Synthesis, characterization and optical non-linearity of two heterobimetallic copper clusters containing tetraselenometalates

Article abstract of DOI:10.1016/S0022-2860(99)00414-7

Cluster [{MoCu₃Se₃Br}(PPh₃)₃Se]·THF·H₂O (1·3THF·H₂O) was prepared from reaction of [Et₄N]₂[MoSe]₄, Cu(PPh₃)₂NO₃ and Et₄N·Br in CH₂Cl₂ solution; also, 1 can be also obtained from the reaction of [MoSe₄Cu₄Py₆Br₂] and excess PPh₃ in a DMFCH₂Cl₂ mixture solvent. The ⁹⁵Mo NMR technique was used to monitor the above two reaction processes. X- ray crystallographic structure determination shows that it contains a strongly distorted cubane-like {MoCu₃Se₃Br} core. The coordination of the central Mo atom and each Cu atom are distorted from tetrahedral. Cluster [{WCu₃Se₃C1}(PPh₃)₃Se] (2) was synthesized by the reaction of [Et₄N]₂[WSe]₄ and Cu(PPh₃)₂Cl in the solid state for nonlinear optical (NLO) studies. Its structure was reported by Ibers (Inorg. Chem. 31 (1992) 4365). The NLO properties of clusters 1 and 2 were studied. Both NLO absorption and refraction were obtained, and their effective third-order non- linearities were detected with α₂ = 3.4 x 10^-10 and 5.9 x 10^-10 m/W and n₂ = - 1.5 X 10^-17 and - 1.3 x 10^-17 m²/W, respectively, for the same concentration CH₂C1₂ solution of 1 and 2. Influence of skeletal atoms to nonlinear absorption is also discussed in the paper. (C) 2000 Elsevier Science B.V.

Full text of DOI:10.1016/S0022-2860(99)00414-7

Journal of Molecular Structure 525 (2000) 79–86
www.elsevier.nl/locate/molstruc
Synthesis, characterization and optical non-linearity of two
heterobimetallic copper clusters containing tetraselenometalates
Qian-Feng Zhanga, Chi Zhanga, Ying-Lin Songb, Xin-Quan Xina,
*
aState Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, Nanjing University, Nanjing 210093,
People’s Republic of China
bDepartment of Physics, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
Received 16 August 1999; accepted 11 October 1999
Abstract
Cluster [{MoCu3Se3Br}(PPh3)3Se]·3THF·H2O (1·3THF·H2O) was prepared from reaction of [Et4N]2[MoSe]4, Cu(PPh3)2NO3
and Et4N·Br in CH2Cl2 solution; also, 1 can be also obtained from the reaction of [MoSe4Cu4Py6Br2] and excess PPh3 in a DMF–
CH2Cl2 mixture solvent. The 95Mo NMR technique was used to monitor the above two reaction processes. X-ray crystal-
lographic structure determination shows that it contains a strongly distorted cubane-like {MoCu3Se3Br} core. The coordination
of the central Mo atom and each Cu atom are distorted from tetrahedral. Cluster [{WCu3Se3Cl}(PPh3)3Se] (2) was synthesized
by the reaction of [Et4N]2[WSe]4 and Cu(PPh3)2Cl in the solid state for nonlinear optical (NLO) studies. Its structure was
reported by Ibers (Inorg. Chem. 31 (1992) 4365). The NLO properties of clusters 1 and 2 were studied. Both NLO absorption
and refraction were obtained, and their effective third-order non-linearities were detected with a2 ˆ 3:4 × 10Ϫ10 and 5:9 ×
10Ϫ10 m=W and n2 ˆ Ϫ1:5 × 10Ϫ17 and Ϫ1:3 × 10Ϫ17 m2=W; respectively, for the same concentration CH2Cl2 solution of 1 and
2. Influence of skeletal atoms to nonlinear absorption is also discussed in the paper. ᭧ 2000 Elsevier Science B.V. All rights
reserved.
Keywords: Synthesis; Crystal structure; 95Mo NMR spectrum; Heteroselenometallic cluster; NLO properties
1. Introduction
years [8,9]. Up to now, a few heteroselenometallic
clusters containing [MSe4]2Ϫ (M ˆ Mo, W) anions
have been synthesized and characterized [10–14],
however, only few non-linear optical (NLO) results
of these clusters have been reported. As part of our
interest in NLO properties of Mo(W)–Cu(Ag,Au)–S
system, we have characterized a series of clusters and
explored their third-order NLO properties [15,16].
