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, Zn4S(C2H2N3)3(NCS)3, which exhibits exceptional multifunctional optical properties: considerable second-harmonic generation (SHG) response [6.8 × KH2PO4 (KDP) @1064 nm], substantial birefringence (Δn = 0.32 @546 nm), high thermal stability (Td = 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 Zn4S(C2H2N3)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).