Sign alteration of the nonlinear refraction of isomorphous cubane-shaped heterothiometallic clusters originating from a skeleton atomic effect

Employing ligand-redistribution reactions, two heterothiometallic cluster compounds [MOS₃ Cu₃(μ₃-X)(2-pic)₃] (M = Mo, X = Br, 1; M = W, X = I, 2) have been synthesized for third-order excited state nonlinear optical studies. Crystallographic investigations show that clusters 1 and 2 adopt an isomorphous neutral cubane-like skeleton. Such a neutral cubic arrangement with pyridine derivative ligands is found for the first time in heterothiometallic cluster compounds. Their optical nonlinearities were measured by the Z-scan technique with a 10 ns pulsed laser at 532 nm. Surprisingly, these two isomorphous clusters exhibit diametrically opposed nonlinear refractive properties, with cluster 1 showing self-defocusing performance (effective nonlinear refractive index n₂ = - 3.8 × 10^-18 m² W^-1) and cluster 2 demonstrating a self-focusing effect (n₂ = 3.5 × 10^-17 m² W^-1). Both clusters additionally show strong reverse saturable absorptive abilities. A new nonlinear refraction model is presented to simulate the experimental results. The Z-scan experimental results imply that excited state molecules also play a major role in nonlinear refraction. The above NLO refractive properties and absorption abilities suggest that clusters 1 and 2 may be very promising candidates for NLO applications.