Organic dyes containing pyrenylamine-based cascade donor systems with different aromatic π linkers for dye-sensitized solar cells: Optical, electrochemical, and device characteristics

Article abstract of DOI:10.1002/asia.201100849

New organic dyes containing pyrenylamine donors in a cascade arrangement and cyanoacrylic acid acceptors have been synthesized and characterized by optical, electrochemical, and theoretical studies. The dyes inherit a D-π¹-D-π²-A (D=donor, A=acceptor) molecular architecture where the π linkers π¹ are changed from phenyl to biphenyl and fluorene, whereas the π linker π² that connects the donor fragment with the acceptor is a phenyl unit. The conjugation pathway linking the two donor segments has been found to play a major role in the optical and electrochemical properties. Shorter π linkers such as phenyl groups facilitate the donor-acceptor interaction while the nonplanar biphenyl spacer decreases the electronic communication between the donors and enhances the oxidation propensity of the corresponding dye. All the dyes display an intense longer wavelength electronic transition,which is attributable to the amine-to-cyanoacrylic acid charge transfer. The extinction coefficient of this peak grows dramatically on increasing the conjugation pathway length between the two donor segments. The dyes were used as sensitizers in nanocrystalline TiO₂-based dye-sensitized solar cells (DSSCs) and the cascade donor system contributed to the enhancement in the device efficiency due to favorable absorption and redox properties.

Full text of DOI:10.1002/asia.201100849

FULL PAPERS
DOI: 10.1002/asia.201100849
Organic Dyes Containing Pyrenylamine-Based Cascade Donor Systems with
Different Aromatic p Linkers for Dye-Sensitized Solar Cells: Optical,
Electrochemical, and Device Characteristics
K. R. Justin Thomas,*[a] Neha Kapoor,[a] Chuan-Pei Lee,[b] and Kuo-Chuan Ho[b]
Abstract: New organic dyes containing
pyrenylamine donors in a cascade ar-
rangement and cyanoacrylic acid ac-
ceptors have been synthesized and
characterized by optical, electrochemi-
cal, and theoretical studies. The dyes
inherit a d-p1-d-p2-A (D=donor, A=
acceptor) molecular architecture where
the p linkers p1 are changed from
phenyl to biphenyl and fluorene,
whereas the p linker p2 that connects
the donor fragment with the acceptor
is a phenyl unit. The conjugation path-
way linking the two donor segments
has been found to play a major role in
the optical and electrochemical proper-
ties. Shorter p linkers such as phenyl
groups facilitate the donor–acceptor in-
teraction while the nonplanar biphenyl
spacer decreases the electronic commu-
nication between the donors and en-
hances the oxidation propensity of the
corresponding dye. All the dyes display
an intense longer wavelength electronic
transition,which is attributable to the
amine-to-cyanoacrylic acid charge
transfer. The extinction coefficient of
this peak grows dramatically on in-
creasing the conjugation pathway
length between the two donor seg-
ments. The dyes were used as sensitiz-
ers in nanocrystalline TiO2-based dye-
sensitized solar cells (DSSCs) and the
cascade donor system contributed to
the enhancement in the device efficien-
cy due to favorable absorption and
redox properties.
Keywords: absorption
density functional calculations
·
amines
·
·
donor–acceptor systems · dye-sensi-
tized solar cells
Introduction
to the tuning of functional properties.[3] As the semiconduc-
tor oxide is not capable of absorbing the sun light by itself,
organic dyes have been adsorbed on the surface of TiO2 to
extend their light-harvesting potential. Organic dyes con-
taining donor–acceptor structural units linked by a suitable
aromatic group and possessing a carboxylic acid group have
been widely employed as sensitizers in excitonic solar
cells.[4] In most efficient dyes, the triarylamines served as
donors while cyanoacrylic acid served as the acceptor.[5–10]
Efforts focused on the optimization of the functional effica-
cy of the organic dyes were directed to study the nature of
donor or p-linker segments to find an effective dye. Organic
dyes featuring various triarylamines and aromatic linkers
such as oligophenyl,[5] fluorene,[6] carbazole,[7] oligothio-
phene,[8] coumarin,[9] etc. have been developed and exam-
ined as sensitizers in dye-sensitized solar cells (DSSCs).
Modification of the triarylamine chromophore by incorpora-
tion of electron-rich aromatic units such as fluorene has
been found to be beneficial for enhancing the absorption
properties and lowering the oxidation potential, which is
necessary for the regeneration of the oxidized dye by the
electrolyte.[10] Incorporation of electron-rich segments in the
conjugation pathway has been found to lower the excited-
state ionization potential, which is essential for facilitating
the electron injection into the conduction band of the
TiO2.[11] Triarylamine-free organic dyes containing pyrenoi-
Generation of electricity from alternative energy resources
has gained immense attention in the past decade owing to
the fear that fossil fuels might vanish rapidly as their con-
sumption rate is growing year by year. Solar cells have been
projected as an efficient alternative as the sun energy is
available aplenty in most of the countries.[1] Photoelectro-
chemical cells fabricated using organic dyes are commonly
termed as organic solar cells (OSC) and have been consid-
ered as the cheaper alternative to the commercialized sili-
con-based solar cells.[2] Organic solar cells based on nano-
crystalline semiconductor oxides such as TiO2 have been in-
tensively studied in recent years owing to their low cost and
their flexibility towards chemical modification, thus leading
[a] Dr. K. R. J. Thomas, N. Kapoor
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. 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.201100849.
midazole,[12]
phenanthrenoimidazole,[13]
imidazole,[14]
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Chem. Asian J. 2012, 7, 738 – 750

pyrene,[15] fluorene,[16] and/or
thiophene[17] have also been
demonstrated as successful
sensitizers.
Pyrene, a flat polyaromatic
hydrocarbon possesses excel-
lent fluorescence properties,
carrier mobilities, and signifi-
cant improved hole-injection
ability than fluorene owing to
its extended conjugation.[18]
Additionally, pyrene has an ex-
ceptionally long fluorescence
lifetime in the range of 400 ns
in deaerated solutions as com-
pared to <10 ns observed for
most organic fluorophores.[19]
Prolongation of excited-state
lifetime will be beneficial for
the excited-state processes
such as electron-transfer reac-
tions, and this is an essential
criterion for the success of
DSSCs. Though pyrene-based
triarylamines[20,21] and their
polysubstituted derivatives[22]
Figure 1. Structures of the dyes.
have been demonstrated as charge-transporting emitters and
semiconductors in organic field-effect transistors (OFET),
their use in photovoltaic devices is rather limited.[6f] Thayu-
manavan and co-workers[23] have used a pyrenylamino
moiety as a donor in a dendritic donor–acceptor architecture
and this showed an efficient photoinduced charge separation
from the donor moiety to the benzothiadiazole-based ac-
ceptor unit. A dye with a pyrenylamine donor, 2-fluore-
nylthiophene linker, and a cyanoacrylic acid acceptor has
been demonstrated as a sensitizer for DSSCs.[6f] A series of
pyrene end-capped organic dyes, with varying degree of con-
jugation linkers with cyanoacrylic acid, have been recently
reported.[15] However, to the best of our knowledge, system-
atic structure–property relationship studies on pyrenylamine
donor-containing organic dyes suitable for DSSCs have not
been undertaken. We have hypothesized that use of pyrenyl-
amine units as donors in organic dyes will be useful for effi-
cient charge separation due to their enhanced donor
strength and enhanced excited-sate lifetime and facile elec-
tron injection into the conduction band of TiO2, owing to
the favorable lowest unoccupied molecular orbital
(LUMO). Thus, we have developed three dyes (Figure 1)
containing two pyrenylphenylamine donor units in a cascade
arrangement with aromatic segments with varying conjuga-
tion pathways such as phenyl, biphenyl, and fluorene. We
have also synthesized two control dyes with a single donor
unit to unravel the impact of the cascade donor system on
the light-harvesting properties. The presence of additional
donors in the DSSC sensitizers have been found to be bene-
ficial for lowering the excited-state oxidation potential,
thereby facilitating efficient injection of the electron into
the conduction band of TiO2. The conjugation pathway of
the aromatic linker between the donors in the diamine
system has been modified from biphenyl to fluorene as the
nonplanar and planar linkers, respectively, are expected to
differ in mediating electronic communication between the
two donors and fine tune the ground- and excited-state
properties of the dyes.[5b] Though the organic sensitizers
based on a cascade donor system[10a,24,25] have been explored
before for applications in DSSCs, the effect of the aromatic
linker between the donors has not been studied in detail.
