New oxindole-bridged acceptors for organic sensitizers: Substitution and performance studies in dye-sensitized solar cells

Article abstract of DOI:10.3390/molecules25092159

New D-π-A configured organic sensitizers featuring halogen-substituted oxindole-bridged acceptor units have been synthesized for dye-sensitized solar cells applications. Among fluorine, bromine, and iodine substitution, the cell based on bromine incorporated dye exhibited the highest efficiency. The oxindoles in these sensitizers were found to assist the electron injection through the chelation of their amide carbonyl groups to the TiO₂ surface. This study provides an alternate approach for future rational dye design to gain excellent DSSC performance.

Full text of DOI:10.3390/molecules25092159

molecules
Article
New Oxindole-Bridged Acceptors for Organic
Sensitizers: Substitution and Performance Studies
in Dye-Sensitized Solar Cells
Yogesh S. Tingare 1,*, Chaochin Su 1,*, Ming-Tai Shen 2, Sheng-Han Tsai 1, Shih-Yu Ho 1
and Wen-Ren Li 2,
*
1
Institute of Organic and Polymeric Materials/Research and Development Center for Smart
Textile Technology, National Taipei University of Technology, Taipei 10608, Taiwan;
kieferboy8@hotmail.com (S.-H.T.); stacy760414@hotmail.com (S.-Y.H.)
Department of Chemistry, National Central University, Chung-Li 32001, Taiwan; moodplayer@hotmail.com
Correspondence: yogeshtingare@yahoo.co.in (Y.S.T.); f10913@mail.ntut.edu.tw (C.S.);
ch01@ncu.edu.tw (W.-R.L.)
2
*
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 5 April 2020; Accepted: 3 May 2020; Published: 5 May 2020
Abstract: New D-
π
-A configured organic sensitizers featuring halogen-substituted oxindole-bridged
acceptor units have been synthesized for dye-sensitized solar cells applications. Among fluorine,
bromine, and iodine substitution, the cell based on bromine incorporated dye exhibited the highest
efficiency. The oxindoles in these sensitizers were found to assist the electron injection through
the chelation of their amide carbonyl groups to the TiO2 surface. This study provides an alternate
approach for future rational dye design to gain excellent DSSC performance.
Keywords: oxindoles; halogens; electron injection; dye-sensitized solar cells
1. Introduction
Energy harvesting directly from the sunlight using the photovoltaic technology is being increasingly
recognized as an essential strategy for future global energy production [1]. Finding energy generating
devices using inexpensive and environment-friendly materials has become a well-focused academic
and industrial research topic. The solar-to-power converting assembly, known as dye-sensitized solar
cells (DSSCs), introduced by O’Regan and Grätzel has attracted a significant amount of attention [2–15].
The technology has shown a great potential to assist as an efficient, low production cost, and clean
energy source. To achieve excellent solar power conversion efficiencies in DSSCs, considerable attention
has been focused on designing and synthesizing various photosensitizers [10
,11,16–20]. For the past
decades, the sensitizers like ruthenium (II) polypyridyl complexes [21 27] and zinc-based porphyrin
–
dyes [28–33] have dominated DSSCs as efficient sensitizers with their outstanding power conversion
efficiencies. But the cost, toxicity and limited availability of the metals, as specially the ruthenium,
have inspired researcher to pursue alternative sensitizing dyes for DSSC. Parallelly, the ongoing
research on perovskite solar cells [34
20%. But the use of environmentally harmful lead-based materials and instability in these devices
have raised some major concerns [36 37]. On the other hand, the metal free organic dyes have kept
,35] is found to be promising with efficiency exceeding over
,
a considerable interest with their robust availability, flexibility of structural tuning, and high molar
extinction coefficients which make them competitive alternatives to the other sensitizers [38,39].
Typically, metal-free organic dyes are configured with donor-π-bridge-acceptor structures.
As per the literature reports, in-depth studies have been directed towards the modification of the
donors and the π-bridge moieties of the organic dyes but only a few developments were made in the
acceptor groups [10,40–42]. The most commonly used acceptors (anchoring groups) are carboxylic acid
Molecules 2020, 25, 2159; doi:10.3390/molecules25092159
www.mdpi.com/journal/molecules

Molecules 2020, 25, 2159
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or cyanoacrylic acid [9]. But the cyanoacrylic acid acceptor was reported to exhibit some disadvantages,
such as lower photostabilities due to presence of double bond and dominating redox properties
because of strong electronegative nitrile moiety [43]. It was also reported that the nitrile group of the
cyanoacrylic acid affected the acidity of the adjacent carboxylic acid which can give a negative influence
on the binding to the TiO2 surface [44
electron-withdrawing cyano group and construct new acceptor systems to achieve high absorptivity
and good photostability [46 49]. The acceptor moiety of the dye shows a significant influence on the
,45]. Several attempts have been made to replace the strong
–
spectral response and the device performance. Therefore, there is a need to further explore various
acceptors and expand “design principles” to help guide metal-free dye discovery.
