Novel metal complexes for D-(A-π-A)2 motif dye sensitizer: Synthesis and photovoltaic application

Article abstract of DOI:10.1002/aoc.6220

Four metal complexes BDTT-PCd, BDTT-PZn, BDTT-PCu, and BDTT-PCo with the symmetrical structure were designed, synthesized, and characterized, which would be used as D-(A-π-A)₂ motif dye sensitizers. These complexes adopt Cd(II), Zn(II), Cu(II), and Co(II) complexes as auxiliary electron acceptors (A), thienylbenzo[1,2-b:4,5-b′]dithiophene (BDTT) as electron donor (D), and the 8-quinolinol derivative as the π bridge and the acceptor (A) of D-(A-π-A)₂ motif dye sensitizers. The photophysical, electrochemical, and photovoltaic properties of these complexes have been investigated. The four complexes-based dye-sensitized solar cells (DSSCs) device exhibited a short-circuit photocurrent density (J_sc) of 17.51, 17.02, 15.07, and 14.38 mA cm^?2 and an attractive power conversion efficiency (η) of 8.82%, 8.32%, 8.03%, and 7.55%, respectively, under the AM 1.5 irradiation (100 mW cm^?2). The photovoltaic conversion efficiency (PCE) and the short-circuit photocurrent density (J_sc) of BDTT-PCd, BDTT-PZn, BDTT-PCu, and BDTT-PCo are sequentially reduced; it attributed to the difference of the electron-withdrawing ability of the auxiliary acceptor resulted by the difference of coordination abilities of metal ions Cd(II), Zn(II), Cu(II), and Co(II) with ligand. These results provide strong evidence that D-(A-π-A)₂ motif complexes has great potential as promising photosensitizers in the applications of DSSCs.

Full text of DOI:10.1002/aoc.6220

Received: 18 November 2020
DOI: 10.1002/aoc.6220
Revised: 24 January 2021
Accepted: 19 February 2021
F U L L P A P E R
Novel metal complexes for D-(A-π-A)2 motif dye sensitizer:
Synthesis and photovoltaic application
Houpeng Zhang
| Xianming Wu
| Shiyu Tang
| Kaixuan Wang
|
Yong Tian | Chaofan Zhong
Key Laboratory of Environmentally
Friendly Chemistry and Applications of
Ministry of Education, College of
Chemistry, Xiangtan University,
Xiangtan, China
Four metal complexes BDTT-PCd, BDTT-PZn, BDTT-PCu, and BDTT-PCo
with the symmetrical structure were designed, synthesized, and characterized,
which would be used as D-(A-π-A)2 motif dye sensitizers. These complexes
adopt Cd(II), Zn(II), Cu(II), and Co(II) complexes as auxiliary electron
acceptors (A), thienylbenzo[1,2-b:4,5-b0]dithiophene (BDTT) as electron donor
(D), and the 8-quinolinol derivative as the π bridge and the acceptor (A) of
D-(A-π-A)2 motif dye sensitizers. The photophysical, electrochemical, and
photovoltaic properties of these complexes have been investigated. The four
complexes-based dye-sensitized solar cells (DSSCs) device exhibited a short-
circuit photocurrent density (Jsc) of 17.51, 17.02, 15.07, and 14.38 mA cm−2
and an attractive power conversion efficiency (η) of 8.82%, 8.32%, 8.03%, and
7.55%, respectively, under the AM 1.5 irradiation (100 mW cm−2). The photo-
voltaic conversion efficiency (PCE) and the short-circuit photocurrent density
(Jsc) of BDTT-PCd, BDTT-PZn, BDTT-PCu, and BDTT-PCo are sequentially
reduced; it attributed to the difference of the electron-withdrawing ability of
the auxiliary acceptor resulted by the difference of coordination abilities of
metal ions Cd(II), Zn(II), Cu(II), and Co(II) with ligand. These results provide
strong evidence that D-(A-π-A)2 motif complexes has great potential as
promising photosensitizers in the applications of DSSCs.
Correspondence
Chaofan Zhong, Key Laboratory of
Environmentally Friendly Chemistry and
Applications of Ministry of Education,
College of Chemistry, Xiangtan
University, Xiangtan, Hunan 411105,
China.
Email: zhongcf798@xtu.edu.cn
Funding information
Open Project Program of Key Laboratory
of Environmentally Friendly Chemistry
and Applications of Ministry of Education,
China, Grant/Award Number: 19HJYH10
K E Y W O R D S
auxiliary acceptor, coordination metal (Cd, Zn, Cu, and Co), D-(A-π-A)2 motif dye sensitizer,
dye-sensitized solar cells
1 | INTRODUCTION
improvement in efficiency, have become a hot research
topic in recent years.
