Additional donor bridge as a design approach for multi-anchoring dyes for highly efficient dye-sensitized solar cells

Article abstract of DOI:10.1016/j.jphotochem.2017.10.033

This work presents an approach for designing efficient multi-anchoring dyes for dye-sensitized solar cells by using additional donor as a bridging group between donor–acceptor (D–A) side arms. With triarylamine as the additional donor and phenothiazine-cyanoacrylic acid as the D–A side arm, the multi-anchoring dyes exhibited higher molar extinction coefficients in the intramolecular charge transfer bands, less dye aggregation on the TiO₂ surface, and better DSSC performance than that of the mono-anchoring reference dye. The best cell efficiency of 7.06% was obtained from the di-anchoring dye with two additional donor units T2P2 showing the largest charge recombination resistance of 99.7 Ω, open circuit voltage of 0.83 V, and short-circuit photocurrent density of 12.72 mA cm^?2. The improvement in performance of the designed dyes suggests that additional donor is a potential bridging group for efficient multi-anchoring sensitizers.

Full text of DOI:10.1016/j.jphotochem.2017.10.033

Journal of Photochemistry and Photobiology A: Chemistry 352 (2018) 86–97
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Invited paper
Additional donor bridge as a design approach for multi-anchoring dyes
for highly efficient dye-sensitized solar cells
Utt Eiampraserta, Jutarat Sudchanhama, Panida Surawatanawonga,b
,
Pasit Pakawatpanuruta,b, Supavadee Kiatisevia,b,
*
a
Department of Chemistry and Center of Excellence for Innovation in Chemistry (PERCH-CIC), Faculty of Science, Mahidol University, 272 Rama VI Road,
Rajthevi, Bangkok 10400, Thailand
b
Center of Sustainable Energy and Green Materials, Faculty of Science, Mahidol University, Salaya, Putthamonthon, Nakhon Pathom 73170, Thailand
A R T I C L E I N F O
A B S T R A C T
Article history:
This work presents an approach for designing efficient multi-anchoring dyes for dye-sensitized solar cells
by using additional donor as a bridging group between donor–acceptor (D–A) side arms. With
triarylamine as the additional donor and phenothiazine-cyanoacrylic acid as the D–A side arm, the multi-
anchoring dyes exhibited higher molar extinction coefficients in the intramolecular charge transfer
bands, less dye aggregation on the TiO2 surface, and better DSSC performance than that of the mono-
anchoring reference dye. The best cell efficiency of 7.06% was obtained from the di-anchoring dye with
Received 18 August 2017
Received in revised form 3 October 2017
Accepted 19 October 2017
Available online xxx
Keywords:
DSSC
Multi-anchoring organic dye
Additional donor
Bridging
two additional donor units T2P2 showing the largest charge recombination resistance of 99.7
V, open
circuit voltage of 0.83 V, and short-circuit photocurrent density of 12.72 mA cmꢀ2. The improvement in
performance of the designed dyes suggests that additional donor is a potential bridging group for
efficient multi-anchoring sensitizers.
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
analogs in terms of light harvesting [15] and electron transfer
[11,16–18].
Molecular engineering of sensitizers for dye-sensitized solar
cells (DSSCs) offers an indispensable tool in an effort of developing
better molecular sensitizers for low-cost, environmentally friend-
ly, and highly efficient photovoltaic devices [1,2]. The organic
Owing to the long-wavelength absorption and the high
absorptivity, most of the reported multi-anchoring dyes gave
large photocurrent [10,19]. However, the low photovoltage caused
by the shift of TiO2 band-edge [20], charge recombination [21], and
donorꢀ
p
bridgeꢀacceptor (Dꢀ
p
ꢀA) structure has recently
strong molecular interaction of highly p-conjugated molecules
become a promising design approach for dye molecules because
of no use of toxic or expensive metal, simple synthesis, and large
structural variation compared with organometallic ruthenium(II)-
based dyes [3–5]. Although record efficiencies were achieved by
[22,23] limits the overall DSSC performance. Thus, various
molecular design strategies have been explored to solve the issue
[11,17,18,24–27]. Among a number of contributing factors that
influence the DSSC performance, the choice of a donor is especially
critical for the design of the multi-anchoring sensitizers [28,29].
using Dꢀ
p
ꢀA dyes [6,7], the long-term stability of DSSCs remains
a central issue for the practical use of these devices [8,9]. The D
For example, di-anchoring D(ꢀ
(ꢀ ꢀA)3 dyes with triarylamine donor frequently showed lower
open-circuit voltage (Voc) [20,21,30–32], while the phenothiazine-
p
ꢀA)2 and tri-anchoring
D
ꢀ
p
ꢀA sensitizers with a single binding contact at the TiO2 surface
p
generally contribute to weak surface adsorption of the dyes that
potentially leads to the dye leaching and subsequently the poor
long-term stability [10–13]. Using multi-anchoring dyes not only
results in stronger bonding to the TiO2 surface [11,14], but it also
provides additional advantages over their mono-anchoring
based D(ꢀ ꢀA)2 dyes containing an alkoxyphenyl group as a bulky
p
substituent at the nitrogen atom of the phenothiazine ring showed
better performance by suppressing dye aggregation and dark
current when compared to their mono-anchoring counterparts
and the benchmark N719 [24,25]. The attachment of
at the (ꢀ ꢀA)2 architecture has been
position in the Dꢀ
demonstrated to enhance electron injection and also suppress dark
current [18]. Nevertheless, such an extension of the -conjugation
further required additional co-adsorbent or a large bulky donor to
p
ꢀA branches
p
p p
* Corresponding author at: Department of Chemistry and Center of Sustainable
Energy and Green Materials, Faculty of Science, Mahidol University, 272 Rama VI
Road, Rajthevi, Bangkok 10400, Thailand.
p
E-mail address: supavadee.mon@mahidol.ac.th (S. Kiatisevi).
https://doi.org/10.1016/j.jphotochem.2017.10.033
1010-6030/© 2017 Elsevier B.V. All rights reserved.

U. Eiamprasert et al. / Journal of Photochemistry and Photobiology A: Chemistry 352 (2018) 86–97
87
prevent dye aggregation [26]. Moreover, employing the di-
2. Results and discussion
anchoring double Dꢀ
p
ꢀA sensitizers ((Dꢀ ꢀA)2) gave the PCE
p
enhancement of 45–94% compared to the corresponding mono-
2.1. Synthesis
anchoring sensitizer [11,17] which was the result of a cross-shaped
geometry of the (Dꢀ
p
ꢀA)2 dye leading to the weak intermolecular
All dyes were synthesized according to the reactions shown in
Scheme 1. Brominated intermediates (1 and 3aꢀd) were prepared
as described in the Supporting information. Phenothiazine and
triarylamine units were used to construct aldehydes 5aꢀd by
SuzukiꢀMiyaura cross coupling reaction as the key synthetic step.
Phenothiazine unit was synthesized from the reaction of 1 and
n-BuLi at ꢀ78 ꢁC and subsequently with N-formylmorpholine to
obtain 2 in 67% yield. Bromoarenes 3aꢀd were converted to the
corresponding boronate esters 4aꢀd by its reaction with bis
(pinacolato)diboron in the presence of palladium-Xphos catalytic
interaction. In contrast, the performances of most tri-anchoring
dyes were lower than the mono-anchoring analog [30–33].
According to the previous reports on mono-anchoring dyes in
DSSCs, the addition of an extra donor (D'), e.g., D'ꢀD ꢀ
p
ꢀA, could
improve the DSSC performance relative to the Dꢀ ꢀA analogs in
p
terms of dye aggregation, charge recombination [31,34], and long-
term stability [35,36]. Apart from the donor strength enhance-
ment, a theoretical study also revealed that the presence of an
additional secondary donor can be beneficial to further increasing
the photocurrent because of the long-range charge shift from the
terminal donor D' to the acceptor via a cascade effect [37].
