5 Hz, 1.5 Hz, 2 H), 7.20−7.24 (m, 4 H), 7.15−
7.19 (m, 2 H), 6.89 (d, J = 8.5 Hz, 1 H), 4.06−4.08 (m, 2 H), 3.66 (s,
2 H), 2.17−2.23 (m, 4 H), 1.04 (t, J = 7.0 Hz, 3 H), 0.79 (t, J = 7.0 Hz,
3 H), 0.42 (t, J = 7.5 Hz, 6 H); 13C NMR (CDCl3, 125.77 MHz) δ
182.8, 154.5, 151.8, 151.7, 146.9, 144.9, 142.3, 141.6, 141.1, 138.1,
137.7, 137.6, 134.5, 134.4, 132.6, 131.3, 131.2, 131.1, 131.0, 130.6,
130.4, 130.1, 129.4, 129.3, 129.2, 129.1, 129.0, 128.8, 128.24, 128.17,
127.0, 126.8, 126.49, 126.45, 126.0, 125.6, 125.0, 124.3, 122.6, 120.5,
56.6, 40.1, 39.6, 33.0, 16.3, 16.1, 8.7; HRMS (ESI) m/z calcd for
C56H49N4OS (M + H) 825.3622, found 825.3629.
5′-(9,9-Diethyl-5,7-bis(1-ethyl-4,5-diphenyl-1H-imidazol-2-yl)-
9H-fluoren-2-yl)-2,2′-bithiophene-5-carbaldehyde (7b). Compound
7b was obtained by following the procedure described above for 7a
using 6a and (5′-(1,3-dioxolan-2-yl)-2,2′-bithiophene-5-yl)-
tributylstannane: dark-yellow solid; yield 0.18 g, 39%; mp 88−90
°C; IR (KBr, cm−1) 1654 (νCO); 1H NMR (CDCl3, 500.13 MHz) δ
9.87 (s, 1 H), 7.89 (dd, J = 10.5 Hz, 1.5 Hz, 2 H), 7.69 (d, J = 4.0 Hz,
1 H), 7.61 (d, J = 7.5 Hz, 2 H), 7.58 (d, J = 1.5 Hz, 1 H), 7.48−7.55
(m, 11 H), 7.45 (dd, J = 7.5 Hz, 2.0 Hz, 2 H), 7.35−7.37 (m, 2 H),
7.28 (d, J = 3.5 Hz, 1 H), 7.22−7.25 (m, 3 H), 7.13−7.21 (m, 3 H),
6.85 (d, J = 8.0 Hz, 1 H), 4.07−4.08 (m, 2 H), 3.66 (s, 2 H), 2.15−
2.25 (m, 4 H), 1.04 (t, J = 7.0 Hz, 3 H), 0.79 (t, J = 7.0 Hz, 3 H), 0.42
(t, J = 7.0 Hz, 6 H); 13C NMR (CDCl3, 125.77 MHz) δ 182.4, 151.6,
151.3, 145.0, 146.9, 146.2, 144.9, 141.4, 141.2, 140.3, 138.0, 137.5,
137.4, 134.9, 134.5, 134.4, 132.9, 131.22, 131.15, 130.93, 130.87,
130.23, 130.18, 129.9, 129.2, 129.1, 129.0, 128.8, 128.6, 128.1, 128.0,
127.1, 126.8, 126.6, 126.3, 126.2, 125.2, 125.1, 124.8, 124.3, 123.9,
122.4, 119.6, 56.4, 39.9, 39.5, 32.9, 16.2, 15.9, 8.6; HRMS (ESI) m/z
calcd for C60H51N4OS2 (M + H) 907.3499, found 907.3508.
