Efficient organic DSSC sensitizers bearing an electron-deficient pyrimidine as an effective π-spacer

Article abstract of DOI:10.1039/c1jm10201j

A series of new push-pull organic dyes incorporating electron-deficient pyrimidine as the π-spacer have been synthesized, characterized, and used as the sensitizers for dye-sensitized solar cells (DSSCs). In comparison with the model compound M-TP, which

Full text of DOI:10.1039/c1jm10201j

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PAPER
Efficient organic DSSC sensitizers bearing an electron-deficient pyrimidine as
an effective p-spacer†
a
b
a
b
b
Li-Yen Lin, Chih-Hung Tsai, Ken-Tsung Wong, Tsung-Wei Huang, Chung-Chih Wu, Shu-Hua Chou,a
*
*
Francis Lin,a Shinn-Horng Chenc and An-I Tsaic
Received 14th January 2011, Accepted 9th February 2011
DOI: 10.1039/c1jm10201j
A series of new push–pull organic dyes incorporating electron-deficient pyrimidine as the p-spacer have
been synthesized, characterized, and used as the sensitizers for dye-sensitized solar cells (DSSCs). In
comparison with the model compound M-TP, which adopts phenylene as the p-spacer, the tailor-made
dyes display enhanced spectral responses in the red portion of the solar spectrum. Through the
introduction of two hexyloxy chains on the diphenylthienylamine donor, the DSSC employing dye
OHexDPTP exhibited high power conversion efficiency up to 7.64% under AM1.5G irradiation.
(cyanoacrylic acid) are typically electron-rich systems, such as
thiophene derivatives,6 furan,7 selenophene,8 and pyrrole.9
Introduction
Dye-sensitized solar cells (DSSCs) have attracted considerable
€
However, such designs only yield limited bathochromic shifts in
1
interest since Gratzel’s pioneering report in 1991. Photosensi-
absorption. Meanwhile, a few organic D–p–A dyes incorpo-
rating electron-withdrawing terephthalonitrile10 or electron-
deficient heteroarenes such as quinoline,11 isoxazole,12 and
thiazole13 moieties as linkages to cyanoacrylic acid, showed more
red-shifted absorption than their parent analogues and thus
enhanced the light harvesting.
tizers certainly play a crucial role for highly efficient DSSCs and
have been extensively investigated. In addition to RuII poly-
pyridyl-based sensitizers,2 metal-free organic dyes have also been
utilized in DSSCs because of the flexibility in tailoring their
molecular structures, low cost, and high molar extinction coef-
ficients.3 Remarkably, organic-based DSSCs with conversion
efficiencies comparable to the Ru-based DSSCs have been
reported recently.4 Due to the effective photoinduced intra-
molecular charge transfer characteristics, most of the efficient
organic sensitizers are modeled on the donor–(p-spacer)–
acceptor (D–p–A) system. For efficient solar energy conversion,
it is desirable to use dyes that possess increased molar extinction
coefficients that extend throughout the visible and into the near-
IR regions. Organic sensitizers with long p-conjugated spacers
had been shown to be operative in augmenting the molar
extinction coefficients as well as in realizing panchromatic light-
harvesting, giving moderate DSSC efficiency and stability.4,5 In
addition to the length of the p-conjugated spacer, the intrinsic
electronic characteristics of the p-conjugated spacer may also
affect the spectral response of the dyes. In most organic D–p–A
dyes, the p-conjugated spacers linked to the acceptor
Here we report a series of organic D–p–A dyes, in which
various diphenylthienylamines donors and cyanoacrylic acid
acceptors are bridged by an electron-deficient pyrimidine ring. A
model compound M-TP with phenylene as the p-spacer was also
synthesized for parallel comparison. In general, the dyes were
designed with the following three structural characteristics:
(i) the pyrimidine-containing p-system which favors planar
geometry necessary for effective intramolecular charge transfer;14
(ii) the acceptor and also the TiO2-anchoring group, cyanoacrylic
acid, is linked to a highly electron-deficient pyrimidine ring;
(iii) the diphenylthienylamine moiety is adopted as a stronger
electron donor compared to the conventional triphenylamine
moieties.15 These three characteristics are all beneficial in
lowering the energy of the intramolecular charge transfer tran-
sition. As a result, these dyes show evident bathochromic shifts in
absorption, and thus effectively improve the light-harvesting
range. To the best of our knowledge, this is the first time that the
pyrimidine ring has been used as the p-conjugated spacer in
organic photosensitizers for DSSCs.
