Coplanar indenofluorene-based organic dyes for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tet.2012.07.045

A series of new organic dyes, comprising indenofluorene moiety as a conjugated bridge, with an extended π-groups, such as thiophene and furan, diphenylamine as donor, cyanoacrylic acid group as an electron acceptor and anchoring group, have been synthesized. Photophysical and electrochemical measurements, and theoretical computation were carried out on these dyes. Dye-sensitized solar cells (DSSCs) using these dyes as the sensitizers exhibited photocurrent density (J_SC), open-circuit voltage (V_OC), and fill factor (FF) in the range of 6.95-8.20 mA/cm², 0.70-0.71 V, and 0.69-0.71, respectively, corresponding to an overall conversion efficiency of 3.36-4.05%. The best efficiency reached 56% of the standard cell based on N719.

Full text of DOI:10.1016/j.tet.2012.07.045

Tetrahedron 68 (2012) 7755e7762
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Coplanar indenofluorene-based organic dyes for dye-sensitized solar cells
*
Sumit Chaurasia, Yung-Chung Chen, Hsien-Hsin Chou, Yuh-Shen Wen, Jiann T. Lin
Institute of Chemistry, Academia Sinica, Nankang, Taipei 11529, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 May 2012
Received in revised form 4 July 2012
Accepted 13 July 2012
Available online 20 July 2012
A series of new organic dyes, comprising indenofluorene moiety as a conjugated bridge, with an ex-
tended
p-groups, such as thiophene and furan, diphenylamine as donor, cyanoacrylic acid group as an
electron acceptor and anchoring group, have been synthesized. Photophysical and electrochemical
measurements, and theoretical computation were carried out on these dyes. Dye-sensitized solar cells
(DSSCs) using these dyes as the sensitizers exhibited photocurrent density (JSC), open-circuit voltage
(VOC), and fill factor (FF) in the range of 6.95e8.20 mA/cm2, 0.70e0.71 V, and 0.69e0.71, respectively,
corresponding to an overall conversion efficiency of 3.36e4.05%. The best efficiency reached 56% of the
standard cell based on N719.
Keywords:
Dye-sensitized solar cells
Indenofluorene
Ó 2012 Elsevier Ltd. All rights reserved.
Metal-free sensitizers
1. Introduction
indacenodithiophene,10 and quinacridone.11 It is important to note,
however, that planarized spacer with sluggish tendency to form
Sun is an obvious source of clean and cheap energy, therefore,
harnessing the power of the Sun with photovoltaic technologies in
large scale manners appears to be a promising approach toward the
energy challenge.1 Among different solar cells with the use of organic
materials, dye-sensitized solar cells (DSSCs) have attracted consid-
quinoid structure may exhibit pep* character dominating over
charge-transfer character upon photo-excitation. Consequently,
charge separation and electron injection were hampered in spite of
intense absorption of the dye. For example, the sensitizer using
a ladder-type pentaphenylene segment as the spacer between di(p-
tolylamino) donor and 2-cyanoacrylic acceptor absorbs at relatively
short wavelength (lmax¼457 nm in CH2Cl2), and the cell efficiency
reaches only 2.3%, corresponding to 50% of N719-based stand-
ard cell (N719¼(cis-di(thiocyanato))bis(2,20-bipyridyl-4,40-dicar-
boxylato)ruthenium(II) bis(tetrabutylammonium)).12 Therefore, we
chose (1,2-b)-indenofluorene moiety as a substitute for pentaphe-
nylene in order to have a better trade-off between charge transfer
and light absorption intensity.
€
erable attention since Gratzel’s seminal report on ruthenium-based
sensitizers in 1991.2 So far, ruthenium(II) polypyridyl complexes
still stand out as the most efficient and stable senstitizers for DSSC’s,
and the record high conversion efficiency (h) of 12.3% based on Zn-
porphyrin complexes have been reached recently.3 In addition to the
more costly ruthenium-based sensitizers, metal-free organic dyes
are also intensively exploited in DSSCs because of relatively low cost
and flexibility in tailoring molecular structures for manipulation of
electronic and photophysical characteristics.4 Conversion efficiency
surpassing 10% has been demonstrated for a metal-free sensitizer-
based DSSC.5 The common skeleton of metal-free sensitizers consists
of electron donor, electron acceptor, and conjugated spacer, in which
the acceptor also functions as the anchor linked to the photoanode.
(1,2-b)-Indenofluorene was widely used as the building block of
functional conjugated polymers for various optoelectronic devices
applications.13 Though there was also report on blue OLED using
dispirofluoreneeindenofluorene based De
our knowledge, indenofluorene has never been used as a coplanar
bridging unit in De eA systems for DSSCs, possibly due to the
p
eA type molecule,14 to
Such a pushepull type donore(
p
-spacer)eacceptor motif (De
peA)
p
not only enhances light harvesting at longer wavelength due to the
charge transfer transition from the donor to the acceptor, but also
synthetic complexity. A broader and stronger absorption band
would be obtained for indenofluoene derivatives with respect to
corresponding fluorene derivatives, due to the extended p-conju-
facilitates electron injection and charge separation.6 In De
peA
systems, planarization of the conjugated spacer can allow more ef-
fective electronic communication between the donor and the ac-
ceptor. Some representative planarized spacers for high perfor-
mance DSSCs include oligoene,7 fluorene,8 spirofluorene,9
gation of indenofluorene compared to fluorene. Additionally, better
solubility and processability of the dye can be anticipated by in-
corporation of four alkyl chains at the indenofluorene moiety. Ap-
propriate incorporation of heteroaromatic rings,15 such as
thiophene and furan in the spacer between the donor and the ac-
ceptor has been used to tune the electronic and optical properties of
the sensitizer, and thus the efficiency of DSSCs. Therefore, we also
* Corresponding author. E-mail address: jtlin@gate.sinica.edu.tw (J.T. Lin).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.07.045

7756
S. Chaurasia et al. / Tetrahedron 68 (2012) 7755e7762
integrated the indenofluorene with thiophene or furan unit as the
spacer of De eA sensitizer. To the best of our knowledge, this is the
first report of DSSCs on indenofluorene-based sensitizers.
