Optical Properties and Metal-Dependent Charge Transfer in Iodido Pentelates

Lead-free heavy halogenido metalates are currently under intense investigation, as they show similarly promising semiconducting properties as their famous but toxic lead relatives. A major interest in this regard is the understanding and control of optical properties with the goal of designing highly efficient photoconducting materials. Here, we present two isostructural iodido pentelates (Hpyz)₃ E₂ I₉  ⋅ 2H₂ O (pyz=pyrazine; E=Sb, Bi). Both compounds are stable up to 100 °C. We observe an inverted order of band gap sizes, 1.91 eV and 1.98 eV for the antimony and bismuth compound, respectively, compared to similar pairs of compounds. We use DFT calculations to confirm that this surprising finding can be traced back to the presence of charge transfer excitations in both compounds.