Synthesis, characterization and optical limiting effect of nickel complexes of multi-sulfur 1,2-dithiolene

Article abstract of DOI:10.1016/j.jpcs.2004.09.015

Three nickel complexes with a new multi-sulfur 1,2-dithiolene ligand, (n-Bu₄N)[Ni(cddt)₂] 1, (Ph₄P)[Ni(cddt) ₂] 2 and [Ni(cddt)₂] 3 (cddt = 4a, 6, 7, 7a-5H-cyclopenta[b]-1,4-dithiin-2,3-dithiolate), h

Full text of DOI:10.1016/j.jpcs.2004.09.015

Journal of Physics and Chemistry of Solids 66 (2005) 207–212
www.elsevier.com/locate/jpcs
Synthesis, characterization and optical limiting effect of nickel
complexes of multi-sulfur 1,2-dithiolene
a
a,
b
a
*
Yong Ji , Jing-Lin Zuo , Lixue Chen , Yun-Qi Tian ,
Yinglin Songb, Yi-Zhi Lia, Xiao-Zeng Youa
aState Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute,
Nanjing University, Han Kou road, Nanjing 210093, People’s Republic of China
bDepartment of Applied Physics, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
Received 16 April 2004; revised 10 August 2004; accepted 1 September 2004
Abstract
Three nickel complexes with a new multi-sulfur 1,2-dithiolene ligand, (n-Bu4N)[Ni(cddt)2] 1, (Ph4P)[Ni(cddt)2] 2 and [Ni(cddt)2] 3
(cddtZ4a, 6, 7, 7a-5H-cyclopenta[b]-1,4-dithiin-2,3-dithiolate), have been synthesized and characterized by electrochemical measurements,
IR, EPR and UV–Vis-NIR spectroscopies. The crystal structure of complex 2 is determined. Their optical nonlinearities are measured by the
Z-scan technique with an 8 ns pulsed laser at 532 nm and all exhibit NLO absorptive abilities. Complexes 1 and 2 both exhibit effective self-
defocusing performance (n2ZK5.81!10K10 esu for 1 and K4.51!10K10 esu for 2). The optical limiting (OL) effects were observed with
nanosecond and picosecond laser pulses. The OL capability of complex 3 is superior to C60 at the same experimental condition in ns
measurements.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: A. Optical materials; B. Crystal growth; D. Crystal structure; D. Optical properties
1. Introduction
[23–32], which are generally regarded as the best candidates
for optical limiting application. As part of our continued
work on exploring better OL materials, we report here the
synthesis and properties of three new nickel complexes
based on a new multi-sulfur dithiolene ligand, 4a, 6, 7, 7a-
5H-cyclopenta[b]-1, 4-dithiin-2, 3-dithiolate (cddt). The
nickel complexes exhibit excellent optical limiting effects
towards nanosecond and picosecond laser pulses. In this
paper, the crystal structure of Ph4P[Ni(cddt)2] 2 is also
presented.
In recent years, bis-1,2-dithiolene complexes of tran-
sition metals have been widely studied due to their
interesting properties and applications in areas such as
conductivity, magnetism, non-linear materials and catalysis
[1–15]. They have larger delocalized systems and can exist
in several clearly defined oxidation states. Also they exhibit
high thermal and photochemical stabilities. The third-order
optical nonlinearity and the optical limiting (OL) ability of
nickel complexes with multi-sulfur 1,2-dithiolene ligands
have been investigated by our group previously [9,10]. An
ideal optical limiter should have the capability of being
transparent under laser radiation of low energies and opaque
at high energies. The most frequently reported materials are
fullerenes (C60) [16–22], and phthalocyanine complexes
2. Experimental
2.1. Synthesis
All solvents were of analytical grade and used without
further purification. All reactions were carried out under N2.
The ligand precursor, cisK4a, 6, 7, 7a-tetrahydro-5H-
cyclopenta[b]-1, 3-dithiolo[4, 5-e][1,4]dithiin-2-thione was
* Corresponding author. Tel.: C86 25 8359 3893; fax: C86 25 8331
7761.
E-mail address: zuojl@nju.edu.cn (J.-L. Zuo).
0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2004.09.015