The oxindoles are natural indole alkaloids and can be isolated from the plants [50]. Although
oxindoles are ubiquitous within the pharmaceutical industry [51–53], the potential of oxindoles to
act as sensitizing materials [54] has not been investigated by other groups and need to be studied
thoroughly. We unprecedentedly visualize oxindoles as new acceptors for the organic sensitizers.
It is our speculation that the amide group of the oxindole dye will not only act as an electron acceptor
but also can direct the electron injection because of its chelation to the TiO2 surface [54]. Furthermore,
it is located near the anchoring group which will help to accelerate the electron transfer to the TiO2
due to the shorter distance. The non-conjugated carboxylic acid group incorporated in the end of the
dye will act as efficient binding group without affect the push–pull capability of the acceptor. Effect of
halogens substituted on the donor moiety of the dye was also studied and was found to effectively
contribute to the enhancement of the open-circuit voltage and the overall cell efficiency [55].
In this article, we designed and synthesized a series of new oxindole sensitizers (TI111
–TI116,
Figure 1a) which exhibited respectable photoelectric conversion efficiencies ( ). These sensitizers
η
have an excellent electron-donating triphenylamine (TPA) donor and the thiophene in the spacer and
are differentiated by various halogen-substituted oxindole acceptors. The electronegative fluorine
(F) element is well known for its application in the field of organic light-emitting diodes as well as
in the artificial amino acid synthesis [56–60]. The alteration in the Fermi levels of the dye could be
achieved by using F substituent in the acceptor of the dye, which usually accounts for an increase in
the open-circuit voltage of the dye by reducing the dark current [58]. We incorporated an F atom in
the parent compound TI111, forming TI112, to alter the photo-physical properties of the oxindole
dye. The comparative studies of different halogen substituents in the donor part of sensitizers have
been evaluated [55], but their effects on the sensitizing properties of dyes when substituted near the
acceptors have not been reported. Therefore we further extended our investigation and designed
TI114, and TI115 in which bromine (Br), and iodine (I) substituents were incorporated, respectively.
Additionally, it is noted that the variation in the position of the substituent with respect to the anchoring
group can deliver different cell performances [58]. Thus, we designed TI114 analogue, TI116, to study
the effect of position of the substitution on the conversion efficiency of the DSSC.
a)
N
S
H
X
Y
TI111
TI112
TI114
TI115
TI116
; X = H, Y = H
; X = F, Y = H
; X = Br, Y = H
; X = I, Y = H
; X = H, Y = Br
O
N
O
OH
Figure 1.
(
a
) Molecular structures of oxindole dyes (TI111−TI116). (
b) ORTEP diagram of methyl ester
of TI112. (c) ORTEP diagram of methyl ester of TI116.

Molecules 2020, 25, 2159
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2. Results and Discussion
Scheme 1 depicts the syntheses of new sensitizers TI111
–
TI115 that started with a Knoevenagel
condensation reaction using amides 1a
(2) [54] to yield condensed compounds 3a
with methyl bromoacetate in the presence of base and tetrabutylammonium bromide (TBAB) to afford
esters 4a . Finally, sensitizers TI111 TI115 were obtained after basic hydrolysis of their corresponding
esters. As shown in Scheme 2, the preparation of the sensitizer TI116 began with a microwave-assisted
Wittig reaction. 6-Bromo-1H-indole-2, 3-dione ( ), aldehyde , PPh3, and toluene were combined in
a microwave reaction vessel and irradiated in the Synthos 3000 microwave oven at 300 W for 4 h to
yield the desired product . Ester was synthesized through alkylation of compound with methyl
bromo acetate. Basic hydrolysis of the resulting ester successfully provided the required sensitizer
TI116. The molecular geometries of TI112 and TI116 dyes were confirmed through single-crystal
structural analysis of their corresponding esters 4b and , respectively (Figure 1b,c; Tables S1 and S2 in
–d
and 5-[4-(diphenylamino)phenyl]thiophene-2-carbaldehyde
–
d
as single Z-isomers. Oxindoles 3a were then alkylated
–d
–d
–
5
2
6
7
6
7
7
the Supporting Information). As shown in the X-ray structures, the stereogenic double bonds present
in oxindoles are arranged in Z configuration.
Scheme 1. Synthesis of TI111-TI115 sensitizers.
Scheme 2. Synthesis of TI116 sensitizer.
The optical properties of these dyes were investigated using UV/Vis absorption spectroscopy
and shown in Figure 2. The absorption spectrum of the parent dye TI111 features
λ
of 469 nm
max
[molar absorption coefficient (
ε
) = 3.77
×
104 M−1 cm−1] which is attributed to the delocalized
π-π*

Molecules 2020, 25, 2159
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transition (Figure 2a). The halogen substituents played an important role in the absorption properties
of the corresponding dyes. The of the dyes TI112 (480 nm, = 3.05
104 M−1 cm−1) with F
substituent was found to red-shift 11 nm. However, decrease in molar extinction coefficient was
observed when compared to the non-substituted dye TI111. Nevertheless, dye TI114 = 483 nm,
= 3.88 max and . Surprisingly, dye TI115
104 M−1 cm−1) with Br substituent exhibited the highest
max = 483 nm, = 3.47 in which Br is replaced with I substituent.