Subsequent condensation of the thus formed aromatic aldehyde
with cyanoacetic acid results in formation of SC-1. For the syntheses
of SC-2 and SC-3, (5-(1,3-dioxolan-2-yl)thiophen-2-yl)tributyl-
stannane or (5-(1,3-dioxolan-2-yl)furan-2-yl)tributylstannane is
allowed to undergo palladium catalyzed Stille coupling reaction
with 4,20 followed by condensation of thus formed aldehyde with
cyanoacetic acid. The dyes are obtained as red powders, and are
soluble in common organic solvents, such as THF, CH2Cl2, and
CHCl3.
p
2. Results and discussion
2.1. Synthesis and characterization
The structures of new metal-free sensitizers are shown in Fig. 1.
The synthetic protocols for compound SC-1eSC-3 are illustrated in
Scheme 1. Indenofluorene was synthesized in excellent yields using
2.2. Photophysical properties
The UVevis absorption spectra of the new dyes in THF are shown
in Fig. 2 and the data are collected in Table 1. The absorption band at
a shorter wavelength (w300 nm) is attributed to localized
pep*
transition of basic chromophore, indenofluorene, as evidenced from
0.6
SC-1
SC-2
SC-3
0.5
SC-1
SC-2
0.4
SC-3
Wavelength (mn)
0.3
0.2
0.1
0.0
Fig. 1. Structures of compounds SC-1, SC-2, and SC-3.
300
400
500
600
a literature reported method.16 The first step is the alkylation of
methylene units of indenofluorene for suppressing their tendency
of oxidation and improving solubility.17 The alkylated indeno-
fluorene then undergoes double bromination using CuBr2 adsorbed
Wavelength (nm)
Fig. 2. UVevis absorption spectra of dyes SC-1, SC-2, and SC-3 in THF solutions and
inset spectra of dyes SC-1, SC-2, and SC-3 adsorbed on TiO2 (2 m).
m
Scheme 1. Synthesis of organic dyes SC-1, SC-2, and SC-3. Reagents and conditions: (a) (i) n-BuLi, dry THF, ꢁ78 ꢂC, N2, (ii) 2-ethylhexyl bromide, ꢁ78 ꢂC to rt, then repeat (i) and (ii)
again; (b) CuBr2/Al2O3, CCl4, reflux, 8 h, N2; (c) Ph2NH, Pd2(dba)3, rac-2,20-bis(diphenylphosphino)-1,10-biphenyl (BINAP) and sodium tert-butoxide, toluene, 80 ꢂC, 24 h, N2; (d) (i) n-
BuLi, dry THF, ꢁ78 ꢂC, N2, (ii) dry DMF, ꢁ78 ꢂC to rt, N2; (e) cyanoacetic acid, NH4OAc, AcOH, 100 ꢂC, 12 h; (f) (i) (5-(1,3-dioxolan-2-yl)thiophen-2-yl)tributylstannane/(5-(1,3-
dioxolan-2-yl)furan-2-yl)tributylstannane, PdCl2(PPh3)2, dry DMF, N2, (ii) glacial acetic acid, H2O, 50 ꢂC, 6 h.
on Al2O3.18 One of the bromo substituent is replaced by dipheny-
lamino entity via palladium catalyzed BuchwaldeHartwig CeN
coupling reaction of the dibromo intermediate with di-
phenylamine.19 The arylamine intermediate, 4, is then formylated.
the absorption spectra of indenofluorene reported in literature,21
ranging from 290 to 350 nm. The broad band ranging from 408 to
430 nm and possessing
a high extinction coefficient (>3.7
ꢀ104 Mꢁ1 cmꢁ1) is attributed to the more delocalized
pep*

S. Chaurasia et al. / Tetrahedron 68 (2012) 7755e7762
7757
Table 1
Electrooptical parameters of the dyes
a
b
a
e
f
Dyes
labs (εꢀ104 Mꢁ1 cmꢁ1
)
[nm]
labs
lem
E1/2 (ox)c
[mV]
HOMO/LUMOd
[eV]
E0e0
[eV]
E0e0
*
Dye loading
[nm]
[nm]
[eV]
[10ꢁ7 mol cmꢁ2
]
SC-1
SC-2
SC-3
310 (2.40), 408 (3.70)
410
425
436
504
534
514
409 (97)
362 (88), 923 (99)
366 (91), 892 (108)
5.51/2.80
5.46/2.90
5.47/2.83
2.71
2.56
2.64
1.60
1.50
1.57
3.34
4.02
3.84
308 (2.40), 372 (2.80), 426 (5.36)
308 (2.70), 370 (3.00), 408 (sh),
430 (5.40), 466 (sh)
a
Recorded in THF (solution).
Absorption maxima of film (adsorbed on TiO2).
b
c
Recorded in CH2Cl2. scan rate¼100 mV sꢁ1; electrolyte¼(n-C4H9)4NPF6; potentials are quoted with reference to the internal ferrocene standard (E1/2¼þ147eþ166 mV vs
Ag/AgNO3).
d
The HOMO and LUMO energies are calculated using formula HOMO¼5.1þ(E1/2ꢁEFc).
e
f
The optical bandgap energy gap, E0e0, was derived from the intersection of absorption and emission spectra.