aDepartment of Chemistry, National Taiwan University, Taipei, 10617,
Taiwan. E-mail: kenwong@ntu.edu.tw; Fax: + 886-2-33661667; Tel:
+ 886-2-33661665
bDepartment of Electrical Engineering, Graduate Institute of Photonics and
Optoelectronics, and Graduate Institute of Electronics Engineering,
National Taiwan University, Taipei, 10617, Taiwan. E-mail: chungwu@
cc.ee.ntu.edu.tw; Fax: +886-2-2367 1909; Tel: +886-2-3366 3636
cEnergy Material Research Group, Eternal Chemical Co., Ltd., Kaohsiung,
821, Taiwan
Results and discussion
The synthetic pathways of new dyes DPTP, OMeDPTP,
OHexDPTP, and model compound M-TP are illustrated in
Scheme 1, and the detailed syntheses are described in the
Experimental. Palladium-catalyzed Negishi coupling reactions of
† Electronic supplementary information (ESI) available: 1H and 13C
NMR spectra for all new compounds. See DOI: 10.1039/c1jm10201j
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Scheme 1 Synthesis of dyes DPTP, OMeDPTP, OHexDPTP, and M-TP
1 with 5-bromo-2-iodopyrimidine afforded 2, which were then
converted to their corresponding carbaldehydes 3 by lithiation
with n-butyl lithium and subsequently quenched with ethyl
formate. Finally, the aldehydes were condensed with cyanoacetic
acid to yield the target compounds DPTP, OMeDPTP, and
€
OHexDPTP via the Knoevenagel reaction in the presence of
ammonium acetate. The model compound M-TP was obtained
via two-step processes that first involved the Stille coupling
reaction of 5-(N,N-diphenylamino)-2-(tri-n-butylstannyl)thio-
€
phene with 4-bromobenzaldehyde, followed by the Knoevenagel
condensation with cyanoacetic acid.
The UV-Vis absorption spectra and emission spectra of dyes in
a tert-butanol–acetonitrile (1 : 1) solution are depicted in Fig. 1.
Their photophysical data are also summarized in Table 1. The
absorption spectrum of DPTP in solution shows an absorption
maximum (lmax) centered at 484 nm (3 ¼ 25300 Mꢀ1 cmꢀ1), which
is assigned to the p–p* transition of the D–p–A conjugated
system. The M-TP that incorporates phenylene as the p-spacer
displays a weaker and blue-shifted absorption band at 438 nm
(3 ¼ 19800 Mꢀ1 cmꢀ1) in comparison with DPTP. It appears that
Fig. 1 Absorption (line + symbol) and emission (symbol) spectra of
DPTP, OMeDPTP, OHexDPTP, and M-TP measured in
tert-butanol–acetonitrile (1 : 1) solution (10ꢀ5 M).
a
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Table 1 Photophysical and electrochemical parameters of the dyes and adsorbed dye amount on TiO2 nanoparticle films
3a (Mꢀ1 cmꢀ1
)
Gc (mol cmꢀ2
)
Eoxe (V)
Eox*f(V)
a
labs (nm)
a
lem (nm)
b
labs on TiO2 (nm)
d
E0–0 (V)
Dye
DPTP
484
502
506
438
25300
31300
29300
19800
560
590
594
549
454
477
467
409
1.8 ꢁ 10ꢀ7
1.9 ꢁ 10ꢀ7
6.0 ꢁ 10ꢀ8
2.5 ꢁ 10ꢀ7
2.20
2.11
2.11
2.32
1.09
0.99
0.98
1.01
ꢀ1.11
ꢀ1.12
ꢀ1.13
ꢀ1.31
OMeDPTP
OHexDPTP
M-TP
a
b
Absorption and emission spectra were measured in a tert-butanol–acetonitrile (1 : 1, v/v) solution (10ꢀ5 M). Absorption spectra on TiO2 were
c
obtained through measuring the dye adsorbed on 7 mm TiO2 nanoparticle films in a chlorobenzene solution. G is the surface density of the dye on
d
the TiO2 nanoparticle film. E0–0 was estimated from the onset point of the absorption spectra. The oxidation potentials of the dyes were
e
measured in CH2Cl2 with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte, scan rate: 100 mV sꢀ1; calibrated with
f
ferrocene/ferrocenium (Fc/Fc+) as an internal reference and converted to NHE by addition of 630 mV.16 Eox* ¼ Eox ꢀ E0–0
.
our molecular scaffold effectively shifts the absorption peak to
a longer wavelength. Furthermore, OMeDPTP and OHexDPTP
exhibit a lmax at 502 (3 ¼ 31300 Mꢀ1 cmꢀ1), and 506 nm (3 ¼
29300 Mꢀ1 cmꢀ1), respectively, which are red-shifted by ca. 20 nm
compared to that of the parent dye DPTP, as a result of the
stronger electron-donating nature of the dialkoxy substituents.
DPTP, OMeDPTP, and OHexDPTP exhibit emission bands
centered at 560, 590, and 594 nm, respectively, when excited on
their absorption maxima.
pyrimidine and cyanoacrylic acid fragments (Fig. 4). This result
suggests that the HOMO–LUMO excitation would shift the
electron density distribution from the diphenylamine unit to the
cyanoacrylic acid moiety, facilitating efficient photoinduced/
interfacial electron injection from excited dyes to the TiO2
electrode.
Quasi-reversible oxidation waves (Eox in Table 1) of the dyes
were observed in the cyclic voltammetry (CV), which can be
attributed to the oxidation of the diphenylthienylamine moieties.