Since O'Regan and Gratzel first proposed dye-sensitized
solar cells (DSSCs) in 1991,[1] they have attracted exten-
sive attention from researchers in recent years[2,3] due to
their simple manufacturing, flexible structure, low cost,
and high conversion efficiency compared with the tradi-
tional silicon-based solar cells.[4,5] Dye sensitizers, as the
core part of DSSCs to complete the photoelectric conver-
sion and the breakthrough point for the greatest
In the early stage of the development of the dye
sensitizers, donor–acceptor (D-A) was the mainstream
structure. Due to its simple structure and relatively
simple synthesis, the structure of donor–acceptor binding
achieved good results.[6,7] Subsequently, the D-π-A struc-
ture of the π bridge was introduced between the donor
and the acceptor.[8] The D-π-A motif improves the
interaction between the electron donor and the electron
Appl Organomet Chem. 2021;35:e6220.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6220

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ZHANG ET AL.
acceptor, resulting in a stronger charge-transfer light
absorption band, which can capture sunlight more
efficiently and effectively convert sunlight into electrical
energy, and the photoelectric conversion efficiency
increases exponentially.[9] However, this structure has
defects, such as poor performance, poor thermal stability,
and narrow absorption range.
Then in 2011, Zhu et al. proposed the concept of the
D-A-π-A motif, which introduced an auxiliary electron
acceptor between the π bridge and the donor. This struc-
ture can enhance the electron absorption capacity, reduce
the reorganization of electrons, and inject more electrons
into titanium dioxide, and can also adjust and ameliorate
the energy level, absorption spectrum, and photovoltaic
performance of the dye sensitizer.[10] In 2014, Grätzel
et al. proposed a new method to synthesize the classic
D-A-π-A structure of the porphyrin dye SM315, with an
efficiency of up to 13%.[11] Wu et al. put forward a view
in 2015[12] that the auxiliary electron acceptor in the
D-A-π-A motif not only helps to adjust the energy level,
absorption spectrum, and photovoltaic performance, but
also greatly improves the photostability of the dye
sensitizer.
between metal and ligand and the strength of coordina-
tion[32,33]; by this way, the push-and-pull electron balance
and intramolecular electron transport ability of the dye
sensitizer are improved, and the photovoltaic perfor-
mance of the dye sensitizer would also be improved.[23]
The structure of D-(A-π-A)2 motif is compared with that
D-A-π-A motif, the addition of one more -A-π-A chain in
the dye is in favor of the performance of the dyes; this is
because of the two receptor parts that can increase the
ability of the acceptor to push–pull electron and adjust
the HOMO and LOMO energy level of dye molecules, so
as to achieve an excellent push–pull electron balance of
dye molecules.[34]
Based on the above speculation, a series of metal
complex for D-(A-π-A)2 motif dye sensitizers by using
imine derivatives and metal complexes as the auxiliary
electron acceptors were designed and synthesized,
systematically studied of the influence of different
strength coordination bond on photovoltaic performance
in this paper. The research has also obtained many mean-
ingful research results and conclusions for adopting the
metal complex as the auxiliary electron acceptor of the
dye sensitizer in dye-sensitized solar cells.
Recently, there are several types of electron-
withdrawing units, such as benzothiadiazole (BTD),[13–16]
benzotriazole (BTZ),[17–19] quinoxaline (Qu),[20–23]
phthalimide,[24] and diketo-pyrrolopyrrole (DPP),[25–27]
that have been systematically exploited as the auxiliary
electron acceptor for the D-A-π-A structures. There
2 | EXPERIMENTAL SECTION
2.1 | Materials
is
a
trend to systematically add benzoxadiazole,
All starting materials in this work were obtained from
the Shanghai Chemical Reagent Co. Ltd (Shanghai,
China) and used as received, except that 2,6-dibromo-
4,8-bis(5-(2-ethylhexyl)-thiophen-2-yl)benzo[1,2-b:4,5-b0]
dithiophene (BDTT) was procured from Suzhou Ge'ao
New Material Co. Ltd. N,N-Dimethylformamide (DMF)
and tetrahydrofuran (THF) were dried over CaH2 and
freshly distilled prior to use. Other reagents and solvents
were purchased commercially as analytical-grade quality
and used without further purification. Column chroma-
tography was performed on silica gel (200–300 mesh).
benzothiadiazole, quinoxaline, and benzotriazole as an
auxiliary electron acceptor to the D-π-A system, and test
and theoretical calculation are performed. The results
show that the greater the electron-withdrawing ability of
the additional acceptor unit, the greater the redshift in
the light response is.[28,29] The obvious difference in
energy levels and photovoltaic performance between
LS-2 and WS-5[24] is a good indication that in addition to
the ability to withdraw electrons, steric hindrance must
also be considered when choosing an auxiliary electron
acceptor.
Because of being affected by the composition and
structure of the organic auxiliary electron acceptor, the
electron-withdrawing ability of organic auxiliary electron
acceptors such as benzoxadiazole and benzothiadiazole
and the ability to adjust the electron-withdrawing capac-
ity of the auxiliary acceptors are limited to a certain
extent. Therefore, it is difficult to further drastically
improve the photovoltaic performance of these organic
dye sensitizers.[30,31] However, if the metal complexes are
used as auxiliary electron acceptor, their electron-
withdrawing ability can be changed and be controlled
drastically by changing the type of coordination bond
2.2 | Instruments and measurements
1H NMR and 13C NMR spectra were recorded on a
Bruker ARX400 (400 MHz) spectrometer, using CDCl3 or
DMSO-d6 as solvent and tetramethylsilane (TMS,
δ = 0.00 ppm) as an internal standard. Elemental ana-
lyses of C, H, N, and S were taken on a Perkin-Elmer
2400 II instrument under nitrogen atmosphere. Fourier
transform infrared spectra (FT-IR) were recorded on a
Perkin-Elmer Spectra One with the spectral resolution of
2.0 cm−1 using KBr pellets in the 4000–400 cm−1 region.

