Zn ₄ S(C ₂ H ₂ N ₃ ) ₃ (NCS) ₃ : Coordination-Driven Structural Distortion Engineering Triggers Exceptional NLO Effect in Mixed-Bonding Zinc Isothiocyanate

Nonlinear optical (NLO) materials (especially second-order NLO crystals) enable pivotal technologies like laser frequency conversion and ultrafast electro-optic modulation. The functional performance of NLO crystals is predominantly governed by two interdependent factors: the ordered arrangement mode of constituent motifs and their inherent structural distortion characteristics. This work pioneers a coordination-driven structural manipulation strategy to regulate the NLO-active tetrahedral units. Leveraging ligands with different coordination capabilities to tetrahedral metal centers modulates the electron density distribution around metal sites through coordination-bond perturbation. Resulting electron redistribution asymmetrically influences the covalent bonds, inducing controlled tetrahedral distortion. Guided by the aforementioned strategy, we synthesized an isothiocyanate with mixed bonding configurations, namely, Zn 4 S(C 2 H 2 N 3 ) 3 (NCS) 3, which exhibits exceptional multifunctional optical properties: considerable second-harmonic generation (SHG) response [6.8 × KH 2 PO 4 (KDP) @1064 nm], substantial birefringence (Δ n = 0.32 @546 nm), high thermal stability ( T d = 415 °C), and wide bandgap (4.72 eV). Theoretical calculations not only validate the efficacy of the coordination-driven structural manipulation strategy but also elucidate that synergistic interactions among constituent functional motifs govern the optical properties. A novel thiocyanate designated as Zn 4 S(C 2 H 2 N 3 ) 3 (NCS) 3 (ZSTN) was engineered through an effective coordination-driven structural distortion strategy, manifesting a mixed-chemical-bonding characteristic that concurrently delivers strong second-harmonic generation response (6.8 × KDP), substantial birefringence (Δn ≈ 0.32), wide bandgap (4.72 eV), and exceptional thermal stability (>400 °C).