Push-pull triphenylamine based chromophores as photosensitizers and electron donors for molecular solar cells

Article abstract of DOI:10.1016/j.tet.2013.05.137

A series of push-pull chromophores (1-6) comprising triphenylamine as donor and methylenethiazole, bearing electron-withdrawing groups in the 2′ position, as terminal acceptor groups have been developed. The resulting dyes were characterized by UV-vis and

Full text of DOI:10.1016/j.tet.2013.05.137

Tetrahedron 69 (2013) 6875e6883
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Pushepull triphenylamine based chromophores as photosensitizers
and electron donors for molecular solar cells
Ana Aljarilla a, Cristina Herrero-Ponce b, Pedro Atienzar b, Susana Arrechea a,
,
b,
*
Pilar de la Cruz a, Fernando Langa a , Hermenegildo García
*
a Instituto de Nanociencia, Nanotecnología y Materiales Moleculares (INAMOL), Univ. de Castilla-La Mancha, 45071 Toledo, Spain
Instituto Universitario de Tecnología Química CSIC-UPV, Univ. Politecnica de Valencia, 46022 Valencia, Spain
b
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 April 2013
Received in revised form 28 May 2013
Accepted 31 May 2013
Available online 21 June 2013
A series of pushepull chromophores (1e6) comprising triphenylamine as donor and methylenethiazole,
bearing electron-withdrawing groups in the 20 position, as terminal acceptor groups have been de-
veloped. The resulting dyes were characterized by UVevis and fluorescence spectroscopies, cyclic and
OSWV voltammetries and thermal analysis. The low energy absorption band of these oligomers was
located at 583e705 nm in solution. A significant positive solvatochromism, with excellent correlation
with the KamleteTaft constant, was found confirming the charge transfer character of this band. The-
oretical studies reveal a partial quinoid character in these compounds and a reduced band gap HO-
MOeLUMO energies of 1e6 derived from electrochemical and optical measurements were found to be
suitable for their use as donor materials in combination with PCBM as acceptor in planar heterojunction
solar cells. The studied devices, based on stacked cells connected electrically, incorporating these low
band gap chromophores exhibited reasonable conversion efficiencies of up to 0.91% under air mass (AM)
1.5G illumination.
Keywords:
Triphenylamine
Methylenethiazole
Pushepull chromophores
Bulk heterojunction solar cells
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
intramolecular charge transfer (ICT). The flexibility in the design
and synthesis of pushepull molecules allows exploring the per-
Organic photovoltaic cells (OPVs)1 offer the advantage, with
respect to analogous devices based on inorganic semiconductors, of
the possibility to apply molecular design and organic synthesis to
prepare a wide array of molecules having the same core structure
but different substituents.2 In this way the energy levels and the
absorption spectra of the organic molecules can be finely tuned to
maximise the efficiency of the devices.3 Nowadays, small molecular
semiconductors for OPV are attracting considerable attention, due
to their well defined molecular structure, definite molecular weight
and high purity without batch to batch variations.4 Organic solar
cells with an overall efficiency of 12% have been prepared with this
type of molecules and C60 as acceptor,5 although efficiencies around
1% are not uncommon for these types of cells.6
formance of related analogues with the aim of rationalizing the
parameters that affect the efficiency of organic solar cells based on
these types of molecules.
Recently it has been shown that pushepull systems can self-
assemble into centrosymmeric dimers cancelling molecular di-
polar moments in solid materials and the supramolecular systems
demonstrating that highly dipolar DeA dyes are applicable as p-
type hole conductors in bulk heterojunction (BHJ) solar cells,
overcoming a serious restriction in the design of small molecules
for OPV∙7
Pushepull molecules having triphenylamine (TPA) have shown
great potential in the field of molecular photovoltaic devices8 as
electron donor moiety,9 stabilizing the separated hole generated
from the exciton and improving hole-transport.10
Methylenethiazole moiety bearing electron-withdrawing
groups in the 20-position are excellent acceptors. Pushepull chro-
mophores with this moiety show clear evidence for the formation
of centro-symmetric dimers in concentrated solution. Self-
assembly has also been found in the solid state for dicyano-
methylenethiazole merocyanines.11 This kind of pushepull sys-
tems gives rise to broad absorption bands, assigned to the intra-
molecular charge transfer (ICT), and have been used in non linear
optics12 as well as dyes in bulk heterojunction (BHJ) solar cells.7
In the area of OPVs there is much current interest in studying
the performance of highly dipolar donoreacceptor (DeA)
substituted
p systems (called pushepull structures) as they can
maximize the efficiency of the cell due to the combination of a high
molar absorptivity with a low band gap allowing an efficient
* Corresponding authors. Tel./fax: þ34 925 268 43 (F.L.); tel./fax: þ34 963 877 000
(H.G.); e-mail addresses: Fernando.Langa@uclm.es (F. Langa), hgarcia@qim.upv.es
(H. García).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.05.137

6876
A. Aljarilla et al. / Tetrahedron 69 (2013) 6875e6883
Solar cells prepared with dye HB238 (Chart 1) and phenyl-
C61butyrate methyl ester (PCBM) reached an efficiency of 1.1%;
spin-coated films of this dye showed
a hole mobility of
4ꢀ10ꢁ4 cm2 Vꢁ1 sꢁ1 indicating that charge transport properties are
not necessarily impaired by large ground-state dipole moments of
monomeric molecules.7
Chart 1. Structure of dye HB238.
Herein, we have prepared a series of pushepull chromophores
(1e6 in Chart 2) based on TPA units as donor and methylenethiazole
with electron withdrawing substituents (cyano and/or carbox-
ymethyl ester) as acceptor units showing panchromatic absorption,
extending from the UV to the near-infrared region, and a narrow
HOMOeLUMO gap.13 Finally, using PCBM as n-type component and
chromophores 1e6 as p-type semiconducting elements, we have
also prepared a series of solution-processed small molecule organic
solar cells (SMOSC), allowing us to draw conclusions about the
properties of these chromophores.
R1
R1
S
N
N
R2
R2
S
N
S
N
NC
Scheme 1. Synthetic route to chromophores 1e6.