208
Y. Ji et al. / Journal of Physics and Chemistry of Solids 66 (2005) 207–212
prepared by the (2C4) cycloaddition reaction as described
in the literature [33].
measurement. UV–VIS spectra were recorded on a
UV-3100 spectrophotometer. The ESR spectrum was
recorded on a Bruker ER 200-D-SRC spectrometer.
2.1.1. Preparation of (n-Bu4N)[Ni(cddt)2] (1)
Potassium hydroxide (2.0 g, 35.7 mmol) and cis-4a, 6, 7,
7a-tetrahydro-5H-cyclopenta[b]-1,3-dithiolo[4,5-e][1,4]
dithiin-2-thione (1.0 g, 3.8 mmol) were added to 20 mL of
ethanol. The reaction mixture was stirred for 1 h at 40 8C.
The resulting pale yellow microcrystals of K2cddt were
isolated by centrifugation. Then it was dissolved instantly in
methanol (20 mL). A solution of NiCl2$2H2O (0.45 g,
1.9 mmol) in 20 mL of methanol was added dropwise to this
solution. After stirring at room temperature for 30 min, the
solution was then exposed to air for 15 min. After filtration,
1 equiv. of tetrabutylammonium bromide was added to the
filtrate, and dark green solids were precipitated immedi-
ately. The solids were collected by filtration and then
recrystallized from acetone. The yield was 0.99g (70%).
Anal. Calc. for C30H52NNiS8 (%): C, 48.58; H, 7.01; N,
1.88; Ni, 7.91. Found: C, 47.92; H, 6.85; N, 1.82; Ni, 8.27.
FT-IR (KBr, cmK1): 2953(m), 1459(m), 1438(m), 1373(s),
881(m).
2.3. Structure determination
Green crystals of 2 were obtained upon recrystallization
from CH2Cl2 and CH3OH. Intensities data were measured
on a Bruker Appex CCD diffractometer with monochro-
˚
mated Mo-Ka (lZ0.71073 A) and processed using Smart
[34]. The structure was solved by direct methods. It belongs
to the triclinic system, P-1 space group. The cell
constants are aZ10.474(2), bZ12.975(2), cZ16.237(2)
˚
A, aZ86.60(1), bZ71.25(1), gZ66.52(1)8, VZ
3
1910.5(6) A , which were confirmed in the subsequent
˚
refinement. The structure was refined using the SHELXTL [34]
program package. All non-hydrogen atoms were refined
anistropically by full-matrix least squares. The hydrogen
atoms were placed in their calculated positions with C–HZ
˚
0.93–0.96 A. All the hydrogen atoms were assigned fixed
isotropic thermal parameters (1.2–1.5 times of the atom they
attached), and allowed to ride on their respective parent
atoms. A full-matrix least squares calculation using 4437
unique reflections (IO2.0s (I)) gave a final refinement with
R factors of RZ0.0533 and RwZ0.1386.
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic Data
Centre, CCDC No. 185926. Copies of this information
may be obtained from The Director, CCDC, 12 Union
Road, Cambridge, CB2 1EZ, UK (fax: C44 1233 336033;
e-mail to: deposit@ccdc.cam.ac.uk or www: http://www.
ccdc.cam.ac.uk).
2.1.2. Preparation of (Ph4P) [Ni(cddt)2] (2)
This complex was synthesized by mixing 1 with excess
Ph4PBr in CH3CN. The green precipitate was collected by
filtration and washed with methanol and ether, and then
dried in vacuo. The yield was 0.42 g (75%). Anal. Calc. for
C38H36NiPS8 (%): C, 54.44; H, 4.32; Ni, 7.00. Found: C,
53.75; H, 4.25; Ni, 7.26. FT-IR (KBr, cmK1): 3050(w),
2926(m), 1624(m), 1437(m), 1374(s), 1108(s), 723(s),
688(s).
2.1.3. Preparation of [Ni(cddt)2] (3)
2.4. Optical measurements
This complex was prepared by the reaction of 1 and
excess iodine in acetone. The black precipitate was
collected by filtration and washed with methanol and
ether, and then dried in vacuo. The yield was 0.28 g
(80%). Anal. Calc. for C14H16NiS8 (%): C, 33.67; H, 3.23;
Ni, 11.75. Found: C, 32.89; H, 3.45; Ni, 11.66. FT-IR (KBr,
cmK1): 2953(m), 1624(m), 1443(m), 1263(s), 1231(s),
1186(s), 824(m).
Complexes 1 and 2 were dissolved in DMF (the neutral
complex 3 in toluene) and contained in 5-mm-thick quartz
cells for the measurements of third-order nonlinear optical
properties and optical limiting effects, which were per-
formed with linearly polarized 8 ns pulsed at 532 nm
generated from a Q-switched frequency-doubled Nd:YAG
laser. Three complexes are all stable toward air and laser
light under experimental conditions. The spatial profiles of
the optical pulses were of nearly Gaussian transverse mode.
The pulsed laser was focused onto the sample cell with a
30 cm focal length mirror. The energy of input and output
pulses was measured simultaneously by precision laser
detectors (818J-09B, Newport Corp.), which were linked to
a computer by a New-port Com. Attenuator. The exper-
imental data were collected utilizing a single shot at a rate of
1 pulse per second to avoid the influence of thermal and
long-term effects.
2.2. Physical measurements
CHN analyses were performed using a Perkin-Elmer
240C analytical instrument. Ni element analysis was
measured on a Jarrell-ash ICP quantimeter. IR spectra
were collected on a Shimadzu 440 spectrometer with KBr
pellets. The cyclic voltammetry was performed by a model
79-1V-Analyser with a electrochemical cell using a
platinum wire as the working electrode, a platinum plate
as auxiliary electrode and Ag/AgCl as reference electrode.
Measurements were made in CH2Cl2 using 0.4 M tetra-
butylammonium perchlorate as the supporting electrolyte,
and nitrogen was passed for 15 min prior to each
The third-order NLO absorptive and refractive proper-
ties of these complexes were determined by performing
Z-scan measurements [35]. The samples were mounted
on a computer-controlled translation stage moving along