104 M−1 cm−1) showed lower
λ
max
ε
×
(λ
max
ε
×
λ
ε
(λ
ε
×
ε
These results revealed that, in comparison to other halogenated dyes, the Br substitution in the oxindole
dye was more effective towards improving the light-harvesting capacity. It is interesting to note that,
changing the position of substituent with respect to the anchoring group in TI116 showed a steep
decrease in the absorption intensity, compared to the TI114 dye which might have resonance interaction
of the substituent with the anchoring group. Mostly, organic dyes featured blue-shifts in the absorption
bands when attached onto the TiO2 surface. Such a phenomenon was attributed to deprotonation of
the carboxylic acid or the formation of aggregation of the sensitizers [61–66]. The UV/Vis absorption
spectra of oxindole dyes anchored onto TiO2 films (Figure 2b) were broadened. Compared to the
solutions, sensitizers TI112–TI115 on films exhibited stronger adsorption than the dye TI111.
Figure 2. (a) Absorption spectra of TI111-TI116 in CH2Cl2. (b) Absorption spectra for transparent
TiO2 films (thickness: 5 µm) coated with 8.0 × 10−5 M solutions of TI111–TI116.
The HOMO and LUMO energy levels of the sensitizer play an important role in affecting the
dye-regeneration and electron-transfer processes in DSSCs. Table 1 reveals that the HOMO levels of
these dyes are lower than that of I−/I3− redox potential, assuring regeneration of the oxidized dyes by
the redox couple in the electrolyte. The LUMO levels of the sensitizers TI111–TI116 were adequate for
efficient electron injection and the halogen substitutions do not alter the energy levels of these dyes.
To gain further insight into the frontier orbital profiles, the locations of the HOMOs and LUMOs of
oxindole dyes were calculated using a semiempirical computation method. The molecular orbital
calculations showed that the HOMO orbital of each dye is localized to the electron-rich TPA moiety
(Figure 3a and Figure S2 in the Supporting Information). Interestingly, in each oxindole dye, the LUMO
level is confined to the amide portion of the molecule and does not extend to the non-conjugated
anchoring carboxylate moiety. These calculations indicate that each oxindole dye is poised for effective
electron injection through the amide group rather than the anchoring carboxylate moiety. To confirm
this inference and explore the electron injection mechanism in the oxindoles, FTIR spectra of TI114
powder and the dye adsorbed on TiO2 were measured. The characteristic band for the amide carbonyl
group of the oxindole dye TI114 powder was observed at 1678 cm−1 (Figure 3b). When the dye was
adsorbed on the TiO2 surface, the peak for C=O stretching disappeared which clearly verified the
chelation of the amide carbonyl group of the oxindole dye to the TiO2 surface. With such chelations,
oxindole sensitizers are expected to improve photo-induced electron injection yields.

Molecules 2020, 25, 2159
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Table 1. Electrochemical properties for TI111–TI116 dyes.
Dye
λabs/nm (ε/L mol−1 cm−1
)
EHOMO[a] (eV)
ELUMO[b] (eV)
E0-0[b] (eV)
TI111
TI112
TI114
TI115
TI116
469 (3.7737)
480 (3.0562)
483 (3.8825)
483 (3.4712)
410 (1.7437)
5.11
5.11
5.11
5.12
5.11
3.02
3.05
3.08
3.09
3.06
2.09
2.06
2.03
2.03
2.05
+
[a] EHOMO = Eox
−
EFc/Fc + 4.8 eV. [b] ELUMO = EHOMO – E0-0. The band gap, E0-0, was estimated from the onset of
absorption spectrum measured in CH2Cl2.
Figure 3. (a) Frontier molecular orbital profiles of TI114 and TI116 at the B3LYP/6-31g (d,p) level.
(b) FTIR spectra of the TI114 dye powder and dye adsorbed on film.
Table 2 summarizes the cell performance of the DSSCs anchored with oxindole dyes: short-circuit
photocurrent densities (Jsc), open-circuit voltages (Voc), fill factors (FFs), and solar-to-electricity
conversion efficiencies (η). Figure 4 presents their corresponding photocurrent density–voltage (I–V)
curves, incident photo-to-current efficiency (IPCE) spectra and electrochemical impedance spectroscopy
(EIS). The parent dye TI111 exhibited Jsc = 10.03 mA cm−2, Voc = 680 mV, and FF = 0.699, corresponding
to an overall
η = 4.76% (Table 2). Furthermore, incorporating the halogen substitutions in the
oxindole dyes enhanced the device conversion efficiencies. The DSSC based on the F-substituted
TI112 sensitizer achieved Jsc = 11.32 mA cm−2, Voc = 690 mV, and FF = 0.695, corresponding to an
overall η = 5.43%, which was approximately 14% higher than that of the non-substituted oxindole
sensitizer TI111. The enhanced
η was due to its increase in the Jsc which could be attributed to
the electronic coupling of F with the anchoring group [58]. Replacing the F with other halogen
substituents in the oxindoles showed further enhancement in the DSSC performance. Among all the
halogen substituted sensitizers reported in this study, the Br incorporated sensitizer TI114 exhibited
the highest photovoltaic parameters (Jsc = 12.46 mA cm−2, Voc = 720 mV, and FF = 0.708) with
an overall conversion efficiency (η) of 6.35%. The improved photocurrent of the TI114 sensitizer
suggests that compared to F substitution, the Br-substituted dye can exhibit better cell performance.
