Syntheses and characterization of several nickel bis(dithiolene) complexes with strong and broad Near-IR absorption

Article abstract of DOI:10.1016/j.ica.2011.07.046

Several nickel bis(dithiolene) complexes with strong and broad absorptions in the Near-IR (NIR) region (700-1100 nm) were synthesized by using green and simple synthetic routes. The physical and chemical properties of these dyes were systematically studied, including structure, optical spectroscopy and electrochemical behavior, etc. These NIR dyes were first applied to dye-sensitized solar cells (DSCs) and the photoelectrochemical performances were also investigated. The effects of different substituent groups on the properties of the dyes and photovoltaic performances of DSCs were discussed. Furthermore, we also applied the synthesized NIR dyes for constructing NIR absorbing filter. With their particular photoelectrochemical properties, the nickel bis(dithiolene) complexes exhibit promising prospects for future application.

Full text of DOI:10.1016/j.ica.2011.07.046

Inorganica Chimica Acta 376 (2011) 619–627
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Syntheses and characterization of several nickel bis(dithiolene) complexes with
strong and broad Near-IR absorption
⇑
Qingqing Miao, Junxiong Gao, Zeqing Wang, Hang Yu, Yi Luo, Tingli Ma
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 May 2011
Received in revised form 22 July 2011
Accepted 27 July 2011
Available online 2 August 2011
Several nickel bis(dithiolene) complexes with strong and broad absorptions in the Near-IR (NIR) region
(700–1100 nm) were synthesized by using green and simple synthetic routes. The physical and chemical
properties of these dyes were systematically studied, including structure, optical spectroscopy and elec-
trochemical behavior, etc. These NIR dyes were first applied to dye-sensitized solar cells (DSCs) and the
photoelectrochemical performances were also investigated. The effects of different substituent groups on
the properties of the dyes and photovoltaic performances of DSCs were discussed. Furthermore, we also
applied the synthesized NIR dyes for constructing NIR absorbing filter. With their particular photoelect-
rochemical properties, the nickel bis(dithiolene) complexes exhibit promising prospects for future
application.
Keywords:
NIR dye
Nickel bis(dithiolene) complex
Crystal stacking
Optical spectroscopy
Photoelectrochemistry
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
total solar energy. The theoretical calculation has proved that the
efficiency of DSCs will be greatly improved with the broadened
Nickel bis(dithiolene) complexes are important Near-IR (NIR)
dyes [1]. They have attracted much attention due to their unique
optical, electronic, magnetic, and electrochemical properties [2–4].
With these particular properties, such as superior photostability,
air-stability, thermal stability, tense and broad absorption in the
NIR region, easy adjustment of the absorption range, high molecular
absorption coefficient [5] and high electron mobility [6], many
applications have been developed. They have been used as NIR pho-
todetectors, Q-switch dye lasers, antioxidants for polymers, light
stabilizers, laser diodes for optical switching devices [5,7–18]. How-
ever, there is little systematic research on the synthesis and charac-
terizationsof thenickel bis(dithiolene) complexes. In addition, as we
know, there is no report on the application in the field of dye-sensi-
tized solar cell (DSC).
DSC is a new type of photovoltaic cell [19]. It has attracted consid-
erable interest due to its unique advantages, such as high efficiency,
low cost and the simple fabrication process. For successful commer-
cialization, it is necessary to further improve the energy conversion
efficiency of DSC. Many attempts have been carried out, however,
the results are not satisfying. One of the most effective methods
for further improving the efficiency is broadening the absorption
spectra of the cells and utilizing the NIR light, which is 45% of the
absorption spectrum [20]. Kuster et al. reported the squaraine NIR
dye for DSC [21]. Mori et al. also reported the DSC based on zinc
phthalocyanine, with the efficiency of 4.6% [22]. The above results
exhibit the promising prospects of NIR dyes for DSC. Furthermore,
the NIR dyes can be used in tandem DSCs, hybrid DSCs and co-sensi-
tized DSCs [23–25]. Our group has successfully developed a new
type of hybrid DSC by using a NIR dye (zinc phthalocyanine) and a
visible dye (pyrazine-dicarboxylic acid) [26]. However, the prob-
lems of the NIR dyes currently used for DSCs are the narrow absorp-
tion bands (the end of the absorption band is usually below 800 nm)
and the low efficiencies. Therefore, it is necessary to develop NIR
dyes with strong and broad absorptions between 800 and
1200 nm for DSCs.
In this work, several nickel bis(dithiolene) complexes with
strong and broad absorptions were synthesized and the structures
were shown in Fig. 1(A–D). The synthesis of A and B were per-
formed using the modified green synthetic routes. C and D were
synthesized by another simple method, especially the synthetic
route of C and D is first reported. We also systematically studied
the properties of optical spectroscopy and electrochemistry of
these complexes. These synthesized nickel bis(dithiolene) com-
plexes were first applied to DSCs. The effects of the different sub-
stituent groups on the photovoltaic performances of DSCs and
the energy level matching were investigated. Furthermore, the
application of the complex for NIR absorbing filter was also
conducted.
⇑
Corresponding author. Tel.: +86 411 84986237; fax: +86 411 84986230.
E-mail address: tinglima@dlut.edu.cn (T. Ma).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.07.046

620
Q. Miao et al. / Inorganica Chimica Acta 376 (2011) 619–627
1361, 1138, 882, 749, 696, 407. Anal. Calc. for C28H20NiS4: C,
61.89; H, 3.71; S, 23.60. Found: C, 61.55; H, 3.55; S, 23.20%. UV–
Vis (CH2Cl2, nm) kmax (e): 270 (38 306), 317 (53 182), 598 (2
219), 857 (33 885).
