Metal-free and fluorescent diketopyrrolopyrrole fluorophores for dye-sensitized solar cells

Article abstract of DOI:10.1002/cplu.201200059

Novel metal-free and fluorescent dipyrrolopyrrole (DPP) dyes consisting of a central bis(lactam)-bearing pendent benzyl or branched hydrocarbon groups as solubilizing fragments and two orthogonal side arms (dimethylaminopropyne and benzoate anion) have been designed and synthesized. The UV/Vis absorption spectra recorded in THF are dominated by intense low-energy π-π absorptions centered at 488 nm or 602 nm, respectively, for the phenyl- or thiophene-based DPP dyes. The fluorescence spectra also display broad bands at 555 nm (τF= 0.57 and ΦF=4.6 ns) for the phenyl- and at 624 nm (ΦF=0.31 and ΦF=3.5 ns) for the thiophene-based molecular structures. Under standard global AM 1.5G irradiation a maximum photon-to-electron conversion efficiency of 2.54% was achieved with dye-sensitized solar cells based on nanocrystalline TiO2 (Jsc=7.5 mAcm-2, Voc=0.49 V, and FF=0.70) for the phenyl-based DPP dye and 1.89% (Jsc=7.1 mAcm-2, Voc= 0.41 V, and FF=0.65) for the thiophene-based DPP dye.

Full text of DOI:10.1002/cplu.201200059

DOI: 10.1002/cplu.201200059
Metal-Free and Fluorescent Diketopyrrolopyrrole
Fluorophores for Dye-Sensitized Solar Cells
Delphine Hablot,[a] Ashraful Islam,*[b] Liyuan Han,[b] and Raymond Ziessel*[a]
Novel metal-free and fluorescent dipyrrolopyrrole (DPP) dyes
consisting of a central bis(lactam)-bearing pendent benzyl or
branched hydrocarbon groups as solubilizing fragments and
two orthogonal side arms (dimethylaminopropyne and ben-
zoate anion) have been designed and synthesized. The UV/Vis
absorption spectra recorded in THF are dominated by intense
low-energy p–p* absorptions centered at 488 nm or 602 nm,
respectively, for the phenyl- or thiophene-based DPP dyes. The
fluorescence spectra also display broad bands at 555 nm (FF =
0.57 and tF =4.6 ns) for the phenyl- and at 624 nm (FF =0.31
and tF =3.5 ns) for the thiophene-based molecular structures.
Under standard global AM 1.5G irradiation maximum
a
photon-to-electron conversion efficiency of 2.54% was ach-
ieved with dye-sensitized solar cells based on nanocrystalline
TiO2 (Jsc =7.5 mAcmꢀ2, Voc =0.49 V, and FF=0.70) for the
phenyl-based DPP dye and 1.89% (Jsc =7.1 mAcmꢀ2, Voc =
0.41 V, and FF=0.65) for the thiophene-based DPP dye.
Introduction
The direct conversion of solar energy into electricity or chemi-
cal energy is one of the main scientific and technological chal-
lenges that humanity will have to face during this century.[1]
The on-going depletion of fossil resources and the related en-
vironmental issues necessitate that the scientific community
should find alternative sources of energy. Among these, solar
energy lies at the forefront owing to its inexhaustible charac-
ter. Silicon-based solar cell use is a well-established technology
that requires massive amounts of energy to produce the sili-
con and thus its widespread development is technically limit-
ed. Dye-sensitized solar cells (DSSCs)[2,3] and bulk heterojunc-
tion solar cells (BHJSCs)[4,5] are alternatives which have received
considerable attention as candidates for renewable energy de-
vices. In this regard, metal-free sensitizers are interesting
owing to their ease of preparation and purification, their high
absorption coefficients and broad absorption wavelengths
spanning from the near-UV to the near infrared, and they are
easy to recycle at the end of their life cycle. Other advantages
of organic dyes versus phosphorescent metal complexes are
the wide range of readily modified molecular structures that
are available and their particular optical and redox properties.
The use of organic dyes to photosensitize mesoporous TiO2
has blossomed over the last decades.[6] Indoline,[7] fluorene,[8]
coumarin,[9] thienothiophene,[10] hemicyanines,[11] triaryla-
mines,[12] heteropolycyclic,[13] porphyrin,[14,15] phthalocyanine,[16]
blue squaraine[17] benzothiadiazole,[18] and thiophene-N,N-di-
phenylhydrazone[19] dyes have all been scrutinized.
transfer or electron transfer. A push–pull zinc porphyrin sensi-
tizer recently achieved very high power conversion efficien-
cy.[20] Our research group has recently introduced the concept
of using highly fluorescent borondipyrromethene (Bodipy)
dyes to photosensitize semiconductor-based solar cells.[19,20]
The absence of the donor (triphenylamine) but the presence of
the anchor (carboxylate) provides highly fluorescent dyes capa-
ble of charge separation under appropriate cell conditions and
the covalent linkage is provided by a carboxylate[21] or an ace-
tylacetonate[22] unit. The absorption properties of the Bodipy
dyes were tuned by changing the delocalization pathway on
the organic dyes and by using double bond side-arms. We
note that D-p-A Bodipy dyes have also been used in DSSC
solar cells.[23]
Related dipyrrolopyrrole (DPP) compounds exhibit pro-
nounced absorption and intense emissive properties with ex-
ceptional outdoor photochemical, mechanical, and thermal sta-
bility which have led to industrial applications in paints, inks,
and plastics.[24] We recently used soluble DPP scaffoldings to
construct donor/acceptor dyads to promote intramolecular
energy-transfer processes which appeared to be extremely fast
(picosecond time scale).[25] The use of DDP subunits to link two
different dyes also favors energy transfer by excitation in the
[a] D. Hablot, Dr. R. Ziessel
Laboratoire de Chimie Molꢀculaire et Spectroscopies Avancꢀes (LCOSA)
UMR 7515 au CNRS, Ecole Europꢀenne de Chimie, Polymꢁres et Matꢀriaux
25 rue Becquerel, 67087 Strasbourg Cedex 02 (France)
E-mail: ziessel@unistra.fr
In fact, most of these organic dyes have the same basic
structure with a donor-(p-conjugated spacer)-acceptor (D-p-A)
skeleton but in some cases remarkable solar-to-electric power
conversion efficiencies reaching around 10% have been ach-
ieved.[6] In all cases, the donor belongs to the triarylamine
family while the molecular anchor is an acrylic acid fragment.