Also, compared to the dye with the traditional cyanoacrylic acid acceptor bearing the same TPA
donor-π-bridge moiety [67], the sensitizer with the Br-substituted oxindole acceptor seems to be more
impressive. Surprisingly, changing the position of the substituent with respect to the anchoring group
exhibited negative effect on the solar cell performance. The TI116 anchored DSSC showed lower
current density (Jsc = 9.66 mA cm−2
, Voc = 630 mV, and FF = 0.690), that is corresponding to an overall
η = 4.21%, which may be due to the absence of electronic coupling of substitution with the anchoring
group. In order to gain more insight into the electron transport of oxindole dyes, the electrochemical
impedance of the cells was measured under illumination and is shown in Figure 4c. The fitted EIS
data, the charge transfer resistance related to the recombination of electrons at the interface (Rk),
the electron transport resistance in the photoanode (Rw), the effective rate constant for recombination
(keff), the electron lifetime in the photoanode (τ), the effective electron diffusion coefficient (Deff),
and the electron diffusion length (Ln) are compiled in Table S4 in SI. The radii of the semicircles in
the EIS spectra of the DSSCs based on oxindole dyes, according to the Nyquist plot, is in the order

Molecules 2020, 25, 2159
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of TI116 > TI111 > TI112 > TI115 > TI114. These results indicate that the charge transport is in the
order of TI114 > TI115 > TI112 > TI111 > TI116, which agrees with the overall device performances.
Also, the life time (
τ
) values shown in Table S4 indicates that the capacity to suppress the back reaction
−
of the electron with the I3 in the electrolyte are affected by the different halogen substituent on
the oxindole moiety. The measured
τ is in the order of TI114 = TI112 > TI115 = TI111 > TI116 and
is consistent with those of Voc determined experimentally (see Table 2). Among all the sensitizers
studied here, the dye TI114 showed the highest charge transport, lowest electron transport resistance
and better electron lifetime which have led enhancement in its Voc and efficiency. The IPCE of the
device using the TI111 sensitizer exceeded 50% in the spectral region ranging from 425 to 565 nm
and reached a maximum of 52% (Figure 4b). Incorporating halogen substituents in the sensitizers
displayed red-shifted ranges and increasing IPCE maxima. The dye TI114 with the Br substituent
exhibited the maximum IPCE value of 60% at 585 nm, which was the highest among the listed oxindole
dyes. Similar to the I–V curve (Figure 4a), altering the position of the substituent also resulted in
a lower IPCE value as well as a blue shift for dye TI116 (a maximum of 43% at 445 nm). Based on
these analyses and the previous report [58], we can estimate that the halogen substituent, particularly
positioned para to the anchoring group, can have strong impact on the device performance of the
sensitizer. We believe that the effectiveness of electron transport, the proper balance of Fermi level,
and controlling affinity of the acceptors towards the semiconductor are the crucial factors affecting
device performances of oxindole-based DSSCs. Such factors cannot be predicted intuitively and their
physical origins need to be further addressed in future work.
Table 2. Photovoltaic parameters of devices incorporating TI111-TI116 sensitizers.
JSC (mA cm−2
)
Sensitizer
VOC (mV)
FF
η (%)
TI111
TI112
TI114
TI115
TI116
10.03
11.32
12.46
11.97
9.66
680
690
720
680
630
0.669
0.695
0.708
0.714
0.692
4.76
5.43
6.35
5.81
4.21
Figure 4.
(a) Current versus potential plots and (b) Monochromatic IPCE spectra for sensitizers
TI111–TI116. (c) EIS spectra measured under illumination of DSSCs based on TI111–TI116 dyes.

Molecules 2020, 25, 2159
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3. Materials and Methods
General procedure for syntheses of compounds 3(a
–
d): In a 100 mL round bottom flask
5-[4-(diphenylamino) phenyl] thiophene-2-carbaldehyde (2) (12 mmol) was treated with the appropriate
oxindole (1a ) (10 mmol) and piperidine (1 mmol) in 20 mL of ethanol. After the reaction mixture
–
d
was refluxed for 6 h, it was cooled and the resulting precipitate was filtered, washed with cold ethanol,
and dried to give the target compounds (70–90% yield).
1
(Z)-3-((5-(4-(Diphenylamino)phenyl)thiophen-2-yl)methylene)indolin-2-one (3a; 3.8 g, 82.79%): H NMR
(300 MHz, DMSO-d6)
7.66 (d, J = 8.7 Hz, 2H); 7.52 (d, J = 3.9 Hz, 1H); 7.35 (t, J = 7.8 Hz, 4H); 7.20 (t, J = 7.2 Hz, 1H); 7.14–7.08
(m, 6H); 7.02–6.97 (m, 3H); 6.88 (d, J = 7.8 Hz, 1H). 13C NMR (75 MHz, DMSO-d6)
167.8; 150.9; 148.1;
δ 10.64 (s, 1H); 8.05 (s, 1H); 7.89 (d, J = 4.2 Hz, 1H); 7.68 (d, J = 7.2 Hz, 1H);
δ
147.1; 140.8; 139.7; 136.4; 130.1; 128.7; 128.5; 127.5; 127.3; 125.1; 124.9; 124.2; 123.6; 122.9; 121.3; 121.3;
119.7; 109.9. HRMS (FAB) (m/z [M + H]+) calcd for C31H23N2OS, 471.1526; found 471.1513.