Recently we have attempted to synthesize the corre-
sponding heterobimetallic selenide clusters by the
solid-state reaction at low heating temperature, and
tried to investigate their optical non-linearities based
on our previous knowledge. An important reason is
that Se-containing compounds are interesting because
The chemistry of [MS4]2Ϫ (M ˆ Mo, W) anions and
their related compounds has been extensively investi-
gated owing to their relevance to the biological
system, rich structural chemistry and special reaction
properties as well as potential applications in
nonlinear optical materials [1–7], whereas that of
[MSe4]2Ϫ (M ˆ Mo, W) anions and their related
compounds have received attention only in recent
* Corresponding author. Tel.: ϩ86-253593132; fax: ϩ86-
253314502.
E-mail address: xxin@netra.nju.edu.cn (X.-Q. Xin)
0022-2860/00/$ - see front matter ᭧ 2000 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(99)00414-7

80
Q.-F. Zhang et al. / Journal of Molecular Structure 525 (2000) 79–86
they may be used as precursors for the preparations of
low-bandgap semiconductors [17]. Herein, we present
synthetic reaction, structural characterization and
NLO properties of the two typical cubane-like clusters
[{MoCu3Se3Br}(PPh3)3Se](M ˆ Mo (1), W(2)).
relative to 85% acid and 2 mol dmϪ3 Na2Mo4 in D2O,
respectively.
2.1.2. Preparation of
[{MoCu3Se3Br}(PPh3)3Se]·3THF·H2O (1·3THF·H2O)
Cu(PPh3)NO3 (0.50 g, 0.75 mmol) in CH2Cl2
(10 ml) was added to a CH2Cl2 solution (10 ml) of
[Et4N]2[MoSe4] (0.21 g, 0.25 mmol). The resulting
brown solution was stirred for 5 min at room tempera-
ture, then Et4N·Br (0.09 g, 0.20 mmol) was added and
subsequently filtered to afford a reddish-brown
filtrate. Block-shaped red crystals of 1·3THF·H2O
were obtained by layering the filtrate with THF.
Yield: 0.18 g (41.6%). Found: C, 47.6; H, 4.29; P,
5.17; Cu, 10.6%; requires for C66H71O4MoCu3-
Se4BrP3: C, 46.4; H, 4.19; P, 5.45; Cu, 11.1%. IR
(KBr, cmϪ1): 3420.5(vs), 1668.4(vs), 1434.4(vs),
2. Experimental
2.1. Synthesis
2.1.1. General
All manipulations were conducted using Schlenk
techniques under an atmosphere of nitrogen.
[Et4N]2[MoSe4] and [Et4N]2[WSe4] were prepared by
a
modification of the literature method [18].
Cu(PPh3)2NO3 and Cu(PPh3)2Cl were obtained by
reaction of Cu(NO3)2·H2O and CuCl, respectively,
with PPh3 in CH2Cl2–MeOH. IR spectra were
measured on a Digilab FTS-40 spectrophotometer.
Electronic spectra were preformed on a Hitachi
U-3410 spectrophotometer. 31P and 95Mo NMR data
were recorded on a Varian Unity-500 spectrometer,
1095.1(vs),
744.2(vs),
703.8(vs),
691.3(vs),
521.3(vs), 505.5(vs), 491.8(vs), 384.8(s), 329.3(s).
