This document is the Accepted Manuscript version of a Published Work that appeared in final form in Dyes and Pigments 123
(2015) 293 e303 DOI: http://dx.doi.org/10.1016/j.dyepig.2015.07.026

Organic sensitizers bearing a trialkylsilyl ether group for liquid dye
sensitized solar cells
'
Raquel Perez-Tejada a, Natalia Martínez de Baroja a, Santiago Franco a, *, Laia Pelleja b, **,
Jesús Orduna a, Raquel Andreu a, Javier Garín a
a
b

'
'
Departamento de Química Organica, Instituto de Ciencia de Materiales de Aragon (ICMA), Universidad de Zaragoza e CSIC, E50009 Zaragoza, Spain
Institute of Chemical Research of Catalonia (ICIQ), Avda Països Catalans 16, Tarragona E-43007, Spain

a b s t r a c t
In this work we present the synthesis, optical characterization and performance of five metal-free sensitizers for dye-sensitized solar cells (DSSC).
All dyes include, for the first time, a trialkylsilyl ether group in the p-conjugated bridge (a thiophene ring). The influence of different donor unities, like
tri- arylamine (TPA), 4H-pyranylidene and dithiafulvene has been evaluated in DSSC with a liquid I3/I- electrolyte, obtaining the best results with the
4H-pyranylidene moiety. The size and the position of the bulky group have a great importance in the efficiency of the final devices.
In order to explain the recombination processes and electron life-time, charge extraction (CE) and transient photovoltage (TPV) experiments have been
carried out.

1. Introduction
Solar energy offers a clean, well-spread and inexhaustible energy source. Although the market is dominated by silicon-based
photovoltaic devices, in recent years the interest on alternatives
more environment-friendly has increased, specially focused on
reducing mass during cell manufacture processes and the thickness
of the final device.
Organic Photovoltaic Cells (OPVs) [1,2] and particularly Dye
Sensitized Solar Cells (DSSCs) [3,4] constitute an interesting alternative due to their low manufacturing cost, flexibility of molecular
design, light-weight and great esthetic features, like color and
transparency. The key element in a DSSC device is probably the
sensitizer dye and over the last years, thousands of new dyes have
been investigated. The most efficient organic sensitizers are based
on Donorep spacereacceptor (DepeA) structures [5], a type of
pushepull systems which lead to effective photoinduced intramolecular charge-transfer (ICT). In these systems small variations

in the different parts of the molecule (mostly in the donor and the
p-bridge) may result in significant changes in the photovoltaic
properties. Triphenylamine (TPA)-based metal free organic dyes are
one of the most common donor groups in DSSCs [6,7], as it presents
several advantages, like a non-planar structure, which suppresses
the formation of aggregates. Furthermore, the physical properties
can be easily modulated by introduction of bulky or donor groups
[8e13]. Recently, proaromatic systems like 4H-pyranylidene
[14e17] and dithiafulvene [18e20] have been introduced as alternative and efficient donor unities in DSSCs, but no studies
comparing their properties have been reported.
When designing a new sensitizer, one important factor to take
into account is related to the minimization of aggregates by pep
stacking. This may be performed by using additives, such as
deoxycholic acid (DCA) [21,22] or by the introduction of bulky
groups both, in the donor or in the p-bridge [12,23e26]. However,
it is relatively difficult to synthesize dyes with bulky chains in the
conjugated spacer, requiring tedious and multiple reaction steps.
Silicon-based dyes have been very promising for DSSCs due to
its photo and thermal stability [27,28]. Examples of organic sensitizers bearing a dithienosilole (DTS) as a p-linker have been
reported with high efficiencies [29e34]. However, to the best of
our knowledge, organic dyes featuring a trialkylsilyl ether

reference. UVeVisible spectra were recorded with a UVeVis UNICAM UV4 spectrophotometer. Pulse differential voltammetry
measurements were performed with a m-Autolab type III 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 L-1). Scan rate was 0.01 V s-1, modulation
amplitude 0.025 V and modulation time 0.05 s-1.
2.2. Synthesis

2.1. General information

2.2.1. 4-((tert-Butyldimethylsilyloxy)methyl)-5-((2,6-diphenyl-4Hpyran-4-ylidene)methyl)thiophene-2-carbaldehyde (7)
A solution of 2,6-diphenyl-(4H-pyran-4ylidene)-diphenylphosphine oxide 5 (680 mg, 1.56 mmol) in anhydrous THF (12 mL) was
prepared, purged with argon and cooled to -78 o C. To this solution,
n-BuLi (1.6 M in hexanes) (1.2 mL, 2.08 mmol) was added dropwise
and the resulting mixture was stirred for 15 min. Then 3-((tertbutyldimethylsilyloxy)methyl)thiophene-2-carbaldehyde 4 (472 mg;
1.84 mmol) in anhydrous THF (5 mL) was added dropwise and the
mixture was warmed to 0 o C for 3 h (TLC monitoring using 10% EtOAc
in hexanes). Saturated NH4Cl solution was added to quench the reaction and the solvent was evaporated under reduced pressure. The
organic layer was extracted with EtOAc (2 x 25 mL) and dried over
anhydrous MgSO4. After the removal of the solvent, the residue was
dissolved in EtOAc/hexanes (3/97) and filtered over silica gel to give
the crude tert-butyl((2-((2,6-diphenyl-4H-pyran-4-ylidene)methyl)
thiophen-3-yl)methoxy)dimethylsilane 6 as an intermediate. Then, a
solution of 2,2,6,6-tetramethylpiperidine (0.27 mg, 1.58 mmol) in
THF (8.4 mL) was prepared, purged with argon and cooled to -78 o C.
To this solution, tBuLi (1.7 M in pentane) (1.01 mL, 1.71 mmol) was
added dropwise and the resulting mixture was stirred for 1 h and
then a solution of 6 (676.0 mg, 1.41 mmol) in THF (33.6 mL) was
added. The resulting mixture was stirred for an additional hour, DMF
(0.28 mL, 3.70 mmol) was added dropwise and the mixture was
warmed to -30 o C. The reaction was quenched by the addition of
saturated NH4Cl solution and the solvent was evaporated under
reduced pressure. The organic layer was extracted with EtOAc
(2 x 20 mL) and dried over anhydrous MgSO4. After the removal of
the solvent, the aldehyde 7 was purified by silica gel column chromatography (6% EtOAc in hexanes). Yield: red oil (583.0 mg,
1.16 mmol; 82%).
IR (neat): cm-1 1648 (C]O), 1573 (C]C). 1H NMR (400 MHz,
CDCl3): d (ppm) 9.81 (s, 1H), 7.90e7.76 (m, 4H), 7.69 (s, 1H),
7.55e7.43 (m, 6H), 7.29 (d, J ¼ 2.0 Hz, 1H), 6.55 (d, J ¼ 2.0 Hz, 1H),
6.07 (s, 1H), 4.75 (s, 2H), 0.96 (s, 9H), 0.14 (s, 6H). 13C NMR
(100 MHz, CDCl3): d (ppm) 182.1, 155.3, 152.6, 146.6, 139.0, 137.9,
137.0, 132.8, 132.5, 132.5, 130.2, 129.7, 128.9, 128.8, 125.3, 124.7,
109.0, 104.7, 102.9, 59.7, 26.0, 18.4, -5.2. HRMS (ESIþ): m/z calcd for
[C30H33O3SSi]þ: 501.1914, found: 501.1914 [M þ H]þ; calcd for
[C30H32NaO3SSi]þ: 523.1734, found: 523.1721 [M þ Na]þ.