Y. Ji et al. / Journal of Physics and Chemistry of Solids 66 (2005) 207–212
209
Fig. 2. Perspective view of [Ni(cddt)2]- with atom numbering scheme. The
thermal ellipsoids are drawn at the 30% probability level.
It shows three redox couples in the recorded region: the first
one-electron reduction assigned to [Ni(cddt)2]2K
/[Ni(cddt)2]K is quasi-reversible (E1/2ZK0.69 V); the
second reversible peak (E1/2Z0.08 V) is assigned to
[Ni(cddt)2]K/[Ni(cddt)2]; the third quasi-reversible peak is
also observed at a more positive potential (E1/2Z0.92 V),
which can be reasonably assigned to oxidation of the ligand.
The frozen-glass ESR spectrum of complex 2 in DMF at
128 K shows three peaks with g1Z2.114, g2Z2.050,
g3Z1.998, which are closer to the values of similar
nicklel-dithiolene complexes [10].
Scheme 1.
the Z-direction with respect to the focal point instead of
being positioned at its focal point. To determine both the
sign and magnitude of the nonlinear refraction, a 2 mm
diameter aperture was placed in front of the transmission
detector and the transmittance recorded as a function of
the sample position on the Z-axis (closed-aperture Z-scan).
To measure the nonlinear absorption, the Z-dependent
sample transmittance was recorded without the aperture
(open aperture Z-scan).
The UV–Vis-NIR spectra of (Ph4P)[Ni(cddt)2] 2 and
[Ni(cddt)2] 3 are shown in Fig. 1. Complex 2 in acetonitrile
solution shows a strong broad absorption at 1034 nm
(3Z11500), which is assigned to a p/p* transition
(2b1u/3b2g) [5]. Complex 3 in benzene solution exhibits
this absorption at 982 nm (3Z46000). Such a strong
intensity is perhaps due to the strongly coordinated donors
in the ligands.
3. Results and discussion
3.2. Crystal structure of complex 2
3.1. Synthesis
The molecular structure of (Ph4P)[Ni(cddt)2] 2 with atom
numbering scheme is shown in Fig. 2, and selected bond
distances and angles are listed in Table 1.
The synthesis of complex 1 and 3 is outlined in Scheme 1.
The electrochemistry of the complex anion [Ni(cddt)2]K
in 1 is investigated by cyclic voltammetry in CH2Cl2.
The four S atoms surround the Ni atom to give a square-
planar geometry with nearly equal Ni–S bond lengths
Table 1
˚
Selected bond distances (A) and angles (8) in (Ph4P)[Ni(cddt)2] 2
Ni1–S2
2.150(1)
1.739(5)
1.750(5)
1.744(5)
1.352(7)
1.512(7)
1.45(1)
Ni1–S1
2.156(1)
1.731(5)
1.803(5)
1.824(5)
1.51(1)
S1–C1
S2–C2
S3–C1
S3–C3
S4–C2
S4–C7
C1–C2
C3–C4
C3–C7
C4–C5
1.49(1)
C5–C6
C6–C7
1.525(7)
103.6(2)
104.4(2)
119.9(4)
122.9(4)
118.8(3)
120.9(4)
107.1(5)
105.6(6)
107.0(5)
108.0(4)
S1–Ni1–S2
Ni1–S2–C2
C2–S4–C7
S1–C1–S3
S4–C7–C3
S2–C2–C1
S3–C3–C7
S3–C3–C4
C4–C5–C6
C3–C7–C6
91.91(4)
103.8(2)
99.5(2)
Ni1–S1–C1
C1–S3–C3
S1–C1–C2
S3–C1–C2
S2–C2–S4
S4–C2–C1
C4–C3–C7
C3–C4–C5
C5–C6–C7
S4–C7–C6
116.9(3)
115.0(4)
120.3(4)
117.6(3)
109.3(5)
110.7(6)
105.3(4)
Fig. 1. UV–VIS-NIR spectra of complex 2 (1.08!10K4 M) in CH3CN (–)
and complex 3 (4!10K5 M) in C6H6 (–) at 25 8C.

