Novel 4H-pyranylidene organic dyes for dye-sensitized solar cells: Effect of different heteroaromatic rings on the photovoltaic properties

Article abstract of DOI:10.1016/j.orgel.2014.09.003

Eight novel Ruthenium-free dyes consisting of a 4H-pyranylidene donor, a 2-cyanoacrylic acid acceptor and several heterocyclic rings with different electronic properties are designed and synthesized. The photovoltaic performances of the related dye-sensitized solar cells are discussed and compared, obtaining the best results with the thiophene followed by the mismatched-oriented thiazole. Dyes containing a pyrrol or a matched-oriented thiazole lead to the lower efficiencies of all the series. Charge extraction (CE) and transient photovoltage (TPV) measurements were conducted in order to investigate the recombination resistance and the difference in voltage for the devices. In order to get further insight in the electronic and optical properties of the studied compounds, theoretical calculations using TD-DFT and the CPCM have been carried out.

Full text of DOI:10.1016/j.orgel.2014.09.003

Organic Electronics 15 (2014) 3237–3250
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Novel 4H-pyranylidene organic dyes for dye-sensitized solar
cells: Effect of different heteroaromatic rings on the
photovoltaic properties
Raquel Pérez Tejada a, Laia Pellejà b, Emilio Palomares b,c, , Santiago Franco a, , Jesús Orduna a,
⇑
⇑
Javier Garín a, Raquel Andreu a
a Departamento de Química Orgánica-Instituto de Ciencia de Materiales de Aragón, Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain
b Institute of Chemical Research of Catalonia (ICIQ), Avda. Països Catalans, 16, Tarragona E-43007, Spain
c ICREA, Passeig Lluis Companys, 23, Barcelona E-08010, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 July 2014
Received in revised form 1 September 2014
Accepted 5 September 2014
Available online 18 September 2014
Eight novel Ruthenium-free dyes consisting of a 4H-pyranylidene donor, a 2-cyanoacrylic
acid acceptor and several heterocyclic rings with different electronic properties are
designed and synthesized. The photovoltaic performances of the related dye-sensitized
solar cells are discussed and compared, obtaining the best results with the thiophene
followed by the mismatched-oriented thiazole. Dyes containing a pyrrol or a matched-
oriented thiazole lead to the lower efficiencies of all the series. Charge extraction (CE)
and transient photovoltage (TPV) measurements were conducted in order to investigate
the recombination resistance and the difference in voltage for the devices. In order to
get further insight in the electronic and optical properties of the studied compounds,
theoretical calculations using TD-DFT and the CPCM have been carried out.
Keywords:
DSSC
4H-pyranylidene
Thiazole
Furan
Ó 2014 Elsevier B.V. All rights reserved.
Pyrrol
Metal-free sensitizer
1. Introduction
properties. Indeed, one of the key components for such
efficient DSSC is the photosensitizer, which is not only
Dye-sensitized solar cells (DSSCs) are among the most
efficient molecular devices in converting sun light into
electrical current [1]. In fact, under low light conditions
(so called indoor conditions) this type of solar cells is capa-
ble to surpass the efficiency of most common solar cells
based on polycrystalline silicon. Efficiencies as high as
13% under standard measurement conditions (100 mW/
cm2 sun simulated 1.5 AM G spectrum) have been recently
reported [2]. Such high efficiencies have been possible with
the use of the so called Ruthenium-free DSSCs, which are
organic sensitizers with unique physical and chemical
the responsible to harvest the light and inject electrons
into the conduction band of the TiO2 semiconductor, but
also to act as a first barrier to prevent back electron trans-
fer from the photo-injected electrons at the TiO2 and the
oxidized electrolyte due to excellent molecular packing at
the TiO2 surface [3].
Based on the pioneering work of Feldt et al. [4] and
Wang et al. [5] most of the organic sensitizers have a typ-
ical electron Donor-p-Acceptor structure (D-p-A) [6], that
leads to an intramolecular charge transfer (ICT) absorption
band with high molecular extinction coefficient in the
visible region of the sun light. Recently, several groups
have proposed different configurations as D–A-
p-A [7],
D- -A [8], D–D- -A [9] or 2D- -A [10] with good
results. Triarylamines moieties are commonly used as
electron donating groups because of its excellent
p
–
p
p
p
⇑
Corresponding authors.
E-mail addresses: epalomares@ICIQ.es (E. Palomares), sfranco@unizar.
es (S. Franco).
http://dx.doi.org/10.1016/j.orgel.2014.09.003
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

3238
R. Pérez Tejada et al. / Organic Electronics 15 (2014) 3237–3250
electron-donating capability and good stability [11]. While
the 4H-pyranylidene ring has proved to be a very good
donor system in NLO applications [12], it has scarcely
explored in DSSCs [13]. The main feature relies on its
pro-aromatic character that improves the charge transfer
process through the gain in aromaticity. Moreover, the
donor character can be modulated by incorporating sub-
stituents with different electronic properties. We think
that this system can be an interesting alternative to other
common donor unities, like triarylamines.
(triplet), q (quartet) and m (multiplet). 1H–1H COSY and
1H-13C-HSQC experiments were recorded on a Bruker
ARX300 or a Bruker AV400 at 300 or 400 MHz in order to
establish peaks assignment and spatial relationships. Elec-
trospray mass spectra were recorded on a Bruker Micro-
ToF-Q spectrometer; accurate mass measurements were
achieved using sodium formate as external reference.
UV–Visible spectra were recorded with an UV–Vis UNICAM
UV4 spectrophotometer. Pulse differential voltammetry
measurements were performed with a
l-Autolab type III
Most of the studies carried out in DSSCs using these
type of sensitizers have been focused on the molecular
potentiostat using a glassy carbon working electrode, Pt
counter electrode, and Ag/AgCl reference electrode. The
experiments were carried out under argon in CH2Cl2, with
Bu4NPF6 as supporting electrolyte (0.1 mol LÀ1). Scan rate
was 0.01 V sÀ1, modulation amplitude 0.025 V and modu-
p-bridge as it plays an important role in tuning the main
photochemical properties of the sensitizer [14]. The use
of electron-excessive heteroaromatic systems like thio-
phene, furan [15] or pyrrole [16] is a common strategy to
expand the solar cell absorption spectrum to the red region
of the sun light, by acting as efficient donor auxiliary sys-
tems and affording a greater photo- and chemical stability.
lation time 0.05 sÀ1
.
2.2. Synthesis
Moreover, other important roles on the
p-bridge such as
2.2.1. 5-((2,6-Di-tert-butyl-4H-pyran-4-
the polarizability (the formation of molecular dipoles at
the surface of the semiconductor metal oxide), the pres-
ence of moieties that may enhance or retard the recombi-
nation reaction mentioned above, among others, must be
also taking into account to design efficient dyes.
Although the thiophene moiety is among the most used
conjugated linkers in organic sensitizers for DSSCs, the
number of comparative studies involving in the same ser-
ies, heterocyclic rings with different electronic properties
(donor and electron-accepting characteristics) is truly
scarce [17].
ylidene)methyl)thiazole (5b)
A solution of (2,6-di-tert-butyl-4H-pyran-4yl)tributyl-
phosphonium perchlorate 1b (607 mg, 1.23 mmol) in
anhydrous THF (10 mL) was prepared, purged with argon
and cooled to À78 °C. To this solution, n-BuLi (1.6 M in
hexanes) (0.77 mL, 1.59 mmol) was added dropwise and
the resulting mixture was stirred for 15 min. Then thia-
zole-5-carbaldehyde 4 (86 mg, 0.94 mmol) in anhydrous
THF (7 mL) was added dropwise and the mixture was pro-
gressively heated to reach 0 °C during 4 h. Saturated NH4Cl
solution was added to quench the reaction and the solvent
was evaporated under reduce pressure. The organic layer
was extracted with dichloromethane, washed with water
and dried over anhydrous MgSO4. After removal of the
solvent, the product was purified by alumina column chro-
matography (30% ethyl acetate in hexanes). Yield: orange
solid (219 mg, 0.75 mmol; 88%).
In this work, we aim to evaluate the potential of 8 novel
Ruthenium-free dyes consisting of
donor, a 2-cyanoacrylic acid acceptor and different hetero-
aromatic rings acting as the -bridge, i.e., N-methylpyrrole,
a 4H-pyranylidene
p
furan (electron-excessive) and thiazole (electron-
deficient). In fact, to our best knowledge, there are not
many examples of DSSCs based on this latter heterocyclic
system [17b,18], for which, due to its unsymmetrical nat-
ure, two regioisomers should be considered (matched
and mismatched) [19].
The photovoltaic performances of the related DSSCs are
discussed and compared with other previously thiophene
reported systems [13] allowing a complete understanding
of the dyes molecular structure device efficiency relation-
ship for this new approach towards Ruthenium-free DSSCs.
Mp 60–61 °C. IR (KBr): cmÀ1 1577 (C@C). 1H NMR
(300 MHz, CD2Cl2): d (ppm) 8.52 (s, 1H), 7.63 (s, 1H),
6.23 (d, J = 2.0 Hz, 1H), 5.81 (s, 1H), 5.69 (d, J = 2.0 Hz,
1H), 1.24 (s, 9H), 1.20 (s, 9H). 13C NMR (75 MHz, CD2Cl2):
d (ppm) 165.7, 163.2, 148.8, 140.2, 137.2, 131.9, 104.6,
99.9, 98.9, 36.2, 35.7, 28.2, 28.1. HRMS (ESI+): m/z calcd
for [C17H24NOS]+: 290.1573, found: 290.1577.
2.2.2. 2-((2,6-Di-tert-butyl-4H-pyran-4-
ylidene)methyl)thiazole (7b)
A solution of (2,6-di-tert-butyl-4H-pyran-4yl)tributyl-
phosphonium perchlorate 1b (607 mg, 1.23 mmol) in
anhydrous THF (10 mL) was prepared, purged with argon
and cooled to À78 °C. To the solution, n-BuLi (1.6 M in
hexanes) (0.76 mL, 1.23 mmol) was added dropwise and
the resulting mixture was stirred for 15 min. Then thia-
zole-2-carbaldehyde (86 mg, 0.94 mmol) in anhydrous
THF (7 mL) was added dropwise and the mixture was pro-
gressively heated to reach room temperature during 18 h.
Saturated NH4Cl solution was added to quench the
reaction and the solvent was evaporated under reduce
pressure. The organic layer was extracted with ethyl ace-
tate, washed with water and dried over anhydrous MgSO4.
After removal of the solvent, the product was purified by
2. Experimental section
2.1. General information
Infrared measurements were carried out in KBr using a
Perkin–Elmer Fourier Transform Infrared 1600 spectrome-
ter. Melting points were obtained on a Gallenkamp appara-
tus in open capillaries and are uncorrected. 1H and 13C
NMR spectra were recorded on a Bruker ARX300 or a Bru-
ker AV400 at 300 or 400 MHz and 75 or 100 MHz respec-
tively; d values are given in ppm (relative to TMS) and J
values in Hz. The apparent resonance multiplicity is
described as s (singlet), br s (broad singlet), d (doublet), t

