Framework Reorganization Governing Optical Transparency and Dielectric Switching in [TMA] ₄ [Zn(SCN) ₄ ][Zn(SCN) ₃ Cl]

An organic–inorganic hybrid thiocyanate, [TMA]4[Zn(SCN)4][Zn(SCN)3Cl], was synthesized, and its structural, vibrational, optical, and dielectric properties were investigated. Single-crystal X-ray diffraction reveals a triclinic centrosymmetric structure (P1̅) comprising two distinct Zn2+ coordination environments, including tetrahedral ZnN4 units and ZnN3Cl2 motifs forming infinite Zn–Cl–Zn chains. The framework is stabilized by tetramethylammonium cations through weak C–H···S and C–H···Cl interactions. Spectroscopic studies confirm N-bound thiocyanate coordination. Thermal analysis shows a reversible first-order phase transition near 292 K. Optical measurements indicate a wide direct band gap of 4.55–4.70 eV and strong deep-UV absorption with high transparency in the visible range. Dielectric studies reveal an unusual inverse dielectric switching across the phase transition, with a decrease in permittivity upon heating. These results identify [TMA]4[Zn(SCN)4][Zn(SCN)3Cl] as a hybrid thiocyanate framework exhibiting thermally tunable dielectric behavior and deep-UV filtering capability.