ZHANG ET AL.
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Ultraviolet-visible (UV-Vis) absorption spectra of the four
polymers were obtained on a Perkin-Elmer Lambada-25
spectrometer (USA). Samples were dissolved in DMF and
diluted to a concentration of 10−5–10−4 mol L−1. Ther-
mogravimetric analyses (TGA) were performed on a
Shimadzu TGA-7 analyzer in 25–800ꢀC under N2
atmosphere with a heating and cooling rate of 20ꢀC/min.
The average molecular weight and polydispersity index
(PDI) of the four dyes were performed on PL-120 gel
permeation chromatography (GPC) analysis with DMF
as eluent and polystyrene as standard. Cyclic voltammetry
(CV) was conducted on an electrochemistry workstation
(CHI630C, Chenhua, Shanghai) in a solution of 0.1-M
tetrabutylammonium tetrafluoroborate (Bu4NBF4) dis-
solved in acetonitrile at a scan rate of 100 mV s−1 at room
temperature. The working electrode was a glassy carbon
electrode; the saturated calomel electrode (SCE) was used
as reference electrode, and the counter electrode was a
platinum wire electrode. Ferrocene/ferrocenium redox
couple was used for the redox potential calibration.[17]
2.4 | Syntheses
2.4.1 | Synthesis of compound 1
About 15.6 ml of salicylaldehyde (0.15 mol) was added to
the solution of concentrated hydrochloric acid (90 ml) and
paraformaldehyde (9 g, 0.3 mol) was stirred at room tem-
perature for 48 h.[19] After filtration, the crude product was
washed with saturated sodium bicarbonate solution until
weakly alkaline, then with distilled water until neutral,
dried, and recrystallized with petroleum ether; a white
needle-like solid product was collected by vacuum drying
(17.8 g,70% yield), m.p. 85–86ꢀC. H-NMR (CDCl3, ppm):
11.08 (s, 1H), 9.90 (s, 1H), 7.57 (s, 1H), 7.55 (d, 1H), 7.01
(d, 1H), 4.60 (s, 2H). 13C-NMR (CDCl3, ppm): 196.22,
161.63, 137.36, 133.66, 129.24, 120.37, 118.34, 45.26.
1
2.4.2 | Synthesis of compound 2
A mixture of chloromethyl salicylaldehyde (6.96 g, 5 mmol)
and triphenylphosphine (10.48 g, 40 mmol) was dissolved
in 70 ml of benzene and was refluxed for 1 h.[20] After
cooled to room temperature and filtration, the product
was washed three times with a small amount of diethyl
ether; white powdery solid (15.8 g, 91% yield) was obtained
by drying in a vacuum oven for 2 days, m.p. 253–254ꢀC.
The above phosphine salt (7.785 g, 18 mmol) was dissolved
in 60 ml of formaldehyde in water (37%), and NaOH solu-
tion (19.5 ml, 12.5 mol/L) was slowly added dropwise with
vigorous stirring. After stirred at room temperature for
4 h, the mixture was extract with chloroform and dry with
anhydrous MgSO4 for 12 h. After filtration and spin off the
solvent, the crude product was purified by column chro-
matography using petroleum ether/ethyl acetate (10/1, v:
v) as eluent to obtain a light yellow solid 2 (2.1 g, 79%
yield), m.p. 19–20ꢀC. 1H-NMR (CDCl3, ppm): 11.01 (s,
1H), 9.90 (s, 1H), 7.62 (d, 1H), 7.54 (s, 1H), 6.97 (d, 1H),
6.64–6.71 (m, 1H), 5.65–5.69 (d, 1H), 5.22–5.25 (d, 1H).
13C-NMR (CDCl3, ppm): 191.04, 161.04, 136.47, 136.12,
130.84, 129.13, 127.71, 118.43, 114.30.
2.3 | Fabrication of DSSCs devices
The DSSCs with the sandwich structure were fabricated
based on TiO2 semiconductors.[18] Fluorine-doped SnO2
conducting glass (FTO) was cleaned by ultrasonication in
water, acetone, ethanol, and isopropanol and subjected to
an O3 treatment for 10 min. The substrates were
immersed in an aqueous 40 mmol L−1 TiCl4 solution at
70ꢀC for 30 min and then rinsed with deionized water
and ethanol. The 20- to 30-nm TiO2 colloidal particles
were coated onto the prepared FTO glass with the sliding
glass rod method and were then sintered at 450ꢀC for
30 min. This process was repeated two more times to
obtain a 15-mm-thick TiO2 film. After cooling to 100ꢀC,
the TiO2 film was soaked in 0.5 mmol L−1 dye (for BDTT-
PCd, BDTT-PZn, BDTT-PCu, and BDTT-PCo) in DMF
solution and kept in the dark for 24 h at room tempera-
ture. Then, the films were cleaned by anhydrous ethanol.