We felt that fine tuning of the donor–donor interaction by
altering the conjugation pathway can be used as a strategy
to modulate the excited-state properties of the dyes and the
stability of the cation radical produced after the electron
ejection. Herein, we report the synthesis, characterization,
optical properties, electrochemical characteristics, and DSSC
studies of the newly developed dyes, which expose the im-
portance of the p linker between the donors.
Results and DiscussionSynthesis
The dyes, 3a–3c, were synthesized as illustrated in
Scheme 1. The diamines (1a–1c) required for the present
ꢀ
study were formed by a palladium-catalyzed C N coupling
reaction[26] of the N-phenylpyrenyl-1-amine with the corre-
sponding dibromoarene. They were then converted into the
required aldehydes (2a–2c) by performing a Vilsmeier–
Haack formylation using POCl3 and N-methylformanilide in
chlorobenzene. The monoformylation selectively occurred at
the para position of the phenyl unit rather than the pyrene
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739

K. R. Justin Thomas et al.
FULL PAPERS
unit is not favorable for the in-
tramolecular charge transfer
from the amine to the cyanoa-
crylic acid unit. Thus, the
peaks appearing for DC1 at
305 and 375 nm are assigned to
pyrene localized p–p* and
amine-to-pyrene charge-trans-
fer transitions, respectively.[23]
The absence of the later peak
in the naphthalene analog
N,N’-bis-(1-naphthyl)-N,N’-di-
phenyl-1,1’-biphenyl-4,4’-dia-
mine (NPB)[27] clearly validates
the above assignment. The
amine-to-pyrene charge-trans-
fer transition exhibits an inter-
esting trend, which is depen-
dent on the donor strength of
the amine. A slight blue-shift is
observed for the compounds
containing electron-withdraw-
Scheme 1. Synthesis of the dyes 3a–3c.
ing groups such as aldehyde 4-
(phenyl(pyren-1-yl)amino)ben-
zaldehyde (PPAB, 2a–2c; see
unit, and this paved the way for the subsequent preparation
of the target dyes (3a–3c) by Knoevenagel condensation
with cyanoacetic acid in the presence of ammonium acetate
as the catalyst. The dyes DC1 and DC2 have also been syn-
thesized as model compounds to study the impact of the
conjugation and auxiliary donor on optical and electrochem-
ical properties from the readily available aldehydes. The
dyes are red in color and moderately soluble in solvents
such as toluene, dichloromethane, tetrahydrofuran (THF),
acetonitrile (MeCN), N,N-dimethylformamide (DMF), di-
methyl sulfoxide (DMSO), and methanol to produce dark
yellow solutions.
below) when compared to that of parent amines. It is likely
that the charge density on the amine unit is competitively
withdrawn by both pyrene and aldehyde units. This would
lead to the reduction in the strength of the donor–acceptor
interaction between the amine and pyrene/aldehyde. Thus,
the position of this peak is highly dependent on the donor
strength of the amine; as the donor strength increases, the
charge-transfer interaction with pyrene also increases
(Table 1). The later peak is probably buried in the intense
charge-transfer transition occurring in the dyes DC2 and
3a–3c at the higher wavelength region (>400 nm).
The dye DC2 shows the most red-shifted absorption in
the series, while dye 3c exhibits absorption with a high-
Optical Properties
The absorption spectra of dyes (3a–3c, DC1, and DC2), the
precursor amines, and aldehydes were examined in dichloro-
methane (Figure 2) and the pertinent data are collected in
Table 1. All the dyes exhibited three prominent absorption
bands and invariably all the bands feature a shoulder at
shorter wavelength, indicative of mixing of transitions. The
higher energy band occurring at 275 nm is observed for all
dyes and can be assigned to a pyrenylphenylamine localized
p–p* transition.[20,23] The incremental increase in the molar
extinction coefficient for this band in the dyes 3a–3c, when
compared to the control dyes (DC1 and DC2) is attributable
to the presence of two pyrenylphenylamine units in the
former dyes. The origin of the higher wavelength peaks ap-
pearing at approximately 320 and 430 nm are easily under-
stood by comparing the absorption spectra of DC1 with
DC2. In DC1, the meta disposition of the cyanoacrylic acid
Figure 2. Absorption spectra of the dyes recorded in dichloromethane.
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Chem. Asian J. 2012, 7, 738 – 750

Organic Dyes Containing Pyrenylamine-Based Cascade Donor Systems
Table 1. Photophysical and electrochemical data of the compounds recorded for a dichloromethane solution.
Dye[a] lmax [nm] (emax, Mꢀ1 cmꢀ1 ꢁ103)
lem [nm] Stokes
shift [cmꢀ1
Eox (DEp) [mV][a]
HOMO [eV] LUMO [eV] E0ꢀ0 [eV] Eox* [eV]
]
DPAP 300 (25.7), 381 (14.1)
469
562
4925
444 (69), 940
96 (48), 588 (64),
1128
280 (64), 532 (70),
992
5.24
4.90
2.36
2.25
2.88
2.65
–
–
1a
1b
1c
323 (55.5), 335 (54.2), 413 (22.5)
334 (76.3), 406 (37.0)
6419
4750
5473
503
542
5.08
4.94
2.28
2.28
2.80
2.66
–
–
337 (47.7), 362 (37.8), 418 (30.4)
140 (66), 492 (67),
1040
PPAB 329 (27.0), 349 (26.9), 378 (23.5)
467
559
5042
8358
604 (76), 976
236 (65), 644 (71),
1124
380 (63), 604 (69),
992
5.40
5.04
2.45
2.28
2.95
2.76
–
–
2a
2b
2c
336 (49.8), 381 (33.7)
332 (50.7), 376 (47.7)
335 (50.2), 390 (52.2)
497
513
6475
6148
5.18
5.06
2.36
2.33
2.82
2.73
–
–
260 (76), 556 (80),
964
DC1
DC2
274 (29.5), 309 (38.6), 377 (15.4)
274 (22.9), 331 (13.6), 376 (13.8), 431
(25.7)
461
484
4833
2541
544 (62), 972
612 (65) , 968
5.34
5.41
2.40
2.71
2.94
2.70
ꢀ1.626
ꢀ1.318
3a
3b
3c
276 (49.0), 323 (39.5), 335 (38.9), 414
(30.0)
276 (44.0), 334 (40.9), 409 (36.1)
561
510
545
6329
4664
5632
244 (48), 636 (48),
1108
392 (44), 608 (64),
1004
276 (47), 556 (64),
1060
5.04
5.19
5.07
2.51
2.52
2.51
2.53
2.67
2.56
ꢀ1.516
ꢀ1.508
ꢀ1.518
276 (51.3), 335 (46.3), 417 (44.8)
[a] from cyclic voltammetry using scan rate 100 mVsecꢀ1 and referenced to a ferrocene redox couple.
molar extinction coefficient. On introduction of the second
amino unit, the peak position shifted slightly to the higher
energy side for 3a–3c. However, this cannot be treated as a
blue-shift as the band showed significant broadening, indica-
tive of the fusion of multiple peaks. Interestingly, the molar
extinction coefficient of the higher wavelength peak pro-
gressively increased on the extension of the conjugation
bridge connecting the two donor units. The rise in the opti-
cal density for the higher wavelength absorption for the
dyes 3a–3c may originate due to the following facts: a) a
two-fold increase in the chromophoric density for the
amine-to-pyrene charge-transfer electronic transition and
b) the extension of the linker conjugation and its planarity,
which may enhance the donor strength of the cascade donor
system and increase the transition probability of the amine-
to-cyanoacetic acid charge transfer. A slight blue-shift ob-
served for the biphenyl-bridged derivative (3b), when com-
pared to the phenyl-bridged (3a) and fluorene-bridged (3c)
derivatives, suggests the presence of an inefficient interac-
tion between the auxiliary amine donor and the pivotal
amine donor, owing to noncoplanarity in the biphenyl
bridge.
