Studies on two interesting microporous polymeric clusters {[Et4N]2[MS4Cu4(CN)4]}n (M = Mo or W) with three-dimensional open frameworks: synthesis, structural characterization, strong optical non-linearities and large optical limiting properties

Abstract

Reactions combining stoichiometric amounts of (Et₄N)₂MS₄ (M = Mo or W) and CuCN (1∶4) in pyridine afforded interesting three-dimensional cluster polymers with open frameworks {[Et₄N]₂[MS₄Cu₄(CN)₄]}_n (M = Mo 1 or W 2). Crystal structure determination shows that the anionic MS₄Cu₄ units bridged by cyanide produce three-dimensional channels running down the crystallographic a axis. An alternative way to view this framework is in terms of the diamond structure, where C has alternately been replaced by a MS₄Cu₄ aggregate and C–C by two parallel cyanide bridging ligands. In these intersecting channels the shortest distances between Cu atoms along the b and c axes are 15.22 and 8.11 Å respectively. Non-linear optical properties of the two clusters were investigated first with an 8 ns pulsed laser at 532 nm. These two clusters exhibit large optical limiting performance with limiting threshold values of 0.28 for 1, 0.15 J cm⁻² for 2 respectively. Both compounds show strong third-order NLO absorption effects (α₂ 1.5 × 10⁻⁹1, 1.6 × 10⁻⁹ m W⁻¹2) and self-focusing performance (n₂ 1.84 × 10⁻¹⁶1, 1.22 × 10⁻¹⁶ m² W⁻¹2) in 3.64 × 10⁻⁵1 and 2.93 × 10⁻⁵ mol dm⁻³2 DMF solution separately. The corresponding effective NLO susceptibilities χ⁽³⁾ are 4.58 × 10⁻⁹1 and 5.12 × 10⁻⁹ esu 2 while the corresponding hyperpolarizabilities (γ₍₁₎ = 1.15 × 10⁻²⁹ and γ₍₂₎ = 1.26 × 10⁻²⁹ esu) are also reported.