After drying, electrolyte containing 0.5 mol L−1 LiI,
0.05 mol L−1 I2, and 0.5 mol L−1 4-tertbutylpyridine was
dripped on the surface of the TiO2 electrodes. A Pt foil
used as a counter electrode was clipped onto the top of
the TiO2 as the working electrode. The dye-coated
semiconductor film was illuminated through the con-
ducting glass supported without a mask. The photo-
electrochemical performance of the solar cell was
measured using a Keithley 2602 SourceMeter controlled
by a computer. The cell parameters were obtained under
an incident light with intensity of 100 mW cm−2, which
was generated by a 500-W Xe lamp passing through an
AM 1.5-G filter with an effective area of 0.25 cm2.
2.4.3 | Synthesis of compound 3
About 1.48 g of 5-vinyl salicylaldehyde (10 mmol) and
0.93 g of aniline (10 mmol) were dissolved in 20 ml of
anhydrous ethanol and were stirred at 80ꢀC for 12 h.[21]
After filtration, the crude product was washed with cold
anhydrous. An orange-yellow solid 3 (1.82 g, 82% yield)
was collected, m.p. 66–67ꢀC. 1H-NMR (CDCl3, ppm):
13.34 (s, 1H), 8.64 (s, 1H), 7.42–7.49 (m, Ar-H), 7.26–7.32
(m, Ar-H), 6.99–7.01 (d, 1H), 6.64–6.72 (m, 1H), 5.62–5.67

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ZHANG ET AL.
(d, 1H), 5.16–5.19 (d, 1H). 13C-NMR (CDCl3, ppm):
162.25, 161.00, 148.37, 135.64, 130.78, 130.19, 129.46,
129.04, 127.03, 121.20, 118.96, 117.53, 111.95.
(CH3COO)2ꢁ4H2O (0.548 g, 2.2 mmol) was used instead
of Cd (CH3COO)2ꢁ2H2O. A khaki solid (0.561 g, 54%
yield) was obtained. FT-IR (KBr, cm−1): 3431 (O-H), 2213
(C≡N), 1598 (C=N), 1568 (C=C), 1114 (C-O-Co),
516 (N-Co). Anal. calcd. for [C28H19N3O4Co]: C, 64.62; H,
3.68; N, 8.07. Found: C, 64.73; H, 3.75; N, 8.01%.
2.4.4 | Synthesis of PCd
A methanol solution (20 ml) of Cd (CH3COO)2ꢁ2H2O
(0.585 g, 2.2 mmol) was slowly dropped into a mix-
ture[22] of 2-cyano-3-(8-hydroxyquinolin-5-yl)acrylic
acid[23] (0.446 g, 2 mmol) and compound 3 (0.48 g,
2 mmol) in dry tetrahydrofuran (50 ml) with stirring.
Then the mixture was carefully adjusted by glacial
acetic acid or ammonia to weak acidity and heated to
reflux for 12 h. After cooled to room temperature, the
crude product was collected by filtration, washed with
deionized water and absolute ethanol, and then rec-
rystallized with anhydrous methanol. A yellow solid
(0.699 g, 61% yield) was obtained by vacuum drying.
FT-IR (KBr, cm−1): 3418 (O-H), 2209 (C≡N), 1603
(C=N), 1559 (C=C), 1100 (C-O-Cd), 493 (N-Cd). Anal.
calcd. for [C28H19N3O4Cd]: C, 58.60; H, 3.34; N, 7.32.
Found: C, 59.30; H, 3.41; N, 7.24%.
2.4.8 | Synthesis of BDTT-PCd
The polymeric metal complex BDTT-PCd is synthesized
according to the Heck coupling reaction.[24] A mixture of
PCd (0.172 g, 0.3 mmol), 2,6-dibromo-4,8-bis(5-(2-eth-
yloctyl)thiophen-2-yl)benzo[1,2-b:4,5-b0]dithiophene (BDTT)
(0.11 g, 0.15 mmol), catalyst palladium acetate (Pd
(CH3COO)2, 0.0021 g, 0.0094 mmol), and tris(2-tolyl)phos-
phine (0.018 g, 0.06 mmol) were dissolved by refined DMF
solvent (8 ml) and dried triethylamine (3 ml) under nitro-
gen. The reactant was gradually heated to 90ꢀC stirred for
48 h. After cooled to room temperature and filtration, the
crude product was washed with anhydrous methanol. A
yellow solid was obtained (0.139 g, 53% yield). FTIR (KBr,
cm−1): 3405 (O-H), 2920, 2846 (C-H), 2205 (C≡N), 1598
(C=N), 1557 (C=C), 1095 (C-O-Cd), 488 (N-Cd). Anal. calcd.
for [C90H76N6O8S4Cd2]: C, 62.75; H, 4.45; N, 4.88; S, 7.45.
Found: C, 62.98; H, 4.58; N, 4.91; S, 7.44%.
Mn = 1.71 kg/mol.
