Improving the photovoltage of dithienopyrrole dye-sensitized solar cells via attaching the bulky bis(octyloxy)biphenyl moiety to the conjugated π-linker

Article abstract of DOI:10.1002/adfm.201203348

The judicious design of 3D giant organic dye molecules to enable the formation of a porous photoactive layer on the surface of titania is one of the viable tactics to abate the adverse interfacial charge recombination in dye-sensitized solar cells (DSCs) employing outer-sphere redox couples. Here 2′,6′-bis(octyloxy)-biphenyl substituted dithieno[3,2-b:2′, 3′-d]pyrrole segment is constructed and employed as the π-linker of a high molar absorption coefficient organic push-pull dye. With respect to its congener possessing the hexyl substituted dithieno[3,2-b:2′,3′-d] pyrrole linker, the new dye can self-assemble on the surface of titania to afford a porous organic coating, which effectively slow down the kinetics of charge recombination of titania electrons with both outer-sphere tris(1,10-phenanthroline)cobalt(III) ions and photooxidized dye molecules, improving the cell photovoltage. In addition, the diminishments of charge recombination via modulating the microstructure of interfacial functional zone can also overcompensate the disadvantageous impact of reduced light-harvesting and evoke an enhanced photocurrent output, bringing forth an efficiency improvement from 7.5% to 9.3% at the 100 mW cm^-2, simulated AM1.5 conditions. Copyright

Full text of DOI:10.1002/adfm.201203348

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Improving the Photovoltage of Dithienopyrrole Dye-
Sensitized Solar Cells via Attaching the Bulky Bis(octyloxy)
biphenyl Moiety to the Conjugated π-Linker
Ning Cai, Jing Zhang, Mingfei Xu, Min Zhang,* and Peng Wang*
On the other side, much effort has been
devoted to metal-free organic D-π-A dyes,
primarily owing to the flexibility of molec-
ular design.[5,6] The elaborate option of con-
jugated π-linkers is one of the executable
tactics towards energy-level engineering
of organic D-π-A dyes,[7] apart from elec-
tron donors and acceptors. In this respect,
a great amount of organic dyes involving
thiophene and its numerous derivative
segments have been exploited[8–14] since
the work by Arakawa et al.[15] on the cou-
marin-thiophene DSC chromophores.
Amongst all metal-free dyes, so far only
a few dyes featuring either dithienosilole
(DTS) or cyclopentadithiophene (CPDT)
have been demonstrated to exhibit over
10% efficiencies.[16–18] However the prepa-
rations of DTS and CPDT typically involve
cryogenic temperatures and hazardous
organolithium reagents.
The judicious design of 3D giant organic dye molecules to enable the for-
mation of a porous photoactive layer on the surface of titania is one of the
viable tactics to abate the adverse interfacial charge recombination in dye-
sensitized solar cells (DSCs) employing outer-sphere redox couples. Here
2′,6′-bis(octyloxy)-biphenyl substituted dithieno[3,2-b:2′,3′-d]pyrrole segment
is constructed and employed as the π-linker of a high molar absorption coeffi-
cient organic push-pull dye. With respect to its congener possessing the hexyl
substituted dithieno[3,2-b:2′,3′-d]pyrrole linker, the new dye can self-assemble
on the surface of titania to afford a porous organic coating, which effectively
slow down the kinetics of charge recombination of titania electrons with both
outer-sphere tris(1,10-phenanthroline)cobalt(III) ions and photooxidized dye
molecules, improving the cell photovoltage. In addition, the diminishments
of charge recombination via modulating the microstructure of interfacial func-
tional zone can also overcompensate the disadvantageous impact of reduced
light-harvesting and evoke an enhanced photocurrent output, bringing forth
an efficiency improvement from 7.5% to 9.3% at the 100 mW cm−2 , simulated
AM1.5 conditions.
Thereby the dithieno[3,2-b:2′,3′-d]pyr-
role (DTP) unit pioneered by Rasmussen
et al.[19] as a promising building block of electronic materials
has been exploited as the electron-donor or π-linker of metal-
free organic dyes.[12,20] It is valuable to note that the triphe-
nylamine-cyanoacrylic acid dye with the DTP linker displays a
higher molar extinction coefficient than its counterparts with
DTS and CPDT, ensuring a comparable photocurrent genera-
tion. Unfortunately, an inferior photovoltage was observed for
the C241 dye with the 4-hexyl-4H-dithieno[3,2-b:2′,3′-d]pyrrole
linker (Figure 1A).[12] Recent studies on DSCs with outer-
sphere electron mediators have suggested that the extraordi-
nary merit of devising bulky organic dyes for high open-circuit
photovoltage cells.[4,18,21–24] In this work, we will synthesize the
4-(2′,6′-bis(octyloxy)biphenyl-4-yl)-4H-dithieno[3,2-b:2′,3′-d]pyr-
role segment and employ it as the π-conjugated linker of a new
dye coded C251 (Figure 1A), and further compare its electro-
chemical, photophysical, and photovoltaic characteristics with
respect to the reference DTP dye C241.