R1
CN
NC
R3
R1
1
2
3
4
R1 = H, R2 = Ph, R3 = CN
5
6
R1 = H, R2 = Ph
R1 = OC6H13, R2 = Ph
R1 = H, R2 = But, R3 = CN
R1 = H, R2 = Ph, R3 = CO2Me
R1 = OC6H13, R2 = Ph, R3 = CN
excellent thermal stability up to 300 ꢂC with Td higher than 314 ꢂC
(Figure S15) that are in principle satisfactory for application in
photovoltaic devices.
Chart 2. New Push-Pull cromophores (1e6).
2.2. Optical and electrochemical properties
2. Results and discussion
2.1. Synthesis
The optical and electrochemical properties of 1e6 have been
analysed by UVevis absorption and fluorescence emission spec-
troscopies; the electrochemical properties were measured by
means of Cyclic (CV) and Osteryoung (OSWV) voltammetries, per-
formed in o-dichlorobenzene (ODCB)-acetonitrile (5:1) solutions in
the presence of tetrabutylammonium perchlorate (0.1 M) as sup-
porting electrolyte. The corresponding results are listed in Table 1.
The synthesis of the target compounds 1e6 is shown in
Scheme 1. Chromophores 1e4 were prepared by acetic anhydride
mediated condensation reaction of thiazoles 9e117,12,13 with the
corresponding aldehyde 7 or 8.14 After purification through column
chromatography on silica gel, compounds 1e4 were obtained in
42e65 % yield. For the synthesis of 5 and 6, intermediate aldehydes
1415 and 15 were initially prepared in 77 and 75% yield, re-
spectively, by Suzuki coupling of bromoanilines 12 and 1316 and 5-
formyl-2-thienyl boronic acid; finally, condensation of 14 and 15
with thiazole 9 afforded 5 and 6 in 40 and 35%, respectively as dark
blue solids; a single isomer (Z) was obtained in all cases, in con-
cordance with the published results for similar systems.12a
Table 1
Data of UVevis and Fluorescence Emission spectroscopiesa and OSWVb for com-
pounds 1e6
lmax (nm)
log (ε)
lem (nm)c
E1red(V)d
E1 (V)
D
E (eV)e
ox
1
2
3
4
5
6
624
583
603
641
656
705
4.71
4.73
4.70
4.61
4.83
4.51
746
721
719
776
814
ꢁ0.94
ꢁ1.04
ꢁ1.01
ꢁ0.98
ꢁ0.81
ꢁ0.83
0.50
0.67
0.60
0.47
0.51
0.27
1.44
1.71
1.61
1.45
1.32
1.10
All compounds were satisfactorily characterized by 1H and 13C
NMR, FT-IR and MALDI-MS spectrometry (see the Experimental
section and the Supporting data for synthetic details and full ana-
lytical and spectroscopic characterization).
The thermal stabilities of compounds 1e6 were evaluated by
thermogravimetric analysis (TGA) under nitrogen, with a heating
rate of 10 ꢂC/min. The decomposition temperatures (Td) were es-
timated from the TGA plot as the temperature of the intercept of the
leading edge of the weight loss curve. Compounds 1e6 display
>900
a
10ꢁ5 M, in dichloromethane.
b
10ꢁ3 M in ODCB-acetonitrile (5:1) versus Fc/Fþc , glassy carbon, Pt counter elec-
trode, 20 ꢂC, 0.1 M Bu4NClO4, scan rate¼100 mV sꢁ1
.
c
lexc lmax of each compound.
¼
d
e
Non-reversible processes.
Estimated from the difference between E1 and E1
.
ox
red

A. Aljarilla et al. / Tetrahedron 69 (2013) 6875e6883
6877
The UVevis absorption spectra of compounds 1e4 (Table 1,
Figure S16), exhibit an intense absorption band in the visible region
with maxima at 624 nm (log ε¼4.71), 583 nm (log ε¼4.73), 603 nm
(log ε¼4.70) and 641 nm (log ε¼4.61) for compounds 1e4, re-
spectively; replacing the tert-butyl group in 2 by a phenyl group in 1
leads to a bathochromic shift of 41 nm; similarly, a bathochromic
shift of 20 nm is observed when replacing the carboxymethyl sub-
stituent in 3 by the stronger electron-acceptor dicyanovinyl group
(1); finally, the introduction of the electron donor hexyloxy moiety
at the TPA (4) leads to a bathochromic shift of 17 nm respect to 1. On
the other hand, compounds 5e6 (Table 1, Figure S17) display
a bathochromic shift respect to 1e4, explained by the more ex-
tended conjugation due to the presence of the thiophene ring. In
fact, 5 and 6 show light absorption through the whole visible region,
including the near-IR, with maxima at 656 nm (log ε¼4.83) and
705 nm (log ε¼4.51), respectively. It is interesting to note that the
absorption of 6 expands up to 900 nm, facilitating the absorption in
this region of the solar emission. Utilization of materials that absorb
light extending into the IR range is one of the strategies to increase
solar light harvesting, thereby increasing power-efficiency.17 The
high molar extinction coefficient absorption bands are as well
positive for light harvesting and hence photocurrent generation. In
all cases, the intense absorption band can be attributed to the ICT
from the TPA moiety (acting as donor) to the methylene substituted
thiazole ring (acting as acceptor), behaviour that is already observed
in related pushepull compounds.12a,18,19
1 and, consequently, better conjugation between the TPA moiety
and the 2,2-dicyano-methylenethiazole acceptor unit.
The redox properties of 1e6 were investigated by CV and OSWV
in ODCBeacetonitrile (5:1) and the potentials were recalculated
over the value of the potential of Fc/Fþc (0.08 V) using as reference
(Table 1, Figures S20eS22). In the cathodic side, compounds 1e6
show a first reversible oxidation wave assigned to the TPA moiety
based on the oxidation potential of TPAeCHO. A second non-
reversible oxidation wave is observed at 0.96 V for 5. A second
and a third non-reversible oxidation waves were observed for 4
(0.88 V and 1.39 V) and 6 (0.76 V and 1.10 V). On the reduction side,
compounds show an irreversible wave, probably due to the meth-
ylenethiazol moiety. The EHOMO values were calculated with respect
to ferrocene (Fc) as reference (EHOMO: ꢁ5.1 eV). Low-lying HOMO
levels enable high open-circuit voltages and are, therefore, desir-
able.21 The data shown in Table 1 indicate that the largest difference
in the EHOMO values, mostly depending on the TPA donor unit, oc-
curs between compounds 2 and 6, while the largest differences in
the ELUMO values occur between compounds 2 and 5. Although
those dyes with stronger acceptor units exhibit low-lying LUMO
levels, ELUMO of the dyes is still higher than that ELUMO of PCBM
(ꢁ4.3 eV). Hence, the energetics of these dyes match quite well
with PCBM to minimize energy waste upon electron transfer from
the donor dye to the fullerene acceptor.
