Fine tuning the performance of DSSCs by variation of the π-spacers in organic dyes that contain a 2,7-diaminofluorene donor

Article abstract of DOI:10.1002/asia.201200752

Organic dyes that contain a 2,7-diaminofluorene-based donor, a cyanoacrylic-acid acceptor, and various aromatic conjugation segments, which are composed of benzene, fluorene, carbazole, and thiophene units, as a π-bridge have been synthesized and characterized by optical, electrochemical, and theoretical investigations. The trends in the absorption and electrochemical properties of these dyes are in accordance with the electron-donating ability of the conjugating segment. Consequently, the dyes that contained a 2,7-carbazole unit in the π-spacer exhibited red-shifted absorption and lower oxidation potentials than their corresponding fluorene- and phenylene-bridged dyes. However, the enhanced power-conversion efficiency that was exhibited by the fluorene-bridged dyes in the DSSCs was attributed to their broader and intense absorption. Despite the longer-wavelength absorption and reasonable optical density, carbazole-bridged dyes exhibited lower power-conversion efficiencies, which were ascribed to the poor alignment of the LUMO level in these dyes, thereby leading to the inhibition of electron injection into the TiO₂ conduction band. The spacer race: Organic dyes that contain a 2,7-diaminofluorene-based donor and a cyanoacrylic-acid acceptor, bridged by various aromatic segments, are efficient sensitizers for DSSCs, owing to their broad and intense absorption profile. Copyright

Full text of DOI:10.1002/asia.201200752

FULL PAPER
DOI: 10.1002/asia.201200752
Fine Tuning the Performance of DSSCs by Variation of the p-Spacers in
Organic Dyes that Contain a 2,7-Diaminofluorene Donor
Abhishek Baheti,[a] K. R. Justin Thomas,*[a] Chuan-Pei Lee,[b] and Kuo-Chuan Ho[b]
Abstract: Organic dyes that contain
2,7-diaminofluorene-based donor,
Consequently, the dyes that contained
a 2,7-carbazole unit in the p-spacer ex-
hibited red-shifted absorption and
lower oxidation potentials than their
corresponding fluorene- and phenyl-
ene-bridged dyes. However, the en-
hanced power-conversion efficiency
that was exhibited by the fluorene-
bridged dyes in the DSSCs was attrib-
uted to their broader and intense ab-
sorption. Despite the longer-wave-
length absorption and reasonable opti-
cal density, carbazole-bridged dyes ex-
hibited lower power-conversion effi-
ciencies, which were ascribed to the
poor alignment of the LUMO level in
these dyes, thereby leading to the in-
hibition of electron injection into the
TiO2 conduction band.
a
a cyanoacrylic-acid acceptor, and vari-
ous aromatic conjugation segments,
which are composed of benzene, fluo-
rene, carbazole, and thiophene units, as
a p-bridge have been synthesized and
characterized by optical, electrochemi-
cal, and theoretical investigations. The
trends in the absorption and electro-
chemical properties of these dyes are
in accordance with the electron-donat-
ing ability of the conjugating segment.
Keywords: density functional calcu-
lations · donor–acceptor systems ·
dyes/pigments
cells
· fluorene · solar
Introduction
found that two factors significantly contribute to a drop in
device efficiency: 1) the formation of dye aggregates and
2) charge recombination between the conduction-band elec-
trons and the oxidized dye or electrolyte. Aggregation at the
TiO2 surface can be hindered by introducing lateral alkyl
chains or other hindering units.[6] The minimization of “dark
current” may be achieved by suitably adjusting the energies
of the dyes and electrolytes.[7] To evolve these methods so as
to minimize the influence of these factors, systematic struc-
ture–property relationship studies are necessary on dyes
that show promising performance in DSSCs.
Organic dyes that contain triarylamine donors and cya-
noacrylic-acid acceptors that are bridged by various aromat-
ic segments, such as oligothiophenes,[8] benzothiadiazoles,[9]
benzotriazoles,[10] etc., have been developed as promising
sensitizers for DSSCs. It has been generally noted that the
incorporation of triarylamines that contain electron-rich
units, such as fluorenes[11] and alkoxy-substituted ben-
zenes,[12] is beneficial for realizing favorable electronic prop-
erties. We have recently found that the introduction of auxil-
iary donors onto fluorene-based triarylamines helped to im-
prove the optical, electrochemical, and photovoltaic proper-
ties of their corresponding dyes.[13] It has also been generally
accepted that a change in the nature of the conjugation
pathway that bridges the donor and acceptor fragments in
the organic dyes is an effective way to fine tune the elec-
tronic properties of the dyes.[1] There is ample evidence in
the literature on the involvement of bridging units in
charge-separation kinetics.[1] Although the coplanar p-conju-
gating segment is beneficial for red-shifting the absorption
wavelength, non-coplanar bridges have enhanced the proba-
bilities of charge separation and charge injection.[14] In addi-
Metal-free organic dyes[1] are considered to be a fascinating
choice for the conversion of solar energy into electricity by
using photoelectrochemical cells, despite the superior per-
formance of ruthenium-based dyes.[2] Ruthenium dyes suffer
from disadvantages such as high cost, lower molar extinction
coefficients, and the need for tedious purification steps in
their synthesis.[2] On the other hand, organic dyes are cheap,
they display larger molar absorption coefficients, and their
chemical modification can be performed by using simple
methods.[1] Most of the currently used organic sensitizers are
constructed from several structural elements, such as
a donor (D) and an acceptor (A) that are bridged by a suita-
ble p-conjugation unit (D–p–A).[1] Apart from this standard
arrangement, in recent years, a number of dyes that featured
other structural formats, such as D–p–D–A,[3] D–D–p–A,[4]
and D–A–p–A,[5] have also been explored in an attempt to
improve the power-conversion efficiency of their corre-
sponding dye-sensitized solar cells (DSSCs). It has been
[a] A. Baheti, Dr. K. R. J. Thomas
Organic Materials Laboratory, Department of Chemistry
Indian Institute of Technology Roorkee
Roorkee–247 667 (India)
Fax : (+91)1332-286202
E-mail: krjt8fcy@iitr.ernet.in
[b] C.-P. Lee, Prof. Dr. K.-C. Ho
Department of Chemical Engineering
National Taiwan University
Taipei 10617 (Taiwan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200752.
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Chem. Asian J. 2012, 7, 2942 – 2954

tion, the introduction of an energy-delocalizing chromo-
phore into the conjugation pathway may cause prolongation
of the excited state and, eventually, could favor electron
ejection. Herein, we have varied the conjugation bridge be-
tween the donor and the acceptor, by incorporating phenyl/
fluorene/carbazole and thiophene units, thereby developing
six new dyes. As an electron-rich unit, fluorene and carba-
zole may improve the redox stability of the dyes and their
dye-regeneration propensities. Carbazole is commonly used
as a donor because of its good thermal, electrochemical sta-
bility, and hole-transporting properties.[15] We used a 2,7-dis-
ubstituted carbazole in the bridge on the assumption that it
would effectively enhance the conjugation and serve as ef-
fective charge-delocalizing segment in comparison to the
3,6-disubsituted carbazole. The newly developed dyes
showed significant enhancement in terms of their optical
and electrochemical properties when compared to their
analogous derivatives that contained a simple phenylene
spacer[13] between the donor and acceptor and, consequently,
served as efficient sensitizers in DSSCs (see above).
Results and Discussion
Synthesis and Characterization
The synthetic pathways that lead to the formation of the tar-
geted dyes are shown in Scheme 1. Fluorenediamine-based
aryl bromides N2-(4-bromophenyl)-9,9-diethyl-N2,N7,N7- tri-
phenyl-9H-fluorene-2,7-diamine
(4a), N2-(7-bromo-9,9-diethyl-
9H-fluoren-2-yl)-9,9-diethyl-
N2,N7,N7-triphenyl-9H-fluorene-
2,7-diamine (4b), and N2-(7-
bromo-9-butyl-9H-carbazol-2-
yl)-9,9-diethyl-N2,N7,N7-triphen-
yl-9H-fluorene-2,7-diamine (4c)
were either synthesized from
9,9-diethyl-N2,N2,N7,N7- tetra-
phenyl-9H-fluorene-2,7-diamine
(3)[16] in a single-step bromina-
tion reaction or from 7-bromo-
9,9-diethyl-N,N-diphenyl-9H-
fluoren-2-amine (1) in two steps
that involved consecutive Pd-
ꢀ
catalyzed C N coupling reac-
tions[17] with aniline and 2,7-di-
bromo-9,9-diethyl-9H-fluorene
or 2,7-dibromo-9-butyl-9H-car-
bazole. These compounds were
then converted into their corre-
sponding substituted thienyl al-
dehydes (5 and 6) through
Stille coupling reactions[18] and
subsequent acid hydrolysis (Scheme 1). Finally, the alde-
hydes were converted into their desired cyanoacrylic-acid
derivatives (7 and 8) through Knoevenagel condensation.[19]
The dyes were thoroughly characterized by 1H and
13C NMR spectroscopy and by MS. These dyes were dark
red in color and were reasonably soluble in common sol-
vents, such as toluene, tetrahydrofuran (THF), EtOAc,
CH2Cl2, MeCN, etc.