(E)-2-Cyano-3-(5-(9,9-diethyl-5,7-bis(1-ethyl-4,5-diphenyl-1H-
imidazol-2-yl)-9H-fluoren-2-yl)thiophene-2-yl)acrylic Acid (IF3). A
mixture of the aldehyde 7a (0.20 g, 0.24 mmol), 2-cyanoacetic acid (25
mg, 0.29 mmol), and a catalytic amount of ammonium acetate in acetic
acid (5 mL) was refluxed for 12 h. After cooling, it was poured into
water and the resulting solid collected by filtration. The solid formed
was thoroughly washed with water and dried. It was crystallized by
using a dichloromethane/hexane mixture to obtain analytically pure
compound: yellow solid; yield 0.17 g, 81%; mp 166−168 °C; IR (KBr,
1
cm−1) 2208 (νCN), 1700, 1582; H NMR (DMSO-d6, 500.13 MHz)
δ 8.30 (s, 1 H), 8.02−8.05 (m, 1 H), 7.90−7.94 (m, 2 H), 7.83 (d, J =
4.0 Hz, 1 H), 7.74−7.78 (m, 1 H), 7.44−7.60 (m, 15 H), 7.20−7.25
(m, 4 H), 7.13−7.17 (m, 2 H), 6.68−6.77 (m, 1 H), 4.05−4.06 (m, 2
H), 3.59 (s, 2 H), 2.24−2.29 (m, 4 H), 1.00 (t, J = 7.0 Hz, 3 H), 0.77
(t, J = 7.0 Hz, 3 H), 0.33 (s, 6 H); 13C NMR (DMSO-d6+TFA, 125.77
MHz) δ 163.5, 152.4, 152.3, 152.2, 152.0, 146.5, 143.2, 143.04, 142.98,
141.3, 141.2, 141.1, 138.3, 135.3, 133.7, 131.2, 131.1, 131.0, 130.7,
130.63, 130.58, 130.5, 130.4, 129.6, 129.5, 129.2, 129.14, 129.09,
128.9, 128.83, 128.78, 127.9, 127.64, 127.60, 127.3, 127.1, 127.0,
126.8, 126.5, 126.4, 126.1, 122.7, 122.3, 120.9, 118.1, 117.7, 56.9, 41.3,
41.1, 31.9, 14.53, 14.49, 8.4; HRMS (FAB) m/z calcd for
C59H48N5O2S (M−H) 890.3534, found 890.3553; m/z calcd for
C59H49N5NaO2S (M + Na) 914.3510, found 914.3515.
(E)-2-Cyano-3-(5′-(9,9-diethyl-5,7-bis(1-ethyl-4,5-diphenyl-
1H-imidazol-2-yl)-9H-fluoren-2-yl)-2,2′-bithiophene-5-yl)-
acrylic Acid (IF4). Compound IF4 was obtained from 7b by
following a procedure described above for IF3: orange solid; yield 0.21
5-(5,7-Bis(4,5-bis(9,9-diethyl-9H-fluoren-2-yl)-1-ethyl-1H-imida-
zol-2-yl)-9,9-diethyl-9H-fluoren-2-yl)thiophene-2-carbaldehyde
(7c). Compound 7c was obtained by following the procedure describe
1
g, 88%; mp >300 °C; IR (KBr, cm−1) 2204 (νCN), 1709, 1585; H
3169
dx.doi.org/10.1021/jo500330r | J. Org. Chem. 2014, 79, 3159−3172

The Journal of Organic Chemistry
Article
NMR (DMSO-d6, 500.13 MHz) δ 8.39 (s, 1 H), 8.03 (d, J = 1.0 Hz, 1
H), 7.91−7.92 (m, 3 H), 7.74 (d, J = 4.0 Hz, 2 H), 7.67 (d, J = 3.5 Hz,
1 H), 7.55−7.61 (m, 9 H), 7.50−7.52 (m, 4 H), 7.44 (d, J = 7.0 Hz, 2
H), 7.20−7.26 (m, 4 H), 7.13−7.17 (m, 2 H), 6.70 (d, J = 8.0 Hz, 1
H), 4.04−4.06 (m, 2 H), 3.58 (s, 2 H), 2.27−2.28 (m, 4 H), 0.99 (t, J
= 7.0 Hz, 3 H), 0.77 (t, J = 7.0 Hz, 3 H), 0.32−0.34 (m, 6 H); 13C
NMR (CDCl3+TFA, 125.77 MHz) δ 154.0, 153.1, 148.8, 145.8, 144.9,
142.6, 141.4, 140.9, 136.6, 135.8, 134.1, 131.7, 131.5, 131.3, 131.1,
131.0, 130.7, 130.6, 130.5, 130.3, 130.22, 130.16, 130.1, 130.0, 129.4,
129.3, 129.2, 128.7, 128.4, 127.3, 127.2, 127.0, 126.0, 125.4, 125.31,
125.25, 124.9, 121.6, 121.4, 120.8, 116.1, 57.5, 42.1, 42.0, 32.2, 15.1,
14.8, 8.0; HRMS (FAB) m/z calcd for C63H50N5O2S2 (M − H)
972.3411, found 972.3414; m/z calcd for C63H51N5NaO2S2 (M + Na)
996.3387, found 996.3378.