1. Introduction
During the past two decades, substantial research passions have
been fascinated by dye-sensitized solar cells (DSCs)[1,2] in the light
of their grand potential to achieve the solar-to-electricity trans-
formation at a low cost, based upon resource-abundant and less
energy-intensive materials as well as simple manufacture tech-
nologies. The energy gap and energy offsets of a dye molecule,
the light-harvesting component of this nanostructured device,
with respect to other materials jointly impact the photocarrier
generation. In addition, the free energy of output electrons could
be considerably modulated by an organic layer mainly composed
of dye molecules, lying between titania nanocrystals and redox
shuttles. The cutting-edge DSC dyes are ruthenium polypyridyl
and zinc porphyrin complexes, exhibiting ≈12% power conver-
sion efficiencies at the air mass 1.5 conditions.[3,4]
N. Cai, J. Zhang, Dr. M. Xu, Dr. M. Zhang, Prof. P. Wang
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
Changchun, 130022, China
E-mail: min.zhang@ciac.jl.cn; peng.wang@ciac.jl.cn
2. Results and Discussion
As depicted in Scheme 1, C251 was prepared according
to a previously reported procedure of the Suzuki−Miyaura
cross-coupling of a pinacol bisarylaminophenyl boronate with
DOI: 10.1002/adfm.201203348
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a bromine substituted aromatic aldehyde,[12] and the widely
deployed Knoevenagel condensation to produce a cyanoacrylic
acid DSC dye.[25,26] Bisoctyloxy substituted iodobenzene 2 was
almost quantitatively obtained by alkylation of 2-iodobenzene-
1,3-bis(olate) in the polar aprotic solvent of acetone. By virtue of
a highly active phosphine ligand of 2-(2′,6′-dimethoxybiphenyl)
dicyclohexylphosphine (SPhos),[27] the hindered iodide 2 was
successfully cross-coupled at a moderate temperature with
4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzenamine in
excellent yield to afford amine 3, which further underwent the
Buchwald−Hartwig amination of 3,3′-dibromo-2,2′-bithiophene
to give the key intermediate 4 in excellent yield. Aldehyde 5 was
obtained in excellent yield by the selective monoformylation of 4
via the Vilsmeier−Haack reaction, and further brominated with
N-bromosuccinimide (NBS) to generate the key intermediates
6. Note that the bromination must be carried out at a low tem-
perature of ≈0 °C to avoid the side reaction on the bis(octyloxy)
phenyl moiety.
The electronic absorption spectra of diluted THF solutions
of C241 and C251 were first recorded to evaluate the molec-
ular-level capacity of solar radiation capture. As depicted in
Figure 1B, a slightly lower maximum molar absorption coef-
ficient (εabsmax) of 61 × 103 M−1 cm−1 at 506 nm was measured
for the C251 dye, compared to that of 65 × 103 M−1 cm−1 at
511 nm for C241. The experimentally measured blue-shifting
of absorption peak and the decrease of absorption coefficient
from C241 to C251 can be nicely reproduced by the TDDFT
calculations on the maximum absorption wavelength (λmax
)
and the corresponding oscillator strengths (f) listed in Table S1
of the Supporting Information. Less intense and blue-shifted
absorption of C251 in comparison to C241 is mainly related
to the small increase from 18 to 20° of the torsion angle
between the phenyl and DTP moieties. Further analyses on the
molecular orbital topologies (Figure 2) involving the S0→S1
transitions disclose that the low-energy absorption bands of
C241 and C251 are mainly assigned to the intramolecular
charge transfer transitions from the triphenylamine donor to
the cyanoacrylic acid acceptor. It is also worthy of noting that
Figure 1. A) Chemical structures of two dithienopyrrole dyes investigated
in this work. B) Electronic absorption spectra of the THF dye solutions.
C) Absorption spectra of the dye-coated titania films.
Scheme 1. Synthetic route of the C251 dye. Reagents and conditions: i) 1-bromooctane, K2CO3, acetone, reflux; ii) 4-(4,4,5,5-tetramethyl-1,3,2-diox-
aborolan-2-yl)benzenamine, Pd(OAc)2, SPhos, K3PO4, dioxane/H2O (v/v, 5/1), rt; iii) 3,3′-dibromo-2,2′-bithiophene, Pd2(dba)3, BINAP, NaOt-Bu,
toluene, reflux; iv) DMF, POCl3, 1,2-dichloroethane, rt; v) NBS, THF, 0 °C; vi) 4,4,5,5-tetramethyl-2-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}-1,3,2-
dioxaborolane, Pd(OAc)2, SPhos, K3PO4, dioxane/H2O (v/v, 5/1), rt; vii) cyanoacetic acid, piperidine, CHCl3, reflux.
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scan rate (ν) in terms of the following equa-
tion of ipa = (2.69 × 105)Ascmn3/2Dapp1/2v1/2 [31]
.
At a given film geometry with invariant elec-
trode area (As), film thickness, and porosity,
the dye concentration (cm) is proportional to
the dye load amount. Herein, n is the trans-
ferred electron number and Dapp refers to
the apparent charge diffusion coefficient.
Through comparing the fitting slopes of data
shown in Figure 3B, the Dapp in the molec-
ular layer composed of the hexyl substituted
DTP dye is ∼4 times higher than that of the
2′,6′-bis(octyloxy)-biphenyl substituted DTP
counterpart, proving a stronger intermo-
lecular electronic interaction for C241 with
respect to C251.