2.3. Theoretical calculations
Fluorescence Emission spectroscopy was employed to get in-
formation on the excited state-solvent interactions (Figure S18) and
the results are reported in Table 1. The emission maxima are above
700 nm for compounds 1e4, 800 nm for 5, and above 900 nm for 6.
The large Stokes shift observed for compounds 1e5 (between
116 and 158 nm) can be attributed to the significant structural re-
organization upon photoexcitation or to the charge transfer nature
of the excited state.
The ground state geometry and the electron density distribu-
tions of HOMO and LUMO have been calculated using DFT at the
B3LYP 6-31G* (d, p) level in gas phase with Gaussian 03W. All the
calculated distances and angles are in concordance with those
determined by X-ray diffraction for analogous phenylthiazol-2-
ylidene malononitrile systems.12a Fig. 2 shows the optimized
structure of compound 4 to exemplify a typical conformation. The
optimized structure of compounds 1e6 confirm that, in all cases,
To gain more detailed information on the pushepull character of
these series of compounds, the UVevis Absorption spectra of
compounds 1, 5 and 6 have been recorded in a series of solvents,
presenting a large variation of polarity (THF, diethyl ether and
the Dep-A system is essentially coplanar, being the dihedral angles
between the corresponding electron donor fragment and the thi-
azole ring z10ꢂ. For the 4-phenylthiazole derivatives (1, 3e6), the
phenyl ring is twisted respect to the thiazole, with a calculated
dihedral angle w35ꢂ, due to the substitution in 5-position of the
thiazole ring, differently to the corresponding unsubstituted 4-
phenylthiazole, which is coplanar.22
dichloromethane) and KamleteTaft p
* constants.20
For all studied compounds, a positive solvatochromism, up to
42 nm for 1, 31 nm for 5 and 35 nm for 6 (Figures S19), is observed
by increasing the solvent polarity (Table S1), confirming the CT
character of this band; moreover, we found an excellent correlation
(correlation coefficient >0.99) of the absorption maxima with the
KamleteTaft
p
* constants of the solvent (Table S2). The slope (S) of
*), indicative of the sol-
the KamleteTaft equation (E¼EoþS
p
vatochromic sensibility, is higher for 1 (S¼6.01) than for 5 (S¼3.91)
and 6 (S¼3.89) (Fig. 1, Table S2) indicating a higher polarizability of
50
49
48
47
46
45
44
43
42
41
40
Fig. 2. Optimized geometry of compound 4.
It is known that the substitution of the conjugated systems by
terminal electron acceptors23 causes that the aromatic character of
the backbone changes to a partially quinoid-like character in which
the degree of bond length alternation (BLA)24 is significantly re-
ꢁ
duced. In our systems, the calculated BLA values (BLA¼0.05 A) re-
veals a partially quinoid character indicating the existence of
intramolecular CT (ICT). As an example, in compound 6 the length
of double bond between the electron acceptor fragment and the
ꢁ
thiazole ring (1.385 A) is significantly longer than the correspond-
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
25
ꢁ
ing bond of the TCNQ (1.373 A). This fact suggests some zwit-
terionic contribution to the ground state.
π
*
The low-lying calculated HOMO levels of compounds 1e6
Fig. 1. Transition Energy of the low energy band for dyes 1 (square), 5 (circle) and 6
(ꢁ5.03 to ꢁ5.44 eV vs vacuum) are localized through the
p-
(triangle) versus Taft constant of solvents.

6878
A. Aljarilla et al. / Tetrahedron 69 (2013) 6875e6883
conjugated system with an important contribution of the TPAs
fragment. Nevertheless both orbitals, HOMO and LUMO (Fig. 3) are
overlapped, favouring the HOMO to LUMO electronic transitions
and so, increasing the double bond character of the single bonds
(vide supra). The theoretical HOMOeLUMO gap is smaller for
compounds 5 and 6 than those found for the series 1e4, in agree-
ment with the experimental values (Fig. 4).
acceptor in the photoinduced charge separation process. The device
is completed by a gold cathode that was coated also with a thin
layer of molybdenum oxide as electron blocking layer.27 The per-
formance of the SMOSCs was optimised with respect to the dye to
PCBM proportion, and also with respect to the thickness of the TiOx
and MoOx layers.
In a related precedent reporting the activity of solar cells for
a series of merocyanine colorants in combination with PCBM, it was
observed that the proportion between the chromophore and the
PCBM plays an important role determining the current density and
the overall efficiency of the cell.28 It was concluded that in that case
the maximum efficiency is achieved for PCBM weight percentages
ranging from 65 to 85 % and there was a significant drop in current
density in the regions below or above these proportions. In our
case, we also proceeded to optimize this proportion between the
dye and PCBM for compounds 1 and 4. Aimed at this optimization
a series of SMOSCs, prepared exactly under the same parameters
and conditions, but in which the PCBM content was varied in the
range of 70e90 %, were characterised. The results obtained indicate
that the most appropriate proportion for maximum efficiency has
a PCBM percentage about 80%, a value, that is, close to that pre-
viously established for merocyanine dyes.29 Fig. 6 shows some ef-
ficiency values indicating that 80 wt % is about the optimal dye/
PCBM proportion for dyes 3 and 4.
Fig. 3. Distribution of orbital coefficients of HOMO and LUMO of compound 5.
-2,5
-2.91
-2.94
-2.99
-3.07
-3.10
-3,0
-3,5
-4,0
-4,5
-5,0
-5,5
-6,0
-6,5
-3.24
-4.3
-5.03
0,5
0,4
-5.17
-5.30
-5.33
-5.44
-5.44
-6.0
0,3
0,2
(b)
(a)
1
2
3
4
5
6
PCBM
Fig. 4. HOMOeLUMO levels and band gap of compounds 1e6 and their relative
position respecting PCBM.