2.2.2. Synthesis of bis [1,2-di(4-methylphenyl)ethylene-1,2-ditholate]
nickel complex (B)
2.2.2.1. Preparation of4,40-dimethylbenzil (5). Compound 5 was pre-
pared by an improved procedure analogous to that for 4,40-disub-
stituted benzyl [29]. In a mechanically stirred and CaCl2 drying
tube dried suspension of anhydrous aluminum chloride (3.33 g,
25 mmol) and toluene (10.6 ml, 100 mmol, as the reactant and sol-
vent) at 0–5 °C was slowly added to oxalyl chloride (1.27 g,
10 mmol) in 5 ml toluene. The rufous reaction mixture was stirred
for 2 h and continued for another 10 h at room temperature. The
resulting mixture was quenched with crushed ice (20 g) mixed
with concentrated hydrochloric acid (2 ml). The organic phase
was separated and the aqueous phase was extracted with petro-
leum ether. The combined organic phases were washed with
water, saturated NaHCO3, water again and then dried over anhy-
drous Na2SO4. The solvent was removed in vacuum and the residue
was purified by recrystallization from 95% ethanol to give 1.67 g of
faint red sheet crystals (compound 5), yield 70.2%; 1H NMR
(400 MHz, d6-DMSO, d/ppm, J/Hz) d: 3.33 (s, 6H), 7.43 (d,
Fig. 1. Structures of the nickel bis(dithiolene) complexes.
2. Experimental
2.1. General analytical measurements
1H NMR spectra were taken with a Bruker Avance||-400 NMR
spectrometer (Switzerland). Chemical shifts were referenced to
TMS(Me4Si) and expressed as d (ppm) values. IR spectra were mea-
sured in KBr pellets with a Nicolet Nexus (USA) FT-IR instrument in
the range of 4000–400 cmꢀ1. Element analysis was recorded with a
vario EL III analyzer. All chemicals were commercially available
and were used without further purification.
J = 8.08 Hz, 4H), 7.80 (d, J = 8.08 Hz, 4H). FT-IR (KBr, cmꢀ1
) m:
2917, 1661, 1604, 1173, 830, 746.
2.2.2.2. Preparation of complex B [30]. Compound 5 (1.19 g, 5 mmol)
was refluxed with P2S5 (1.67 g, 7.5 mmol) in 15 ml of dioxane for
6 h. During this period, the thiophosphoric ester was formed. The
red brown reaction mixture was filtered to remove the excess
P2S5 and a solution of NiCl2ꢁ6H2O (0.58 g, 2.5 mmol) in 2.3 ml dis-
tilled water was added to the filtrate. The reaction mixture was re-
fluxed for 2–4 h and cooled by an ice-water bath. The dark green
crystal of the complex was formed and collected by filtration,
washed with a minimal amount of dioxane, water, ethanol, ether,
and then dried to give the crude complex B. Purification is effected
by recrystallization from dichloromethane to afford 0.93 g of dark
green solid (complex B), yield 62.1%; 1H NMR (400 MHz, CDCl3/
TMS, d/ppm, J/Hz) d: 2.33 (s, 12H), 7.09 (d, J = 8.10 Hz, 8H), 7.29
(d, J = 8.10 Hz, 8H). 13C NMR (100 MHz, CDCl3, d/ppm) d: 21.46
(methyl carbon), 128.80 (aryl carbon), 129.12 (aryl carbon),
138.69 (aryl carbon), 139.04 (aryl carbon), 181.35 (vinyl carbon).
2.2. Synthesis and characterization
2.2.1. Synthesis of bis(1,2-diphenylethylene-1,2-ditholate) nickel
complex (A)
2.2.1.1. Preparation of benzoin (2) [27]. A mixture of thiamine (VB1,
3.31 g, 11 mmol), distilled water (7 ml) and 95% ethanol (30 ml)
was stirred and cooled in the ice-water bath. The precooled 10%
NaOH solution (10 ml) and fresh benzaldehyde (compound 1,
21.22 g, 200 mmol) were slowly added to the mixture in turn. After
stirring, the obtained mixture was refluxed at 70 °C for 1.5 h in the
pH of 9–10. The hot reaction mixture was allowed to cool down to
room temperature and then immersed in an ice-water bath over-
night. The yellowish powder was collected by filtration and the
crude product was washed with water several times. Purification
was performed by recrystallization from 95% ethanol to give
12.69 g of white crystals (compound 2), yield 59.8%; 1H NMR
(400 MHz, CDCl3/TMS, d/ppm, J/Hz) d: 4.54 (s, 1H), 5.95 (s, 1H),
7.27–7.42 (m, 7H), 7.50–7.54 (m, 1H), 7.91 (d, J = 8.54 Hz, 2H).
FT-IR (KBr, cmꢀ1
807, 408. Anal. Calc. for C32H28NiS4: C, 64.11; H, 4.71; S, 21.39.
) m: 2972, 2916, 1603, 1407, 1358, 1141, 887,
Found: C, 63.72; H, 4.54; S, 21.29%. UV–Vis (CH2Cl2, nm) kmax (e):
276 (32 279), 320 (36 866), 600 (1 704), 884 (27 655).
FT-IR (KBr, cmꢀ1
) m: 3414, 3378, 3083, 3059, 3028, 2932, 1679,
1595, 1578, 1490, 1449, 1262, 1207, 1068, 755, 704.