Consistent with these findings, this work presents a new approach
for designing multi-anchoring dyes by adding D' at the bridging
position among two and three donor side arms. As shown in Fig. 1,
the core structure of the designed dyes consists of triarylamine as
additional donor (D'), phenothiazine as primary donor (D), and
cyanoacrylic acid as acceptor (A). Both donors were selected since
their non-planar geometries are known to reduce the molecular
aggregation [38–40]. The combination of phenothiazine and
arylamine as the double donor unit has been shown to increase
light harvesting ability [41–43]. All phenyl groups on triarylamine
were capped with electronically neutral methyl groups at the para-
position to reduce the reactivity in bromination reaction. The
series of the new organic dyes comprises two di-anchoring dyes
(TP2 and T2P2) and a tri-anchoring dye (TP3). Their DSSC
performances were compared with the mono-anchoring analog,
TP. TP2 and T2P2 consisting of three and four donor units,
respectively, were designed to investigate the effect of the number
of additional donors. Furthermore, the concept of using the double
donor in tri-anchoring, as in TP3, was investigated. All synthesized
dyes were characterized experimentally and computationally, and
their photoelectrochemical and photovoltaic characteristics in the
DSSC devices were also studied.
system. Without
a chromatographic purification, the crude
products of boronate esters were coupled with 2 to obtain the
corresponding aldehydes 5aꢀd. The aldehydes were reacted with
cyanoacetic acid in the presence of NH4OAc in acetic acid to give
the final dye products TP, TP2, T2P2, and TP3 in high yields. The
dye structures were confirmed by NMR and HRMS, and their NMR
spectra are shown in Figs. S1–S4 of the Supporting information.
2.2. Optical properties
The photophysical properties of all dyes were investigated by
UV–vis absorption spectroscopy in chloroform solution and on
TiO2 film. The spectra of dyes are shown in Fig. 2, and the
corresponding data are summarized in Table 1. Di- and tri-
anchoring dyes showed two separate absorption bands in nearly
identical regions as the reference mono-anchoring dye. The bands
in high-energy region (<400 nm) correspond to the
p p*
ꢀ
transitions of the conjugated backbone, and the low-energy bands
(>400 nm) can be assigned to the intramolecular charge transfer
(ICT) transitions. The absorption maxima (lmax) of the ICT bands
varied only in a small range (lmax 467–471 nm) for the multi-
anchoring dyes. In addition, the molar extinction coefficients (e) of
these ICT bands substantially increased in the order of TP
(1.37 ꢂ104 Mꢀ1 cmꢀ1), TP2 (3.16 ꢂ104 Mꢀ1 cmꢀ1), and TP3
(4.48 ꢂ 104 Mꢀ1 cmꢀ1), which are linearly dependent on the
Fig. 1. Molecular structures of di-anchoring (TP2, T2P2), tri-anchoring (TP3) and mono-anchoring (TP) dyes.

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U. Eiamprasert et al. / Journal of Photochemistry and Photobiology A: Chemistry 352 (2018) 86–97
Scheme 1. Synthetic routes of TP, TP2, T2P2, and TP3. Reagents and conditions: (a) n-BuLi, N-formylmorpholine, THF, ꢀ78 ꢁC; (b) bis(pinacolato)diboron, KOAc, Pd2(dba)3,
Xphos, THF or 1,4-dioxane, 85ꢀ100 ꢁC; (c) 2, K3PO4, Pd2(dba)3, Xphos, toluene or 1,4-dioxane, water, 100 ꢁC; (d) cyanoacetic acid, NH4OAc, AcOH, 100 ꢁC.
increasing number of phenothiazine units. The
e values appeared
to be comparable between TP2 and T2P2 (3.40 ꢂ104 Mꢀ1 cmꢀ1
)
that contain the same number of phenothiazine units. The results
suggest that the phenothiazine primary donor played an important
role in the hyperchromic shift of the absorption maxima—whereas
such a shift was scarcely affected by the increasing number of
triarylamine donor units. The dramatic enhancement of the
absorption intensity indicates better light-harvesting capability
of the multi-anchoring dyes.
In Fig. 2b, the normalized absorption spectra of the dyes coated
on TiO2 film show blue-shifted lmax compared to those in solution.
The blue shift is a result of the deprotonation of the dyes and/or the
formation of H-aggregation [44]. The formation of ꢀCOOꢀ
increased the LUMO energy level leading to a wider HOMOꢀ
LUMO gap and lower acceptor strength. The absorption of the
ꢀCOOꢀ form was investigated by recording the absorption spectra
in the presence of triethylamine to generate the fully deprotonated
species [8]. We found that the absorption maxima of the
deprotonated dyes showed a large blue shift (ꢀ43 to ꢀ45 nm) to
the wavelength region of 422ꢀ426 nm (Table S1) compared with
the lmax in chloroform. On the TiO2 surface, the blue shifts varied
from ꢀ49 nm (TP), ꢀ34 nm (TP2), ꢀ36 nm (T2P2), to ꢀ20 nm
(TP3). The fact that the smallest shift was observed in TP3 possibly
indicates a decreased tendency toward aggregation and a better
photovoltaic performance compared to the other dyes [45].
Furthermore, the spectra of the dyes adsorbed on TiO2 surface
showed broader absorption bands according to the large full-width
at half maximum (FWHM) of the absorption band (TP:148 nm,
TP2:166 nm, T2P2:170 nm,TP3:172 nm) compared to those in
solution (Table 1). The spectrum broadening of the absorption
bands on TiO2 may be attributed to strong intermolecular
interactions, such as dipolar and nonbonding interactions (e.g.
ꢀꢀSꢃꢃꢃNꢀꢀ) between dye molecules in the solid state [46].
Fig. 2. (a) UV–vis absorption spectra of dyes in chloroform. (b) Normalized
absorption spectra of dyes on transparent TiO2 film.

U. Eiamprasert et al. / Journal of Photochemistry and Photobiology A: Chemistry 352 (2018) 86–97
89
Table 1
solvation model using chloroform as the solvent parameter. The
hexyl group substituent was replaced by a methyl group to
maximize the calculation time efficiency. As shown in Fig. 3,
HOMO is uniformly distributed along the triphenylamine unit
extending toward the adjacent ring of phenothiazine, whereas
LUMO is distributed on the phenothiazine-substituted anchoring
unit. The calculated absorption spectra (see Fig. S5) showed the
blue shifts of absorption maxima in the ICT bands (393–397 nm,
Table 2) comparable to the experimental values and those reported
in the literature [55,56]. The calculated magnitudes of the
Photophysical properties of dyes.
Dye
in chloroform (10ꢀ5 M)
on TiO2 film
lmax/nm
lmax/nm (
e
/104 Mꢀ1 cmꢀ1
)
FWHM/nm
FWHM/nm
TP
466 (1.37)
471 (3.16)
470 (3.40)
114
104
106
108
417
437
434
447
148
166
170
172
TP2
T2P2
TP3
467 (4.48)
oscillator strength (f) are in the same trend as the
e values
2.3. Computational calculation
obtained from the experiment. The f values of the di- and tri-
anchoring dyes (TP2 and TP3) were 1.505 and 2.379, respectively,
which are almost 2- and 3-fold larger than that of the mono-
anchoring analog TP (f = 0.769). In addition, the ICT absorption
band of the dye containing a phenothiazine primary donor, e.g., TP,
exhibited the slightly enhanced f value and the blue-shift of the ICT
The absorption properties of all dyes were studied by the
density functional theory (DFT) using G09 software package [47].