Abstract in Hindi:
Photophysical Properties
The absorption spectra of the dyes in CH2Cl2 are presented
in Figure 1. All of the dyes exhibit two prominent absorp-
tion peaks in the regions 374–384 nm and 487–506 nm
(Table 1). The peaks at shorter wavelengths (<400) are as-
signed to localized p–p* transitions that originate from the
diphenylamine, aminofluorene, and p-linker segments,
whilst the peaks at longer wavelengths (>485 nm) are at-
Chem. Asian J. 2012, 7, 2942 – 2954
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2943

K. R. Justin Thomas et al.
FULL PAPER
Scheme 1. Synthesis of dyes 7 and 8.
sponsible for better donor–acceptor interactions when com-
pared to other congeners that have fluorene or carbazole
moieties in their conjugation pathways. The red-shift that is
observed for the carbazole-linked derivatives (8c and 7c)
compared to their fluorene analogues (8b and 7b) is attrib-
uted to the electron-rich nature of the carbazole moiety.
Similarly, a slight red-shift for dyes that contain a bithio-
phene unit in the spacer (8a–8c) when compared to their
analogous monothiophene dyes (7a–7c) is also attributed to
the more electron-rich conjugation pathway, owing to the
presence of an additional thiophene unit.[20] The effect of
donor–acceptor interactions on the longer-wavelength ab-
sorption can be understood by comparing the absorption
wavelengths of the dyes (7 and 8) with that of their precur-
sor bromides (4) and aldehydes (5 and 6; Table 1). For simi-
lar structural organization, the longer-wavelength band is
red-shifted on the introduction of electron-withdrawing al-
dehyde or cyanoacrylic-acid units.
Figure 1. Absorption spectra of dyes 7 and 8 in CH2Cl2.
tributed to a charge-transfer transition mixed with a delocal-
ized p–p* transition. In general, the features of the charge-
transfer band are heavily controlled by the nature of the
electron donor, the electron acceptor, and the linker. Thus,
the peak positions of the lower-energy band in the dyes
follow the order 8a>7a>8c>7c>8b>7b. In an individu-
al series (7 or 8), the charge-transfer band shifts batho-
chromically in the order a>c>b. This shift is probably due
to the shorter linker length in dyes 8a and 7a, which is re-
The molar extinction coefficient at the maximum absorp-
tion of the dyes falls in the range 24100–33200mꢀ1 cmꢀ1,
which is notably larger than those of prominent Ru-based
dyes N3[21] and N719[22] (13900 and 14000mꢀ1 cmꢀ1, respec-
tively). The order of emax for the dyes that possess monothio-
phene and bithiophene units in the linker follows the order
7c>7b>7a and 8c>8b>8a, respectively. This order
agrees fairly well with the general observations that the ex-
tension of conjugation and increase in electron-donating
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Chem. Asian J. 2012, 7, 2942 – 2954

p-Spacers in Organic Dyes that Contain 2,7-Diaminofluorenes
Table 1. Optical and electrochemical properties of the dyes and their precursors.
sured in solvents of different
polarity. A representative ex-
ample is illustrated in Figure 2
and the corresponding data are
compiled in Table 2. All of the
dyes showed negative solvato-
chromism for the longer-wave-
length absorption band and no
significant changes for the
shorter-wavelength band (about
375 nm). The most red-shifted
absorption was observed in
CH2Cl2 and the absorption
peak shifted to shorter wave-
length on increasing the polari-
ty. This result suggests an effec-
tive solvation of the dyes with
increasing solvent polarity. The
unusual red-shift in CH2Cl2,
which caused a deviation from
linearity for a plot of absorp-
tion energy against the Reich-
ardt solvent parameter (ET(30),
Figure 3), probably originates
from the instant stabilization of
polarizable electrons during ex-
citation, which is induced by
the larger polarizability of the
solvent.[24]
Compound labs [nm]
(eꢁ103
[mꢀ1 cmꢀ1])[a]
lem
[nm][b]
Eox [mV]
(DEp [mV])[c]
HOMO
[eV][d]
LUMO
[eV][e]
E0–0
E0–0*
G
N
[eV][f]
[V][g]
ACHTUNGTRENNUNG
4a
4b
4c
5a
375 (50.1),
314 (37.6)
370 (40.2),
310 (27.9)
381 (39.9),
271 (29.6)
420 (26.6),
369 (33.1),
311 (24.1)
420 (37.1),
374 (50.1)
408 (35.6),
375 (49.3),
274 (36.1)
421 (27.1),
374 (40.6)
431 (42.2),
381 (50.8)
434 (39.7),
379 (48.3),
269 (36.1)
500 (24.1),
374 (36.4),
318 (23.7)
487 (24.2),
383 (42.9)
492 (31.0),
380 (45.4),
267 (42.2)
506 (24.6),
379 (35.7)
491 (30.4),
386 (41.1),
311 (22.5)
493 (33.2),
383 (44.6),
263 (38.4)
417
417
421
520
208 (90),
441 (80)
127 (70),
427 (74)
133 (76),
428 (76)
207 (78),
524 (80)
5.008
4.927
4.933
5.007
1.941
1.954
1.870
2.536
3.067
2.973
3.063
2.471
ꢀ2.09
ꢀ2.08
ꢀ2.16
ꢀ1.49
5b
5c
510
433
124 (68),
446 (72)
131 (78),
425 (72)
4.924
4.931
2.484
2.487
2.440
2.444
ꢀ1.55
ꢀ1.54
6a
6b
6c
529
523
522
187 (74),
469 (72)
133 (70),
433 (70)
114 (80),
411 (74)
4.987
4.933
4.914
2.616
2.577
2.576
2.371
2.356
2.338
ꢀ1.41
ꢀ1.45
ꢀ1.45
7a
578
227.5 (91), 5.027
458.0 (84)
2.919
2.108
ꢀ1.11
7b
7c
581
583
158.5 (70), 4.959
449.0 (69)
143.5 (75), 4.940
2.869
2.927
2.089
2.013
ꢀ1.16
ꢀ1.09
419 (72)
8a
8b
599
584
217.0 (78), 5.017
449.5 (89)
164.5 (74), 4.965
411.5 (70)
2.979
2.887
2.038
2.077
ꢀ1.05
ꢀ1.14
The blue-shift in polar sol-
vents may also be contributed
to by the acid–base equilibrium
that is exhibited by the dyes.
We speculate that the carboxyl-
ic-acid unit is partially depro-
tonated in polar solvents that
are capable of abstracting the
acidic hydrogen atom, either by
hydrogen bonding or owing to
8c
578
161 (81),
440 (82),
773
4.961
2.967
1.994
ꢀ1.05
[a] Recorded in CH2Cl2. [b] Recorded in toluene. [c] Potentials are quoted with reference to an internal ferro-
cene standard. [d] HOMO=4.8+Eox. [e] LUMO=HOMOꢀE0–0. [f] Obtained from the optical edge. [g] Excit-
ed-state oxidation potential with reference to NHE.
ability of the donor enhance the transition probability for
the p–p* and charge-transfer transitions, respectively.
2,7-Carbazole-bridged dyes 7c and 8c exhibit higher
molar extinction coefficients than the other dyes in each
series, owing to their electron-rich and rigid 2,7-disubstituted
carbazole framework,[23] which enhances the transition prob-
ability for both the p–p* and charge-transfer transitions.
Lower molar extinction coefficients for dyes 7a and 8a are
attributed to non-planarity in the conjugation pathway,
which retards the charge-transition probability. In between
these two classes, the dyes that contain a bithiophene unit in
the linker (8) show larger emax values when compared to
their monothiophene analogues (7). This result indicates
that the electron-donating ability of the p-bridge in the dyes
plays an important role in increasing their light-harvesting
capability.
To confirm the charge-transfer nature of the lower-energy
absorption band, absorption spectra of the dyes were mea-
Figure 2. Absorption spectra of dye 7c in various solvents.
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K. R. Justin Thomas et al.