0,1
0
In a photovoltaic cell, VOC is proportional to the difference be-
tween the HOMO of the donor and the LUMO of the acceptor.21b In
compounds 1e6, the difference between the HOMO level of the
dyes and the LUMO level of PCBM (ꢁ4.3 eV) are in the range of
1.41e0.73 eV, values, which should enable high open-circuit volt-
ages. The LUMO levels vary by only 0.33 eV, being in the range
ꢁ2.91 to ꢁ3.24 eV, higher than that of PCBM, in agreement to the
electrochemically determined values.
70
75
80
85
90
wt% PCBM
Fig. 6. Optimization of dye/PCBM ratio for compounds 3 (a) and 4 (b).
The existence of an optimal PCBM proportion with respect to the
dye could be correlated with the quantum yield of the charge sepa-
ration step and the need of an efficient quenching of the excited state
of the dye by PCBM. This efficient quenching leading to charge sep-
aration may require a high PCBM content to minimise other decay
pathways of the dye excited state alternative to charge separation.
After having optimised the composition of the photoactive layer,
we proceeded to optimise the thickness of the dye/PCBM active
layer as well as those of the hole and electron blocking layers. Plots
of efficiency versus the thickness of these three layers are presented
in Figs. 7 and 8. In the case of the donor-PCBM layer chlorobenzene
2.4. Preparation and characterization of SMOSC
The configuration of the device is shown in Fig. 5.
Fig. 5. Device configuration for dye/PCBM bulk heterojunction solar cells. MoOx and
TiOx act as electron and hole blocking layer, respectively. TCO stands for a transparent
conductive oxide acting as electrodes.
In the cell configuration used here the surface of transparent
conducting electrode (TCO) is coated with a nanometric film of TiOx
that should act as hole blocking layer.26 The photoactive part of the
device consists in a blend of the pushepull molecule acting as
chromophore and electron donor molecule and PCBM as electron
Fig. 7. Optimization of the dye/PCBM layer thickness for dyes 3 (a) and 4 (b).

A. Aljarilla et al. / Tetrahedron 69 (2013) 6875e6883
6879
Table 3
(a)
0,4
0,3
0,2
0,1
0
Comparison of energetic levels and open-circuit voltage for compounds 1e6
exp
oc
EHOMO (eV)a
V
(mV)b
V
(mV)
teo
oc
Compound
1
2
3
4
5
6
ꢁ5.60
ꢁ5.77
ꢁ5.70
ꢁ5.57
ꢁ5.61
ꢁ5.37
1000
1170
1100
970
1010
770
695.00
813.27
805.99
632.55
822.56
739.10
a
Calculated from EOx measurements calibrated against Ferrocene/Ferrocenium
(Fc/Fcþ) redox couple (ꢁ5.1 eV).
0
50
100
Thickness TiOx (nm)
b
Theoretical VOC calculated using the formula VOC¼(1/e)(ELUMOPCBMꢁ
EHOMO
ꢁ0.3 V), where ELUMOPCBM¼ꢁ4.3 V30 and EHOMO
¼EHOMO obtained by
Donor
Donor
(b)
0,4
CV.
0,3
0,2
0,1
0
Overall a comparison of the data presented in Table 2 with those
reported in the literature indicates that the efficiency of the cells
prepared with the dyes 1e6 is mostly limited by the low current
density at short circuit (Jsc) measured for our cells.
In contrast, the VOC values measured in our cells are in agree-
ment with the expected energy levels of the HOMO (Dye) LUMO
(PCBM) involved in the process and they are similar or even higher
than those reported in the literature for organic solar cells.31 Also
the fill factor for our devices is not significantly lower than the
reported values.32 Therefore, from the performance data it can be
concluded that the limiting factor of the efficiency of our cells is the
low JSC current.
The main reason of the low density current is probably the lack of
complete light absorption due to the transparency of the cells. Fig. 9
shows a photograph of the solar cells prepared with dye 1 in which
it can be seen that the cell is visually transparent. The transparency
of the cell was quantitatively assessed by quantitative transmission
optical spectroscopy that shows that the prepared cells with these
dyes exhibit a low absorbance. In fact when the photoresponse of
the cells in terms of incident photon-to-current conversion effi-
ciency (IPCE) versus wavelength (Fig. 10) is corrected by the absor-
bance of the cell or light-harvesting efficiency (LHE) giving the
absorbed photon-to-current conversion efficiency (APCE), a better
understanding of the reasons of the efficiency data is obtained.
These corrected photoresponses of some representative cells are
also shown in Fig. 10. These APCE curves revealed high photon to
electron conversion in a large range of wavelengths, indicating that
the dyes exhibit a high efficiency in the conversion of light into
current, similar or higher to those in the state of the art for SMOSC.
According to these plots one of the main limitations of the dyes is
the need to increase light harvesting, for instance, through an ade-
quate engineering of the cell to increase sufficiently the thickness of
the photoactive layer. However we have already commented that an
increase of the thickness of the donor-PCBM layers has a negative
influence derived from the higher charge recombination rate within
the layer. Other factors like phase segregation with insufficient
blending of the dyes with PCBM or the energy offset of chromophore
LUMO to the LUMO of PCBM must also be considered and optimized
in order to improve the efficiency of the SMOSC.
0
10
20
30
Thickness MoOx(nm)
Fig. 8. (a) Optimization of the hole blocking layer thickness. (b) Electron blocking layer
thickness.
solutions of a mix of dye and PCBM in a proportion 20:80 wt % were
cast by spin coating into TCO coated with TiOx at different speeds
and the resulting thickness determined by optical profilometry. It
was established that the optimal thickness of the photoactive layer
should be around 50 nm.
Concerning the TiOx thickness in the range 0e80 nm it was
observed that there is a region between 20 and 60 nm where the
performance of the cell is the maximum, decreasing if the thickness
is thinner or thicker than these values (Fig. 8a). Similar trend was
observed for the thickness of the MoOx layer and also from this
study a suitable thickness for this electron blocking layer was se-
lected (Fig. 8b).