31P NMR (CDCl3): d ˆ 12:7 ppm: 95Mo NMR
(CDCl3): d ˆ 1301:4 ppm:
[MoSe4Cu4Py6Br2] was prepared according to the
literature method [19]. A mixture of CuBr (0.16 g,
Table 1
Data collections and processing parameters for 1·3THF·H2O and 2
Compound
Formula
1·3THF·H2O
C66H71O4P3BrCu3Se4Mo
1703.45
Dark-red, prism
0:35 × 0:30 × 0:28
Trigonal
2
C54H45P3ClCu3Se4W
1512.57
Formula weight
Color, habit
Red, prism
0:48 × 0:24 × 0:22
Orthorhombic
P212121 (no. 19)
13.0809(3)
18.0122(4)
22.8324(5)
5379.7(2)
Crystal size, mm3
Crystal system
Space group
R3 (no. 146)
15.706(2)
˚
a, A
˚
b, A
˚
c, A
23.887(5)
5103(14)
3
˚
V, A
Z
3
4
F(000)
2523
1.667
3.947
2912
1.868
6.182
D(calcd), g cmϪ3
m, mmϪ1
Collection range; 2umax, Њ
Unique data measured
Observed data, n
No. of variables, p
R1
23.28
28.28
3195 (Rint ˆ 2.61%)
3007 ꢀI Ͼ 2:0sꢀI††
221
13 293 (Rint ˆ 5.01%)
10 622 ꢀI Ͼ 2:0sꢀI††
595
0.0381
0.0349
wR2
0.1010
1.102
0.0545
1.030
S (goodness of fit on F2)
Largest diff. peak and
hole, e A
0.849 and Ϫ0.398
0.777 and Ϫ0.793
Ϫ3
˚

Q.-F. Zhang et al. / Journal of Molecular Structure 525 (2000) 79–86
81
Table 2
(15 ml) and the solution was stirred for 4 h, the
reaction mixture was filtered to afford a dark-red
filtrate. Analytically pure crystals were obtained by
Et2O vapor diffusion after two days. The product
was washed with EtOH and dried under
vacuum. Anal. Found: C, 45.7; H, 3.22; P, 6.45%.
These elements analytical results are identical to
those of 1.
Atomic coordinates ( × 104) and equivalent isotropic displacement
parameters ( × 103) for 1·3THF·H2O
Atom
X
y
z
U (eq)
Mo
Cu
Se(1)
Br(1)
Se(2)
P(1)
3333
6667
5894(1)
7583(1)
6667
12 431(1)
13 276(1)
12 781(1)
14 142(1)
11 483(1)
13 630(1)
13 105(4)
13 201(5)
12 803(5)
12 303(5)
12 198(5)
12 588(4)
13 984(4)
14 218(4)
14 512(6)
14 555(6)
14 295(6)
14 036(5)
14 142(4)
13 991(6)
14 346(7)
14 877(8)
15 043(6)
14 668(4)
36(1)
55(1)
46(1)
56(1)
73(1)
47(1)
49(2)
80(3)
90(4)
76(3)
80(3)
66(3)
52(2)
70(3)
91(4)
102(4)
100(4)
84(4)
3940(1)
2613(1)
3333
3333
6667
4632(2)
4896(6)
4786(9)
5014(10)
5363(8)
5504(9)
5280(8)
5815(6)
5995(8)
6890(9)
7558(9)
7398(9)
6511(9)
3871(7)
2962(8)
2383(11)
2726(14)
3580(15)
4188(10)
6667
5070(2)
4384(6)
3485(8)
3000(9)
3433(9)
4343(9)
4821(8)
5836(7)
6722(8)
7325(9)
7001(11)
6165(11)
5560(9)
4130(6)
3405(8)
2654(9)
2645(11)
3338(13)
4080(9)
3333
2.1.3. Preparation of [{WCu3Se3Cl}(PPh3)3Se] (2)
A well-ground mixture of Cu(PPh3)2Cl (1.12 g,
1.5 mmol) and [Et4N]2[WSe4] (0.38 g, 0.5 mmol)
was put into a Schlenk tube and heated at 80ЊC for
6 h under a nitrogen atmosphere. After extracting the
resultant black solid CH2Cl2 (30 ml), the extract was
filtered to afford a reddish-orange filtrate. Block-
shaped red crystals were obtained for 2 days by
diffusing the filtrate with diethyl ether. Analytical
results of IR spectroscopy and crystal cell dimensions
are same as those in Ref. [10].
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
C(21)
C(22)
C(23)
C(24)
C(25)
C(26)
C(31)
C(32)
C(33)
C(34)
C(35)
C(36)
O
56(2)
86(3)
2.2. X-ray structure determinations
103(5)
112(6)
113(5)
81(3)
152(9)
272(10)
220(12)
322(22)
251(15)
244(15)
Well-shaped single crystals for 1·3THF·H2O and 2
were selected for X-ray data collections which were
preformed on a Siemens Smart CCD area-detecting
14 217(11)
5905(10)
6099(14)
6535(18)
6646(13)
6237(15)
O(01)
C(02)
C(03)
C(04)
C(05)
9222(20)
9411(20)
8721(33)
8029(24)
8268(24)
3496(22)
4418(19)
4332(25)
3272(27)
2701(18)
diffractormeter
using
graphite-monochromated
ꢀ
MoKa ꢀl ˆ 0:71073 A† radiation with v scan mode
at 293 K. All calculations were carried on a Siemens
Computer Station with shelxtl-97 program [20].
