The title compounds [Et4N]2[MS4Cu4(SCN)4(2-pic)4] (M = W 1, Mo 2) have been synthesized by the reaction of (Et4N)2MS4, Cu(SCN) and 2-picoline (2-pic, 2-methylpyridine). Single crystal X-ray diffraction data show that the anion clusters [MS4Cu4(SCN)4(2-pic)4]2− have the planar ‘open’ structure with four Cu atoms in three kinds of coordination modes. Nonlinear optical properties of these two clusters are investigated with a 8 ns pulsed laser at 532 nm. The two clusters exhibit large optical limiting performance, with limiting threshold values of 0.3 J cm−2 for 1, 0.5 J cm−2 for 2, and self-defocusing effects, effective nonlinear refractive index n2 = −6.84 × 10−12 esu (esu = 7.162 × 1013 m5 v−2) 1 and n2 = −8.48 × 10−12 esu 2 respectively. Both compounds show reverse saturable absorption: α2 = 3.1 × 10−6 m W−1 for 1 and α2 = 3.2 × 10−6 m W−1 for 2 in 6.98 × 10−4 mol dm−3 and 7.44 × 10−4 mol dm−3 DMF solution respectively. The corresponding effective NLO susceptibilities χ(3) are 6.5 × 10−8 esu 1 and 8.9 × 10−8 esu 2 while the corresponding hyperpolarizabilities (γ(1) = 9.42 × 10−32 esu and γ(2) = 1.29 × 10−31 esu) are also reported.
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