After having optimized some parameters of the SMOSC for some
selected dyes, we proceeded to the preparation under these con-
ditions of photovoltaic devices for all the dyes 1e6 under study
with the aim of obtaining data of the relative performance of the
corresponding solar cells in order to establish a valid comparison of
efficiency depending on the chemical structure of the dye. A
summary of the results obtained is collected in Table 2.
Table 2
Performance of TPA-based SMOSCs using compounds 1e6 as dyes and electron
donor moieties
Compound
JSC (mA/cm2)
VOC (mV)
Fill factor (%)
Efficiency (%)
1
2
3
4
5
6
1.16
1.59
2.12
1.62
1.55
0.87
1.75
695.00
813.27
805.99
632.55
822.56
739.10
1407
30.31
35.47
30.08
32.08
30.32
27.94
29.17
0.25
0.46
0.51
0.33
0.39
0.18
0.72
Two stacked
cells (dye 3)
Three stacked
cells (dye 3)
1.54
2016
29.28
0.91
As it can be seen in this Table 3 although the theoretical VOC
values are higher than the experimental ones, as expected, the
relative order predicted by theoretical calculations agrees reason-
ably well with the experimental VOC values except for compounds 5
and 6.
Fig. 9. (a) Photographs of conventional P3HT:PCBM devices. (b) Dye 1: PCBM. The
purpose is to provide a visual observation of the transparency of SMOSC prepared with
dye 1.

6880
A. Aljarilla et al. / Tetrahedron 69 (2013) 6875e6883
each experiment, which was run under argon atmosphere. Exper-
(a)
10.0
7.5
5.0
2.5
0.0
iments were done in a one-compartment cell equipped with
a platinum working microelectrode (B¼2 mm) and a platinum wire
counter electrode. An Ag/AgNO3 (0.01 M in CH3CN) electrode was
used as reference and checked against the Fc/Fþc before and after
each experiment.
The thermal stability was evaluated by TGA on Mettler Toledo
TGA/DSC Starte System under nitrogen, with a heating rate of 10 ꢂC/
min. Heating of crystalline samples leads to melting of the solids,
but no recrystallization was observed.
300
400
500
600
700
800
Compounds 4-(diphenylamino)benzaldehyde (7), 5-formylthio
phene-2-yl-2-boronic acid and 4-bromo-N,N-bis(phenyl)aniline
(12) are commercially available and were used without further
purification.
Wavelegth (nm)
(b)
20
Compounds 8,15 9,7,12a 10,7,13 11,7 1317 and 1416 were synthesized
according to literature procedures.
15
10
5
3.1.2. General procedure for the synthesis of 14 and 15. To a solution
of corresponding bromide (1 mmol) and PdCl2(dppf) (0.1 mmol) in
dry toluene (8 mL/mmol) was added a solution of 5-formylthien-2-
yl-2-boronic acid (2 mmol) and K2CO3 (5 mmol) in dry methanol
(8 mL/mmol). The mixture was refluxed for 18 h. The reaction was
quenched by the addition of water (20 mL) and extracted with
CH2Cl2 (3ꢀ25 mL). The combined organic extract was dried over
anhydrous MgSO4 and filtered. Solvent was removed by rotary
evaporation. The product was purified by column chromatography
(silica gel, hexaneeCHCl3, 1:1).
0
300
400
500
600
700
800
Wavelength (nm)
Fig. 10. (a) Photoresponse spectra plotted as incident photon-to-current conversion
efficiency (IPCE). (b) APCE measured for SMOSCs based as dyes 1e6 as photosensitizers
and electron donors and PCBM. Note the significant higher values of the efficiency in
plot b.
D: dye 1, -: dye 2, C: dye 3, ,: dye 4 B: dye 5 and :: dye 6.
3.1.2.1. 5-((4-(Diphenyl)amino)phenyl)thiophene-2-carbaldehyde
(14). 1.00 g of 4-Bromo-N,N-bis(phenyl)aniline (12) (3.1 mmol) and
966 mg of 5-formylthiophene-2-yl-2-boronic acid (6.20 mmol)
were reacted according to the general procedure giving 846 mg of
In an attempt to optimise the performance of the SMOSCs by
increasing photoabsorption we performed measurements in which
a series of transparent cells were collated and connected in such
a way that the combined response of two or three individual
SMOSCs was determined. Under these conditions we have been
able to increase the overall efficiency up to 0.91%, see Table 2 for
two and three stacked cells connected electrically.
14 as yellow oil (77% yield). 1H NMR (400 MHz, CDCl3)
d/ppm: 10.0
(s, 1H), 7.70 (d, 1H, J¼4.0 Hz), 7.50 (m, 2H), 7.30 (d, 1H, J¼4.0 Hz),
7.25 (m, 4H), 7.11 (m, 4H), 7.06 (m, 2H), 7.04 (m, 2H). 13C NMR
(100 MHz, CDCl3) d/ppm: 182.7, 154.6, 149.1, 147.0, 141.3, 137.8,
135.2, 129.5, 127.3, 126.1, 125.2, 125.0, 123.9, 122.9, 122.4. MS (m/z)
(MALDI-TOF): calculated C23H17NOS: 355.10; found (MþHþ):
355.20, 356.20, 357.20.
3. Experimental section
3.1. Synthesis
3.1.2.2. 5-((4-(Bis(4-hexyloxy)phenyl)amino)phenyl)thiophene-2-
carbaldehyde (15). 1.00 g of 4-Bromo-N,N-bis(4-(hexyloxy)phenyl)
aniline (13) (1.91 mmol) and 595 mg of 5-formylthiophene-2-yl-2-
boronic acid (3.82 mmol) were reacted according to the general
procedure giving 795 mg of 15 as yellow oil (75% yield). 1H NMR
3.1.1. General synthetic procedures. Synthetic procedures were
carried out under inert argon atmosphere, in dry solvent unless
otherwise noted. All reagents and solvents were reagent grade and
were used without further purification. Chromatographic purifi-
cations were performed using silica gel 60 SDS (particle size
0.040e0.063 mm). Analytical thin-layer chromatography was per-
formed using Merck TLC silica gel 60 F254. 1H NMR spectra were
obtained on Bruker TopSpin AV-400 (400 MHz) spectrometer.