57.0, 32.4, 8.0; HRMS (ESI) m/z calcd for C32H28BrN2 (M + H)
521.1415, found 521.1426.
2-(7-Bromo-9,9-diethyl-9H-fluoren-2-yl)-1-ethyl-4,5-diphenyl-1H-
imidazole (9). Compound 9 was synthesized following the procedure
as described for the synthesis of 6a, by using 8 (52 g, 1 mmol) and
bromoethane (0.16 g, 2 mmol): light-yellow solid; yield 0.47 g, 78%;
1
mp 183−185 °C; H NMR (CDCl3, 500.13 MHz) δ 7.79 (d, J = 8.0
Hz, 1 H), 7.67−7.70 (m, 2 H), 7.60−7.62 (m, 1 H), 7.43−7.54 (m, 9
H), 7.21 (t, J = 7.5 Hz, 2 H), 7.14 (tt, J = 7.5 Hz, 1.5 Hz, 1 H), 3.96 (q,
J = 7.0 Hz, 2 H), 2.02−2.11 (m, 4 H), 1.01 (t, J = 7.0 Hz, 3 H), 0.35 (t,
J = 7.5 Hz, 6 H); 13C NMR (CDCl3, 125.77 MHz) δ 152.5, 149.8,
147.7, 141.1, 140.0, 137.9, 134.6, 131.6, 131.1, 130.5, 130.2, 129.6,
129.1, 128.7, 128.3, 128.1, 126.9, 126.4, 126.3, 123.8, 121.6, 121.4,
119.9, 56.7, 39.8, 32.8, 16.1, 8.6; HRMS (ESI) m/z calcd for
C34H32BrN2 (M + H) 549.1729, found 549.1739.
(E)-3-(5-(5,7-Bis(4,5-bis(9,9-diethyl-9H-fluoren-2-yl)-1-ethyl-1H-
imidazol-2-yl)-9,9-diethyl-9H-fluoren-2-yl)thiophene-2-yl)-2-cya-
noacrylic Acid (IF5). Compound IF5 was obtained from 7c by
following a procedure described above for IF3: yellow solid; yield 0.30
5-(9,9-Diethyl-7-(1-ethyl-4,5-diphenyl-1H-imidazol-2-yl)-9H-fluo-
ren-2-yl)thiophene-2-carbaldehyde (10a). Compound 10a was
obtained by following the procedure as described for the synthesis
of 7a by using 9 in place of 6a: yellow solid; yield 0.18 g, 63%; mp
214−216 °C; IR (KBr, cm−1) 1655 (νCO); 1H NMR (CDCl3, 500.13
MHz) δ 9.90 (s, 1 H), 7.85 (d, J = 8.0 Hz, 1 H), 7.76−7.80 (m, 2 H),
7.70−7.73 (m, 3 H), 7.66 (d, J = 1.5 Hz, 1 H), 7.43−7.54 (m, 8 H),
7.22 (t, J = 7.5 Hz, 2 H), 7.15 (tt, J = 7.5 Hz, 1.0 Hz, 1 H), 3.99 (q, J =
7.0 Hz, 2 H), 2.12−2.16 (m, 4 H), 1.03 (t, J = 7.0 Hz, 3 H), 0.40 (t, J =
7.5 Hz, 6 H); 13C NMR (CDCl3, 125.77 MHz) δ 182.8, 155.0, 151.4,
150.7, 147.6, 142.5, 142.1, 141.3, 137.8, 137.6, 134.3, 132.3, 131.3,
131.1, 130.3, 129.7, 129.1, 128.8, 128.3, 128.1, 127.0, 126.4, 125.8,
124.04, 123.96, 120.8, 120.7, 120.3, 56.7, 39.9, 32.8, 16.1, 8.6; HRMS
(ESI) m/z calcd for C39H35N2OS (M + H) 579.2465, found 579.2472.