2.2.3. Synthesis of bis [1,2-di(4-methoxycarbonylphenyl)ethylene-1,2-
ditholate] nickel complex (C)
2.2.1.2. Preparation of complex A [28]. Benzoin (2.12 g, 10 mmol)
was refluxed with P2S5 (3.33 g, 15 mmol) in 30 ml of dioxane for
10 h. During this period, the thiophosphoric ester of dithiobenzoin
was formed. The hot reaction mixture was filtered to remove the
excess P2S5 and a solution of NiCl2ꢁ6H2O (1.16 g, 4.9 mmol) in
8 ml distilled water was added to the filtrate. The reaction mixture
was refluxed for 2–4 h and cooled by an ice-water bath. Green-
black crystal of the complex was formed and collected by filtration,
washed with a minimal amount of dioxane, water, ethanol, and
ether. It was then dried to give a crude complex A. Purification
was conducted by recrystallization from dichloromethane to afford
1.63 g of green-black rod-like crystals (complex A), yield 60.0%; 1H
NMR (400 MHz, CDCl3/TMS, d/ppm, J/Hz) d: 7.29 (d, J = 7.47 Hz,
4H), 7.31 (d, J = 7.47 Hz, 4H), 7.37–7.38 (m, 8H), 7.39–7.40 (m,
4H). 13C NMR (100 MHz, CDCl3, d/ppm) d: 128.40 (aryl carbon),
128.93 (aryl carbon), 128.96 (aryl carbon), 141.21 (aryl carbon),
2.2.3.1. Preparation of 4,40-bis(methyloxycarbonyl)benzoin (7). Com-
pound 7 was synthesized by an improved scheme, combining the
two methods in previous studies [31,32]. Potassium cyanide
(0.98 g, 15 mmol) was added to the solution of methyl 4-form-
ylbenzoate (6) (8.21 g, 50 mmol) stirred in 99% ethanol (35 ml)
and water (9 ml). The reaction mixture was stirred at 30 °C for
15 min. The product was collected by filtration, washed with
water, dried under reduced pressure, and recrystallized from etha-
nol to give 6.70 g of pale yellow needles, yield 81.6%; 1H NMR
(400 MHz, d6-DMSO, d/ppm, J/Hz) d: 3.81 (s, 3H), 3.86 (s, 3H),
6.17 (s, 1H), 6.48 (s, 1H), 7.55 (d, J = 8.30 Hz, 2H), 7.90 (d,
J = 8.30 Hz, 2H), 8.01 (d, J = 8.50 Hz, 2H), 8.10 (d, J = 8.50 Hz, 2H).
2.2.3.2. Preparation ofcomplex C. Complex C was synthesized by our
methods. Compound 7 (0.98 g, 3 mmol) was refluxed with P2S5
(0.10 g, 4.5 mmol) in 8 ml of dioxane for 8 h. During this period,
181.64 (vinyl carbon). FT-IR (KBr, cmꢀ1
) m: 1571, 1493, 1442,

Q. Miao et al. / Inorganica Chimica Acta 376 (2011) 619–627
621
the thiophosphoric ester was formed. The red brown reaction mix-
ture was filtered to remove the excess P2S5 and a solution of
NiCl2ꢁ6H2O (0.36 g, 1.5 mmol) in 2 ml distilled water was added
to the filtrate. The reaction mixture was refluxed for 2 h, cooled
by an ice-water bath, and concentrated to remove the solvent.
The residue was collected by filtration, washed with a minimal
amount of dioxane, water, ethanol, ether, and dried to give the
crude product. Purification was performed by column chromatog-
raphy using silica gel and dichloromethane–petroleum ether mix-
ture (1/1; v/v) as the eluent to give 0.23 g of bottle green solid
(complex C), yield 19.8%; 1H NMR (400 MHz, CDCl3/TMS, d/ppm,
J/Hz) d: 4.30 (s, 12H), 7.41 (d, J = 8.26 Hz, 8H), 8.12 (d, J = 8.26 Hz,
performed at room temperature using an OTTLE cell with a Ag ref-
erence, a Pt counter and a Pt working electrode on a HP8453 (USA)
UV–Vis spectrophotometer. The CH2Cl2 solution was degassed by
bubbling argon through the cell for 10 min and tetrabutylammo-
nium hexafluorophosphate ([nBu4N][PF6], TBAHFP, Fluka, puriss
electrochemical grade, P99.0%) was used as supporting electrolyte
in a small ArCN solution. The bias was performed by a computer
controlled potentiostat (Zenium, Zahner, Germany). Electrochemi-
cal measurements were carried out on a BAS100W (USA) electro-
chemistry workstation at room temperature under argon, using
1 mM analyte in dry, deoxygenated dichloromethane (distilled
from CaH2) solution, supporting electrolyte (0.1 M [nBu4N][PF6]),
a glassy carbon working electrode, a Pt auxiliary electrode, an
Ag/AgCl reference electrode. The scan rate was 100 mV/s. Ferro-
cene was added and the ferrocenium/ferrocene (Fc+/Fc) redox cou-
ple was used as the internal reference for calibration. The
geometrical and electronic properties of the dyes were performed
with DFT calculations using the GAUSSIAN 03 software by B3LYP
and 6-31G (d). Current–voltage (I–V) curves of DSCs were con-
ducted by a Keithley digital source meter (Keithley 2601, USA) un-
der a 300 W solar simulator simulating the AM 1.5 spectrum
(100 mW/cm2, Solar Light Co., Inc., USA). The incident light inten-
sity was calibrated with a solar power meter (TES-1333R, Taiwan)
and a standard amorphous silicon solar cell (BS-520, Bunkoh-Keiki
Co., Ltd., Japan).
8H). FT-IR (KBr, cmꢀ1
) m: 2924,2853, 1717, 1637, 1596, 1464,
1404,1363, 1355, 1273, 1230, 1142, 1113, 1064, 1014, 889, 805,
409. Anal. Calc. for C36H28NiO8S4: C, 55.75; H, 3.64; S, 16.54. Found:
C, 55.70; H, 3.58; S, 16.42%. UV–Vis (CH2Cl2, nm) kmax (e): 261
(19 284), 328 (40 909), 601 (1034), 868 (18 518).
2.2.4. Synthesis of bis [1,2-di(4-carboxylphenyl)ethylene-1,2-
ditholate] nickel complex (D)
2.2.4.1. Preparation of 4,40-bis(methyloxycarbonyl)benzil (8) [31].
4.7 ml 48% aqueous hydrobromic acid was added slowly to the
solution of 7 (3.28 g, 10 mmol) in DMSO (23 ml) under stirring.
The solution was heated to 55 °C for 24 h followed by adding
25 ml water. The product was filtered, washed with water and
dried at 70 °C in vacuum. Yield: 3.23 g (99%), yellow solid. 1H
NMR (400 MHz, CDCl3/TMS, d/ppm, J/Hz) d: 3.99 (s, 6H), 8.07 (d,
J = 8.15 Hz, 4H), 8.20(d, J = 8.15 Hz, 4H).