The lower oxidation potential of OMeDPTP and OHexDPTP
vs. DPTP can be ascribed to the presence of stronger electron-
donating dialkoxy substituents. The zero–zero excitation energy
(E0–0), which was estimated from the onset point of the
absorption spectra, and the ground-state oxidation potential
(Eox) were used to calculate the excited-state oxidation potential
Eox* (¼ Eox ꢀ E0–0). It is worth mentioning that both ground-
state and excited-state oxidation potentials of DPTP are more
positive than those of M-TP, yet the difference in the excited-
state oxidation potentials of the two dyes is larger than that in
ground-state oxidation potentials. It clearly suggests that the
introduction of the electron-deficient pyrimidine as the p-spacer
has a significant influence on the excited-state potential but
a more limited effect on the ground-state oxidation potential of
DPTP. The different influence on the ground-state and excited-
state oxidation potentials by incorporating pyrimidine leads to
a narrow energy gap of DPTP. The deduced Eox* values of these
dyes are more negative than the conduction band edge of the
Upon anchoring on the 7 mm porous TiO2 nanoparticle films,
the absorption of these dyes blue-shifted in comparison with
those in the tert-butanol–acetonitrile (1 : 1) solution (Fig. 2). The
blue shift may stem from the deprotonation of carboxylic acid
upon adsorption onto the TiO2 surface and the resulting
carboxylate–TiO2 unit is a weaker electron acceptor compared to
the carboxylic acid.6g Similarly to many metal-free dyes,6g,7,13,17
the absorption of the dyes exhibit negative solvatochromism, i.e.
a blue shift of lmax in polar solvents (see Fig. 3, taking DPTP as
an example). This phenomenon may be attributed to the inter-
action of polar solvent molecules with the carboxylic acid, which
decreases the electron-withdrawing strength of the COOH
group.
Density function theory (DFT) calculations show a coplanar
conformation between thiophene and pyrimidine due to the lack
of ortho–ortho steric interactions, and the HOMOs of both
sensitizers are mainly populated on diphenylamine and thio-
phene moieties, whereas LUMOs are sizably delocalized through
Fig. 2 Absorption spectra of the dyes anchoring on the 7 mm porous
TiO2 nanoparticle films.
Fig. 3 Absorption spectra of DPTP in different solvents.
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Fig. 4 Frontier molecular orbitals (HOMO and LUMO) of the dyes calculated with DFT on a B3LYP/6-31G(d) level.
TiO2 (ꢀ0.5 V vs. NHE),18 indicating that the electron injection
process shall be energetically favorable. In addition, the ground-
state oxidation potentials of the dyes are more positive than the
redox potential of the iodide/triiodide couple (0.4 V vs. NHE),
suggesting that the oxidized dyes shall be able to accept electrons
from Iꢀ thermodynamically for effective dye regeneration and
minimize the charge recombination between oxidized dyes and
photoinjected electrons in TiO2.
450 nm, indicating highly efficient DSSC performances.
In agreement with the trend in absorption spectra, the IPCE
spectra of DPTP, OMeDPTP and OHexDPTP all exhibit higher
peak values and extend to longer wavelengths than that of the
model compound M-TP. Moreover, with dialkoxy substituents,
IPCE spectra of OMeDPTP and OHexDPTP extend to longer
wavelengths than that of DPTP. As listed in Table 2, the short-
circuit photocurrent density (Jsc), open-circuit voltage (Voc), and
fill factor (FF) of the OHexDPTP cell are 13.79 mA cmꢀ2, 0.77 V,
and 0.72, respectively, yielding an overall conversion efficiency
(h) of 7.64%. Although both DPTP and OMeDPTP cells possess
Jsc similar to (or even slightly higher than) that of the
OHexDPTP cell, yet with relatively lower Voc, they both exhibit
inferior h values of 6.72% and 7.05%, respectively. For a fair
comparison, the N719-sensitized DSSC was also fabricated and
tested under similar conditions (Table 2). The efficiency of the
OHexDPTP cell reaches ꢂ90% of the N719 cell efficiency.
Interestingly, DPTP and OHexDPTP show nearly the same
Jsc despite the larger molar extinction coefficient and more red-
shifted absorption of OHexDPTP. This is likely to be due to the
smaller density of OHexDPTP adsorbed on the TiO2 surface
The photovoltaic characteristics of these dyes as the sensitizers
for DSSCs were evaluated with a sandwich DSSC cell using
0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 0.05 M LiI,
0.03 M I2, 0.5 M 4-tert-butylpyridine, and 0.1 M guanidinium
thiocyanate in
a
mixture of acetonitrile–valeronitrile
(85 : 15, v/v) as the redox electrolyte (details of the device prep-
aration and characterization are described in the experimental).
The incident monochromatic photon-to-current conversion
efficiency (IPCE) spectra and the current–voltage (J–V) curves
under standard global AM 1.5 solar irradiation of the DSSCs are
plotted in Fig. 5 and Fig. 6, respectively. The DSSCs using
DPTP, OMeDPTP, and OHexDPTP show IPCEs > 70% from
380–400 nm to 570 nm and IPCE maxima of 90–92% around
Fig. 6 Photocurrent density vs. voltage for DSSCs under AM1.5G
simulated solar light (100 mW cmꢀ2).
Fig. 5 IPCE spectra of DSSCs.