The ground-state geometries were calculated by B3LYP [48–50]/6-
31G(d) [51–53] in gaseous phase and the time-dependent (TD) DFT
was performed by CAM-B3LYP/6-31G(d) [54] in the continuum
Fig. 3. Selected frontier molecular orbitals (FMO) of dyes (isodensity value = 0.03) by CAM-B3LYP/6-31G(d). The triarylamine and phenothiazine units were filled with blue
and pink, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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U. Eiamprasert et al. / Journal of Photochemistry and Photobiology A: Chemistry 352 (2018) 86–97
Table 2
The calculated electronic transition energies of the dyes in the intramolecular charge transfer bands by CAM-B3LYP/6-31G(d).
Dye
Transition
l
/nm
f
Orbital description
a
b
Type I
Type II
TP
TP2
S0!S1
S0!S1
S0!S2
S0!S1
S0!S2
397
397
394
397
396
0.769
0.155
1.350
0.439
1.037
Hꢀ1!L+ 0 (60%)
Hꢀ0!L+0 (29%)
Hꢀ0!L+0 (16%)
Hꢀ0!L+1 (16%)
Hꢀ1!L+1 (16%)
Hꢀ1!L+0 (15%)
Hꢀ1!L+1 (35%), Hꢀ2!L+0 (19%)
Hꢀ1!L+0 (39%), Hꢀ2!L+0 (20%)
Hꢀ3!L+1 (18%), Hꢀ2!L+0 (14%)
Hꢀ2!L+0 (24%), Hꢀ3!L+0 (17%),
Hꢀ2!L + 1 (12%)
T2P2
TP3
S0!S1
S0!S2
396
396
0.944
1.281
Hꢀ1!L+2 (34%), Hꢀ2!L+0 (13%)
Hꢀ2!L+0 (20%), Hꢀ2!L+1 (14%),
Hꢀ1!L+1 (13%)
–
–
S0!S3
393
0.154
Hꢀ1!L+1 (22%), Hꢀ2!L+0 (21%)
Hꢀ0!L+2 (11%)
a
Transitions from HOMO localizing on phenothiazine unit.
Transitions from HOMO localizing on triarylamine unit.
b
transition when compared with the mono-donor phenothiazine-
based analog [35] (see Fig. S8). This result shows good agreement
with the experimental data in that the ICT bands of the double
donor dyes were mainly influenced by the absorption of the
primary donor unit.
lower in energy than the first one has electrons distributed over
phenothiazine unit extending toward the anchoring group. In
terms of the percentage contribution, it was found that the
transitions from the pink-labeled MOs have the higher contribu-
tion than that of the transitions from the blue-labeled MOs levels.
These transitions are assigned in Table 2 as Type I and II,
respectively. The result implies that the electron transfer in this
system is more favorable from phenothiazine, the primary donor,
than from triarylamine, the additional donor, to cyanoacrylic acid
The calculation results also provided two relationships: (i)
between the number of anchoring group and the number of LUMO
degeneracy and (ii) between the number of electron donating
group and the number of occupied MOs associated with the ICT
transition (Scheme 2). For example, the degeneracy of two LUMOs
(LUMO and LUMO+1) was observed in both TP2 and T2P2 which
contain two anchoring units, whereas TP3 containing three
anchoring units displayed the degeneracy of three LUMOs (LUMO,
LUMO+1, and LUMO+2). The degeneracy facilitates transitions at
the similar energy as well as the charge injection since the LUMOs
are mainly localized around the anchoring groups, as shown in
Fig. 3. Notably, the number of the occupied MOs associated with
the ICT transitions related to the number of electron donating units
in dye molecules as follows: two donor units-two occupied MOs
(TP–HOMO and HOMO–1), three donor units-three occupied MOs
(TP2–HOMO, HOMO–1, and HOMO–2), and four donor units-four
occupied MOs (T2P2 and TP3–HOMO, HOMO–1, HOMO–2, and
HOMO–3).
acceptor. Similar observations were found in D0ꢀDꢀ
ꢀA dyes in
p
which the highest oscillator strength was contributed by the
charge transfer from the primary donor (D) to the acceptor [34,57].
The electron transfer from the primary donor to the acceptor
seems highly efficient because of the shorter distance and the
smaller dihedral angle of phenothiazine and cyanoacrylic acid
(0.9–2.5ꢁ) compared with those of triarylamine and phenothiazine
(33.9–35.7ꢁ), as shown in Fig. S6 of the Supporting information.
Although the introduction of triarylamine additional donor
showed no major contribution to the absorption in ICT band,
the large dihedral angle between arylꢀaryl rings, i.e., arylamineꢀ
phenothiazine and arylamineꢀarylamine, could aid in reducing
dye aggregation.
The frontier occupied MOs can be categorized into two sets on
the basis of their electronic distribution as shown in the frontier
molecular orbitals (Fig. 3) and their energy levels (Scheme 2). The
first set (blue-labeled HOMOs) provides the electron delocalization
around the triarylamine unit extending toward the adjacent phenyl
ring of phenothiazine as seen in the HOMOs of all dyes including
the HOMO–1 of T2P2. The second set (pink-labeled MOs) which is
2.4. Electrochemical properties
The possibilities of the electron transfer from dye to TiO2 and
the dye regeneration by the Iꢀ/I3 redox electrolyte were
ꢀ
investigated by cyclic voltammetry. The resulting cyclic voltammo-
grams are shown in Fig. S7 of the Supporting information, and the
electrochemical data are summarized in Table 3. The first oxidation
Scheme 2. Molecular orbital (MO) energy diagram of TP, TP2, T2P2, and TP3.

U. Eiamprasert et al. / Journal of Photochemistry and Photobiology A: Chemistry 352 (2018) 86–97
91
Table 3
Phillips’ rule [62–64]. In addition, the signal of the cyano group
n(CRN)) in both protonated and deprotonated forms appeared at
Electrochemical data of the dyes.
(
the same wavenumber (2220 cmꢀ1), indicating that this group was
not involved in the dye adsorption process. Notably, the absence of
free carboxylic group signals based on the FTIR data is a good
evidence of di- and tri-anchoring adsorption of TP2, T2P2, and TP3
on the TiO2 surface [65]. This may reflect the largest binding area of
TP3 on the TiO2 surface compared to the other dyes.
a
Dye
E0 /eV
ꢀ0
Eonset(ox)/V vs. Fc/Fc+
E/V vs. NHE
EHOMO
ELUMO
TP
2.21
2.22
2.20
2.22
0.36
0.34
0.29
0.36
0.99
0.97
0.92
0.99
ꢀ1.19
ꢀ1.22
ꢀ1.27
ꢀ1.21
TP2
T2P2
TP3
a
The optical energy gap, E0–0, was estimated from the onset absorption edge
lonset) of the dyes in solution. The energy (eV) was derived from the equation E0–0
2.6. Photovoltaic properties
(
(eV) = 1242.39/lonset [21].
The DSSCs based on the mono-, di-, and tri-anchoring organic
dyes and the benchmark dye N719 were fabricated, and their
photovoltaic performances were investigated under the standard
AM 1.5G condition (100 mW cmꢀ2). The current–voltage (J–V)
curves and the corresponding data are displayed in Fig. 5 and
Table 4, respectively. The T2P2-based device yielded the highest
potential (Eonset) was determined from the intersection of the
tangents of the baseline and the current signal [58], and the redox
potential was calibrated with ferrocene/ferrocenium (Fc/Fc+) as the
internal standard. The Eonset was converted to the ground-state
oxidation potential (EHOMO
) by equation: EHOMO (V) = Eonset
power conversion efficiency (
80% of that of N719. Under the same condition, TP2 yielded slightly
lower (6.60%), whereas TP3 and TP produced significantly lower
values of 4.78% and 3.41%, respectively. The large values of both
di-anchoring dyes can be attributed to high open-circuit voltage
(Voc) and high short-circuit current density (Jsc).