R. Pérez Tejada et al. / Organic Electronics 15 (2014) 3237–3250
3239
silica gel column chromatography (20% ethyl acetate in
hexanes). Yield: dark yellow oil (250 mg, 0.86 mmol, 91%).
IR (KBr): cmÀ1 1666 (C@C). 1H NMR (400 MHz, CD2Cl2):
d (ppm) 7.66 (d, J = 3.3 Hz, 1H), 7.35 (d, J = 1.8 Hz, 1H), 7.04
(d, J = 3.3 Hz, 1H), 5.84 (s, 1H), 5.76 (d, J = 1.8 Hz, 1H), 1.27
(s, 9H), 1.22 (s, 9H). 13C NMR (100 MHz, CD2Cl2): d (ppm)
167.9, 166.9, 164.9, 143.3, 135.4, 115.0, 104.6, 102.5,
100.8, 36.3, 36.0, 28.2, 28.1. HRMS (ESI+): m/z calcd for [C17-
H24NOS]+: 290.1573, found: 290.1589.
(30% ethyl acetate in hexanes). Yield: brown oil (276 mg,
0.88 mmol; 79%).
IR (KBr): cmÀ1 1653 (C@O), 1578 (C@C). 1H NMR
(300 MHz, CD2Cl2):
d (ppm) 9.35 (s, 1H), 6.88 (d,
J = 4.2 Hz, 1H), 6.31 (d, J = 4.2 Hz, 1H), 6.25 (d, J = 2.0 Hz,
1H), 5.77 (d, J = 2.0 Hz, 1H), 5.37 (s, 1H), 3.86 (s, 1H), 1.22
(s, 9H), 1.21 (s, 9H). 13C NMR (75 MHz, CD2Cl2): d (ppm)
177.9, 166.0, 163.8, 143.0, 135.0, 131.3, 125.7, 108.9,
105.2, 99.4, 97.2, 36.2, 35.8, 32.7, 28.2, 28.1. HRMS (ESI+):
m/z calcd for [C20H28NO2]+: 314.2115, found: 314.2104;
calcd for [C20H27NO2Na]+: 336.1934, found: 336.1911.
2.2.3. 5-((2,6-Diphenyl-4H-pyran-4-ylidene)methyl)-1-
methyl-1H-pyrrole-2-carbaldehyde (8a)
2.2.5. 5-((2,6-Diphenyl-4H-pyran-4-ylidene)methyl)furan-2-
carbaldehyde (9a)
A solution of 2,6-diphenyl(4H-pyran-4ylidene)diphen-
ylphosphine oxide 1a (706 mg, 1.63 mmol) in anhydrous
THF (14 mL) was prepared, purged with argon and cooled
to À78 °C. To the solution, n-BuLi (1.6 M in hexanes)
(1.1 mL, 1.79 mmol) was added dropwise and the resulting
mixture was stirred for 20 min. Then 1-methyl-5-(5,5-
dimethyl-1,3-dioxan-2-yl)-1H-pyrrole-2-carbaldehyde
(363 mg; 1.63 mmol) in anhydrous THF (9 mL) was added
dropwise and the mixture was progressively heated to
reach room temperature during 24 h (TLC monitoring
using 30% ethyl acetate in hexanes). The reaction was
quenched with HCl (2.4 N) (10 mL) and the mixture was
stirred for 30 min at room temperature. The organic layer
was extracted with dichloromethane, washed with water
and dried over anhydrous MgSO4. After removal of the sol-
vent, the aldehyde was purified by silica gel column chro-
matography (10% ethyl acetate in hexanes). Yield: red solid
(253 mg, 0.71 mmol; 44%).
A solution of 2,6-diphenyl(4H-pyran-4ylidene)diphen-
ylphosphine oxide 1a (1.55 g, 3.57 mmol) in anhydrous
THF (30 mL) was prepared, purged with argon and cooled
to À78 °C. To the solution, n-BuLi (1.6 M in hexanes)
(2.5 mL, 4.00 mmol) was added dropwise and the resulting
mixture was stirred for 20 min. Then 5-(1,3-dioxolan-2-
yl)furan-2-carbaldehyde (600 mg, 3.57 mmol) in anhy-
drous THF (20 mL) was added dropwise and the mixture
was progressively heated to reach 0 °C during 4 h (TLC
monitoring using 50% ethyl acetate in hexanes). The reac-
tion was quenched with HCl (2.4 N) (10 mL) and the mix-
ture was stirred for 15 min at room temperature. The
organic layer was extracted with dichloromethane, washed
with water and dried over anhydrous MgSO4. After
removal of the solvent, the aldehyde was purified by silica
gel column chromatography (20% ethyl acetate in hex-
anes). Yield: red solid (750 mg, 2.20 mmol; 62%).
Mp 174–175 °C. IR (KBr): cmÀ1 1645 (C@O), 1581
(C@C). 1H NMR (400 MHz, CD2Cl2): d (ppm) 9.43 (s, 1H),
7.82–7.78 (m, 4H), 7.50–7.43 (m, 6H), 7.01 (d, J = 1.6 Hz,
1H), 6.96 (d, J = 4.2 Hz, 1H), 6.55 (d, J = 1.6 Hz, 1H), 6.50
(d, J = 4.2 Hz, 1H), 5.88 (s, 1H), 3.93 (s, 3H). 13C NMR
(75 MHz, CD2Cl2): d (ppm) 178.4, 154.3, 152.2, 142.0,
133.5, 133.3, 133.2, 131.8, 130.2, 130.0, 129.2, 129.2,
125.5, 125.5, 125.1, 109.6, 108.7, 103.0, 100.7, 32.8. HRMS
(ESI+): m/z calcd for [C24H20NO2]+: 354.1489, found:
354.1485; calcd for [C24H20NO2Na]+: 376.1308, found:
376.1290.
Mp 134–135 °C. IR (KBr): cmÀ1 1648 (C@O), 1580
(C@C). 1H NMR (300 MHz, CD2Cl2): d (ppm) 9.50 (s, 1H),
7.94–7.91 (m, 2H), 7.84–7.80 (m, 2H), 7.33 (d, J = 1.9 Hz,
1H), 7.55–7.44 (m, 6H), 7.26 (d, J = 3.8 Hz, 1H), 6.55 (d,
J = 1.9 Hz, 1H), 6.36 (d, J = 3.8 Hz, 1H), 5.75 (s, 1H). 13C
NMR (100 MHz, CD2Cl2): d (ppm) 175.4, 161.4, 154.5,
153.0, 151.2, 133.8, 133.1, 132.9, 130.2, 130.0, 129.2,
129.1, 125.5, 125.2, 125.0, 110.0, 108.3, 103.8, 100.4. HRMS
(ESI+): m/z calcd for [C23H17O3]+: 341.1172, found:
341.1168; calcd for [C23H16O3Na]+: 363.0992, found:
363.0999.
2.2.4. 5-((2,6-Di-tert-butyl-4H-pyran-4-ylidene)methyl)-1-
methyl-1H-pyrrole-2-carbaldehyde (8b)
2.2.6. 5-((2,6-Di-tert-butyl-4H-pyran-4-
ylidene)methyl)furan-2-carbaldehyde (9b)
A solution of (2,6-di-tert-butyl-4H-pyran-4yl)tributyl-
phosphonium perchlorate 1b (714 mg, 1.44 mmol) in anhy-
drous THF (12 mL) was prepared, purged with argon and
cooled to À78 °C. To the solution, n-BuLi (1.6 M in hexanes)
(1.00 mL, 1.60 mmol) was added dropwise and the resulting
mixture was stirred for 15 min. Then 1-methyl-5-(5,5-
dimethyl-1,3-dioxan-2-yl)-1H-pyrrole-2-carbaldehyde (248
mg, 1.11 mmol) in anhydrous THF (10 mL) was added drop-
wise and the mixture was progressively heated to reach
room temperature during 24 h (TLC monitoring using 30%
ethyl acetate in hexanes). The reaction was quenched with
HCl (1 N) (75 mL) and the mixture was stirred for 10 min
at room temperature. The organic layer was extracted with
dichloromethane, washed with water and dried over
anhydrous MgSO4. After removal of the solvent, the
aldehyde was purified by silica gel column chromatography
A solution of (2,6-di-tert-butyl-4H-pyran-4yl)tributyl-
phosphonium perchlorate 1b (589 mg, 1.19 mmol) in
anhydrous THF (10 mL) was prepared, purged with argon
and cooled to À78 °C. To the solution, n-BuLi (1.6 M in hex-
anes) (0.74 mL, 1.19 mmol) was added dropwise and the
resulting mixture was stirred for 15 min. Then 1-methyl-
5-(5,5-dimethyl-1,3-dioxan-2-yl)-1H-pyrrole-2-carbalde-
hyde (154 mg, 0.91 mmol) in anhydrous THF (7 mL) was
added dropwise and the mixture was stirred for additional
30 min (TLC monitoring using 30% ethyl acetate in hex-
anes). AmberlystÒ 15 ionic exchange resin was then added
to quench the reaction and the mixture was stirred for
30 min at room temperature. The resin was filtered off
and washed with dichloromethane. The organic layer
was dried over MgSO4 and the solvent was evaporated
under reduced pressure. Pure compound was obtained by