FULL PAPER
Table 2. Absorption data for the dyes in various solvents.[a]
Dye
labs [nm] (eꢁ103 [mꢀ1 cmꢀ1])[b]
TOL
TOL+TFA
TOL+TEA
CH2Cl2+TFA
CH2Cl2+TEA
THF
EtOAc
DMF
MeCN[c]
7a
7b
7c
8a
8b
7a
7b
8c
489 (29.6),
375 (41.2),
315 (26.2)
477 (28.0),
383 (38.5)
530 (28.2),
376 (29.9),
314 (24.2)
512 (19.2),
384 (36.5)
434 (32.3),
374 (39.9),
316 (27.6)
428 (32.7),
380 (41.3)
513 (23.7),
373 (37.7),
316 (24.4)
499 (24.8),
383 (41.5)
438 (28.7),
372 (38.3),
315 (27.4)
433 (43.8),
379 (59.3)
470 (32.9),
372 (48.1),
316 (30.7)
458 (32.0),
380 (50.6)
470 (28.2),
370 (38.4),
313 (24.7)
456 (30.7),
378 (46.5)
421 (32.1),
370 (37.9),
312 (26.6)
420 (46.7),
377 (55.6),
312 (29.8)
422 (53.1),
376 (60.4),
310 (32.4)
441 (40.1),
376 (60.4),
315 (26.1)
437 (51.3),
384 (49.9),
310 (26.1)
421 (32.1),
370 (37.9),
312 (26.6)
420 (46.7),
377 (55.6),
312 (29.8)
437 (54.3),
379 (54.2)
477,
368,
315
427,
378
483 (28.5),
382 (48.6)
520 (24.1),
385 (41.6
431 (38.1),
319 (47.9)
504 (33.1),
381 (56.3),
267 (45.1)
515 (26.2),
379 (34.5)
435 (40.5),
377 (54.4),
265 (39.8)
449 (33.5),
378 (41.3)
465 (37.8),
380 (61.1)
461 (34.3),
378 (52.9)
463,
376
497 (25.7),
381 (34.4),
315 (18.9)
484 (34.3),
387 (43.8)
528 (28.2),
381 (28.1),
308 (17.7)
499 (30.9),
387 (42.0)
447 (33.9),
379 (38.6),
318 (22.1)
441 (43.9),
384 (43.8)
478 (29.6),
378 (40.0)
477 (34.9),
376 (45.5)
482,
375
492 (34.4),
386 (41.9),
310 (24.3)
513 (23.7),
373 (37.7),
316 (24.4)
499 (24.8),
383 (41.5)
444 (49.3),
383 (52.9)
468 (44.2),
384 (54.5),
309 (28.1)
470 (32.9),
372 (48.1),
316 (30.7)
458 (32.0),
380 (50.6)
465 (44.1),
382 (52.6),
308 (26.9)
470 (28.2),
370 (38.4),
313 (24.7)
456 (30.7),
378 (46.5)
456,
382,
311
477,
368,
315
489 (29.6),
375 (41.2),
315 (26.2)
477 (28.0),
383 (38.5)
530 (28.2),
376 (29.9),
314 (24.2)
512 (19.2),
384 (36.5)
434 (32.3),
374 (39.9),
316 (27.6)
428 (32.7),
380 (41.3)
438 (28.7),
372 (38.3),
315 (27.4)
433 (43.8),
379 (59.3)
427,
378
455 (30.5),
383 (44.3)
475 (27.5),
384 (45.0)
442 (42.4),
381 (49.3)
502
N
448 (45.1),
381 (49.7)
459 (32.6),
382 (43.7)
446 (29.5),
379 (39.4)
436,
376
380 (41.3)
[a] Abbreviations: toluene (TOL), trifluoroacetic acid (TFA), triethylamine (TEA). [b] Recorded at the concentration of 2.0ꢁ10ꢀ5 m. [c] Molar extinction
coefficient values could not be obtained owing to poor solubility.
Figure 4. Absorption spectra of dye 7c in CH2Cl2 in the presence of TEA
Figure 3. Correlation between the absorption maxima for dye 7c in dif-
or TFA.
ferent solvents with the ET(30) parameter.
the basic nature of the solvent. This abstraction may, in-
turn, impede the donor–acceptor interactions in the dye and
produce a hypsochromic shift of the charge-transfer transi-
tion.[25] This theory is supported by the addition of triethyla-
mine (TEA) or trifluoroacetic acid (TFA) to solutions of
the dyes in toluene/CH2Cl2. The addition of TEA leads to
a significant blue-shift in the absorption, owing to the inef-
fective charge transfer that is caused by deprotonation of
the carboxylic acid, whilst TFA causes a red-shift (Figure 4).
The absorption spectra of the dyes in N,N-dimethylforma-
mide (DMF) displayed a prominent blue-shift for the
charge-transfer transition compared to those in other sol-
vents, which was consistent with the basicity of DMF.
It is interesting to compare the optical properties of dyes
7a and 8a with those of (E)-2-cyano-3-(4-((7-(diphenylami-
no)-9,9-diethyl-9H-fluoren-2-yl)ACHTUNTRGNEUNG(phenyl) amino) phenyl)a-
crylic acid (JD1),[13] which we reported previously. The in-
sertion of a thiophene (7a) or bithiophene moiety (8a) into
the conjugation pathway between the donor and acceptor
triggers a red-shift of 53 and 59 nm, respectively. The effect
of the auxiliary donor group is evident by comparing dye 7a
with (E)-2-cyano-3-(5-(4-(diphenylamino)phenyl)thiophen-
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p-Spacers in Organic Dyes that Contain 2,7-Diaminofluorenes
2-yl)acrylic acid[26] and dye 8a with (E)-2-cyano-3-(5’-(4-(di-
phenylamino)phenyl)-[2,2’-bithiophen]-5-yl)acrylic acid.[27]
The presence of an additional diphenylaminofluorene donor
group leads to hyperchromism and a red-shift of the charge-
transfer transition. Similar effects have also been observed
for the fluorene-conjugated derivatives (7b and 8b) when
compared with their analogous monoamine derivatives. All
of these observations clearly indicate that the extension of
conjugation between the donor and the acceptor, and the in-
troduction of an additional donor in the form of diphenyla-
minofluorene, are beneficial for the light-harvesting proper-
ties.
The dyes are weakly emissive in toluene (Table 1) and the
emission wavelength is insensitive to the nature of the bridg-
ing segment. The emission of the dyes is completely
quenched when dissolved in polar solvents, such as MeCN,
CH2Cl2, etc. This result probably indicates the presence of
a noticeable dipole in the dye molecules, owing to extensive
polarization, which may interact with the solvent dipole and
facilitate nonradiative relaxation of the excited state by an
electron-transfer mechanism.[28]
series (7 and 8). Apparently, the electronic communication
between the amine donor and the cyanoacrylic-acid acceptor
significantly affects the oxidation potential: The stronger in-
teractions result in less-facile oxidation of the aryl amine.
Stronger donor–acceptor interactions were speculated for
dyes 7a and 8a based on their absorption spectra (see
below). Longer conjugating segments, such as fluorene and
carbazole moieties, lessened the interactions between the
amine and the cyanoacrylic acid and cathodically shifted the
oxidation potentials of the corresponding dyes, 7b, 8b, 7c,
and 8c, when compared to the dyes with phenylene linkages
(7a and 8a). The more electron-rich fluorene and carbazole
groups, compared to the phenyl unit, may be also responsi-
ble for this shift. Consequently, carbazole, which is suffi-
ciently electron rich, facilitates the oxidation of dyes 7c and
8c by increasing the electron density over the donor amine
moieties. The oxidation potentials are in the range 0.91–
1.00 V versus the normal hydrogen electrode (NHE) and
the Eox* values for the dyes are between ꢀ1.05 and ꢀ1.19 V
versus NHE. These values are favorable for facile electron
injection from the dye into the conduction band of TiO2 and
for regeneration of the oxidized dyes by the I2/Iꢀ electro-
lyte.[30] The oxidation potentials are significantly lower than
that of the closely related dyes that contain monoamine
donors and clearly reveal the role of the auxiliary
donor.[13,26,27]
The effect of the addition of TFA on the electrochemical
properties of the dyes was also analyzed by CV (Figure 6).
A strong acid, such as TFA, is expected to either shift the
acid–base equilibrium of the dyes toward their acidic form
or to protonate the amine unit. Both effects, in principle,
could make the amines difficult to oxidize. As expected, the
redox potentials that correspond to both oxidations of the
dyes increased upon the addition of TFA to the dye solu-
tions (Figure 6). This result confirms that the charge-transfer
interactions are spread over the entire molecule. However,
the positive shift for the first oxidation potential is less pro-
nounced than that of the second oxidation potential. This
result suggests that the donor amine moiety, which is adja-
Electrochemical Properties
Cyclic voltammetry was carried out to determine the oxida-
tion potentials of the dyes in the ground (Eox) and excited
states (Eox*). The excited-state oxidation potential, Eox*, can
be obtained by subtracting the 0–0 transition energy (E0–0
)
from Eox and the required E0–0 value is derived from the ab-
sorption edge. Two quasi-reversible one-electron oxidations
were detected for the dyes (Figure 5). These oxidation po-
tentials are more positive than the ferrocene/ferrocenium
redox couple.[29] The first oxidation (131.0–227.5 mV) is at-
tributed to the removal of an electron from the auxiliary
donor. The second oxidation wave, at higher potential, is at-
tributed to the oxidation of the amine donor, which is in
direct electronic communication with the cyanoacrylic-acid
acceptor unit. Both oxidations follow a similar trend, owing
to the variation in the nature of the conjugation segment.
The oxidation potentials follow the order c<b<a for both
Figure 6. Cyclic voltammograms of dye 8c in CH2Cl2 before and after the
Figure 5. Cyclic voltammograms of dyes 7 and 8 in CH2Cl2.
addition of TFA.