2.5. Fabrication of DSCs
The TiO2 paste of commercial P25 powder (Degussa) using poly-
ethylene glycol 600 (PEG 600) as a dispersant was prepared,
according to the procedure developed by our group [33]. A
screen-printing method was used to fabricate TiO2 films on the
cleaned fluorine-doped tin oxide conducting glass (FTO glass, Asahi
Glass Co. Ltd.; sheet resistance: 10 ohm/square). The obtained elec-
trodes coated with the TiO2 paste were sintered at 500 °C for
30 min. While cooling to 40 °C, they were immerged into the dye
solutions and kept at room temperature for 20 h. The dye solutions
of the three nickel bis(dithiolene) complexes were saturated solu-
tions dissolved in dichloromethane. A sandwich cell was prepared
2.2.4.2. Preparation of 4,40-bis(hydroxycarbonyl)benzil (9) [31]. The
mixture of 8 (1.6 g, 4.9 mmol) in acetic acid (120 ml) and a 4:1
H2SO4/H2O solution (55 ml) was refluxed and stirred for 10 h. Then
80 ml water was added and the mixture solution was cooled on ice.
The filtration was performed. The product was washed with water,
and dried at 70 °C in vacuum. Yield: 1.4 g (96%), pale yellow solid.
1H NMR (400 MHz, DMSO/TMS, d/ppm, J/Hz) d: 8.08 (d, J = 8.20 Hz,
4H), 8.15 (d, J = 8.20 Hz, 4H), 13.38 (s, 2H).
2.2.4.3. Preparation ofbis [1,2-di(4-carboxylphenyl)ethylene-1,2-ditho
late] nickel complex (D). Compound 9 (0.89 g, 3 mmol) was refluxed
with P2S5 (0.10 g, 4.5 mmol) in 8 ml of dioxane for 8 h. During this
period, the thiophosphoric ester was formed. The red brown reac-
tion mixture was filtered to remove the excess P2S5 and a solution
of NiCl2ꢁ6H2O (0.36 g, 1.5 mmol) in 2 ml distilled water was added
to the filtrate. The reaction mixture was refluxed for 2 h, cooled by
an ice-water bath, and concentrated to remove the solvent. The
residue was collected by filtration, washed with a minimal amount
of dioxane, water, ethanol, ether, and dried to give the green prod-
uct. 1H NMR (400 MHz, DMSO/TMS, d/ppm, J/Hz)d: 8.07 (d,
J = 8.37 Hz, 8H), 8.14 (d, J = 8.37 Hz, 8H), 13.54 (s, 4H). FT-IR (KBr,
using the dye sensitized TiO2 film (15 l
m, 0.2 cm2), a Pt-sputtered
FTO counter electrode and the electrolyte containing 0.03 M I2,
0.06 M LiI, 0.6 M 1,2-dimethyl-3-propylimidazolium iodine (BMII),
0.1 M guanidinium thiocyanate and 0.5 M 4-tert-butylpyridine (4-
TBP) in acetonitrile.
2.6. Fabrication of NIR absorbing filter
Polymethylmethacrylate (PMMA) was solved in CH2Cl2 to form
a transparent and viscous solution, followed by adding the nickel
bis(dithiolene) complex. After stirring, the solution was kept in
vacuum and injected into the mold. The solvent was removed in
the oven to afford the green NIR absorbing filter. For contrast,
PMMA was fabricated in the same procedure without adding the
nickel bis(dithiolene) complex.
cmꢀ1
) m: 3429, 1670, 1596, 1404, 1268, 1204, 1174, 1047, 881,
840, 730, 696, 405. UV–Vis (CH2Cl2, nm) kmax: 315, 595, 890.
2.3. X-ray crystal structure determination
X-ray crystal structures were characterized by Bruker Smart
APEX||(Germany) CCD X-ray single crystal diffractometer. The
structures were solved by the SHELXL software. Detailed crystal data
and experimental parameters of complex B are given in Tables S1–
S3 (Supplementary material).
3. Results and discussion
3.1. Synthesis
The nickel bis(dithiolene) complexes were synthesized by ben-
zoin and benzil methods. A and C were obtained by benzoin meth-
ods, whereas B and D were afforded from benzil methods (Scheme
1). The synthesis of A and B were performed with corresponding
benzoin or benzyl, which followed the modified green synthetic
routes. When increasing the reflux time of the benzoin or benzyl
2.4. Physical measurements
The UV–Vis–NIR absorption spectra were recorded using a
HP8453 (USA) UV–Vis spectrophotometer in a 1 ꢂ 1 cm quartz
absorption cell. The spectroelectrochemical measurements were

622
Q. Miao et al. / Inorganica Chimica Acta 376 (2011) 619–627
Scheme 1. The synthetic routes of (a) A, (b) B, (c) C, and (d) D.
with P2S5, yields of the product were improved. There was no
report on the syntheses of C and D. We found that it was difficult
to obtain the corresponding complexes by the green synthetic
routes. Finally, C and D were successfully obtained by a simple
method as described in Scheme 1.
with a very limited tetrahedral distortion of B was found and all
the distances of the four Ni–S bonds are equal within experimental
error, which is consistent with the literature on the subject [34].
This means that the Ni center, four S atoms and two C@C are in a
square-planar environment (plane Ni), which results in the p-con-
jugated delocalization of the nine atoms. The four Ni–S bonds are
approximately equal (2.103(3)–2.108(3) Å) with the angles of
nearly 90° for S–Ni–S. The average bond lengths for S–C and C@C
are 1.692(5) and 1.390(6) Å, respectively. The benzyl groups are
not in the same plane with the square-planar Ni–S–C ring, rather,
they make torsion angles. The angles between the phenyl rings
and the metallacycle are not identical, ranging from 36–44°
(35.7°, 38.6°, 43.5°, 44.0°).
Fig. 3 shows the crystal stacking of A and B. Viewing along the a,
b and c-axis, the varying but orderly projection views can be ob-
tained for both A and B. The view of the unit cell of A shows that
the plane Ni stack in order along the a-axis, in which the shortest
intermolecular Niꢁ ꢁ ꢁNi distance is 5.955 Å. When it stacks along the
b-axis and c-axis, the shortest distances change to 11.096 and
18.640 Å, corresponding to the cell parameters. The same conclu-
sions can be made for the Sꢁ ꢁ ꢁS and Cꢁ ꢁ ꢁC in the plane. This further
confirms the fact that all the nine atoms containing the Ni center,
the four S atoms and the four C atoms are in the same plane. The
shortest intra-stack Niꢁ ꢁ ꢁNi distance of 4.888 Å for B is obtained
when it is stacked along the a-axis. However, the values are
11.395 and 15.220 Å while viewing along the b and c-axis,
respectively.