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Table 2 DSSC performance parameters.a
Dye
Jsc (mA cmꢀ2
)
Voc (V)
FF
h (%)
DPTP
13.72
14.01
13.79
11.19
16.12
0.69
0.71
0.77
0.66
0.76
0.71
0.71
0.72
0.70
0.70
6.72
7.05
7.64
5.22
8.57
OMeDPTP
OHexDPTP
M-TP
N719
a
The concentration of the organic dyes was maintained at 0.5 mM in
chlorobenzene solution, with 0.5 mM deoxycholic acid (DCA) as
a
co-adsorbent [N719 was fabricated in tert-butanol–acetonitrile
(1 : 1, v/v) solution under the same conditions]. Performances of
DSSCs were measured with a 0.125 cm2 working area. Irradiating light:
AM1.5G (100 mW cmꢀ2).
with greater steric congestion of the dihexyloxy-substituted
diphenylthienylamine moiety at the donor. By desorbing the dyes
adsorbed onto the TiO2 surface with basic solution, the
adsorbing densities were determined to be 1.8 ꢁ 10ꢀ7 and 6.0 ꢁ
10ꢀ8 mol cmꢀ2 for DPTP and OHexDPTP, respectively, sup-
porting such a mechanism. On the other hand, the surface
density of OMeDPTP is similar to that of DPTP due to their
similar molecular sizes, and thus the largest Jsc of OMeDPTP
among the dyes can be ascribed to its simultaneously better light-
harvesting ability and larger surface density on TiO2.
Through the introduction of two long aliphatic hexyloxy
chains on the diphenylthienylamine donor, one also notes that
the OHexDPTP-based cell gives an enhanced Voc in contrast to
the DPTP- and OMeDPTP-based cells. Electrochemical
impedance spectroscopy (EIS) was conducted to elucidate these
phenomena. In this study, the impedance spectroscopy was
carried out by subjecting the cell to the constant AM1.5G
100 mW cmꢀ2 illumination and to the bias at the open-circuit
voltage Voc of the cell (namely, under the condition of no dc
electric current), for better manifesting the process at the TiO2/
electrolyte interface of a cell under operation.19 Fig. 7a shows the
EIS Nyquist plots (i.e. minus imaginary part of the impedance
ꢀZ00 vs. the real part of the impedance Z0 when sweeping the
frequency) for DSSCs based on DPTP, OMeDPTP and
OHexDPTP dyes. For the frequency range investigated (20 Hz
to 1 MHz), in all cells, a larger semicircle occurs in the lower
frequency range (ꢂ20 Hz to 1 kHz) and a smaller semicircle
occurs in the higher frequency range. With the bias illumination
and voltage applied, the larger semicircle at lower frequencies
corresponds to the charge transfer processes at the TiO2/dye/
electrolyte interface, while the smaller semicircle at higher
frequencies corresponds to the charge transfer processes at the
Pt/electrolyte interface. The three cells hardly show any differ-
ence in smaller semicircles at higher frequencies, but the differ-
ence in larger semicircles at lower frequencies between the
OHexDPTP and DPTP (or OMeDPTP) cells is significant. The
larger width of the lower-frequency semicircle of the
OHexDPTP cell indicates a larger charge-transfer resistance at
the TiO2/dye/electrolyte interface.
Fig. 7 Electrochemical impedance spectroscopy (a) Nyquist plots;
(b) Bode phase plots for DSSCs based on DPTP, OMeDPTP, and
OHexDPTP dyes.
at the Pt/electrolyte interface, and the one at lower frequencies
corresponds to the charge transfer at the TiO2/dye/electrolyte
interface. Again, the difference in Bode plots of the three cells
mainly occurs in the lower-frequency range. The characteristic
frequency of the lower-frequency peak in the Bode plot is related
to the charge recombination rate, and its reciprocal is associated
with the electron lifetime.20 As can be seen in Fig. 7b, the lower-
frequency peak of the OHexDPTP cell shifts to a significantly
lower frequency (compared to the DPTP and OMeDPTP cells),
indicating that the electron lifetime was effectively prolonged by
the two longer hexyloxy chains near the TiO2/dye/electrolyte
interface. EIS results clearly suggest that the two longer hexyloxy
chains of OHexDPTP act as effective blocking units between the
TiO2 surface and the electrolyte, preventing hydrophilic I3ꢀ ions
approaching the TiO2 surface and thereby leading to suppressed
charge recombination/dark current and lengthened electron
lifetimes.21 These factors may in turn lead to the higher Voc of the
OHexDPTP cell than those of the DPTP and OMeDPTP cells.
Fig. 7b shows the EIS Bode plots (i.e. the phase of the
impedance vs. the frequency) of the DSSCs based on DPTP,
OMeDPTP, and OHexDPTP dyes. For the frequency range
investigated, both EIS Bode plots also exhibit two peak features;
the one at higher frequencies corresponds to the charge transfer
Conclusions
In summary, a series of new push–pull (D–p–A) DSSC sensi-
tizers have been synthesized and characterized, in which different
diphenylthienylamines bearing various terminal substituents
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were chosen as donor (D) that were bridged by electron-deficient
pyrimidine rings to the acceptor (A), cyanoacrylic acid. By taking
advantage of the characteristic planarity and electronic
deficiency of pyrimidine, the new dyes efficiently yield the bath-
ochromic shift in absorption and effectively improve the light-
harvesting. The introduction of two long hexyloxy chains on the
diphenylthienylamine donor appears to suppress charge recom-
bination at the TiO2/dye/electrolyte interface, leading to a higher
open-circuit voltage of the solar cell. A solar cell device based on
the sensitizer OHexDPTP yielded an overall conversion effi-
ciency of 7.64% under AM1.5G irradiation. This new molecular
design strategy could trigger the future development of other
simple organic photosensitizers for efficient DSSCs.
