Polymorphism and Red Photoluminescence Emission from 5s² Electron Pairs of Sb(III) in a New One-Dimensional Organic-Inorganic Hybrid Based on Methylhydrazine: MHy₂SbI₅

We explore the crystal structure and luminescent properties of a new 1D organic-inorganic hybrid, MHy(2)SbI(5), based on methylhydrazine. The compound reveals the red photoluminescence (PL) originating from the 5s(2) electron pairs of Sb(III) as well as complex structural behavior. MHy(2)SbI(5) crystalizes in two polymorphic forms (I and II) with distinct thermal properties and structural characteristics. Polymorph I adopts the acentric P2(1)2(1)2(1) chiral space group confirmed by SHG, and, despite a thermally activated disorder of MHy, does not show any phase transitions, while polymorph II undergoes reversible low-temperature phase transition and high-temperature reconstructive transformation to polymorph I. The crystal structures of both forms consist of 1D perovskite zig-zag chains of corner-sharing SbI6 octahedra. The intriguing phase transition behavior of II is associated with the unstable arrangement of the [SbI5](2-)(infinity) chains in the structure. The energy band gap (E-g) values, estimated based on the UV-Vis absorption spectra, indicate that both polymorphs have band gaps, with E-g values of 2.01 eV for polymorph I and 2.12 eV for polymorph II.