Chemical shifts are reported in parts per million (ppm) relative to
the solvent residual peak (CDCl3, 7.27 ppm). 13C NMR chemical
(400 MHz, CDCl3)
d
/ppm: 9.84 (s,1H), 7.70 (d,1H, J¼4.0 Hz), 7.46 (m,
2H), 7.26 (d, 1H, J¼4.0 Hz), 7.09 (m, 4H), 6.90 (m, 2H), 6.85 (m, 4H),
3.95 (t, 4H, J¼6.5 Hz), 1.52e1.42 (m, 4H), 1.40e1.32 (m, 8H), 0.92 (t,
6H, J¼7.0 Hz). 13C NMR (100 MHz, CDCl3)
d/ppm: 182.5, 156.1, 155.2,
150.0, 140.7, 139.7, 137.9, 127.2, 127.1, 124.0, 122.2, 119.2, 115.4, 68.2,
31.6, 29.3, 25.7, 22.6, 14.0. MS (m/z) (MALDI-TOF): calculated
C35H41NO3S: 555.28; found (MþHþ): 555.37, 556.34, 557.37.
shifts (d) are reported relative to the solvent residual peak (CDCl3,
77.0 ppm). UVevis measurements were carried out on a Shimadzu
UV 3600 spectrophotometer. For extinction coefficient de-
termination, solutions of different concentration were prepared in
CH2Cl2 (HPLC grade) with absorption between 0.1 and 1 of absor-
bance using a 1 cm UV cuvette. The emission measurements were
carried out on Cary Eclipse fluorescence spectrophotometer. Mass
spectra (MALDI-TOF) were recorded on a VOYAGER DEÔ STR
mass spectrometer using dithranol as matrix. Melting points are
uncorrected.
Cyclic voltammetry was performed in ODCBeacetonitrile (5:1)
solutions. Tetrabutylammonium perchlorate (0.1 M as supporting
electrolyte) were purchased from Acros and used without purifi-
cation. Solutions were deoxygenated by argon bubbling prior to
3.1.3. 2-(5-(4-(Diphenylamino)benzylidene)-4-phenylthiazol-2-
(5H)-ylidene)malonitrile (1). To a solution of 4-(diphenylamino)
benzaldehyde (7) (Aldrich, 97%) (333 mg, 1.20 mmol) in 1.7 mL of
acetic anhydride (1.5 mL/mmol) and 11 mL of ODCB (10 mL/mmol)
under argon atmosphere was added to a stirred solution of 2-(4-
phenylthiazol-2-yl)malononitrile (9) (250 mg, 1.11 mmol). The
mixture was stirred at 140 ꢂC for 6 h. After cooling to room tem-
perature, the reaction was extracted with diethyl ether, and the
organic layer was dried over MgSO4 (anhydrous). After filtration of
the drying agent, the solvent was removed under vacuum. Com-
pound 1 was purified by column chromatography (silica gel, hex-
aneeethyl acetate, 7:3). The final product 1 was recrystallized in
CH2Cl2ehexane as blue solid with 60% yield (320 mg). Mp: >300

A. Aljarilla et al. / Tetrahedron 69 (2013) 6875e6883
6881
with decomposition. 1H NMR (400 MHz, CDCl3)
d
/ppm: 7.80 (m,
over MgSO4 (anhydrous). After filtration of the drying agent, the
solvent was removed under vacuum. Compound 4 was purified by
column chromatography (silica gel, hexaneeethyl acetate, 7:3). The
final product 4 was recrystallized in CH2Cl2epentane as blue solid
with 42% yield (317 mg). Mp: >300 with decomposition. 1H NMR
2H), 7.66 (m, 1H), 7.60 (s, 1H), 7.58 (m, 2H), 7.48 (m, 2H), 7.4 (m, 4H),
7.25 (m, 2H), 7.22 (m, 4H), 7.03 (m, 2H). 13C NMR (100 MHz, CDCl3)
d/ppm: 182.4, 180.6, 152.3, 145.2, 141.6, 133.6, 132.3, 132.0, 131.7,
130.5, 130.0, 129.0, 162.6, 126.1, 125.0, 119.4, 115.1, 113.2, 65.7.
UVevis (CH2Cl2) lmax/nm (log ε): 624 (4.7), 425.0 (4.0), 339 (4.2). IR
n
(400 MHz, CDCl3)
d
/ppm: 7.78 (m, 2H), 7.63 (mt, 1H, J¼2.10 Hz), 7.57
/cmꢁ1: 3061, 2925, 2210, 1590, 1562, 1495, 1422, 1335, 1299, 1177,
(m, 3H), 7.44 (m, 2H), 7.13 (m, 4H), 6.91 (m, 4H), 6.89 (m, 2H), 3.96
(t, 4H, J¼6.6 Hz), 6.89 (d, 2H, J¼9.0 Hz), 3.96 (t, 4H, J¼6.6 Hz),1.8 (dt,
4H, J¼14.8, 6.6 Hz), 1.47 (m, 4H), 1.38e1.32 (m, 8H), 0.92 (t, 6H,
1096, 700. MS (m/z) (MALDI-TOF): calculated C31H20N4S: 480.14;
found (MþHþ): 480.2, 481.20, 482.2, 483.2, 484.2.
J¼7.0 Hz). 13C NMR (100 MHz, CDCl3)
d/ppm: 182.4, 180.2, 157.5,
3.1.4. 2-(5-(4-(Diphenylamino)benzylidene)-4-tert-butylthiazol-
2(5H)-ylidene)malonitrile (2). A solution of 4-(diphenylamino)
benzaldehyde (7) (423 mg, 1.55 mmol) in 1.0 mL of acetic anhydride
(1.5 mL/mmol) was added under argon atmosphere to a stirred
solution of 2-(4-tert-butylthiazol-2-yl)malononitrile (10) (200 mg,
0.77 mmol). The mixture was stirred at 140 ꢂC for 2 h. After cooling
to room temperature, the reaction was extracted with diethyl ether,
and the organic layer was dried over MgSO4 (anhydrous). After
filtration of the drying agent, the solvent was removed under
vacuum. Compound 2 was purified by column chromatography
(silica gel, hexaneeethyl acetate, 7:3). The final product 2 was
recrystallized in CH2Cl2ehexane as blue solid with 65% yield
153.1, 141.9, 137.5, 134.0, 132.0, 131.9, 131.0, 130.5, 128.9, 128.0, 123.7,
117.5, 115.5, 115.4, 113.5, 68.4, 31.5, 29.2, 27.7, 22.6, 14.0. UVevis
(CH2Cl2) lmax/nm (log ε): 641 (4.6), 431 (3.8), 334 (4.1). IR n :
/cmꢁ1
2950, 2929, 2857, 2207, 1501, 1449, 1421, 1333, 1234, 1178, 1157,
1094. MS (m/z) (MALDI-TOF): calculated C43H44N4O2S: 680.32;
found (MþHþ): 680.4, 681.4, 682.4, 683.4, 684.4.