To investigate the correlation between the Jsc and light response
of the dyes, the incident photon-to-current efficiency (IPCE)
spectra of DSSCs fabricated using these dyes were recorded. As
shown in Fig. 6, the cells based on TP2 and T2P2 exhibited the
higher IPCE values in the range of 360ꢀ630 nm than those based on
h) of 7.06%, which is approximately
(V) + 0.63 [21]. By subtracting EHOMO with the optical energy gap
(E0–0), the corresponding excited state potential (ELUMO) was
obtained. The EHOMO values of TP, TP2, T2P2, and TP3 were
estimated to be 0.99, 0.97, 0.92, and 0.99 V vs. NHE, respectively.
These values are more positive than the redox potential of the Iꢀ/
I3ꢀ redox mediator (Eredox: 0.4 V vs. NHE) [21], indicating that the
regeneration of oxidized dyes is thermodynamically favorable. On
the other hand, the ELUMO values of all dyes are more negative than
that of the conduction band of TiO2 (ECB: ꢀ0.5 V vs. NHE). The
magnitudes of ECBꢀELUMO and EredoxꢀEHOMO gaps are larger than
the minimum requirements (0.2 V) for fast excited-state injection
and dye regeneration [59,60].
h
h
h
2.5. Infrared (IR) spectroscopy
The IR spectra of dye powder and dyes adsorbed on the
transparent TiO2 film were recorded to investigate the anchoring
properties (Fig. 4). The dye powder gave the signals of carboxylic
acid functional group consisting of the carbonyl stretching (n(C
¼
O)
ꢄ1690 cmꢀ1) and the stretching of carbonꢀoxygen bond (n(CꢀꢀOH)
ꢄ1245 cmꢀ1) [61]. In case of the dyes adsorbed on the TiO2 surface,
the signals of carboxylate functional group were observed. This
deprotonated form exhibited two vibrations of the carboxylate
group, which are the asymmetric stretching (n(COO )as) around
1600 cmꢀ1 and the symmetric stretching
(n(COO)s) around
1380 cmꢀ1 [14,21]. The smaller difference (ꢄ220 cmꢀ1) between
n(COO–)as and n(COO )s compared to that between n(C
and n(C OH)
¼O)
(ꢄ455 cmꢀ1) suggests that the dye molecules were adsorbed onto
Fig. 5. Current–voltage (J–V) curves of DSSCs based on different dyes.
the TiO2 surface via the bridging bidentate mode, as predicted by
Fig. 4. The IR spectra of (a) dye powder and (b) dyes adsorbed on the TiO2 film.

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U. Eiamprasert et al. / Journal of Photochemistry and Photobiology A: Chemistry 352 (2018) 86–97
Table 4
Photovoltaic parameters of the DSSCs using different dyes.a
Dye
h
(%)
Voc/V
Jsc/mA cmꢀ2
FF
Γ
/10ꢀ7 mol cmꢀ2
Jsc(mol)
b
/107 mA molꢀ1
TP
3.41 ꢅ 0.18
6.60 ꢅ 0.13
7.06 ꢅ 0.09
4.78 ꢅ 0.03
8.95 ꢅ 0.44
0.68 ꢅ 0.01
0.81 ꢅ 0.00
0.83 ꢅ 0.00
0.77 ꢅ 0.00
0.77 ꢅ 0.01
7.07 ꢅ 0.43
12.13 ꢅ 0.21
12.72 ꢅ 0.21
10.42 ꢅ 0.15
17.04 ꢅ 0.52
0.71 ꢅ 0.02
0.65 ꢅ 0.01
0.67 ꢅ 0.01
0.60 ꢅ 0.02
0.68 ꢅ 0.03
1.18
1.10
1.27
0.71
–
5.99
11.03
10.02
14.68
–
TP2
T2P2
TP3
N719
c
a
The performance parameters were averaged from five independent cells with 0.25 cm2 active area.
Short-circuit current density generated per mole of dye molecules was calculated using the equation Jsc(mol) = J
The concentration of 0.3 mM in ethanol was used.
b
c
sc/G.
TP3 and TP. These results were consistent with the trend of Jsc
(TP < TP3 < TP2 < T2P2). Interestingly, although the molar extinc-
tion coefficient of T2P2 at the wavelength region below 400 nm is
higher than that of TP2 (Fig. 2a), it was found that TP2 showed
higher IPCE values in that region than T2P2. However, the broader
IPCE curve toward the long wavelength region of T2P2 possibly
resulted in the comparable Jsc (TP2: 12.13 mA cmꢀ2
, T2P2:
12.73 mA cmꢀ2). The high Jsc observed for TP2 and T2P2 was
further evidenced from their high dye loading values (Γ) of
1.10 ꢂ10ꢀ7 mol cmꢀ2 and 1.27 ꢂ10ꢀ7 mol cmꢀ2, respectively. The
reverse phenomenon was observed for tri-anchoring dye (TP3) due
to the fact that a large TiO2 surface area is required for a successful
binding of the dye molecule. In addition, the relationship between
e
and the generated photocurrent of individual dye molecules was
determined by comparing the molar short-circuit current (Jsc(mol)
among different dyes. The highest Jsc(mol) was obtained with the
cell using TP3 (14.68 mA molꢀ1) in spite of its low
. These Jsc(mol)
values showed a similar trend to the values of the corresponding
)
Fig. 6. IPCE spectra of DSSCs based on different dyes.
h
e
dyes in that the Jsc(mol) of tri- and di-anchoring dyes gave almost
three- and two-fold, respectively, higher than the mono-anchoring
analog.
On comparing the di-anchoring dyes (TP2 and T2P2), it seems
that the introduction of an additional triarylamine donor in T2P2
did not benefit both light harvesting and Jsc. Herein, the high Voc of
T2P2 should be responsible for its high
h. Voc for the devices
follows an order: TP < TP3 < TP2 < T2P2. The highest Voc (0.83 V)
observed for the dye T2P2 containing bis(triarylamine) donor
(larger than N719, 0.77 V) is probably due to the bulkiness which
prevents the electrolyte penetration to the TiO2 surface [66]. The
TiO2/dye/electrolyte interfacial properties of the devices were
further studied using the Nyquist and Bode phase plots as shown in
Fig. 7. The measurements were carried out in the dark under
forward bias of ꢀ0.8 V in the range of 50,000ꢀ0.1 Hz which
resulted in three semicircles for TP and two semicircles for TP2,
T2P2, and TP3. The small semicircles (left region, 50,000–1000 Hz)
represent the charge transfer resistance at the counter electrode/
electrolyte interface, while the large semi-circles (middle region,
1000–1 Hz) correspond to the charge transfer resistance (Rct) at the
TiO2/dye/electrolyte (Table 5). The third semicircle at the lowest
frequency range (right region, 1–0.1 Hz) in TP can be attributed to
the diffusion resistance in the electrolyte. The calculated Rct values
assumed an order: TP (21.1 ꢅ1.1
V
) < TP2 (67.6 ꢅ 6.2
V
) ꢆ TP3
(72.8 ꢅ 1.6 ) < T2P2 (99.7 ꢅ1.7
V
V). The high Rct observed for T2P2
reflects the slow charge recombination and thus the high Voc, and
could be a result of the high T2P2 dye loading, which led to the
formation of a dense organic monolayer that impeded the
electrolyte approaching the TiO2 surface [67]. Despite the low
dye loading, TP3 also showed the high Rct possibly as
a
consequence of the high number of alkyl chains which makes it
effective in preventing the iodide ions from the semiconductor
surface [68].
Fig. 7. Electrochemical impedance spectra: (a) Nyquist plot with the equivalent
circuit and (b) Bode phase plot for DSSCs based on different dyes under dark
conditions.