(Z)-3-((5-(4-(Diphenylamino)phenyl)thiophen-2-yl)methylene)-5-fluoroindolin-2-one (3b; 3.4 g, 70.69%):
1H NMR (300 MHz, DMSO-d6)
δ
10.63 (s, 1H); 8.14 (s, 1H); 7.89 (d, J = 3.9 Hz, 1H); 7.67 (d, J = 8.7 Hz, 2H);
7.60 (dd, J = 9, 2.4 Hz, 1H); 7.55 (d, J = 3.9 Hz, 1H); 7.37 (t, J = 7.6 Hz, 4H); 7.14 (d, J = 7.2 Hz, 2H)
7.10 (d, J = 8.1 Hz, 5H); 7.01 (d, J = 8.4 Hz, 2H); 6.84 (dd, J = 8.4, 4.5 Hz, 1H). 13C NMR (75 MHz,
DMSO-d6) 167.9; 159.8; 156.7; 151.7; 148.2; 147.0; 140.3; 137.0; 136.1; 130.1; 127.4; 127.3; 126.5;
;
δ
126.4; 125.1; 124.2; 123.7; 122.8; 120.8 (d, J = 7.5 Hz); 114.7 (d, J = 22.5 Hz); 110.4 (d, J = 7.5 Hz);
107 (d, J = 24.7 Hz). HRMS (FAB) (m/z [M]+) calcd for C31H21FN2OS, 488.1353; found 488.1350.
(Z)-5-bromo-3-((5-(4-(Diphenylamino)phenyl)thiophen-2-yl)methylene)indolin-2-one (3c; 4.2 g, 76.45%): [54
]
;
:
1H NMR (300 MHz, DMSO-d6)
δ
10.75 (s, 1H); 8.22 (s, 1H); 7.93 (s, 1H); 7.89 (d, J = 3.6 Hz, 1H)
7.69 (d, J = 8.7 Hz, 2H); 7.57 (d, J = 3.9 Hz, 1H); 7.40–7.35 (m, 5H); 7.16–7.10 (m, 6H);
7.02 (d, J = 8.4 Hz, 2H); 6.83 (d, J = 8.4 Hz, 1H). 13C NMR (75 MHz, DMSO-d6)
δ
167.4; 151.9; 148.3;
147.0; 140.6; 139.7; 136.2; 130.7; 130.4; 130.2; 127.5; 127.3; 127.2; 125.1; 124.3; 123.8; 122.7; 122.4; 119.8;
113.3; 111.7. HRMS (FAB) (m/z [M + H]+) calcd for C31H22BrN2OS, 549.0631; found 549.0620.
(Z)-3-((5-(4-(Diphenylamino)phenyl)thiophen-2-yl)methylene)-5-iodoindolin-2-one (3d; 5.2 g, 86.95%):
1H NMR (300 MHz, DMSO-d6)
δ 10.71 (s, 1H); 8.19 (s, 1H); 8.07 (d, J = 1.5 Hz, 1H); 7.90 (d, J = 3.9 Hz, 1H);
7.68 (d, J = 8.4 Hz, 2H); 7.56 (d, J = 3.9 Hz, 1H); 7.51 (dd, J = 8.1, 1.5 Hz, 1H); 7.37 (t, J = 7.8 Hz, 4H); 7.15
(d, J = 7.5 Hz, 2H); 7.11 (d, J = 7.5 Hz, 4H); 7.02 (d, J = 8.4 Hz, 2H); 6.72 (d, J = 8.1 Hz, 1H). 13C NMR
(75 MHz, DMSO-d6) δ 167.2; 151.8; 148.2; 147.0; 140.5; 140.1; 136.5; 136.2; 130.1; 127.9; 127.7; 127.4; 127.2;
125.1; 124.2; 123.7; 122.8; 119.6; 112.2; 84.3. HRMS (FAB) (m/z [M]+) calcd for C31H21IN2OS, 596.0414;
found 596.0413.
General procedure for syntheses of compounds 4(a–d): To a solution of compound 3(a–d)
(10 mmol) in dimethylformamide (50 mL), potassium carbonate (12 mmol) was added and stirred for
30 min. Methyl bromoacetate (12 mmol) and tetrabutylammonium bromide (1 mmol) were then added
and reaction mixture was continued stirring at room temperature for 48 h. The reaction mixture was
diluted by ethyl acetate (30 mL) and the salt formed was removed by filtration and washed with ethyl
acetate (6 mL). The solution was then concentrated under reduced pressure. The resulting residue was
purified by column chromatography (SiO2; CH2Cl2) to yield ester compounds (60–65% yield).
