Two interesting heterobimetallic compounds [MOS₃Cu₃(4-pic)₆]·Br (M = Mo, W) with cationic cluster: Synthesis, structural characterization, non-linear response and large optical limiting properties

Two heterobimetallic cluster compounds [MOS₃Cu₃(4-pic)₆]·Br (M = Mo, W) with cationic cluster skeleton and halide anion Br^- synthesized by the reaction of (NH₄)₂MOS₃, CuBr and 4-picoline (4-pic) are presented. Their structures have been crystallographically determined and such nest-shaped cationic clusters, obtained for the first time, are interesting in comparison with the analogical nest-shaped clusters with a neutral skeleton or anionic skeleton. The non-linear optical (NLO) properties of these two clusters are investigated with an 8 ns pulsed laser at 532 nm. These two clusters exhibit large optical limiting abilities with limiting thresholds F_th = 0.18 J cm^-2 (1) and F_th = 0.15 J cm^-2 (2), respectively. Both compounds show strong third-order NLO absorption effects (α₂-value of 1.6 × 10^-10 m W^-1 (1), 2.8 × 10^-10 m W^-1 (2)) and self-focusing performance (n₂-value of 4.56 × 10^-11 esu (1), 4.62 × 10^-11 esu (2)) in 3.86 × 10^-4 mol dm^-3 (1) and 8.89 × 10^-4 mol dm^-3 (2) DMF solution separately. The corresponding effective NLO susceptibilities χ⁽³⁾ are 5.4 × 10^-12 esu (1) and 5.5 × 10^-12 esu (2) while the corresponding hyperpolarizabilities (γ₍₁₎ = 2.51 × 10^-31 esu and γ₍₂₎ = 1.03 × 10^-31 esu) are also reported.