In these dyes, the fluorescence is heavily quenched by charge
Homepage: http://www-lmspc.u-strasbg.fr/lcosa
[b] Dr. A. Islam, L. Han
Advanced Photovoltaics Center
National Institute for Materials Science (NIMS)
1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan)
462
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Dye-Sensitized Solar Cells
DPP chromophore, the direction
of which can be switched by
changing the environment.[26]
A
recent study on organic
bulk heterojunction (BHJ) has
demonstrated the viability of so-
lution-processed heterojunctions
composed entirely of small mol-
ecules.[4c,c,27] Along these lines
DPP dyes have been widely in-
vestigated as law-band-gap ma-
terials in solar cells.[28] When
blended with PC61BM (a modi-
fied fullerene as electron accept-
or), BHJ solar cells generated
photon-to-current conversion ef-
ficiencies (PCEs) up to 4.4% with
high JSC =10 mAcmꢀ2 and VOC
=
0.92 V.[29] This incredible efficien-
cy was ascribed to the good film
morphology,
to
adequate
HOMO/LUMO energy scaling of
the DPP levels with respect to
the electron acceptor and the
working potential of the hole ex-
tracting electrode, but also to
the hole mobility. Narrow-band-
gap DPP polymers for solar cells
Scheme 1. Outline of the preparation of phenyl-based DPP dyes. Reaction conditions: 1) benzylbromide
(10 equiv), DMF, K2CO3 (11 equiv), 1208C, 2 h (36% yield); 2) dimethylaminopropyne (1 equiv), benzene, triethyla-
mine, [Pd(PPh3)2Cl2] (10 mol%), CuI (15 mol%), 408C, 60 h (35% (3) and 20% (4) yield); 3) benzene, ethanol, trie-
thylamine, [Pd(PPh3)2Cl2] (10 mol%), CO flux, 708C, 15 h (90% yield); 4) NaOH, THF/H2O, 808C; 5) DMF, water, trie-
thylamine, [Pd(PPh3)2Cl2] (10 mol%), CO flux, 1108C, 15 h (47% yield). DMF=N,N-dimethylformamide.
have also been produced for high photovoltaic performan-
ces.[30]
quent hydrolysis under basic conditions (Scheme 1). Unfortu-
nately, here this protocol was not effective owing to the fact
that the basic conditions destroyed the bis(lactam) framework.
To reach the target molecule 6 we used a carboxylation reac-
tion promoted by palladium catalysis under a flow of carbon
monoxide, water as the nucleophile; and triethylamine as base
to quench the nascent acid. Compound 6 was isolated by
column chromatography in 47% yield.
To the best of our knowledge, there are only two recent
contributions concerning DPP dyes based on the general con-
cept of D-p-A in DSSC applications.[31,32] Herein, we have built
highly fluorescent DPP dyes through modification of the DPP
periphery by introduction an acid function on one side for co-
valent linking to mesoporous TiO2 and a dimethylaminopro-
pyne fragment on the other side to disfavor aggregation of
the dyes.
A similar strategy was developed for the preparation of thio-
phene-based DPP dyes (Scheme 2).[34] Alkylation of the insolu-
ble bis(lactam) compound 7 was achieved using 2-ethylhexyl
bromide under basic conditions. Bromation of the thiophene
subunit was possible using NBS and the use of dimethylamino-
propyne allowed the preparation of a mixture of compounds
10 and 11, which could be easily separated owing to the pres-
ence of the polar dimethylamino group. Unfortunately, the car-
boxylation conditions developed in Scheme 1 and applied to
derivative 11 led to intractable mixtures of dyes which could
not be separated by conventional chromatography. The solu-
tion came from the use of the benzoate derivative 12 which
could be satisfactorily linked to the DDP dye through a palladi-
um(0)-mediated cross-coupling reaction.
Results and Discussion
Synthesis
The preparation of the target dyes is illustrated in Scheme 1
and Scheme 2. Solubilization of 1 was made feasible using
excess benzylbromide. Orthogonal functionalization of 2 re-
quired the use of dimethylaminopropyne and Pd0 catalysis.
Owing to the high polarity imported by the dimethylamino
fragment, the chromatographic separation of 3 and 4 was
facile, and provided the pivotal monosubstituted derivative 3
in 35% yield. The disubstituted derivative 4 (more polar com-
pound), was isolated pure during the same procedure in
a yield of 20%. The yield ratio 3:4 depends on the reaction
conditions and also on the stoichiometry of the reactants. It
was anticipated that the most reliable way to generate the car-
boxylic acid would be to prepare first the diester 5 using a pal-
ladium(0)-catalyzed carboalkoxylation reaction,[33] and subse-
Optical properties
Photophysical measurements were performed on DPP dye 6 in
THF solution. The absorption spectroscopic profile is broad
and centered at 488 nm with a moderately high absorption co-
efficient log e=4.15 (Table 1). The breadth of the band is indi-
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463

R. Ziessel et al.
vibronic spacing of approximate-
ly 820 cmꢀ1. On comparing the
absorption spectroscopic profile
with the emission profile, it is
clear that there is a major reor-
ganization in the excited state,
confirming the possibility of
charge transfer. This is in strong
contrast with Bodipy dyes, which
display emission profiles that
mirror the absorption spectra
(Figure 1).[37]
For the second dye 13, the
low-energy absorption band is
shifted to the red by about
114 nm with a larger absorption
coefficient log e=4.57 (Table 1).