5′-(9,9-Diethyl-7-(1-ethyl-4,5-diphenyl-1H-imidazol-2-yl)-9H-flu-
oren-2-yl)-2,2′-bithiophene-5-carbaldehyde (10b). Compound 10b
was obtained by following the procedure as described for the synthesis
of 7a by using 9 and (5′-(1,3-dioxolan-2-yl)-2,2′-bithiophene-5-
yl)tributylstannane: light-brown solid; yield 0.22 g, 68%; mp 160−
1
g, 84%; mp >300 °C; IR (KBr, cm−1) 2207 (νCN), 1699, 1582; H
NMR (DMSO-d6, 500.13 MHz) δ 8.02−8.08 (m, 2 H), 7.90−7.98 (m,
5 H), 7.82−7.84 (m, 1 H), 7.70−7.76 (m, 5 H), 7.55−7.65 (m, 7 H),
7.49−7.51 (m, 2 H), 7.40 (s, 6 H), 7.21−7.34 (m, 7 H), 6.88−6.96
(m, 1 H), 4.15 (s, 2 H), 3.62−3.66 (m, 2 H), 2.27−2.36 (m, 4 H),
2.06−2.15 (m, 8 H), 1.82−1.91 (m, 4 H), 1.65−1.75 (m, 4 H), 1.05 (t,
J = 7.0 Hz, 3 H), 0.85 (t, J = 7.0 Hz, 3 H), 0.23−0.37 (m, 18 H),
0.13−0.18 (m, 12 H); 13C NMR (DMSO-d6 + TFA, 125.77 MHz) δ
163.5, 152.7, 152.4, 152.3, 151.8, 150.6, 150.5, 149.8, 149.74, 149.70,
149.52, 149.49, 146.4, 143.4, 143.2, 142.8, 142.7, 142.2, 142.1, 141.1,
140.8, 140.6, 139.8, 138.1, 135.3, 133.9, 131.5, 131.04, 130.97, 130.9,
130.7, 130.6, 130.0, 129.7, 128.2, 127.9, 127.2, 127.0, 126.6, 126.3,
126.0, 125.5, 125.3, 125.2, 124.7, 124.6, 122.9, 122.8, 122.5, 122.2,
121.7, 120.8, 120.4, 120.3, 120.19, 120.15, 120.1, 120.0, 118.0, 116.9,
56.0, 55.9, 55.59, 55.56, 41.4, 41.3, 31.7, 31.5, 31.2, 14.3, 14.2, 8.4,
8.08, 8.05, 7.9, 7.8; HRMS (FAB) m/z calcd for C103H96N5O2S (M −
H) 1466.7279, found 1466.7286.