210
Y. Ji et al. / Journal of Physics and Chemistry of Solids 66 (2005) 207–212
the conditions used in this work, can be represented by
equations (1) and (2) [35,37]:
ð
CN
2
1
pffiffiffi
ð
TðzÞ Z
qðzÞ Z
ln½1 CqðzÞeKt ꢀdt
(1)
(2)
pqðzÞ
KN
ꢀ
ꢁ
ð
CN CN
2
2
I0
½K2ðr=u0Þ Kðt=t0Þ ꢀ
e
a2
2
1 Cðz=z0Þ
KN
0
ꢀ
ꢁ
1 KeKa L
0
!
r dr dt
a0
where light transmittance, T, is a function of the sample’s
Z-position (against focal point zZ0), z is the distance of the
sample from the focal point, L is the sample thickness, I0 is
the peak irradiation intensity at focus, z0 Zpu20=l (where u0
is the spot radius of the laser pulse at focus and l is the laser
wave length), r is the radial coordinate, t is the time, t0 is the
pulse width, and a2 is effective third-order NLO absorptive
coefficient.
Fig. 3. Packing diagram of 2 looking down the a axis.
and S–Ni–S angles. The Ni atom lies at the origin, and the
S2CaCS2 unit in the cddt ligand is planar. The gross
geometry of [Ni(cddt)2]K is very similar to that of
[Ni(dddt2)]K [15]. The average C–S, Ni–S and CZC
bond lengths are 1.735(5), 2.153(1) and 1.352(7) A,
respectively. The corresponding values in the (Bu4N)
The nonlinear refractive properties of these complexes
were accessed under the closed-aperture configuration while
the absorptive properties under the open-aperture configur-
ation. The valley–peak pattern of the normalized transmit-
tance curve under the closed-aperture configuration shows
characteristic self-defocusing behavior of the samples 1 and
2.The non-linear refraction of complex 3 is negligible. The
effective third-order nonlinear refractive index n2 of
complexes 1 and 2 can be derived from the difference
between the normalized transmittance values at the valley
and peak positions (DTvKp) using Eq. (3) [38]
˚
˚
[Ni(dddt)2] are 1.75, 2.137 and 1.37 A [15]. The difference
in bond lengths can be explained as the result of steric and
electronic effects of the external units of the ligand. The
cyclopentene ring adopts an envelope conformation with the
trans orientation.
Owning to the planar geometry, the [Ni(cddt)2]K units
can be stacked along the a-axis to generate columns which
are separated from one another by Ph4PC ions, as shown in
la0
0:812pIð1 KeKa L
n2 Z
DTvKp
(3)
˚
0
Fig. 3. The closest Ni/Ni distance is 10.474(2) A, which is
Þ
the length of the a axis. The shortest intermolecular S/S
˚
contact of 5.019(3) A occurs between S3 and S5, which is
˚
larger than the sum of the van der Waals radii (3.70 A) [36].
The NLO absorptive coefficients (a2) of complexes 1–3
were calculated to be 2.201!10K11, 1.467!10K11 and
4.991!10K11 mWK1, respectively. From the different
values between normalized transmittance at valley and
peak portions, the NLO refractive index (n2) were
calculated to be K5.81!10K10 and K4.51!10K10 esu
for 1 and 2.The negative values of the third-order nonlinear
refraction of complexes 1 and 2 also indicate that there are
self-defocusing effects in the NLO refractive behaviors of
these two bis-1,2-dithiolene complexes.
3.3. Nonlinear optical absorption and refraction
The third-order nonlinear optical properties of the three
complexes were investigated with 532 nm laser pulses of
8 ns duration (I0Z4.48!1012 W mK2) in each solution,
respectively. The results from the Z-scan experiments [34]
for these three complexes are summarized in Table 2.
Typical results from the Z-scan experiments for complexes
3 are displayed in Fig. 4. The nonlinear absorption
components were evaluated by the Z-scan method under
an aperture configuration and the NLO absorptive exper-
imental data for complexes 1–3 obtained under
3.4. Optical limiting capabilities
It has been documented that, generally, the OL behavior
of the NLO materials is affected by their NLO absorptive
Table 2
Nonlinear optical parameters of complexes 1–3 measured at 532 nm with ns laser pulses
Complex
a2 (m WK1
)
Im c(3) (esu)
n2 (esu)
Re c(3) (esu)
1
2
3
2.201!10K11
1.467!10K11
4.991!10K11
7.218!10K13
4.812!10K13
18.01!10K13
K5.81!10K10
K4.51!10K10
1.468!10K13
1.142!10K13