The positions of all metal and selenium atoms were
determined by direct methods, and successive differ-
ence electron density maps located the remaining non-
hydrogen atoms. All non-hydrogen atoms except THF
group and O of H2O in 1·3THF·H2O were refined by
full-matrix least-squares procedures with anisotropic
thermal parameters. The THF group in 1·3THF·H2O
was rigidly refined. Further details of the structure
analyses are listed in Table 1. Because structure of 2
was reported by Ibers, we present only its crystallo-
graphic details, which are better than those reported
previously. Description of this structure is not too
necessary. Positional parameters and selected bond
lengths and angles of 1·3THF·H2O are given in Tables
2 and 3.
1.10 mmol) and [Et4N]2[MoSe4] (0.17 g, 0.25 mmol)
in 20 ml pyridine(Py)–DMF (3:1) was stirred for 2 h
at room temperature. After filtration, the brown pre-
cipitate was removed off to afford a red-brown filtrate.
Dark-red microcrystals was obtained by diffusing
Et2O into the filtrate after 3 days. The crystal were
washed with EtOH and dried under vacuum. Yield:
0.15 g (38.7%). Found: C, 21.6; H, 1.82; N, 7.15%;
requires for C20H20N6MoCu4Se4Br2: C, 20.5; H, 1.72;
N, 7.18%. IR (KBr, cmϪ1): 1595.4(vs), 1442.2(vs),
1217.1(vs), 1152.2(vs), 1070.2(vs), 1037.9(vs),
1006.4(vs), 752.2(vs), 699.1(vs), 650.3(m), 627.1(s),
418.8(s), 334.8(s), 310.7(w), 258.4(w).
To a freshly slurry of [MoSe4Cu4Py6Br2] (0.19 g,
0.15 mmol) in DMF (8 ml) was slowly added a
solution of PPh3 (0.14 g, 0.5 mmol) in CH2Cl2
2.3. Optical measurements
A CH2Cl2 solution of 1:5 × 10Ϫ3 mol dmϪ3 of

82
Q.-F. Zhang et al. / Journal of Molecular Structure 525 (2000) 79–86
Table 3
˚
Selected bond lengths (A) angles (Њ) for 1·3THF·H2O
Mo–Se(2)
2.265(2)
Mo–Se(1)
2.3831(9)
2.7577(12)
2.231(2)
2.3917(12)
1.832(9)
Mo–Se(1)a
2.3832(9)
2.7578(12)
2.3896(13)
2.795(2)
Mo–Cua
Mo–Cu
Cu–P(1)
Cu–Se(1)a
Cu–Se(1)b
Cu–Br(1)
P(1)–C(11)
P(1)–C(21)
1.839(9)
110.54(3)
P(1)–C(31)
1.828(9)
108.38(3)
Se(2)–Mo–Se(1)
Se(1)a–Mo–Cub
Se(2)–Mo–Cu
Se(1)–Mo–Cu
Cua–Mo–Cu
P(1)–Cu–Se(1)b
Se(1)b–Cu–Mo
Se(1)a–Cu–Br(1)
Mo–Cu–Br(1)
Mo–Se(1)–Cua
Cua–Br(1)–Cu
C(11)–P(1)–Cu
Se(1)–Mo–Se(1)b
Se(1)b–Mo–Cub
Se(1)a–Cu–Mo
Se(1)a–Mo–Cu
P(1)–Cu–Se(1)a
Se(1)a–Cu–Se(1)b
P(1)–Cu–Br(1)
Se(1)b–Cu–Br(1)
Mo–Se(1)–Cub
Cub–Se(1)–Cua
C(31)–P(1)–Cu
C(21)–P(1)–Cu
54.87(3)
137.03(3)
112.43(5)
72.36(5)
117.97(7)
54.58(3)
98.70(4)
94.76(4)
70.56(3)
71.25(5)
113.6(3)
112.38(5)
54.60(3)
54.81(3)
119.82(8)
107.89(5)
109.96(7)
98.65(4)
70.59(3)
85.83(6)
114.3(3)
115.0(3)
a Symmetry code: Ϫy ϩ 1; x Ϫ y ϩ 1; z.
b Symmetry code: Ϫx ϩ y; Ϫx ϩ 1; z.
cluster 1 or 2 was placed in a 1-mm quartz cuvette for
optical measurements. Their nonlinear absorption and
nonlinear refraction were measured with a linearly
polarized laser light ꢀl ˆ 532 nm; pulse width ˆ 7
ns) generated from a Q-switched and frequency-
doubled Nd:YAG laser. The spatial profiles of the
optical pulses were nearly Gaussian. The laser beam
was focused with a 25-cm focal-length focussing
mirror. The radius of the laser beam waist was
measured to be 30 mm (half-width at 1/e2 maximum).