We further carried out the incident
photon-to-collected
electron
conversion
efficiency (IPCE) measurement of a dye-
coated titania film in conjunction with an
acetonitrile (MeCN) electrolyte containing
the tris(1,10-phenanthroline)cobalt(II)/(III)
redox couple. The cell details are described
in the experimental section. As shown in
Figure 4A, the C251 cell exhibits a broad pla-
teau with similar IPCE summit compared
Figure 2. Distribution of frontier molecular orbitals involving the S0→S1 transitions of C241
and C251 in THF. All the iodensity surface values are fixed at 0.03.
there is nearly no distribution of these frontier orbitals on the
2′,6′-bis(octyloxy)-biphenyl moiety, which could be considered
as an insulating spacer to reduce the intermolecular exciton
annihilation in the assembly of dye molecules grafted on the
surface of titania.
The attenuation of intermolecular interaction in the assembly
of DTP dye molecules via attaching the 2′,6′-bis(octyloxy)-
biphenyl group was further supported by recording the progres-
sive blue-shifting of absorption peaks (Figure S1, Supporting
Information) during the dyeing of titania. The shift of absorp-
tion peak is less evident for C251 during the dying process
than C241, suggesting that C241 is prone to aggregate on the
surface of titania. We also measured the electronic absorption
of a stained titania film reaching the stage of adsorption-des-
orption equilibrium, which is more closely related to the DSC
light-harvesting capacity. As presented in Figure 1C, lower
absorption coefficients (a) in the whole visible region are noted
for a dyed film with C251 than C241. The measured dye load
amounts of C241 and C251 are 2.8 × 10−8 and 1.4 × 10−8 mol
cm−2 μm−1 , respectively, which could be rationalized in terms
of the geometries (Figure S2, Supporting Information) of C241
and C251.
The substituent related intermolecular interaction was further
assessed by measuring the cyclic voltammograms of dye-coated
titania films at various scan rates, to probe the diffusive lateral
charge transfer characteristics[28–30] in the electroactive dye layer
+
on titania. The ground-state redox potentials ED/D of C241
and C251 grafted on titania can be precisely derived through
averaging the anodic and cathodic peak potentials shown in
Figure 3A, being 0.28 and 0.32 V versus the ferrocene/ferro-
cenium (Fc/Fc+) reference, respectively. Furthermore, the peak
current of anodic wave (ipa) can be nicely correlated with the
Figure 3. A) Cyclic voltammograms of the dye-coated titania films in
1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. Scan
rate: 60 mV s−1 . B) Plots of anodic peak current (ipa) as a function of the
square root of scan rate (ν1/2). The solid lines are fitting in terms of equa-
tion ipa = (2.69 × 105)Ascmn3/2Dapp1/2v1/2
.
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T a b le 1 . Photovoltaic parameters obtained at the simulated AM1.5
conditions.a)
sc [mA cm−2
]
η
[%]
dye
calculated jsc
Voc
[V]
FF
j
[mA cm−2
]
C241
C251
12.33
12.53
13.53
0.82
0.96
0.73
0.72
7.5
9.3
13.40
a)
The validity of our photovoltaic data is confirmed by comparing the calculated
sc via wavelength integration of the product of the standard AM1.5 emission spec-
j
trum (ASTM G173-03) and measured IPCE spectrum with the experimental jsc
,
showing a less than 5% error. Also note that all our cells show a linear dependence
of photocurrent on light intensity as shown in Figure S4, Supporting Information.
the jsc of DSCs is measured at the condition of a considerable
low electron density in the mesoporous titania film, which can
significantly reduce the charge recombination flux, thereby it is
roughly proportional to the photocarrier generation flux. This
estimation has motivated us to compare the dye structure corre-
lated Voc variation at a certain jsc by measuring j−V curves under
various metal-mesh attenuated lights and plotting Voc as a func-
tion of jsc, as presented in Figure 4C. It is noted that at a given
jsc, the C251 cell exhibits an obviously higher Voc than C241.
For an invariable redox electrolyte in DSCs, the rise or fall
of Voc originates from an upward or downward displacement
of electron quasi-Fermi-level (EF,n) in titania, which intrinsically
stems from a change of titania conduction band edge (Ec) and/
or a fluctuation of electron density.[32–35] At a given photocar-
rier generation flux, the electron density is determined by the
interfacial recombination rate of titania electrons with electron
accepting species in electrolytes and/or dye cations. We further
performed charge extraction and transient photovoltage decay
measurements to scrutinize the origins of Voc variation. As
depicted in Figure 5A, two type of cells made with C241 and
C251 display a similar extracted electron density (dn) at the same
potential bias, indicative of a fixed conduction-band edge of
titania with respect to the electrolyte Fermi-level. As Figure 5B
presents, an over one order of magnitude longer charge recom-
bination lifetime (τn) for the C251 cell is however measured
compared to the C241 congener at a given dn, accounting for
the aforementioned superior photovoltage at a given jsc.
Obviously, albeit an evident reduction of dye load amount,
the replacement of the linear hexyl group with the bulky
2′,6′-bis(octyloxy)-biphenyl moiety must endow dye molecules
to form a more favorite organic layer to inhibit the charge
recombination, at the interface between the redox shuttle and
titania. To further picture the microstructure of the dye mol-
ecule layers, we first measured their damping effect on the
escape of photoelectrons from titania to vacuum. Note that in
some previous work[18,36,37] the intensity (I) of Ti2p3/2 signal
in X-ray photoelectron spectroscopy (XPS) measurements has
been correlated to the charge recombination at the titania/elec-
trolyte interface. Herein evident reductions of both Ti2p3/2 and
Ti2p1/2 signal intensities (Figure 6) can also be perceived for the
C241 or C251 dye-coated titania film in comparison with the
bare substrate.