This bathochromic shift is in-
duced by the thiophene moiet-
ies which favor delocalization
along the thiophene–lactam
main axis.[38] Remarkably, the ab-
sorption band is quite broad,
spanning over 200 nm (Figure 2).
Here the Stokes shift is weaker
(586 cmꢀ1) and the emission pro-
file resembles the absorption
with a vibronic sequence of ap-
proximately 1363 cmꢀ1
.
The
short lifetime and the shape of
the emission band are in keep-
Scheme 2. Outline of the preparation of thiophene-based DPP dyes. Reaction conditions: 1) 2-ethylhexylbromide
(4.6 equiv), DMF, K2CO3 (4 equiv), 1458C, 15 h (49% yield); 2) NBS, anhydrous chloroform, RT, argon (42% yield);
3) dimethylaminopropyne (1 equiv), benzene, triethylamine, [Pd(PPh3)4] (10 mol%), 608C, 15 h (26% (10) and 46%
(11) yield); 4) DMF, water, triethylamine, [Pd(PPh3)2Cl2] (10 mol%), CO flux, 708C, 15 h, eventually followed by sapo-
nification; 5) DMF, triethylamine, [Pd(PPh3)2Cl2] (10 mol%), 608C, 15 h (99% yield).
ing with the properties of a sin-
glet emitter.
When absorbed on a transpar-
ent thin TiO2 film, dyes 6 and 13
Table 1. Spectroscopic features of dyes 6 and 13
show broad absorption spectra
similar to that in solution, where-
Cmpd
lmax [nm][a]
log emax
lflu [nm]
FF
tS [ns]
S
+/0 [V][b]
E0ꢀ0 [eV][c]
S
+/* [V][d]
as the absorption peaks are
6
13
488
602
4.15
4.57
555
624
0.57
0.31
4.6
3.5
+0.92
+0.88
2.13
1.65
ꢀ1.21
ꢀ0.77
slightly red-shifted owing to the
interaction between the carbox-
[a] Absorption peaks and molar extinction coefficients (e) were measured in THF. [b] Ground-state oxidation po-
tentials, S+/0, were determined by cyclic voltammetry in deoxygenated dichloromethane solutions, containing
0.1m TBAPF6 (TBA=tetrabutylammonium), at a solute concentration of approximately 1.5 mm and at room
temperature. Potential was standardized versus ferrocene (Fc) as internal reference and converted to the SCE
ylate group and TiO2 (Figure 3).
This broadening of the absorp-
tion spectrum is desirable for
scale assuming that E1/2 (Fc/Fc+)= +0.38 V (DEp =60 mV) vs. SCE. Error in half-wave potentials is ꢁ10 mV For
harvesting the solar spectrum
.
irreversible processes the peak potentials (Eap or Ecp) are quoted. All reversible redox steps result from one-elec-
tron processes. [c] Optical energy gaps, E0ꢀ0, were estimated from the 10% maximum absorption intensity of
the absorption spectra on transparent TiO2. [d] Excited-state oxidation potentials, S+/*, were calculated from
and leads to a large photocur-
rent.
the expression S+/*=S+/0ꢀE0ꢀ0
.
To evaluate the thermodynam-
ics of electron injection from the
excited-state of the dye to the
cative that it involves two overlapping sets of transitions.
There is a large Stokes shift of approximately 2470 cmꢀ1, which
indicates that the excited state may possess considerable
charge-transfer character.[35] The fluorescence quantum yield is
rather high but the emission lifetime is comparable to those
recorded for the Bodipy dyes and lies in the nanosecond
range.[36] The fluorescence spectroscopic profile is broad and
can be analyzed in terms of a single transition with an average
conduction band of the TiO2 electrode and dye regeneration
through electron donation from the electrolyte, a cyclic vol-
tammogram was obtained in a typical three-electrode electro-
chemical cell in deoxygenated and anhydrous dichlorome-
thane containing 0.1m tetra-n-butylammonium hexafluoro-
phosphate as the supporting electrolyte (Table 2). A Pt micro-
disk was used as a working electrode and ferrocene was used
as an internal reference. The ground-state oxidation potential
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Dye-Sensitized Solar Cells
values, S+/0, of dye 6 and 13 were measured to be 0.92 and
0.88 V (vs SCE), respectively, and these values were low
enough for efficient regeneration of the oxidized dyes through
reaction with iodide (0.07 V vs SCE).[39] The onset of the optical
energy gap (E0ꢀ0) of dye 6 was approximately 2.13 eV, deter-
mined from the 10% maximum absorption intensity of the ab-
sorption spectrum on transparent TiO2 films, whereas this gap
is estimated to 1.65 eV for dye 13. The excited-state oxidation
potentials, S+/*, of dye 6 and 13 were estimated to ꢀ1.21 V
and ꢀ0.77 V (versus SCE), respectively. The S+/* of dye 6 lies
above the conduction band edge (approximately ꢀ0.82 V vs
SCE)[40] of the nanocrystalline TiO2. Therefore, an efficient excit-
ed-state injection into the conduction band of TiO2 is expected
to occur with dyes 6. For dye 13, S+/* lies about 0.1 eV lower
than the conduction band edge of TiO2. Hence, the excited
state of dye 13 is not energetically favorable for efficient elec-
tron injection.
Figure 1. Absorption (black line), emission (dashed line), and excitation
(dotted line) spectra of dye 6 in THF at room temperature.
The photovoltaic performance parameters of cells sensitized
by dyes 6 and 13 were studied using electrolytes E1, E2, and
E3 with various LiI and TBP (4-tert-butylpyridine) concentra-
tions under standard AM 1.5G irradiation (100 mWcmꢀ2). The
short-circuit current density (Jsc), open-circuit voltage (Voc), fill
factors (FF), and overall solar-to-electric energy conversion effi-
ciencies (h) for TiO2 electrodes coated with dyes 6 and 13 are
summarized in Table 2. The IPCEs for the solar cells, plotted as
a function of excitation wavelength, were calculated from
Equation (1),
IPCEðlÞ ¼ 1240ðIsc=lfÞ
ð1Þ
where Isc is the photocurrent density at short circuit (mAcmꢀ2
)
Figure 2. Absorption (black line), emission (dashed line), and excitation
(dotted line) spectra of dye 13 in THF at room temperature.
under monochromatic irradiation, l is the wavelength of inci-
dent radiation (nm) and f is the incident radiative flux (Wmꢀ2).