Chem. Asian J. 2012, 7, 2942 – 2954
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2947

K. R. Justin Thomas et al.
FULL PAPER
Table 3. Computed vertical transition energies, oscillator strengths (f), and their assignment for the dyes.[a]
Dye
Theory
B3LYP (vacuum)
Configuration
MP2K (vacuum)
Configuration
MP2K (THF)
Configuration
lmax [nm]
f
lmax [nm]
f
lmax [nm]
f
7a
7b
7c
8a
8b
587
0.36 HOMO!LUMO (100%) 426
1.03
1.02
0.96
1.26
1.35
HOMO!LUMO (71%), 452
1.18
HOMO!LUMO (67%),
HOMOꢀ1!LUMO (26%)
HOMO!LUMO (70%),
HOMOꢀ1!LUMO (17%)
HOMO!LUMO (71%),
HOMOꢀ1!LUMO (16%)
HOMO!LUMO (64%),
HOMOꢀ1!LUMO (25%)
HOMO!LUMO (54%),
HOMOꢀ1!LUMO (21%),
HOMOꢀ2!LUMO (18%)
HOMO!LUMO (56%),
HOMOꢀ1!LUMO (21%),
HOMOꢀ2!LUMO (13%)
HOMOꢀ1!LUMO (22%)
HOMO!LUMO (73%), 454
HOMOꢀ1!LUMO (16%)
HOMO!LUMO (74%), 449
HOMOꢀ1!LUMO (16%)
HOMO!LUMO (64%), 477
HOMOꢀ1!LUMO (25%)
HOMO!LUMO (61%), 469
HOMOꢀ1!LUMO (19%),
HOMOꢀ2!LUMO (13%)
HOMO!LUMO (63%), 467
HOMOꢀ1!LUMO (19%),
HOMOꢀ2!LUMO (12%)
639
634
645
680
0.30 HOMO!LUMO (99%)
0.27 HOMO!LUMO (99%)
0.38 HOMO!LUMO (99%)
0.28 HOMO!LUMO (99%)
433
428
450
447
1.12
1.13
1.43
1.57
8c
681
0.24 HOMO!LUMO (99%)
445
1.26
1.52
[a] Contributions of less than 10% are omitted.
cent to the cyanoacrylic acid unit, is largely affected by the
addition of TFA. Because it is less basic than the secondary
donor amine, it is safe to conclude that the addition of TFA
leads to the population of the more neutral acidic form of
the dyes in solution.
HOMO and the LUMO, which suggests better donor–ac-
ceptor interactions in the molecules. The higher-wavelength
electronic transitions for the dyes, as predicted theoretically,
have major contributions from the HOMO-to-LUMO elec-
tronic excitation and possess reasonable oscillator strength.
In view of this result, the longer-wavelength absorption for
the dyes may be safely assigned to a charge-transfer transi-
tion. From the electronic distributions of the molecular orbi-
tals that are involved in the shorter-wavelength absorptions
(not shown); these absorptions can be inferred to as origi-
nating from localized p–p* transitions. The peak positions
as predicted by the MPW1K[34] model closely resembled
those observed in THF (Table 3). The experimental trend
for the HOMO (a>b>c) of the dyes is consistent with the
theoretical values and, in turn, supports a progressive
change in the electron richness of the bridging segment.
Electronic Structure of the Dyes
The electronic structures of the dyes were simulated in
a vacuum by using density functional theory (DFT)[31] with
the B3LYP[32] model and the 6-31G** basis set, as imple-
mented in Gaussian 09.[33] The lowest-energy transitions that
were derived from the theoretical computations, along with
their oscillator strengths and their compositions in terms of
molecular orbitals, are listed in Table 3. The electronic dis-
tributions in the HOMO and LUMO for the selected dyes
are shown in Figure 7. The HOMO is mainly spread on the
2,7-diarylaminofluorene unit with a slight delocalization
over the phenyl, fluorene, and carbazole moieties. The
LUMO is mainly constructed from the cyanoacrylic-acid
segment and is contributed to, to some extent, by the thio-
phene moiety. There is clear charge separation between the
Characteristics of the DSSCs
The photovoltaic performance of the newly synthesized dyes
(7 and 8) were evaluated by using the Grꢂtzel photoelectro-
chemical cell.[30] The photovoltaic performance parameters
for the dyes are collected in Table 4. Figure 8 shows the I–V
characteristics and incident photon-to-current conversion ef-
ficiency (IPCE) action spectra for the DSSCs that were
based on these dyes. The IPCE spectra show maximum con-
version efficiencies in the region 400–550 nm. The IPCEs of
dyes 7b and 8b are more than 80% and 70%, respectively,
in the range 400–450 nm. The IPCEs for dyes 7a, 8a, 7c,
and 8c reach 67, 67, 66, and 66.5% at 440 nm, respectively.
The onset IPCE for dyes 7a–8a and 7b–8c starts close to
about 680 nm.
The short-circuit photocurrent density and efficiency for
the devices increase in the order 7c<7a<7b and 8c<8a<
8b. The short-circuit current is related to the molar extinc-
tion coefficient of the dyes. For dyes 7b and 8b, a higher
molar extinction coefficient yields a higher short-circuit cur-
rent. However, for dyes 7c and 8c, the short-circuit current
Figure 7. Energy-level diagram of the frontier molecular orbitals of dyes
7b, 7c, 8b, and 8c.
2948
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Chem. Asian J. 2012, 7, 2942 – 2954

p-Spacers in Organic Dyes that Contain 2,7-Diaminofluorenes
Table 4. Photovoltaic-performance parameters of the dyes.
Dye
VOC [mV]
JSC [mAcmꢀ2
]
ff
h [%]
Rrec [W]
Rct2 [W]
te [ms]
7a
7b
7c
8a
8b
8c
628
668
634
616
642
636
687
12.8
13.7
12.6
13.1
15.1
11.8
19.4
0.60
0.61
0.58
0.61
0.61
0.56
0.63
4.85
5.52
4.62
4.94
5.91
4.20
8.36
20.75
32.50
22.61
15.50
16.50
25.22
31.24
19.43
18.49
19.28
18.68
17.92
20.27
9.05
4.63
33.76
8.41
5.65
8.41
5.65
15.27
N719
Figure 8. a) I–V characteristics and b) IPCE plots for the devices under illumination at 100 mWcmꢀ2
.
is low, irrespective of their high molar extinction coefficient.
This result may be ascribed to an ineffective overlap of the
LUMO level of the dye with the conduction band of TiO2,
which probably retards electron injection.[35] Dyes 7a and 8a
show lower efficiencies than dyes 7b and 8b, owing to the
lower molar coefficients of the former dyes; in addition,
they show broader absorption bands than the latter dyes
when adsorbed onto a TiO2 surface. Thus, the aggregation-
induced quenching in the excited state may lead to a weaker
light-harvesting ability of the dyes.[36]
Electrochemical Impedance Spectroscopy
Because the electronic coupling between the sensitizer and
the TiO2 nanoparticles can alter the quasi-Fermi level of
TiO2, the Voc value can be different for cells that are made
from different sensitizers. To elucidate the effect of the
nature of the p-linker on the Voc value, electrochemical im-
pedance spectroscopy (EIS) measurements were conducted.
Figure 9 shows Nyquist plots for DSSCs that were construct-
ed from the sensitizers under dark conditions. In general,
the Nyquist plots show three semicircles, which are assigned
to the electrochemical reaction at the Pt/electrolyte inter-
face, charge transfer at the TiO2/dye/electrolyte interface,
Figure 9. Nyquist plots for the DSSCs that were fabricated by using the
dyes under dark conditions.
charge-transfer resistance because of the short length of
available Iꢀ diffusion with the thin spacer and owing to the
low viscosity of the solvents that are used in the electrolyte.
The smaller semicircle corresponds to charge-transfer resist-
ance at the counter electrode and the larger semicircle cor-
responds to charge-transfer resistance at the TiO2/dye/elec-
trolyte interface (Rrec). The radius of this semicircle follows
the order 8a<8b<7a<7c<8c<7b, which indicates that
dye 7b possesses the highest charge-recombination resist-
ꢀ
and to the Warburg diffusion process (Iꢀ/I3 ) in the electro-
lyte.[37] In our DSSCs, we only observed two semicircles. We
assume that, in these DSSCs, the conventional diffusion re-
sistance of the redox couple is greatly overlapped by the
Chem. Asian J. 2012, 7, 2942 – 2954
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2949

K. R. Justin Thomas et al.
FULL PAPER
ance, whereas dye 8a has the lowest resistance. This order is
largely in agreement with the trend for Voc (8a<7a<7c<
8c<8b<7b), with the exception of dye 8b. Smaller Rrec
values indicate faster charge recombination and larger dark
current, which result in a comparatively small open-circuit
voltage for dye 8a. The open-circuit voltage in DSSCs is
mainly determined by the conduction-band edge of TiO2,
because the redox potential of the electrolyte is unlikely to
undergo changes between devices that are fabricated under
similar conditions. Factors that can influence the electron
density on the TiO2 surface can significantly affect the Voc
value. The oxidation potential of the dye and the charge-re-
combination resistance can alter the electron density in
TiO2. Dyes that undergo oxidation more easily may increase
the polarizability of the molecule and enhance the electron
density on TiO2. The lowest oxidation potentials, for dyes
7c, 7b, and 8c, support this argument. The reason for the
higher Voc value for dye 8b, despite it having lower charge-
recombination resistance in the device, is unclear.
8c>7a>7c>8a>7b>8b>N719. We expect that a lower
value of electron-transfer resistance would support the elec-
tron collection and, consequently, would play an important
role in increasing the efficiency of a cell. The electron life-
time can be extracted from the angular frequency (wmin) at
the mid-frequency peak in the Bode phase plot (Figure 10b)
[38]
by using te =1/wmin
.
The obtained te values follow the
order 7b>8b>7c>8cꢁ7a>8a, which is consistent with
the order of the Voc values (see above). The increase in the
electron lifetimes for the devices that are based on fluorene-
containing dyes (7b and 8b) suggests a more effective sup-
ꢀ
pression of the back reaction of injected electrons with I3
ions in the electrolyte, which leads to an improvement in
both the Jsc and Voc values, as well as to a substantial en-
hancement in the PCE of the DSSC. This result may suggest
that the presence of the fluorene moiety in the spacer is
beneficial for electronic coupling and for the suppression of
back-electron transfer when compared to the phenyl/carba-
zole moieties.