Figure 4. A) Photocurrent action spectra. B) j–V characteristics measured
at an irradiation of 100 mW cm−2 AM1.5 sunlight. C) Plots of open-circuit
photovoltage as a function of short-circuit photocurrent density. Cells
were tested using a metal mask with an aperture area of 0.158 cm2. An
antireflection film was adhered to the testing cell during measurements.
to the C241 counterpart. However, it is valuable to notice that
the C241 confers relatively lower IPCEs from 550 to 630 nm, in
spite of higher absorptions of the C241 dye-coated titania film
in this spectral region (Figure S3, Supporting Information),
suggesting that the C251 dye can endow a titania film to display
an improved charge collection efficiency compared to C241.
The solar-to-electricity conversion efficiencies of DSCs made
with C241 and C251 as the photosensitizers were examined by
recording the photocurrent density−voltage ( j−V) characteris-
tics at the 100 mW cm−2 , simulated AM1.5 conditions, and the
detailed photovoltaic parameters were compiled in Table 1. As
Figure 4B presents, the short-circuit photocurrent density (jsc),
open-circuit photovoltage (Voc), and fill factor (FF) of the C241
cell are 12.53 mA cm−2 , 0.82 V, and 0.73, respectively, affording a
power conversion efficiency (η) of 7.5%. Furthermore, a notably
improved Voc of 0.96 V concomitant with an increased jsc of
13.53 mA cm−2 was achieved with the C251 dye, contributing
to a higher η of 9.3%. Note that the efficiency of the N719 dye
under the same device condition is only ≈3%. It is known that
If the dye coating on titania is compact and vertically
homogeneous, the Ti2p3/2 intensity from a dye-coated titania
film can be quantitatively described with a two-layer model
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Figure 7. ATR-FTIR spectra of C241 (a), C241Na (b), C251 (d), and
C251Na (e) spin-coated on flat quartz as well as C241 (c) and C251
(f) grafted on mesoporous titania.
with respect to C241, the three-dimensional giant C251 dye
molecules should form a larger porosity organic coating on the
surface of titania, however the diameter of resultant micropores
should be still smaller than that of tris(1,10-phenanthroline)
cobalt(III) ions. This finding has highlighted the necessity of
constructing an efficient porous dye layer on the surface of
titania through judicious molecular engineering to restrain
the adverse interfacial charge recombination with outer-sphere
redox shuttles.[35]
Figure 5. Plots of A) extracted electron density in titania against open-
circuit photovoltage and B) electron lifetime as a function of extracted
electron density.
I = I0exp(−d/λsinα),[38] where I0 is the intensity of a titania sub-
strate without dye coating, α is the electron take-off angle of
90°, λ is the inelastic mean free path (IMFP) of electrons in
a compact organic solid composed of dye molecules, and d is
the mean thickness of a compressed dye layer. The λ values of
C241 and C251 calculated by means of semi-empirical equa-
tions and parameters[39] are 31.3 and 31.2 Å, respectively. On
the basis of Ti2p3/2 intensities compiled in Table S2, Supporting
Information, the compressed mean thicknesses of dye coatings
on titania are calculated to be 25.9 and 21.2 Å for C241 and
C251. Note that there is not a positive correlation between the
thickness estimated by XPS and the electron lifetime evaluated
by transient photovoltage decay. This scenario suggests that
The chemical bonding behaviors[40,41] of C241 and C251
on titania were further investigated with the aid of attenu-
ated total reflection Fourier transform infrared (ATR-FTIR)
spectrum measurements. In contrast to dye films spin-coated
on a flat quartz showing the characteristic absorption bands
of carboxylic acid (1677 cm−1 for C241; 1679 cm−1 for C251,
Figure 7), both the corresponding deprotonated sodium salt
forms on quartz and dye-coated titania films present two new
absorption bands (1603 and 1361 cm−1 for C241Na; 1602 and
1360 cm−1 for C251Na; 1592 and 1367 cm−1 for C241/titania;
1589 and 1372 cm−1 for C251/titania), which can be assigned
to the asymmetric and symmetric stretching of carboxylate
group.[42] The carboxylic acid absorption bands are not observed
for two dye-coated titania films, which can rule out the possi-
bility of dye multilayer formation however still can not confirm
a monolayer.
The relative mean tilt angles of dye molecules with respect
to the titania surface were analyzed by measuring the charge
recombination kinetics of photoinjected electrons in titania
and oxidized dye molecules. The validity of this analysis was
based on the similar electronic backbone of these two photo-
sensitizers as well as the comparable distribution of titania
surface states in our cells. In our experiments, we selected the
probe light of 765 nm in joint consideration of strong absorp-
tions in the near-infrared region of these oxidized dye mol-
ecules[12] and the signal sensitivity of our photomultiplier tube
detector. To ensure a similar charge carrier distribution profile
in the testing samples, the excitation wavelengths were care-
fully chosen according to an optical density of ∼0.2 for the
Figure 6. Ti2p3/2 and Ti2p1/2 photoelectron signals of titania films
uncoated (a) and coated by (b) C241 and (c) C251.