similar to that of 2a, except that the eluent of column purification
was dichloromethane/hexane (v/v, 3 : 4).ꢃ 2b was isolated as
a yellow solid (56% yield). M.p. 145–147 C; IR (KBr) n 3071,
3020, 2958, 2837, 1606, 1550, 1298, 1168, 1030, 829 cmꢀ1; H
1
NMR (CD2Cl2, 400 MHz) d 8.53 (s, 2H), 7.65 (d, J ¼ 4.0 Hz,
1H), 7.21 (dd, J ¼ 2.4, 6.8 Hz, 4H), 6.88 (dd, J ¼ 2.4, 6.8 Hz, 4H),
6.24 (d, J ¼ 4.0 Hz, 1H), 3.80 (s, 6H); 13C NMR (CD2Cl2,
100 MHz) d 160.6, 160.1, 157.7, 157.5, 140.4, 130.5, 127.9, 126.8,
115.1, 115.0, 112.0, 56.0; HRMS (m/z, FAB+) Calcd. for
C22H1879BrN3O2S 467.0303, found 467.0299. Calcd. for
C22H1881BrN3O2S 469.0283, found 469.0281.
Synthesis of [5-(5-bromo-pyrimidin-2-yl)-thiophen-2-yl]-N,N-
bis(4-hexyloxyphenyl)amine (2c). The synthetic procedure was
similar to that of 2a, except that the eluent of column purification
was dichloromethane/hexane (v/v, 2 : 3). 2c was isolated as
a yellow solid (81% yield). M.p. 86–88 ꢃC; IR (KBr) n 3040, 2954,
Experimental
Synthesis and materials
2856, 1605, 1547, 1240, 1009, 821 cmꢀ1 1H NMR (CDCl3,
;
All chemicals and reagents were used as received from
commercial sources without purification. Solvents for chemical
synthesis were purified by distillation. All chemical reactions
were carried out under an argon or nitrogen atmosphere. The
starting materials 2-(N,N-diphenylamino)thiophene (1a),22
2-[N,N-bis(4-methoxyphenyl)amino]thiophene (1b),23 2-[N,N-
bis-(4-hexyloxyphenyl)amino]thiophene (1c),23 5-bromo-2-iodo-
pyrimidine,24 and 5-(N,N-diphenylamino)-2-(tri-n-butylstannyl)
thiophene22 were synthesized according to the published
procedures.
400 MHz) d 8.53 (s, 2H), 7.67 (d, J ¼ 4.0 Hz, 1H), 7.20 (dd, J ¼
2.0, 6.8 Hz, 4H), 6.85 (dd, J ¼ 2.0, 6.8 Hz, 4H), 6.27 (d, J ¼
4.0 Hz, 1H), 3.94 (t, J ¼ 6.8 Hz, 4H), 1.79 (m, 4H), 1.47 (m, 4H),
1.35 (m, 8H), 0.93 (t, J ¼ 6.8 Hz, 6H); 13C NMR (CDCl3, 100
MHz) d 160.3, 159.6, 157.1, 156.4, 139.7, 130.1, 127.0, 126.3,
115.2, 114.2, 111.5, 68.2, 31.7, 29.3, 25.8, 22.7, 14.2; HRMS (m/z,
FAB+) Calcd. for C32H3879BrN3O2S 607.1868, found 607.1862.
Calcd. for C32H3881BrN3O2S 609.1848, found 609.1843.
Synthesis of 2-(5-N,N-diphenylaminothiophen-2-yl)-pyrimidine-
5-carbaldehyde (3a). To a stirring solution of 2a (4.90 g, 12 mmol)
in 160 mL anhydrous THF was added dropwise to n-BuLi
(1.6 M, 7.73 mL, 12.36 mmol) at ꢀ100 ꢃC under an argon
atmosphere. The resulting solution was stirred for 20 min, after
which dry ethyl formate (1.03 mL, 12.72 mmol) was added
dropwise over 5 min. After stirring for 20 min, the reaction was
quenched with 1.5 M HCl in THF solution (8 mL, 12 mmol). The
cold bath was removed, and the reaction mixture was stirred for
2 h. The THF was removed under reduced pressure and the
reaction mixture was extracted with chloroform, the combined
extracts were washed with brine, dried over anhydrous magne-
sium sulfate, and filtered. The solvent was removed by rotary
evaporation, and the crude product was purified by column
chromatography on silica gel with dichloromethane/hexane (v/v,
2 : 1) as eluent to afford 3a as an orange solid (1.33 g, 31% yield).