Upon illumination at 100 mWcmꢀ2 under open-circuit
conditions, from the radius of the intermediate-frequency
semicircle in the Nyquist plot (Figure 10a), we found that
the electron-transport resistance (Rct2) followed the order
Conclusions
We have developed several new organic dyes that contain
a fluorene-2,7-diamine-based donor and a cyanoacrylic-acid
acceptor; these units were bridged by various aromatic con-
jugated segments that were composed of benzene, fluorene,
carbazole, and thiophene units. These dyes were character-
ized by optical spectroscopy, cyclic voltammetry, and
TDDFT computations. The absorption and electrochemical
properties of the dyes were in accordance with the electron-
donating ability of the conjugating segment. Consequently,
the 2,7-carbazole-bridged dyes exhibited red-shifted absorp-
tions and lower oxidation potentials than their correspond-
ing fluorene- and phenylene-bridged dyes. The enhanced
power-conversion efficiencies that were observed for the flu-
orene-bridged dyes were attributed to their broader and
more intense absorption profiles. Their enhanced optical
properties contributed to the increase in their photocurrent
density and their comparatively larger HOMO value facili-
tated the regeneration of the dyes by the electrolyte, as well
as suppressing the back reaction of the injected electrons
with the electrolyte constituents. Despite the longer-wave-
length absorption and reasonable optical density that was
observed for the carbazole-bridged dyes, their relatively
lower power-conversion efficiencies were ascribed to the
poor alignment of the LUMO level of the dye, which proba-
bly inhibited the electron injection into the TiO2 conduction
band. The success of this design strategy to improve device
performance indicates that more detailed studies on the
structure–property relationships in the organic dyes is essen-
tial to understand the more complicated physicochemical
processes that are involved in DSSCs and to optimize them
to increase their power-conversion efficiency.
Figure 10. a) Nyquist and b) Bode phase plots for the devices that were
fabricated by using dyes under illumination at 100 mWcmꢀ2
.
2950
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Chem. Asian J. 2012, 7, 2942 – 2954

p-Spacers in Organic Dyes that Contain 2,7-Diaminofluorenes
CH2Cl2. The combined organic layer was dried over anhydrous sodium
sulfate and evaporated under vacuum to afford the crude product, which
was adsorbed onto neutral alumina and purified by column chromatogra-
phy (hexanes/CH2Cl2). The desired bromo derivative was collected as the
second fraction after recovery of the unreacted excess aryldibromide.
Experimental Section
General Methods
All reactions and manipulations were carried out under a N2 atmosphere
by using standard Schlenk techniques. Solvents were dried according to
standard procedures. Column chromatography was performed on either
100–200 mesh silica gel or on neutral alumina as the stationary phase
(column length: 30 cm, diameter: 2.0 cm). 1H and 13C NMR spectroscopy
were measured on a 500 MHz spectrometer (Bruker). MS was recorded
in positive-ion mode on an ESI TOF high-resolution mass spectrometer
(Bruker Daltonics). Cyclic voltammetry (CH Instruments) was carried
out at RT on a conventional three-electrode configuration that consisted
of a glassy carbon working electrode, a platinum-wire auxiliary electrode,
and a non-aqueous Ag/AgNO3 reference electrode. E1/2 values were de-
N2-(7-Bromo-9,9-diethyl-9H-fluoren-2-yl)-9,9-diethyl-N2,N7,N7-triphenyl-
9H-fluorene-2,7-diamine (4b)
Compound 4b was synthesized from 2,7-dibromo-9,9-diethyl-9H-fluorene
and 9,9-diethyl-N2,N2,N7-triphenyl-9H-fluorene-2,7-diamine according to
the general procedure described above. White solid; yield: 1.11 g (57%);
m.p. 124–1268C; 1H NMR (500 MHz, CDCl3): d=7.39–7.53 (m, 6H),
7.22–7.23 (m, 6H), 6.97–7.15 (m, 15H), 1.77–1.92 (m, 8H), 0.33–0.39 ppm
(m, 12H); 13C NMR (125.77 MHz, CDCl3): d=151.7, 139.4, 130.2, 126.2,
121.5, 121.1, 56.7, 32.6, 8.4 ppm; HRMS (ESI): m/z calcd for C52H47BrN2:
779.2995 [M+H]+; found: 779.2996.
c
a
c
termined as 1/2ACHTUNGTRENNUNG
(Epa+Ep ), where Ep and Ep are the anodic and cathodic
peak potentials, respectively; the potentials are quoted against an inter-
nal ferrocene standard. CH2Cl2 was used as the solvent in all of the ex-
periments and the supporting electrolyte was 0.1m tetrabutylammonium
perchlorate. Electronic absorption spectra were obtained on a UV/Vis
spectrophotometer (Shimadzu) by using freshly prepared solutions.
N2-(7-Bromo-9-butyl-9H-carbazol-2-yl)-9,9-diethyl-N2,N7,N7-triphenyl-9H-
fluorene-2,7-diamine (4c)
Compound 4c was synthesized from 2,7-dibromo-9-butyl-9H-carbazole
and 9,9-diethyl-N2,N2,N7-triphenyl-9H-fluorene-2,7-diamine according to
the general procedure described above. White solid; yield: 1.28 g (66%);
m.p. 140–1428C; 1H NMR (500 MHz, CDCl3): d=7.87 (d, J=8.5 Hz,
1H), 7.82 (d, J=8.5 Hz, 1H), 7.49–7.52 (m, 2H), 7.46 (d, J=1.0 Hz, 1H),
7.26–7.30 (m, 4H), 7.23–7.25 (m, 3H), 7.17 (d, J=8.5 Hz, 2H), 7.07–7.14
(m, 8H), 6.98 (m, 5H), 4.06 (t, J=7.0 Hz, 2H), 1.81 (dd, J=14.5 Hz,
7.5 Hz, 4H), 1.71 (t, J=7.5 Hz, 2H), 1.29 (dd, J=15.5 Hz, 7.5 Hz, 2H),
0.85–0.88 (m, 3H), 0.39 ppm (t, J=7.5 Hz, 6H); 13C NMR (125 MHz,
CDCl3): d=151.32, 151.25, 148.5, 148.1, 147.0, 146.8, 146.6, 141.8, 141.7,
136.7, 136.7, 129.2, 123.9, 122.4, 122.1, 122.0, 120.83, 120.76, 119.80, 119.6,
119.5, 118.3, 117.7, 116.9, 111.6, 104.2, 56.1, 42.8, 32.6, 31.0, 20.5, 13.8,
8.6 ppm; HRMS (ESI): m/z calcd for C51H46BrN3: 780.2948 [M+H]+;
found: 780.2945.
Synthesis of 9,9-diethyl-N2,N2,N7-triphenyl-9H-fluorene-2,7-diamine (2)
In
amine (3.3 g, 7.0 mmol) and aniline (0.72 g, 7.7 mmol) were mixed with
sodium tert-butoxide (1.2 g, 12.4 mmol), [Pd(dba)2] (dba=(1E,4E)-1,5-di-
a pressure tube, 7-bromo-9,9-diethyl-N,N-diphenyl-9H-fluoren-2-
AHCTUNGTRENNUNG
phenylpenta-1,4-dien-3-one; 100 mg, 0.18 mmol), and 1,2-bis(diphenyl-
phosphino)ferrocene (dppf; 100 mg, 0.18 mmol) in toluene (10 mL)
under a nitrogen atmosphere. The mixture was heated at 808C for 36 h.
After the completion of the reaction, the volatile compounds were re-
moved by evaporation. The residue was triturated with water and extract-
ed with CH2Cl2. The combined organic layer was dried over anhydrous
sodium sulfate and evaporated under vacuum to produce a crude prod-
uct, which was adsorbed onto neutral alumina and purified by column
chromatography (hexanes/CH2Cl2). White solid; yield: 1.72 g (51%);
m.p. 122–1248C; 1H NMR (500 MHz, CDCl3): d=7.52 (d, J=8.0, 1H),
7.24–7.31 (m, 7H), 6.93–7.17 (m, 13H), 5.30 (s, 1H), 1.85–1.92 (m, 4H),
0.39–0.45 ppm (m, 6H); 13C NMR (125 MHz, CDCl3): d=151.4, 151.2,
147.2, 146.1, 145. 9, 136.8, 132.07, 132.05, 129.3, 129.1, 124.7, 124.7, 123.9,
123.7, 122.9, 122.9, 122.4, 120.0, 119.9, 119.6, 119.4, 114.4, 114.3, 56.0,
32.5, 8.6 ppm; HRMS (ESI): m/z calcd for C35H32N2: 480.2565 [M+H]+;
found: 480.2560.