1
162 °C; IR (KBr, cm−1) 1653 (νCO); H NMR (CDCl3, 500.13
MHz) δ 9.88 (s, 1 H), 7.83 (d, J = 7.5 Hz, 1 H), 7.77 (d, J = 8.0 Hz, 1
H), 7.69−7.73 (m, 3 H), 7.64 (dd, J = 8.0 Hz, 1.5 Hz, 1 H), 7.58 (d, J
= 1.5 Hz, 1 H), 7.54−7.55 (m, 2 H), 7.48−7.51 (m, 3 H), 7.44−7.46
(m, 2 H), 7.37 (q, J = 4.0 Hz, 2 H), 7.29 (d, J = 4.0 Hz, 1 H), 7.21 (t, J
= 7.5 Hz, 2 H), 7.14 (tt, J = 7.5 Hz, 1.5 Hz, 1 H), 3.98 (q, J = 7.0 Hz, 2
H), 2.13 (q, J = 7.5 Hz, 4 H), 1.03 (t, J = 7.0 Hz, 3 H), 0.40 (t, J = 7.5
Hz, 6 H); 13C NMR (CDCl3, 125.77 MHz) δ 182.4, 151.2, 150.3,
147.6, 147.1, 146.6, 141.4, 141.3, 141.2, 137.7, 137.4, 134.7, 134.5,
132.5, 131.4, 131.0, 130.2, 129.5, 129.0, 128.6, 128.1, 128.0, 127.1,
126.7, 126.1, 124.9, 124.0, 123.9, 123.6, 120.5, 120.0, 119.9, 56.4, 39.7,
32.7, 16.0, 8.5; HRMS (ESI) m/z calcd for C43H37N2OS2 (M + H)
661.2342, found 661.2354.
(E)-3-(5′-(5,7-Bis(4,5-bis(9,9-diethyl-9H-fluoren-2-yl)-1-ethyl-1H-
imidazol-2-yl)-9,9-diethyl-9H-fluoren-2-yl)-2,2′-bithiophene-5-yl)-2-
cyanoacrylic Acid (IF6). Compound IF6 was obtained from 7d by
following a procedure described above for IF3: orange solid; yield 0.32
1
g, 85%; mp >300 °C; IR (KBr, cm−1) 2205 (νCN), 1699, 1585; H
NMR (DMSO-d6, 500.13 MHz) δ 8.38 (s, 1 H), 8.03−8.08 (m, 2 H),
7.90−7.98 (m, 6 H), 7.81 (d, J = 9.5 Hz, 1 H), 7.70−7.76 (m, 5 H),
7.66−7.68 (m, 4 H), 7.55−7.63 (m, 4 H), 7.49−7.51 (m, 2 H), 7.39−
7.41 (m, 6 H), 7.33 (t, J = 7.5 Hz, 2 H), 7.25−7.30 (m, 4 H), 6.90 (d, J
= 8.0 Hz, 1 H), 4.14−4.16 (m, 2 H), 3.66 (s, 2 H), 2.31−2.36 (m, 4
H), 2.04−2.14 (m, 8 H), 1.82−1.89 (m, 4 H), 1.65−1.75 (m, 4 H),
1.06 (t, J = 7.5 Hz, 3 H), 0.85 (t, J = 7.0 Hz, 3 H), 0.30−0.38 (m, 18
H), 0.11−0.18 (m, 12 H); 13C NMR (DMSO-d6 + TFA, 125.77 MHz)
δ 163.5, 152.6, 152.4, 150.6, 150.5, 149.9, 149.7, 149.5, 146.2, 145.2,
144.5, 143.4, 143.2, 142.7, 142.2, 142.1, 141.2, 140.7, 139.8, 137.0,
135.1, 134.5, 134.2, 131.4, 131.1, 131.0, 130.7, 130.0, 129.7, 128.4,
128.2, 127.8, 127.2, 127.0, 126.6, 126.4, 126.3, 125.9, 125.4, 125.3,
124.9, 124.7, 122.8, 122.2, 121.7, 120.8, 120.6, 120.3, 120.1, 120.0,
116.5, 116.4, 56.8, 56.0, 55.9, 55.6, 41.3, 31.7, 31.5, 31.2, 14.3, 13.5,
8.4, 8.0, 7.9; HRMS (FAB) m/z calcd for C107H98N5O2S2 (M − H)
1548.7156, found 1548.7162.