Methyl (Z)-2-(3-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)-2-oxoindolin-1-yl)acetate (4a; 2.81 g,
1
64.15%): H NMR (300 MHz, DMSO-d6)
δ 8.17 (s, 1H); 7.89 (d, J = 4.2 Hz, 1H); 7.75 (d, J = 6.9 Hz, 1H);
7.67 (d, J = 8.7 Hz, 2H); 7.56 (d, J = 3.9 Hz, 1H); 7.35 (t, J = 7.8 Hz, 4H); 7.25 (t, J = 7.9 Hz, 1H) 7.14–7.09
(m, 7H); 7.06 (d, J = 8.1 Hz, 1H); 7.01 (d, J = 5.4 Hz, 2H); 4.69 (s, 2H); 3.69 (s, 3H). 13C NMR (75 MHz,
CDCl3)
δ 168.5; 166.3; 152.5; 148.3; 147.2; 140.4; 139.1; 136.2; 129.4; 129.0; 128.0; 127.4; 127.1; 124.9;
124.1; 123.5; 122.9; 122.5; 122.1; 119.2; 118.7; 107.9; 52.5; 41.0. HRMS (FAB) (m/z [M + H]+) calcd for
C34H27N2O3S, 543.1737; found 543.1732.

Molecules 2020, 25, 2159
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Methyl (Z)-2-(3-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)-5-fluoro-2-oxoindolin-1-yl)acetate
(4b; 2.39 g, 61.43%): 1H NMR (300 MHz, DMSO-d6)
8.29 (s, 1H); 7.91 (d, J = 4.2 Hz, 1H);
7.71 (d, J = 8.7 Hz, 2H); 7.61 (d, J = 3.9 Hz, 1H); 7.38 (t, J = 7.6 Hz, 4H); 7.15-7.10 (m, 9H);
7.10 (d, J = 3.9 Hz, 2H); 4.72 (s, 2H); 3.72 (s, 3H). 13C NMR (75 MHz, CDCl3)
168.4; 166.2; 161.2;
159.8; 153.4; 148.4; 147.1; 139.8; 136.3; 135.8; 130.1; 129.4; 127.1; 124.9; 123.6; 122.7; 122.6; 118.6; 114.1
(d, J = 22.5 Hz)
108.3 (d, J = 7.5 Hz); 106.1 (d, J = 22.5 Hz); 52.5; 41.1. HRMS (FAB) (m/z [M]+) calcd
δ
δ
;
for C34H25FN2O3S, 560.1564; found 560.1568.
Methyl (Z)-2-(5-bromo-3-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)-2-oxoindolin-1-yl)acetate
(4c; 3.02 g, 63.84%) 1H NMR (300 MHz, DMSO-d6)
δ 8.35 (s, 1H); 8.03 (s, 1H); 7.91 (d, J = 3.6 Hz, 1H);
7.70 (d, J = 8.4 Hz, 2H); 7.60 (d, J = 3.9 Hz, 1H) 7.44 (d, J = 8.4 Hz, 1H); 7.38 (t, J = 7.6 Hz, 4H); 7.17–7.10
(m, 6H); 7.06 (d, J = 8.4 Hz, 1H); 7.01 (d, J = 8.4 Hz, 2H); 4.72 (s, 2H); 3.71 (s, 3H). 13C NMR (75 MHz,
CDCl3)
δ 168.3; 165.8; 153.5; 148.4; 140.1; 139.0; 135.8; 130.2; 130.2; 129.4; 127.1; 126.1; 125.0; 123.6;
122.7; 121.6; 117.6; 114.8; 109.3. HRMS (FAB) (m/z [M + H]+) calcd for C34H26BrN2O3S, 621.0842;
found 621.0841.
Methyl (Z)-2-(3-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)-5-iodo-2-oxoindolin-1-yl)acetate
(4d; 4.63 g, 64.84%) 1H NMR (300 MHz, DMSO-d6)
δ 8.28 (s, 1H); 8.13 (s, 1H); 7.89 (d, J = 3.9 Hz, 1H);
7.66 (d, J = 8.7 Hz, 2H); 7.57 (d, J = 8.4 Hz, 1H); 7.55 (d, J = 3.9 Hz, 1H); 7.36 (t, J = 7.8 Hz, 4H);
7.15–7.08 (m, 6H); 6.99 (d, J = 8.7 Hz, 2H); 6.92 (d, J = 8.4 Hz, 1H); 4.69 (s, 2H); 3.71 (s, 3H). 13C NMR
(75 MHz, DMSO-d6) δ 169.0; 165.5; 152.5; 148.4; 147.0; 141.2; 140.3; 136.4; 136.1; 131.3; 130.1; 127.8; 127.4;
127.0; 126.6; 125.2; 124.3; 123.8; 122.6; 117.6; 111.5; 85.4; 52.7; 41.2. HRMS (FAB) (m/z [M]+) calcd for
C34H25IN2O3S, 668.0625; found 668.0622.
General procedure for syntheses of sensitizers TI111–TI115: Compound 4(a–d) (9 mmol) was
dissolved in THF (6 mL) and 0.5 M NaOH(aq.) (18 mmol) was added to the solution, which was then
stirred at room temperature for 6 h. After the hydrolysis was completed, the solvent was evaporated.