The solar cell sensitized by dye 6 showed high power con-
version efficiency, with Jsc of 7.5 mAcmꢀ2, Voc of 0.49 V, FF of
0.70, corresponding to an overall conversion efficiency (h) of
2.54% with electrolyte composition of 0.6m dimethylpropyli-
midazolium iodide (DMPII), 0.05m I2 and 0.1m LiI (E1; Figure 4
and Table 2). The Jsc of the dye-sensitized solar cell (DSC;
8.0 mAcmꢀ2) with an electrolyte composition of 0.6m DMPII,
0.05m I2 and 0.5m LiI (E2) was slightly higher than that of the
DSC with E1, owing to the higher concentration of LiI com-
pared to that of E1. However, both Voc and FF with E2 were
lower than those with E1, resulting in a lower h value (2.36%)
for E2. The open-circuit voltage increased at a TBP concentra-
tion of 0.5m with electrolyte composition E3 (0.6m DMPII,
0.05m I2, 0.1m LiI, 0.5m TBP). The TBP probably adsorbed on
the bare TiO2 surface and suppressed the recombination be-
Figure 3. Absorption spectra of dyes 6 (c) and 13 (a) anchored on
a transparent TiO2 film.
ꢀ
Table 2. Photovoltaic performance of DSCs based on dye 6.
tween the injected electrons and I3 ions. In contrast, the
short-circuit photocurrent decreased significantly as the TBP
concentrations increased owing to higher energy shift of the
TiO2 conduction band edge. The solar cell sensitized by dye 13
showed poor power conversion efficiency compare to that of
dye 6. Photovoltaic parameters Jsc, Voc, FF, and h of the dye 13
with E2 were 7.1 mAcmꢀ2, 0.41 V, 0.65, and 1.89%, respectively.
Although dye 13 has better light harvesting efficiency than
Cmpd
TBP (M)/LiI (M)
Jsc [mAcmꢀ2
]
Voc [V]
FF
h [%]
6
6
6
13
13
13
0:0.5
0:0.1
0.5:0.1
0:0.5
0:0.1
0.5:0.1
8.0
7.5
4.0
7.1
5.1
3.6
0.44
0.49
0.53
0.41
0.46
0.52
0.67
0.70
0.78
0.65
0.68
0.74
2.36
2.54
1.66
1.89
1.60
1.39
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465

R. Ziessel et al.
rent conversion efficiency in this range reaches about 90%.
The onset of the IPCE spectrum of dye 6 is close to 650 nm. As
shown in Figure 5, dye 13 achieved moderate sensitization of
nanocrystalline TiO2 across the whole visible range and into
the near-IR region as far as 900 nm, and displayed an IPCE
value of about 40% between 350 and 650 nm.
Conclusion
Novel fluorescent DPP dyes have been synthesized with two
orthogonal functions. The dimethylaminopropyne group intro-
duces solubility and polarity which facilitates the purification
procedure and the acid function is required for anchoring to
transparent thin TiO2 films. The novel dyes are fluorescent and
not constructed on a push–pull principle. The redox potential
of the excited dye is such as to allow injection of an electron
into the conduction band of nanocrystalline TiO2. The best
device is produced with a TBP/LiI ratio of 0:0.1, providing
a conversion of 2.54% with a Jsc of 7.5 mAcmꢀ2 and a Voc of
0.49 V. The absorption of dye 6 is not optimal above 600 nm
on the nanocrystalline TiO2 film, and the absorption of dye 13
provides a larger light absorption cross-section. Unfortunately,
dye 13 shows lower incident-photon-to-current conversion effi-
ciency (IPCE) compared to dye 6, resulting in a total solar-to-
electric conversion efficiency (h) of 1.89%. An efficiency im-
provement has not yet been attained for dye 13 owing to
poor excited-state electron injection to the TiO2 conduction
band. However, this study demonstrates that a DPP-based fluo-
rescent molecule could be used as a photosensitizer in dye-
sensitized solar cells. The development of highly efficient or-
ganic sensitizers should be feasible through improved molecu-
lar design, and the fact that a strong luminescence is main-
tained in the dyes is not detrimental for the photosensitization
process. Results from this study strongly indicate that the ap-
plication of this class of DPP-based organic dyes as panchro-
matic sensitizers for DSCs is interesting, and opens new ave-
nues for the design of more efficient dyes by fine-tuning of
the ground and excited states through structural changes.
Figure 4. Photocurrent voltage characteristics of DSCs sensitized with the
dye 6 at AM 1.5G irradiation (light intensities: 100 mWcmꢀ2). The redox elec-
trolyte consisted of a solution of (E1) 0.6m DMPII, 0.05m I2, 0.1m LiI in aceto-
nitrile (solid line); (E2) 0.6m DMPII, 0.05m I2, 0.5m LiI in acetonitrile (dashed
line); and (E3) 0.6m DMPII, 0.05m I2, 0.1m LiI, 0.5m TBP in acetonitrile
(dashed-dotted line).
dye 6, it shows a lower cell performance compared to that of
dye 6 because of poor excited-state electron injection to the
TiO2 conduction band. The concentration of LiI in the electro-
lyte has strong effect on electron injection of this dye in DSCs.
Figure 5 shows the monochromatic incident photon-to-cur-
rent conversion efficiency (IPCE) for DSCs based on dyes 6 and
13 with an electrolyte having a LiI concentration of 0.5m (E2).
The newly developed material 6 shows excellent sensitization
of nanocrystalline TiO2 from 350 to 600 nm with a maximum
value of 78% in the plateau region. The action spectrum of
dye 6 is consistent with the absorption spectrum on a transpar-
ent TiO2 film (Figure 3). Taking into account the reflection and
absorption losses by the conducting glass, the photon-to-cur-
Experimental Section
Synthesis
Compounds 3,[23] 9,[41] and 12[42] were prepared and purified ac-
cording to the literature.