3.2. Structural properties
There are 10
numbered rings of the molecule. The metal center provides the
unoccupied orbital for the -electron delocalization, which belongs
p electrons for the nine atoms in the two five-
p
to the Hückel (4n + 2) system.
The dark green rod-like single crystals of A, B, and C were ob-
tained from the dichloromethane–hexane mixture. The crystal
structures of A and B were determined. With the small needles,
it was difficult to detect the X-ray crystal structure of C. The X-
ray crystal structure of B is shown in Fig. 2. The crystal system is
ꢀ
triclinic with the space group of P1. A square-planar configuration
3.3. IR spectra
The characteristic absorption peaks in the IR spectra of the com-
plexes are similar. The peaks at ca. 1358 cmꢀ1 can be assigned to
the stretching vibration of C@C bonds (1361 cmꢀ1 for A,
1358 cmꢀ1 for B, 1355 cmꢀ1 for C), whereas C@S bonds for A, B,
and C were recorded at 1138, 1141 and 1142 cmꢀ1, respectively.
The peaks near 885 cmꢀ1 originate from the stretching vibration
Fig. 2. The X-ray crystal structure of B.

Q. Miao et al. / Inorganica Chimica Acta 376 (2011) 619–627
623
Fig. 3. Projection views of the unit cells of A (top) and B (bottom) along the a, b and c-axis. The distances between the atoms are calculated by the diamond software.
of the couple mode C@S and C–Ph. The Ni–S stretching bonds of A,
B, and C appear at 407, 408 and 409 cmꢀ1, respectively. For com-
plex A, the mono-substituted benzene peak appears at 749 and
696 cmꢀ1. The symbolized peaks for para-substituted benzene in
B and C are 807 and 805 cmꢀ1, respectively. In addition, the peaks
below 3000 cmꢀ1 originate from the stretching vibrations of C–H of
the methyl group. As for C, the peaks at 1457, 1363, 1273, and
1065 cmꢀ1 are assigned to the bending vibration of C–H and
stretching vibration of C–O in COOCH3 [28].
3.4.2. The substituent effect
Fig. 4a shows the effect of the functional group attached to ben-
zene ring on the absorption maximum. It can be observed that the
methyl-substituted phenyl derivative B exhibits the maximum
absorption band at 884 nm in CH2Cl2. It is red-shifted by 27 and
16 nm compared with H-substituted complex A and methoxycar-
bonyl-substituted phenyl derivative C, respectively. This can be ex-
plained by the fact that the methyl group, being the electron
donating substituent, increases the electron density of the dithio-
lene moiety and decreases the gap of HOMO and LUMO, which
then results in the red-shift of the NIR absorption maximum of B.
The result is consistent with the conclusion of the recent review
[37]. For C, it is blue-shifted by 16 nm compared with B and red-
shifted by 11 nm compared with A. This can be explained as fol-
lows: the methoxycarbonyl group, being the electron withdrawing
substituent, decreases the electron density of the dithiolene moiety
as compared with B. When compared with A, the methoxycarbonyl
group of C increases the conjugation of the whole molecular plane
3.4. UV–Vis–NIR spectra
3.4.1. The solvent effect
A, B, and C exhibit intense absorption bands below 400 nm and
broad absorption bands with high molar extinction coefficients in
the NIR region (700–1100 nm) (Fig. 4a). The bands below 400 nm
originate from the ligand-to-metal charge-transfer (LMCT) bands
and intraligand transitions [35]. The intense feature absorption
and extends
p delocalization. The results show that the kmax of
bands in the NIR region are generally assigned to the
p ?
p⁄ tran-
nickel bis(dithiolene) complexes are effected by the substituted
groups, which are the integration of conjugation effect, induction
effect and field effect. On the other hand, the absorption spectra
of D in solution and on TiO2 film were shown in Fig. 4b. Both the
spectra of D in solution and on TiO2 film showed intense and broad
absorptions centered at around 890 nm in the NIR region. When
the dye was deposited on TiO2 film, the absorption band became
broader and the maximum absorption data slightly shifted to a
longer wavelength region. This can be attributed to the aggregation
of the dyes on the surface of TiO2 film [38].
sition between HOMO and LUMO [36]. The kmax values of A, B and C
are 857, 884 and 868 nm, respectively. The high molar extinction
coefficients are 33 885, 27 655, and 18 518 L molꢀ1 cmꢀ1
respectively.
,
The maximum absorption data of A, B, and C in different sol-
vents are shown in Table 1. The positive solvatochromism is con-
firmed as increasing the polarity of the solvent. From this, one
can interpret that the polar excited molecules of the complex are
more susceptible to the solvation effect than ground state mole-
cules when combined with the polar solvent, resulting in the lower
energy of the excited molecules. Correspondingly, the decreased
energy difference is achieved and the red-shifted kmax can be ob-
served. Furthermore, C suffers a larger effect when DMF and DMSO
were used, causing the peak to fall to 944 nm in DMF and 957 nm
in DMSO.
3.4.3. Spectroelectrochemistry
Fig. 5a displays the general formula of the ligands in nickel
bis(dithiolene) complexes. Different from the other complexes,
the spectroelectrochemistry of the nickel bis(dithiolene)

624
Q. Miao et al. / Inorganica Chimica Acta 376 (2011) 619–627
Fig. 5. (a) The general formula of the ligands in nickel bis(dithiolene) complexes;
(b) Absorption spectra of neutral A (solid line) and its electrochemically generated
monoanion and dianion (dotted line) in CH2Cl2 solution. The spectra were recorded
every 5 s.
nearly without the LLCT absorption [39]. This behavior is similar
to those reduced forms of square-planar bis(benzene-1,2-dithio-
lato) metal complexes [35] and is consistent with the results that
the absorption intensity in the NIR region of neutral complexes is
generally weakened upon oxidation or reduction [40,41].