3240
R. Pérez Tejada et al. / Organic Electronics 15 (2014) 3237–3250
alumina column chromatography (30% ethyl acetate in
hexanes). Yield: brown oil (100 mg, 0.33 mmol; 36%).
IR (KBr): cmÀ1 1655 (C@O), 1583 (C@C). 1H NMR
(d, J = 1.8 Hz, 1H), 1.30 (s, 9H), 1.25 (s, 9H). 13C NMR
(100 MHz, CD2Cl2): d (ppm) 181.9, 175.4, 169.4, 167.6,
154.2, 141.0, 134.6, 105.6, 102.5, 102.10, 36.7, 36.3, 28.2,
28.1. HRMS (ESI+): m/z calcd for [C18H24NO2S]+: 318.1522,
found: 318.1505.
(400 MHz, CD2Cl2):
d (ppm) 9.40 (s, 1H), 7.20 (d,
J = 3.8 Hz, 1H), 6.91 (d, J = 2.0 Hz, 1H), 6.20 (d, J = 3.8 Hz,
1H), 5.78 (d, J = 2.0 Hz, 1H), 5.46 (s, 1H), 1.28 (s, 9H), 1.22
(s, 9H). 13C NMR (100 MHz, CD2Cl2): d (ppm) 175.1,
167.1, 165.2, 162.3, 150.9, 136.0, 124.9, 108.5, 105.0,
100.7, 97.4, 36.4, 36.0, 28.2, 28.1. HRMS (ESI+): m/z calcd
for [C19H25O3]+: 301.1798, found: 301.1792; calcd for [C19-
H24O3Na]+: 323.1618, found: 323.1620.
2.2.9. 3-(5-((2,6-Diphenyl-4H-pyran-4-ylidene)methyl)-1-
methyl-1H-pyrrol-2-yl)-2-cyanoacrylic acid (12a)
To
a
solution of 5-((2,6-diphenyl-4H-pyran-4-yli-
8a
dene)methyl)-1-methyl-1H-pyrrole-2-carbaldehyde
(120 mg, 0.33 mmol) and 2-cyanoacetic acid (45 mg,
0.53 mmol) in chloroform (12 mL) was added piperidine
(219 ll; 2.19 mmol). The mixture was refluxed for 5 days
2.2.7. 5-((2,6-Di-tert-butyl-4H-pyran-4-
ylidene)methyl)thiazole-2-carbaldehyde (10b)
under argon atmosphere and then cooled down to room
temperature. Pure compound was obtained by reverse
C18 column chromatography (70% AcNH4, 20 mM in aceto-
nitrile). Yield: deep red solid (88 mg, 0.21 mmol; 63%).
Mp 230–231 °C. IR (KBr): cmÀ1 2210 (C„N), 1653
A
solution of 5-((2,6-di-tert-butyl-4H-pyran-4-yli-
dene)methyl)thiazole 5b (199 mg, 0.69 mmol) in anhy-
drous THF (13 mL) was prepared, purged with argon and
cooled to À78 °C. To the solution, n-BuLi (1.6 M in hexanes)
(0.76 mL, 1.22 mmol) was added dropwise and the result-
(C@O), 1558 (C@C). 1H NMR (300 MHz, dmso-d6):
d
ing mixture was stirred for 1 h. Then DMF (161 ll;
(ppm) 7.99 (s, 1H), 7.95–7.86 (m, 4H), 7.67 (d, J = 4.4 Hz,
1H), 7.59–7.45 (m, 6H), 7.05 (br s, 1H), 7.01 (bs, 1H), 6.97
(d, J = 4.4 Hz, 1H), 6.04 (s, 1H), 3.67 (s, 3H). 13C NMR
(75 MHz, dmso-d6): d (ppm) 165.3, 153.5, 150.8, 141.0,
137.4, 132.3, 132.0, 131.9, 130.1, 129.6, 128.9, 128.8,
127.0, 125.1, 124.3, 119.5, 118.7, 112.6, 108.8, 102.1,
101.4, 89.1, 30.2. HRMS (ESI+): m/z calcd for
[C27H20N2O3]+: 420.1468, found: 420.1460.
2.1 mmol) was added dropwise and the mixture was then
warmed to room temperature (TLC monitoring using 30%
ethyl acetate in hexanes). Saturated NH4Cl solution was
added to quench the reaction and the solvent was evapo-
rated under reduce pressure. The organic layer was
extracted with dichloromethane and dried over anhydrous
MgSO4. After removal of the solvent, the aldehyde was
purified by alumina column chromatography (30% ethyl
acetate in hexanes). Yield: deep red solid (150 mg,
0.47 mmol; 69%).
2.2.10. 3-(5-((2,6-Di-tert-butyl-4H-pyran-4-ylidene)methyl)-
1-methyl-1H-pyrrol-2-yl)-2-cyanoacrylic acid (12b)
To a solution of 5-((2,6-di-tert-butyl-4H-pyran-4-
ylidene)methyl)-1-methyl-1H-pyrrole-2-carbaldehyde
(270 mg, 0.86 mmol) and 2-cyanoacetic acid (114 mg,
1.34 mmol) in chloroform (32 mL) was added piperidine
Mp 143–144 °C. IR (KBr): cmÀ1 1661 (C@O), 1564
(C@C). 1H NMR (300 MHz, CD2Cl2): d (ppm) 9.84 (s, 1H),
7.80 (s, 1H), 6.38 (d, J = 2.0 Hz, 1H), 5.91 (s, 1H), 5.87 (d,
J = 2.0 Hz, 1H), 1.29 (s, 9H), 1.23 (s, 9H). 13C NMR
(75 MHz, CD2Cl2): d (ppm) 183.7, 165.4, 159.7, 146.4,
143.3, 136.6, 105.8, 100.5, 100.0, 36.5, 36.0, 28.2, 28.1.
HRMS (ESI+): m/z calcd for [C18H24NO2S]+: 318.1522,
found: 318.1518; calcd for [C18H23NO2SNa]+: 340.1342,
found: 340.1337.
(614 ll; 5.52 mmol). The mixture was refluxed for 5 days
under argon atmosphere and then cooled down to room
temperature. Pure compound was obtained by reverse
C18 column chromatography (70% AcNH4, 20 mM in aceto-
nitrile). Yield: red-pink solid (312 mg, 0.82 mmol; 95%).
Mp 168–169 °C. IR (KBr): cmÀ1 2206 (C„N), 1667
(C@O), 1551 (C@C). 1H NMR (400 MHz, acetone-d6): d
(ppm) 8.02 (s, 1H), 7.76 (d, J = 4.7 Hz, 1H), 6.65 (d,
J = 4.7 Hz, 1H), 6.36 (d, J = 1.8 Hz, 1H), 5.98 (d, J = 1.8 Hz,
1H), 5.69 (s, 1H), 3.69 (s, 3H), 1.25 (s, 9H), 1.23 (s, 9H).
13C NMR (75 MHz, CD3COCD3-d6): d (ppm) 167.6, 166.8,
165.2, 144.6, 139.4, 136.9, 128.8, 122.0, 120.1, 113.6,
107.0, 101.0, 99.8, 89.6, 37.4, 37.0, 31.6, 29.1, 29.0. HRMS
(ESI+): m/z calcd for [C23H29N2O3]+: 381.2173, found:
381.2155; calcd for [C23H28N2O3Na]+: 403.1992, found:
403.1965.
2.2.8. 2-((2,6-Di-tert-butyl-4H-pyran-4-
ylidene)methyl)thiazole-5-carbaldehyde (11b)
A
solution of 2-((2,6-di-tert-butyl-4H-pyran-4-yli-
dene)methyl)thiazole 7b (247 mg, 0.85 mmol) in anhy-
drous THF (16 mL) was prepared, purged with argon and
cooled to À45 °C. To this solution, n-BuLi (1.6 M in hex-
anes) (0.90 mL, 1.43 mmol) was added dropwise and the
resulting mixture was stirred for 1 h. Then DMF (199 ll;
2.56 mmol) was added dropwise and the mixture was
warmed to room temperature overnight (TLC monitoring
using 20% ethyl acetate in hexanes). Saturated NH4Cl solu-
tion was added to quench the reaction and the solvent was
evaporated under reduce pressure. The organic layer was
extracted with ethyl acetate and dried over anhydrous
MgSO4. After removal of the solvent, the aldehyde was
purified by silica gel column chromatography (20% ethyl
acetate in hexanes). Yield: deep orange solid (192 mg,
0.60 mmol; 71%).
2.2.11. 3-(5-((2,6-Diphenyl-4H-pyran-4-
ylidene)methyl)furan-2-yl)-2-cyanoacrylic acid (13a)
To a solution of 5-((2,6-diphenyl-4H-pyran-4-ylidene)
methyl)furan-2-carbaldehyde 9a (167 mg, 0.49 mmol)
and 2-cyanoacetic acid (65 mg, 0.76 mmol) in chloroform
(12 mL) was added piperidine (324 ll, 3.25 mmol). The
Mp 166–169 °C. IR (KBr): cmÀ1 1651 (C@O), 1591(C@C).
1H NMR (300 MHz, CD2Cl2): d (ppm) 9.87 (s, 1H), 8.32
(s, 1H), 7.61 (d, J = 1.8 Hz, 1H), 5.93 (s, 1H), 5.92
mixture was refluxed for 8 h under argon atmosphere and
then cooled down to room temperature. Pure compound
was obtained by reverse C18 column chromatography