Preparation of compound 5: A solution of 3 (100.0 mg, 1 equiv,
0.159 mmol) and [Pd(PPh3)2Cl2] (5.6 mg, 0.05 equiv, 0.008 mmol) in
benzene/EtOH/triethylamine (10:2:1 mL) was heated to 708C over-
night (15h) under a flux of CO at atmospheric pressure. After cool-
ing to RT, the solvents were removed under vacuum. The residual
solid was dissolved in dichloromethane. The solution was washed
with H2O (1ꢁ30 mL) and the combined water washes back-extract-
ed with dichloromethane (3ꢁ30 mL). The combined organic
phases were dried over magnesium sulfate. The solution was fil-
tered and evaporated to dryness. The residue was purified by
column chromatography on silica gel, eluting with dichlorome-
thane then dichloromethane/EtOH (from 98:2 to 95:5) to give
88.6 mg of orange compound 5 (yield 90%). 1H NMR (300 MHz,
Figure 5. Photocurrent action spectrum obtained with dye 6 (c) and 13
(a) attached to a nanocrystalline TiO2 film. The incident photon-to-current
conversion efficiency is plotted as a function of wavelength. A sandwich-
type sealed cell configuration was used to measure this spectrum. The elec-
trolyte composition was 0.6m DMPII, 0.05m I2, and 0.5m LiI in acetonitrile
(E2).
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Dye-Sensitized Solar Cells
CDCl3): d=8.01 (d, J=8.5 Hz, 2H), 7.73 (d, J=8.3 Hz, 2H), 7.65 (d,
J=8.3 Hz, 2H), 7.40 (d, J=8.3 Hz, 2H), 6.99–7.29 (10 lines m, 10H),
4.89 (s, 4H), 4.30 (q, J=7.3 Hz, 2H), 3.40 (s, 2H), 2.27 (s, 6H),
1.30 ppm (t, J=7.1 Hz, 3H); 13C NMR (75 MHz, CDCl3): d=165.51,
162.55, 162.37, 149.04, 147.35, 137.08, 137.07, 132.48, 131.97,
131.63, 129.82, 128.89, 128.85, 128.77, 128.48, 128.32, 127.45,
126.88, 126.61, 126.57, 110.64, 109.86, 88.11, 84.70, 61.23, 48.53,
45.64, 45.56, 44.17, 14.18 ppm; ESI-MS (CH2Cl2 +1% TFA) m/z (in-
tensity, molecular peak): 622.2 (100, [M+H]+); elemental analysis
(%) calcd for C40H35N3O4: C 77.27, H 5.67, N 6.76; found: C 77.04,
H 5.32, N 6.43.
Compound 13: To a degassed solution of 11 (99.2 mg, 1 equiv,
0.145 mmol) and 12 (29.2 mg, 1.2 equiv, 0.174 mmol) in DMF/trie-
thylamine (20;14 mL) was added [Pd(PPh3)4] (8.4 mg, 0.05 equiv,
0.007 mmol). The reaction mixture was heated to 608C overnight
(15h) then was cooled to RT. The maximum amount of DMF was
removed under vacuum. The residue was dissolved in a water/
methanol mixture and slow addition of diethyl ether resulted in
the precipitation of compound 13 as a violet solid (111.8 mg, quan-
titative yield). 1H NMR (400 MHz, DMSO): d=0.71–0.91 (m, 12H),
1.07–1.38 (m, 16H), 1.64–1.84 (m, 2H), 2.25 (s, 6H), 3.58 (s, 2H),
3.85–4.03 (m, 4H), 7.55 (d, J=4.1 Hz, 1H), 7.67 (d, J=8.2 Hz, 2H),
7.71 (d, J=4.1 Hz, 1H), 7.96 (d, J=8.6 Hz, 2H), 8.71 (d, J=4.1 Hz,
1H), 8.74 ppm (d, J=4.1 Hz, 1H); ESI-MS in dichloromethane/meth-
anol+1% TFA, negative mode, m/z (%): 748.2 (100); elemental
analysis (%) calcd for C44H50N3NaO4S2H2O: C 66.89, H 6.63, N 5.32;
found: C 66.63, H 6.42, N 5.19.
Preparation of compound 6: A solution of 3 (102.0 mg, 1 equiv,
0.162 mmol) and [Pd(PPh3)2Cl2] (5.7 mg, 0.05 equiv, 0.008 mmol) in
DMF/H2O/triethylamine (6:2:1 mL) was heated at 1108C overnight
(15h) under a flux of CO at atmospheric pressure. After cooling to
RT, the solution was taken to dryness under vacuum. The crude re-
sidual solid was purified by column chromatography on silica gel,
eluting with dichloromethane (+1% formic acid) then dichlorome-
thane/EtOH (+1% formic acid, from 98:2 to 8:2) to give 45.2 mg
1
of orange/red compound 6 (yield 47%). H NMR (200 MHz, CDCl3):
Cell fabrication and characterization
d=8.04 (d, J=8.3 Hz, 2H), 7.58–7.82 (3 lines m, 4H), 7.48 (d, J=
8.3 Hz, 2H), 6.94–7.34 (m, 10H), 4.91 (s, 4H), 3.57 (s, 2H), 2.85 ppm
(s, 6H), ESI-MS (CH2Cl2 +1% TFA) m/z (intensity, molecular peak):
594.1 (100, [M+H]+); elemental analysis (%) calcd for C38H31N3O4:
C 76.88, H 5.26, N 7.08; found: C 76.46, H 5.07, N 6.82.