The electron lifetime (te) calculated from the Rct and the
chemical capacitance (C ) follows the order: TP (17.0 ꢅ1.3 ms) <
m

U. Eiamprasert et al. / Journal of Photochemistry and Photobiology A: Chemistry 352 (2018) 86–97
93
Table 5
were recorded with an HR-TOF-MS Micromass model VQTOF2
mass spectrometer. UV–vis spectra were recorded on a AGILENT
CARY 60 UV–vis spectrophotometer. Attenuated total reflection
Fourier transform infrared (ATR-FTIR) spectra were recorded on
Bruker ALPHA FTIR spectrometer. The cyclic voltammetry was
performed on a computer–controlled AUTOLAB potentiostat/
galvanostat with NOVA 1.5 software with the conventional
three-electrode system such as platinum counter electrode, Ag/
AgCl pseudo-reference electrode, and glassy carbon working
electrode. The measurements were carried out in THF solution
containing 0.1 mM dye and 0.1 M tetra-n-butylammonium hexa-
fluorophosphate (TBAPF6) as the supporting electrolyte. The scan
Electrochemical parameters of the DSSCs based on different dyes.a
a
b
c
Dye
R
/
ct V a
C
m
/mF
te/ms
te/ms
TP
21.1 ꢅ 1.1
67.6 ꢅ 6.2
99.7 ꢅ 1.7
72.8 ꢅ 1.6
0.81 ꢅ 0.05
0.82 ꢅ 0.06
0.74 ꢅ 0.04
1.16 ꢅ 0.11
17.0 ꢅ 1.3
55.1 ꢅ 4.2
73.5 ꢅ 5.0
84.8 ꢅ 9.7
4.9 ꢅ 0.7
17.8 ꢅ 2.3
24.6ꢅ 3.7
23.3 ꢅ 3.0
TP2
T2P2
TP3
a
Charge transfer resistance (Rct) and chemical capacitance (C ) were derived
m
from fitting the largest semi-circle of the Nyquist plot with the equivalent circuit
represented in Fig. 7a. The data were averaged from three independent cells.
b
The electron lifetime (te) was calculated from the equation te = Rct ꢂ C . Both Rct
m
and C were obtained from the Nyquist plot.
m
c
The electron lifetime (te) was calculated from the equation te = 1/2pf. The
rate was 10 mV sꢀ1
.
frequency (f) was obtained by fitting the middle frequency of the Bode phase plot
shown in Fig. 7b. The data were averaged from five independent cells.
4.2. Synthesis
TP2 (55.1 ꢅ4.2 ms) < T2P2 (73.5 ꢅ 5.0 ms) ꢆ TP3 (84.8 ꢅ 9.7 ms)
which increased with increasing number of donor units and
anchoring moieties. This trend is consistent with the order
obtained from the Bode phase plot. The significantly increased
lifetime of an electron for the dyes T2P2 and TP3 was caused by the
suppression of the recombination of electrons due to the
hydrophobic repulsion exerted by both triarylamine and pheno-
thiazine segments [66]. Compared with the di-anchoring dyes, TP3
exhibited the lower Voc in spite the highest electron lifetime. This
was possibly affected by higher proton releasing of the adsorbed
tri-anchoring dyes, leading to the positive shift of the TiO2
conduction band [20].
4.2.1. 7-bromo-10-hexyl-10H-phenothiazine-3-carbaldehyde (2)
To a solution of 1 (6.07 g, 13.6 mmol) in THF (20 mL), a solution
of 1.6 M n-BuLi in hexane (8.68 mL, 13.89 mmol) was added
dropwise via syringe at ꢀ78 ꢁC under argon atmosphere. The
reaction was stirred at this temperature for 30 min and N-
formylmorpholine (1.40 mL,13.9 mmol) was added dropwise. After
stirred at ꢀ78 ꢁC for 15 min, the reaction was allowed to warm up
to room temperature. A 1 M HCl solution was added to adjust pH
(1–3). The organic phase was separated, washed with brine, dried
over anhydrous Na2SO4 and evaporated to dryness. The residue
was purified by silica gel column chromatography (10–50% CH2Cl2
in hexanes) to afford 3.6 g (67%) of 2 as orange solid. 1H NMR
(400 MHz, CDCl3): d 9.78 (s,1H), 7.63 (dd, J = 8.4, 1.9 Hz,1H), 7.54 (d,
3. Conclusion
J = 1.9 Hz, 1H), 7.23 (dd, J = 8.6, 2.3 Hz, 1H), 7.19 (d, J = 2.2 Hz, 1H),
6.88 (d, J = 8.4 Hz, 1H), 6.69 (d, J = 8.6 Hz, 1H), 3.82 (t, J = 7.2 Hz, 2H),
1.80–1.73 (m, 2H), 1.45–1.38 (m, 2H), 1.30–1.27 (m, 4H), 0.87 (t,
Three multi-anchoring dyes (TP2, T2P2, and TP3) containing
triarylamine as the additional donor and the bridging group were
designed and synthesized to demonstrate the significance of the
double donor unit toward the DSSC performance compared to the
mono-anchoring type (TP). These multi-anchoring dyes produced
strong hyperchromic shift of the ICT absorption. Although the di-
anchoring dyes showed good conversion efficiencies of 6.60–7.06%,
even better DSSC performance was found when T2P2 bearing two
additional donors was used as sensitizer, which was due to its high
Voc and high charge transfer resistance. Despite the long electron
lifetime, the cell efficiency of the tri-anchoring dye TP3 was limited
by reduced dye loading as a result of its large binding area on the
TiO2 surface. These results suggest that an additional donor unit
can be a potential bridging group between the side arms of multi-
anchoring dyes. Further modification of multi-anchoring dye
structures through the introduction of more efficient component is
therefore indispensible to achieve both high efficiency and long-
term stability for practical DSSC applications.
J = 6.9 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d
189.99, 150.36, 142.69,
131.31, 130.37, 130.31, 129.82, 128.46, 126.15, 124.39, 117.16, 115.85,
115.06, 48.16, 31.42, 26.66, 26.52, 22.64, 14.07. IR: 2962, 1683
(C
O), 1593, 1459, 1241, 1193, 798 cmꢀ1. HRMS (ESI) m/z: calcd. for
19H2079BrNNaOS [M + Na]+, 412.0347; found, 412.0348.
n
¼
C
4.2.2. 4-methyl-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
phenyl)-N-(p-tolyl)aniline (4a)
A mixture of 3a (352 mg, 1.00 mmol), bis(pinacolato)diboron
(257 mg, 1.01 mmol), Pd2(dba)3 (dba = dibenzylideneacetone;
14 mg, 0.015 mmol), 2-dicyclohexylphosphino-20,40,60-triisopro-
pylbiphenyl (Xphos; 29 mg, 0.061 mmol), KOAc (0.29 g, 3.0 mmol)
and THF (3 mL) under argon atmosphere was heated at 85 ꢁC for
2 h. After cooled down to room temperature, the reaction was
diluted with EtOAc (10 mL) and washed with water (2 ꢂ 20 mL).
The organic layer was dried over anhydrous Na2SO4 and
evaporated in vacuum to afford the crude product of 4a which
was used in the further step.
4. Experimental details
4.2.3. 4-methyl-N,N-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)phenyl)aniline (4b)
4.1. Materials and methods
Following the procedure described for 4a, a mixture of 3b
(417 mg, 1.00 mmol), bis(pinacolato)diboron (513 mg, 2.02 mmol),
Pd2(dba)3 (27.5 mg, 0.03 mmol), Xphos (57.2 mg, 0.12 mmol) and
KOAc (590 mg, 6.0 mmol) was heated to 85 ꢁC in THF (6 mL) with
stirring for 2 h. The crude product 4b was used in the further step
without chromatographic purification.