Infrared measurements were carried out in KBr or neat using a
PerkineElmer Fourier Transform Infrared 1600 spectrometer.
Melting points were obtained on a Gallenkamp apparatus in open
capillaries and are uncorrected. 1H and 13C NMR spectra were
recorded on a Bruker ARX300 or a Bruker AV400 at 300 or 400 MHz
and 75 or 100 MHz respectively; d values are given in ppm (relative
to TMS) and J values in Hz. The apparent resonance multiplicity is
described as s (singlet), d (doublet), and m (multiplet). 1He1H COSY
and 1He13C-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. Electrospray mass spectra
were recorded on a Bruker Microtof-Q spectrometer; accurate mass
measurements were achieved using sodium formate as external

2.2.2. 4-((tert-Butyldimethylsilyloxy)methyl)-5-((4,5-dimethyl-1,3dithiol-2ylidene)methyl)thiophene-2-carbaldehyde (10)
A solution of tributyl(4,5-dimethyl-1,3-dithiol-2-yl)phosphonium
hexafluorophosphate 8 (317.7 mg, 0.66 mmol) and 3-((tert-butyldimethylsilyloxy)methyl)thiophene-2-carbaldehyde 4 (131.0 mg,
0.51 mmol) in anhydrous THF (16 mL) was prepared, purged with
argon and cooled to -78 o C. To this solution, Et3N (352.4 mL,
2.51 mmol) was added dropwise and the resulting mixture was stirred for 15 min. After the removal of the solvent, the residue was
dissolved in EtOAc/hexanes (1:9) and filtered over neutral aluminum
oxide to give compound 9 as an intermediate. Then, a solution of 9 in
THF (10 mL) was prepared, purged with argon and cooled to -45 o C.
To this solution, n-BuLi (1.6 M in hexanes) (0.53 mL, 0.85 mmol) was

Fig. 1. Molecular structures of TBDMS organic sensitizers.

Fig. 2. Molecular structures of 4H-pyranylidene organic sensitizers.

(R1R2R3SiO-) were never used to preclude the p-aggregation on
the TiO2 in DSSCs. This popular protecting group can be easily
introduced from alcohols [35] and the overall size and the stability
depend on the nature of the R1, R2 or R3. Moreover, the silyl ether
group greatly enhances the solubility of the sensitizer that facilitates its adsorption on the TiO2 surface.
In the present work, a series of five new metal-free organic
sensitizers for DSSCs with a trialkylsilyloxy group have been
designed, synthesized and characterized. In order to evaluate the
donor influence on the photovoltaic properties a TPA and two
proaromatic donor unities (4H-pyranylidene and dithiafulvene)
have been used (Fig. 1).
The influence of the size and the relative position on the heterocyclic linker of two bulky groups, tert-butyldimethyl (TBDMS) or
tert-butyldiphenyl silyl ether (TBDPS) is also studied (Fig. 2). Finally,
the photophysical properties, molecular orbital calculations and the
performance of DSSCs based on these organic dyes are reported.
2. Experimental section