Ethanol (5 mL) was added and the resulting precipitate was filtered, washed with cold ethanol (0.5 mL),
and dried. The precipitate were then dissolved in water (10 mL) and 0.5 M HCl(aq.) was added to
precipitate the product. The solid was filtered, washed with water (3
to afford target sensitizers (80–90% yield).
×
6 mL) and dried under vacuum
1
Dye TI111 (2.35 g, 86.08%): H NMR (500 MHz, DMSO-d6)
δ 8.32 (d, J = 7.6 Hz, 1H); 7.85 (s, 2H); 7.73
(d, J = 8.6 Hz, 2H); 7.60 (d, J = 4 Hz, 1H); 7.37–7.32 (m, 5H); 7.14–7.05 (m, 8H); 6.99 (d, J = 8.5 Hz, 2H);
4.54 (s, 2H). 13C NMR (125 MHz, DMSO-d6)
δ
169.9; 168.6; 150.0; 148.4; 146.9; 143.1; 139.4; 135.5; 130.1;
129.8; 128.5; 127.6; 126.5; 125.2; 124.4; 124.3; 123.6; 122.6; 122.4; 121.2; 120.6; 109.4; 41.6. HRMS (FAB)
(m/z [M + H]+) calcd for C33H25N2O3S, 529.1580; found 529.1570.
Dye TI112 (2.38 g, 80.13%): 1H NMR (300 MHz, DMSO-d6)
7.71 (d, J = 8.7 Hz, 2H); 7.66 (d, J = 2.1 Hz, 1H); 7.59 (d, J = 3.9 Hz, 1H); 7.38 (t, J = 7.6 Hz, 4H); 7.17–7.11
(m, 8H); 7.02 (d, J = 8.7 Hz, 2H); 4.52 (s, 2H). 13C NMR (125 MHz, DMSO-d6)
169.8; 168.4; 159.1; 157.3;
δ 8.24 (s, 1H); 7.91(d, J = 3.9 Hz, 1H);
δ
150.8; 148.5; 146.9; 140.2; 139.4; 135.0; 130.1; 127.6; 126.2; 125.2; 124.6; 124.4; 122.4; 121.5; (d, J = 8.8 Hz);
120.5; 115.9 (d, J = 40 Hz); 110.5 (d, J = 41.2 Hz); 41.7. HRMS (FAB) (m/z [M]+) calcd for C33H23FN2O3S,
546.1408; found 546.1405.
Dye TI114 (2.63 g, 89.15%): 1H NMR (500 MHz, DMSO-d6)
7.64 (d, J = 8.2 Hz, 2H); 7.59 (s, 1H); 7.49 (d, J = 8 Hz, 1H); 7.34 (t, J = 7.3 Hz, 4H); 7.13–7.04 (m, 7H);
6.97 (d, J = 8.2 Hz, 2H); 4.53 (s, 2H). 13C NMR (125 MHz, DMSO-d6)
169.7; 168.1; 150.7; 148.7; 146.9;
δ 8.29 (s, 1H); 7.87 (s, 2H);
δ
142.2; 140.3; 135.0; 132.0; 130.2; 127.6; 126.1; 125.3; 124.8; 124.4; 122.7; 122.4; 120.0; 113.9; 111.4; 41.7.
HRMS (FAB) (m/z [M]+) calcd for C33H23BrN2O3S, 606.0613; found 606.0603.
Dye TI115 (3.23 g, 87.53%): 1H NMR (500 MHz, DMSO-d6)
δ 8.28 (s, 1H); 8.11 (s, 1H);
7.88 (d, J = 3.7 Hz, 1H); 7.68 (d, J = 8.5 Hz, 2H); 7.56 (d, J = 3.7 Hz, 1H); 7.34 (d, J = 7.3 Hz, 4H);
7.11-7.08 (m, 7H); 6.98 (d, J = 3.7 Hz, 2H); 6.89 (d, J = 8.2 Hz, 1H); 4.53 (s, 2H). 13C NMR (125 MHz,
DMSO-d6) δ 169.9; 165.5; 152.3; 148.3; 147.0; 141.0; 140.7; 136.3; 136.1; 131.0; 130.1; 127.7; 127.4; 127.1;

Molecules 2020, 25, 2159
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126.5; 125.3; 125.2; 124.3; 123.8; 122.6; 117.9; 111.5; 85.2; 41.5. HRMS (FAB) (m/z [M]+) calcd for
C33H23IN2O3S, 654.0469; found 654.0467.