(E)-2-Cyano-3-(5-(9,9-diethyl-7-(1-ethyl-4,5-diphenyl-1H-imida-
zol-2-yl)-9H-fluoren-2-yl)thiophene-2-yl)acrylic acid (IF1). Com-
pound IF1 was obtained from 10a by following a procedure described
for IF3: yellow solid; yield 0.13 g, 82%; mp 204−206 °C; IR (KBr,
1
cm−1) 2206 (νCN), 1698, 1582; H NMR (DMSO-d6, 500.13 MHz)
δ 8.51 (s, 1 H), 8.02−8.06 (m, 3 H), 7.90−7.93 (m, 2 H), 7.82−7.85
(m, 2 H), 7.74 (dd, J = 7.5 Hz, 1.5 Hz, 1 H), 7.54−7.59 (m, 3 H),
7.48−7.50 (m, 2 H), 7.42−7.43 (m, 2 H), 7.21 (t, J = 7.5 Hz, 2 H),
7.13 (tt, J = 7.0 Hz, 1.5 Hz, 1 H), 3.96 (q, J = 7.0 Hz, 2 H), 2.18 (q, J =
7.5 Hz, 4 H), 0.95 (t, J = 7.0 Hz, 3 H), 0.30 (t, J = 7.5 Hz, 6 H); 13C
NMR (DMSO-d6 + TFA, 125.77 MHz) δ 163.5, 152.9, 151.4, 150.7,
146.5, 144.3, 143.8, 141.0, 140.9, 134.7, 132.7, 130.9, 130.5, 130.2,
129.5, 129.4, 129.2, 129.0, 128.7, 128.0, 127.6, 126.7, 126.2, 125.9,
125.5, 124.6, 121.8, 121.6, 121.0, 120.6, 116.4, 56.5, 41.2, 31.5, 14.3,
8.2; HRMS (FAB) m/z calcd for C42H34N3O2S (M−H) 644.2377,
found 644.2389; m/z calcd for C42H35N3NaO2S (M + Na) 668.2353,
found 668.2366.
2-(7-Bromo-9,9-diethyl-9H-fluoren-2-yl)-4,5-diphenyl-1H-imida-
zole (8). Compound 8 was synthesized following the general
procedure as described for the synthesis of 5a and 5b by using 7-
bromo-9,9-diethyl-9H-fluorene-2-carbaldehyde (2b) (0.66 g, 2 mmol)
and benzil (0.42 g, 2 mmol): cream-colored solid; yield 0.78 g, 75%;
1
mp >300 °C; H NMR (acetone-d6, 500.13 MHz) δ 11.79 (s, 1 H),
8.21 (s, 1 H), 8.19 (dd, J = 8.0 Hz, 1.5 Hz, 1 H), 7.92 (d, J = 8.0 Hz, 1
H), 7.81 (d, J = 8.0 Hz, 1 H), 7.69−7.70 (m, 2 H), 7.66 (d, J = 1.5 Hz,
1 H), 7.54−7.57 (m, 3 H), 7.44 (t, J = 7.5 Hz, 2 H), 7.36−7.39 (m, 1
H), 7.32 (t, J = 7.5 Hz, 2 H), 7.24 (t, J = 7.5 Hz, 1 H), 2.16 (q, J = 7.5
Hz, 4 H), 0.33 (t, J = 7.5 Hz, 6 H); 13C NMR (CDCl3+TFA, 125.77
MHz) δ 152.9, 151.5, 145.6, 144.8, 144.7, 138.5, 130.7, 130.3, 129.5,
129.3, 128.1, 126.6, 126.0, 125.8, 123.4, 122.1, 121.2, 121.0, 119.9,
(E)-2-Cyano-3-(5′-(9,9-diethyl-7-(1-ethyl-4,5-diphenyl-1H-imida-
zol-2-yl)-9H-fluoren-2-yl)-2,2′-bithiophene-5-yl)acrylic Acid (IF2).