2.4.5 | Synthesis of PZn
Metal complex PZn was synthesized by the similar
method that described for PCd, except that Zn
(CH3COO)2ꢁ2H2O (0.42 g, 2.2 mmol) was used instead of
Cd (CH3COO)2ꢁ2H2O. Am orange solid (0.547 g, 52%
yield) was collected. FT-IR (KBr, cm−1): 3420 (O-H), 2207
(C≡N), 1611 (C=N), 1563 (C=C), 1111 (C-O-Zn),
506 (N-Zn). Anal. calcd. for [C28H19N3O4Zn]: C, 63.83; H,
3.63; N, 7.98. Found: C, 63.75; H, 3.52; N, 7.99%.
2.4.9 | Synthesis of BDTT-PZn
BDTT-PZn was obtained following the similar method
that described for BDTT-PCd from a mixture of metal
complex PZn, BDTT, palladium acetate, and tris(2-tolyl)
phosphine, DMF, and triethylamine as a reddish brown
solid (0.114 g, 46% yield). FTIR (KBr, cm−1): 3410 (O-H),
2920, 2851 (C-H), 2205 (C≡N), 1598 (C=N), 1559 (C=C),
1108 (C-O-Zn), 497 (N-Zn). Anal. calcd. for
[C90H76N6O8S4Zn2]: C, 66.37; H, 4.70; N, 5.16; S, 7.88.
Found: C, 66.81; H, 4.92; N, 5.11; S, 7.93%.
Mn = 1.64 kg/mol.
2.4.6 | Synthesis of PCu
Metal complex PCu was synthesized by the similar
method that described for PCd, except that Cu
(CH3COO)2ꢁH2O (0.438 g, 2.2 mmol) was used instead of
Cd (CH3COO)2ꢁ2H2O. A dark green solid (0.586 g, 56%
yield) was collected. FT-IR (KBr, cm−1): 3427 (O-H), 2209
(C≡N), 1615 (C=N), 1563 (C=C), 1113 (C-O-Cu),
515 (N-Cu). Anal. calcd. for [C28H19N3O4Cu]: C,
64.06; H, 3.65; N, 8.00. Found: C, 64.14; H, 3.71; N, 8.12%.
2.4.10 | Synthesis of BDTT-PCu
BDTT-PCu was obtained following the similar method
that described for BDTT-PCd from a mixture of metal
complex PCu, BDTT, palladium acetate and tris(2-tolyl)
phosphine, DMF, and triethylamine as a black solid
(0.106 g, 43% yield). FTIR (KBr, cm−1): 3418 (O-H), 2920,
2851 (C-H), 2205 (C≡N), 1598 (C=N), 1561 (C=C), 1110
2.4.7 | Synthesis of PCo
Metal complex PCo was synthesized by the similar
method that described for PCd, except that Co
(C-O-Cu),
511
(N-Cu).
Anal.
calcd.
for
[C90H76N6O8S4Cu2]: C, 66.52; H, 4.71; N, 5.17; S, 7.89.

ZHANG ET AL.
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Found: C, 65.79; H, 4.63; N, 5.13; S, 7.62%.
Mn = 1.63 kg/mol.
reaction of aniline and 5-vinyl salicylaldehyde, the metal
complex BDTT-PM is prepared by the coordination reac-
tion of compound 3 and 5-vinyl-8-hydroxyquinoline with
Cd(II), Cu(II), Zn(II). or Co(II) acetate, and the coupled
metal complexes BDTT-PCd, BDTT-PZn, BDTT-PCu, and
BDTT-PCo were gained by Heck coupling reaction of
metal complexes PCd, PZn, PCu, and PCo with donor
BDTT. The 1H NMR spectra of compounds 1, 2, and 3 are
show in Figures S1, S2, and S3. The coupled metal
complexes are tested by UV-Vis absorption spectrum,
thermogravimetry, cyclic voltammetry, and FT-IR
spectra, and the detailed data are collected in Table 1.
The Fourier-transform infrared spectra (FT-IR) of
metal complexes and metal complexes are shown in
Figures S4 and S5. According to Figure S4, the peaks
around 3400 cm−1 are hydroxyl peaks, which are broad
2.4.11 | Synthesis of BDTT-PCo
BDTT-PCo was obtained following the similar method
that described for BDTT-PCd from a mixture of metal
complex PCo, BDTT, palladium acetate and tris(2-tolyl)
phosphine, DMF, and triethylamine as a black solid
(0.115 g, 47% yield). FTIR (KBr, cm−1): 3422 (O-H), 2922,
2850 (C-H), 2206 (C≡N), 1599 (C=N), 1564 (C=C), 1112
(C-O-Co),
514
(N-Co).
Anal.
calcd.
for
[C90H76N6O8S4Co2]: C, 66.90; H, 4.74; N, 5.20; S, 7.94.
Found: C, 67.31; H, 4.99; N, 5.21; S, 7.93%.