R. Pérez Tejada et al. / Organic Electronics 15 (2014) 3237–3250
3241
(50% AcNH4, 20 mM in acetonitrile). Yield: dark violet solid
chloroform (15 mL) was added piperidine (277 ll,
(133 mg, 0.33 mmol; 66%).
2.58 mmol). The mixture was refluxed for 4 days under
argon atmosphere and then cooled down to room temper-
ature. Pure compound was obtained by reverse C18 col-
Mp 164–165 °C. IR (KBr): cmÀ1 2217 (C„N), 1687
(C@O), 1546 (C@C). 1H NMR (300 MHz, dmso-d6):
d
(ppm) 8.09–8.06 (m, 2H), 7.91–7.88 (m, 3H), 7.58–7.49
umn
chromatography
(70%
AcNH4,
20 mM
in
(m, 8H), 7.05 (d, J = 1.5 Hz, 1H), 6.79 (d, J = 3.9 Hz, 1H),
acetonitrile). Yield: dark violet solid (100 mg, 0.26 mmol;
67%).
5.94 (s, 1H). 13C NMR (100 MHz, dmso-d6):
d (ppm)
164.5, 161.1, 154.8, 152.1, 147.1, 135.9, 133.6, 131.8,
131.6, 130.3, 129.9, 128.9, 128.6, 125.5, 124.5, 122.5,
117.1, 113.1, 108.7, 103.7, 100.6, 92.2. HRMS (ESI+): m/z
Calcd for [C23H18NO4]+: 408.1230, found: 408.1211.
Mp 219–220 °C. IR (KBr): cmÀ1 2212 (C„N), 1659
(C@O), 1542 (C@C). 1H NMR (300 MHz, acetone-d6): d
(ppm) 8.31 (s, 1H), 7.98 (s, 1H), 6.52 (d, J = 1.9 Hz, 1H),
6.21 (s, 1H), 6.11 (d, J = 1.9 Hz, 1H), 1.32 (s, 9H), 1.26 (s,
9H). 13C NMR (75 MHz, acetone-d6):
d (ppm) 168.1,
2.2.12. 3-(5-((2,6-Di-tert-butyl-4H-pyran-4-
ylidene)methyl)furan-2-yl)-2-cyanoacrylic acid (13b)
To a solution of 5-((2,6-di-tert-butyl-4H-pyran-4-yli-
165.0, 162.9, 152.4, 146.1, 145.1, 143.5, 136.1, 120.1,
115.9, 105.6, 101.2, 99.7, 87.4, 35.9, 35.4, 27.1, 27.0. HRMS
(ESI+): m/z Calcd for [C21H25N2O3S]+: 385.1580, found:
385.1578. Calcd for [C21H24N2O3SNa]+: 407.1400, found:
407.1389.
dene)methyl)furan-2-carbaldehyde
0.40 mmol) and 2-cyanoacetic acid (51 mg, 0.60 mmol) in
chloroform (10 mL) was added piperidine (265 l,
9b
(120 mg,
l
2.65 mmol). The mixture was refluxed for 24 h under argon
atmosphere and then cooled down to room temperature.
Pure compound was obtained by reverse C18 column chro-
matography (50% AcNH4, 20 mM in acetonitrile). Yield:
dark violet solid (81 mg, 0.22 mmol; 55%).
2.2.15. 3-(2-((2,6-Diphenyl-4H-pyran-4-
ylidene)methyl)thiazol-5-yl)-2-cyanoacrylic acid (15a)
To
dene)methyl)thiazole-2-carbaldehyde
0.33 mmol) and 2-cyanoacetic acid (44.5 mg, 0.52 mmol)
in chloroform (10 mL) was added piperidine (222 l,
a
solution of 2-((2,6-diphenyl-4H-pyran-4-yli-
11a (120 mg,
Mp 162–163 °C. IR (KBr): cmÀ1 2214 (C„N), 1664
(C@O), 1539 (C@C). 1H NMR (400 MHz, acetone-d6): d
(ppm) 7.87 (s, 1H), 7.53 (d, J = 4.0 Hz, 1H), 6.81 (d,
J = 1.8 Hz, 1H), 6.52 (d, J = 4.0 Hz, 1H), 6.01 (d, J = 1.8 Hz,
1H), 5.66 (s, 1H), 1.33 (s, 9H), 1.26 (s, 9H). 13C NMR
l
2.22 mmol). The mixture was refluxed for 4 days under
argon atmosphere and then cooled down to room temper-
ature. Pure compound was obtained by reverse C18 col-
umn
chromatography
(50%
AcNH4,
20 mM
in
(100 MHz, acetone-d6):
d (ppm) 169.2, 167.0, 165.9,
acetonitrile). Yield: dark red solid (75.5 mg, 0.18 mmol;
53%).
164.6, 148.7, 138.4, 138.2, 128.6, 118.4, 113.2, 107.3,
103.1, 99.6, 91.1, 38.0, 37.3, 29.3, 29.2. HRMS (ESI+): m/z
calcd for [C22H26NO4]+: 368.1856, found: 368.1847; calcd
for [C22H25NO4Na]+: 390.1676, found: 390.1655.
Mp 263–265 °C. IR (KBr): cmÀ1 2213 (C„N), 1649
(C@O), 1545 (C@C). 1H NMR (400 MHz, dmso-d6):
d
(ppm) 8.57 (s, 1H), 8.49 (s, 1H), 8.45 (d, J = 1.4 Hz, 1H),
7.95–7.89 (m, 4H), 7.61–7.52 (m, 6H), 7.15 (d, J = 1.4 Hz,
1H), 6.37 (s, 1H). 13C NMR (100 MHz, dmso-d6): d (ppm)
173.3, 163.4, 156.5, 155.9, 154.4, 144.3, 138.3, 131.6,
131.4, 130.5, 130.2, 129.0, 128.9, 127.0, 125.1, 124.8,
116.4, 108.2, 104.5, 104.1, 97.6. HRMS (ESI+): m/z calcd
for [C25H17N2O3S]+: 425.0954, found: 425.0954; calcd for
[C25H16N2O3SNa]+: 447.0774, found: 447.0776.
2.2.13. 3-(5-((2,6-Diphenyl-4H-pyran-4-
ylidene)methyl)thiazol-2-yl)-2-cyanoacrylic acid (14a)
To
dene)methyl)thiazole-2-carbaldehyde
0.19 mmol) and 2-cyanoacetic acid (26 mg, 0.30 mmol) in
chloroform (6 mL) was added piperidine (129 l,
a
solution of 5-((2,6-diphenyl-4H-pyran-4-yli-
10a (70 mg,
l
1.29 mmol). The mixture was refluxed for 3 days under
argon atmosphere and then cooled down to room temper-
ature. Pure compound was obtained by reverse C18
column chromatography (70% AcNH4, 20 mM in
acetonitrile). Yield: dark blue solid (60 mg, 0.14 mmol;
47%).
2.2.16. 3-(2-((2,6-Di-tert-butyl-4H-pyran-4-
ylidene)methyl)thiazol-5-yl)-2-cyanoacrylic acid (15b)
To a solution of 2-((2,6-di-tert-butyl-4H-pyran-4-yli-
dene)methyl)thiazole-2-carbaldehyde
0.60 mmol) and 2-cyanoacetic acid (80 mg, 0.93 mmol) in
chloroform (25 mL) was added piperidine (397 l,
11b
(190 mg,
Mp 228–229 °C. IR (KBr): cmÀ1 2215 (C„N), 1650
(C@O), 1578 (C@C). 1H NMR (300 MHz, dmso-d6):
d
l
(ppm) 8.23 (s, 1H), 8.19 (s, 1H), 7.91–7.86 (m, 4H), 7.55–
7.48 (m, 6H), 7.08 (d, J = 1.6 Hz, 1H), 7.04 (d, J = 1.6 Hz,
1H), 6.40 (s, 1H). 13C NMR (75 MHz, dmso-d6): d (ppm)
163.2, 154.9, 152.5, 152.1, 144.7, 144.4, 143.6, 133.6,
131.6, 131.5, 130.6, 130.0, 129.0, 128.9, 125.0, 124.6,
116.1, 108.6, 103.8, 102.1, 100.7. HRMS (ESI+): m/z calcd
for [C25H17N2O3S]+: 425.0954, found: 425.0949.
3.97 mmol). The mixture was refluxed for 4 days and a half
under argon atmosphere and then cooled down to room
temperature. Pure compound was obtained by reverse
C18 column chromatography (70% AcNH4, 20 mM in aceto-
nitrile). Yield: red-pink solid (181.6 mg, 0.47 mmol; 79%).
Mp198–202 °C. IR (KBr): cmÀ1 2214 (C„N), 1660
(C@O), 1549 (C@C). 1H NMR (300 MHz, acetone-d6): d
(ppm) 8.45 (s, 1H), 8.44 (s, 1H), 7.83 (d, J = 1.4 Hz, 1H),
6.19 (d, J = 1.4 Hz, 1H), 6.12 (s, 1H), 1.32 (s, 9H), 1.29 (s,
2.2.14. 3-(5-((2,6-Di-tert-butyl-4H-pyran-4-
ylidene)methyl)thiazol-2-yl)-2-cyanoacrylic acid (14b)
To a solution of 5-((2,6-di-tert-butyl-4H-pyran-4-yli-
9H). 13C NMR (75 MHz, acetone-d6):
d (ppm) 168.8,
167.2, 163.3, 156.7, 144.7, 140.9, 126.8, 116.3, 106.2,
105.4, 102.2, 102.1, 97.2, 35.9, 35.6, 27.2, 27.1. HRMS
(ESI+): m/z calcd for [C21H25N2O3S]+: 385.1580, found:
dene)methyl)thiazole-2-carbaldehyde
10b
(124 mg,
0.39 mmol) and 2-cyanoacetic acid (52 mg, 0.61 mmol) in