3.1.7. 2-(5-((5-(4-(Diphenylamino)phenyl)thiophen-2-yl) methylene)-
4-phenylthiazol-2(5H)-ylidene)malonitrile (5). A solution of 5-(4-
(diphenylamino)phenyl)thiophene-2-carbaldehyde (14) (433 mg,
1.22 mmol) in 1.7 mL of acetic anhydride (1.5 mL/mmol) and 11 mL
ODCB (10 mL/mmol) under argon atmosphere was added to a stir-
red solution of 2-(4-phenylthiazol-2-yl)malononitrile (9) (250 mg,
1.11 mmol) in acetic anhydride under argon atmosphere. The
mixture was stirred at 140 ꢂC for 6 h. After cooling to room tem-
perature, the reaction was extracted with diethyl ether, and the
organic layer was dried over MgSO4 (anhydrous). After filtration of
the drying agent, the solvent was removed under vacuum. Com-
pound 5 was purified by column chromatography (silica gel, hex-
aneeethyl acetate, 7:3). The final product 5 was recrystallized in
CH2Cl2-hexane as blue solid with 40% yield (249 mg). Mp: >300
(260 mg). 1H NMR (400 MHz, CDCl3)
2H), 7.38 (m, 4H), 7.23 (m, 2H), 7.21 (m, 4H), 7.05 (m, 2H), 1.58 (d,
9H). 13C NMR (100 MHz, CDCl3)
/ppm: 190.8, 181.9, 151.6, 145.4,
d/ppm: 7.89 (s, 1H), 7.49 (m,
d
137.7, 133.1, 130.9, 129.9, 126.4, 125.7, 124.7, 119.7, 114.8, 112.9, 66.6,
38.9, 31.2. Mp: 347e350 ꢂC. UVevis (CH2Cl2) lmax/nm (log ε): 683
(4.7), 405 (3.9), 298 (4.3). IR n
/cmꢁ1: 3055, 2971, 2214, 1610, 1592,
1566, 1507, 1448, 1433, 1335, 1302, 1197, 1174, 1137, 699. MS (m/z)
(MALDI-TOF): calculated C29H24N4S: 460.17; found (MþHþ):
460.20, 461.3, 462.3, 463.3.
with decomposition. 1H NMR (400 MHz, CDCl3)
d/ppm: 7.81 (m,
3.1.5. Methyl 2-cyano-2-(5-(4-(diphenylamino)benzylidene)-4-
phenylthiazol-2(5H)-ylidene)acetate (3). A solution of 4-(dipheny-
lamino)benzaldehyde (7) (366 mg, 1.34 mmol) in 1.8 mL of acetic
anhydride (1.5 mL/mmol) and 12.0 mL of ODCB (1.5 mL/mmol)
under argon atmosphere was added to a stirred solution of
methyl cyano(4-phenyl-1,3-thiazol-2-yl)acetate (11) (250 mg,
1.22 mmol). The mixture was stirred at 140 ꢂC for 2 h. After
cooling to room temperature, the reaction was extracted with
diethyl ether, and the organic layer was dried over MgSO4 (an-
hydrous). After filtration of the drying agent, the solvent was
removed under vacuum. Compound 3 was purified by column
chromatography (silica gel, hexaneeethyl acetate, 7:3). The final
product 3 was recrystallized in CH2Cl2epentane as blue solid with
52% yield (292 mg). Mp: >300 with decomposition. 1H NMR
3H), 7.67 (m, 1H), 7.60 (m, 2H), 7.58 (m, 2H), 7.52 (m, 1H), 7.39 (m,
1H), 7.33 (m, 4H), 7.17 (m, 4H), 7.14 (m, 2H), 7.08 (m, 2H). 13C NMR
(100 MHz, CDCl3) d/ppm: 181.3, 179.7, 157.2, 149.9, 146.5, 139.7,
136.9, 132.8, 132.5, 131.4, 130.2, 129.6, 129.5, 129.1, 127.4, 125.5, 125.1,
124.4, 121.7, 115.0, 113.1, 66.5. UVevis (CH2Cl2) lmax/nm (log ε): 656
(4.8), 474 (4.3), 363 (4.5), 300 (4.5). IR n
/cmꢁ1: 2924, 2852, 2212,
1559, 1488, 1408, 1326, 1287, 1188, 1092, 1069, 696. MS (m/z)
(MALDI-TOF): calculated C35H22N4S2: 562.13; found (MþHþ): 562.2,
563.2, 564.2, 465.2, 565.2, 566.2.
3.1.8. 2-(5-((5-(4-(Bis(4-(hexyloxy)phenyl)amino)phenyl) thiophen-
2-yl)methylene)-4-phenylthiazol-2(5H)-ylidene) malonitrile (6). A
solution of 5-((4-(bis(hexyloxy)phenyl)amino)phenyl)thiophene-
2-carbaldehyde (15) (677 mg, 1.22 mmol) in 1.7 mL of acetic an-
hydride (1.5 mL/mmol) and 11 mL ODCB (10 mL/mmol) under argon
atmosphere was added to a stirred solution of 2-(4-phenylthiazol-
2-yl)malononitrile (9) (250 mg, 1.11 mmol) in acetic anhydride
under argon atmosphere. The mixture was stirred at 140 ꢂC for 6 h.