M.p. 200–202 ꢃC; IR (KBr) n 3064, 2923, 2821, 1687, 1587, 1445,
Synthesis of [5-(5-bromo-pyrimidin-2-yl)-thiophen-2-yl]-N,N-
diphenylamine (2a). To a stirring solution of 1a (5.03 g, 20 mmol)
in anhydrous THF (60 mL) was added dropwise to n-BuLi
ꢃ
(1.6 M, 13.13 mL, 21 mmol) at ꢀ78 C under an argon atmo-
sphere. The reaction mixture was warmed to ꢀ35 ꢃC and stirred
for 15 min. ZnCl2 (22 mL of a 1 M solution in THF, 22 mmol)
was then added to the reaction mixture in one portion, then the
reaction mixture was warmed to room temperature and stirred
for 30 min. To the above resulting zinc reagent, 5-bromo-2-
iodopyrimidine (5.70 g, 20 mmol), Pd(PPh3)4 (1.15 g, 1 mmol),
and THF (80 mL) was added. The whole mixture was heated to
reflux under argon for 3 h. After cooling to room temperature,
the reaction mixture was poured into water and extracted with
ethyl acetate, the combined extracts were washed with brine,
dried over anhydrous magnesium sulfate, and filtered. The
solvent was removed by rotary evaporation, and the crude
product was purified by column chromatography on silica gel
with dichloromethane/hexane (v/v, 1 : 1) as eluentꢃto afford 2a as
a yellow solid (5.83 g, 71% yield). M.p. 160–162 C; IR (KBr) n
1217, 1058, 835 cmꢀ1 1H NMR (CD2Cl2, 400 MHz) d 9.94
;
(s, 1H), 8.90 (s, 2H), 7.90 (d, J ¼ 4.4 Hz, 1H), 7.39–7.18 (m, 10H),
6.50 (d, J ¼ 4.4 Hz, 1H); 13C NMR (CD2Cl2, 100 MHz) d 188.6,
164.4, 161.6, 158.8, 146.9, 133.1, 130.0, 129.7, 125.8, 125.4, 125.2,
115.5; HRMS (m/z, FAB+) Calcd. for C21H15N3OS 357.0936,
found 357.0935.
1
3068, 3028, 1587, 1547, 1468, 1293, 1120, 810 cmꢀ1; H NMR
(CD2Cl2, 400 MHz) d 8.59 (s, 2H), 7.74 (d, J ¼ 4.0 Hz, 1H), 7.33
(t, J ¼ 7.6 Hz, 4H), 7.24 (d, J ¼ 7.6 Hz, 4H), 7.14 (t, J ¼ 7.6 Hz,
2H), 6.53 (d, J ¼ 4.0 Hz, 1H); 13C NMR (CD2Cl2, 100 MHz)
d 159.8, 158.0, 157.8, 147.4, 131.6, 129.9, 129.8, 124.9, 124.6,
117.4, 115.9; HRMS (m/z, FAB+) Calcd. for C20H1479BrN3S
407.0092, found 407.0102. Calcd. for C20H1481BrN3S 409.0071,
found 409.0080.
Synthesis of 2-[5-N,N-bis(4-methoxyphenyl)aminothiophen-2-
yl]-pyrimidine-5-carbaldehyde (3b). The synthetic procedure was
similartothatof3a, andtheeluentusedforcolumnpurificationwas
dichloromethane to afford 3b as an orange solid (46% yield). M.p.
183–185 ꢃC; IR (KBr) n 3061, 3033, 2820, 2734, 1692, 1592, 1503,
1299, 1159, 825 cmꢀ1; 1H NMR (CD2Cl2, 400 MHz) d 9.89 (s, 1H),
8.84 (s, 2H), 7.84 (d, J ¼ 4.4 Hz, 1H), 7.26 (dd, J ¼ 2.4, 6.8 Hz, 4H),
6.91 (dd, J ¼ 2.4, 6.8 Hz, 4H), 6.25 (d, J ¼ 4.4 Hz, 1H), 3.81 (s, 6H);
Synthesis of [5-(5-bromo-pyrimidin-2-yl)-thiophen-2-yl]-N,N-
bis(4-methoxyphenyl)amine (2b). The synthetic procedure was
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13C NMR (CD2Cl2, 100 MHz) d 188.5, 164.5, 164.2, 158.8, 158.1,
139.7, 133.9, 127.4, 126.9, 124.7, 115.3, 111.3, 56.1; HRMS (m/z,
FAB+) Calcd. for C23H19N3O3S 417.1147, found 417.1144.
Synthesis of 4-(5-N,N-diphenylaminothiophen-2-yl)-benzalde-
hyde (4). A mixture of 4-bromobenzaldehyde (1.48 g, 8.0 mmol),
5-(N,N-diphenylamino)-2-(tri-n-butylstannyl)thiophene (5.40 g,
10.0 mmol), and PdCl2(PPh3)2 (281 mg, 0.4 mmol) in THF
(50 mL) was stirred and heated to reflux under argon for 1.5 h.
After cooling to room temperature, the solvent was removed by
rotary evaporation, and the crude product was purified by
column chromatography on silica gel with dichloromethane/
hexane (v/v, 1 : 2) as eluent to afford 4 as a yellow solid (2.5 g,
88% yield). M.p. 122–124 ꢃC; IR (KBr) n 3061, 3033, 2820, 2734,
1692, 1592, 1503, 1299, 1159, 1061, 825 cmꢀ1; 1H NMR (DMSO-
d6, 400 MHz) d 9.93 (s, 1H), 7.85 (d, J ¼ 8.4 Hz, 2H), 7.73 (d, J ¼
8.4 Hz, 2H), 7.55 (d, J ¼ 4.0 Hz, 1H), 7.34 (t, J ¼ 7.6 Hz, 4H),
7.14–7.09 (m, 6H), 6.67 (d, J ¼ 4.0 Hz, 1H); 13C NMR (CDCl3,
100 MHz) d 191.1, 153.7, 147.2, 140.2, 134.3, 134.2, 130.3, 129.2,
124.7, 124.1, 123.7, 123.2, 119.9; HRMS (m/z, FAB+) Calcd. for
C23H17NOS 355.1031, found 355.1028.
