amate ligands. With nanosecond-duration laser pulses, the limit-
ing thresholds for the two clusters were obtained and compared
with those of C60. The non-linear responses for the clusters in
DMF have also been studied in picosecond time-resolved
pump-probe experiments. Influences of structural types and
peripheral ligands on the non-linear optical properties were
discussed and renewed. We are currently investigating more
versatile co-ordination geometries and structure types of
heteroselenometallic clusters and further studying their NLO
properties so that some desired NLO functions will be achieved.
Acknowledgements
This research was supported by grants from the National
Science Foundation and the State Key Laboratory of Struc-
tural Chemistry, Fujian Institute of Research on Structure of
Matter, Chinese Academy of Sciences.
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Fig. 5 Pump-probe result for C60 in toluene solution (filled circles) and
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nest-shaped clusters [MoCu3OS3BrCl2]2Ϫ and [MoCu3OS3-
(NCS)3]2Ϫ 10a
has a relatively high threshold. From previous
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study, the nest-shaped cluster is positioned over a six-membered
Cu–S–Cu–S–Cu–S ring which appears like a doughnut, and
the M᎐O group is not responsible for the non-linear refractive
᎐
property change. Thus, this kind of metal cluster has a relatively
small optical limiting capability. The T-frame shaped cluster
has two different types of co-ordinated Se atoms: two- and
three-co-ordinated. The electronic interactions between µ-Se
and µ3-Se bridged metal atoms may result in different electron
delocalizations in the cluster molecule, which needs a higher
pump energy to generate the plasma, further leading to degrad-
ation of the limiting effect. In comparison with the limiting
effects of the cross frame clusters with planar WCu4, cluster 2
Ϫ
containing ligand Et2NCS2 has a higher threshold than those
of clusters with similar structure, such as [WCu4S4(SCN)2(py)6]
and [WSe4{Cu(dppm)}4][ClO4]2. A previous conclusion was
that only minor modifications of the NLO effects were
observed when all of the three replaceable peripheral ligands
of the cubane cage shaped cluster were replaced.11b It is now
preferable to reconsider the limiting effects influenced by the
properties of the peripheral ligands on the basis of the present
study on optical response. The strong σ-donor ligands, such as
py, PPh3 and dppm, result in transfer of electron density from
the copper-complex fragments toward the [WE4]2Ϫ (E = S or Se)
Ϫ
moiety. However, the π-conjugated ligand R2NCS2 leads to
delocalization of electron density to both Cu and W atoms. In
other words, the change of the extent of electron delocalization
between the HOMOs and the LUMOs in the cluster is more
significant. Consequently, a greater change in the strength of
interaction between the cluster molecules and propagation
optical field is induced by photoexcitation.31 Further theoretical
analyses are underway in our laboratories.
13 (a) W. Ji, S. Shi, H. J. Du, P. Ge, S. H. Tang and X. Q. Xin, J. Phys.
Chem., 1995, 99, 17297; (b) M. K. M. Low, H. W. Hou, H. G.
Zheng, W.-T. Wong, G. X. Jin, X. Q. Xin and W. Ji, Chem.
Commun., 1998, 505.
14 See any issue of The Bulletin of the Selenium–Tellurium Development
Association, Grimbergen, Belgium.
In summary, we have synthesized and characterized two
heteroselenometallic cluster compounds containing dithiocarb-
J. Chem. Soc., Dalton Trans., 2000, 605–610
609