Mn = 1.60 kg/mol.
peaks due to the presence of hydrogen bonds; 2209 cm−1
,
2207 cm−1, 2209 cm−1, and 2213 cm−1 are C≡N on the
auxiliary ligand absorption peak. The absorption peaks of
C=N are at 1603 cm−1, 1611 cm−1, 1615 cm−1, and
1598 cm−1, and the absorption peaks of the four com-
plexes C-O-M appear at 1100 cm−1 (C-O-Cd), 1111 cm−1
(C-O-Zn), 1113 cm−1 (C-O-Cu), 1114 cm−1 (C-O-Co);
N-M absorption peaks appear at 493 cm−1 (N-Cd),
506 cm−1 (N-Zn), 515 cm−1 (N-Cu), and 516 cm−1
(N-Co), which is due to the absorption peak is inversely
proportional to the reduced mass of coordination metal,
while the reduced mass generated by Cd(II), Zn(II),
3 | RESULTS AND DISCUSSION
3.1 | Synthesis and characterization
The synthesis routes of the dyes BDTT-PM are shown in
Scheme 1, compound 1 attained by salicylaldehyde was
reacted with paraformaldehyde in the presence of
hydrogen chloride under air, compound 2 was prepared
from alkenylation of 5-chloromethylsalicylic aldehyde,
compound 3 was obtained by amine condensation
SCHEME 1 Synthetic routes of
coupled metal complex BDTT-PCd,
BDTT-PZn, BDTT-PCu, and BDTT-
PCo

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ZHANG ET AL.
Cu(II), and Co(II) decreases in turn. Therefore, the
absorption peak wave numbers of C-O-M and N-M of
metallic bonds increase in turn. From Figure S5, the
C-O-M absorption peaks of BDTT-PCd, BDTT-PZn,
BDTT-PCu, and BDTT-PCo appear at 1095 cm−1 (C-O-
Cd), 1108 cm−1 (C-O-Zn), 1110 cm−1 (C-O-Cu), and
1112 cm−1 (C-O-Co); N-M absorption peaks appear at
488 cm−1 (N-Cd), 497 cm−1 (N-Zn), 511 cm−1 (N-Cu),
and 514 cm−1 (N-Co). Compared with their respective
complexes, the metal bonds have a redshift of 2–9
wavenumbers, which is due to the introduction of the
donor BDTT with good planarity, increasing the degree
of conjugation; 2846–2920 cm−1 is the absorption peak of
methyl and methylene groups on the donor alkyl chain.
It can be known from FT-IR spectra and other standard
data that the four target dyes have been successfully
synthesized.
3.2 | Photophysical properties
The higher the absorption capacity of dye sensitizers to
solar energy, the higher the photoelectric conversion
efficiency is. Therefore, the wavelength range of dye
sensitizers excited by sunlight and generated electrons
overlapped with the maximum wavelength range of solar
radiation energy more and the higher the light absorption
intensity, the higher the possibility of obtaining high
efficiency. UV-Vis absorption of four metal complexes
and four dye sensitizers was tested, their absorption
spectra were summarized in Figure 1, and the relevant
data were listed in Table 1.
According to the UV-Vis spectrum of PM and
BDTT-PM (Figure 1), the maximum absorption wave-
length of the complex PCd (384 nm) is larger than the
complex PZn (375 nm), the complex PCu (369 nm),
TABLE 1 Photophysical and electrochemical property of the BDTT-PM
λa,max
λb,max
εmax
(L mol−1 cm−1
Ered
(V)
Eox
(V)
HOMO
(eV)
LUMO
(eV)
Eg
(eV)
Td
(ꢀC)
Dyes
(nm)
(nm)
)
BDTT-PCd 514
BDTT-PZn 508
BDTT-PCu 500
BDTT-PCo 481
432
388
372
366
19,520
18,418
17,565
16,880
−1.018
−1.052
−1.031
−1.028
1.007
1.026
1.082
1.126
−5.347
−5.366
−5.422
−5.466
−3.322
−3.288
−3.309
−3.312
2.025
2.078
2.113
2.154
340
323
328
309
aThe absorption peaks of metal complexes and the first absorption peaks of polymers for UV-Vis spectra in DMF solution.
bThe second absorption peaks of polymers for UV-Vis spectra in DMF solution.
FIGURE 1 UV-Vis
absorption spectra of PCd, PZn,
PCu, and PCo and BDTT-PCd,
BDTT-PZn, BDTT-PCu, and
BDTT-PCo

ZHANG ET AL.
7 of 11
and the complex PCo (351 nm); this may be due to
the radius of the Cd(II) ion that is significantly larger
than that of the other three metal ions, and the
electron interaction with the ligand is stronger, so the
coordination bond is stronger. After coordination, it is
conducive to the formation of a better planar configu-
ration and more favorable to the electron transporta-
tion within the molecule. The maximum absorption
wavelength of the complexes of the three metals in the
same period PZn > PCu > PCo is affected by the
electron-withdrawing ability of metal ions. The stron-
ger the electron-withdrawing ability is, the more stable
the coordination bond.