added dropwise and the resulting mixture was stirred for 1 h. DMF
(0.09 mL, 1.2 mmol) was added dropwise and then, the mixture was
warmed to room temperature over 24 h. Saturated NH4Cl solution
was added to quench the reaction and the solvent was evaporated
under reduced pressure. The organic layer was extracted with CH2Cl2
and dried over anhydrous MgSO4. After the removal of the solvent, the
aldehyde 10 was purified by silica gel column chromatography (20%
ethyl acetate in hexanes). Yield: red oil (136.2 mg, 0.34 mmol; 67%).
IR (neat): cm-1 1649 (C]O), 1492 (C]C). 1H NMR (400 MHz,
CDCl3): d (ppm) 9.80 (s, 1H), 7.65 (s, 1H), 6.82 (s, 1H), 4.69 (s, 2H),
2.09 (s, 3H), 2.03 (s, 3H), 0.92 (s, 9H), 0.09 (s, 6H). 13C NMR
(100 MHz, CDCl3): d (ppm) 181.9, 146.6, 142.5, 137.9, 137.3, 135.8,
124.4, 123.4, 102.9, 59.6, 25.9, 18.3, 13.8, 13.1, -5.2. HRMS (ESIþ): m/z
calcd for [C18H27O2S3Si]þ: 399.0937, found: 399.0961 [M þ H]þ;
þ
þ
calcd for [C18H26NaO2S3Si] : 421.0756, found: 421.0784 [M þ Na] .
2.2.3. 4-(3-((tert-Butyldimethylsilyloxy)methyl)thiophen-2-yl)N,N-diphenylaniline (13)
A solution of the stannyl derivative 12 (990.0 mg, 1.87 mmol)
and compound 11 (668.2 mg, 1.87 mmol) in anhydrous toluene
(24 mL) was prepared, purged with argon for 15 min before addition of Pd(PPh3)4 (112.3 mg, 0.10 mmol). The reaction mixture was
refluxed for 15 h under an argon atmosphere. After addition of
water (30 mL), the mixture was extracted with toluene (2 x 30 mL).
The organic phase was washed with a saturated aqueous solution of
NH4Cl (30 mL) and water (3 x 10 mL) and dried over anhydrous
MgSO4. After the removal of the solvent, the compound was purified by silica gel column chromatography (2% diethyl ether in
hexanes). Yield: light yellow oil (392.4 mg, 0.83 mmol; 44%).
IR (neat): cm-1 1592 (C]C). 1H NMR (300 MHz, CD2Cl2): d (ppm)
7.39e7.33 (m, 2H), 7.32e7.24 (m, 4H), 7.21 (d, J ¼ 5.2 Hz, 1H),
7.15e7.02 (m, 9H), 4.68 (s, 2H), 0.89 (s, 9H), 0.05 (s, 6H). 13C NMR
(100 MHz, CD2Cl2): d (ppm) 148.1, 148.0, 140.6, 137.6, 130.4, 130.3,
129.9, 128.4, 125.2, 123.7, 59.9, 26.3, 18.8, -5.02. HRMS (ESIþ): m/z
calcd for [C29H34NOSSi]þ: 472.2125, found: 472.2109 [M þ H]þ;
calcd for [C29H33NNaOSSi]þ: 494.1944, found: 494.1930 [M þ Na]þ.
2.2.4. 4-((tert-Butyldimethylsilyloxy)methyl)-5-(4(diphenylamino)phenyl)thiophene-2-carbaldehyde (14)
A solution of 13 (352.4 mg, 0.75 mmol) in anhydrous THF
(17 mL) was prepared, purged with argon and cooled to -30 o C. To
this solution, n-BuLi (1.6 M in hexanes) (0.78 mL, 1.25 mmol) was
added dropwise and the resulting mixture was stirred for 1 h. Then,
DMF (173 mL, 2 mmol) was added dropwise and the mixture was
warmed to room temperature (TLC monitoring using 2% diethyl
ether in hexanes). Saturated NH4Cl solution was added to quench
the reaction and the solvent was evaporated under reduced pressure. The organic layer was extracted with diethyl ether (15 x 3 mL)
and dried over anhydrous MgSO4. After removal of the solvent, the
aldehyde 14 was purified by column chromatography on silica gel
(2% ethylic ether in hexanes). Yield: yellow oil (298.6 mg,
0.50 mmol; 80%).
IR (neat): cm-1 1670 (C]O), 1445 (C]C). 1H NMR (400 MHz,
CD2Cl2): d (ppm) 9.84 (s, 1H), 7.80 (s, 1H), 7.41e7.38 (m, 2H),
7.33e7.29 (m, 4H), 7.16e7.05 (m, 8H), 4.72 (s, 2H), 0.91 (s, 9H), 0.08
(s, 6H). 13C NMR (100 MHz, CD2Cl2): d (ppm) 183.2, 150.3, 149.4,
147.7, 141.0, 139.5, 139.2, 130.4, 130.0, 126.5, 125.7, 124.4, 122.7, 60.0,
26.3, 18.8, -4.5. HRMS (ESIþ): m/z calcd for [C30H34NO2SSi]þ:
500.2074, found: 500.2045 [M þ H]þ.
2.2.5. 3-(4-((tert-Butyldimethylsilyloxy)methyl)-5-((2,6-diphenyl4H-pyran-4-ylidene)methyl) thiophen-2-yl)-2-cyanoacrylic acid (1)
To a solution of aldehyde 7 (122 mg, 0.24 mmol) and 2cyanoacetic acid (33 mg, 0.38 mmol) in chloroform (5 mL) was
added piperidine (160 mL; 1.62 mmol). The mixture was refluxed for

24 h under an argon atmosphere and then cooled down to room
temperature. HCl (1 N) was added until pH ¼ 3, the organic phase
was dried over MgSO4 and concentrated under reduced pressure.
Then, hexane was added and a solid appeared. This compound was
filtered under reduced pressure and washed with a mixture hexane/CH2Cl2 (9/1) to afford 1 Yield: dark purple solid (134 mg,
0.24 mmol; 96%).
Mp 215e218 o C. IR (KBr): cm-1 3100e2600 (COOeH), 2210
(C^N), 1672 (C]O), 1652 (C]C). 1H NMR (400 MHz, dmso-d6):
d (ppm) 8.33 (s, 1H), 8.02e7.89 (m, 4H), 7.86 (s, 1H), 7.63e7.45 (m,
6H), 7.37 (d, J ¼ 1.5 Hz, 1H), 7.11 (d, J ¼ 1.5 Hz, 1H), 6.37 (s, 1H), 4.76
(s, 2H), 0.90 (s, 9H), 0.11 (s, 6H). 13C NMR (75 MHz, dmso-d6):
d (ppm) 164.1, 154.8, 151.9, 145.7, 144.9, 139.3, 132.2, 131.6, 131.5,
130.5, 130.0, 129.9, 129.1, 128.9, 124.8, 124.5, 117.7, 109.1, 105.3, 102.4,
58.7, 25.7, 17.9, -5.4. HRMS (ESIþ): m/z calcd for [C33H34NO4SSi]þ:
568.1972, found: 568.1945 [M þ H]þ.
2.2.6. 3-(4-((tert-Butyldimethylsilyloxy)methyl)-5-((4,5-dimethyl1,3-dithiol-2-ylidene)methyl)thiophen-2-yl)-2-cyanoacrylic acid (2)
To a solution of aldehyde 10 (136.4 mg, 0.34 mmol) and 2cyanoacetic acid (45.4 mg, 0.53 mmol) in chloroform (32 mL) was
added piperidine (226.4 mL, 2.25 mmol). The mixture was refluxed
for 85 h under an argon atmosphere and then cooled down to room
temperature. The reaction crude was chromatographied on reverse
C18 silica gel (50% AcONH4 (20 mM) in acetonitrile). After the
addition of acetic acid a solid appeared. This compound was filtered
under reduced pressure to afford 2. Yield: dark purple solid
(68.7 mg, 0.15 mmol; 43%).
Mp 170e174 o C. IR (KBr): cm-1 2211 (C^N), 1673 (C]O), 1531
(C]C). 1H NMR (300 MHz, CDCl3): d (ppm) 8.21 (s, 1H), 7.68 (s, 1H),
6.84 (s, 1H), 4.69 (s, 2H), 2.14 (s, 3H), 2.07 (s, 3H), 0.92 (s, 9H), 0.09
(s, 6H). 13C NMR (75 MHz, CDCl3): d (ppm) 169.1, 149.1, 146.5, 145.5,
139.8, 136.6, 130.9, 125.6, 124.5, 116.9, 103.1, 91.5, 59.5, 25.9, 18.3,
14.0, 13.1, -5.3. HRMS (ESIþ): m/z calcd for [C21H28NO3S3Si]þ:
466.0995, found: 466.1040 [M þ H]þ. HRMS (ESIþ): m/z calcd for
[C21H27NNaO3S3Si]þ: 488.0815, found: 488.0793 [M þ Na]þ.
2.2.7. 3-(4-((tert-Butyldimethylsilyloxy)methyl)-5-(4(diphenylamino)phenyl)thiophen-2-yl)-2-cyanoacrylic acid (3)
To a solution of aldehyde 14 (139.2 mg; 0.29 mmol) and 2cyanoacetic acid (45.4 mg; 0.53 mmol) in chloroform (26 mL) was
added piperidine (200.6 mL; 1.90 mmol). The mixture was refluxed
for 5 days under an argon atmosphere and then cooled down to
room temperature. The reaction crude was chromatographied on
reverse C18 silica gel (50% AcONH4 (20 mM) in acetonitrile). After
the addition of acetic acid a solid appeared. This compound was
filtered under reduced pressure to afford 3. Yield: pinkish solid
(113.1 mg, 0.20 mmol; 70%).
Mp 196e200 o C. IR (KBr): cm-1 2223 (C^N), 1679 (C]O), 1566
(C]C). 1H NMR (400 MHz, acetone-d6): d (ppm) 8.42 (s, 1H), 8.05 (s,
1H), 7.58e7.53 (m, 2H), 7.39e7.33 (m, 4H), 7.18e7.11 (m, 6H)
7.11e7.06 (m, 2H), 4.83 (s, 2H), 0.91 (s, 9H), 0.11 (s, 6H). 13C NMR
(100 MHz, acetone-d6): d (ppm) 167.8, 151.5, 150.9, 148.9, 148.3,
143.4, 140.7, 134.9, 131.8, 131.5, 127.4, 127.2, 126.0, 123.6, 117.8, 100.1,
60.9, 27.3, 19.8, -4.1. HRMS (ESI-): m/z calcd for [C33H33N2O3SSi]-:
565.1987, found: 565.2014 [M - H]-.
2.2.8. 3-((tert-Butyldiphenylsilyloxy)methyl)thiophene-2carbaldehyde (18)
A solution of tert-butyldiphenyl(thiophen-3-ylmethoxy)silane
17 (2.2 g, 6.2 mmol) in anhydrous diethyl ether (10 mL) was prepared, purged with argon and cooled to -10 o C. To this solution, tBuLi (1.7 M in pentane) (4 mL, 6.2 mmol) was added dropwise for a
period of 20 min and the resulting mixture was stirred at -10 o C for
1 h. Then, DMF (0.75 mL, 9.6 mmol) was added dropwise and the