It is interesting to compare the absorption spectra of the
dyes with the precursor amines and aldehydes (see Figur-
es S4 and S5 in the Supporting Information) as they reveal
the origin of certain transitions and are useful to understand
the impact of the electron-accepting group. Firstly, the ab-
sorption peaks of amines 1a–1c and aldehydes 2a–2c are
red-shifted when compared to that of the control amine,
N,N-diphenylpyren-1-amine (DPAP) and the aldehyde, 4-
(phenyl(pyren-1-yl)amino)benzaldehyde (PPAB), respec-
tively. This red-shift originates because of the auxochromic
effect exerted by the additional amino segment linked
through an aromatic unit such as phenyl, biphenyl, or fluo-
rene. Secondly, the optical density of the dye precursors 1a–
1c and 2a–2c are significantly larger than those of the con-
trol precursors DPAP and PPAB, respectively. This suggests
that multiple transitions arising from different chromo-
phore-localized electronic excitations overlap in the shorter
wavelength region, thus resulting in the increase in molar
extinction coefficient. Thirdly, the longer wavelength ab-
sorption grows in intensity on moving from amines (1a–1c)
to aldehydes (2a–2c) with a concomitant blue-shift in the
peak positions. This suggests that on introduction of an alde-
hyde functionality, an additional electronic transition corre-
sponding to the amine-to-aldehyde charge-transfer appears;
however, it merges with the transition attributable to the
amine-to-pyrene charge-transfer from the higher energy
side.
To understand the effect of environments on the electron-
ic properties of the dyes, we have studied the absorption
spectra of the dyes in solvents such as toluene, THF, di-
chloromethane, acetonitrile, and methanol with different po-
larity indices (Table 2). The changes in the absorption pro-
files for the dyes with solvent polarity are illustrated in
Figure 3 for the dyes DC2 and 3a. In general, all the dyes
exhibited red-shifted absorption in nonpolar solvents such
as toluene and shorter wavelength absorption in the polar
solvents such as methanol. The solvatochromism was most
pronounced for DC2 compared to that of dyes 3a–3c. The
dielectric constant of the solvents increases in the order of
toluene<tetrahydrofuran<dichloromethane<methanol<
acetonitrile. Considering the progressive increase in the ef-
fective solvation of the dye with solvent polarity, more red-
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Table 2. Absorption properties of dyes in different solvent environments.
Dyes lmax, nm (emax, Mꢀ1 cmꢀ1 ꢁ103)
DCM
DCM+TFA
DCM+TEA
MeOH
MeOH+TFA MeOH+NaOH TOL THF
273 (32.4), 303 272 (35.2), 296 309, 300,
MeCN
300,
DC1 274 (29.5), 309
273 (30.3), 311 (42.4), 376
(17.1)
296 (35.6), 377
(15.2)
272 (36.9),
296 (40.5),
376 (16.0)
272 (31.9),
329 (22.2),
405 (36.9)
(38.6), 377 (15.4)
(40.6), 376
(16.9)
(38.9), 376
(15.6)
379[a] 377[a] 376[a]
DC2 274 (22.9), 331
(13.6), 376 (13.8),
431 (25.7)
274 (23.9), 327 (13.6), 360
(12.9), 448 (31.4)
332 (16.1), 402
(23.2)
271 (32.5), 329 272 (25.5), 329 332,
332,
376,
330,
374,
(19.3), 423
(41.3)
(18.1), 403
(29.6)
375,
431[a] 411[a] 420[a]
3a
276 (49.0), 323
(39.5), 335 (38.9),
414 (30.0)
276 (44.0), 334
(40.9), 409 (36.1)
276 (51.3), 335
(46.3), 417 (44.8)
276 (58.6), 322 (43.9), 333
(43.9), 381 (29.5), 408 (30.9), (39.9), 336 (39.9),
460 (28.1)
275 (51.3), 331 (47.0), 378
(32.3), 401 (29.6), 460 (36.3) (42.4), 405 (42.8)
276 (54.6), 334 (48.2), 380 322 (38.6), 336
(36.2), 416 (40.9), 463 (33.2) (38.7), 409 (32.8)
276 (48.9), 322
274, 319, 333, 274, 319, 332, 274, 319, 333,
324,
337,
322,
335,
320,
333,
407[a]
412[a]
406[a]
410 (33.9)
276 (44.0), 335
415[a] 413[a] 406[a]
3b
3c
273, 331,
403[a]
273, 330, 411[a] 273, 331, 403[a] 334,
411[a] 410[a] 400[a]
274, 332, 412[a] 274, 333, 410[a] 337,
335, 336,
419[a] 415[a] 420[a]
333,
332,
274, 333,
410[a]
[a] Molar extinction coefficients could not be obtained due to poor solubility.
tonitrile<dichloromethane<tetrahydrofuran<methanol).
It is probable that the hydrogen-bonding interaction of the
solvents such as tetrahydrofuran and methanol with the dye
might alter the acceptor ability of the carboxylic acid unit,
analogous to the interaction of the dye with the TiO2 sur-
face. To further confirm the interaction of the dye with
methanol, absorption spectra of the dye DC2 in acetonitrile
with different amounts of methanol were recorded
(Figure 4). A significant blue-shift was noticed on increasing
the methanol concentration; this is consistent with the
above proposition.
Figure 4. Absorption spectra of DC2 recorded in acetonitrile and metha-
nol mixtures.
The dyes have also shown changes in the absorption upon
addition of trifluoroacetic acid (TFA) and triethylamine
(TEA) to the dichloromethane and methanol solutions, indi-
cative of the presence of acid–base equilibria (Figure 5).
The dyes exhibited a red-shift upon addition of TFA to the
dichloromethane solution, while addition of TEA to the so-
lution resulted in a blue-shift. Such behavior has been no-
ticed for several organic dyes, developed for DSSC applica-
tions, which contain a cyanoacrylic acid acceptor unit.[17]
Figure 3. Absorption spectra recorded for a) DC2 and b) 3a in different
solvents.
shifted absorption in toluene and shorter wavelength ab-
sorption in acetonitrile were expected. However, the dye
DC2 showed significant blue-shifted absorption peaks in tet-
rahydrofuran and methanol. This is in agreement with the
hydrogen-bonding parameters of the solvents (toluene<ace-
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Organic Dyes Containing Pyrenylamine-Based Cascade Donor Systems
the cyanoacrylic acid acceptor would cause a migration of
electron density from the auxiliary donor toward the accept-
or, while this would be less pronounced in 3b. The net
result is a more polar excited state for 3a, which would facil-
itate a dipolar relaxation with the solvent dipoles when com-
pared to the dye 3b. The higher wavelength shoulder ob-
served in the emission spectra of DC2 was speculated to
originate from the excimer. This assignment was confirmed
by analyzing the concentration dependence of the emission
profile. At higher concentrations the shoulder observed at
550 nm has grown in intensity and appeared as a prominent
peak (Figure 7). It is interesting to note that such excimer
Figure 5. Changes in the absorption spectra of the dyes DC2 and 3b in
dichloromethane upon addition of TFA or TEA.