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respect to C241 there is a smaller tilt angle of the C251 dye
molecules anchored on the surface of titania probably due to
its 3D steric encumbrance, as pictorially illustrated in Scheme
2, which comes up based upon the preceding theoretical and
experimental information.
3. Conclusions
To summarize, we have synthesized a new push-pull dye fea-
turing the 4-(2′,6′-bis(octyloxy)biphenyl-4-yl)-4H-dithieno[3,2-
b:2′,3′-d]pyrrole π-linker, which presents a power conversion
efficiency 9.3% at the 100 mW cm−2 , simulated AM1.5 condi-
tions. The attachment of this insulating three-dimensional
branched moiety to the conjugated electronic backbone of dye
molecules brings forth enhanced photocurrent and photo-
voltage in comparison with the linear hexyl group. Transient
absorption and photovoltage decay measurements have dis-
closed that the new dye can form a powerful porous photoac-
tive layer on the surface of TiO2 nanocrystals to decelerate the
charge recombination kinetics involving photoinjected titania
electrons. The strategy presented here on the construction of a
three-dimensional dithienopyrrole dye could be extended to the
design of other infrared absorption DSC dyes for more efficient
cells employing the single-electron redox shuttles or solid hole-
transporters, on account that the significant control of charge
recombination in these devices is a big challenge.
Figure 8. Absorption decay traces recorded at 765 nm of 13-μm-thick,
C241 (a) and C251 (b) coated titania films in contact with an inert elec-
trolyte upon pulsed laser excitation. Laser pulse fluence: 11 μJ cm−2
;
excitation wavelength: 663 nm (C241) and 652 nm (C251). Smooth lines
are stretched exponential fittings over raw data obtained by averaging
500 laser shots.
dye-coated titania films infiltrated with an inert electrolyte con-
sisting of 0.5 M 4-tert-butylpyridine (TBP) and 0.1 M lithium
bis(trifluoromethanesulfonyl)imide (LiTFSI) in MeCN. The
absorption traces (Figure 8) can be fitted to a stretched expo-
nential decay function ΔA ∝ A0exp[−(t/τ)α], where A0 is the
pre-exponential factor, α is the stretching parameter, and τ is
the characteristic time. By use of the gamma function Γ(x), the
4. Experimental Section
mean times of these charge recombination reaction (<τ>rec
)
Materials: MeCN, tert-butanol (t-BuOH), phosphorus oxychloride
(POCl3), and DMF were distilled before use. Pd(OAc)2, SPhos,
were derived through <τ> = (τ/α)Γ(1/α),[43] being 2.7 and 5.7 ms
for C241 and C251, respectively. It is obvious that replacing
the hexyl unit with 2′,6′-bis(octyloxy)-biphenyl brings forth an
evident deceleration of charge recombination, although the
C251 dye has a 40 meV larger Gibbs free energy for this elec-
tron transfer reaction, which is estimated based on the afore-
NBS,
tris(dibenzylideneacetone)dipalladium
(Pd2(dba)3),
(BINAP)
and
were
( )-2,2′-bis(diphenylphosphino)-1,1′-binaphthalene
purchased from Sigma-Aldrich and used without any purification.
C241,[12]
phenyl}-1,3,2-dioxaborolane,[46]
4,4,5,5-tetramethyl-2-{4-[N,N-bis(4-hexyloxyphenyl)amino]
2-iodobenzene-1,3-diol
(1),[47]
and
4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzenamine,[48]
3,3′-dibromo-2,2′-bithiophene[49] were synthesized according to the
corresponding literature methods. The synthetic details of C251 are
described as follows.
+
ED/D
mentioned
in Figure 3. By correlating the reduced elec-
tron transfer rate to a longer tunneling distance between titania
electrons and triphenylamine holes,[44,45] we image that with
2-Iodo-1,3-bis(octyloxy)benzene (2) : To a stirred solution of 1 (10.000 g,
42.371 mmol) and K2CO3 (29.280 g, 211.855 mmol)
in acetone (200 mL) was added 1-bromooctane
(32.730 g, 169.484 mmol). The mixture was
refluxed under argon overnight and then cooled
to room temperature before filtering and washing
with dichloromethane. After evaporation of the
filtrate under reduced pressure, the crude product
was purified by column chromatography over
silica gel with petroleum ether as eluent to obtain
a colorless oil as the desired product 2 (18.729 g,
96% yield). 1H NMR (600 MHz, DMSO-d6, δ): 7.24
(t, J = 8.4 Hz, 1H), 6.58 (d, J = 8.4 Hz, 2H), 4.00 (t,
J = 6.3 Hz, 4H), 1.71 (m, 4H), 1.47 (m, 4H), 1.26
(m, 16H), 0.86 (t, J = 6.9 Hz, 6H). 13C NMR (150
MHz, DMSO-d6, δ): 158.42, 129.93, 105.26, 78.67,
68.62, 31.16, 28.62, 28.56, 25.52, 22.03, 13.91. MS
(ESI) m/z calcd. for C22H37IO2: 460.2. Found: 461.1
([M+H]+). Anal. calcd. for C22H37IO2: C, 57.39; H,
Scheme 2. Pictorial adsorptions of C241 and C251 on the surface of titania proposed on the
basis of joint experimental and theoretical data. The tunneling distances of two charge transfer
channels of photoinjected electrons in titania with the tris(1,10-phenanthroline)cobalt(III) ions
(d1) and dye cations (d2) are both longer for C251 than C241.