The incident and transmitted pulse energy were
measured simultaneously by two Laser Precision
detectors (RjP-735 energy probes) communicating
to a computer via an IEEE interface [21]. The
details of the set-up can be found elsewhere
[22].
3. Results and discussion
3.1. Synthesis and reaction
In the present reaction system, the reaction of
[Et4N]2[MoSe4] and Cu(PPh3)2NO3 in a ratio of 1:3
in CH2Cl2, [MoSe4]2Ϫ anion was coordinated by
[Cu(PPh3)]ϩ fragment to afford an intermediate
product [MoSe4(CuPPh3)3]ϩ which has a non-coordi-
nation terminal Mo–Se bond; when halide anion is
additionally supplied, the neutral cubane-like cluster
was formed (Scheme 1).
When cluster [MoSe4Cu4Py6Br2] is reacted with
excess PPh3 in CH2Cl2, the same cubane-like cluster
is also obtained (see Scheme 2). The reaction
processes involve: (i) a change of the cluster
Scheme 1.

Q.-F. Zhang et al. / Journal of Molecular Structure 525 (2000) 79–86
83
Scheme 2.
skeleton from the cross MoSe4Cu4 to the cubane-like
(Se)MoSe3Cu3; (ii) the substitute reaction of Py by
PPh3; (iii) the formation of the Cu–P bond enhancing
the ability for the Br atom to assume a m3-position.
For the furtherance of understanding the reactivity
and reaction mechanism in the above two reactions,
the 95Mo NMR technique was used to monitor the
reaction processes. Fig. 1(a) shows 95Mo NMR spec-
trum of the sample drawn from the synthetic reaction
mixture for [Et4N]2[MoSe4] and Cu(PPh3)2NO3 in a
molar ratio of 1:3 in CH2Cl2, only one resonance
peak at 1316 ppm ꢀDn1=2 ˆ 125 Hz†; which may be
assigned to [MoSe4(CuPPh3)3]ϩ intermediate [23].
After the addition of Et4N·Br salt to the above reaction
mixture, nearly no change of the 95Mo NMR signal
was observed. This fact may infer that the final
product [{MoCu3Se3Br}(PPh3)3Se] was formed
through the BrϪ anion attaching to the [MoSe4-
(CuPPh3)3]ϩ species. The 95Mo NMR spectrum of
the sample [MoSe4Cu4Py6Br2] in DMF is depicted in
Fig. 2(a), only one resonance peak at Ϫ1.92 ppm
ꢀDn1=2 ˆ 48 Hz† was found, which is similar to the
95Mo NMR results of the cluster compounds
containing MoCu4 core plane [14]. When an excess
PPh3 ligand was reacted with [MoSe4Cu4Py6Br2] in
DMF–CH2Cl2 for 2 h, the single at Ϫ1.92 ppm dis-
appeared and a new one at 1307 ppm appeared, and
the signal was independent of the reaction time. Based
on the above results, it may be suggested that the great
change from the high field to the low field for the Mo
nucleus deshielding effect came from the cluster
skeleton transformation. Thus, the reaction processes
are undergoing structural change from MoCu4 core
cross-plane to MoCu3 core distorted-cubane.
Although cluster 2 was previously prepared by the
solution reaction method [10], it can be readily
synthesized by the solid state reaction at low heating
temperature. The fact that the reaction was carried out
in the solid state, may be inferred from the starting
material [Et4N]2[WSe4] color varying from a violet-
red to a red-brown, accompanied by disappearance of
white complex Cu(PPh3)2Cl and gas evolution gener-
ated from partial decomposition of [Et4N]2[WSe4].
Different from the molybdenum-containing cluster
1, the crystal product of tungsten-containing cluster
2 is very air- and moisture-stable.