Similarly, addition of TFA to the methanolic solution of the
dyes also led to a prominent red-shift but the addition of
sodium hydroxide or TEA did not alter the absorption peak
(see the Supporting Information). This suggests the presence
of the deprotonated form for the dyes in the methanol solu-
tion.
The emission spectra of the dyes were recorded in di-
chloromethane and are displayed in Figure 6. The shortest
wavelength emission peak was observed for DC1 while the
longest wavelength emission was found for 3a. While the
absorption wavelengths are similar for the dyes 3a–3c, the
Figure 7. Emission spectra of DC2 recorded for dichloromethane solu-
significant difference observed in the emission peaks (3a>
3c>3b) is surprising. It appears that the nature of the aro-
matic segment that bridges the auxiliary donor with the
main donor plays a major effect in the excited state. In 3a,
through the shorter phenylene linkage, the auxiliary donor
enhances the donor strength of the main donor, while in 3b
the twisted biphenyl bridge decouples the auxiliary donor
from the main donor. Thus in 3a, the electronic excitation,
which arises due to the charge transfer from the amine to
tions of different concentrations.
formation owing to the intermolecular interaction of pyrene
units was prohibited in the dyes 3a–3c due to the introduc-
tion of an auxiliary triarylamine segment, which effectively
inhibits the approach of two pyrene units. It is likely that
the dyes, 3a–3c, are prone to resist aggregation at the sur-
face of the TiO2 due to the nonplanar structural elements,
and this property will be beneficial for the photocurrent
generation.
Electrochemical Properties
The dyes were analyzed by cyclic voltammetric experiments
to understand the redox propensity of the dyes. In order for
the dyes to effectively function as a sensitizer in DSSCs,
they must possess ground-state oxidation potentials lower
than that of the electrolyte and the first excited-state oxida-
tion potential above the conduction band of TiO2.[4] The
dyes fulfilling the above criteria will effectively inject elec-
trons into the conduction band of TiO2 from the excited
state and will allow regeneration of the dye by the electro-
lyte. The cyclic voltammetric measurements were performed
in dichloromethane solutions (2ꢁ10ꢀ4 m) using tetrabutylam-
monium hexafluorophosphate as a supporting electrolyte.
Figure 6. Emission spectra of the dyes recorded in dichloromethane.
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K. R. Justin Thomas et al.
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The cyclic voltammograms recorded for the dyes DC2, 3b,
and 3c along with their amine precursors (DPAP, 1b, and
1c) in the presence of ferrocene are shown in Figure 8 and
the relevant data are collected in Table 1. The dyes DC1
and DC2 exhibited a quasi-reversible oxidation couple at
1.16 V vs. NHE). The values are more positive than the
iodide/triiodide redox couple (ca. 0.42 V vs NHE). This en-
sures the efficient regeneration of the dye from the oxidized
dye by the receipt of an electron from the electrolyte. The
redox potentials of dyes 3a–3c match well with that of the
electrolyte, as compared to the dyes DC1 and DC2, owing
to the presence of two triarylamine donor segments in the
former dyes, which enhance the donor strength and substan-
tially lower the highest occupied molecular orbital (HOMO)
energy levels of the dyes (3a–3c).
We have also estimated the excited-state oxidation poten-
tials (Eox*) of the sensitizers, which correspond to the
LUMO levels of the dyes, to estimate the thermodynamic
driving force for the injection of electrons from the dye into
the conduction band of TiO2. The excited-state potential
was calculated from the first oxidation potential (Eox) at the
ground-state and the zero–zero electronic transition energy
*
(E0ꢀ0) from Eox ¼ Eox ꢀ E0ꢀ0. The optical band gap was de-
rived from the absorption edge. A dye must possess a more
negative excited-state potential than the energy level of the
TiO2 conduction band edge (ꢀ0.5 V vs NHE) for energeti-
cally favorable downhill electron injection.[24] The Eox*
values observed for the dyes (ꢀ1.32 to ꢀ1.63 V vs NHE)
are more negative than the conduction-band-edge energy
level of the TiO2 electrode. Thus, these dyes are expected to
effectively inject electrons into the TiO2 conduction band.
Figure 8. Cyclic voltammograms recorded in dichloromethane at the scan
rate 100 mV/sec using ferrocene as an internal standard for the dyes
DC2, 3b, and 3c. The cyclic voltammograms of their parent amines
DPAP, 1b, and 1c (solid lines) are also shown for comparison.
higher positive potentials than that of ferrocene, which is at-
tributable to the oxidation of the triarylamine segment. The
diamine-based dyes 3a–3c displayed two quasi-reversible
oxidation couples, which arise from the two different amino
groups. The redox couple located at the lower oxidation po-
tential is assigned to the triarylamine segment that is further
away from the cyanoacrylic unit, while the oxidation occur-
ring at the higher potential is attributed to the triarylamine
segment bearing the cyanoacrylic acid unit, which will be
difficult to oxidize. Facile first oxidation observed for the
dyes 3a and 3c when compared to 3b is suggestive of a
stronger electronic interaction between the donor segments
through the short phenylene unit in 3a and planar fluorene
segment in 3c. Whereas in 3b, the twisted biphenyl unit is
probably inhibiting the electronic interaction between the
donor units. The highest second oxidation potential ob-
served for 3a in the series is corroborative of the above ar-
guments. The oxidation potentials of the dyes are more posi-
tive when compared to the corresponding values of the
parent amines (DPAP, 1a–1c), and this confirms the pres-
ence of intramolecular charge-transfer interactions between
donor and acceptor units and within the cascade donor seg-
ment.[24] This also points out that the interaction between
the donors in the cascade donor system is affected by the
nature of the bridging unit.
Theoretical Investigations
Density functional theory (DFT) calculations[28] have been
employed to optimize the geometry of the compounds and
rationalize the origin of the absorption spectra observed for
these dyes (DC1, DC2, and 3a–3c). The prominent higher
wavelength vertical transitions, their oscillator strengths (f),
and dipole moments obtained from the computations are
collected in Table 3.
Electronic distributions in the frontier molecular orbitals
of the dyes, 3a–3c, are displayed in Figure 9 while the corre-
sponding results for the control dyes are shown in Fig-
ure S15 in the Supporting Information. In DC1, HOMO and
HOMO-1 are mainly contributed by the pyrenylphenyla-
mine segment, whereas in DC2 they are delocalized over
the entire molecule. The LUMO is located on the cyanoa-
crylic unit and is extended up to the phenyl ring, while the
LUMO+1 is constituted primarily by the pyrene unit in
DC1 and DC2. The longer wavelength vertical transition of
DC2 is mainly due to the electronic excitation from the
HOMO to LUMO, while in DC1 it arises due to electronic
promotion from the HOMO to LUMO+1 with little contri-
bution from HOMO-1 to LUMO and LUMO+1. This clear-
ly suggests that in DC1 the prominent excitation leads to
the charge transfer from the amine to pyrene segments,
while in DC2 the charge transfer occurs from the amine to
cyanoacrylic acid. The shorter wavelength vertical transition
observed for DC1 when compared to DC2 is also consistent
with the experimental observation (see above).