The DSCs were fabricated as follows: A double-layer TiO2 photo-
electrode (thickness 15 mm; area 0.25 cm2) was used as a working
electrode. A 10 mm main transparent layer with titania particles
(ꢂ25 nm) and
a 5 mm scattering layer with titania particles
(ꢂ400 nm) were screen-printed on a fluorine-doped tin oxide con-
ducting glass substrate.[43] A solution of dye 6, (3ꢁ10–4m) in di-
chloromethane (CH2Cl2) was used to coat the TiO2 film with the
dye. The electrodes were immersed in the dye solutions and then
kept at 258C for 24 h to adsorb the dye onto the TiO2 surface. Pho-
tovoltaic measurements were performed in a two-electrode sand-
wich-type sealed-cell configuration. The dye-coated TiO2 film was
used as the working electrode, and platinum-coated conducting
glass was used as the counter-electrode. The two electrodes were
separated by a Surlyn spacer (40 mm thick) and sealed by heating
the polymer frame. The electrolytes were composed of (E1) 0.6m
dimethylpropylimidazolium iodide (DMPII), 0.05m I2 and 0.1m LiI
in acetonitrile; (E2) 0.6m DMPII, 0.05m I2, and 0.5m LiI in acetoni-
trile; and (E3) 0.6m DMPII, 0.05m I2, 0.1m LiI, and 0.5m TBP in ace-
tonitrile. The current-voltage characteristics were measured using
a black metal mask with an aperture area of 0.25 cm2 under stan-
dard AM 1.5G irradiation (100 mWcmꢀ2, WXS-155S-10: Wacom
Denso Co. Japan).[44] Monochromatic incident photon-to-current
conversion efficiency spectra were measured with monochromatic
incident light of 1ꢁ1016 photons cmꢀ2 under 100 mWcmꢀ2 in di-
rector current mode (CEP-2000BX, Bunko-Keiki).
Preparation of compounds 10 and 11: To a degassed solution of 9
(214.8 mg, 1 equiv, 0.314 mmol) and [Pd(PPh3)2Cl2] (11.0 mg,
0.05 equiv, 0.016 mmol) in benzene/triethylamine (18:3 mL) was
added CuI (2.9 mg, 0.05 equiv, 0.016 mmol) and 1-dimethylamino-
2-propyne (34 mL, 1 equiv, 0.314 mmol). The reaction mixture was
heated to 608C overnight (15h) then cooled to RT. The reaction
mixture was filtered through Celite then taken to dryness under re-
duced pressure. The residue was dissolved in dichloromethane and
the solution was washed with saturated aqueous NaCl (1ꢁ20 mL).
The combined washes were back-extracted with dichloromethane
(3ꢁ30 mL) and the combined organic phases then dried over
MgSO4. The solution was filtered and evaporated to dryness. The
residue was purified by column chromatography on silica gel, elut-
ing with dichloromethane then dichloromethane/EtOH (gradient
from 99:1 to 97:3) to give 11 (99 mg, 46%) and then dichlorome-
thane/EtOH (gradient from 95:5 to 85:15) to give 10 (57 mg, 26%).
Compound 11: 1H NMR (400 MHz, CDCl3): d=0.76–0.97 (m, 12H),
1.10–1.47 (m, 16H), 1.76–1.94 (m, 2H), 2.72 (s, 6H), 3.82–4.09 (m,
6H), 7.25 (d, J=4.1 Hz, 1H), 7.38 (d, J=4.1 Hz, 1H), 8.70 (d, J=
4.1 Hz, 1H), 8.77 ppm (d, J=4.1 Hz, 1H); 13C NMR (75 MHz, CDCl3):
d=10.42, 13.99, 23.01, 23.51, 28.29, 30.13, 39.08, 43.93, 46.00,
48.66, 78.96, 108.16, 108.60, 119.05, 127.79, 128.39, 128.55, 130.34,
131.12, 131.45, 131.88, 131.92, 132.01, 132.14, 133.20, 135.20,
135.44, 139.32, 139.61, 161.45 ppm; ESI-MS in dichloromethane/
methanol+1% TFA, positive mode, m/z (%): 684.1 (90), 686.1
(100); elemental analysis (%) calcd for C35H46BrN3O2S2: C 61.39,
H 6.77, N 6.14; found: C 61.13, H 6.53, N 5.82.
Electrochemical measurements
Electrochemical studies employed cyclic voltammetry with a con-
ventional three-electrode system using a BAS CV-50W voltammetric
analyzer equipped with a Pt microdisk (2 mm2) working electrode
and a silver wire counter-electrode. Ferrocene was used as an in-
ternal standard and was calibrated against a saturated calomel ref-
erence electrode (SCE) separated from the electrolysis cell by
a glass frit presoaked with electrolyte solution. Solutions contained
the electro-active substrate (about 1.5 mm) in deoxygenated and
anhydrous dichloromethane containing tetra-n-butylammonium
hexafluorophosphate (0.1m) as supporting electrolyte. The quoted
half-wave potentials were reproducible within approximately
15 mV. See Table 3 for data.
Compound 10: 1H NMR (300 MHz, CDCl3): d=0.66–0.98 (m, 12H),
1.05–1.52 (m, 16H), 1.76–1.95 (m, 2H), 2.47 (s, 12H), 3.65 (s, 4H),
3.84–4.09 (m, 4H), 7.31 (d, J=4 Hz, 2H), 8.83 ppm (d, J=4 Hz, 2H);
13C NMR (75 MHz, CDCl3): d=8.62, 10.42, 13.99, 23.01, 23.50, 28.29,
29.67, 30.11, 39.09, 46.02, 108.78, 127.91, 130.27, 133.17, 135.28,
139.57, 161.48 ppm; ESI-MS in dichloromethane/methanol+1%
TFA, positive mode, m/z (%): 687.2 (100); elemental analysis (%)
calcd for C40H54N4O2S2: C 69.93, H 7.92, N 8.16; found: C 69.74,
H 7.75, N 7.85.
ChemPlusChem 2012, 77, 462 – 469
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chempluschem.org
467

R. Ziessel et al.
[8] S. Kim, J. W. Lee, S. O. Kang, J. Ko, J. H. Yum, S. fantacci, F. De Angelis, D.