mixture was warmed to 0 o C and stirred at this temperature
overnight (TLC monitoring using 20% hexanes in dichloromethane).
Saturated NH4Cl solution was added to quench the reaction and the
solvent was evaporated under reduced pressure. The organic layer
was extracted with diethyl ether (10 x 3 mL) and dried over
anhydrous MgSO4. After removal of the solvent, the aldehyde 18
was purified by silica gel column chromatography (20% hexanes in
dichloromethane). Yield: yellow oil (454.9 mg, 1.20 mmol; 20%).
IR (neat): cm-1 1664 (C]O), 1588 (C]C). 1H NMR (400 MHz,
CD2Cl2): d (ppm) 9.95 (d, J ¼ 0.9 Hz, 1H), 7.68e7.66 (m, 5H),
7.48e7.37 (m, 6H), 7.25 (d, J ¼ 5.0 Hz, 1H), 5.06 (s, 2H), 1.08 (s, 9H).
13
C NMR (100 MHz, CD2Cl2): d (ppm) 182.9, 150.9, 138.1, 136.1, 134.5,
133.5, 130.5, 130.0, 128.5, 61.4, 27.2, 19.7. HRMS (ESIþ): m/z calcd for
[C22H25O2SSi]þ: 381.1339, found: 381.1313 [M þ H]þ; calcd for
þ
þ
[C22H24NaO2SSi] : 403.1158, found: 403.1139 [M þ Na] .
2.2.9. 4-((tert-Butyldiphenylsilyloxy)methyl)thiophene-2carbaldehyde (19)
A solution of tert-butyldiphenyl(thiophen-3-ylmethoxy)silane
17 (1.6 g, 4.5 mmol) in anhydrous THF (39 mL) was prepared,
purged with argon and cooled to -10 o C. To this solution, n-BuLi
(1.6 M in hexanes) (2.8 mL, 4.5 mmol) was added dropwise and the
resulting mixture was stirred at -10 o C for 1 h and 30 min. Then,
DMF (0.54 mL, 6.9 mmol) was added dropwise and the mixture was
warmed to 0 o C and stirred at this temperature overnight (TLC
monitoring using 20% ethyl acetate in hexanes). Saturated NH4Cl
solution was added to quench the reaction and the solvent was
evaporated under reduced pressure. The organic layer was extracted with diethyl ether (15 x 2 mL) and dried over anhydrous
MgSO4. After removal of the solvent, the aldehyde 19 was purified
by silica gel column chromatography (20% ethyl acetate in hexanes). Yield: white solid (465.3 mg, 1.22 mmol; 21%).
Mp 76e78 o C. IR (KBr): cm-1 1666 (C]O), 1466 (C]C). 1H NMR
(400 MHz, CD2Cl2): d (ppm) 9.86 (d, J ¼ 1.3 Hz, 1H), 7.69e7.65 (m,
5H), 7.59e7.58 (m, 1H), 7.47e7.37 (m, 6H), 4.77 (d, J ¼ 0.6 Hz, 2H),
1.08 (s, 9H). 13C NMR (100 MHz, CD2Cl2): d (ppm) 183.5, 144.7, 144.5,
136.1, 135.8, 133.7, 130.8, 130.4, 128.4, 62.2, 27.1, 19.7. HRMS (ESIþ):
m/z calcd for [C22H25O2SSi]þ: 381.1339, found: 381.1320 [M þ H]þ;
þ
þ
calcd for [C22H24NaO2SSi] : 403.1158, found: 403.1144 [M þ Na] .
2.2.10. 4-((tert-Butyldiphenylsilyloxy)methyl)-5-((2,6-diphenyl4H-pyran-4-ylidene)methyl)thiophene-2-carbaldehyde (21)
A solution of 5 (22.6 mg, 0.51 mmol) in anhydrous THF (5 mL)
was prepared, purged with argon and cooled to -78 o C. To this
solution, n-BuLi (1.6 M in hexanes) (0.32 mL, 0.51 mmol) was added
dropwise and the resulting mixture was stirred for 20 min. Then 3((tert-butyldiphenylsilyloxy)methyl)thiophene-2-carbaldehyde 18
(151.1 mg; 0.40 mmol) in anhydrous THF (5 mL) was added dropwise and the mixture was warmed to room temperature for 21 h
(TLC monitoring using 10% dichloromethane in hexanes). Saturated
NH4Cl solution was added to quench the reaction and the solvent
was evaporated under reduced pressure. The organic layer was
extracted with hexanes (25 x 2 mL) and dried over anhydrous
MgSO4. After the removal of the solvent, the residue was dissolved
in hexanes/CH2Cl2 (9/1) and filtered over silica gel to give the
compound 20 as an intermediate. A solution of 20 in THF (10 mL)
was prepared, purged with argon and cooled to -78 o C. To this
solution, n-BuLi (1.6 M in hexanes) (0.36 mL, 0.58 mmol) was added
dropwise and the resulting mixture was stirred for 1 h and 30 min.
DMF (0.08 mL, 1.03 mmol) was added dropwise and the mixture
stirred for 2 h. Saturated NH4Cl solution was added to quench the
reaction and the solvent was evaporated under reduced pressure.
The organic layer was extracted with hexanes (15 x 2 mL) and dried
over anhydrous MgSO4. After the removal of the solvent, the