3242
R. Pérez Tejada et al. / Organic Electronics 15 (2014) 3237–3250
384.1560; calcd for [C21H24N2O3SNa]+: 407.1400, found:
407.1380.
2.4. Computational details
Theoretical calculations were performed using Gaussian
09 [20] and the default parameters provided by this
software package. Solvent effects were estimated using a
Conductor-like Polarizable Continuum Model (CPCM)
[21]. Equilibrium geometries in solution were obtained
using the M06-2x hybrid meta-GGA exchange correlation
functional [22] and the medium size 6-31G* basis set
[23] both for ground and first excited states. Excitation
energies were calculated by time-dependent single point
calculations using the M06-2x/6-311+G (2d,p) model
chemistry. Absorption spectra were calculated from the
vertical excitations at the optimized ground state geome-
try, emission spectra from the vertical excitations at the
optimized excited state geometry and adiabatic excitation
energies (E0–0) were calculated subtracting the ground
state energy from the excited state energy both calculated
at their optimized geometry. Molecular Orbital contour
plots were obtained using the Avogadro software [24] at
0.04 isosurface value.
2.3. Device preparation and characterization
The working and counter electrodes consisted of TiO2
and thermalized platinum films, respectively, and were
deposited onto F-doped tin oxide (FTO, Pilkington Glass
Inc., with 15
strates. DSC devices were made using 8
sisting of 20 nm TiO2 nanoparticles (DyesolÓ paste) and a
scattering layer of 4 m of 400 nm TiO2 particles (DyesolÓ
X
sqÀ1 sheet resistance) conducting glass sub-
l
m thick films con-
l
paste). Prior to the deposition of the TiO2 paste, the con-
ducting glass substrates were immersed in a solution of
TiCl4 (40 mM) at 70 °C for 30 min and then dried. The
TiO2 nanoparticle paste was deposited onto a conducting
glass substrate using the screen-printing technique. The
TiO2 electrodes were gradually heated under airflow at
325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min and
500 °C for 15 min. The heated TiO2 electrodes were
immersed again in a solution of TiCl4 (40 mM) at 70 °C
for 30 min and then washed with ethanol. The electrodes
were heated again at 500 °C for 30 min and cooled before
dye adsorption. The active area for devices was 0.16 cm2.
The counter electrode was made by spreading a 5 mM
solution of H2PtCl6 in isopropyl alcohol onto a conducting
glass substrate containing a small hole to allow the intro-
duction of the liquid electrolyte using vacuum, followed
by heating at 390 °C for 15 min. Films were sensitized in
0.3 mM dye solutions in dichloromethane containing
0.2 mM for 3 h at room temperature. Finally, the working
and counter electrodes were sandwiched together using a
thin thermoplastic (Surlyn) frame that melts at 100 °C.
The electrolyte for all devices measured was the LP1 as
described later on this work.
3. Results and discussion
3.1. Synthesis and structural characterization
Aldehydes (8À9)a were prepared by a Horner reaction
of (2,6-diphenyl-4H-pyran-4-yl)diphenylphosphine oxide
1a [25] with the reported monoprotected aldehydes 2
[26] and 3 [27] respectively. Thiazole aldehydes 10a and
11a were prepared from a procedure previously reported
in our laboratory [19] (see Scheme 1).
Tert-butyl aldehydes 8b, 9b were obtained by a Wittig
reaction from the phosphonium salt 1b [28] and the corre-
sponding monoprotected aldehydes as described before.
The IV characteristics of cells were measured using a
Sun 2000 Solar Simulator (150 W, ABET Technologies).
The illumination intensity was measured to be 100 mW/
m2 with a calibrated silicon photodiode. The appropriate
filters were utilized to faithfully simulate the AM 1.5G
spectrum. The applied potential and cell current were
measured with a Keithley 2400 digital source meter. The
IPCE (Incident Photon to Current conversion Efficiency)
was measured using a homemade set up consisting of a
150 W Oriel Xenon lamp, a motorized monochromator
and a Keithley 2400 digital source meter.
Concerning aldehydes bearing
a matched 10b or a
mismatched thiazole ring 11b, they were also prepared
in two steps from phosphonium salt 1b with the commer-
cially available thiazolecarbaldehydes 4 or 6 and then,
followed by lithiation and reaction with anhydrous DMF.
Finally, the desired dyes (12–15)a,b were obtained by
Knoevenagel condensation of the aforementioned alde-
hydes with 2-cyanoacetic acid in the presence of piperi-
dine as shown in Scheme 2. All intermediates and the
final organic dyes have been confirmed by 1H NMR, 13C
NMR and HRMS.
Transient photovoltage and charge extraction measure-
ments were carried out on a home-built system. In charge
extraction, white light from a series of LEDs was used as
the light source. When the LEDs are turned off the cell is
immediately short circuited and the charge is extracted
allowing electron density in the cells to be calculated. By
changing the LEDs intensity the electron density can be
estimated as a function of cell voltage. In transient photo-
voltage measurements, in addition to the white light
applied by the LEDS, constant background voltage is
applied to the cells and again, a diode pulse (660 nm,
10 mW) is then applied to the sample inducing a change
of 2-3 mV within the cell. The resulting photovoltage decay
3.2. Optical properties
The absorption spectra of the dyes in CH2Cl2 are shown
in Fig. 2 and the photophysical data are collected in Table 1.
In order to compare with the new systems we are also
included in this study data of the previously reported dyes
with a thiophene ring in the p-spacer (16a,b) [13] (Fig. 1).
All dyes exhibit a strong and broad band ranging from 450
to 650 nm that can be assigned to an ICT process. The
energy of the ICT band is dependent on the heterocycle
employed as spacer and does not follow a general rule.
transients are collected and the
fitting the data to the equation exp(Àt/
s
values are determined by
).
All dyes have molar extinction coefficients (
e) ranging from
s
24600 to 52,000 MÀ1 cmÀ1, higher than those of standard