3.2. Structure of 1·3THF·H2O
The molecular cluster of 1·3THF·H2O crystallized
Fig. 2. (a) The 95Mo NMR spectrum of the sample [MoSe4Cu4-
Py6Br2] in DMF ꢀd ˆ Ϫ1:92 ppm; Dn1=2 ˆ 48 Hz†: (b) 95Mo
NMR spectrum of the sample drawn from the reaction mixture of
[MoSe4Cu4Py6Br2] and excess PPh3 in DMF–CH2Cl2 ꢀd ˆ
1307 ppm; Dn1=2 ˆ 119 Hz†:
Fig. 1. (a) 95Mo NMR spectrum of the sample drawn from the
reaction mixture of [Et4N]2MoSe4 with 3 equiv of Cu(PPh3)2NO3
in CH2Cl2 ꢀd ˆ 1316 ppm; Dn1=2 ˆ 125 Hz†: (b) Addition of equal
equiv of Et4N·Br salt to (a) ꢀd ˆ 1301 ppm; Dn1=2 ˆ 113 Hz†:

84
Q.-F. Zhang et al. / Journal of Molecular Structure 525 (2000) 79–86
in the trigonal R3 space group possible due to the
presence of solvent molecules THF and H2O. There
are no any contacts among the solvent and cluster
molecules. The molecular structure possesses a crys-
tallographic C3 axis passing through the terminal
Se(2), Mo, Br(1) and O atoms. The neutral cluster
[(m3–Cl)(m3–WSe4)(CuPPh3)3] [10]. The Cu–Br
distance of 2.797(1) A is normal. The Set–Mo–Seb
˚
angles are ca. 2.1Њ more obtuse than the Seb–Mo–
Seb angles, each of which along with the average
Se–Cu–Se angle of 107.9(1)Њ is near 109.4Њ of tetra-
hedral angle. The Br atom is at one vertex of [MoCu3-
Se3Br]ϩ cubane, however, the average Cu–Br–Cu
bond angle of 71.2(2)Њ is much deviated from 90Њ of
the standard cubane angle. A view of the structure is
presented in Fig. 3.
molecular [{MoCu3Se3Br}(PPh3)3Se] contains
a
highly distorted cubane-like [MoCu3Se3Br]ϩ cluster
core with Se2Ϫ terminal ligand to the Mo, and PPh3
ligand bound to each Cu atom. The Mo–Seb
˚
(bridging) distance of 2.384(1) A is greater than the
˚
Mo–Set (terminal) distance of 2.264(2) A. The
3.3. NLO properties
˚
average Mo–Cu distance of 2.757(1) A in
1·3THF·H2O is slightly longer than that of
2.712(2) A in [(m3-Cl)(m3-MoS4)(CuPPh3)3] [24],
The electronic spectra of clusters 1 and 2 exhibit a
moderately intense absorption band positioned at 381
and 338 nm, respectively. The bands have been
assigned as charge-transfer bands of the type
ꢀpꢀSe† ! dꢀM†† arising from the MSe4 moiety
(M ˆ Mo, W). It is interesting to note that the
Se ! M charge-transfer transition peaks are
˚
and shorter than the sum of the van der Waals radii
of Mo and Cu atoms, which may be considered that an
interaction exists between two metal atoms. The
˚
average Cu–Se bond length of 2.391(1) A in
˚
1·3THF·H2O agrees well with that of 2.394(3) A in
Fig. 3. A view of the inner core of [{MoCu3Se3Br}(PPh3)3Se] in 1·3THF·H2O. Only inequivalent atoms are labeled. Displacement ellipsoids are
plotted at the 50% probability level.

Q.-F. Zhang et al. / Journal of Molecular Structure 525 (2000) 79–86
85
lines in Figs. 4(a) and 5(a) are the theoretical curve
calculated with a2 ˆ 3:4 × 10Ϫ10 and 5:9 ×
10Ϫ10 m=W; respectively, for the same concentration
CH2Cl2 solution of 1 and 2. The non-linear refractive
components of clusters 1 and 2 were assessed by
dividing the normalized Z-scan data obtained in the
close-aperture configuration by those obtained in the
open-aperture configuration. They are plotted in Figs.
4(b) and 5(b) for 1 and 2, respectively; the solid
curves are an eye guide for comparison where the
effective NLO refractivity n2 values estimated there-
fore are Ϫ1:5 × 10Ϫ17 m2=W for 1 and Ϫ1:3 ×
10Ϫ17 m2=W for 2, respectively.