aldehyde 21 was purified by silica gel column chromatography (20%
ethyl acetate in hexanes). Yield: red oil (136.4 mg, 0.22 mmol; 55%).
IR (neat): cm-1 1643 (C]O), 1518 (C]C). 1H NMR (400 MHz,
CD2Cl2): d (ppm) 9.77 (s, 1H), 7.92e7.89 (m, 2H), 7.80e7.78 (m, 2H),
7.75e7.67 (m, 4H), 7.64 (s, 1H), 7.55e7.38 (m, 12H), 7.27 (d, J ¼ 2.0
Hz, 1H), 6.47 (d, J ¼ 2.0 Hz, 1H), 6.00 (s, 1H), 4.81 (s, 2H), 1.09 (s, 9H).
13
C NMR (100 MHz, CD2Cl2): d (ppm) 182.4, 155.8, 153.0, 147.1, 139.1,
138.5, 137.8, 136.2, 133.8, 133.2, 133.1, 133.0, 130.7, 130.4, 130.2,
129.4, 129.3, 128.4, 125.9, 125.2, 109.5, 105.4, 103.3, 60.9, 27.2, 19.7.
HRMS (ESIþ): m/z calcd for [C40H37O3SSi]þ: 625.2227, found:
625.2235 [M þ H]þ.
2.2.11. 3-((tert-Butyldiphenylsilyloxy)methyl)-5-((2,6-diphenyl-4Hpyran-4-ylidene)methyl)thiophene-2-carbaldehyde (23)
A solution of 2,6-diphenyl(4H-pyran-4ylidene)diphenylphosphine oxide 5 (222.6 mg, 0.51 mmol) in anhydrous THF (5 mL) was
prepared, purged with argon and cooled to -78 o C. To the solution,
n-BuLi (1.6 M in hexanes) (0.32 mL, 0.51 mmol) was added dropwise and the resulting mixture was stirred for 20 min. Then 4((tert-butyldiphenylsilyloxy)methyl)thiophene-2-carbaldehyde
19
(150.0 mg; 0.39 mmol) in anhydrous THF (5 mL) was added dropwise and the mixture was warmed to room temperature for 21 h
(TLC monitoring using 20% ethyl acetate in hexanes). Saturated
NH4Cl solution was added to quench the reaction and the solvent
was evaporated under reduced pressure. The organic layer was
extracted with EtOAc and dried over anhydrous MgSO4. After the
removal of the solvent, the residue was dissolved in EtOAc/hexanes
(2:8) and filtered over silica gel to give the crude tert-butyl((5-((2,6diphenyl-4H-pyran-4-ylidene)methyl)thiophen-3-yl)methoxy)
diphenylsilane 22 as an intermediate. A solution of 22 in THF
(10 mL) was prepared, purged with argon and cooled to -78 o C. To
this solution, n-BuLi (1.6 M in hexanes) (0.42 mL, 0.68 mmol) was
added dropwise and the resulting mixture was stirred for 2 h. DMF
(0.09 mL, 1.2 mmol) was added dropwise and the mixture stirred
for 2 h. Saturated NH4Cl solution was added to quench the reaction
and the solvent was evaporated under reduced pressure. The
organic layer was extracted with EtOAc (15 x 3 mL) and dried over
anhydrous MgSO4. After the removal of the solvent, the aldehyde
23 was purified by silica gel column chromatography (20% ethyl
acetate in hexanes). Yield: dark red oil (138.9 mg, 0.22 mmol; 56%).
IR (neat): cm-1 1643 (C]O), 1579 (C]C). 1H NMR (400 MHz,
CD2Cl2): d (ppm) 9.86 (s, 1H), 7.90e7.88 (m, 2H), 7.82e7.80 (m, 2H),
7.73e7.70 (m, 4H), 7.50e7.41 (m, 12H), 7.22 (d, J ¼ 1.9 Hz, 1H), 7.05
(s, 1H), 6.55 (d, J ¼ 1.9 Hz, 1H), 6.14 (s, 1H), 5.02 (s, 2H), 1.11 (s, 9H).
13
C NMR (100 MHz, CD2Cl2): d (ppm) 181.4, 155.6, 153.1, 151.5, 151.3,
136.1, 133.7, 133.5, 133.3, 133.2, 133.1, 130.7, 130.5, 130.2, 129.4,
129.3, 128.4, 127.8, 125.9, 125.3, 109.1, 107.6, 103.2, 61.2, 27.2, 19.7.
HRMS (ESIþ): m/z calcd for [C40H37O3SSi]þ: 625.2227, found:
625.2244 [M þ H]þ.
2.2.12. 3-(4-((tert-Butyldiphenylsilyloxy)methyl)-5-((2,6-diphenyl4H-pyran-4-ylidene)methyl)thiophen-2-yl)-2-cyanoacrylic acid
(15)
To a solution of aldehyde 21 (97.9 mg, 0.157 mmol) and 2cyanoacetic acid (20.8 mg, 0.24 mmol) in chloroform (16 mL) was
added piperidine (109.9 mL, 1.04 mmol). The mixture was refluxed
for 72 h under an argon atmosphere and then cooled down to room
temperature. The reaction crude was chromatographied on reverse
C18 silica gel (started 50% AcONH4 (20 Mm) in acetonitrile and
finished 30%). After the addition of acetic acid a solid appeared. This
compound was filtered under reduced pressure to afford 15. Yield:
dark purple solid (83.4 mg, 0.12 mmol; 77%).
Mp 208e211 o C. IR (KBr): cm-1 2207 (C^N), 1652 (C]O), 1539
(C]C). 1H NMR (300 MHz, THF-d8): d (ppm) 8.29 (s, 1H), 8.01e7.98
(m, 2H), 7.88e7.85 (m, 2H), 7.79 (s, 1H), 7.76e7.70 (m, 4H),