8.10. Found: C, 57.31; H, 8.14.
4-Amino-2′,6′-bis(octyloxy)-biphenyl
(3) :
To
a stirred solution of 2 (2.000 g, 4.343 mmol),
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4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzenamine (0.793 g,
3.620 mmol), and K3PO4 (3.842 g, 18.100 mmol) in dioxane/H2O (106
mL, v/v, 5/1) was added Pd(OAc)2 (0.016 g, 0.072 mmol) and SPhos
(0.030 g, 0.072 mmol). The resulting mixture was stirred at 45 °C under
argon for 24 h and then cooled to room temperature. The solution was
extracted with ethyl acetate before the organic phase was washed with
water and dried over anhydrous sodium sulfate. After evaporation of
the solvent under reduced pressure, the crude product was purified by
column chromatography over silica gel with dichloromethane/petroleum
ether (v/v, 2/1) as eluent to obtain a colorless oil as the desired product
3 (1.418 g, 92% yield). 1H NMR (600 MHz, DMSO-d6, δ): 7.12 (t, J =
8.4 Hz, 1H), 6.91 (d, J = 8.4 Hz, 2H), 6.63 (d, J = 8.4 Hz, 2H), 6.51 (d,
J = 8.4 Hz, 2H), 4.94 (s, 2H), 3.83 (t, J = 6.3 Hz, 4H), 1.52 (m, 4H), 1.27
(m, 8H), 1.22 (m, 12H), 0.86 (t, J = 7.2 Hz, 6H). 13C NMR (150 MHz,
DMSO-d6, δ): 156.69, 146.79, 131.34, 127.50, 120.91, 120.00, 112.70,
105.56, 67.73, 31.14, 28.63, 28.47, 25.35, 22.04, 13.92. MS (ESI) m/z
calcd. for (C28H43NO2): 425.3. Found: 426.4 ([M+H]+). Anal. calcd. for
C28H43NO2: C, 79.01; H, 10.18; N, 3.29. Found: C, 79.11; H, 10.24; N,
3.17.
4-(2′,6′-Bis(octyloxy)biphenyl-4-yl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole (4) :
To a solution of 3,3′-dibromo-2,2′-bithiophene (0.326 g, 1.006 mmol) in
toluene (9 mL) was added NaOt-Bu (0.230 g, 2.414 mmol), Pd2(dba)3
(0.022 g, 0.025 mmol), and BINAP (0.062 g, 0.101 mmol) under argon.
The resulting mixture was stirred for 30 min before 3 (0.430 g, 1.011
mmol) was added. The reaction was refluxed for 20 h and then cooled
to room temperature. Water (10 mL) was added to quench the reaction
and the solution was extracted with ethyl acetate before the organic
phase was washed with water and dried over anhydrous sodium sulfate.
After evaporation of the solvent under reduced pressure, the crude
product was purified by column chromatography over silica gel with
dichloromethane/petroleum ether (v/v, 1/2) as eluent to obtain a colorless
oil as the desired product 4 (0.561 g, 95% yield). 1H NMR (600 MHz,
DMSO-d6, δ): 7.63 (d, J = 8.4 Hz, 2H), 7.47 (d, J = 4.8 Hz, 2H), 7.46 (d, J
= 8.4 Hz, 2H), 7.30 (d, J = 5.4 Hz, 2H), 7.27 (t, J = 8.4 Hz, 1H), 6.74 (d, J
= 8.4 Hz, 2H), 3.92 (t, J = 6.6 Hz, 4H), 1.56 (m, 4H), 1.25 (m, 4H), 1.13
(m, 16H), 0.72 (t, J = 6.6 Hz, 6H). 13C NMR (150 MHz, DMSO-d6, δ):
156.59, 143.26, 137.13, 132.48, 131.76, 129.02, 124.58, 120.43, 118.44,
116.22, 112.43, 105.62, 68.05, 31.09, 28.65, 28.56, 25.44, 21.97, 13.74.
MS (ESI) m/z calcd. for (C36H45NO2S2): 587.3. Found: 588.4 ([M+H]+).
Anal. calcd. for C36H45NO2S2: C, 73.55; H, 7.72; N, 2.38. Found: C, 73.39;
H, 7.80; N, 2.29.
4-(2′,6′-Bis(octyloxy)biphenyl-4-yl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole-
2-carbaldehyde (5) : To a solution of 4 (0.470 g, 0.799 mmol) and DMF
(0.174 mL, 2.237 mmol) in 1,2-dichloroethane (10 mL) was added POCl3
(0.087 mL, 0.945 mmol). The resulting mixture was stirred at room
temperature under argon for 10 h and then saturated sodium acetate
aqueous solution (20 mL) was added. After stirred overnight at room
temperature, the solution was extracted with dichloromethane before
the organic phase was washed with water and dried over anhydrous
sodium sulfate. After evaporation of the solvent under reduced pressure,
the crude product was purified by column chromatography over silica
gel with dichloromethane/petroleum ether (v/v, 2/1) as eluent to afford
a yellow solid as the desired product 5 (0.448 g, 91% yield). 1H NMR
(600 MHz, DMSO-d6, δ): 9.92 (s, 1H), 8.26 (s, 1H), 7.78 (d, J = 5.4 Hz,
1H), 7.69 (d, J = 8.4 Hz, 2H), 7.50 (d, J = 8.4 Hz, 2H), 7.37 (d, J = 5.4 Hz,
1H), 7.28 (t, J = 8.4 Hz, 1H), 6.75 (d, J = 8.4 Hz, 2H), 3.93 (t, J = 6.3 Hz,
4H), 1.56 (m, 4H), 1.24 (m, 4H), 1.10 (m, 16H), 0.69 (t, J = 7.2 Hz,
6H). 13C NMR (150 MHz, DMSO-d6, δ): 184.16, 156.58, 147.22, 142.51,
140.32, 136.24, 132.58, 130.40, 129.13, 123.71, 122.97, 120.88, 118.38,
116.43, 112.32, 105.68, 68.12, 31.07, 28.62, 28.56, 25.41, 21.94, 13.68.