To identify the feasibility of dye regeneration by the
iodine/iodide electrolyte system, we have measured the
ground-state oxidation potentials of the dyes (see Table 1),
which fall in the range 1.31–1.38 V versus the normal hydro-
gen electrode (NHE) for the control dyes (DC1 and DC2),
however, the dyes 3a–3e exhibited lower values (1.01–
744
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Chem. Asian J. 2012, 7, 738 – 750

Organic Dyes Containing Pyrenylamine-Based Cascade Donor Systems
Table 3. Predicted (TDDFT B3LYP/6-31GACTHUNGTRENNUNG
Dye labs [nm]
f
DC1 408.6
326.4
0.26 7.33
0.17
0.19
HOMO!LUMO+1 (+77%), HOMO-1!LUMO (13%), HOMO-1!LUMO+1 (+8%)
HOMO-1!LUMO+1 (+73%), HOMO-2!LUMO+1 (6%), HOMO!LUMO+1 (6%), HOMO-2!LUMO+2
(+6%), HOMO!LUMO+2 (5%)
310.0
305.5
0.36
HOMO!LUMO+3 (+62%), HOMO-2!LUMO+1 (11%), HOMO!LUMO+2 (+9%), HOMO-1!LUMO+2
(+9%)
HOMO-3!LUMO (54%), HOMO-4!LUMO (41%)
DC2 440.2
407.0
0.43 8.19
0.11
0.48
0.21
0.40 9.58
0.31
0.12
0.15
0.12
0.23
HOMO!LUMO (+95%)
HOMO!LUMO+1 (+78%), HOMO-1!LUMO (13%), HOMO-1!LUMO+1 (+7%)
HOMO-1!LUMO (+81%), HOMO-1!LUMO+1 (+7%), HOMO!LUMO+1 (+7%)
HOMO-1!LUMO+1 (+72%), HOMO!LUMO+1 (8%), HOMO-2!LUMO+2 (6%)
HOMO!LUMO (+98%)
368.1
333.3
3a
489.1
451.2
420.3
382.4
369.4
351.0
348.2
HOMO!LUMO+1 (+92%)
HOMO-1!LUMO (+94%), HOMO-2!LUMO (5%)
HOMO-1!LUMO+1 (56%), HOMO-2!LUMO (+32%)
HOMO-2!LUMO (59%), HOMO-1!LUMO+1 (24%)
HOMO-3!LUMO (+48%), HOMO!LUMO+4 (18%), HOMO-2!LUMO+1 (10%), HOMO!LUMO+3
0.26
(+10%)
HOMO-3!LUMO (+35%), HOMO!LUMO+3 (26%)
3b
3c
487.1
446.5
434.8
378.7
362.8
347.9
516.3
468.6
444.2
435.6
368.9
362.4
356.5
0.31 9.59
0.41
0.41
0.29
0.38
0.35
0.21 8.31
0.48
0.28
0.24
HOMO!LUMO (+98%)
HOMO!LUMO+1 (+71%), HOMO!LUMO+2 (+21%),
HOMO-1!LUMO (+68%), HOMO!LUMO+2 (+20%)
HOMO-2!LUMO (+73%), HOMO-3!LUMO (11%), HOMO-1!LUMO+1 (+7%)
HOMO-1!LUMO+2 (+43%), HOMO-3!LUMO (+35%), HOMO-2!LUMO (+9%)
HOMO!LUMO+3 (+51%), HOMO-2!LUMO+1 (14%)
HOMO!LUMO (+99%)
HOMO!LUMO+1 (+88%), HOMO!LUMO+2 (+7%)
HOMO!LUMO+2 (+75%), HOMO-1!LUMO (+14%)
HOMO-1!LUMO (+81%), HOMO!LUMO+2 (+12%)
HOMO-3!LUMO (+58%), HOMO!LUMO+4 (+26%), HOMO-2!LUMO (+8%)
HOMO-1!LUMO+2 (+82%), HOMO!LUMO+4 (7%), HOMO!LUMO+5 (+7%)
HOMO!LUMO+3 (+34%), HOMO!LUMO+4 (+24%), HOMO-2!LUMO+1 (+13%), HOMO-1!
LUMO+3 (+11%), HOMO-3!LUMO (+6%)
0.51
0.11
0.16
For the diamine-based dyes (3a–3c), the picture is entire-
mine dyes (3a–3c) compared to that of control dyes (DC1
and DC2), thus implying a strong dipolar interaction in the
diamine dyes due to the presence of a cascade donor
system.
ly different. The HOMO is mainly distributed over the
amine segment that is further away from the acceptor unit
for 3b and 3c, while in 3a it is spread up to the amine con-
taining the acceptor unit. The LUMO in the dyes is mainly
delocalized over the phenylcyanoacrylic acceptor unit and
the LUMO+1 is composed of the pyrene segment. There
are two prominent electronic excitations predicted for these
dyes. The longer wavelength transition is due to the elec-
tronic excitation from the HOMO to LUMO, while the next
absorption is from the HOMO to LUMO+1 electronic tran-
sition. This clearly indicates that in these dyes the longer
wavelength transition is due to the charge transfer from the
amine to cyanoacrylic acid acceptor, whereas the second
transition stems from the charge transfer from the amine to
the pyrene unit. On changing the conjugation pathway of
the linker that separates the two amine units from phenyl to
biphenyl or fluorene decreases the oscillator strength of the
first absorption with a concomitant increment in the second
absorption. It is interesting to note that extension of the
conjugation between the amine units in this class of com-
pounds is disadvantageous for the desired charge-transfer
transition due to the competitive charge migration from the
amine to the pyrene unit, with the latter being prominent
for the dyes with an elongated conjugation pathway. The
theoretically computed dipole moment is larger for the dia-
Photovoltaic Device Performance
The DSSCs were constructed by using the dyes (DC1, DC2,
and 3a–3c) as the sensitizer for nanocrystalline anatase
TiO2. Typical solar cells, with an effective area of 0.25 cm2,
were fabricated with an electrolyte composed of 0.05m I2/
0.5m LiI/0.5m tert-butylpyridine in acetonitrile solution. The
device performance statistics were obtained under AM 1.5
solar irradiation and collected in Table 4. The current–volt-
age curves of the DSSCs sensitized by the dyes are shown in
Figure 10.
Among the control dyes, DC2 shows better efficiency due
to the donor–acceptor structure, whereas in DC1 the cya-
noacrylic acid is delinked from the donor by meta disposi-
tion. The DSSCs sensitized by the dyes 3a and 3c have ex-
hibited the higher h values 4.28% (Jsc =10.2 mAcmꢀ2, Voc =
0.595 V, ff=0.71) and 3.89% (Jsc =9.51 mAcmꢀ2, Voc =
0.583 V, ff=0.70), respectively. The dyes featuring diamines
(3a–3c) show higher current conversion efficiency than the
control dyes DC1 and DC2, which is attributed to the larger
Chem. Asian J. 2012, 7, 738 – 750
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745

K. R. Justin Thomas et al.
FULL PAPERS
Figure 9. Electronic distribution observed for the frontier orbitals for the dyes 3a-3c.
Table 4. Performance characteristics of the DSSCs fabricated using the
dyes.
Dyes JSC [mAcmꢀ2
]
VOC [mV] ff
h [%] Rct2
[ohm]
fmax
te [ms]
DC1 2.48
DC2 6.27
472
516
577
595
583
0.43 0.51
86.36
19.84
27.93
17.50
17.24
34.36 4.63
112.98 1.41
92.68 1.72
0.71 2.29
0.69 2.88
0.71 4.28
0.70 3.89
3a
3b
3c
7.20
10.2
9.51
7.01
8.55
22.69
18.61
molar extinction coefficient for the charge-transfer transi-
tion in the former dyes, and this is a result of the cascade
donor system. In fact the enhancement of donor strength
owing to the presence of the cascade donor system in 3a–3c
is manifested in the incident photo-to-current conversion ef-
ficiency (IPCE) spectra (Figure 11). The dyes 3a–3c showed
higher power conversion efficiencies at longer wavelength
Figure 10. The current–voltage (I–V) characteristics of the DSSCs fabri-
cated using the dyes.