The bandgap E0e0: the excited-state oxidation potential versus NHE.
*
transition with charge-transfer character. The charge-transfer
character of the band is also confirmed by theoretical computation
(vide infra). The absorption spectra of SC-2 (lmax¼426 nm) and SC-3
0.00003
0.00002
0.00001
0.00000
-0.00001
-0.00002
SC-1
SC-2
SC-3
(lmax¼430 nm) show considerable red shifts as compared with that
of SC-1 (lmax¼408 nm). This result clearly indicates the importance
Fc
of an extended conjugation via incorporation of a heteroaromatic
p
moiety. The molar extinction coefficient is also increased signifi-
cantly in SC-2 and SC-3. The UVevis absorption spectra maximum in
CH2Cl2 solution was measured to be 448, 462, and 472 nm for SC-1,
SC-2, and SC-3, respectively. Compared with the pentaphenylene
dye (lmax¼457 nm),12 a red shift of the absorption for SC-2 and SC-3
was observed. This may be due to weaker electronic communication
between the donor and the acceptor in the pentaphenylene dye.
Fig. 2 (Inset) depicts the absorption spectra of the organic dyes
adsorbed on TiO2 surface. They are slightly red-shifted compared
with the absorption spectra in THF solution, and may be attributed
to J-aggregation of the dye molecules. The dyes are weakly emissive
with the emission maximum ranging from 504 to 534 nm, which
follow the trend SC-1<SC-3<SC-2 (Fig. S1 see Supplementary data).
The larger dihedral angle between the indenofluorene and the
thiophene rings in the ground state may be the cause of its larger
Stokes shift of the emission spectra.
Fc
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Potential (V)
Fig. 3. Cyclic voltammograms of SC-1, SC-2, and SC-3 recorded in CH2Cl2.
chains were replaced by isobutyl groups to save computational
efforts. The computed frontier orbitals of the compounds and their
corresponding energy states are included in Figs. S2 and S3 (see
Supplementary data), respectively. The configurations of transi-
tions in gas-phase were also conducted using time-dependent DFT
and the results were summarized in Table S1 (see Supplementary
data). Fig. 4 displays the HOMO and LUMO orbitals of SC-1eSC-3.
The HOMO orbitals mainly reside on the arylamine extending to the
conjugated spacer, and LUMO orbitals mainly reside on 2-
cyanoacrylic acid extending to the conjugated spacer. Therefore,
charge-transfer character is evident for the S0/S1 transition,
which solely comes from HOMO to LUMO. Fig. 5 displays the di-
hedral angles between the neighboring groups in the conjugated
spacer. It is interesting to note that the dihedral angle between the
thiophene and the indenofluorene rings in SC-2 is significantly
larger than that between the furan and the indenofluorene rings in
SC-3 possibly due to more pronounced steric hindrance arose from
larger sulfur atom. The planarity of SC-3 was further established
from their crystal structure (Fig. S4, see Supplementary data),
which shows that the bridging unit (indenofluorene) and furan are
almost coplanar, also confirming the validity of computation.
2.3. Electrochemical properties
The electrochemical properties of the dyes were studied by
cyclic voltammetry (CV) and differential pulse voltammetry (DPV).
The relevant CV data are collected in Table 1 and the cyclic vol-
tammograms are shown in Fig. 3. All the dyes are redox-stable,
exhibiting reversible oxidation couples. Only one quasi-reversible
redox wave was observed for SC-1, while two quasi-reversible re-
dox waves were observed for SC-2 and SC-3. The first redox wave
may be attributed to the oxidation of the arylamine, while the
second quasi-reversible redox wave is most likely stemmed from
the oxidation of the conjugated segment. The first oxidation po-
tential of the compounds decreases in the order of SC-1>SC-2>SC-
3. This can be rationalized by the electronic influence of electron
excessive thiophene (electron density of carbon: ꢁ0.011; ꢁ0.074)
or furan (electron density of carbon: ꢁ0.019; ꢁ0.105)22 on the
arylamine. The energy levels of the HOMOs (highest occupied
molecular orbitals) in these materials were calculated with refer-
ence to ferrocene (5.1 eV)23 and ranged from 5.46 to 5.51 eV. This
together with the energy gap (E0e0, 2.56e2.71 eV) obtained from
the edge of absorption spectra were utilized to derive the LUMO
(lowest unoccupied molecular orbital) energy.
2.5. Photovoltaic properties
Dye-sensitized solar cells were fabricated with the use of these
new dyes and nanocrystalline anatase TiO2. The cells had an ef-
fective area of 0.25 cm2, and the electrolyte used was composed of
0.05 M I2/0.5 M LiI/0.5 M tert-butylpyridine (TBP) in acetonitrile
solution. The device performance statistics under standard global
AM 1.5 solar condition are presented in Table 2. The photo-
currentevoltage (JeV) curves, dark current and the incident
2.4. Theoretical approach
Density functional calculations at the B3LYP/6-31G* level was
conducted for the compounds to correlated between dye structures
and corresponding physical properties, The excessive 2-ethylhexyl

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S. Chaurasia et al. / Tetrahedron 68 (2012) 7755e7762
Fig. 4. HOMOs and LUMOs of the dyes.
18
16
14
12
10
8
SC-1
SC-2
SC-3
N719
6
4
2
0
-2
-4
-6
-8
Fig. 5. Dihedral angles between the neighboring units of the dyes.