7.59e7.32 (m, 13H), 6.76 (d, J ¼ 1.6 Hz, 1H), 6.12 (s, 1H), 4.87 (s, 2H),
1.09 (s, 9H). 13C NMR (100 MHz, THF-d8): d (ppm) 164.9, 156.6,
153.8, 147.8, 146.1, 139.8, 139.8, 136.6, 134.3, 134.0, 133.7, 133.5, 131.9,
130.9, 130.6, 130.0, 129.7, 128.8, 126.2, 125.7, 117.9, 110.2, 106.2,
104.0, 96.6, 61.3, 27.4, 20.1. HRMS (ESIþ): m/z calcd
for
[C43H38NO4SSi]þ: 692.2285, found: 692.2248 [M þ H]þ; calcd for
[C43H37NNaO4SSi]þ: 714.2105, found: 714.2066 [M þ Na]þ.
2.2.13. 3-(3-((tert-Butyldiphenylsilyloxy)methyl)-5-((2,6-diphenyl4H-pyran-4-ylidene)methyl)thiophen-2-yl)-2-cyanoacrylic acid
(16)
To a solution of aldehyde 23 (120.4 mg, 0.192 mmol) and 2cyanoacetic acid (25.6 mg, 0.30 mmol) in chloroform (18 mL) was
added piperidine (134.6 mL, 1.28 mmol). The mixture was refluxed
for 20 h under an argon atmosphere, then cooled down to room
temperature. The reaction crude was chromatographed on reverse
C18 silica gel (started 50% AcONH4 (20 mM) in acetonitrile and
finished 33%). After the addition of acetic acid a solid appeared. This
compound was filtered under reduced pressure to afford 16. Yield:
dark purple solid (86.0 mg, 0.12 mmol; 65%).
Mp 219e226 o C. IR (KBr): cm-1 2209 (C^N), 1651 (C]O), 1541
(C]C).1H NMR (400 MHz, THF-d8): d (ppm) 8.51 (s, 1H), 7.99e7.88
(m, 4H), 7.74e7.72 (m, 4H), 7.49e7.34 (m, 14H), 6.99 (s, 1H), 6.81 (br
s, 1H), 4.92 (s, 2H), 1.08 (s, 9H). 13C NMR (100 MHz, THF-d8): d (ppm)
164.9, 156.4, 154.0, 152.5, 151.7, 143.7, 136.6, 134.3, 134.0, 133.7,
133.5, 131.9, 131.2, 130.9, 130.7, 130.0, 129.8, 128.8, 128.1, 126.2,
125.8, 118.1, 109.8, 108.4, 103.9, 95.8, 61.3, 27.4, 20.0. HRMS (ESIþ):
m/z calcd for [C43H38NO4SSi]þ: 692.2285, found: 692.2313.
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 U sq-1 sheet
resistance) conducting glass substrates. Efficient DSC devices were
made using 9 mm thick films consisting of 20 nm TiO2 nanoparticles
(Dyesol© paste) and a scattering layer of 4 mm of 400 nm TiO2
particles (Dyesol© paste). Prior to the deposition of the TiO2 paste,
the conducting glass substrates were immersed in a solution of
TiCl4 (40 mM) 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 an air flow at 325 o C for 5 min, 375 o C for 5 min,
450 o C for 15 min and 500 o C for 15 min. The heated TiO2 electrodes
were immersed again in a solution of TiCl4 (40 mM) at 70 o C for
30 min and then washed with ethanol. The electrodes were heated
again at 500 o 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 introduction of the liquid electrolyte using vacuum, followed by
heating at 390 o C for 15 min. All films were sensitized in 0.3 mM
dye solutions in dichloromethane for 2 h at room temperature
(optimized dye loading conditions). The sensitized electrodes were
washed with dichloromethane and dried under air. Finally, the
working and counter electrodes were sandwiched together using a
thin thermoplastic (Surlyn) frame that melts at 100 o C. Electrolyte
LP1 was used: consisted of 0.5 M 1-butyl-3-methylimidazolium
iodide (BMII), 0.1 M lithium iodide, 0.05 M iodine and 0.5 M tertbutylpyridine in acetonitrile.
The J/V curves of the cells were measured using a Sun 2000 solar
simulator equipped with a 150 W xenon lamp. The illumination
intensity was measured to be 100 mW cm-2 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 using a Keithley 2400 digital source meter.
The IPCE (incident photon to current conversion efficiency) was
measured using a home-made set up consisting of a 150 W Oriel
xenon lamp, a motorized monochromator and a Keithley 2400
digital source meter.
Transient photovoltage (TPV) and charge extraction (CE) measurements were carried out alike reported before [36]. In CE measurements, white light from a series of LEDs was used as the light
source. When the LEDs are turned off, the cell is immediately shortcircuited and the charge is extracted allowing the electron density
in the cells to be calculated. By changing the intensity of the LEDs,
the electron density can be estimated as a function of cell voltage. In
TPV measurements, in addition to the white light applied by the
LEDS, a diode pulse (660 nm, 10 mW) is applied to the sample
inducing a change of 2e3 mV within the cell. The resulting photovoltage decay transients are collected and the t values are
determined by fitting the data to the equation exp(-t/t).
2.4. Computational details
Density Functional Theory (DFT) calculations were performed
using Gaussian 09 [37] with the ultrafine integration grid. Solvent
effects were estimated using a Conductor-like Polarizable Continuum Model (CPCM) [38,39]. Equilibrium geometries were optimized using the M06-2x hybrid meta-GGA exchange correlation
functional [40] and the medium size 6e31G* base [41]. Ground
state geometries were characterized as minima by frequency calculations. Excitation energies were calculated by time-dependent
single point calculations using the M06-2x/6e311 þ G (2d,p)
model chemistry. Absorption spectra were estimated through the
calculation of vertical excitations at the ground state geometry and
emission spectra, through the calculation of vertical excitations at
the excited state geometry. Due to the large computational cost
required to calculate excited state vibrational energies, E0-0 were
approached to adiabatic excitation energies (EAdia). Ground state
oxidation potentials (Eox) were calculated using the M06-2x/
6e311 þ G (2d,p) energies and calculating the thermal corrections
to Gibbs free energy at the M06-2x/6e31G* level. Excited state
oxidation potentials were estimated subtracting E0-0 from EOX.
Molecular Orbital contour plots were obtained using the Avogadro software [42] at 0.04 isosurface value.
3. Results and discussion
3.1. Synthesis
The target compounds 1, 2, and 3 were prepared via condensation of the corresponding aldehyde derivative (7, 10, 14) and
cyanoacetic acid in the presence of piperidine (Scheme 1).
Aldehydes 7 and 10 were synthesized by a WittigeHorner or
Wittig reaction of the diphenylphosphine oxide 5 [43] or the
phosphonium salt 8 [44] with aldehyde 4 [45], followed by lithiation of the resulting intermediates 6 and 9 and reaction with
anhydrous DMF (Scheme 2). A different approach was adopted for
the aldehyde 14 (Scheme 3) which was prepared through a Stille
reaction between the stannane 12 [46] and the compound 11 [47],
followed by lithiation and formylation with DMF.
3.2. Optical properties
The absorption spectra of the synthesized dyes were studied in
10-5 M CH2Cl2 solution (Fig. 3) and the relevant optical data are
listed in Table 1. Two absorption bands can be observed for all dyes,
one between 300 and 400 nm assigned to a pep* transition and the
main band between 400 and 700 nm due to the ICT among the

