New Self-Defocusing Materials: Synthesis, Structures and Nonlinear Optical Properties of Nest Shaped Clusters
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S. SHI, X.Q. XIN, W. JI, AND W. XIE Optical Crystal Laboratory and Physics Department, National University of Singapore, Singapore 0511.
ABSTRACT Mixed metal clusters (n-Bu 4N)2[MoCu 3OS 3X3] (X = SCN, Cl and Br) were synthesized by solid state reactions. X-ray single crystal diffraction data show that the anionic clusters assume nest shaped structures. The clusters exhibit large self-defocusing effect as demonstrated by their Z-scan traces. The effective third-order polarizability of -YI = 4.8 x 10-29 esu was determined for (n-Bu 4N)[MoCu 3OS 3(SCN)3] by a degenerate four-wave mixing technique with 7-ns laser pulses of 532 nm wavelength. An irradiance-dependent transmission measurement was conducted to assess the contribution of nonlinear absorption to the -y-value. These results in combination with the self-defocusing property of the cluster yielded a negative value for the real part of -y,Re-y = - 4.8 x 10-29 esu. These numerical values are in good agreement with those obtained in a Z-scan experiment. Even though the clusters absorb laser light at 532 nm significantly, the results of temporal profile analysis on transmitted pulses show that the solvent thermal effect makes little contribution to the observed nonlinear optical phenomenon.
INTRODUCTION A strong push in searching for new materials with large third-order nonlinear optical (NLO) effects comes from the prospect of ultra-fast switch and optical data processing.' In the past, efforts were largely devoted to exploring NLO properties of semiconductors, conjugated polymers and discrete organic molecules. 2 Recently, C6. has also attracted much attention.' By contraries, inorganic clusters, a very promising group of compounds, seemed to be largely overlooked. Only a limited number of papers have appeared dealing with nonlinear optical 4 properties of well defined inorganic clusters. We recently discovered strong optical limiting effect of a group of cubic cage shaped mixed metal clusters (n-Bu4N) 3[MM' 3BrX 3S4] (M = Mo, W; M'= Ag, Cu; X = Cl, I) where 5 the NLO effect is derived mainly from a dissipative (nonlinear absorption) process. Here we report synthesis, structures and NLO properties of mixed metal clusters (n-Bu4N) 2[MoCu 3OS 3X 3] (X = SCN, Cl and Br) as representative examples of a new type of nonlinear optical materials, nest shaped clusters.
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Mat. Res. Soc. Symp. Proc. Vol. 374 01995 Materials Research Society
EXPERIMENTAL Compound (n-Bu 4N)2[MoCu 3OS 3(NCS) 3] (I) was synthesized by a solid state reaction. 6 A well-ground solid mixture of 1.0 mmol of (NH4) 2MoOS 3 , 2.0 mmol of CuSCN, 1.0 mmol of KSCN and 4.0 mmol of (n-Bu) 4NBr were heated at 110 'C for 12 h under argon. Desired compound I was extracted from a black raw product into 40 ml of CH 2C12 / DMF mixture (3:1 in v:v). Dark red crystals of the compound were obtained after slow evaporation of the solvents. The crystal product was washed with ethanol and ether and air dried. Yield: 35%. Elemental analysis confirmed the formula of compound I satisfactorily. Anal. (calcd.) for co
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