Synthesis of 2-[5-N,N-bis(4-hexyloxyphenyl)aminothiophen-2-
yl]-pyrimidine-5-carbaldehyde (3c). The synthetic procedure was
similar to that of 3a, and the eluent used for column purification
was dichloromethane to afford 3c as an orange solid (54% yield).
ꢃ
M.p. 93–95 C; IR (KBr) n 3046, 2951, 2854, 2722, 1691, 1587,
1542, 1238, 1067, 831 cmꢀ1; 1H NMR (CD2Cl2, 400 MHz) d 9.88
(s, 1H), 8.83 (s, 2H), 7.83 (d, J ¼ 4.4 Hz, 1H), 7.24 (dd, J ¼ 2.0,
6.8 Hz, 4H), 6.89 (dd, J ¼ 2.0, 6.8 Hz, 4H), 6.23 (d, J ¼ 4.4 Hz,
1H), 3.96 (t, J ¼ 6.8 Hz, 4H), 1.78 (m, 4H), 1.47 (m, 4H), 1.37 (m,
8H), 0.93 (t, J ¼ 6.8 Hz, 6H); 13C NMR (CD2Cl2, 100 MHz)
d 188.5, 164.5, 164.4, 158.8, 157.7, 139.5, 133.9, 127.4, 126.7,
124.6, 115.8, 111.1, 68.9, 32.2, 29.8, 26.3, 23.3, 14.5; HRMS (m/z,
FAB+) Calcd. for C33H39N3O3S 557.2712, found 557.2714.
Synthesis of 2-cyano-3-[2-(5-N,N-diphenylaminothiophen-2-yl)-
pyrimidin-5-yl] acrylic acid (DPTP). A mixture of 3a (100 mg,
0.28 mmol), cyanoacetic acid (71 mg, 0.84 mmol), ammonium
acetate (14 mg) in glacial acetic acid (5 mL) was stirred and
heated at 80 ꢃC for 12 h. After cooling to room temperature, the
resulting precipitate was collected by filtration and thoroughly
washed with water, methanol, and hexane to afford DPTP as
Synthesis of 2-cyano-3-[4-(5-N,N-diphenylaminothiophen-2-yl)-
phenyl] acrylic acid (M-TP). The synthetic procedure was similar
to that of DPTP and afforded M-TP as a dark red solid (28%).
M.p. 223–225 ꢃC; IR (KBr) n 3063, 3042, 2228, 1688, 1592, 1504,
1462, 1270, 1184, 1054, 937 cmꢀ1 1H NMR (DMSO-d6, 400
;
MHz) d 8.26 (s, 1H), 8.02 (d, J ¼ 8.4 Hz, 2H), 7.73 (d, J ¼ 8.4 Hz,
2H), 7.58 (d, J ¼ 4.0 Hz, 1H), 7.35 (t, J ¼ 7.6 Hz, 4H), 7.15–7.10
(m, 6H), 6.68 (d, J ¼ 4.0 Hz, 1H); 13C NMR (DMSO-d6,
100 MHz) d 163.5, 153.3, 153.0, 146.8, 138.2, 133.8, 131.6, 129.7,
129.6, 125.6, 124.7, 124.0, 123.0, 120.2, 116.5, 102.0; HRMS (m/
z, FAB+) Calcd. for C26H18N2O2S 422.1089, found 422.1086.
ꢃ
a black solid (80 mg, 67%). M.p. 233–235 C; IR (KBr) n 3032,
1
2928, 2229, 1687, 1585, 1491, 1247, 1083, 858 cmꢀ1; H NMR
(DMSO-d6, 400 MHz) d 9.15 (s, 2H), 8.26 (s, 1H), 7.85 (d, J ¼ 4.0
Hz, 1H), 7.43 (t, J ¼ 7.6 Hz, 4H), 7.30 (d, J ¼ 7.6 Hz, 4H), 7.24 (t,
J ¼ 7.6 Hz, 2H), 6.44 (d, J ¼ 4.0 Hz, 1H); 13C NMR (DMSO-d6,
100 MHz) d 162.8, 161.4, 160.2, 158.5, 148.3, 146.0, 132.4, 129.9,
128.6, 125.7, 124.9, 121.7, 116.2, 114.4, 104.1; HRMS (m/z,
FAB+) Calcd. for C24H16N4O2S 424.0994, found 424.1004.
Solvent effect measurement
The absorption spectra of the two dyes were measured in
different organic solvents while maintaining the concentrations
of the dye solutions below 1 ꢁ 10ꢀ5 M.
Synthesis of 2-cyano-3-{2-[5-N,N-bis(4-methoxyphenyl)amino-
thiophen-2-yl]-pyrimidin-5-yl} acrylic acid (OMeDPTP). The
synthetic procedure was similar to that of DPTP and afforded
OMeDPTP as a black solid (62%). M.p. 236–238 ꢃC; IR (KBr) n
Theoretical calculations
Density functional theory (DFT) calculations were evaluated by
the B3LYP hybrid functional for the geometry optimizations. All
the molecular orbital levels of HOMO and LUMO were achieved
with the 6–31G(d) basis set implemented in the Gaussian
03 package.