Fig. 4. UV–Vis–NIR spectra of (a) A (red line), B (blue line), and C (green line) in
CH2Cl2 solution; (b) D in CH2Cl2 solution (blue line) and coated on the TiO2 film (red
line). (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this paper.)
3.5. Electrochemical properties
Table 1
In Fig. 6, cyclic voltammetry and differential pulse voltammetry
(CV/DPV) are applied to study the electron exchange in the com-
plexes. With excellent reversibility, three pairs of reversible redox
waves are observed, as shown in Fig. 6a and b. The peaks in Fig. 6a
(from left to right) are assigned to the couples of [A]1+/[A]0, [A]0/
[A]1ꢀ and [A]1ꢀ/[A]2ꢀ. The same conclusion can be made for B
[5,42,43]. It is known that the first oxidation potential corresponds
to the HOMO level of the dye [44]. The HOMO levels of A and B can
be obtained by using ferrocene as an internal standard [45]. The
values are 5.40 and 5.29 eV for A and B, respectively.
Maximum absorption data of the three nickel complexes in different solvents.
Samples
Solvents (polarity)
Et2O
(2.9)
CH2Cl2
(3.4)
THF
(4.2)
CHCl3
(4.4)
Dioxane DMF
DMSO
(7.2)
(4.8)
(6.4)
Complex 844
A
Complex 869
B
Complex 861
C
857
884
868
856
880
867
858
883
871
856
871
871
896
957
880
869
895
944
Fig. 6c shows the mono- and dielectron reduction of A, B and C.
The reversible peaks near 0 V show the monoelectron reduction of
the complexes with the half-wave potentials of E1/2(A)1 = 0.11 V,
E1/2(B)1 = 0.05 V and E1/2(C)1 = 0.24 V for A, B and C. In the same
way, the values of the dielectron reduction can be obtained as fol-
lows: E1/2(A)2 = ꢀ0.72 V, E1/2(B)2 = ꢀ0.78 V and E1/2(C)2 = ꢀ0.54 V.
These values are consistent with the results of DPV (Fig. 6d).
Fig. 6c also reveals the substituent effect of different groups on
the redox potentials. It is interesting to find that the complexes
are oxidized in the following order: B, A, C. The potentials of B
are the most negative, which can be explained by the fact that
the electron density at the center core is slightly increased due
to the electron-donating property of methyl groups. Thus, among
the three complexes, B can be oxidized most easily. With the
complexes is interesting and the absorption spectra of the electro-
chemically generated mono- and dianions of A are shown in
Fig. 5b. When the bias is performed, a drastic decrease in the LLCT
absorption band with the maxima of 852 nm is observed, which is
assigned to the neutral complex A ([Ni||(LÅ)(L)]). Its reduced mono-
anion form [Ni||(LÅ)(L)]ꢀ, on the other hand, shifts this maxima to
930 nm. The decreased bands at 592 nm, the increased bands near
400 nm, the new band at 529 nm, and the decreased band at
930 nm of the reduced anion form can be observed over time. Sub-
sequently, the corresponding dianion [Ni||(LÅ)(L)]2ꢀ is formed

Q. Miao et al. / Inorganica Chimica Acta 376 (2011) 619–627
625
Fig. 6. Cyclic voltammograms of (a) A and (b) B with Fc+/Fc; (c) Cyclic voltammograms and (d) differential pulse voltammograms of A (red line), B (blue line) and C (green
line) in CH2Cl2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
electron-withdrawing group, C is the most difficult to oxidize, due
to the decreased electron density.
3.7. Applications
3.7.1. DSCs
The NIR dyes of A, B, C and D were applied to fabricate DSCs. The
I–V curves are shown in Fig. 8. The open circuit voltage (VOC) of the
cell A, B, C, and D were 433, 454, 424, and 411 mV, respectively.
The short circuit current density (JSC) of 0.32, 0.32, 0.55, and
0.51 mA/cm2 were obtained for cell A, B, C, and D, and the energy
conversion efficiencies were in the range of 0.07–0.11%. The imper-
fect performance of DSCs based on nickel bis(dithiolene) com-
plexes can be attributed to the following reasons: First, the
efficiencies of DSCs based on NIR dyes are usually low due to the
lower energy of the NIR light. Second, for cell A, B and C, it is the
lack of chemical bonds between the dyes and TiO2 [46], which
can stimulate the excited electrons of dye to effectively inject into
the conduction band of TiO2. As for D with COOH group, in spite of
the bond between dye and TiO2, the energy level of the complex
cannot well match that of the DSC system. The LUMO of the dye
used in DSC must be higher than the conduction band of TiO2
(ECB = 3.9–4.2 eV [47,48]) and the HOMO level should be lower
than the redox potential of Iꢀ/I3ꢀ (4.85 eV) [47]. The subsequent
DFT calculations also show that the LUMO level of D is 4.27 eV. This
value is similar to the ECB of TiO2 and the excited electrons of D
cannot be injected effectively into the ECB of TiO2. As a result, the
electron cycle in the DSC system cannot possibly be proceed, there-
fore resulting in the low Jsc and the efficiency. Third, the DFT results
show a high localization of the electrons in the metallic ring plane
3.6. DFT calculations
Fig. 7 shows the geometric and electronic properties of the dyes
performed with DFT calculations. The LUMO and HOMO energy
levels (ELUMO and EHOMO) are obtained. The ELUMO are 3.79, 3.65,
4.16 and 4.27 eV for A, B, C and D, with the corresponding EHOMO
values of 5.34, 5.16, 5.73 and 5.85 eV, respectively. These results
are consistent with the substituent effect. The electron-donating
group increases the electron density of the dithiolene moiety with
the higher values of EHOMO and ELUMO, but more improvement is
obtained for EHOMO. Thereby the gap of HOMO and LUMO is de-
creased [5]. The electron distribution in the figure further validates
the planar structure and delocalized p-electrons of the complexes.