Comparing the NLO data of 1 and 2, the non-linear
absorption capability of 2 is obviously stronger than
that of 1, while their non-linear refractive effects are
almost identical. Such a significant improvement of
non-linear absorption capability by replacing skeletal
Mo atom with W atom implies a heavy atom
effect, which is similar to those observed in the
same structure clusters [MS4(AuAsPh3)2] and
Fig. 4. Z-scan data of 1:5 × 10Ϫ3 M of 1 in CH2Cl2 at 532 nm with I0
being 2:6 × 1012 W=m2 : (a) collected under the open aperture
configuration showing NLO absorption; (b) obtained by dividing
the normalized Z-scan data obtained under the closed aperture
configuration by the normalized Z-scan data in (a) (it shows the
self-defocusing effect).
[n-Bu4N]3[WS4M0 Br3] (M ˆ Mo, W; M0 ˆ Cu, Ag)
3
[25,26]. Together with electronic spectral data of 1
and 2, it may be inferred that substitution of Mo
obviously blue shifted when Mo is replaced by W in
the clusters. Peak at ϳ380 nm was shifted by ϳ40 nm
to shorter wavelength. This is a demonstration of
metal dependence on the NLO performances of
clusters.
The NLO properties of clusters 1 and 2 were
investigated by using the Z-scan technique. Two
clusters show both non-linear absorption and refrac-
tion. The nonlinear absorption component was
evaluated under an open aperture configuration.
Theoretical curves of transmittance against the
Z-position, Eqs. (1) and (2), were
Z∞
2
1
TꢀZ† ˆ
ln‰1 ϩ qꢀz†Š eϪt dt
ꢀ1†
p
1=2qꢀZ†
Ϫ ∞
ꢀ1 Ϫ eϪa L
†
0
qꢀZ† ˆ a2IiꢀZ†
ꢀ2†
a0
Fig. 5. Z-scan data of 1:5 × 10Ϫ3 M of 2 in CH2Cl2 at 532 nm with I0
being 2:9 × 1012 W=m2 : (a) collected under the open aperture
configuration showing NLO absorption; (b) obtained by dividing
the normalized Z-scan data obtained under the closed aperture
configuration by the normalized Z-scan data in (a) (it shows the
self-defocusing effect).
fitted to the observed Z-scan data by varying the effec-
tive third-order NLO absorptivity a2 value, where the
experimentally measured a0 (linear absorptivity), L
(the optical path of sample) and Ii(Z) (the on-axis
irradiance at Z-position) were adopted. The solid

86
Q.-F. Zhang et al. / Journal of Molecular Structure 525 (2000) 79–86
atom by heavier W atom allows more efficient spin–
orbital coupling and ionization/geminate recombina-
tion between the W and Se atoms than between the
Mo and Se atoms. However, non-linear refraction
effects of 1 and 2 are not influenced by substitution
of the center atom. Based on our previous results from
the theoretical analyses, the reason may be that an
intense charge transfer may not necessarily give rise
to a large refractive index [27].
References
¨
[1] A. Mu¨ller, E. Diemann, R. Josters, H. Bogge, Angew. Chem.,
Int. Ed. Engl. 20 (1981) 934.
[2] I.G. Dance, K. Fisher, Prog. Inorg. Chem. 41 (1994) 637.
[3] Q. Huang, X.T. Wu, Q.M. Wang, T.L. Sheng, J.X. Lu, Angew
Chem., Int. Ed. Engl. 35 (1996) 985.
[4] R.H. Holm, J.M. Berg, Acc. Chem. Res. 19 (1986) 363.
[5] D. Coucouvanis, Acc. Chem. Res. 24 (1991) 1.
[6] S. Shi, W. Ji, S.H. Tang, J.P. Lang, X.Q. Xin, J. Am. Chem.
Soc. 116 (1994) 3615.
The negative values of non-linear refraction in 1
and 2 indicate that there are self-defocusing effects
in NLO behaviors of the present cubane clusters. A
combination of their NLO refractive properties (self-
defocusing) and NLO absorptive properties is in
contrast to the behavior of the corresponding sulfur
cubane-like cluster compound, [MCu3S4X4]3Ϫ
(M ˆ Mo, W) [28], which typically shows a combina-
tion of strong NLO absorptive and very weak self-
focusing characteristics. Thus, a slightly significant
improvement of NLO absorptive capability is seen
when S atoms are replaced by Se atoms in the cluster
skeleton. Heavy atom effect arises from the fact that
Se atoms can facilitate intersystem crossing more
effectively via spin–orbit coupling than S atoms
[29]. Meanwhile, NLO refractive effects change
from self-focusing of S-containing clusters to self-
defocusing of Se-containing clusters although they
are of a similar structural type. A considerable inter-
ruption may be that the bigger the atomic radius is, the
looser the structure shows. This makes the cage struc-
ture to have bigger electronic displacements coming
from larger bond parameters.