3106, 3005, 2927, 2833, 2217, 1713, 1589, 1545, 1076, 860 cmꢀ1
;
1H NMR (DMSO-d6, 400 MHz) d 9.10 (s, 2H), 8.22 (s, 1H), 7.79
(d, J ¼ 4.0 Hz, 1H), 7.32 (d, J ¼ 8.8 Hz, 4H), 7.00 (d, J ¼ 8.8 Hz,
4H), 6.13 (d, J ¼ 4.0 Hz, 1H), 3.78 (s, 6H); 13C NMR (DMSO-d6,
100 MHz) d 163.3, 163.0, 161.6, 158.6, 157.5, 148.5, 138.7, 133.4,
127.3, 125.6, 120.9, 116.4, 115.1, 109.9, 102.8, 55.4; HRMS (m/z,
FAB+) Calcd. for C26H20N4O4S 484.1205, found 484.1209.
Measurement of absorption spectra of dye-loaded nanoporous
TiO2 films
Synthesis of 2-cyano-3-{2-[5-N,N-bis(4-hexyloxyphenyl)ami-
nothiophen-2-yl]-pyrimidin-5-yl} acrylic acid (OHexDPTP). The
synthetic procedure was similar to that of DPTP and afforded
OHexDPTP as a black solid (86%). M.p. 142–144 ꢃC; IR (KBr) n
A 7 mm thick transparent porous TiO2 nanoparticle layer
(adopting 20 nm anatase TiO2 nanoparticles) was coated on
a glass plate by the doctor-blade method. After sintering at 500
ꢃC for 30 min, the TiO2 film was immersed into dye solutions of
0.5 mM dye in chlorobenzene at room temperature for 24 h.
Then the UV-Vis absorption spectrum of the dye-loaded TiO2
film was recorded on a spectrophotometer.
3067, 2953, 2857, 2226, 1693, 1584, 1537, 1213, 1009, 852 cmꢀ1
;
1H NMR (CD2Cl2, 400 MHz) d 9.05 (s, 2H), 8.08 (s, 1H), 7.88 (d,
J ¼ 4.4 Hz, 1H), 7.25 (d, J ¼ 8.8 Hz, 4H), 6.90 (d, J ¼ 8.8 Hz,
4H), 6.25 (d, J ¼ 4.4 Hz, 1H), 3.96 (t, J ¼ 6.4 Hz, 4H), 1.77 (m,
4H), 1.47 (m, 4H), 1.36 (m, 8H), 0.92 (t, J ¼ 6.4 Hz, 6H); 13C
NMR (CD2Cl2, 100 MHz) d 165.7, 163.2, 159.5, 158.2, 150.1,
139.5, 135.1, 127.7, 126.5, 120.7, 116.1, 111.7, 101.1, 69.0, 32.2,
29.8, 26.3, 23.2, 14.4; HRMS (m/z, FAB+) Calcd. for
C36H40N4O4S 624.2770, found 624.2769.
Fabrication of dye-sensitized solar cells
To prepare the DSSC working electrodes, the FTO glass plates
were first cleaned in a detergent solution using an ultrasonic bath
for 15 min, and then rinsed with water and ethanol. A layer of
5956 | J. Mater. Chem., 2011, 21, 5950–5958
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20 nm sized anatase TiO2 nanoparticles for the transparent
nanocrystalline layer was first coated onto the FTO glass plates
by the doctor-blade method. After drying the film at 120 ꢃC,
another layer of 400 nm sized anatase TiO2 nanoparticles was
then deposited as the light scattering layer of the DSSC. The
resulting working electrode was composed of a 12 mm thick
transparent TiO2 nanoparticle layer (particle size: 20 nm) and a 4
mm thick TiO2 scattering layer (particle size: 400 nm). The
nanoporous TiO2 electrꢃodes were then seqꢃuentially heated at
applying a bias at the open-circuit voltage Voc of the cell (namely,
under the condition of no dc electric current) and by using an ac
amplitude of 10 mV.
Acknowledgements
This work was financially supported by the National Science
Council of Taiwan.
ꢃ
150 C for 10 min, 300 C for 10 min, 400 C for 10 min, and
Notes and references
finally, at 500 ꢃC for 30 min. After cooling, the nanoporous TiO2
electrodes were immersed into a chlorobenzene solution con-
taining organic dyes (0.5 mM) or acetonitrile/tert-butanol
mixture (1 : 1) containing N719 (0.5 mM) with deoxycholic acid
(0.5 mM, DCA) as a co-adsorbent at room temperature for 24 h.
Counter electrodes of the DSSC were prepared by depositing
40 nm thick Pt films onto the FTO glass plates by e-beam
evaporation. The dye-adsorbed TiO2 working electrode and
a counter electrode were then assembled into a sealed DSSC cell
with a sealant spacer between the two electrode plates. A drop of
electrolyte solution [0.6 M 1-butyl-3-methylimidazolium iodide
(BMII), 0.05 M LiI, 0.03 M I2, 0.5 M 4-tert-butylpyridine, and
0.1 M guanidinium thiocyanate in a mixture of acetonitrile–
valeronitrile (85 : 15, v/v)] was injected into the cell through
a drilled hole. Finally, the hole was sealed using the sealant and
a cover glass. An anti-reflection film was adhered to the DSSC. A
mask with an aperture area of 0.125 cm2 was covered on a testing
cell during photocurrent–voltage and incident photon-to-current
conversion efficiency measurements.
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