The electrons are distributed on both the dithiolene moiety and the
phenyl ring for HOMO. During electronic excitation at LUMO level,
the electrons move from the ligands to the nickel plane. The differ-
ent substituent groups affect the electron density in the phenyl
ring, with increased electron density for electron-donating group
and decreased electron density for electron-withdrawing group.
Furthermore, the HOMO levels of DFT are approximate to the val-
ues obtained by CV. For A, EHOMO by DFT was 5.34 eV, with the EHO-
of 5.40 eV by CV. For B, the HOMO level was 5.16 eV by DFT,
MO
with the EHOMO of 5.29 eV by CV. The results show that there is
consistency between DFT and CV.

626
Q. Miao et al. / Inorganica Chimica Acta 376 (2011) 619–627
Fig. 7. Frontier molecular orbital pictures of A, B, C and D base on DFT/B3LYP.
Fig. 9. Transmittance spectra of PMMA (blue line) and the NIR absorbing filters
Fig. 8. I–V curves of the DSCs based on A, B, C and D.
based on PMMA-A (red line). (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this paper.)
of the nickel bis(dithiolene) complexes, which is unfavorable for
charge separation. In general, for effective electron injection and
charge separation, the electrons at LUMO should distribute near
the COOH groups [49]. Although we introduced the electron with-
drawing substituent groups of COOMe and COOH into the struc-
ture, the symmetrical structures and the high localization of the
electrons of C and D reduces the electron withdrawing ability.
Our results demonstrated that the nickel bis(dithiolene) complexes
are promising NIR dyes due to their particular photoelectrochemi-
cal properties, however, to further improve the efficiency of DSCs,
it is necessary to modify the energy levels and the structures of
dithiolene complexes, such as changing metal center, introducing
stronger electron withdrawing group, and developing unsymmet-
rical structure. The related work is in progress.
the spectrum of PMMA. The green NIR absorbing filter based on
PMMA-A showed strong and broad absorption band in the range
of 700–1000 nm. In other words, the fabricated NIR absorbing filter
can effectively filter the NIR light in the range of 700–1000 nm.
This is very useful in many optical devices for martial applications,
such as filtering the interferential light of the light-emitting device
to the Night Vision Imaging System (NVIS) – compatible lighting.
With the superior properties, such as good photothermal stability,
excellent solubility, low toxicity, the nickel bis(dithiolene) com-
plex exhibits the great potential in the application of NIR absorbing
filters.
4. Conclusions
3.7.2. The NIR absorbing filters
The transmittance spectra of the NIR absorbing filter based on
complex A (PMMA-A) was shown in Fig. 9. It can be clearly seen
that there is no absorption band between 400 and 1100 nm in
We synthesized several nickel bis(dithiolene) complexes with
strong and broad absorptions in the NIR region (700–1100 nm).
The structure, optical spectroscopy, and electrochemical properties

Q. Miao et al. / Inorganica Chimica Acta 376 (2011) 619–627
627
[12] H. Shiozaki, H. Nakazumi, T. Kitao, J. Soc. Dyers Colour. 104 (1988) 173.
[13] S. Oliver, C. Winter, Adv. Mater. 4 (1992) 119.
[14] C.S. Winter, S.N. Oliver, J.D. Rush, C.A.S. Hill, A.E. Underhill, Mol. Cryst. Liq.
Cryst. 235 (1993) 181.
[15] G.C. Anyfantis, G.C. Papavassiliou, N. Assimomytis, A. Terzis, V. Psycharis, C.P.
Raptopoulou, P. Kyritsis, V. Thoma, I.B. Koutselas, Solid State Sci. 10 (2008)
1729.
[16] T.D. Anthopoulos, G.C. Anyfantis, G.C. Papavassiliou, D.M. de Leeuw, Appl.
Phys. Lett. 90 (2007) 122105.
[17] T.D. Anthopoulos, S. Setayesh, E. Smits, M. Cölle, E. Cantatore, B. de Boer,
P.W.M. Blom, D.M. de Leeuw, Adv. Mater. 18 (2006) 1900.
[18] T. Taguchi, H. Wada, T. Kambayashi, B. Noda, M. Goto, T. Mori, K. Ishikawa, H.
Takezoe, Chem. Phys. Lett. 421 (2006) 395.
[19] B. O’Regan, M. Grätzel, Nature 353 (1991) 737.
[20] H.J. Snaith, Adv. Funct. Mater. 20 (2010) 13.
[21] Simon Kuster, Frédéric Sauvage, Md.K. Nazeeruddin, Michael Grätzel, Frank A.
Nüesch, Thomas Geiger, Dyes Pigments 87 (2010) 30.
[22] Shogo Mori, Morio Nagata, Yuki Nakahata, Kazumasa Yasuta, Ryota Goto,
Mutsumi Kimura, Minoru Taya, J. Am. Chem. Soc. 132 (2010) 4054.
[23] T. Yamaguchi, Y. Uchida, S. Agatsuma, H. Arakawa, Sol. Energy Mater. Sol. Cells
93 (2009) 733.
[24] F. Inakazu, Y. Noma, Y. Ogomi, S. Hayase, Appl. Phys. Lett. 93 (2008) 093304.
[25] D.B. Kuang, P. Walter, F. Nüesch, S. Kim, J. Ko, P. Comte, S.M. Zakeeruddin, M.K.
Nazeeruddin, M. Grätzel, Langmuir 23 (2007) 10906.
[26] Q.Q. Miao, L.Q. Wu, J.N. Cui, M.D. Huang, T.L. Ma, Adv. Mater. 23 (2011) 2764.
[27] R. Breslow, J. Am. Chem. Soc. 80 (1958) 3719.
of these dyes were systematically studied. These NIR dyes were
first applied to DSCs and the photovoltaic performances of the de-
vices were investigated. With the essential characters of dyes used
in DSCs, such as high molar extinction coefficients and excellent
reversibility, the nickel bis(dithiolene) complexes exhibit the po-
tential for application in DSC. We found that the structure and
the proper energy level of the dyes are important factors that affect
the efficiency of DSCs. Although the efficiencies of the fabricated
DSCs are unsatisfied, our results are useful and helpful for design-
ing and modifying the structures of dyes to further improve the
efficiencies of DSCs based on the NIR dyes. There is also another
possibility for the NIR dyes with the visible dyes to be used in tan-
dem DSCs, hybrid DSCs and co-sensitized DSCs. Furthermore, the
great potential of the synthesized complexes for the application
of NIR absorbing filters is verified. With their particular photoelect-
rochemical properties, the nickel bis(dithiolene) complexes exhibit
promising prospects for future application.