Di Censo, M. K. Nazeeruddin, M. Grꢂtzel, J. Am. Chem. Soc. 2006, 128,
16701–16707.
Table 3. Electrochemical data for dyes 6 and 13 and related compound-
s.[a]
[9] Z.-S. Wang, Y. Cui, K. Hara, Y. Dan-oh, C. Kasada, A. Shinpo, Adv. Mater.
2007, 19, 1138–1141.
[10] M. Wang, M. Xu, S. Shi, R. Li, F. Gao, G. Zhang, Z. Yi, R. Humphry-Baker,
P. Wang, S. M. Zakeeruddin, M. Grꢂtzel, M. Grꢂtzel, Adv. Mater. 2008, 20,
4460–4463.
Cmpd E0’(ox, soln) [V],
DE [mV]
E0’(red, soln) [V],
DE [mV]
Electro-gap
2.03 V
3
+1.23 (70)
+1.11 (irr.)
ꢀ1.10 (70)
ꢀ1.74 (irr.)
onset potential +1.03 V onset potential ꢀ1.00 V
[11] Y. S. Chen, C. Li, Z. H. Zeng, W. B. Wang, X. S. Wang, B. W. Zhang, J.
Mater. Chem. 2005, 15, 1654–1661.
6
+1.32 (80)
+1.01 (irr.)
ꢀ1.17 (80)
1.97 V
[12] W. Wu, J. Yang, J. Hua, J. Tang, L. Zhang, Y. Long, H. Tian, J. Mater.
Chem. 2010, 20, 1772–1779.
–
onset potential +0.92 V onset potential ꢀ1.05 V
[13] Y. Ooyama, Y. Shimada, Y. Kagawa, I. Imae, Y. Harima, Org. Biomol. Chem.
2007, 5, 2046–2054.
11
13
+1.32 (80) +1.16 (irr.)
+1.00 (60)
onset potential +0.89 V onset potential ꢀ1.00 V
ꢀ1.14 (70)
1.89 eV
1.83 eV
[14] H. Imahori, T. Umeyama, S. Ito, Acc. Chem. Res. 2009, 42, 1809–1818.
[15] K. D. Seo, M. J. Lee, H. M. Song, H. S. Kang , H. K. Kim, Dyes & Pigments
2012, 94, 143-.149.
[16] S. Eu, T. Katoh, T. Umeyama, Y. Matano, H. Imahori, Dalton Trans. 2008,
5476–5483.
[17] a) A. Burke, L. Schmidt-Mende, S. Ito, M. Grꢂtzel, Chem. Commun. 2007,
234–236; b) J. Warnan, F. Buchet, Y. Pellegrin, E. Blart, F. Odobel, Org.
Lett. 2011, 13, 3944–3947.
[18] D. H. Lee, M. J. Lee, H. M. Song, B. J. Song, K. D. Seo, M. Pastore, C. An-
selmi, S. Fantacci, F. De Angelis, M. K. Nazeeruddin, M. Grꢂetzel, H. K.
Kim, Dyes Pigm. 2011, 91, 192–198.
–
+1.30 (70)
+0.95 (60)
ꢀ1.09 (70)
–
onset potential +0.88 V onset potential ꢀ0.95 V
[a] Potentials determined by cyclic voltammetry in deoxygenated di-
chloromethane solution, containing 0.1m TBAPF6, [electrochemical
window from +1.6 to ꢀ2.2 V], at a solute concentration of approximately
1.5 mm and at room temperature. Potentials were standardized versus
ferrocene (Fc) as internal reference and converted to the SCE scale as-
suming that E1/2 (Fc/Fc+)= +0.38 V (DEp=60 mV) vs. SCE. Error in half-
[19] P. Shen, X. Liu, S. Jiang, L. Wang, L. Yi, D. Ye, B. Zhao, S. Tan, Dyes Pigm.
2012, 92, 1042–1051.
[20] A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin,
E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin, M. Grꢂtzel, Science 2011, 334,
629–634.
wave potentials is ꢁ10 mV For irreversible processes the peak potentials
.
(Eap or Ecp) are quoted. All reversible redox steps result from one-electron
processes. irr.=irradiation
[21] D. Kumaresan, R. P. Thummel, T. Bura, G. Ulrich, R. Ziessel, Chem. Eur. J.
2009, 15, 6335–6338.
[22] J.-H. Olivier, A. Haefele, P. Retailleau, R. Ziessel, Org. Lett. 2010, 12, 408–
411.
Acknowledgements
[23] a) S. Erten-Ela, D. Yilmaz, B. Icli, Y. Dede, S. Icli, U. E. Akkaya, Org. Lett.
2008, 10, 3299–3302; b) S. Kolemen, Y. Cakmak, S. Erten-Ela, Y. Altay, J.
Brendel, M. Thelakkat, E. U. Akkaya, Org. Lett. 2010, 12, 3812–3815.
[24] Y. Zhu, A. R. Radidranath, T. Beyerslein, B. Tieke, Macromolecules 2007,
40, 6981–6989.
[25] D. Hablot, P. Retailleau, R. Ziessel, Chem. Eur. J. 2010, 16, 13346–13351.
[26] D. Hablot, A. Harriman, R. Ziessel, Angew. Chem. 2011, 123, 7979–7982;
Angew. Chem. Int. Ed. 2011, 50, 7833–7836.
We thank the Centre National de la Recherche Scientifique
(CNRS) for financial support of this study and Professor Jack Har-
rowfield (ISIS, Strasbourg) for a critical reading of this manuscript
prior to publication.
Keywords: dipyrrolopyrroles
materials · solar cells · thiophenes · TiO2
· fluorescence · mesoporous
[27] A. Mishra, P. Bꢂuerle, Angew. Chem. 2012, 124, 2060–2109; Angew.
Chem. Int. Ed. 2012, 51, 2020–2067.
[28] A. B. Tamayo, M. Tantiwiwat, B. Walker, T.-Q. Nguyen, J. Phys. Chem. C
2008, 112, 15543–15552.