-10
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (V)
monochromatic photo-to-current conversion efficiency (IPCE) plots
of the cells are shown in Fig. 6 and Fig. 7, respectively. The short-
circuit photocurrent density (JSC), open-circuit voltage (VOC) and
fill factor (FF) of the devices are in the range of 6.95e8.20 mA/cm2,
0.70e0.71 V and 0.69e0.71, respectively, corresponding to an
overall conversion efficiency of 3.36e4.05%. DSSC of SC-3 has the
best power conversion efficiency (4.05%) among all, reaching 56% of
N719-based standard cell (conversion efficiency¼7.18%) fabricated
and measured under similar conditions. Action spectra of the in-
cident photon-to-current conversion efficiency (IPCE) as a function
of wavelength were measured to evaluate the photoresponse in the
whole region, and the data are plotted in Fig. 7. High IPCE perfor-
mance (above 80%) was observed in the range from 400 to 550 nm
for dyes SC-2 and SC-3, with maximum values of 80% at 480 nm and
88% at 460 nm, which is much higher than the pentaphenylene
based dye (w70%).12 The higher efficiency of SC-2 and SC-3 is at-
Fig. 6. The current densityevoltage and dark-current curves of DSSCs based on
SC-1eSC-3.
100
90
80
70
60
50
40
30
20
10
0
SC-1
SC-2
SC-3
N719
tributed to the better light harvesting due to extended
p-conju-
gation and/or the higher dye density of SC-2 (4.02ꢀ10ꢁ7 mol/cm2)
and SC-3 (3.84ꢀ10ꢁ7 mol/cm2) than SC-1 (3.34ꢀ10ꢁ7 mol/cm2) on
TiO2 surface.
400
500
600
700
800
Wavelength (nm)
Fig. 7. IPCE plots for the DSSCs.
Table 2
DSSCs performance parameters of the dyes
Sample
VOC (V)
JSC (mA/cm2)
h
(%)
FF
Rct (U)
(4,9-dihydro-4,4,9,9-tetrakis(4-methylphenyl)-s-indaceno[1,2-b:5,
6-b0]dithiophene) as the spacer was reported to have a low cell ef-
ficiency of 2.31%, though the dye has longer absorption maximum
(500 nm) and larger extinction coefficient (84,000 Mꢁ1 cmꢁ1) than
SC-2 and SC-3.24 Possibly the bulkiness of the molecule results in
a lower dye density of the former on TiO2 surface.
SC-1
SC-2
SC-3
N719
0.70
0.70
0.71
0.75
6.95
8.10
8.20
15.5
3.36
4.04
4.05
7.18
0.69
0.71
0.70
0.62
34.6
26.8
26.1
22.1
The electrochemical impedance measurement under illumina-
tion was shown in Fig. 8. In general, electrochemical impedance
measured under illumination can provide information on electron
injection and transport inside TiO2.25 Upon illumination of
100 mW cmꢁ2 under open circuit conditions, the radius of the in-
termediate frequency semicircle in the Nyquist plot (Fig. 8) repre-
sents the electron transport resistance. The lower electron
The slightly lower cell efficiency (47% of N719 standard cell) of
SC-1 compared to pentaphenylene based dye (50% of N719 standard
cell) can be attributed to the inferior light harvesting of the former.12
The higher cell efficiency of SC-2 and SC-3 (56% of N719 standard
cell) than pentaphenylene based dye may be also due to the better
light harvesting of the former (vide supra). It is interesting to
note that a ladder-type dye with thiophene-implanted planarized

S. Chaurasia et al. / Tetrahedron 68 (2012) 7755e7762
7759
transport resistance (Rct) will assist electron collection and improve
the cell efficiency. Therefore, the order of decreasing Rct (Table 2),
SC-1>SC-2>SC-3, is consistent with the trend of the cell
performance.
reaches 4.05%, which is 56% of the efficiency of the reference cell
based on ruthenium dye N719, and surpasses that of ladder-type
pentaphenylene- or indaceno[1,2-b:5,6-b0]dithiophene-based dye.
4. Experimental section
4.1. General information
22
20
SC-1
SC-2
18
1H NMR and 13C NMR spectra were taken on a Bruker AMX-400
or Bruker AV-400 spectrometer using chloroform-d1 (CDCl3), or
acetone-d6 as the solvent. Cyclic Voltammetric (CV) measurement
was performed on BAS-100 using CH2Cl2 as solvent. UVevis (Ul-
traviolet-Visible) spectra were recorded on a Dynamica DB-20
Spectrophotometer. PL (Fluorescence) spectra were recorded on
a Hitachi F-4500 Spectrophotometer. Elemental analysis was per-
formed on a PerkineElmer 2400. Fast atom bombardment mass
spectrometry (FABMS) analysis was performed on a JEOL Tokyo
Japan JMS-700 mass spectrometer equipped with the standard FAB
source. The photoelectrochemical characterizations on the solar
cells were carried out using an Oriel Class A solar simulator (Oriel
91195A, Newport Corp.). Photocurrentevoltage characteristics of
the DSSCs were recorded with a potentiostat/galvanostat (CHI650B,
CH Instruments, Inc.) at a light intensity of 100 mW cmꢁ2 calibrated
by an Oriel reference solar cell (Oriel 91150, Newport Corp.). The
monochromatic quantum efficiency was recorded through
a monochromator (Oriel 74100, Newport Corp.) at short circuit
condition. The intensity of each wavelength was in the range of
1e3 mW cmꢁ2. Electrochemical impedance spectra (EIS) were
SC-3
N719
16
14
12
10
8
6
4
2
0
10
20
30
40
50
60
Z' (Ohm)
Fig. 8. Electrochemical impedance spectra (Nyquist plots) of DSSC for dyes measured
under illumination (AM 1.5).
