Extending lead-free organic–inorganic semiconducting materials to new polymeric structures: 2,2′-bipyridinium iodoantimonate and quinoxalinium iodobismuthate

We report on the synthesis, crystal structures and optoelectronic properties of two novel organic-inorganic group V iodometalates - (bpy-H)SbI₄ (1) and (Qx-H)₂[BiI₄]₂·CH₃OH (2) (bpy-H: 2,2'-bipyridinium; Qx-H: quinoxalinium). Crystals of both compounds were grown solvothermally using as starting reagents antimony(iii) iodide and 2,2'-bipyridine, for 1, and bismuth(iii) iodide and quinoxaline, for 2. Both compounds were structurally analyzed by single crystal X-ray diffraction at 100 K and their structures feature one-dimensional edge-sharing anionic chains of MI₆-octahedra, with short interchain I⋯I contacts giving rise to the possibility of three-dimensional charge transport capability. The cavities between the one-dimensional [MI₄ ^-] _<i>n</i> chains are occupied by the rigid <i>N</i>-protonated organic amines - 2,2'-bipyridinium (bpy-H), for 1, and quinoxalinium (Qx-H), for 2 - to balance the charge and stabilize the structures. The study of their supramolecular 3D structures assisted by a Hirshfeld surface analysis revealed that the latter are due to several types of intermolecular interactions, such as intermolecular hydrogen-bonding (H⋯I and H⋯O) and additional π⋯π and I⋯I contacts. Solid-state optical studies performed on powders and thin films were carried out and revealed that the materials display semiconductive optical band gap energies of around 2.17 eV, for 1, and 2.03 eV, for 2, which is encouraging for optoelectronic applications. The bulk crystals of 1 exhibit a broad dual-band emission which originates from both free excitons (FE) and self-trapped excitons (STE), as proven by its long lifetime (146.32 ns) and the temperature dependences of broadband emission. In contrast, bulk crystals of 2 exhibit broadband emission centred at 454 nm, common to the salt (Qx-H)Cl, originating from ligand-centered transition. These are proven by their short lifetimes (3.86 ns for 2 and 1.07 ns for (Qx-H)Cl).