Exploring ultra-high optical anisotropy in polyiodides

Abstract

High anisotropy birefringent materials are urgently needed as essential elements for modern optical technology. The integration of organic π-conjugated moieties with inorganic active units produces hybrid materials exhibiting pronounced optical anisotropy and tunable birefringence characteristics. However, the exploration and development of materials exhibiting large birefringence present ongoing challenges. In this work, three new organic–inorganic hybrid polyiodides, namely, C₄H₆N₃I₂, (C₅H₇N₂)I·I₃ and (C₅H₆NO)₂I₄, have been synthesized by a solution method. It has been established that the structures of all three compounds contain two types of polyiodide anions (I₃⁻ and I₄²⁻), which are formed by different I–I bonds. It is noteworthy that C₄H₆N₃I₂, (C₅H₇N₂)I·I₃, and (C₅H₆NO)₂I₄ exhibit significant birefringence at 1064 nm, with calculated values of 1.384, 0.829, and 0.891, respectively. Theoretical calculations indicate that the combination of linear polyiodide anions with appropriately arranged π-conjugated groups results in high optical anisotropy. This work demonstrates that polyiodides are promising candidates for the development of superior birefringent materials.