The Nyquist plots of DSSCs under a forward bias of ꢁ0.50 V in
the dark are shown in Fig. 9. The semicircle in the Nyquist plots can
be used to derive the charge recombination resistance on the TiO2
surface (Rrec) by fitting curves using Z-view software. The larger Rrec
value implies the smaller dark current, and the smaller Rrec value
means more facile recombination of the electron in the conduction
band with the oxidized electrolyte, and thus a lower VOC. The Rrec
values of SC-3 and SC-2 are significantly higher than that of SC-1,
implying that there is more effective suppression of the electron
recombination with the electrolyte. However, only minimal de-
crease of dark current and increase of VOC values were observed for
SC-3 and SC-2 compared to SC-1. We speculate that the Fermi level
of TiO2 adsorbed with SC-3 or SC-2 was shifted in a downward
direction compared to that adsorbed with SC-1.
recorded for DSSC under illumination at open-circuit voltage (VOC
)
or dark at ꢁ0.55 V potential at room temperature. The frequencies
explored ranged from 10 mHz to 100 kHz. The TiO2 nanoparticles
and the reference compound, N719, were purchased from Solar-
onix, S.A., Switzerland.
4.2. Synthetic details and characterization
4.2.1. 6,6,12,12-Tetrakis(2-ethylhexyl)-6,12-dihydroindeno[1,2-b]fluo-
rene (2). To a stirred solution of indenofluorene 115 (2.54 g,
10 mmol) in dry THF (30 mL) under nitrogen, 15 mL of n-butyl-
lithium (n-BuLi) (24 mmol, 1.6 M in hexane) was added dropwise at
ꢁ78 ꢂC. The solution turned to deep purple. The solution was stir-
red for an additional 1.5 h at ꢁ78 ꢂC and 1-bromo-2-ethylhexane
(5.4 mL, 30 mmol) was added dropwise and the solution was
warmed to room temperature. After stirring for 1.5 h, the solution
was cooled again to ꢁ78 ꢂC and second portion of n-BuLi (15 mL,
24 mmol) was added dropwise and stirred again for an additional
1.5 h before the second portion of 1-bromo-2-ethylhexane (6 mL,
33 mmol) was added. The solution was warmed to room temper-
ature and stirred for 12 h. It was quenched by adding water. The
mixture was extracted using EtOAc and washed with H2O. The or-
ganic layer was dried over anhydrous MgSO4 and then concen-
trated under reduced pressure to afford the crude product. Further
purification was performed by column chromatography to provide
a thick oil (excess 2-ehthylhexylbromide was removed under vac-
uum at 80 ꢂC for 8e10 h). Yield¼6.35 (90%). 1H NMR (chloroform-
2000
SC-1
1500
SC-2
SC-3
N719
1000
500
0
0
500
1000
1500
2000
2500
3000
Z' (Ohm)
Fig. 9. Electrochemical impedance spectra (Nyquist plots) of DSSC for SC-1eSC-3 dyes
measured in the dark under ꢁ0.50 V bias.
d1, 400 MHz):
(m, 2H), 7.31 (t, J¼7.4 Hz, 2H), 7.37 (d, J¼7.4 Hz, 2H), 7.65 (s, 2H),
7.68e7.72 (m, 2H); 13C NMR (chloroform-d1, 100 MHz):
10.25,
d 0.41e0.93 (m, 60H), 1.95e2.10 (m, 8H), 7.22e7.26
3. Conclusion
d
10.46, 10.60, 14.20, 14.32, 22.91, 22.97, 26.89, 27.05, 27.17, 28.20,
28.39, 28.53, 33.81, 33.91, 34.03, 34.10, 34.74, 44.56, 44.70, 44.99,
45.14, 45.32, 54.68, 115.10, 119.34, 124.21, 126.20, 126.82, 140.58,
141.92, 149.90, 151.04. FABMS (m/z): 703.0 (Mþþ1).
In summary, we have successfully synthesized a series of metal-
free organic DSSC sensitizers by exploiting indenofluorene as a co-
planar bridging unit, thiophene and furan as extended
p-spacer
group in the De eA structure, using the conventional diarylamine
p
as donor and cyanoacetic acid as acceptor. DSSCs using these dyes
as the sensitizers exhibited high IPCE performance (above 80%) in
the range from 400 to 550 nm. The highest conversion efficiency
4.2.2. 2,8-Dibromo-6,6,12,12-tetrakis(2-ethylhexyl)-6,12-dihydroind-
eno[1,2-b]fluorene (3). To a stirred solution of 2 (5.0 g, 7.13 mmol) in
carbon tetrachloride (50 mL) at room temperature was added

7760
S. Chaurasia et al. / Tetrahedron 68 (2012) 7755e7762
CuBr2/Al2O3 (20 g) and mixture was stirred at 80 ꢂC under N2 at-
mosphere and was monitored through 1H NMR. After complete
consumption of starting material (after 6e8 h), the reaction mix-
ture was filtered, and the precipitate was washed with dichloro-
methane (100 mLꢀ3). The combined organic extracts were
concentrated under reduced pressure, and the crude product was
purified by flash chromatography to give compound 3 as colorless
oil. Yield: 6.0 g (98%). 1H NMR (chloroform-d1, 400 MHz):
(109 mg, 51%). 1H NMR (chloroform-d1, 400 MHz):
d 0.50e1.0 (m,
60H), 1.88e1.93 (m, 2H), 2.03e2.15 (m, 6H), 7.02e7.07 (m, 2H),
7.10e7.16 (m, 6H), 7.25e7.30 (m, 4H), 7.65e7.67 (m, 2H), 7.72 (s, 1H),
7.82e7.86 (m, 1H), 8.03e8.07 (m, 1H), 8.19e8.22 (m, 1H), 8.41 (s, 1H).