[29] B. Walker, A. B. Tamayo, X. D. Dang, P. Zalar, J. H. Seo, A. Garcia, M. Tanti-
wiwat, T.-Q. Nguyen, Adv. Funct. Mater. 2009, 19, 3063–3069.
[30] M. M. Wienk, M. Turbiez, J. Gilot, R. A. J. Janssen, Adv. Mater. 2008, 20,
2556–2560.
[31] S. Qu, W. Wu, J. Hua, C. Kong, Y. Long, H. Tian, J. Phys. Chem. C 2010,
114, 1343–1349.
[32] S. Qu, B. Wang, F. Guo, J. Li, W. Wu, C. Kong, Y. Long, J. Hua, Dyes Pigm.
2012, 92, 1384–1393.
[33] a) A. El Ghayoury, R. Ziessel, J. Org. Chem. 2000, 65, 7757–7763; b) L. J.
Charbonniꢄre, N. Weibel, R. F. Ziessel, J. Org. Chem. 2002, 67, 3933–
3936.
[34] H. Bꢅrckstꢅmmer, A. Weissenstin, D. Bialas, F. Wꢅrthner, J. Org. Chem.
2011, 76, 2426–2432.
[35] a) H. M. Rosenberg, E. C. Eimutis, J. Phys. Chem. 1966, 70, 3494–3499;
b) G. Briegleb, J. Trencsꢃni, W. Herre, Chem. Phys. Lett. 1969, 3, 146–
150; c) D. V. Matyushov, G. A. Voth, J. Phys. Chem. A 2000, 104, 6485–
6486.
[1] a) N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. USA 2006, 103, 15729–
15735; b) N. Armaroli, V. Balzani, Angew. Chem. 2007, 119, 52–67;
Angew. Chem. Int. Ed. 2007, 46, 52–66; c) J. Liu, G. Cao, Z. Yang, D.
Wang, D. Dubois, X. Zhou, G. L. Graff, L. R. Pederson, J.-G. Zhang, Chem-
SusChem 2008, 1, 676–697.
[2] a) A. Hagfeldt, M. Grꢂtzel, Chem. Rev. 1995, 95, 49–68; b) L. M. Gon-
Åalves, V. de Zea Bermudez, H. A. Ribeiro, A. M. Mendes, Energy Environ.
Sci. 2008, 1, 655–667; c) A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pet-
tersson, Chem. Rev. 2010, 110, 6595–6663.
[3] a) M. Grꢂtzel, Nature 2001, 414, 338–344; b) L. Han, A. Islam, H. Chen,
C. Malapaka, B. Chiranjeevi, S. Zhang, X. Yang, M. Yanagida, Energy Envi-
ron. Sci. 2012, unpublished results.
[4] a) J. Roncali, Acc. Chem. Res. 2009, 42, 1719–1730; b) P. M. Beaujuge,
J. M. J. Frꢃchet, J. Am. Chem. Soc. 2011, 133, 20009–20029; c) P.-L. Bou-
dreault, A. Najari, M. Leclerc, Chem. Mater. 2011, 23, 456–469; d) H.
Zhou, L. Yang, W. You, Macromolecules 2012, 45, 607–632.
[5] B. Walker, C. Kim, T.-Q. Nguyen, Chem. Mater. 2011, 23, 470–482.
[6] a) J.-H. Yum, D. P. Hagberg, S.-J. Moon, K. M. Karlsson, T. Marinado, L.
Sun, A. Hagfeldt, M. K. Nazeeruddin, M. Grꢂtzel, Angew. Chem. 2009,
121, 1604–1608; Angew. Chem. Int. Ed. 2009, 48, 1576–1580; b) A.
Mishra, M. K. R. Fischer, P. Bꢂuerle, Angew. Chem. 2009, 121, 2510–2536;
Angew. Chem. Int. Ed. 2009, 48, 2474–2499.
[36] R. Ziessel, G. Ulrich, J.-H. Olivier, T. Bura, A. Sutter, Chem. Commun.
2010, 46, 7978–7980.
[37] a) A. Harriman, L. J. Mallon, K. J. Elliot, A. Haefele, G. Ulrich, R. Ziessel, J.
Am. Chem. Soc. 2009, 131, 13375–13386; b) R. Ziessel, P. Retailleau, K. J.
Elliott, A. Harriman, Chem. Eur. J. 2009, 15, 10369–10374.
[38] R. Ziessel, P. Bauerle, M. Ammann, A. Barbieri, F. Barigelletti, Chem.
Commun. 2005, 802–804.
[7] L. Schmidt-Mende, U. Bach, R. Humphry-Baker, T. Horiuchi, H. Miura, S.
Ito, S. Uchida, M. Grꢂtzel, Adv. Mater. 2005, 17, 813–815.
468
www.chempluschem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 2012, 77, 462 – 469

Dye-Sensitized Solar Cells
[39] G. Oskam, B. V. Bergeron, G. J. Meyer, P. C. Searson, J. Phys. Chem. B
2001, 105, 6867–6873.
G. B. Deacon, C. A. Bignozzi, M. Grꢂtzel, J. Am. Chem. Soc. 2001, 123,
1613–1624.
[40] A. Hagfeldt, M. Grꢂtzel, Chem. Rev. 1995, 95, 49–68.
[41] L. Huo, J. Hou, H.-Y. Chen, S. Zhang, Y. Jiang, T. L. Chen, Y. Yang, Macro-
molecules 2009, 42, 6564–6571.
[44] N. Koide, L. Han, Rev. Sci. Instrum. 2004, 75, 2828–2831.
[42] A. P. Melissaris, M. H. Litt, J. Org. Chem. 1992, 57, 6998–6999.
[43] M. K. Nazeeruddin, P. Pꢃchy, T. Renouard, S. M. Zakeeruddin, R. Humph-
ry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia,
Received: March 15, 2012
Published online on April 11, 2012
ChemPlusChem 2012, 77, 462 – 469
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chempluschem.org
469