3.3 | Electrochemical properties
The highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) energy
levels are important parameters in the design of photovol-
taic materials. Cyclic voltammograms (CV) can be esti-
mated from the initial oxidation potential (Eox) and the
initial reduction potential (Ered).[35] To enable the excited
electrons of the dye sensitizer to be effectively injected
into the conduction band of TiO2, its LUMO energy level
should be higher than that of TiO2 (−4.26). While the dye
sensitizer in the oxidation state can be regenerated, the
HOMO energy level of the dye is required to be lower
than the redox charge pair energy level (−4.83) of the
From the absorbance data in Table 1, compared
with the maximum absorption peak of PM in the
visible light region, BDTT-PM has a certain degree of
red shift, indicating that the introduction of BDTT not
only improves the electron-donating ability of the
entire molecule but also improves the conjugation
degree of the molecular structure. The maximum
absorption wavelength of the complex BDTT-PCd
(514 nm) is larger than the complex BDTT-PZn
(508 nm), BDTT-PCu (500 nm), and BDTT-PCo
(481 nm); this may be due to the electronic structure
configuration of the coordinated metal ion that has
an influence on the light absorption ability, which
makes range redshift of the light absorption. All of
the dye sensitizer exhibit a high molar extinction
coefficient, in which the absorption coefficient of
the coupled product at the maximum wavelength is
all above 14,000 L mol−1 cm−1; BDTT-PCd achieved
the highest absorption coefficient, exceeding 19,500
electrolyte I−/I3− [36]
When these two conditions are met,
.
the energy band gap of the dye sensitizer should be as
small as possible to be more easily excited by sunlight.
The cyclic voltammetry curve and the HOMO and
LUMO energy levels were shown in Figure 2, and
the46.4881ptrelevant data are listed in Table 1. The
LUMO levels of the dyes of BDTT-PCd, BDTT-PZn,
BDTT-PCu, and BDTT-PCo are −3.322, −3.388, −3.309,
and −3.312, respectively, which are higher than
the Fermi level (−4.26 eV) of titanium dioxide, that sat-
isfies the conditions for the dye to inject electrons into
the titanium dioxide; the HOMO levels are −5.447,
−5.366, −5.422, and −5.446, both lower than the redox
potential of I−/I3− (−4.83 eV), which satisfies the condi-
tions for dye reduction and regeneration. The energy
band gaps are 2.025, 2.078, 2.113, and 2.154 eV, respec-
tively. The narrower of the band gap means that the
charge separation is easier when excited, which is the
same as the result of the photophysical test.
L mol−1 cm−1
.
FIGURE 2 Cyclic voltammetry curve (a) and HOMO and LUMO energy level (b) of polymers BDTT-PCd, BDTT-PZn, BDTT-PCu, and
BDTT-PCo

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ZHANG ET AL.
FIGURE 3 Thermogravimetric curve of
BDTT-PCd, BDTT-PZn, BDTT-PCu, and
BDTT-PCo
FIGURE 4 IPCE spectra of BDTT-PCd,
BDTT-PZn, BDTT-PCu, and BDTT-PCo
3.4 | Thermal properties
of the four dye sensitizers are all above 300ꢀC, which can
fully meet the requirements of dye-sensitized solar cells
to work outdoors for a long time.
Since the working environment of dye-sensitized solar
cells is exposed to sunlight for a long time, the equipment
will inevitably generate heat, so the thermal stability of
dye-sensitized solar cells is the significant factor to
improve the service life of dye-sensitized solar cells. The
thermal properties of the four dyes were studied by
thermogravimetric analyses (TGA). The four dyes
(BDTT-PCd, BDTT-PZn, BDTT-PCu, and BDTT-PCo)
show excellent thermal stability, as shown in Figure 3
and Table 1, with the onset decomposition temperatures
(Td, the temperature at which 5% weight loss under
nitrogen occurs) at 340, 323, 328, and 309ꢀC, respectively.
According to the test results, it can be known that the Td
3.5 | Photovoltaic properties
The incident photon to current conversion efficiency
(IPCE) spectra and the photocurrent density-voltage
curves (J-V) of DSSCs based on BDTT-PCd, BDTT-PZn,
BDTT-PCu, and BDTT-PCo are shown in Figures 4 and
5, and the relevant performance parameters are summa-
rized in Table 2.
The IPCE curves shows that the maximum external
quantum efficiency of the four dyes sensitized is all above

ZHANG ET AL.
9 of 11
FIGURE 5 Current density-voltage
characteristics of BDTT-PCd, BDTT-PZn,
BDTT-PCu, and BDTT-PCo
TABLE 2 Photovoltaic parameters
Dyes
solvent
DMF
Jsc (mA/cm2)
17.51
Voc(V)
0.79
ff (%)
69.25
66.78
69.17
69.53
η (%)
of devices with sensitizers in DSSCs at
full sunlight (AM 1.5 G, 100 mW cm−2
)
BDTT-PCd
BDTT-PZn
BDTT-PCu
BDTT-PCo
8.82
8.32
8.03
7.55
DMF
17.02
0.78
DMF
15.07
0.76
DMF
14.38
0.74
65%, and the maximum monochromatic light efficiency
of BDTT-PCd is more than 80%. The light absorption
the highest efficiency of 8.82%; this result is consistent with
the absorption spectrum of the four dye sensitizers, which
is due to the fact that BDTT-PCd has the widest absorption
spectrum, so that it can produce more photocurrent.
intensity of BDTT-PCd
>
BDTT-PZn
>
BDTT-
PCu > BDTT-PCo shows the same trend as the absorp-
tion spectrum, and BDTT-PCd has better absorption in
the long wavelength region, which is due to the influence
of light absorption intensity and region; BDTT-PCd
shows the highest monochromatic light photoelectric
conversion efficiency 82.5%.
