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

Abstract

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.