All reagents were used as purchased. Tetrahydrofuran (THF)
was distilled from sodium-benzophenone. Toluene was dried with
CaH2 and stored under 4Å molecular sieve. 1,4-Dioxane was
distilled from sodium. Column chromatography was carried out on
Merck silica gel 60 (70–230 mesh). Nuclear Magnetic Resonance
(NMR) spectra were recorded on Bruker AscendTM400 Spectrome-
0
4.2.4. N4,N4 -bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
ter. The chemical shift (d) was reported in parts per million (ppm)
40
phenyl)-N4,N -di-p-tolyl-[1,10-biphenyl]-4,40-diamine (4c)
relative to the signals for the residual undeuterated solvent peaks
[69]. The coupling constant (J) was reported in hertz (Hz) and
multiplicity of the signals was indicated as “s”, “d”, “dd”, “t”, “m”, or
“br” for singlet, doublet, doublet of doublets, triplet, multiplet, or
broad signal, respectively. High-resolution mass spectra (HRMS)
Following the procedure described for 4a, a mixture of 3c
(337 mg, 0.5 mmol), bis(pinacolato)diboron (257 mg, 1.01 mmol),
Pd2(dba)3 (13.7 mg, 0.015 mmol), Xphos (28.6 mg, 0.06 mmol) and
KOAc (295 mg, 3.0 mmol) was heated to 85 ꢁC in THF (3 mL) with

94
U. Eiamprasert et al. / Journal of Photochemistry and Photobiology A: Chemistry 352 (2018) 86–97
stirring for 2 h. The crude product 4c was used in the further step
without chromatographic purification.
16H), 6.91 (dd, J = 8.6, 2.7 Hz, 4H), 3.90 (t, J = 7.3 Hz, 4H), 2.34 (s, 6H),
1.88–1.80 (m, 4H), 1.48–1.43 (m, 4H), 1.34–1.31 (m, 8H), 0.89
(t, J = 6.9 Hz, 6H). 13C NMR (100 MHz, CDCl3):
d 190.15, 150.62,
4.2.5. tris(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)
amine (4d)
147.32, 146.74, 144.97, 142.12, 136.37, 134.76, 133.37, 133.24, 131.07,
130.30, 130.20, 128.53, 127.41, 127.23, 125.73, 125.49, 125.36,
124.63, 124.11, 123.99, 123.59, 116.19, 114.77, 48.22, 31.52, 26.82,
Following the procedure described for 4a, a mixture of 3d
(482 mg, 1.00 mmol), bis(pinacolato)diboron (838 mg, 3.30 mmol),
Pd2(dba)3 (13.7 mg, 0.015 mmol), Xphos (29.2 mg, 0.061 mmol) and
KOAc (883 mg, 9.00 mmol) was heated at 100 ꢁC in anhydrous 1,4-
dioxane (12 mL) for 16 h. The crude product 4d was used in the
further step without chromatographic purification.
26.66, 22.72, 21.03, 14.13. IR:
1460, 1318, 1242, 1194, 809 cmꢀ1. HRMS (ESI) m/z: calcd. for
76H70N4O2S2Na [M + Na]+, 1157.4838; found, 1157.4834.
n
2917, 1685 (C¼O), 1599, 1578, 1490,
C
4.2.9. 7,70,700-(nitrilotris(benzene-4,1-diyl))tris(10-hexyl-10H-
phenothiazine-3-carbaldehyde) (5d)
4.2.6. 7-(4-(di-p-tolylamino)phenyl)-10-hexyl-10H-phenothiazine-3-
carbaldehyde (5a)
Following the procedure described for 5a, a mixture of 4d,
Pd2(dba)3 (13.7 mg, 0.015 mmol), Xphos (29 mg, 0.06 mmol), K3PO4
(1.06 g, 5.00 mmol), 2 (1.21 mg, 3.10 mmol), 1,4-dioxane (10 mL)
and water (1 mL) was heated to 100 ꢁC under argon atmosphere.
The crude product was purified by column chromatography
A solution of 2 (390 mg, 1.0 mmol) in dried toluene (2.5 mL) was
added to a mixture of the 4a, Pd2(dba)3 (9.2 mg, 0.01 mmol), Xphos
(19.1 mg, 0.04 mmol), K3PO4 (530 mg, 2.5 mmol) under argon
atmosphere. The mixture was stirred at room temperature for
5 min prior to addition of pure water (0.5 mL). The reaction was
heated to 100 ꢁC for 4 h. After completion of the reaction, the
mixture was diluted with EtOAc. The organic phase was washed
with brine and dried over anhydrous Na2SO4. The crude product
was purified by EtOAc/hexanes (5:95 v/v) to afford 0.36 g (65%) of a
(CH2Cl2) to provide 0.78 g (67%) of
a bright orange solid.
1H NMR (400 MHz, CDCl3):
d
9.78 (s, 3H), 7.63 (dd, J = 8.4, 1.9 Hz,
3H), 7.58 (d, J = 1.9 Hz, 3H), 7.43 (d, J = 8.7 Hz, 6H), 7.36 (dd, J = 8.5,
2.1 Hz, 3H), 7.31 (d, J = 2.1 Hz, 3H), 7.17 (d, J = 8.7 Hz, 6H), 6.91ꢀ6.88
(m, 6H), 3.89 (t, J = 7.2 Hz, 6H), 1.87ꢀ1.79 (m, 6H), 1.47ꢀ1.42
(m, 6H), 1.33ꢀ1.31 (m, 12H), 0.88 (t, J = 7.1 Hz, 9H). 13C NMR
bright orange solid. 1H NMR (400 MHz, CDCl3):
d
9.79 (s, 1H), 7.65
(100 MHz, CDCl3): d 190.09, 150.53, 146.76, 142.29, 136.15, 134.16,
(dd, J = 8.4,1.9 Hz,1H), 7.59 (d, J = 1.9 Hz,1H), 7.36–7.32 (m, 3H), 7.29
(d, J = 2.1 Hz, 1H), 7.09–7.01 (m, 10H), 6.89 (d, J = 8.5 Hz, 2H), 3.89
(t, J = 7.3 Hz, 2H), 2.32 (s, 6H), 1.87–1.80 (m, 2H), 1.59–1.42 (m, 2H),
1.34–1.31 (m, 4H), 0.88 (t, J = 8.0 Hz, 3H). 13C NMR (100 MHz,
131.10, 130.33,128.46, 127.41, 125.79, 125.53,124.59,124.55, 124.18,
116.20, 114.79, 48.21, 31.51, 26.81, 26.64, 22.70, 14.11. IR: 2921,
n
1681 (C
O), 1598, 1578, 1462, 1241, 1193, 804 cmꢀ1. HRMS (ESI) m/
¼
z: calcd. for C75H72N4O3S3 [M]+, 1172.4767; found 1172.4760.
CDCl3):
d 190.16, 150.66, 147.73, 145.27, 142.01, 136.50, 132.82,
132.54,131.06,130.28,130.05,128.56,127.10,125.67,125.45,124.85,
124.66, 124.07, 122.73, 116.17, 114.76, 48.22, 31.53, 26.83, 26.67,
4.2.10. (E)-2-cyano-3-(7-(4-(di-p-tolylamino)phenyl)-10-hexyl-10H-
phenothiazin-3-yl)acrylic acid (TP)
22.72, 20.97, 14.12. IR:
n
2917, 1682 (C
¼
O), 1600, 1578, 1505, 1463,
A mixture of compound 5a (100 mg, 0.17 mmol), cyanoacetic
acid (73 mg, 0.86 mmol) and NH4OAc (66 mg, 0.86 mmol) under
argon atmosphere was heated to 100 ꢁC in a degassed glacial acetic
acid (8 mL) for 5 h. After cooled down to room temperature, excess
water was poured into the reaction flask and stirred for additional
2 h. The precipitate was subjected to vacuum filtration and the
solid was dried by heating at 50 ꢁC under reduced pressure. 106 mg
(95%) of a brown solid was obtained. 1H NMR (400 MHz, (CD3)2CO):
1319, 1241, 1193, 810 cmꢀ1. HRMS (ESI) m/z: calcd. for C39H39N2OS
[M + H]+, 583.2783; found, 583.2788.