MS (ESI) m/z calcd. for C37H45NO3S2: 615.3. Found: 616.2 ([M+H]+).
Anal. calcd. for C37H45NO3S2: C, 72.16; H, 7.36; N, 2.27. Found: C, 72.28;
H, 7.45; N, 2.13.
After evaporation of the solvent under reduced pressure, the crude
product was purified by column chromatography over silica gel with
dichloromethane/petroleum ether (v/v, 2/1) as eluent to obtain a yellow
solid as the desired product 6 (0.192 g, 85% yield). 1H NMR (400 MHz,
DMSO-d6, δ): 9.92 (s, 1H), 8.23 (s, 1H), 7.68 (d, J = 8.4 Hz, 2H), 7.54 (s,
1H), 7.49 (d, J = 8.4 Hz, 2H), 7.28 (t, J = 8.4 Hz, 1H), 6.75 (d, J = 8.4 Hz,
2H), 3.93 (t, J = 6.0 Hz, 4H), 1.55 (m, 4H), 1.24 (m, 8H), 1.17 (m, 12H),
0.68 (t, J = 6.8 Hz, 6H). 13C NMR (150 MHz, CDCl3, δ): 182.94, 157.13,
145.68, 142.58, 140.88, 136.38, 133.67, 133.00, 129.05, 124.35, 121.57,
120.90, 118.69, 116.64, 115.75, 115.66, 105.34, 68.64, 31.75, 29.26, 29.17,
29.10, 26.00, 22.59, 14.01. MS (ESI) m/z calcd. for C37H44BrNO3S2:
693.2. Found: 694.4 ([M+H]+). Anal. calcd. for C37H44BrNO3S2: C, 63.96;
H, 6.38; N, 2.02. Found: C, 64.05; H, 6.47; N, 1.99.
6-{4-[N,N-Bis(4-hexyloxyphenyl)amino]phenyl}-4-(2′,6′-bis(octyloxy)
biphenyl-4-yl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole-2-carbaldehyde
(7) :
To a stirred solution of 6 (0.146 g, 0.210 mmol), 4,4,5,5-tetramethyl-
2-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}-1,3,2-dioxaborolane
(0.144 g, 0.252 mmol), and K3PO4 (0.233 g, 1.050 mmol) in dioxane/
H2O (6 mL, v/v, 5/1) was added Pd(OAc)2 (1 mg, 0.004 mmol) and
SPhos (2 mg, 0.004 mmol). The resulting mixture was stirred overnight
at room temperature under argon and then the solution was extracted
with ethyl acetate before the organic phase was washed with water and
dried over anhydrous sodium sulfate. After evaporation of the solvent
under reduced pressure, the crude product was purified by column
chromatography over silica gel with ethyl acetate/petroleum ether (v/v,
1/10) as eluent to obtain an orange solid as the desired product 7
(0.220 g, 99% yield). 1H NMR (400 MHz, DMSO-d6, δ): 9.89 (s, 1H),
8.21 (s, 1H), 7.72 (d, J = 8.4 Hz, 2H), 7.56 (d, J = 8.4 Hz, 2H), 7.56
(s, 1H), 7.49 (d, J = 8.4 Hz, 2H), 7.28 (t, J = 8.4 Hz, 1H), 7.04 (d, J =
8.8 Hz, 4H), 6.91 (d, J = 8.8 Hz, 4H), 6.76 (m, 4H), 3.93 (m, 8H), 1.70
(m, 4H), 1.55 (m, 4H), 1.41 (m, 4H), 1.31 (m, 8H), 1.18, (m, 12H), 1.08
(m, 8H), 0.88 (t, J = 6.8 Hz, 6H), 0.66 (t, J = 6.8 Hz, 6H). 13C NMR (100
MHz, CDCl3, δ): 182.72, 157.15, 155.84, 149.05, 148.76, 148.39, 142.88,
140.08, 139.92, 136.87, 133.13, 132.88, 128.97, 126.90, 126.39, 126.24,
125.44, 121.60, 120.81, 119.88, 118.82, 115.34, 114.81, 106.32, 105.35,
68.64, 68.26, 31.73, 31.59, 29.70, 29.30, 29.24, 29.15, 29.08, 25.97, 25.75,
22.59, 14.01. MS (ESI) m/z calcd. for C67H82N2O5S2: 1058.6. Found:
1059.7 ([M+H]+). Anal. calcd. for C67H82N2O5S2: C, 75.95; H, 7.80; N,
2.64. Found: C, 75.87; H, 7.91; N, 2.60.
2-Cyano-3-{6-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}-4-(2′,6′-
bis(octyloxy)biphenyl-4-yl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole-2-yl}acrylic acid
(C251) : To a stirred solution of 7 (0.200 g, 0.189 mmol) and cyanoacetic
acid (0.048 g, 0.566 mmol) in chloroform (10 mL) was added piperidine
(0.130 mL, 1.323 mmol). The resulting mixture was refluxed for 12 h
under argon and then cooled to room temperature before acidified with
2 M hydrochloric acid aqueous solution. The solution was extracted
with chloroform before the organic phase was washed with water and
dried over anhydrous sodium sulfate. After evaporation of the solvent
under reduced pressure, the crude product was purified by column
chromatography over silica gel with methanol/chloroform (v/v, 1/50)
as eluent to afford a purple-black powder as the desired product C251.