746
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Chem. Asian J. 2012, 7, 738 – 750

Organic Dyes Containing Pyrenylamine-Based Cascade Donor Systems
Figure 11. IPCE spectra recorded for the DSSCs fabricated using the
dyes, DC2, 3a-3c.
incident light when compared to the control dye DC2. This
suggests that the light harvesting at the lower energy radia-
tion is facilitated for the dyes 3a–3c due to the presence of
a stronger donor–acceptor interaction, a consequence of the
cascade donor system. The highest device efficiency ob-
served for 3b in the series is attributed to the tilting in the
biphenyl segment, which inhibits the intramolecular charge
recombination, while for the planar segments phenyl and
fluorene, the intramolecular charge recombination may be
easier.
The Nyquist plot displayed in Figure 12a shows two semi-
circles, which correspond to the charge-transfer resistances
at the counter electrode and TiO2/dye/electrolyte interface,
respectively. The radius of the bigger semicircle of the dye
3a is significantly larger than the other dyes (DC2, 3b, and
3c), and this indicated the enlarged charge-transfer resist-
ance for this dye. The charge-transfer resistance assumes the
order of 3b<DC2<3c<3a. The Voc for the devices increas-
es in the order of DC2<3a<3c<3b. It is probable that the
dyes based on cascade donors significantly alter the Fermi
level of photoanode, which manifests in the Voc. In the
Bode-phase plot (Figure 12b), the prominent lower frequen-
cy peak of the dyes 3a and DC2 are shifted to higher fre-
quency when compared to those of other dyes (3b and 3c).
This corresponds to a decrease in the electron lifetime for
the 3a and DC2 sensitized DSSCs. The electron lifetime can
be extracted from the angular frequency (wmin) at the mid-
Figure 12. a) Nyquist and b) Bode-phase plots of the DSSCs fabricated
using the dyes (DC2, 3a–3c).
tributed to the lower number of photoinduced electron gen-
eration and the reduced electron injection efficiency of 3c.
The high-frequency peak observed in the Bode plot corre-
sponds to charge transfer at the counter electrode. Slight
changes observed for this peak indicate a probable reaction
at the counter electrode/electrolyte interface.
Conclusions
In summary, we have synthesized new dyes containing 1-pyr-
enylphenylamino donors and cyanoacrylic acid acceptors.
Dyes containing two cascade 1-pyrenylphenylamino donors
have also been successfully synthesized and demonstrated as
efficient sensitizers for DSSCs. The incorporation of an ad-
ditional donor increases the absorption parameters such as
molar extinction coefficient. Tuning of the optical and elec-
trochemical properties has been achieved by the insertion of
an aromatic segment of varied conjugation pathway length
between the two donor units.
frequency peak in the Bode-phase plot by using te =1/wmin
.
The electron lifetime measured for the dyes 3a, 3b, and 3c
are 1.7, 22.7, and 18.6 ms, respectively, and follows the
trend, DC2ꢁ3a<3c<3b. The increase in electron lifetime
for 3b supports more effective suppression of the back reac-
ꢀ
tion of the injected electrons with the I3 in the electrolyte
by alternation of the HOMO of the sensitizer, which leads
to the improvement in the photocurrent and photovoltage
and to the substantial enhancement of the device efficiency.
Despite the enhancement in the electron lifetime, the lower
device efficiency observed for the DSSC based on 3c is at-
The oxidation potentials of the diamine-based dyes were
higher than the monoamine-based control dyes, thereby il-
lustrating the importance of cascade donor system to tune
the HOMO of the dyes. Theoretical calculations revealed
Chem. Asian J. 2012, 7, 738 – 750
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747

K. R. Justin Thomas et al.
FULL PAPERS
and heated at 908C for 12 h. After completion of the reaction, as evi-
denced by the disappearance of aldehyde, the reaction mixture was
poured into ice water and neutralized by sodium bicarbonate. The solu-
tion was extracted with dichloromethane, washed with brine solution,
dried over Na2SO4 followed by evaporation of solvent. The desired prod-
uct was purified by silica-gel column chromatography eluting with DCM/
EtOAc mixture (1:9). The obtained product was further recrystallized
with dichloromethane and methanol: yellow solid; yield 0.28 g (41%);
the presence of two charge-transfer transitions from the
amine to cyanoacrylic acid and pyrene units, respectively.
The dye (3b) possessing a biphenyl linker between the
donors exhibited higher photocurrent conversion efficiency
h=4.28%. It appears that the retardation of intramolecular
charge recombination at the excited state owing to the tilt-
ing in the biphenyl segment is beneficial for the light-har-
vesting properties.
m.p. 192–1948C; IR (KBr): n˜C O =1686 cmꢀ1
;
1H NMR (CDCl3,
=
500 MHz): d=9.75 (s, 1H), 8.23–8.21 (m, 2H), 8.18–8.16 (m, 3H), 8.14–
8.12 (m, 2H), 8.10–8.08 (m, 3H), 8.05 (d, J=1.5 Hz, 2H), 8.04–7.99 (m,
3H), 7.95 (d, J=9.5 Hz, 1H), 7.90–7.85 (m, 2H), 7.63 (d, J=9.0 Hz, 2H),
7.21–7.15 (m, 4H), 7.07 (dd, J=1.0, 7.5 Hz, 2H), 7.03–7.01 (m, 2H),
6.95–6.94 (m, 1H), 6.87 ppm (d, J=8.5 Hz, 2H); 13C NMR (CDCl3,
125 MHz): d=190.4, 154.3, 148.3, 145.8, 140.4, 139.8, 138.8, 131.6, 131.24,
131.17, 131.0, 130.9, 130.4, 129.7, 129.3, 128.7, 128.3, 128.20, 128.16, 128.1,
127.8, 127.6, 127.4, 127.3, 127.2, 127.1, 126.5, 126.4, 126.3, 126.23, 126.19,
126.1, 125.7, 125.5, 125.4, 125.2, 124.8, 124.7, 123.2, 122.8, 122.7, 122.0,
117.0, 112.4 ppm.
Experimental Section
General details regarding the synthesis, characterization, physical meas-
urements, theoretical computations, device fabriction, and characteriza-
tion are similar to that reported in our earlier publication.[10a]
Synthesis of N1,N4-diphenyl-N1,N4-di(pyren-1-yl)benzene-1,4-diamine (1a)
A mixture of 1,4-dibromopyrene (0.5 g, 2.1 mmol), N-phenylpyren-1-
Synthesis of 4-((4’-(phenyl(pyren-1-yl)amino)biphenyl-4-yl)(pyren-1-
yl)amino)benzaldehyde (2b)
amine[21b] (1.4 g, 4.7 mmol), [Pd
ACTHUNTRGNE(UGN dba)2] (dba=(1E,4E)-1,5-diphenylpenta-
1,4-dien-3-one; 49 mg, 0.09 mmol), 1,1’-bis(diphenylphosphino)ferrocene
(dppf; 47 mg, 0.09 mmol), sodium tert-butoxide (0.6 g, 6.4 mmol), and tol-
uene (30 mL) was put in a pressure tube. It was heated at 808C and
stirred for 48 h. After completion of the reaction, it was quenched by the
addition of water and extracted with dichloromethane. The combined ex-
tracts were washed with a brine solution and dried over anhydrous
Na2SO4. Rotary evaporation of the extracts gave the crude product,
which was purified by silica-gel column chromatography using a mixture
of hexanes/dichloromethane (1:4) to produce a yellow solid; yield 0.9 g
(65%); m.p. 320–3228C; 1H NMR (CDCl3, 500 MHz): d=8.17–8.16 (m,
6H), 8.12 (d, J=7.5 Hz, 2H), 8.04 (s, 4H), 7.98 (t, J=7.8 Hz, 3H), 7.95
(d, J=9.5 Hz, 1H), 7.87–7.85 (m, 2H), 7.18–7.14 (m, 4H), 7.01–6.98 (m,
8H), 6.89–6.86 ppm (m, 2H); 13C NMR (CDCl3, 125 MHz): d=143.0,
131.1, 130.9, 129.2, 128.9, 127.6, 127.3, 127.0, 126.8, 126.1, 126.0, 125.8,
125.0, 124.8, 124.6, 123.7, 123.2, 120.7, 120.6 ppm.