5-(4-((7-(Diphenylamino)-9,9-diethyl-9H-fluoren-2-
yl)ACHTNUTRGNEUNG(phenyl)amino)phenyl)-thiophene-2-carbaldehyde (5a)
A
solution of (5-(1,3-dioxolan-2-yl)thiophen-2-yl)tributylstannane
(0.65 mmol) and N2-(4-bromophenyl)-9,9-diethyl-N2,N7,N7-triphenyl-9H-
fluorene-2,7-diamine (4a; 0.35 g, 0.55 mmol) in dry DMF (4 mL) was de-
gassed with nitrogen, followed by the addition of bis(triphenylphosphi-
ne)dichloropalladium(II) ([PdACHTNUGTRENUNG(PPh3)2Cl2]; 4 mg, 1 mol%). The reaction
mixture was heated at 808C for 24 h under a nitrogen atmosphere. After
that, the mixture was poured into water and extracted with CH2Cl2. The
organic layer was washed with brine and water and dried over anhydrous
Na2SO4. The solvent was evaporated and the solid residue was dissolved
in glacial acetic acid (5 mL). The solution was stirred for 30 min at 608C,
then water (10 mL) was added. Heating was continued for a further 6 h.
After cooling to RT, water was added and the mixture was extracted with
CH2Cl2. The organic layer was washed with a liberal amount of water
and dried over anhydrous Na2SO4. After removal of the solvent under
vacuum, the residue was purified by column chromatography on silica gel
(hexanes/CH2Cl2). Orange solid; yield: 0.165 g (45%); m.p. 117–1208C;
1H NMR (500 MHz, CDCl3): d=9.86 (s, 1H), 7.71 (d, J=3.5 Hz, 1H),
7.50–7.54 (m, 4H), 7.27–7.34 (m, 4H), 7.23–7.25 (m, 2H), 6.97–7.17 (m,
16H), 1.83 (dd, J=14.0 Hz, 7.0 Hz, 4H), 0.39 ppm (t, J=7.0 Hz, 6H);
13C NMR (125 MHz, CDCl3): d=182.6, 154.7, 151.6, 151.4, 149.4, 148.1,
147.2, 146.9, 145.5, 141.3, 137.8, 136.4, 132.2, 129.5, 129.2, 127.3, 125.9,
124.9, 124.8, 124.7, 124.7, 124.1, 123.82, 123.75, 123.7, 123.6, 122.8, 122.5,
122.2, 122.1, 120.4, 120.1, 119.4, 56.2, 56.1, 32.6, 8.6 ppm; IR (KBr): n˜ =
1658 cmꢀ1 (C=O); HRMS (ESI): m/z calcd for C46H38N2OS: 667.2778
[M+H]+; found: 667.2782.
Synthesis of N2-(4-bromophenyl)-9,9-diethyl-N2,N7,N7-triphenyl-9H-
fluorene-2,7-diamine (4a)
In a round-bottomed flask, 9,9-diethyl-N2,N2,N7,N7-tetraphenyl-9H-fluo-
rene-2,7-diamine (1.0 g, 1.79 mmol) and N-bromosuccinimide (0.36 g,
1.87 mmol) were dissolved in CCl4 (15 mL). This mixture was heated at
reflux for 4 h, cooled to RT, and filtered. The solvent was evaporated
from the filtrate and the solid was recrystallized from EtOH. White
solid; yield: 0.75 g (65%); m.p. 118–1208C; 1H NMR (500 MHz, CDCl3):
d=7.40–7.42 (m, 2H), 7.20–7.26 (m, 3H), 7.16–7.18 (m, 5H), 6.87–7.03
(m, 15H), 1.71–1.74 (m, 4H), 0.26–0.30 ppm (m, 6H); 13C NMR
(125 MHz, CDCl3): d=151.4, 151.2, 147.5, 147.2, 146.7, 146.1, 145.9,
137.1, 136.8, 132.2, 132.07, 132.05, 129.3, 129.1, 124.7, 124.7, 123.9, 123.9,
123.7, 122.94, 122.87, 122.4, 120.0, 119.9, 119.7, 119.6, 119.4, 114.4, 114.3,
56.1, 56.0, 32.5, 8.6 ppm; HRMS (ESI): m/z calcd for C41H35BrN2:
635.2056 [M+H]+; found: 635.2056.
General Synthesis of the Bromo Derivatives of the Diamines (4b and 4c)
An appropriate aryldibromide (2,7-dibromo-9,9-diethyl-9H-fluorene or
2,7-dibromo-9-butyl-9H-carbazole; 5.0 mmol) was mixed with sodium
tert-butoxide (0.72 g, 7.5 mmol), [PdACHTNUTRGNENG(U dba)2] (dba=(1E,4E)-1,5-diphenyl-
5-(7-((7-(Diphenylamino)-9,9-diethyl-9H-fluoren-2-yl)
ACHTUNGTREN(NUNG phenyl)amino)-9,9-
penta-1,4-dien-3-one; 29 mg, 0.05 mmol), 1,2-bis(diphenylphosphino)fer-
rocene (dppf; 28 mg, 0.05 mmol), and 9,9-diethyl-N2,N2,N7-triphenyl-9H-
fluorene-2,7-diamine (1.20 g, 2.5 mmol) in toluene (10 mL) under a nitro-
gen atmosphere. This mixture was heated at 808C for 36 h. After the
completion of the reaction, the volatile compounds were removed by
evaporation. The residue was triturated with water and extracted with
diethyl-9H-fluoren-2-yl)thiophene-2-carbaldehyde (5b)
Compound 5b was synthesized according to a procedure that was similar
to that of compound 5a, except that compound 4b was used in the place
of compound 4a. Orange solid; yield: 0.28 g (62%); m.p. 138–1418C;
1H NMR (500 MHz, CDCl3): d=9.89 (s, 1H), 7.75 (d, J=4.0 Hz, 1H),
Chem. Asian J. 2012, 7, 2942 – 2954
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
2951

K. R. Justin Thomas et al.
FULL PAPER
7.63–7.67 (m, 2H), 7.56–7.59 (m, 2H), 7.49–7.52 (m, 2H), 7.45 (d, J=
4.0 Hz, 1H), 7.26–7.30 (m, 2H), 7.23–7.26 (m, 4H), 7.16 (d, J=7.5 Hz,
2H), 6.98–7.12 (m, 13H), 1.96–2.03 (m, 2H), 1.77–1.92 (m, 6H), 0.36–
0.40 ppm (m, 12H); 13C NMR (125 MHz, CDCl3): d=182.6, 151.6, 151.2,
151.0, 150.6, 148.1, 147.9, 146.44, 146.36, 143.0, 141.6, 137.4, 136.6, 136.5,
130.6, 129.1, 129.0, 125.5, 123.9, 123.7, 123.5, 123.4, 122.6, 122.5, 122.2,
120.6, 120.4, 119.7, 119.4, 119.1, 118.0, 56.1, 56.0, 32.6, 32.5, 8.5 ppm; IR
(KBr): n˜ =1636 cmꢀ1 (C=O); HRMS (ESI): m/z calcd for C57H50N2OS:
811.3717 [M+H]+; found: 811.3716.
1H NMR (500 MHz, CDCl3): d=9.87 (s, 1H), 7.98 (d, J=7.0 Hz, 1H),
7.89–7.92 (m, 1H), 7.69 (d, J=5.5 Hz, 1H), 7.47–7.52 (m, 4H), 7.35–7.38
(m, 1H), 7.27–7.29 (m, 3H), 7.23–7.26 (m, 4H), 7.18 (d, J=8.5 Hz, 1H),
7.07–7.15 (m, 9H), 6.98–7.03 (m, 5H), 4.14–4.17 (m, 2H), 1.75–1.84 (m,
6H), 0.87–0.90 (m, 5H), 0.39–0.42 ppm (m, 6H); 13C NMR (125 MHz,
CDCl3): d=182.5, 151.3, 151.2, 148.4, 148.1, 147.8, 147.5, 147.1, 146.8,
146.6, 142.4, 141.3, 141.2, 137.5, 136.7, 136.6, 134.5, 130.0, 129.2, 127.3,
123.9, 123.8, 123.8, 123.7, 123.3, 122.9, 122.4, 120.9, 120.1, 119.8, 119.6,
119.5, 117.9, 117.5, 116.8, 105.7, 104.0, 56.1,42.7, 32.6, 31.1, 20.5, 13.9,
8.6 ppm; IR (KBr): n˜ =1660 cmꢀ1 (C=O); HRMS (ESI): m/z calcd for
C60H51N3OS2: 893.3474 [M+H]+; found: 893.3439.