(0.190 g, 89% yield) 1H NMR (400 MHz, DMSO-d6, δ): 13.38 (s, 1H),
8.51 (s, 1H), 8.22 (s, 1H), 7.70 (d, J = 8.4 Hz, 2H), 7.55 (m, 3H), 7.49
(d, J = 8.4 Hz, 2H), 7.28 (t, J = 8.4 Hz, 1H), 7.04 (d, J = 8.8 Hz, 4H), 6.91
(d, J = 9.2 Hz, 4H), 6.76 (m, 4H), 3.93, (m, 8H), 1.70 (m, 4H), 1.55 (m,
4H), 1.41 (m, 4H), 1.31 (m, 8H), 1.15, (m, 20H), 0.88 (t, J = 6.8 Hz, 6H),
0.66 (t, J = 6.8 Hz, 6H). 13C NMR (100 MHz, CDCl3, δ): 168.99, 157.12,
155.91, 150.51, 150.22, 149.29, 148.54, 143.63, 139.91, 136.54, 133.34,
133.03, 132.76, 128.96, 128.42, 126.97, 126.44, 125.82, 122.03, 121.49,
119.61, 118.69, 116.78, 115.35, 115.01, 106.02, 105.30, 91.98, 68.63,
68.26, 31.73, 31.59, 29.69, 29.30, 29.24, 29.15, 29.06, 25.96, 25.74, 22.58,
14.01. HR-MS (ESI) m/z calcd. for C70H83N3O6S2: 1125.57233. Found:
1124.56597 ([M-H]−). Anal. calcd. for C70H83N3O6S2: C, 74.63; H, 7.43;
N, 3.73. Found: C, 74.52; H, 7.49; N, 3.66. IR-FTIR (KBr): 1677 cm−1
(COOH), 2214 cm−1 (CN).
4-(2′,6′-Bis(octyloxy)biphenyl-4-yl)-6-bromo-4H-dithieno[3,2-b:2′,3′-d]
pyrrole-2-carbaldehyde (6) : To a stirred solution of 5 (0.200 g, 0.325 mmol)
in THF (15 mL) at 0°C was added NBS (0.058 g, 0.325 mmol). The
resulting mixture was stirred at the same temperature under argon
for 24 h and then acetone (20 mL) was added to quench the reaction.
Spectral, Electrochemical, and XPS Measurements: Static electronic
absorption and vibration spectra were measured with an Agilent G1103A
spectrometer and a BRUKER Vertex 70 FTIR spectrometer, respectively.
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Transient absorption decay traces were recorded with a LP920 laser flash
photolysis spectrometer using a tunable OPOLett 355II laser to supply
the nanosecond excitation pulse. The ground-state redox potentials of
dyes grafted on titania were measured on a CHI660C electrochemical
workstation in conjunction with a three-electrode electrochemical cell
having a platinum gauze, a silver wire, and a dye-coated titania film on
FTO. All potentials were reported with Fc/Fc+ as the internal reference.
XPS spectra were recorded with an ESCALAB 250 spectrometer equipped
with a hemispherical electron energy analyzer using the Al Kα radiation
(hν = 1486.6 eV) and an energy step of 0.1 eV. The electron take-off
angle was 90°.
Computational Details: The ground state geometries were optimized
employing the hybrid B3LYP[50] function and 6-31G∗∗ basis set. The
solvent effect of THF on geometry has been simulated by the C-PCM
method.[51] Frequency analysis was also performed during the structure
optimization to ensure that there is no imaginary frequency. Vertical
excitation energies were calculated by TDDFT at the MPW1K/6-31G∗∗
levels.[52–54] All calculations were carried out with Gaussian 09 program
packages.[55]
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made via screen-printing on a pre-cleaned fluorine-doped tin oxide
(FTO) conducting glass (Nippon Sheet Glass, Solar, 4 mm thick) was
deployed as the negative electrode of DSCs presented in this paper. A
4.2-μm-thick translucent layer of 25 nm sized titania particles was first
deposited on a FTO glass and further covered with a 5-μm-thick light-
scattering layer of 350−450 nm sized titania particles (WER4-O, Dyesol).
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cobalt(III) tris[bis(trifluoromethanesulfonyl)imide], 0.5
M TBP, and
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Acknowledgements
N.C. and J.Z. contributed equally to this work. The National Science
Foundation of China (No. 91233206, No. 51125015, No. 51203150,
and No. 51103146), the National 973 Program (No. 2011CBA00702),
the National 863 Program (No. 2011AA050521), and the Key Scientific
and Technological Program of Jilin Province (No. 10ZDGG012) are
acknowledged for financial support.
Received: November 14, 2012
Revised: December 16, 2012
Published online:
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