After cooling to room temperature, the reaction was extracted with
diethyl ether, and the organic layer was dried over MgSO4 (anhy-
drous). After filtration of the drying agent, the solvent was removed
under vacuum. Compound 6 was purified by column chromatog-
raphy (silica gel, hexaneeethyl acetate, 7:3). The final product 6 was
recrystallized in CH2Cl2epentane as blue solid with 35% yield
(295 mg). Mp: >300 with decomposition. 1H NMR (400 MHz,
(400 MHz, CDCl3)
7.359e7.53 (m, 5H), 7.38 (md, 4H, J¼1.7 Hz), 7.23 (m, 2H), 7.19 (m,
4H), 7.02 (m, 2H), 3.92 (s, 3H). 13C NMR (100 MHz, CDCl3)
/ppm:
d
/ppm: 7.82 (m, 2H), 7.62 (mt, 1H, J¼2.3 Hz),
d
179.6, 179.6, 166.2, 151.5, 145.5, 140.9, 134.1, 133.5, 132.3, 131.7,
130.5, 129.8, 128.8, 126.4, 125.9, 125.6, 119.7, 116.0, 87.1, 52.7.
UVevis (CH2Cl2) lmax/nm (log ε): 603 (4.7), 582 (4.7), 417 (3.9),
332 (4.1), 286 (4.3). IR n
/cmꢁ1: 3059, 3029, 2943, 2214, 1700, 1564,
1489, 1462, 1280, 1191, 1175, 1079, 701. MS (m/z) (MALDI-TOF):
calculated C32H23N3O2S: 513.15; found (MþHþ): 513.2, 514.2,
515.2, 516.2, 517.2.
3.1.6. 2-(5-(4-(Bis(4-(hexyloxy))phenylamino)benzylidene)-4-
phenylthiazol-2(5H)-ylidene)malonitrile (4). A solution of 4-bis(4-
(hexyloxy)phenylamino)benzaldehyde (8) (60 mg, 1.22 mmol) in
1.6 mL of acetic anhydride (1.5 mL/mmol) and 11 mL ODCB (10 mL/
mmol) under argon atmosphere was added to a stirred solution of
2-(4-phenylthiazol-2-yl)malononitrile (9) (250 mg, 1.11 mmol) in
acetic anhydride under argon atmosphere. The mixture was stirred
at 140 ꢂC for 6 h. After cooling to room temperature, the reaction
was extracted with diethyl ether, and the organic layer was dried
CDCl3)
d
/ppm: 7.80 (m, 3H), 7.65 (m, 1H), 7.59 (mt, 2H, J¼1.4 Hz),
7.51 (m, 2H), 7.50 (d, 1H, J¼4.2 Hz), 7.34 (d, 1H, J¼4.2 Hz), 6.91 (m,
2H), 6.88 (m, 4H), 3.96 (t, 4H, J¼6.6 Hz), 1.80 (dt, 4H, J¼15.0, 6.6 Hz),
1.50 (m, 4H), 1.36 (m, 8H), 1.1 (t, 6H, J¼7.0 Hz). 13C NMR (100 MHz,
CDCl3) d/ppm: 181.3, 179.4, 158.1, 156.5, 140.0, 139.0, 136.4, 132.9,
131.6, 130.2, 129.1, 127.4, 123.2, 118.7, 115.4, 115.2, 113.3, 68.5, 67.8,
31.6, 29.2, 25.7, 22.6, 14.0. UVevis (CH2Cl2) lmax/nm (log ε): 705
(4.5), 494 (4.1), 372 (4.3), 294 (4.3). IR
2210, 1589, 1558, 1504, 1405, 1322, 1241, 1185, 1066, 825. MS (m/z)
n
/cmꢁ1: 2951, 2927, 2856,

6882
A. Aljarilla et al. / Tetrahedron 69 (2013) 6875e6883
(MALDI-TOF): calculated C47H46N4O2S2: 762.31; found (MþHþ):
Supplementary data
762.4, 763.4, 765.4, 766.4, 767.4.
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.tet.2013.05.137.
3.2. Preparation of SMOSCs
All samples were prepared on ITO-coated glass substrates with
sheet resistance 15
References and notes
U
sqꢁ1 from Psiotec Ltd., which were first
cleaned by ultrasonic agitation in acetone and isopropanol. Freshly
cleaned substrate was then covered with a dense TiOx layer using
spin coating (50 nm).27 Solutions of dye/PCBM were prepared at
20:80 wt % and 30 mg/mL in chlorobenzene and were deposited by
spin coating on top of the TiOx dense layer. MoOx layer was de-
posited by thermal evaporation (20 nm) using MoO2 powder as
precursor. Finally, a layer of gold was deposited by thermal evap-
oration (100 nm) using a shadow mask obtaining an active area
pixel of 0.048 cm2. The thickness of all of the films was measured by
a MicroXAM-100 3D surface profilometer.
1. (a) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11, 15e26;
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Stacked cells were prepared by stacking two or three single
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To determine the JSCꢁVOC plots, the cell was connected to
a sourceMeter (Keithley 2601). The voltage scan was controlled
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ferred to a PC that controlled the experiment and at the same time
provided data storage capability to the system. The solar simulator
(Sun 2000, ABET Technologies) was equipped with an AM 1.5G
filter and the nominal power for the measurements was 100 mW/
cm2. The same cells were used to record the IPCE spectra. In IPCE
measurements the sample was excited with a 150 W xenon lamp
through a CzernyeTurner monochromator. The current output at
short circuit was measured by a potentiostat (AMEL), which
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ꢂ
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The present work has shown that a series of pushepull chro-
mophores containing TPA can exhibit moderate intrinsic efficiency
as photoactive component in SMOSCs based on PCBM according to
the absorbed photon to current efficiency. The limiting factor effi-
ciency is the low photon absorption due to the transparency of the
cell derived from the thinness of the photoactive layer. However, the
positive effect of preparing thicker films is counter balanced by the
negative effect of charge recombination due to the short free mean
paths of the charge carrier. Other reasons for the low efficiency could
be phase segregation of dye and PCBM or an inadequate match of the
LUMO energy of the dye and PCBM. The efficiency can be somewhat
improved by designing stacked devices constituted by two in-
dependent cells in series. Nevertheless, optimization of the design of
the dyes by further increasing their molar absorption coefficient
without modifying other parameters related to photovoltaic activity
such as HOMO energyand photoinduced charge separation quantum
yield is likely to be reflected in a higher efficiency of the cell. Further
work in this direction in currently undertaken in our laboratory.
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