Scheme 1. Synthesis of dyes 1e3.

Scheme 2. Synthesis of aldehydes 7, 10.

Scheme 3. Synthesis of aldehyde 14.

Table 2
Photovoltaic properties of DSSCs constructed using the dyes 1e3.
Dye

Jsc (mA cm-2)

Voc (V)

ff

h (%)

1
2
3

11.74
6.99
7.26

0.644
0.609
0.759

74
69
74

5.56
2.91
4.09

3.4. 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 optimized 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
acetonitrile) as electrolyte. The relevant photovoltaic parameters
Voc, Jsc, ff, and solar-to-electrical energy conversion efficiencies (h)
are collected in Table 2. Moreover, current densityevoltage (JeV)
curves and incident photon-to- current conversion efficiencies
(IPCE) of devices based on these dyes are represented in Fig. 4.
The IPCE tendencies of these dyes are in accordance with their
UVevis absorption spectra on the TiO2 film. The onset of the IPCE
spectra for dyes 1 and 2 are significantly broadened compared with
the obtained for dye 3. These results confirm the higher Jsc value
obtained for devices based on dye 1, with a broad and roughly
constant photoresponse (~60%) in the range 450e600 nm. Dye 3
presents a maximum IPCE value of 93% at 475 nm and the highest
Voc value (0.759 V) of all the molecules studied, suggesting a
decrease in the charge recombination processes for this molecule.
However, its Jsc is 60% lower than obtained for compound 1, probably due to the narrower IPCE observed band.
Data in Table 2 indicate that the more efficient donor system is
the 4H-pyranylidene (h ¼ 5.56%), followed by the triarylamine
system (h ¼ 4.09%) and finally the 1,3-dithiole (h ¼ 2.91%).

Fig. 3. Normalized UVevis absorption of compounds 1e3.

donor and the acceptor units. The inclusion of a proaromatic donor
like 4H-pyranylidene or dithiafulvene results in a red-shift of the
absorption when compared to the triarylamine dye. The molar
extinction coefficients of the dyes increase in the order
triarylamine < 4H-pyranylidene < dithiafulvene, being for the last
-1
-1
one 37,108 M cm . When the dyes are attached to TiO2 surface,
they present broader bands than in solution and the maximum
absorption peaks are around 30 nm blue-shifted. In general, the
blue shifts of the absorption spectra on TiO2 are ascribed to the
deprotonation of the carboxylic acid when anchored on the titanium surface and/or to the formation of H-aggregates [48].
3.3. Electrochemical properties
The oxidation potential (Eox) of each dye was determined in
solution by differential pulse voltammetry (DPV) methods. E* ox was
estimated from Eox - E0-0 (Table 1, see also energy diagram shown
in SI Fig. S-33).
The donor unit affects significantly the oxidation potential
values, with an increase on the ease of oxidation in the order
3 < 2 < 1, pointing to the superior donor ability of the 4H-pyranylidene moiety. The regeneration of the oxidized dye after electron injection into the conduction band of TiO2 is guaranteed in all
cases, as the Eox values are more positive than the potential of the
iodide/triiodide redox couple (þ0.42 vs. NHE), indicating that the
oxidized dyes formed after the electron injection into the TiO2
electrode could thermodynamically accept electrons from I- ions.
The resulting values for all the dyes were almost similar and
sufficiently more negative than the energy conduction band edge
energy level of TiO2 (-0.5 V vs. NHE), indicating an efficient injection of electrons into the TiO2 from the excited dyes.

3.5. Structural modification of the thiophene ring and physical
properties
With the aim of understanding the influence of a different bulky
silyl ether group and its position in the thiophene ring, dyes 15 and
16 were synthesized by a Knoevenagel condensation of the unreported aldehydes 21 and 23 with 2-cyanoacetic acid (Scheme 4).
We have chosen the pyranylidene system as donor group due to the
best results obtained in the previous section. These precursors were
obtained in a three-step synthetic route starting in both cases from
compound 17 [49]. Lithiation conditions (base and solvent) were
finely tuned in order to activate position 2 (tert-BuLi, Et2O) or position 5 (BuLi, THF) (Scheme 5). Finally, compounds 18 and 19 were
obtained by reaction of the organolithium intermediates with DMF.
Next, a Horner reaction of diphenylphosphine oxide 5 with 18 or 19
afforded the corresponding pyranylidene-thiophene derivatives 20

Table 1
Optical and electrochemical properties of dyes 1e3.
a

Dye

labs , nm ( , M

1
2
3

570 (30,370)
568 (37,108)
460 (23,958)

a

-1

-1

cm

b

)

c

d

e

*

f
ox

V (vs. NHE)

labs (nm)

lem (nm)

Eox , V (vs. NHE)

E0-0 (eV)

E

522
518
432

659
630
611

0.87
0.93
1.22

1.97
2.06
2.29

-1.10
-1.12
-1.08

Absorption maxima in CH2Cl2 solution.
Absorption maxima on TiO2 films.
c
Emission spectra were recorded in CH2Cl2.
d
First oxidation potentials were measured from 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
Zerothezeroth 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.
b

Fig. 4. JeV curves of compounds 1e3 (left) and IPCE curves of 1e3 (right).