5-(9-Butyl-7-((7-(diphenylamino)-9,9-diethyl-9H-fluoren-2-
yl)ACHTUNGTRENNUNG(phenyl)amino)-9H-carbazol-2-yl)thiophene-2-carbaldehyde (5c)
(E)-2-Cyano-3-(5-(4-((7-(diphenylamino)-9,9-diethyl-9H-fluoren-2-
Compound 5c was synthesized according to a procedure that was similar
to that of compound 5a, except that compound 4c was used in the place
of compound 4a. Orange solid; yield: 0.25 g (55%); m.p. 138–1408C;
1H NMR (500 MHz, CDCl3): d=9.90 (s, 1H) 7.99 (d, J=8.0 Hz, 1H),
7.91 (d, J=8.5 Hz, 1H), 7.73 (d, J=4.0 Hz, 1H), 7.59 (d, J=1.0 Hz, 1H),
7.49–7.54 (m, 4H), 7.28–7.29 (m, 2H), 7.23–7.26 (m, 4H), 6.98–7.12 (m,
13H), 4.15 (t, J=7.0 Hz, 3H), 1.80–1.84 (m, 3H), 1.74–1.77 (m, 2H),
1.29–1.32 (m, 2H), 0.86–0.90 (m, 4H), 0.38–0.41 ppm (m, 6H); 13C NMR
(125 MHz, CDCl3): d=182.7, 156.1, 151.4, 151.3, 148.3, 148.1, 147.5,
146.7, 146.62, 142.59, 141.8, 141.1, 137.6, 136.8, 136.6, 129.4, 129.2, 124.3,
124.0, 123.9, 123.7, 123.7, 122.6, 122.4, 121.1, 120.2, 119.8, 119.60, 119.55,
117.8, 117.7, 116.8, 106.4, 103.8, 56.1, 42.7, 32.6, 31.2, 20.5, 13.8, 8.6 ppm;
IR (KBr): n˜ =1660 cmꢀ1 (C=O); HRMS (ESI): m/z calcd for
C56H49N3OS: 811.3596 [M+H]+; found: 811.3592.
yl)
ACHTUNGTREN(NUGN phenyl)amino) phenyl)thiophen-2-yl)acrylic acid (7a)
5-(4-((7-(diphenylamino)-9,9-diethyl-9H-fluoren-2-yl)-
A
(5a;
0.10 g,
0.15 mmol), cyanoacetic acid (0.019 g, 0.22 mmol), acetic acid (5 mL),
and ammonium acetate (4 mg) were mixed together and heated at reflux
for 12 h. The resulting red solution was poured into ice-cold water to pro-
duce an orange precipitate. This precipitate was filtered, washed thor-
oughly with water, and dried. The solid was further recrystallized with
CHCl3. Red-brown solid; yield: 0.089 g (81%); m.p. 168–1708C;
1H NMR (500 MHz, [D6]DMSO): d=8.43 (s, 1H), 7.95 (d, J=4.5 Hz,
1H), 7.67–7.73 (m, 4H), 7.63 (d, J=4.5 Hz, 1H), 7.35–7.38 (m, 2H),
7.28–7.31 (m, 3H), 6.91–7.17 (m, 16H), 1.77–1.86 (m, 4H), 0.29 ppm (t,
J=7.0 Hz, 6H); 13C NMR (125 MHz, CDCl3): d=163.0, 156.2, 154.5,
151.4, 149.8, 148.0, 146.4, 145.6, 145.0, 140.5, 136.3, 134.2, 132.2, 130.4,
129.5, 129.4, 129.2, 127.8, 127.5, 125.0, 124.9, 124.1, 123.8, 122.5, 121.7,
5’-(4-((7-(Diphenylamino)-9,9-diethyl-9H-fluoren-2-
yl)ACHTUNGTRENNUNG(phenyl)amino)phenyl)-2,2’-bithiophene-5-carbaldehyde (6a)
120.4, 120.0, 119.5, 56.2, 32.6, 8.6 ppm; IR (KBr): n˜ =2217 cmꢀ1 (C N);
ꢂ
HRMS (ESI): m/z calcd for C49H39N3O2S: 734.2836 [M+H]+; found:
Compound 6a was prepared from the reaction between (5’-(1,3-dioxolan-
2-yl)-2,2’-bithiophen-5-yl)tributylstannane and N2-(4-bromophenyl)-9,9-
diethyl-N2,N7,N7-triphenyl-9H-fluorene-2,7-diamine (4a) according to the
procedure described above for compound 5a. The residue was purified
by column chromatography on neutral alumina (hexanes/CH2Cl2).
734.2846.
(E)-2-Cyano-3-(5-(7-((7-(diphenylamino)-9,9-diethyl-9H-fluoren-2-
yl)ACTHNUTRGNEUG(N phenyl)amino)-9,9-diethyl-9H-fluoren-2-yl)thiophen-2-yl)acrylic acid
(7b)
1
Orange solid; yield: 0.206 g (50%); m.p. 140–1428C; H NMR (500 MHz,
CDCl3): d=10.00 (d, J=5.5 Hz, 1H), 7.81–7.82 (m, 1H), 7.61–7.69 (m,
4H), 7.39–7.52 (m, 10H), 7.28–7.34 (m, 5H), 7.12–7.25 (m, 9H), 1.98–
1.99 (m, 4H), 0.53–0.56 ppm (m, 6H); 13C NMR (125 MHz, CDCl3): d=
182.4, 154.5, 151.3, 148.2, 149.1, 147.6, 147.5, 147.2, 146.8, 146.4, 146.2,
146.0, 145.8, 141.3, 137.5, 137.4, 137.3, 136.4, 134.0, 132.1, 129.4, 129.2,
128.4, 127.3, 127.1, 126.9, 126.8, 126.6, 126.2, 124.8, 124.4, 124.4, 124.3,
124.0, 123.9, 123.8, 123.7, 123.3, 123.2, 123.2, 123.1, 123.0, 122.8, 122.7,
122.4, 120.1, 119.9, 119.6, 119.5, 114.5, 56.2, 32.6, 8.6 ppm; IR (KBr): n˜ =
1659 cmꢀ1 (C=O); HRMS (ESI): m/z calcd for C50H40N2OS2: 749.2655
[M+H]+; found: 749.2655.
Compound 7b was prepared from compound 5b according to a procedure
that was similar to that described above for compound 7a. Red-brown
solid; yield: 0.11 g (85%); m.p. 260–2628C; 1H NMR (500 MHz,
[D6]DMSO): d=8.48 (s, 1H), 8.01 (d, J=4.0 Hz, 1H), 7.76–7.84 (m, 4H),
7.67 (dd, J=8.0 Hz, 3.0 Hz, 1H), 7.27–7.35 (m, 6H), 7.08–7.11 (m, 4H),
7.05–7.06 (m, 2H), 6.98–7.03 (m, 8H), 6.95 (dd, J=8.5 Hz, 2.0 Hz, 1 H),
2.03–2.08 (m, 2H), 1.75–1.88 (m, 6H), 0.26–0.31 ppm (m, 6H); 13C NMR
(125 MHz, [D6]DMSO): d=163.6, 153.6, 121.3, 150.8, 150.7, 150.4, 147.7,
147.44, 147.37, 146.4, 146.1, 142.5, 141.3, 136.2, 136.1, 134.7, 134.2, 130.4,
129.5, 129.4, 125.7, 125.1, 124.9, 1123.6, 123.4, 123.1, 122.9, 122.6, 122.3,
121.4, 120.3, 120.2, 120.0, 116.2, 118.7, 117.6, 116.6, 55.8, 55.5, 31.8, 31.8,
5’-(7-((7-(Diphenylamino)-9,9-diethyl-9H-fluoren-2-yl)ACTHNUTRGNE(UNG phenyl)amino)-
9,9-diethyl-9H-fluoren-2-yl)-2,2’-bithiophene-5-carbaldehyde (6b)
8.5, 8.4 ppm; IR (KBr): n˜ =2221 cmꢀ1 (C N); HRMS (ESI): m/z calcd
ꢂ
for C60H51N3O2S: 878.3775 [M+H]+; found: 878.3779.
Compound 6b was synthesized according to a procedure that was similar
to that for compound 6a, except that compound 4b was used in the place
of compound 4a. Red-orange solid; yield: 0.32 g (65%); m.p. 190–1928C;
1H NMR (500 MHz, CDCl3): d=9.87 (d, J=0.5 Hz, 1H), 7.69 (dd, J=
4.0 Hz, 0.5 Hz, 1H), 7.56–7.63 (m, 3H), 7.50–7.52 (m, 3H), 7.32–7.33 (m,
1H), 7.26–7.29 (m, 3H), 7.23–7.25 (m, 4H), 7.16 (d, J=8.5 Hz, 2H),
6.93–7.12 (m, 13H), 2.00 (dd, J=14.0 Hz, 7.0 Hz, 2H), 1.79–1.91 (m,
6H), 0.37–0.40 ppm (m, 12H); 13C NMR (125 MHz, CDCl3): d=182.3,
159.0, 151.2, 150.5, 147.9, 146.4, 141.9, 141.3, 137.4, 136.5, 134.3, 131.2,
129.1, 127.0, 124.8, 123.7, 123.7, 122.4, 122.2, 119.8, 119.6, 119.4, 119.3,
119.0, 118.3, 56.2, 55.9, 32.6, 32.5, 8.50, 8.48 ppm; IR (KBr): n˜ =1662 cmꢀ1
(C=O); HRMS (ESI): m/z calcd for C61H52N2OS2: 893.3594 [M+H]+;
found: 893.3591.