2b was obtained in 29% yield as a yellow solid from N4,N4’-diphenyl-
N4,N4’-di(pyren-1-yl)biphenyl-4,4’-diamine by following a procedure simi-
lar to that described above for 2a. M.p. 196–1988C; IR (KBr): n˜C
=
=
O
1685 cmꢀ1 1H NMR (CDCl3, 500 MHz): d=9.79 (s, 1H), 8.22 (d, J=
;
8.0 Hz, 2H), 8.19–8.15 (m, 4H), 8.13–8.02 (m, 6H), 8.01–7.99 (m, 3H),
7.94 (d, J=9.5 Hz, 1H), 7.88 (d, J=8.0 Hz, 1H), 7.86 (d, J=8.0 Hz, 1H),
7.67–7.65 (m, 2H), 7.49–7.47 (m, 2H), 7.40–7.38 (m, 2H), 7.30–7.28 (m,
2H), 7.24–7.21 (m, 2H), 7.13–7.12 (m, 2H), 7.09–7.08 (m, 2H), 6.99–
6.97 ppm (m, 3H); 13C NMR (CDCl3, 125 MHz): d=190.5, 153.9, 148.3,
148.0, 145.1, 140.6, 138.8, 136.9, 133.2, 131.6, 131.24, 131.16, 131.1, 130.9,
130.5, 129.7, 129.3, 128.80, 128.75, 128.3, 128.2, 128.0, 127.8, 127.7, 127.6,
127.5, 127.4, 127.2, 127.1, 126.5, 126.4, 126.3, 126.2, 126.1, 125.7, 125.6,
125.3, 125.2, 125.1, 124.8, 124.7, 123.3, 122.6, 122.5, 122.2, 121.9,
118.2 ppm.
Synthesis of N4,N4’-diphenyl-N4,N4’-di(pyren-1-yl)biphenyl-4,4’-diamine
(1b)
Synthesis of 4-((9,9-diethyl-7-(phenyl(pyren-1-yl)amino)-9H-fluoren-2-
yl)(pyren-1-yl)amino)benzaldehyde (2c)
2c was obtained in 20% yield as a yellow solid from 9,9-diethyl-N2,N7-di-
phenyl-N2,N7-di(pyren-1-yl)-9H-fluorene-2,7-diamine by following a pro-
cedure similar to that described above for 2a. M.p. 220–2228C; IR
Derivative 1b was prepared by following a procedure similar to that de-
scribed above for 1a: yellow solid; yield 80%; m.p. 310–3128C; H NMR
1
(CDCl3, 500 MHz): d=8.17 (d, J=8.5 Hz, 5H), 8.14 (s, 1H), 8.11 (d, J=
7.5 Hz, 2H), 8.06 (s, 4H), 7.99–7.97 (m, 2H), 7.93 (d, J=8.0 Hz, 2H),
7.86 (d, J=8.0 Hz, 2H), 7.39–7.37 (m, 4H), 7.23–7.19 (m, 4H), 7.12–7.07
(m, 8H), 6.96 ppm (t, J=7.0 Hz, 2H); 13C NMR (CDCl3, 125 MHz): d=
147.5, 146.4, 139.7, 132.8, 130.2, 130.0, 128.7, 128.5, 128.2, 127.7, 127.6,
127.1, 126.9, 126.6, 126.2, 126.14, 126.08, 125.5, 125.3, 125.2, 125.0, 124.2,
124.1, 123.8, 122.3, 121.3, 121.2, 121.1, 120.8 ppm.
(KBr): n˜C O =1685 cmꢀ1 1H NMR (CDCl3, 500 MHz): d=9.78 (s, 1H),
;
=
8.22–8.20 (m, 2H), 8.18–8.15 (m, 3H), 8.13–8.08 (m, 5H), 8.06 (s, 2H),
8.02–7.97 (m, 3H), 7.89–7.83 (m, 3H), 7.65 (d, J=8.5 Hz, 2H), 7.47 (d,
J=8.0 Hz, 1H), 7.42 (d, J=8.5 Hz, 1H), 7.28 (d, J=1.5 Hz, 1H), 7.21 (t,
J=7.5 Hz, 2H), 7.10–7.07 (m, 4H), 6.96–6.93 (m, 4H), 1.81–1.72 (m,
4H), 0.35 ppm (t, J=7.25 Hz, 6H); 13C NMR (CDCl3, 125 MHz): d=
190.5, 154.4, 151.53, 151.46, 148.9, 148.2, 144.9, 141.2, 139.2, 138.5, 131.6,
131.3, 131.2, 131.1, 130.9, 130.2, 129.4, 129.2, 128.54, 128.47, 127.9, 127.8,
127.7, 127.32, 127.25, 127.2, 127.0, 126.5, 126.4, 126.3, 126.23, 126.15,
126.0, 125.7, 125.5, 125.2, 125.1, 124.9, 124.4, 123.5, 122.8, 122.1, 121.7,
120.2, 120.0, 119.8, 118.0, 117.1, 56.2, 32.6, 8.6 ppm.
Synthesis of 9,9-diethyl-N2,N7-diphenyl-N2,N7-di(pyren-1-yl)-9H-fluorene-
2,7-diamine (1c)
Derivative 1c was prepared by following a procedure similar to that de-
1
scribed above for 1a: yellow solid; yield 74%; m.p. 250–2538C; H NMR
(CDCl3, 500 MHz): d=8.17–8.13 (m, 6H), 8.09 (d, J=7.5 Hz, 2H), 8.05
(s, 4H), 8.00–7.97 (m, 2H), 7.89 (d, J=9.0 Hz, 2H), 7.83 (d, J=8.5 Hz,
2H), 7.39 (d, J=8.5 Hz, 2H), 7.22–7.16 (m, 4H), 7.10 (d, J=2.0 Hz, 2H),
7.07–7.05 (m, 4H), 6.96–6.94 (m, 4H), 1.69 (q, J=7.5 Hz, 4H), 0.34 ppm
(t, J=7.5 Hz, 6H); 13C NMR (CDCl3, 125 MHz): d=151.0, 149.0, 147.3,
141.2, 135.8, 131.1, 130.9, 129.1, 129.0, 128.1, 127.6, 127.4, 127.1, 126.7,
126.2, 126.0, 125.8, 124.9, 124.8, 124.7, 123.4, 121.9, 121.6, 121.2, 119.5,
117.3, 55.9, 32.4, 8.3 ppm.
Synthesis of (E)-2-cyano-3-(4-((4-(phenyl(pyren-1-
yl)amino)phenyl)(pyren-1-yl)amino)phenyl)acrylic acid (3a)
A mixture of 2a (1.38 g, 2 mmol), cyanoacetic acid (0.21 g, 2.48 mmol),
ammonium acetate (53 mg, 0.69 mmol), and glacial acetic acid (25 mL)
was heated under reflux for 12 h during which time the aldehyde 2a was
completely consumed as indicated by TLC. Cooling the reaction mixture
to room temperature produced a red precipitate, which was filtered and
washed with water and a diethyl ether/hexanes mixture (1:1) successively.
