Two novel anion cluster compounds with a planar 'open' structure [Et4N]2[MS4Cu4(SCN)4(2-pic)4] (M = W, Mo; 2-pic = 2-methylpyridine): Synthesis, structural characterization, nonlinear response and large optical limiting properties

The title compounds [Et4N]2[MS4Cu4(SCN)4(2-pic)4] (M = W l, 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 cm2 for 1, 0.5 J cm2 for 2, and self-defocusing effects, effective nonlinear refractive index /;2 = -6.84 x 1012 esu (esu = 7.162 x 10 m5 v2) 1 and 2 = -8.48 x 1012 esu 2 respectively. Both compounds show reverse saturable absorption: a2 = 3.1 x l(T6 m W1 for 1 and a2 = 3.2 x 106 m W' for 2 in 6.98 x 104 mol dm3 and 7.44 x 10 mol dm3 DMF solution respectively. The corresponding effective NLO susceptibilities %m are 6.5 x 108 esu 1 and 8.9 x 108 esu 2 while the corresponding hyperpolarizabilities (y(I) = 9.42 x 1032 esu and ym = 1.29 x 1031 esu) are also reported.