(E)-3-(5-(9-butyl-7-((7-(diphenylamino)-9,9-diethyl-9H-fluoren-2-
yl)ACTHNUTRGNEUG(N phenyl)amino)-9H-carbazol-2-yl)thiophen-2-yl)-2-cyanoacrylic acid
(7c)
Compound 7c was prepared from compound 5c according to a procedure
that was similar to that described above for compound 7a. Red solid;
yield: 0.108 g (82%); m.p. 210–2128C; 1H NMR (500 MHz, [D6]DMSO):
d=8.49 (s, 1H), 8.12 (d, J=8.0 Hz, 1H), 8.04–8.07 (m, 2H), 7.98 (s, 1H),
7.90 (d, J=4.5 Hz, 1H), 7.67 (dd, J=8.5 Hz, 5.0 Hz, 2H), 7.57 (dd, J=
8.0 Hz, 1.5 Hz, 1H), 7.27–7.33 (m, 6H), 7.16 (d, J=1.5 Hz, 1H), 7.13 (d,
J=1.5 Hz, 1H), 7.10 (d, J=7.5 Hz, 2H), 7.05–7.06 (m, 2H), 6.99–7.03 (m,
7H), 6.95 (dd, J=8.0 Hz, 2.0 Hz, 1H), 6.90 (dd, J=8.5 Hz, 1.5 Hz, 1H),
4.27 (t, J=6.5 Hz, 2H), 1.79 (dd, J=14.5 Hz, 7.0 Hz, 6H), 1.64 (t, J=
7.0 Hz, 2H), 1.19 (dd, J=15.0 Hz, 7.5 Hz, 2H), 0.78 (t, J=7.0 Hz, 3H),
0.29–0.32 ppm (m, 6H); 13C NMR (125 MHz, [D6]DMSO): d=163.7,
158.7,154.5, 150.8, 150.8, 147.7, 147.4, 146.8, 146.4, 146.2, 146.0, 142.1,
141.2, 140.8, 136.2, 136.1, 134.0, 129.4, 128.8, 124.9, 123.8, 123.6, 123.4,
123.1, 122.7, 122.6, 121.5, 120.5, 120.3, 119.1, 117.7, 117.2, 116.7, 116.0,
106.5, 103.5, 55.5, 54.8, 38.9, 30.5, 19.6, 13.5, 8.4 ppm; IR (KBr): n˜ =
5’-(9-Butyl-7-((7-(diphenylamino)-9,9-diethyl-9H-fluoren-2-
yl)ACHTUNGTRENNUNG(phenyl)amino)-9H-carbazol-2-yl)-2,2’-bithiophene-5-carbaldehyde
(6c)
Compound 6c was synthesized according to a procedure that was similar
to that for compound 6a, except that compound 4c was used in the place
of compound 4a. Orange solid; yield: 0.17 g (35%); m.p. 108–1108C;
2952
www.chemasianj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 2942 – 2954

p-Spacers in Organic Dyes that Contain 2,7-Diaminofluorenes
2216 cmꢀ1 (C N); HRMS (ESI): m/z calcd for C59H50N4O2S: 878.3654
Fabrication and Characterization of the DSSCs
ꢂ
[M+H]+; found: 878.3652.
The DSSCs were fabricated and characterized according to procedures
that we have reported previously.[13] The dye-sensitized photoanodes
were formed by immersing the TiO2-coated ITO plates (about 15 mm
thickness and 20 nm particle size TiO2 layer/4 mm scattering TiO2 layer)
in solutions of the dyes in a mixed-solvent system (MeCN/tBuOH/
DMSO, 3.5:3.5:3 v/v). The electrolyte was composed of 0.1m LiI, 0.6m 1-
propyl-2,3-dimethylimidazolium iodide (DMPII), 0.05m I2, and 0.5m tert-
butylpyridine (TBP) in MeCN/3-methoxypropionitrile (MPN, 1:1 v/v).
The absorption spectra of the dye-coated TiO2 films were obtained on
a UV/Vis spectrophotometer (JASCO) that was equipped with an inte-
grating sphere accessory. The baseline was corrected by using a bare-
TiO2-coated FTO substrate.
(E)-2-Cyano-3-(5’-(4-((7-(diphenylamino)-9,9-diethyl-9H-fluoren-2-
yl)ACHTUNGTRENNUNG(phenyl)amino)phenyl)-2,2’-bithiophen-5-yl)acrylic acid (8a)
Compound 8a was prepared from compound 6a according to a procedure
that was similar to that described above for compound 7a. Red-brown
solid; 0.097 g (80%); m.p. 226–2288C; 1H NMR (500 MHz, CDCl3): d=
8.47 (s, 1H), 7.96 (d, J=4.0 Hz, 1H), 7.66–7.71 (m, 2H), 7.61–7.63 (m,
3H), 7.57 (d, J=4.0 Hz, 1H), 7.49 (d, J=3.5 Hz, 1H), 7.27–7.36 (m, 6H),
6.96–7.13 (m, 14H), 6.90–6.95 (m, 1H), 1.79–1.82 (m, 4H), 0.28–0.31 ppm
(m, 6H); 13C NMR (125 MHz, CDCl3): d=164.3, 157.2, 153.7, 152.5,
151.5, 147.7, 144.9, 143.7, 141.8, 140.4, 138.7, 136.9, 135.2, 133.9, 132.1,
129.3, 129.2, 128.1, 126.6, 124.8, 124.4, 124.0, 123.8, 122.7, 122.4, 120.2,
119.5, 56.1, 32.6, 8.6 ppm; IR (KBr): n˜ =2217 cmꢀ1 (C N); HRMS (ESI):
ꢂ
m/z calcd for C53H41N3O2S2: 816.2713 [M+H]+; found: 816.2716.
Acknowledgements
(E)-2-Cyano-3-(5’-(7-((7-(diphenylamino)-9,9-diethyl-9H-fluoren-2-
K.R.J.T. is thankful to the Department of Science and Technology, New
Delhi, India, for financial support (Ref. No. DST/TSG/ME/2010/27). K.-
C.H. acknowledges a grant-in-aid from the National Science Council,
yl)ACHTUNGTRENNUNG(phenyl)amino)-9,9-diethyl-9H-fluoren-2-yl)-2,2’-bithiophen-5-yl)acrylic
acid (8b)
Compound 8b was prepared from compound 6b according to a procedure
that was similar to that described above for compound 7a. Red solid;
yield: 0.128 g (89%); m.p. 209–2118C; 1H NMR (500 MHz, [D6]DMSO):
d=8.45 (s, 1H), 7.95 (d, J=3.5 Hz, 1H), 7.77 (t, J=8.0 Hz, 3H), 7.66–
7.71 (m, 5H), 7.61 (d, J=4.0 Hz, 1H), 7.27–7.34 (m, 6H), 6.97–7.09 (m,
14H), 6.95 (dd, J=8.0 Hz, 1.5 Hz, 1H), 2.03–2.08 (m, 2H), 1.75–1.87 (m,
6H), 0.26–0.31 ppm (m, 12H); 13C NMR (125 MHz, [D6]DMSO): d=
156.4, 154.2, 151.1, 150.8, 150.7, 150.3, 147.4, 147.3, 146.2, 146.0, 145.3,
141.2, 141.0, 136.2, 136.0, 135.1, 133.9, 133.6, 130.9, 129.5, 129.4, 128.2,
125.1, 124.7, 123.7, 123.5, 123.1, 122.8, 122.5, 121.1, 120.3, 119.6, 119.2,
118.6, 117.9, 116.8, 55.8, 55.5, 31.8, 31.8, 8.5, 8.4 ppm; IR (KBr): n˜ =
Taiwan. A.B. acknowledges
CSIR, New Delhi.
a Senior Research Fellowship from the
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ꢂ
[M+H]+; found: 960.3658.
(E)-3-(5’-(9-Butyl-7-((7-(diphenylamino)-9,9-diethyl-9H-fluoren-2-
yl)ACHTUNGTRENNUNG(phenyl)amino)-9H-carbazol-2-yl)-2,2’-bithiophen-5-yl)-2-cyanoacrylic
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Compound 8c was prepared from compound 6c according to a procedure
that was similar to that described above for compound 7a. Red-brown
solid; yield: 0.115 g (80%); m.p. 240–2438C; 1H NMR (500 MHz,
[D6]DMSO): d=8.47 (s, 1H), 8.03 (dd, J=16.0 Hz, 8.0 Hz, 2H), 7.96 (d,
J=4.0 Hz, 1H), 7.88 (s, 1H), 7.72 (d, J=3.5 Hz, 1H), 7.60–7.69 (m, 4H),
7.51 (d, J=9.0 Hz, 1H), 7.26–7.31 (m, 6H), 7.13 (d, J=11.0 Hz, 2H), 7.07
(d, J=8.0 Hz, 2H), 6.99–7.04 (m, 6H), 6.92–6.94 (m, 1H), 6.80 (d, J=
9.0 Hz, 1H), 4.25 (s, 2H), 1.76 (d, J=7.0 Hz, 4H), 1.62 (s, 2H), 1.18 (d,
J=8.0 Hz, 2H), 0.76 (t, J=7.0 Hz, 3H), 0.30 ppm (t, J=7.0 Hz, 6H);
13C NMR (125.77 MHz, [D6]DMSO): d=163.6, 150.8, 150.8, 147.8, 147.4,
146.9, 146.3, 146.0, 141.9, 140.9, 136.2, 136.1, 133.8, 133.5, 129.5, 129.4,
128.3, 123.7, 123.2, 123.1, 122.5, 121.3, 119.2, 119.0, 117.5, 117.1, 116.1,
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+
ꢂ
(C N); HRMS (ESI): m/z calcd for C63H52N4O2S2: 960.3532 [M+H] ;
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Computational Methods
All of the computations were performed with the Gaussian 09[33] program
package on a computer workstation. The ground-state geometries were
fully optimized without any symmetry constraints at the DFT level with
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singlet transitions at the optimized geometry in the ground state were ob-
tained by TD-DFT calculations by using the same basis set as for the ge-
ometry minimization.
Chem. Asian J. 2012, 7, 2942 – 2954
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
2953

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Received: August 16, 2012
Published online: September 28, 2012
2954
www.chemasianj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 2942 – 2954