R. Pérez Tejada et al. / Organic Electronics 15 (2014) 3237–3250
3243
Scheme 1. Synthesis of aldehydes (8–11)a,b.
Scheme 2. Synthesis of 4H-pyranylidene organic dyes (12–15)a,b.
ruthenium dyes N3 [29] and N719 [30] (13,900 and
14,000 MÀ1 cmÀ1 respectively) except the compound 15a
with a molar extinction coefficients around 11,000 MÀ1
cmÀ1. It is interesting to note that compound 15b has the
highest molar extinction coefficient of all the series
(51,929 MÀ1 cmÀ1).
When the dyes are attached to TiO2 surface, the maxi-
mum absorption peaks are blue-shifted (in some cases
more than 50 nm) as compared to those in solution. In gen-
eral, the blue shifts of the absorption spectra on TiO2 could
be ascribed to deprotonation and formation of H-aggre-
gates (extended head to tail stacking) [31] of the dyes on
the TiO2 surface, as it was described for other organic mol-
ecules [17d,32].
3.3. Theoretical calculations
In order to get further insight in the electronic and opti-
cal properties of the studied compounds, theoretical calcu-
lations using TD-DFT (Time Dependent Density Functional
Theory) and the CPCM (Conductor-like Polarizable Contin-
uum Model) solvation method have been made. Calcula-
tions were first performed at the ground state geometry
to obtain the ground state dipole moments (l0), orbital