4.2.7. 7,7'-((p-tolylazanediyl)bis(4,1-phenylene))bis(10-hexyl-10H-
phenothiazine-3-carbaldehyde) (5b)
Following the procedure described for 5a, a mixture of 4b,
Pd2(dba)3 (18 mg, 0.02 mmol), Xphos (38 mg, 0.08 mmol), K3PO4
(1.1 g, 2.5 mmol) and 2 (780 mg, 2.0 mmol) was heated to 100 ꢁC
under argon atmosphere in toluene (5 mL) and water (1 mL). The
crude product was purified by column chromatography (80–90%
CH2Cl2/hexanes) to afford 0.35 g (40%) of a bright orange solid. 1H
d
8.13 (s, 1H), 7.94 (dd, J = 8.7, 2.1 Hz, 1H), 7.86 (d, J = 2.1 Hz, 1H), 7.50
(d, J = 8.7 Hz, 2H), 7.46 (dd, J = 8.5, 2.2 Hz,1H), 7.37 (d, J = 2.1 Hz,1H),
7.16ꢀ7.10 (m, 6H), 7.00ꢀ6.96 (m, 6H), 4.03 (t, J = 7.2 Hz, 2H), 2.29 (s,
6H), 1.87ꢀ1.79 (m, 2H), 1.52ꢀ1.45 (m, 2H), 1.37ꢀ1.28 (m, 4H), 0.85
NMR (400 MHz, CDCl3):
d
9.78 (s, 2H), 7.63 (dd, J = 8.4, 1.9 Hz, 2H),
(t, J = 7.1 Hz, 3H). 13C NMR (100 MHz, (CD3)2CO):
d 164.25, 153.61,
7.58 (d, J = 1.9 Hz, 2H), 7.39 (d, J = 8.6 Hz, 4H), 7.35 (dd, J = 8.5, 2.1 Hz,
2H), 7.30 (d, J = 2.1 Hz, 2H), 7.12–7.05 (m, 8H), 6.91–6.88 (m, 4H),
3.89 (t, J = 7.2 Hz, 4H), 2.34 (s, 3H), 1.87–1.79 (m, 4H), 1.49–1.42 (m,
4H), 1.34–1.31 (m, 8H), 0.88 (t, J = 6.9 Hz, 6H). 13C NMR (100 MHz,
150.09, 148.49, 146.04, 142.56, 136.86, 133.54, 133.05, 132.62,
130.82,130.49, 130.18, 127.74, 126.72,126.37, 125.53 (2 ꢂ C), 124.63,
124.22, 123.19, 117.42, 117.20, 116.20, 100.14, 48.35, 32.11, 27.30,
27.02, 23.27, 20.83, 14.27. IR:
n
2921, 2217 (CRN), 1694 (C O),
¼
CDCl3):
d
190.12, 150.59, 147.19, 144.86, 142.16, 136.31, 133.52,
1564, 1505, 1469, 1293, 1245 (CꢀꢀOH), 1198, 817 cmꢀ1. HRMS (ESI)
m/z: calcd. for C42H39N3O2S [M]+, 649.2763; found, 649.2765.
133.44,131.08,130.31,130.23,128.50,127.26,125.74,125.49,125.45,
124.62, 124.13, 123.76, 116.19, 114.77, 48.21, 31.52, 26.81, 26.65,
22.71, 21.03, 14.12. IR:
n
2919, 1681 (C
¼
O), 1599, 1578, 1460, 1241,
4.2.11. (2E,20E)-3,30-(((p-tolylazanediyl)bis(4,1-phenylene))bis(10-
hexyl-10H-phenothiazine-7,3-diyl))bis(2-cyanoacrylic acid) (TP2)
Following the procedure described for TP, a mixture of 5b
(100 mg, 0.11 mmol), cyanoacetic acid (97 mg, 1.1 mmol), NH4OAc
(88 mg, 1.1 mmol) in a degassed acetic acid (5 mL) was heated at
110 ꢁC for 2 h. 113 mg (98%) of a dark red powder was obtained. 1H
1193, 806 cmꢀ1. HRMS (ESI) m/z: calcd. for C57H55N3O2S2 [M]+,
877.3736; found, 877.3738.
4.2.8. 7,7'-((p-tolylazanediyl)bis(4,1-phenylene))bis(10-hexyl-10H-
phenothiazine-3-carbaldehyde) (5c)
Following the procedure described for 5a, a mixture of 4c,
Pd2(dba)3 (9.2 mg, 0.01 mmol), Xphos (19 mg, 0.04 mmol), K3PO4
(0.53 g, 2.5 mmol) and 2 (0.39 g, 1.0 mmol) under argon atmo-
sphere was heated to 100 ꢁC in toluene (2.5 mL) and water (0.5 mL).
The crude product was purified by chromatography (CH2Cl2/
hexanes, 4:1 v/v) to afford 0.18 g (32%) of a bright orange solid.
NMR (400 MHz, (CD3)2CO): d 8.16 (s, 2H), 7.98 (dd, J = 8.7, 2.0 Hz,
2H), 7.89 (d, J = 2.0 Hz, 2H), 7.58 (d, J = 8.6 Hz, 4H), 7.50 (dd, J = 8.5,
2.0 Hz, 2H), 7.42 (d, J = 2.0 Hz, 2H), 7.20–7.15 (m, 6H), 7.11
(d, J = 8.6 Hz, 4H), 7.05 (d, J = 8.3 Hz, 2H), 4.06 (t, J = 7.1 Hz, 4H),
2.34 (s, 3H),1.91–1.83 (m, 4H),1.53–1.48 (m, 4H),1.38–1.30 (m, 8H),
0.88 (t, J = 7.0 Hz, 6H). 13C NMR (100 MHz, (CD3)2CO):
d 164.16,
1H NMR (400 MHz, CDCl3):
d
9.79 (s, 2H), 7.64 (dd, J = 8.4, 1.9 Hz,
153.60, 149.99, 147.91, 145.62, 142.59, 136.69, 134.23, 133.92,
132.55,131.03,130.24,127.91, 126.69,126.48,126.21,125.61,124.59,
124.37, 124.17, 117.37, 117.20, 116.15, 100.06, 48.41, 32.13, 27.30,
2H), 7.59 (d, J = 1.9 Hz, 2H), 7.45 (d, J = 8.6 Hz, 4H), 7.40 (d, J = 8.6 Hz,
4H), 7.35 (dd, J = 8.5, 2.1 Hz, 2H), 7.31 (d, J = 2.1 Hz, 2H), 7.14–7.09 (m,

U. Eiamprasert et al. / Journal of Photochemistry and Photobiology A: Chemistry 352 (2018) 86–97
95
27.04, 23.29, 20.96, 14.31. IR:
n
2922, 2219 (CRN), 1685 (C
¼
O),
The sintered film was treated with the TiCl4 solution and annealed
again. After cooling down to 80 ꢁC, the substrate was immersed
into a 0.2 mM dye bath in acetone solution and maintained in the
dark at room temperature for 24 h. The dye-adsorbed electrode
was rinsed with acetone and dried. To prepare the counter
electrode, a small hole was drilled on the FTO glass for electrolyte
injection. The perforated glass was cleaned in an ultrasonic bath
using the same procedure as mentioned above. Pt catalyst
(Platisol T/SP, Solaronix) was deposited on the glass substrate by
the screen printing method and annealed at 500 ꢁC for 15 min. The
working and counter electrodes were assembled into a sandwich
type cell and sealed with Surlyn (Meltonix 1170-25, Solaronix). The
redox electrolyte containing 1 M 1-methyl-3-propylimidazolium
iodide, 0.03 M I2, 0.05 M LiI, 0.5 M 4-tert-butylpyridine, 0.1 M
guadinium thiocyanate in a mixture of acetonitrile and valeroni-
trile (85:15 v/v) was injected to the cell via vacuum backfilling. The
hole was sealed using the sealing film (Meltonix 1170-25,
Solaronix) and covered with a thin glass slide. Finally, silver paint
was spread on the electrode to enhance the electrical contact. The
current–voltage (J–V) characteristics were recorded with the
Keithley current source model 2420 under AM 1.5G illumination
of 100 mW cmꢀ2. The IPCE spectra of the device under short-circuit
conditions were performed with an Oriel 150 W Xe lamp fitted
with a Cornerstone 130 1/8 m monochromator as a monochromatic
light source, a Newport 818-UV silicon photodiode as power
density calibration, and a Keithley 6485 picoammeter. All IPCE
measurements were obtained using a black plastic mask with an
aperture area of 0.16 cm2 and no mismatch correction for the
conversion efficiency data. The EIS measurements were carried out
in the dark (50,000–0.1 Hz) with an AUTOLAB potentiostat/
1569, 1464, 1286, 1244 (CꢀꢀOH), 1170, 803 cmꢀ1. HRMS (ESI) m/z:
calcd. for C63H58N5O4S2 [M + H]+, 1012.3930; found, 1012.3929.