(Z)-6-bromo-3-((5-(4-(Diphenylamino)phenyl)thiophen-2-yl)methylene)indolin-2-one (
6
): 6-Bromo-1H-indole-
) (6.29 g,
2,3-dione ( ) (2 g, 8.84 mmol), 5-[4-(diphenylamino)phenyl]thiophene-2-carbaldehyde (
5
2
17.69 mmol), and PPh3 (4.64 g, 17.69 mmol) were combined and dissolved in 20 mL of toluene in
a microwave reaction vessel containing a magnetic stir bar. The reaction vessel was irradiated in
◦
the Synthos 3000 microwave oven at 300 W for 4 h (up to 110 C). After the removal of solvent,
the crude product was purified by column chromatography (SiO2; CH2Cl2) to afford compound
(2.88, 60% yield). 1H NMR (300 MHz, DMSO-d6)
10.76 (s, 1H); 8.13 (s, 1H); 7.91 (d, J = 3.6 Hz, 1H)
7.68 (d, J = 7.8 Hz, 1H); 7.64 (d, J = 8.4 Hz, 2H); 7.55 (d, J = 3.9 Hz, 1H); 7.37 (t, J = 7.5 Hz, 4H);
7.19 (d, J = 8.1, 1H); 7.16-7.09 (m, 6H); 7.03–7.00 (m, 3H). 13C NMR (75 MHz, DMSO-d6)
167.5; 151.6;
6
δ
;
δ
148.2; 147.0; 141.9; 140.4; 136.2; 130.2; 129.7; 127.4; 127.3; 125.1; 124.3; 24.0; 123.8; 122.8; 121.4; 120.9;
120.0; 112.5. HRMS (FAB) (m/z [M + H]+) calcd for C31H22ON2BrS, 549.0631; found 549.0632.
Methyl (Z)-2-(6-bromo-3-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)-2-oxoindolin-1-yl)acetate
(
7
): Compound
7
was prepared from compound
6
using a method similar to that described for the
1
synthesis of compound
4
to get 1.86 g (57%) final ester. H NMR (300 MHz, DMSO-d6)
δ
8.23 (s, 1H);
7.90 (d, J = 3.6 Hz, 1H); 7.69 (d, J = 7.2 Hz, 1H); 7.67 (d, J = 7.8 Hz, 2H); 7.56 (d, J = 3.9 Hz, 1H); 7.39–7.32
(m, 5H); 7.27 (d, J = 8.1 Hz, 1H); 7.14–7.07 (m, 6H); 6.98 (d, J = 7.8 Hz, 2H); 4.71 (s, 2H); 3.69 (s, 3H).
13C NMR (75 MHz, CDCl3)
δ 168.1; 166.0; 153.2; 148.4; 147.1; 141.2; 139.5; 136.0; 129.5; 129.3; 127.2;
127.0; 124.9; 123.5; 123.1; 122.7; 122.6; 121.3; 119.6; 118.0; 111.2; 52.6; 41.0. HRMS (FAB) (m/z [M]+) calcd
for C34H25BrN2O3S, 620.0769; found 620.0770.
Synthesis of sensitizer TI116: Dye TI116 was prepared from compound
to that described for the synthesis of sensitizer TI111 to get 1.03 g (56%) 1H NMR (500 MHz, DMSO-d6)
8.22 (d, J = 8.1 Hz, 1H); 7.89 (d, J = 4.2 Hz, 2H); 7.73 (d, J = 8.7 Hz, 2H); 7.61 (d, J = 3.9 Hz, 1H);
7 using a method similar
δ
7.40–7.28 (m, 6H); 7.14–7.08 (m, 6H); 6.98 (d, J = 8.4 Hz, 2H); 4.56 (s, 2H). 13C NMR (125 MHz, DMSO-d6)
δ 169.8; 168.5; 150.6; 148.5; 146.9; 144.5; 140.0; 135.3; 130.1; 129.3; 127.6; 126.4; 125.2; 124.9; 124.6; 124.4;
122.6; 122.4; 119.9; 119.8; 112.6; 41.8. HRMS (FAB) (m/z [M]+) calcd for C33H23BrN2O3S, 606.0607;
found 606.0606.
4. Conclusions
In conclusion, we have designed and synthesized new acceptors bearing halogen-substituted
oxindoles for the application in DSSCs. Among all the halogen-substituted acceptors studied in this
artical, the sensitizer with Br exhibited superior photo-physical properties. Also, the amide carbonyl
groups of these acceptors can have partial chelation to the TiO2 surface providing an added benefit
towards effective electron injection. The new oxindole building block can provided a new gateway for
future rational dye design for applications in DSSC. Evidently, further enhancement in the absorption
profiles of oxindole sensitizers can be obtained by using different electron withdrawing substituents
and adding efficient donating moieties. Such structural modification of oxindole-based sensitizers to
gain superior performances in DSSC is currently underway in our laboratory.
Supplementary Materials: The following are available online. Figure S1: Theoretical calculation for the location
of HOMOs and LUMOs, Table S1: Bond lengths [Å] and angles [◦] for ester 4b, Table S2: Bond lengths [Å] and
angles [◦] for ester 7.
Author Contributions: Y.S.T. Conceptualization, performed the synthesis and wrote the manuscript.;
C.S. validation, supervision and helped draft the manuscript.; M.-T.S. participated in the synthesis; S.-H.T. and
S.-Y.H. carried out device characterization.; W.-R.L. validation, project administration and helped draft the
manuscript. All authors have read and agreed to the published version of the manuscript.
Funding: We thank the Ministry of Science and Technology, ROC, for financial support (grants MOST
108-2113-M-027-002-MY, MOST 107-2113-M-027-004, and MOST 106-2113-M-008-007).
Conflicts of Interest: The authors declare no conflict of interest.

Molecules 2020, 25, 2159
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Sample Availability: Samples of the compounds not available from the authors.
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