3244
R. Pérez Tejada et al. / Organic Electronics 15 (2014) 3237–3250
Table 1
Optical and electrochemical properties of the dyes.
a
d
e
* f
Dye
kabs, nm (
e
, MÀ1 cmÀ1
)
kabs, nmb
kem, nmc
Eox
,
V (vs NHE)
E0–0, eV
Eox , V (vs NHE)
12a
12b
13a
13b
14a
14b
15a
15b
16a
16b
514 (37067)
515 (45550)
568 (31194)
538 (26761)
566 (24679)
573 (27785)
533 (10934)
531 (51929)
553 (41071)
544 (33558)
463
481
491
507
506
508
467
461
487
497
587
571
651
621
660
646
611
589
655
588
0.75
0.77
0.77
0.83
0.94
0.97
0.95
1.10
0.81
0.86
2.22
2.27
1.99
2.08
1.98
2.00
2.12
2.19
2.00
2.16
À1.45 (À1.26)
À1.49 (À1.37)
À1.22 (À1.10)
À1.24 (À1.19)
À1.02 (À0.89)
À1.02 (À0.95)
À1.16 (À1.13)
À1.10 (À1.33)
À1.19 (À1.17)
À1.19 (À1.22)
a
b
c
Absorption maxima in CH2Cl2 solution.
Absorption maxima on TiO2 films (4 m).
Emission spectra were recorded in CH2Cl2.
l
d
First oxidation potentials were measured from a three electrode electrochemical cell in CH2Cl2 containing 0.1 M TBAPF6. A glassy carbon, Ag/AgCl (KCl
3 M), and Pt were used as working, reference, and counter electrode respectively.
e
Zeroth-zeroth transition energies estimated from the intersection of normalized absorption and emission spectra in CH2Cl2 solution.
f
Excited-state oxidation potentials of the dyes obtained from Eox – E0–0
.
As previously mentioned, the lowest excitation involves
an ICT and therefore, it is expected an increased contribu-
tion from the zwitterionic structure on passing from the
ground to the first excited state. These changes are not
only responsible for a change in charge distribution but
also in structural parameters. The excited state displays a
more aromatized 4H-pyranylidene ring, a more quinoidal
heterocyclic spacer and changes in the alternation of single
and double bond lengths. If we take compound 15b as an
example, the sum of Mulliken charges at the donor, the
spacer and the acceptor are +0.476, À0.388 and À0.087
respectively in the ground state and +0.674, À0.413 and
À0.260 in the excited state and the bond lengths are those
depicted in Fig. 5. As expected, both the charge distribution
and the changes in bond lengths point to a larger contribu-
tion of the zwitterionic structure to the first excited state.
COOH
Z
CN
Y
X
NCCH2COOH, 65º
CHCl3, Piperidine
(8-11)a,b
R
R
O
(12-15)a,b
12:
13:
14:
15:
16:
X = N-CH3; Y = CH; Z = CH
X = O; Y = CH; Z = CH
X = S; Y = CH; Z = N
X = S; Y = N; Z = CH
X = S; Y = CH; Z = CH
a
R = Ph for series
t
b
R = Bu for series
Fig. 1. Molecular structures of studied organic dyes 12–15 and analogues
16a,b previously reported[13].
energies and topologies, absorption wavelengths (kabs) and
oscillator strengths (f), and then, the first excited state
geometries were optimized to obtain the emission wave-
length (kem) and the adiabatic excitation energy (E0–0).
The results of these calculations are gathered in Table 2.
Compared to experimental results, theoretical calculations
provide a reasonable accuracy with mean absolute errors
of 18 nm (0.09 eV) in kabs, 12 nm (0.04 eV) in kem and
0.10 eV in E0–0, and reproduce the experimental trends.
According to the calculations, the first excited state is
mainly contributed from a one electron HOMO to LUMO
transition. The topology of these orbitals (Fig. 3) accounts
for an ICT from the 4H-pyranylidene donor to the cyanoac-
rylic acid acceptor. There is however a large HOMO-LUMO
spatial overlap that is responsible for the high oscillator
3.4. Electrochemical properties
The electrochemical properties of dyes adsorbed on
TiO2 films were studied by using differential pulse voltam-
metry (DPV) methods. Representative voltammograms
were performed under argon, in CH2Cl2 using 0.1 M tetra-
butylammonium hexafluorophosphate as the supporting
electrolyte, a glassy carbon working electrode, a Pt counter
electrode and a Ag/AgCl reference electrode. The relevant
electrochemical data are listed in Table 1 and in order to
establish a comparison we have also included parameters
for the previously reported dyes with a thiophene ring
[13].
strengths (f) and molar extinction coefficients (
e
). In good
The electronic nature of the heteroaromatic rings in the
conjugated spacer affects significantly to the oxidation
potential values. It is interesting to note that dyes with a
more electron rich ring like pyrrol are the easier oxidized
in both phenyl and tert-butyl series. The stronger elec-
tron-donating ability of this system resulted in a higher
HOMO energy level (estimated from its first oxidation
potential). Thus, the HOMO energy decreased (more
positive Eox) in the order: Eox (pyrrol) < Eox(furan) < Eox
(thiophene) < Eox(thiazole matched) < Eox (thiazole mis-
matched). A more positive value for de Eox could show a
agreement with the experimental trend, the highest exci-
tation energies corresponds to pyrrole derivatives 12
which also display the largest HOMO-LUMO gap followed
by the matched thiazole derivatives 15.
Theoretical calculations also provide useful information
on the changes of the geometry and charge distribution
when passing from the ground to the first excited state.
The studied compounds can be represented by using two
limiting resonance structures (Fig. 4) displaying a neutral
or a zwitterionic charge distribution.

R. Pérez Tejada et al. / Organic Electronics 15 (2014) 3237–3250
3245
0,6
0,6
0,5
0,4
0,3
0,2
0,1
0,0
(12a)
(13a)
(14a)
(15a)
(16a)
(12b)
(13b)
(14b)
(15b)
(16b)
0,5
0,4
0,3
0,2
0,1
0,0
350 400 450 500 550 600 650 700
350 400 450 500 550 600 650 700
Wavelength (nm)
Wavelength (nm)
0,55
0,50
0,45
0,40
0,35
0,30
0,25
0,20
0,15
0,10
0,05
0,00
0,55
0,50
0,45
0,40
0,35
0,30
0,25
0,20
0,15
0,10
0,05
0,00
(12b)
(13b)
(14b)
(15b)
(16b)
(12a)
(13a)
(14a)
(15a)
(16a)
400
500
600
700
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 2. Absorption spectra of organic dyes (12–16)a,b in CH2Cl2 (top) and on TiO2 transparent films (bottom).
Table 2
Results of DFT Calculations in dichloromethane solution.a
b
c
d
d
c
d
Dye
l0
kabs
EHOMO
ELUMO
kem
E0–0
f
12a
12b
13a
13b
14a
14b
15a
15b
16a
16b
14.5
15.5
8.4
8.0
9.2
10.1
8.1
8.8
9.8
10.5
483 (2.57)
477 (2.60)
545 (2.28)
528 (2.35)
568 (2.18)
553 (2.24)
521 (2.38)
502 (2.47)
550 (2.25)
533 (2.33)
1.79
1.59
0.94
0.80
1.24
1.14
1.30
1.20
1.29
1.19
À6.39
À6.33
À6.39
À6.34
À6.52
À6.48
À6.69
À6.67
À6.43
À6.39
À1.91
À1.80
À2.18
À2.01
À2.50
À2.38
À2.37
À2.23
À2.30
À2.15
615 (2.01)
569 (2.18)
651 (1.91)
595 (2.09)
673 (1.84)
634 (1.96)
612 (2.03)
569 (2.18)
652 (1.90)
603 (2.06)
1.95
2.38
2.03
2.21
2.00
2.08
2.20
2.32
2.08
2.18
a
b
c
M06-2x/6-311+G(2p,d)//M06-2x/6-31G* model chemistry using the CPCM solvation model.
In Debye.
In nm with parenthesized energy in eV.
eV.
d
Fig. 3. Contour plot of the HOMO (left) and LUMO (right) of compound 15b.