Y. Ji et al. / Journal of Physics and Chemistry of Solids 66 (2005) 207–212
211
Fig. 4. Z-scan results for complex 3 (in 2.19!10K4 M) measured without
the aperture with the ns pulses.
and refractive performances. The optical limiting effects of
the three complexes were measured under nanosecond and
picosecond laser pulse, respectively. Fig. 5 shows the
normalized transmittance of complex 3 measured with ns
and ps pulses. In order to compare the optical limiting
capability conveniently, we select the same or nearly the
same linear transmittance of 83%. The limiting threshold,
which is defined as the incident fluence at which the
transmittance fall to 50% of the corresponding linear
transmittance, were summarized in Table 3. As shown in
Fig. 5, the optical limiting ability of complex 3 is better than
that of C60 in ns measurements while it is nearly the same as
that of C60 in ps measurements. The limiting threshold was
measured as 0.12 and 0.09 J cmK1 for complex 3 under ns
and ps laser pulses, respectively. At the same experimental
condition, the limiting thresholds of C60 are determined to
be 0.20 and 0.10 J cmK1. Complex 3 has the capability of
being more transparent than C60 under laser radiation of low
energies and more opaque at high energies. For the non-
linear refraction of the complexes 3 is negligible, the
observed optical limiting behavior of this complex can
mostly be attributed to non-linear absorptive processes.
Complexes 1 and 2 display almost the same OL ability,
which suggests that the counter ions have no significant
effects on the third-order susceptibility c(3) and OL
behavior. It is noting that the neutral complex 3 show the
best OL performance in this series of compounds, which
may stem from its higher oxidation state and the tight
structure. For the advantages that they have highly
delocalized p-systems and can exist in several clearly
defined oxidation states; high thermal and photochemical
stabilities; almost transparent in the visible and near-IR
region (400–900 nm), these nickel–dithiolene complexes
are promising candidates for optical limiting applications.
The next work in our laboratory is to investigate the OL
properties of those bis(dithiolene) complexes with heavier
Fig. 5. Fluence-dependent transmittance of complex 3 (in the open
triangles, 6) and C60 (in the filled block, &) measured at 532 nm. (a) the
data measured with ns laser pulses for C60 (2.50!10K4M) and 3 (3.85!
10K4M). (b) the data measured with ps laser pulses for C60 (2.50!10K4M)
and 3 (3.85!10K4M), respectively.
Table 3
The limiting thresholds of some optical limiting materials measured at
532 nm with nano- and picosecond-duration laser pulses
Compound
Solvent
Laser
pulse
width
Linear
trans-
mission
(%)
Limiting
threshold
(J cmK2
)
C60
Toluene
DMF
Ns
Ns
Ns
Ns
Ps
Ps
Ps
Ps
80
85
85
83
83
85
85
83
0.20
1.0
(Bu4N)[Ni(cddt)2]
(Ph4P)[Ni(cddt)2]
[Ni(cddt)2]
DMF
1.0
Toluene
Toluene
DMF
0.12
0.10
0.20
0.20
0.09
C60
(Bu4N)[Ni(cddt)2]
(Ph4P)[Ni(cddt)2]
[Ni(cddt)2]
DMF
Toluene

212
Y. Ji et al. / Journal of Physics and Chemistry of Solids 66 (2005) 207–212
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