4 | CONCLUSION
In summary, four novel D-(A-π-A)2 motif dyes of metal
complexes with Cd(II), Zn(II), Cu(II), and Co (II) of
imine derivative as auxiliary acceptors, BDTT derivatives
as donor (D), and 8-hydroxyquinoline derivative as
π-bridge and acceptor were designed and synthesized.
According to the results of photovoltaic test of four
polymeric metal complexes, the thermogravimetric curve
shows that the four dye sensitizers exhibit excellent ther-
mal stability and can fully meet the DSSC requirements
for dye sensitizers (Td > 300ꢀC). The power conversion
efficiency (PCE) of BDTT-PCd (8.82%) is higher than
BDTT-PZn (8.32%), BDTT-PCu (8.03%), and BDTT-PCo
(7.55%). This is primarily affected by the radius of the
metal ion and the ability of electron-withdrawing. The
more stable the coordination bond formed by the metal
and the ligand, and the effect of the metal radius on the
performance of the dye sensitizer is significantly higher
than the effect of the electron-withdrawing ability on its
The J-V curves illustrate that the short-circuit
current densities (Jsc) follow the order of BDTT-PCd
(17.51 mA/cm2) > BDTT-PZn (17.02 mA/cm2) > BDTT-
PCu (15.07 mA/cm2) > BDTT-PCo (14.38 mA/cm2), which
is consistent with the trend of increasing IPCE. The open
circuit voltage (Voc) is 0.79, 0.78, 0.76, and 0.74 V, respec-
tively, and the fill factor (ff) is 69.25%, 66.78%, 69.17%, and
69.53% respectively. The efficiency difference is mainly
caused by short circuit current density and open circuit
voltage, so the influence of different metal ion coordination
ability on its performance is mainly discussed. Compared
with the dye sensitizers containing metals of the same
main group, the photoelectric conversion efficiency of
BDTT-PCd is higher than BDTT-PZn. With containing the
same cycle sensitizer comparison of three kinds of metal,
the order of photoelectric conversion efficiency is BDTT-
PZn > BDTT-PCu > BDTT-PCo, and BDTT-PCd achieved

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ZHANG ET AL.
performance. The properties of metal ions as radius and
change number affect the stronger of coordination bonds
of metal ions and ligand; the larger the radius of metal
ions is, the stronger the coordination bond is in the same
number of changes, which enhances the electron-
withdrawing ability of the auxiliary acceptor A of metal
complexes and transfer electron ability between donor
(BDTT) and the electron acceptor. The strengthen of
electron-withdrawing and the transfer electron ability
decrease the Eg of metal complexes that cause red shift of
λmax can increase the JSC and PCE. The J-V curves shows
that the short-circuit current densities (Jsc) follow the
order of BDTT-PCd (17.51 mA/cm2) > BDTT-PZn
(17.02 mA/cm2) > BDTT-PCu (15.07 mA/cm2) > BDTT-
PCo (14.38 mA/cm2), which is consistent with the trend
of increasing IPCE. The results shown that from
BDTT-PCo, BDTT-PCu, BDTT-PZn, to BDTT-PCd, the
coordination bonds were strengthened, and the electron-
withdrawing of the auxiliary receptor (A) was enhanced,
leading to the increase of Jsc and PCE. For the ligand with
the same type, the stronger the metal's ability to accept
electron pairs, the stronger the coordination bond, and
the stronger the electron absorption ability. Of course, in
addition to the composition and structure of the metal
ions (charge, radius, and electron configuration), the
composition and structure of the ligands also affect the
strength of the coordination bonds, which is what we are
studying next.
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AUTHOR CONTRIBUTIONS
Xianming Wu: Data curation; formal analysis; software.
Shiyu Tang: Data curation; formal analysis; software.
Kaixuan Wang: Data curation; formal analysis; soft-
ware. Yong Tian: Data curation; formal analysis;
software. Chaofan Zhong: Conceptualization; data
curation; formal analysis; funding acquisition; investiga-
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ORCID
Houpeng Zhang https://orcid.org/0000-0003-4646-1163
Xianming Wu https://orcid.org/0000-0003-3219-8309
Shiyu Tang https://orcid.org/0000-0002-0443-3191
Kaixuan Wang https://orcid.org/0000-0002-2799-061X
Yong Tian https://orcid.org/0000-0001-7877-5840
Chaofan Zhong https://orcid.org/0000-0002-2422-2711
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How to cite this article: Zhang H, Wu X, Tang S,
Wang K, Tian Y, Zhong C. Novel metal complexes
for D-(A-π-A)2 motif dye sensitizer: Synthesis and
photovoltaic application. Appl Organomet Chem.
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