13C NMR (chloroform-d1, 100 MHz)
d 10.06, 10.42, 10.86, 14.16, 14.32,
22.98, 23.12, 24.05, 27.03, 27.16, 27.34, 28.23, 28.28, 28.45, 28.98,
29.91, 30.63, 33.70, 33.90, 34.03, 34.13, 34.36, 34.95, 39.06, 44.78,
45.29, 54.88, 55.09, 68.34, 99.71,114.89,116.30,119.89,120.73,120.92,
121.26, 122.69, 122.78, 123.99, 124.14, 124.39, 124.72, 126.82, 129.04,
129.12, 129.40, 131.09, 132.14, 136.58, 136.71, 138.51, 142.78, 142.83,
147.39, 148.23, 150.65, 150.69, 152.13, 152.96, 153.06, 156.79, 168.01,
168.23. HRMS (FAB) m/z [Mþ] calcd for C68H88N2O2: 965.6924;
found: 965.6942.
d
0.40e0.95 (m, 60H), 1.92e2.10 (m, 8H), 7.45 (d, J¼8.0 Hz, 2H), 7.50
(s, 2H), 7.55e7.59 (m, 2H), 7.61 (m, 2H); 13C NMR (chloroform-d1,
100 MHz): 10.29, 10.33, 10.49, 14.16, 14.23, 22.85, 22.91, 26.97,
d
27.04, 27.12, 28.07, 28.19, 28.37, 33.65, 33.76, 33.88, 34.72, 34.79,
44.29, 44.43, 44.62, 44.83, 44.98, 54.93, 115.17, 120.39, 120.68,
127.46, 127.57, 129.99, 139.78, 140.51, 149.80, 153.22, 153.30. FABMS
(m/z): 861 (Mþþ1).
4.2.6. 5-(8-(Diphenylamino)-6,6,12,12-tetrakis(2-ethylhexyl)-6,12-
dihydroindeno[1,2-b]fluoren-2-yl)thiophene-2-carbaldehyde
(6a). To a flame dried two neck flask fitted with condenser charged
with 4 (0.50 g, 0.53 mmol), (5-(1,3-dioxolan-2-yl)thiophen-2-yl)
tributylstannane (0.353 g, 0.79 mmol), Pd(PPh3)2Cl2 (8.0 mg,
0.01 mmol), and dry DMF (15 mL) were added under N2 atmo-
sphere. The mixture was heated to 80 ꢂC for 24 h. After completion
the reaction mixture was cooled and extracted with ether/brine
and the organic layers were combined and dried over Na2CO3.
Evaporation of the solvent gave crude dioxolane derivative.
The resulting dioxolane derivative was suspended in glacial
acetic acid and heated to 50 ꢂC. After a clear solution is formed,
1 mL of water was added and the temperature of the reaction
mixture was maintained at 50 ꢂC for 5e6 h. It was cooled and
poured into crushed ice, orange precipitate formed was filtered and
washed with H2O. The aldehyde product (6a) was further purified
by flash chromatography using CH2Cl2/hexane (60:40 by vol) as the
eluent to yield an orange solid 370 mg (71%). 1H NMR (chloroform-
4.2.3. 8-Bromo-6,6,12,12-tetrakis(2-ethylhexyl)-N,N-diphenyl-
6,12-dihydroindeno[1,2-b]fluoren-2-amine (4). A mixture of di-
phenylamine (0.591 g, 3.5 mmol), 3 (3.0 g, 3.5 mmol), Pd2(dba)3
(0.064 g, 0.070 mmol), rac-2,20-bis(diphenylphosphino)-1,10-bi-
phenyl (BINAP) (0.043 g, 0.14 mmol), and sodium tert-butoxide
(0.504 g, 5.25 mmol) in toluene was stirred at 80 ꢂC under N2 for
24 h. After completion of the reaction toluene was evaporated
under vacuum and reaction mixture was poured into water,
extracted with ether and the combined organic phase was washed
with brine (30 mLꢀ2) and then dried over MgSO4. After removal of
solvent, the crude product was purified through column chroma-
tography on silica gel using hexane as the eluent to give the purified
product 4 as colorless oil in 64% yield (2.13 g). 1H NMR (chloroform-
d1, 300 MHz)
8H), 7.18e7.25 (m, 4H), 7.39e7.60 (m, 6H); 13C NMR (chloroform-d1,
100 MHz) 10.05, 10.46, 10.78, 10.87, 14.25, 14.34, 22.97, 23.03,
d 0.4e0.10 (m, 60H), 1.72e2.10 (m, 8H), 6.91e7.15 (m,
d
23.13, 23.21, 26.56, 26.93, 27.12, 27.25, 27.38, 27.54, 28.21, 28.26,
28.33, 28.49, 29.01, 29.92, 30.57, 33.69, 33.95, 34.40, 34.84, 44.77,
44.87, 45.33, 54.83, 55.04, 114.74, 115.14, 120.05, 120.11, 120.31,
120.54, 121.24, 121.31, 122.45, 122.53, 123.76, 123.92, 124.62, 127.50,
129.34, 129.99, 138.84, 140.86, 140.93, 146.75, 148.35, 149.75,
150.25, 150.30, 152.66, 152.76, 153.31; HRMS (FAB): (m/z) Mþ calcd
for C64H86BrN, 948.6022; found, 948.6045.