The analytically pure product was obtained by recrystallization from a
toluene/hexanes mixture (1:3, 20 mL) as a red solid; yield 1.29 g (85%);
Synthesis of 4-((4-(phenyl(pyren-1-yl)amino)phenyl)(pyren-1-
yl)amino)benzaldehyde (2a)
N1,N4-diphenyl-N1,N4-di(pyren-1-yl)benzene-1,4-diamine (0.66 g, 1 mmol)
was dissolved in chlorobenzene (20 mL) and N-methyl-N-phenylforma-
mide (NMF; 0.2 mL, 1.5 mmol). Then, POCl3 (0.14 mL, 1.5 mmol) was
added dropwise at room temperature. The reaction mixture was stirred
m.p. 220–2228C; IR (KBr): n˜CꢂN =2208 cmꢀ1
500 MHz): d=8.39 (d, J=8.0 Hz, 1H), 8.35–8.32 (m, 2H), 8.29 (t, J=
7.75 Hz, 2H), 8.24–8.22 (m, 3H), 8.17–8.15 (m, 3H), 8.11–8.04 (m, 6H),
7.99 (d, J=8.0 Hz, 2H), 7.88 (d, J=8.0 Hz, 1H), 7.81 (d, J=9.0 Hz, 2H),
;
1H NMR (CDCl3,
748
www.chemasianj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 738 – 750

Organic Dyes Containing Pyrenylamine-Based Cascade Donor Systems
7.25–7.20 (m, 4H), 6.98 (d, J=9.0 Hz, 4H), 6.94 (t, J=7.5 Hz, 1H),
6.76 ppm (d, J=9.0 Hz, 2H); 13C NMR (CDCl3, 125 MHz): d=152.3,
148.3, 145.5, 140.4, 139.8, 138.8, 132.7, 131.14, 131.09, 130.9, 130.8, 130.3,
129.9, 129.7, 129.1, 128.5, 128.1, 128.02, 127.96, 127.6, 127.2, 127.1, 127.0,
126.9, 126.3, 126.0, 125.8, 125.7, 124.4, 124.3, 123.1, 123.0, 122.7, 122.4,
121.9, 117.3 ppm.
con cell. Photocurrent–voltage curves of the DSSCs were obtained with a
potentiostat/galvanostat. The thickness of the TiO2 film was judged by
scanning electron microscopic images (SEM). For UV absorption spectra,
dye molecules were coated on the TiO2 films, and the corresponding
spectra were obtained by using an UV/Vis spectrophotometer equipped
with an integrating sphere. Electrochemical impedance spectra (EIS)
were obtained from the potentiostat/galvanostat, equipped with a FRA2
module, under a constant light illumination of 100 mWcmꢀ2. The fre-
quency range explored was 10 mHz to 65 kHz. The applied bias voltage
was set at the open-circuit voltage of the DSSC between the ITO-Pt
counter electrode and the FTO-TiO2 dye working electrode, starting
from the short-circuit condition; the corresponding alternating current
(AC) amplitude was 10 mV. The impedance spectra were analyzed by
using an equivalent circuit model. IPCE curves were obtained under
short-circuit conditions. The light source was a class A quality solar simu-
lator (PEC-L11, AM 1.5 G); light was focused through a monochromator
onto the photovoltaic cell. The monochromator was incremented through
the visible spectrum to generate the IPCE (l) as defined by IPCE (l)=
1240 (JSC/lf), where l is the wavelength, JSC is the short-circuit photocur-
rent density (mAcmꢀ2) recorded with a potentiostat/galvanostat, and f is
the incident radiative flux (Wmꢀ2) measured with an optical detector and
a power meter.
Synthesis of (E)-2-cyano-3-(4-((4’-(phenyl(pyren-1-yl)amino)biphenyl-4-
yl)(pyren-1-yl)amino)phenyl)acrylic acid (3b)
Compound 3b was prepared from 2b by following a similar procedure as
described for 3a: brown solid; yield 60%; m.p. 257–2628C; IR (KBr):
n˜CꢂN =2216 cmꢀ1 1H NMR (CDCl3, 500 MHz): d=8.36–8.28 (m, 4H),
;
8.23–8.17 (m, 6H), 8.11–8.01 (m, 6H), 7.92 (d, J=8.0 Hz, 2H), 7.85 (d,
J=8.5 Hz, 1H), 7.77 (d, J=8.5 Hz, 2H), 7.49 (d, J=8.5 Hz, 2H), 7.42 (d,
J=8.5 Hz, 2H), 7.25–7.18 (m, 4H), 7.01 (d, J=8.0 Hz, 2H), 6.98–6.92 (m,
3H), 6.87 ppm (d, J=8.5 Hz, 2H); 13C NMR (CDCl3, 125 MHz): d=
148.1, 140.5, 132.1, 131.1, 130.9, 130.0, 129.8, 128.6, 128.2, 128.1, 127.7,
127.2, 127.1, 126.9, 126.0, 125.8, 124.7, 124.3, 123.1, 122.7, 122.5, 121.7,
119.0 ppm; HRMS m/z: calcd for C60H37N3O2 854.2783 [M+Na+]; found
854.2932.
Synthesis of (E)-2-cyano-3-(4-((9,9-diethyl-7-(phenyl(pyren-1-yl)amino)-
9H-fluoren-2-yl)(pyren-1-yl)amino)phenyl)acrylic acid (3c)
Compound 3c was prepared from 2c by following a similar procedure as
described for 3a: red solid; yield 50%; m.p. 277–2798C; IR (KBr): n˜Cꢂ
N =2212 cmꢀ1 1H NMR (CDCl3, 500 MHz): d=8.36–8.26 (m, 4H), 8.23–
;
Acknowledgements
8.15 (m, 6H), 8.07–7.95 (m, 8H), 7.83 (d, J=8.5 Hz. 1H), 7.78 (d, J=
7.0 Hz, 2H), 7.58–7.52 (m, 2H), 7.36 (s, 1H), 7.23–7.20 (m, 2H), 7.08–
7.06 (m, 2H), 6.97–6.92 (m, 3H), 6.85–6.81 (m, 3H), 1.69–1.62 (m, 4H),
0.23–0.20 ppm (m, 6H); 13C NMR (CDCl3, 125 MHz): d=156.7, 156.2,
153.6, 152.7, 145.8, 144.1, 142.7, 140.0, 136.0, 135.9, 135.6, 134.9, 134.6,
134.2, 132.4, 132.2, 131.8, 130.8, 130.7, 130.4, 129.2, 129.1, 127.0, 126.7,
125.8, 125.6, 123.2, 60.8, 36.8, 13.6 ppm; LRMS m/z: calcd for C65H45N3O2
922.3 [M+Na+]; found 922.2.
KRJT is thankful to Department of Science and Technology, New Delhi,
India for financial support (Ref. No. DST/TSG/ME/2010/27). K.-C.H ac-
knowledges a grant-in-aid from National Science Council, Taiwan. N.K.
is thankful to University Grants Commission, New Delhi, India for re-
search fellowship.
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A fluorine-doped SnO2 conducting glass electrode (FTO, 7 W sqꢀ1, trans-
mittance ca. 80%) was first cleaned with a neutral cleaner and then
washed with deionized water, acetone, and isopropyl alcohol, sequential-
ly. The conducting surface of the FTO was treated with a solution of tita-
nium tetraisopropoxide (1 g) in 2-methoxyethanol (3 g) to obtain a good
mechanical contact between the conducting glass and TiO2 film, as well
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750
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Chem. Asian J. 2012, 7, 738 – 750