[7] H.W. Hou, X.R. Ye, X.Q. Xin, J. Liu, M.Q. Chen, S. Shi,
Chem. Mater. 7 (1995) 472.
[8] M.A. Ansari, J.A. Ibers, Coord. Chem. Rev. 100 (1990) 223.
[9] L.C. Roof, J.W. Kolis, Chem. Rev. 93 (1993) 1037.
[10] C.C. Christuk, M.A. Ansari, J.A. Ibers, Inorg. Chem. 31
(1992) 4365.
[11] C.C. Christuk, M.A. Ansari, J.A. Ibers, Angew. Chem., Int.
Ed. Engl. 31 (1992) 1477.
[12] R.J. Salm, A. Misetic, J.A. Ibers, Inorg. Chim. Acta 240
(1995) 239.
[13] Q.F. Zhang, M.C. Hong, W.P. Su, R. Cao, H.Q. Liu, Polyhe-
dron 16 (1997) 1433.
[14] M.C. Hong, Q.F. Zhang, R. Cao, D.X. Wu, J.T. Chen, W.J.
Zhang, H.Q. Liu, J.X. Lu, Inorg. Chem. 36 (1997) 6251.
[15] H.W. Hou, X.Q. Xin, S. Shi, Coord. Chem. Rev. 153 (1996)
25.
[16] W. Ji, S. Shi, H.J. Du, P. Ge, S.H. Tang, X.Q. Xin, J. Phys.
Chem. 99 (1995) 17 297.
[17] See any issue of The Bulletin of the Selenium–Tellurium
Development Association, Grimbergen, Belgium.
[18] S.C. O’Neal, J.W. Kolis, J. Am. Chem. Soc. 110 (1988) 1971.
[19] R. Cao, Q.F. Zhang, W.P. Su, M.T. Bao, Y. Zheng, M.C.
Hong, Polyhedron 18 (1999) 333.
[20] G.M. Sheldrick, shelxtl-97 Version 5.03, Siemens Analy-
tical X-Ray Instruments Inc., Madison, WI, 1997.
[21] M. Sheik-bahae, A.A. Said, T.H. Wei, D.J. Hahan, E.W. Van
Stryland, IEEE J. Quant. Electron. 26 (1990) 760.
[22] H.W. Hou, B. Liang, X.Q. Xin, K.B. Yu, P. Ge, W. Ji, S. Shi,
J. Chem. Soc., Faraday Trans. 92 (1996) 2343.
[23] Q.F. Zhang, R. Cao, M.C. Hong, D.X. Wu, W.J. Zhang, Y.
Zhen, H.Q. Liu, Inorg. Chim. Acta 271 (1998) 93.
4. Supplementary materials
¨
[24] A. Mu¨ller, H. Bogge, U. Schimanski, Inorg. Chim. Acta 69
Crystal data (excluding structure factors) for the
structures in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supple-
mentary publication no. CCDC-133092/3(0119) and
also available from the authors.
(1983) 5.
[25] H.G. Zheng, W. Ji, M.L.K. Low, G. Sakane, T. Shibahara,
X.Q. Xin, J. Chem. Soc., Dalton Trans. (1997) 2357.
[26] S. Shi, W. Ji, J.P. Lang, X.Q. Xin, J. Phys. Chem. 98 (1994)
3570.
[27] S. Shi, Z. Lin, Y. Mo, X.Q. Xin, J. Phys. Chem. 100 (1996)
10 696.
[28] Z.R. Chen, H.W. How, X.Q. Xin, K.B. Yu, S. Shi, J. Phys.
Chem. 99 (1995) 8717.
[29] T.H. Wei, D.J. Hangan, M.J. Sence, E.W. Van Styland, J.W.
Perryand, D.R. Coulter, Appl. Phys. B 26 (1992) 760.
Acknowledgements
Acknowledgement is made to the research funds
from the National Science Foundation (No.
29631040) and Education Department of China.