d1, 400 MHz): d 0.45e1.05 (m, 60H), 1.78e1.87 (m, 2H), 1.97e2.11
(m, 6H), 6.94e7.02 (m, 2H), 7.03e7.13 (m, 6H), 7.19e7.24 (m, 4H),
7.42 (s, 1H), 7.56e7.66 (m, 5H), 7.68e7.76 (m, 2H), 9.88 (s, 1H). 13C
NMR (chloroform-d1, 100 MHz):
d 9.94, 10.39, 10.67, 10.76, 14.02,
14.24, 22.82, 23.02, 23.10, 26.43, 26.83, 27.07, 27.23, 27.41, 28.08,
28.22, 28.39, 28.88, 29.81, 33.44, 33.57, 33.89, 33.98, 34.27, 34.78,
44.64, 44.74, 45.17, 54.79, 68.34, 99.71, 114.89, 116.30, 119.89,
120.73, 120.92, 121.26, 122.69, 122.78, 123.99, 124.14, 124.39, 124.72,
126.82, 129.04, 129.12, 129.40, 131.09, 132.14, 136.58, 136.71, 138.51,
142.78, 142.83, 147.39, 148.23, 150.65, 150.69, 152.13, 152.96, 153.06,
156.79, 168.01, 168.23. HRMS (FAB) m/z [Mþ] calcd for C69H89N2OS:
980.6743; found: 980.6758.
4.2.4. 8-(Diphenylamino)-6,6,12,12-tetrakis(2-ethylhexyl)-6,12-
dihydroindeno[1,2-b]fluorene-2-carbaldehyde (5). Under a nitrogen
atmosphere and at ꢁ78 ꢂC, n-BuLi (0.500 mL, 1.6 M in hexane)
was added dropwise to a dry THF (30 mL) solution containing 4
(0.50 g, 0.58 mmol). After 1 h stirring, 0.1 mL of N-methyl-
formamide was added slowly to the reaction solution. The tem-
perature of the solution was brought back to room temperature,
and stirred over night. The reaction was then quenched with 2 N
HCl. The solution was extracted with ethyl acetate and subjected
to flash column chromatography. A light yellow thick oil was
obtained in 41% yield (368 mg). 1H NMR (chloroform-d1,
4.2.7. 2-Cyano-3-(5-(8-(diphenylamino)-6,6,12,12-tetrakis(2-
ethylhexyl)-6,12-dihydroindeno[1,2-b]fluoren-2-yl)thiophen-2-yl)
acrylic acid (SC-2). A mixture of 6a (300 mg, 0.31 mmol), cyano-
acetic acid (40 mg, 0.46 mmol), ammonium acetate (8.0 mg) and
acetic acid (10 mL) was heated at 100 ꢂC for 12 h. The solution was
cooled by using crushed ice, the resulting solid was filtered, washed
thoroughly with water, dried and purified through flash chroma-
tography using 2% methanol in dichloromethane and further re-
precipitated from dichloromethane by pouring into methanol to
give a red solid (SC-2) (248 mg, 78%). 1H NMR (acetone-d6,
400 MHz)
d 0.44e1.05 (m, 60H), 1.80e2.11 (m, 8H), 6.95e7.01 (m,
2H), 7.04e7.11 (m, 6H), 7.19e7.25 (m, 4H), 7.57e7.63 (m, 2H), 7.67
(s, 1H), 7.79e7.86 (m, 2H), 7.87e7.90 (m, 1H), 10.03 (s, 1H). FABMS
(m/z): 898 (Mþþ1).
400 MHz):
d 0.50e1.10 (m, 60H), 1.90e2.00 (m, 2H), 2.15e2.25 (m,
4.2.5. 2-Cyano-3-(8-(diphenylamino)-6,6,12,12-tetrakis(2-ethylhexyl)-
6,12-dihydroindeno[1,2-b]fluoren-2-yl)acrylicacid(SC-1). A mixture of
8-(diphenylamino)-6,6,12,12-tetrakis(2-ethylhexyl)-6,12-dihydroin-
deno[1,2-b]fluorene-2-carbaldehyde 5 (200 mg, 0.223 mmol), cya-
noacetic acid (28 mg, 0.334 mmol), ammonium acetate (5.0 mg), and
acetic acid (8 mL) was heated at 100 ꢂC for 12 h. The solution was
then cooled using crushed ice, resulting solid was filtered, washed
thoroughly with water, followed by flash chromatography using
dichloromethane (with 2% methanol) and re-precipitated from
dichloromethane by pouring into methanol to give a red solid
6H), 7.02e7.15 (m, 7H), 7.19e7.24 (m,1H), 7.29e7.35 (m, 4H), 7.76 (s,
1H), 7.81e7.86 (m, 2H), 7.92e8.04 (m, 5H), 8.46 (s, 1H). 13C NMR
(100 MHz, CDCl3):
d 9.84, 10.27, 10.31, 10.66, 13.64, 13.96, 14.16,
16.96, 22.71, 22.77, 26.33, 26.75, 26.97, 27.12, 27.32, 27.76, 27.97,
28.10, 28.28, 28.77, 29.70, 33.32, 33.46, 33.76, 34.56, 34.67, 44.59,
44.76, 45.07, 54.67, 114.59, 115.30, 119.67, 120.21, 120.91, 120.99,
121.32, 121.91, 122.58, 123.73, 123.84, 123.88, 125.70, 129.14, 130.28,
134.50, 136.93, 137.06, 138.65, 139.71, 141.19, 143.77, 146.65, 148.08,
150.15,150.20, 150.52, 151.97,152.04, 152.53,152.63. HRMS (FAB) m/
z [Mþ] calcd for C72H90N2O2S: 1048.6880; found: 1048.6876.