Acknowledgments
[28] G.N. Schrauzer, V.P. Mayweg, J. Am. Chem. Soc. 87 (1965) 1483.
[29] B. Mohr, V. Enkelmann, G. Wegner, J. Org. Chem. 59 (1994) 635.
[30] K. Sung, R.H. Holm, J. Am. Chem. Soc. 124 (2002) 4312.
[31] M.H. Petersen, S.A. Gevorgyan, F.C. Krebs, Macromolecules 41 (2008) 8986.
[32] F. Akutsu, M. Inoki, K. Sunouchi, Y. Sugama, Y. Kasashima, K. Naruchi, M.
Miura, Polymer 39 (1998) 1637.
This research is financially supported by the National Natural
Science Foundation of China (No. 50773008) and the National High
Technology Research and Development Program for Advanced
Materials of China (No. 2009AA03Z220).
[33] T. Ma, T. Kida, M. Akiyama, Electrochem. Commun. 5 (2003) 369.
[34] R. Perochon, L. Piekara-Sady, W. Jurga, R. Clérac, M. Fourmigué, Dalton Trans.
(2009) 3052.
Appendix A. Supplementary material
[35] K. Ray, T. Weyhermu1ller, F. Neese, K. Wieghardt, Inorg. Chem. 44 (2005) 5345.
[36] H. Shiozaki, H. Nakazumi, Y. Nakado, Chem. Lett. (1987) 2393.
[37] B.G. Bonneval, K.I. Moineau-Chane Ching, F. Alary, T.-T. Bui, L. Valade, Coord.
Chem. Rev. 254 (2010) 1457.
[38] Xiao-Feng Wang, Osamu Kitao, Haoshen Zhou, Hitoshi Tamiaki, Shin-ichi
Sasaki, J. Phys. Chem. C 113 (2009) 7954.
[39] S. Kokatam, K. Ray, J. Pap, E. Bill, W.E. Geiger, R.J. LeSuer, P.H. Rieger, T.
Weyhermuller, F. Neese, K. Wieghardt, Inorg. Chem. 46 (2007) 1100.
[40] P. Chaudhuri, C.N. Verani, E. Bill, E. Bothe, T. Weyhermuller, K. Wieghardt, J.
Am. Chem. Soc. 123 (2001) 2213.
CCDC 814307 contains the supplementary crystallographic data
for B this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif. Supplementary data associated with this arti-
cle can be found, in the online version, at doi:10.1016/
j.ica.2011.07.046.
References
[41] D. Herebian, E. Bothe, E. Bill, T. Weyhermuller, K. Wieghardt, J. Am. Chem. Soc.
123 (2001) 10012.
[42] G.C. Papavassiliou, G.C. Anyfantis, C.P. Raptopoulou, V. Psycharis, N. Ioannidis,
V. Petrouleas, P. Paraskevopoulou, Polyhedron 28 (2009) 3368.
[43] S.S. Alix, G.B. Bénédicte, I.C. Kathleen, V. Lydie, J. Electroanal. Chem. 624 (2008)
84.
[44] Z. Kong, H. Zhou, J. Cui, T. Ma, X. Yang, L. Sun, J. Photochem. Photobiol. A:
Chem. 213 (2010) 152.
[45] Robert R. Gagné, Carl A. Koval, George C. Lisensky, Inorg. Chem. 19 (1980)
2855.
[46] T. Ma, K. Inoue, H. Noma, K. Yao, E. Abe, J. Photochem. Photobiol. A: Chem. 152
(2002) 207.
[47] F. Lenzmann, J. Krueger, S. Burnside, K. Brooks, M. Grätzel, D. Gal, S. Rühle, D.
Cahen, J. Phys. Chem. B 105 (2001) 6347.
[48] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 110 (2010)
6595.
[49] M.K. Nazeeruddin, F.D. Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito,
B. Takeru, M. Grätzel, J. Am. Chem. Soc. 127 (2005) 16835.
[1] G.N. Schrauzer, V. Mayweg, J. Am. Chem. Soc. 84 (1962) 3221.
[2] W. Paw, R. Eisemberg, Inorg. Chem. 36 (1997) 2287.
[3] P.L. Hill, L.Y. Lee, T.R. Younkin, S.D. Orth, L.M. White, Inorg. Chem. 36 (1997)
5655.
[4] D. Belo, M. Almeida, Coord. Chem. Rev. 254 (2010) 1479.
[5] T.T. Bui, B. Garreau-de Bonneval, K.I. Moineau-Chane Ching, New J. Chem. 34
(2010) 337.
[6] J.Y. Cho, B. Domercq, S.C. Jones, J.S. Yu, X.H. Zhang, Z.S. An, M. Bishop, S. Barlow,
S.R. Marder, B. Kippelen, J. Mater. Chem. 17 (2007) 2642.
[7] M.C. Aragoni, M. Arca, F.A. Devillanova, F. Isaia, V. Lippolis, A. Mancini, L. Pala,
G. Verani, T. Agostinelli, M. Caironi, D. Natali, M. Sampietro, Inorg. Chem.
Commun. 10 (2007) 191.
[8] K.H. Drexhage, G.A. Reynolds, Opt. Commun. 10 (1974) 18.
[9] H. Shiozaki, H. Nakazumi, Bull. Chem. Soc. Jpn. 63 (1990) 265.
[10] H. Shiozki, H. Nakazumi, Bull. Chem. Soc. Jpn. 63 (1990) 2653.
[11] I.H. Song, C.H. Rhee, S.H. Park, S.L. Lee, D. Grudinin, K.H. Song, J. Choe, Org.
Process Res. Dev. 12 (2008) 1012.