Compound IF2 was obtained from 10b by following a procedure
described for IF3: orange solid; yield 0.15 g, 86%; mp 168−170 °C; IR
1
(KBr, cm−1) 2203 (νCN), 1708, 1585; H NMR (DMSO-d6, 500.13
MHz) δ 8.51 (s, 1 H), 7.96−8.03 (m, 3 H), 7.88 (s, 1 H), 7.76−7.80
3170
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Article
(m, 3 H), 7.71−7.73 (m, 2 H), 7.66 (d, J = 4.0 Hz, 1 H), 7.54−7.59
(m, 3 H), 7.49 (d, J = 7.5 Hz, 2 H), 7.42 (d, J = 7.5 Hz, 2 H), 7.21 (t, J
= 7.5 Hz, 2 H), 7.13 (t, J = 7.5 Hz, 1 H), 3.94−3.98 (m, 2 H), 2.14−
2.22 (m, 4 H), 0.95 (t, J = 7.0 Hz, 3 H), 0.30 (t, J = 7.5 Hz, 6 H); 13C
NMR (DMSO-d6+TFA, 125.77 MHz) δ 163.6, 151.3, 150.6, 146.2,
145.53, 145.46, 144.4, 144.0, 141.2, 139.7, 134.3, 133.9, 133.2, 130.9,
130.4, 130.2, 129.5, 129.4, 129.2, 128.9, 128.7, 128.1, 127.6, 126.7,
126.2, 125.7, 124.9, 124.7, 124.4, 121.6, 121.2, 121.0, 120.6, 119.8,
116.4, 56.4, 41.2, 31.5, 14.3, 8.2; HRMS (FAB) m/z calcd for
C46H36N3O2S2 (M − H) 726.2254, found 726.2262; m/z calcd for
C46H37N3NaO2S2 (M + Na) 750.2230, found 750.2232.
Phys. Chem. C 2012, 116, 21190−21200. (b) Ci, Z.; Yu, X.; Wang, C.;
Ma, T.; Bao, M. Dyes Pigm. 2014, 104, 8−14.
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ASSOCIATED CONTENT
(15) (a) Liu, J.; Li, R.; Si, X.; Zhou, D.; Shi, Y.; Wang, Y.; Jing, X.;
Wang, P. Energy Environ. Sci. 2010, 3, 1924−1928. (b) Fischer, M. K.
■
S
* Supporting Information
Copies of 1H and 13C NMR spectra, solvatochromic plots, and
Cartesian coordinates of the optimized structures. This material
is available free of charge via the Internet at http://pubs.acs.org.
R.; Wenger, S.; Wang, M.; Mishra, A.; Zakeeruddin, S. M.; Gratzel, M.;
̈
Bauerle, P. Chem. Mater. 2010, 22, 1836−1845. (c) Feng, Q.; Zhang,
̈
Q.; Lu, X.; Wang, H.; Zhou, G.; Wang, Z. S. ACS Appl. Mater.
Interfaces 2013, 5, 8982−8990.
(16) (a) Choi, H.; Raabe, I.; Kim, D.; Teocoli, F.; Kim, C.; Song, K.;
AUTHOR INFORMATION
Corresponding Author
*E-mail: krjt8fcy@iitr.ac.in. Phone: +91-1332-285376.
■
Yum, J. H.; Ko, J.; Nazeeruddin, M. K.; Gratzel, M. Chem.Eur. J.
̈
2010, 16, 1193−1201. (b) Chen, C. Y.; Pootrakulchote, N.; Hung, T.
H.; Tan, C. J.; Tsai, H. H.; Zakeeruddin, S. M.; Wu, C. G.; Gratzel, M.
̈
Notes
J. Phys. Chem. C 2011, 115, 20043−20050.
(17) Kwon, T. H.; Armel, V.; Nattestad, A.; MacFarlane, D. R.; Bach,
U.; Lind, S. J.; Gordon, K. C.; Tang, W.; Jones, D. J.; Holmes, A. B. J.
Org. Chem. 2011, 76, 4088−4093.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from Council of Scientific and Industrial
Research to K.R.J.T. is gratefully acknowledged.
■
(18) (a) Venkateswararao, A.; Thomas, K. R. J.; Lee, C. P.; Ho, K. C.
Tetrahedron Lett. 2013, 54, 3985−3989. (b) Sudyoadsuk, T.; Pansay,
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