S. Chaurasia et al. / Tetrahedron 68 (2012) 7755e7762
7761
Elemental analysis calcd (%) for C72H90N2O2S$H2O: C, 81.16; H, 8.70;
N, 2.63; found: C, 80.73; H, 8.60; N, 2.79.
electrode. The TiO2 thin film was dipped into the THF solution
containing 3ꢀ10ꢁ4 M dye sensitizers for at least 12 h. After rinsing
with THF, the photoanode adhered with a polyester tape of 60 mm
4.2.8. 5-(8-(Diphenylamino)-6,6,12,12-tetrakis(2-ethylhexyl)-
6,12-dihydroindeno[1,2-b]fluoren-2-yl)furan-2-carbaldehyde
(6b). To a flame dried two neck flask fitted with condenser charged
with 4 (0.50 g, 0.53 mmol), (5-(1,3-dioxolan-2-yl)furan-2-yl)tribu-
tylstannane(0.340 g, 0.79 mmol), Pd(PPh3)2Cl2 (8.0 mg, 0.010 mmol)
and dry DMF 15 mL was added under N2 atmosphere. The mixture
was heated to 80 ꢂC for 24 h. After completion the reaction mixture
was cooled and extracted with ether/brine, and the organic layers
were combined and dried over Na2CO3. Evaporation of the solvent
gave crude dioxolane derivative.
The resulting dioxolane derivative was suspended in glacial
acetic acid and heated to 50 ꢂC. After a clear solution is formed,
1 mL of water was added and the temperature of the mixture was
maintained at 50 ꢂC for 5e6 h. It was then cooled and poured into
crushed ice, orange precipitate formed was formed, filtered and
washed with H2O. The aldehyde product 6b was further purified by
flash chromatography using CH2Cl2/hexane (60:40 by vol) as the
eluent to form an orange solid (326 mg, 64%). 1H NMR (acetone-d6,
in thickness and with a square aperture of 0.36 cm2 was placed on
top of the counter electrode and tightly clipping them together to
form a cell. Electrolyte was then injected into the space and then
sealing the cell with the Torr Seal cement (Varian, MA, USA). The
electrolyte was composed of 0.5 M lithium iodide (LiI), 0.05 M io-
dine (I2), and 0.5 M 4-tert-butylpyridine that was dissolved in
acetonitrile.
4.4. Quantum chemistry computation
The computation were performed with Q-Chem 4.0 software.27
Geometry optimization of the molecules were performed using
hybrid B3LYP functional and 6-31G* basis set. For each molecule,
a number of possible conformations were examined and the one
with the lowest energy was used. The same functional was also
applied for the calculation of excited states using time-dependent
density functional theory (TD-DFT). There exist a number of pre-
vious works that employed TD-DFT to characterize excited states
with charge-transfer character.28 In some cases underestimation of
the excitation energies was seen.29 Therefore, in the present work,
we use TD-DFT to visualize the extent of transition moments as
well as their charge-transfer characters, and avoid drawing con-
clusions from the excitation energy.
400 MHz)
d 0.50e1.10 (m, 60H), 1.94e2.01 (m, 2H), 2.18e2.28 (m,
6H), 7.05e7.16 (m, 7H), 7.21e7.28 (m, 2H), 7.31e7.37 (m, 4H),
7.59e7.60 (m, 1H), 7.82e7.88 (m, 1H), 7.93e8.04 (m, 4H), 8.10e8.15
(m, 1H), 9.71 (s, 1H). 13C NMR (chloroform-d1, 100 MHz)
d 10.07,
10.38, 10.52, 10.80, 10.89, 13.91, 14.20, 23.11, 23.19, 23.33, 27.42,
27.59, 27.65, 27.74, 27.85, 28.05, 28.55, 28.78, 34.11, 34.37, 34.46,
34.78, 34.92, 35.29, 35.35, 44.59, 44.80, 44.92, 45.31, 45.45, 55.32,
55.39, 108.20, 115.61, 115.66, 116.37, 120.48, 121.13, 121.51, 121.63,
121.68, 123.09, 123.19, 123.27, 124.11, 124.29, 124.41, 124.91, 129.87,
139.82, 141.81, 144.34, 147.45, 148.81, 150.88, 152.90, 177.29. FABMS
(m/z): 963 (Mþ).
Acknowledgements
We thank the Institute of Chemistry, Academia Sinica and Na-
tional Science Council, Taiwan, for financial support.
Supplementary data
4.2.9. 2-Cyano-3-(5-(8-(diphenylamino)-6,6,12,12-tetrakis(2-
ethylhexyl)-6,12-dihydroindeno[1,2-b]fluoren-2-yl)furan-2-yl)acrylic
acid (SC-3). A mixture of 6b (250 mg, 0.26 mmol), cyanoacetic acid
(33 mg, 0.34 mmol), ammonium acetate (6 mg), and acetic acid
(10 mL) was heated at 100 ꢂC for 12 h. The solution was cooled by
using crushed ice, the resulting solid was filtered, washed thor-
oughly with water, dried, and purified through flash chromatog-
raphy using 2% methanol in dichloromethane and further re-
precipitated from dichloromethane by pouring into methanol to
give a red solid (190 mg, 72%). 1H NMR (chloroform-d1, 400 MHz):
This material can be found in the online version. Contained
within are the details of quantum chemistry computation and the
1H and 13C NMR spectra. Supplementary data related to this article
can be found online at http://dx.doi.org/10.1016/j.tet.2012.07.045.
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