3246
R. Pérez Tejada et al. / Organic Electronics 15 (2014) 3237–3250
Table 3
N
C
N
C
Photovoltaic properties of DSSCs constructed using the dyes (12–16)a,b.
Z
Z
Y
Y
H
CO2
H
CO2
Dye
Jsc (mA cmÀ2
)
Voc (V)
ff
g (%)
X
X
12a
12b
13a
13b
14a
14b
15a
15b
16a
16b
3.10
5.44
6.48
9.06
2.93
4.89
2.11
10.51
8.91
12.06
0.564
0.554
0.544
0.609
0.569
0.609
0.569
0.689
0.594
0.654
65
70
72
72
70
76
60
73
68
74
1.13
2.13
2.52
4.00
1.16
2.28
0.72
5.35
3.62
5.85
R
R
O
R
R
O
Fig. 4. Resonance structures of compounds 12–16.
.
1 311
.
1 431
.
1 361
.
1 340
.
1 387
.
1 336
N
.
1 418
.
1 430
.
1 376
.
1 396
.
conditions for these dyes are determined to be 0.3 mM of
dye in dichloromethane solution, 2 h of immersion and
LP1 (0.5 M BMII/0.05 M I2/0.5 M TBP/0.1 M LiI in acetoni-
trile) as electrolyte.
As it can be seen, the IPCE performance of the DSSCs
with bulky tert-butyl groups in the donor system is higher
than those with phenyl groups (Fig. 6). It should be noted
that for dyes 15b and 16b the IPCE is roughly constant
(70–78%) in the range of 400–600 nm, while for the rest
of compounds
450 nm. Surprisingly, the performance of the IPCE spec-
trum of dye 15a (solar cell efficiency, = 0.72%) is much
lower than 15b (solar cell efficiency = 5.35%). This result
1 447
.
1 365
.
1 359
.
1 348
S
CO2H
tBu
.
1 392
.
1 397
.
1 424
.
1 418
NC
.
1 428
.
1 358
.
1 445
.
O
1 362
.
1 360
tBu
.
1 351
.
1 355
.
1 354
a maximum can be observed around
Fig. 5. Selected Bond lengths of compound 15b in the ground (Blue) and
first excited (Red) states. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
g
g
was not observed when comparing the other couples of a,b
compounds. This result can be explained in terms of the
lower molar extinction coefficient of the phenyl system
(15a: 10950 MÀ1 cmÀ1 vs 15b: 51929 MÀ1 cmÀ1) as well
as the presence of aggregates of type H (see figure S-53
in supporting information) in the TiO2 loading conditions
for 15a, despite the use of the chenoxycholic acid as
co-adsorbent. Our hypothesis has been confirmed by the
measurement of the absorption at different concentrations
of both dyes. Thus, whereas 15b is linear in the range of
measured concentrations (10À4 M) 15a do not show Lam-
ber-Beer behaviour from relatively low concentrations
(0.3 Â 10À4 M) (Fig. 8). Finally, it can be noted that elec-
trodes from dye 15a were quickly discolored, even in pres-
ence of the electrolyte, which indicates certain instability
of the dye 15a on the TiO2 surface.
As listed in Table 3, the tert-butyl series give higher
energy conversion efficiencies than the corresponding phe-
nyl ones. In general, devices from tert-butyl substituents in
the 4H-pyranylidene ring showed a higher open-circuit
voltage than their phenyl equivalents. The improvement
of Voc can be attributed to the reduction of the charge
recombination processes between the photo-injected elec-
trons at the TiO2 and the oxidized electrolyte. The presence
of sterically hindered tert-butyl groups reduces the aggre-
gation phenomena and also block the approach of I- to the
TiO2 surface due to the improved molecular packing of the
molecules at the surface of the semiconductor metal oxide
as reported in other literature examples [33].
better performance of the dispositive because of its higher
difference with the iodide/triiodide redox couple (+0.42 vs
NHE) and then, a better regeneration of the oxidize dye
after electron injection into the conduction band of TiO2.
Concerning the thiazole dyes, oxidation potential values
for the mismatched ones are slightly higher than their
matched isomers, in agreement with the previous results
[19]. These results are fairly consistent with the theoretical
calculations using a polarizable continuum model. The
excited state potential (E*ox) was deduced from the expres-
sion (Eox–E0–0), where the band gap energies (E0–0) were
estimated from the intercept of the normalized absorption
and emission spectra (Table 2). The E*ox values were in all
cases sufficiently more negative than the energy conduc-
tion band edge energy level of TiO2 (À0.5 V vs NHE) which
can indicate that the electrons can be efficiently injected
into the TiO2 from the excited dyes. These results are
probably related with the delocalization energies of the
heterocyclic
rings
(furan
16 Kcal molÀ1
,
pyrrole
21 Kcal molÀ1 and thiazole 25 Kcal molÀ1).
3.5. Photovoltaic properties of DSSCs
The device performance (with an effective area of
0.16 cm2) were measured under sun-simulated AM 1.5 G
irradiation (100 mW/cm2). The open-circuit photovoltage
(Voc), short-circuit photocurrent density (Jsc), fill factor
(ff), and solar-to-electrical energy conversion efficiencies
In contrast, the lower performances of DSSCs based on
the phenyl dyes are mostly due to the lower Jsc values
compared to tert-butyl series.
(g) are listed in Table 3. Moreover, current density-voltage
(J–V) curves and incident photon- to- current conversion
efficiencies (IPCE) of devices based on these dyes are
It is well established that in
a DSSC, the Voc is
shown in Figs.
6 and 7, respectively. The optimized
determined by the energy difference between the quasi-

R. Pérez Tejada et al. / Organic Electronics 15 (2014) 3237–3250
3247
70
60
50
40
30
20
10
0
80
70
12b
13b
14b
15b
16b
12a
13a
14a
15a
16a
60
50
40
30
20
10
0
400 450 500 550 600 650 700 750 800
400 450 500 550 600 650 700 750 800
Wavelength (nm)
Wavelength (nm)
Fig. 6. Photocurrent action spectra of the TiO2 electrodes sensitized: phenyl series (a), tert-butyl series (b).
15
10
5
15
10
5
0
0
12b
12a
13b
13a
14b
14a
-5
-5
-10
-15
15b
15a
16b
16a
Dark
Dark
Dark
Dark
Dark
Dark
Dark
Dark
Dark
Dark
-10
-15
0
0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8
0
0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8
Voltage (V)
Voltage (V)
Fig. 7. I/V curves recorded under AM 1.5G illumination and in the dark for DSC devices.
15a
15b
the semiconductor metal oxide and the oxidized electro-
lyte species. To investigate the difference in voltage for
these devices, charge extraction and transient photovolt-
age measurements were conducted. Both techniques have
been widely used in the study of the limiting interfacial
charge transfer reactions in DSSC due to the fast data
acquisition which prevents the degradation of the devices
during the measurement period. In fact, the use of both
techniques implies no more than 20 min for each device.
As can be seen in Fig. 9, the experimental data showed a
shift for the electron density vs light bias (the solar cell
photovoltage when illuminated at different light intensi-
ties) of the exponential curve. The shift is more pro-
nounced for the 12a with almost 50 mV when compare
to devices made using 16a. This shift corresponds to a
change on the position of the TiO2 conduction band-edge
but does not explain by itself the observed differences in
Voc (Table 3). Taking into account that all devices are pre-
pared under the same conditions and using the same sol-
vent and electrolyte the shift can only be attributed to
the differences on surface molecular dipole promoted by
the sensitizers at the surface of the semiconductor nano-
particles. This is in good agreement with the calculated
4
3
2
1
0
1,0x10-4
2,0x10-4
3,0x10-4
0,0
Concentration (M)
Fig. 8. Absorption vs concentration for dyes 15a and 15b.
Fermi-level of the metal oxide semiconductor, which is
correlated with its conduction band (CB) edge and the
electron density, and the oxidation potential of the electro-
lyte used. The electron density is itself dependent on rate
of recombination between the photo-injected electrons at

3248
R. Pérez Tejada et al. / Organic Electronics 15 (2014) 3237–3250
Fig. 9. Electron density as a function of cell voltage.
0,1
12a
13a
14a
15a
16a
12b
13b
14b
16b
0,01
0,01
0,001
0,001
1e+17
1e+18
1e+19
7e+16
1e+17
4e+17
e-density(e-/cm3)
e-density(e-/cm3)
Fig. 10. Device electron lifetime
s
as function of charge density (eÀ/cm3).
molecular dipole moments of these dyes (Table 2) larger
for compounds 12 compared to 16. Moreover, as depicted
in Fig 10, the slower recombination kinetics by the devices
made using the sensitizer 16a also influences the different
photovoltage observed. Furthermore, it is worthy to men-
tion that in all cases the b series shows slower recombina-
tion with the electrolyte than the a series as measured by
TPV, in good agreement with the better molecular packing
of the b series that includes bulky groups in each structure
acting as an effective barrier preventing the contact
between the oxidized electrolyte and the TiO2 nanoparti-
cles surface.
resistance and Voc. The observed values can be attributed
to the presence of sterically hindered tert-butyl groups
and to the differences on molecular dipoles of the sensi-
tizer at the TiO2 surface. Matched orientation of a thiazole
ring in the
p-bridge leads to sensitizers less efficient. The
lower performance measured for pyrrol derivatives could
be due to the higher HOMO energy values for dyes contain-
ing this heterocycle.
Acknowledgements
We gratefully acknowledge the financial support from
the Spanish Ministry of Science and Innovation, MICINN-
FEDER (project CTQ2011-22727) and the Gobierno de
Aragón-Fondo Social Europeo (E39). A predoctoral fellow-
ship to R. Pérez Tejada (CSIC, JAE 2011) is also
acknowledged.
4. Conclusions
In summary, we have designed and prepared eight new
Ruthenium-free dyes with a 4H-pyranylidene ring as the
donor moiety to evaluate the effects of several aromatic
p
linkers (pyrrol, furan, thiazole and thiophene) on the
electrochemical and photovoltaic properties of DSSCs in
D- -A systems. The most effective bridge has to be found
Appendix A. Supplementary material
p
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.orgel.2014.09.003.
the thiophene, followed by the mismatched-oriented
thiazole. These systems lead to a higher recombination

R. Pérez Tejada et al. / Organic Electronics 15 (2014) 3237–3250
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