4.2.12. (2E,20E)-3,30-((([1,10-biphenyl]-4,40-diylbis(p-tolylazanediyl))
bis(4,1-phenylene))bis(10-hexyl-10H-phenothiazine-7,3-diyl))bis(2-
cyanoacrylicacid) (T2P2)
Following the procedure described for TP, a mixture of 5c
(100 mg, 0.09 mmol), cyanoacetic acid (74 mg, 0.88 mmol) and
NH4OAc (68 mg, 0.88 mmol) in a degassed acetic acid (5 mL) was
heated at 110 ꢁC overnight. 98 mg (88%) of a dark red powder was
obtained. 1H NMR (400 MHz, (CD3)2SO):
d 8.15 (s, 2H), 7.91 (d,
J = 8.6 Hz, 2H), 7.82 (d, J = 1.6 Hz, 2H), 7.57ꢀ7.54 (m, 8H), 7.48 (dd,
J = 8.6, 1.8 Hz, 2H), 7.42 (d, J = 1.6 Hz, 2H), 7.18ꢀ7.16 (m, 6H), 7.12 (d,
J = 8.8 Hz, 2H), 7.04ꢀ7.00 (m, 12H), 3.96 (t, J = 6.6 Hz, 4H), 2.28 (s,
6H), 1.72ꢀ1.69 (m, 4H), 1.40 (br, 4H), 1.29ꢀ1.25 (m, 8H), 0.83 (t,
J = 6.8 Hz, 6H). 13C NMR (100 MHz, (CD3)2SO):
d 163.77, 152.22,
148.39, 146.49, 146.03, 144.18, 141.24, 134.97, 133.58, 132.96, 132.17,
131.53, 130.15, 128.98, 126.96, 126.89, 125.50, 125.45, 125.01,
124.39, 123.33, 122.95, 122.71, 122.56, 116.87, 116.49, 115.39, 99.77,
46.98, 30.78, 26.01, 25.69, 22.05, 20.41, 13.79. IR:
n 2919, 2220
(CRN), 1703 (C
¼O), 1556, 1491, 1461, 1317, 1243 (C ꢀ OH), 1169,
807 cmꢀ1. HRMS (ESI) m/z: calcd. for C82H73N6O4S2 [M + H]+,
1269.5135; found, 1269.5141.
4.2.13. (2E,20E,2”E)-3,30,3”-((nitrilotris(benzene-4,1-diyl))tris(10-
hexyl-10H-phenothiazine-7,3-diyl))tris(2-cyanoacrylic acid) (TP3)
Following the procedure described for TP, a mixture of 5d
(102 mg, 0.087 mmol), cyanoacetic acid (110 mg, 1.31 mmol) and
NH4OAc (100 mg, 1.31 mmol) in degassed acetic acid (10 mL) was
heated at 100 ꢁC for 5 h. 100 mg (84%) of a black solid was obtained.
galvanostat using FRA software. The dye loading (Γ) was estimated
1H NMR (400 MHz, (CD3)2CO):
d
8.06 (s, 3H), 7.88 (d, J = 7.9 Hz, 3H),
from UV–vis spectra of a known volume of the solution of desorbed
dyes in 0.1 M NaOH in THF/H2O (1:1 v/v). The amount of desorbed
dye was calculated by comparing with the spectrum of freshly
prepared dye in the same solvent at known concentration [71]. The
TiO2 films without the scattering layer were prepared for recording
the UV–vis and IR absorption spectra of the adsorbed dyes. The
UV–vis and the ATR-FTIR absorption spectra of the adsorbed dyes
were obtained by subtracting the spectrum of the blank TiO2 film
from the spectrum of the dye-coated film.
7.70 (s, 3H), 7.41 (d, J = 7.6 Hz, 6H), 7.30 (d, J = 7.4 Hz, 3H), 7.16 (s, 3H),
7.03ꢀ6.98 (m, 9H), 6.93 (d, J = 8.2 Hz, 3H), 3.87 (br, 6H), 1.73 (br,
6H), 1.38ꢀ1.23 (m, 18H), 0.80 (t, J = 6.7 Hz, 9H). 13C NMR (100 MHz,
(CD3)2CO):
d 164.14, 153.66, 149.90, 147.49, 142.62, 136.50, 134.65,
132.50,130.32,128.11,126.67,126.58,125.67,125.26,124.55,124.15,
117.32, 117.22, 116.12, 99.96, 48.49, 32.15, 27.32, 27.09, 23.32, 14.34.
IR:
n
2928, 2221 (CRN), 1686 (C
¼
O), 1570, 1465, 1405, 1392, 1287,
HRMS (ESI) m/z: calcd. for
1244 (CꢀꢀOH), 1171, 803 cmꢀ1
.
C
84H75N7O6S3 [M]+, 1373.4941; found, 1373.4951.
Acknowledgements
4.3. Computational study
This research was financially supported by the Faculty of
Science, Mahidol University, the Thailand Research Fund (TRF)
(Grant RSA6080065), the Center of Excellence for Innovation in
Chemistry (PERCH-CIC), Office of the Higher Education Commis-
sion, Ministry of Education (OHEC) and RA scholarship from the
Faculty of Graduate Studies, Mahidol University Academic Year
2016. We would like to thank Asst. Prof. Dr. Taweesak Sudyoadsuk,
Prof. Dr. Vinich Promarak, Mr. Kasempong Srisawad, Dr. Krittaphat
Wongma, and Assoc. Prof. Dr. Tienthong Thongpanchang for their
helpful suggestions and supports. We also thank the Central
Instrument Facility (CIF), the Faculty of Science, Mahidol Universi-
ty.
All calculations were performed with Gaussian 09 program
package [47] with the default convergence criteria. The B3LYP
functional and 6-31G(d) basis set were used for the geometry
optimization. TDDFT were carried out using CAM-B3LYP/6-31G(d)
level with the 10 lowest singlet–singlet transition. The effect of
chloroform solvent was considered by conductor-like polarizable
continuum (CPCM) model.
4.4. Preparation and characterization of DSSCs
To prepare the working electrode, the F-doped SnO2 (FTO)
conducting glass substrate (7
V/sq, TCO22-7, Solaronix) was
Appendix A. Supplementary data
cleaned in an ultrasonic bath with detergent, water, methanol
(15 min for each step) and then rinsed with acetone. The substrate
was treated with TiCl4 solution (40 mM TiCl4 in 10% HCl) at 70 ꢁC
for 30 min [70]. A transparent layer of TiO2 (20 nm paste, Ti-
nanoxide T/SP, Solaronix) was screen-printed on the substrate and
dried for 10 min at 130 ꢁC. This procedure was repeated twice and
further coated by a layer of TiO2 (400 nm paste, Ti-nanoxide R/SP,
Solaronix) as scattering layer. The double-layer TiO2 electrode
(active area: 0.5 cm ꢂ 0.5 cm) was annealed at 500 ꢁC for 15 min.
Supplementary data associated with this article can be found, in
the
online
version,
at
https://doi.org/10.1016/j.jphoto-
chem.2017.10.033.
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