Scheme 4. Synthesis of dyes 15, 16.

and 22, which by lithiation and reaction with DMF yielded final
aldehydes 21 and 23 respectively (Scheme 6).
The substitution of a TBDMS group (1) by a tert-butyldiphenylsilyl (TBDPS (15)) does not change significantly the maximum
absorption peak (570 nm for 1 and 573 nm for 15). By contrast, the
molar extinction coefficient ( ) is enhanced on passing from TBDMS
to TBDPS (30,370 M-1 cm-1 (1) to 33,618 M-1 cm-1 (15)), probably
due to the incorporation of two additional phenyl groups. Moreover, a bigger enhancement of the is found when the silyloxy
substituent is located at the position 3 of the thiophene ring instead
of the position 4 (from 33,618 (15) to 39,963 M-1 cm-1 (16)).
Regarding the electrochemical properties, it has to be pointed
that the Eox values for systems 1, 15 and 16 are very similar
(þ0.87, þ0.90, and þ0.88 V respectively, measured in conditions
indicated in Table 1) and hence, the influence of the bulky substituent and its position is fairly null (see SI Fig. S-34).
We have tested the photovoltaic response of dyes 15 and 16
(Fig. 5) in DSSCs and their results were compared with those of
compound 1 (Table 3). The substitution of a TBDMS group (1) by the
bulkier group (TBDPS) (15) improves slightly the efficiency (from
5.56% to 5.86%), mostly due to a better value of the Voc for the last

one. This result suggests that a TBDPS group retards the recombination of TiO2 conduction band electrons with the electrolyte as it
has been shown from charge extraction and transient photovoltage
experiments (see below) [50].
The influence of the position of the silyl ether group was also
studied and we have found that the solar cell based on dye 16 (with
a higher molecular extinction coefficient) shows an efficiency of
3.42%, 42% lower that obtained for dye 15. The decrease in the efficiency is mainly due to a lower Jsc and Voc, to a lesser extent. The
remarkable decrease of Jsc was attributed to the lower injection
efficiency, probably affected by the amount of dye absorbed to the
TiO2 surface.
Desorption experiments (0.1 M NaOH, EtOH:H2O (1:1)) determined that dye 16 is less adsorbed (106 nmol/cm2) than dye 15
(140 nmol/cm2) and this observation was attributed to the steric
hindrance of the TBDPS group which is closer to the anchor group.
Moreover, a lower amount of dye absorbed could result in higher
concentration in the interface TiO2/electrolyte, increasing the
recombination processes.
Fig. 6 illustrates the charge density (charge accumulated at the
device under different light bias) and the charge lifetime at a given

Scheme 5. Synthesis of thiophene-aldehydes 18, 19.

Scheme 6. Synthesis of aldehydes 21, 23.

charge. As it can be seen, the solar cells made using dye 15 have the
slowest charge recombination under working conditions. Moreover, it is worthy to mention that a shift on the measured charge vs.
voltage is observed for the solar cells made using dye 16. This shift
can be attributed to the lower coverage of dyes at the surface of the
TiO2, which has as a consequence the lower concentration of protons at the surface of the TiO2. In fact, this is also in good agreement
with the lower photocurrent measured.
3.6. Theoretical calculations
The electronic structures of the new synthesized compounds
have been studied using density functional theory methods and the
most relevant parameters derived from these calculations are
gathered in Table 4. There is a good agreement with the

experimental values described above, with estimated error of less
than 0.25 eV in the excitation energies, less than 0.1 V in Eox and
less than 0.2 V in E*ox.
Calculations describe the first excited state as the consequence
of a one-electron transition from the HOMO to the LUMO (see
Fig. 7). Excitation involves some charge transfer from the donor to
the cyanoacetic acid acceptor, with a large HOMOeLUMO overlap
that gives rise to large oscillator strengths and therefore to great
molar extinction coefficients.
Unfortunately, a direct correlation between the oscillator
strengths (proportional to the peak area) and the molar extinction
coefficients (related to peak height) is not possible for compounds
giving rise to absorption bands of different width. Thus, compound
2 having the largest gives rise to the narrowest absorption band
and also the lowest f.
4H-pyranylidene compounds 1, 15 and 16 display similar HOMO
and LUMO energies and therefore their excitation energies and
oxidation potentials are also similar. Compound 2, with a weak
donor group has a lower HOMO energy compared to the other
studied dyes and, therefore, displays higher excitation energies and
EOX.
4. Conclusions
A series of five new metal free sensitizers (DepeA) for DSSC
with a trialkylsilyl ether group in the spacer have been synthesized
and characterized. Several donor unities have been studied,
Table 3
Photovoltaic properties of DSSCs constructed using the dyes 1, 15 and 16.
Dye

Fig. 5. JeV curves of compounds 1, 15 and 16.

Jsc (mA cm-2)

Voc (V)

ff

h (%)

1
15
16

11.74
11.44
7.71

0.644
0.689
0.624

74
74
71

5.56
5.86
3.42

Fig. 6. Charge extraction (left) and transient photovoltage (right) curves of compounds 1, 15 and 16.

Table 4
Results of DFT calculations.
EHOMO (eV)

a

lAbs (nm)

fa

lem (nm)

E0-0 (eV)

Eox, V (vs. NHE)

E*OX , V (vs. NHE)

-6.38
-6.38
-6.58
-6.39
-6.40

1
2
3
15
16

ELUMO (eV)
-2.29
-2.13
-2.11
-2.30
-2.33

558
528
431
559
564

1.24
1.00
1.23
1.23
1.25

666
605
565
666
663

2.03
2.18
2.50
2.03
2.03

0.95
1.00
1.24
0.93
0.95

-1.08
-1.18
-1.26
-1.10
-1.08

Oscillator strength.

Appendix A. Supplementary material

References
[1]
[2]
[3]
[4]
[5]
Fig. 7. HOMO (left) and LUMO (right) for compound 15.
[6]

obtaining the best performances with 4H-pyranylidene, followed
by triphenylamine (TPA) and dithiafulvene moieties respectively.
Two trialkylsilyl ether groups (TBDMS and TBDPS) were compared
in order to study the influence of their molecular size in the
photovoltaic properties. The best efficiencies values were obtained
for a TBDPS group at the position 4 of the thiophene ring. This
observation indicates that trialkylsilyl ethers can be promising
substituents for future more efficient sensitizers.

[7]
[8]

[9]

[10]

[11]

Acknowledgments
We gratefully acknowledge the financial support from the
Spanish Ministry of Science and Innovation, MICINN-FEDER (project CTQ2011-22727), MINECO (project CTQ2014-52331-R) and the
'
Gobierno de Aragon-Fondo Social Europeo (E39). A predoctoral
'
fellowship to R. Perez-Tejada (CSIC, JAE 2011) is also acknowledged.
The authors thank Prof. Emilio Palomares for the use of the
equipment at ICIQ and the helpful comments on the results. Dr. Laia
Pelleja thanks MICINN for the Severo Ochoa Excellence Accreditation 2014.2018 (SEV-2013-0319).

[12]

[13]

[14]

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