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)E2I9 center dot 2H(2)O (pyz=pyrazine; E=Sb, Bi). Both compounds are stable up to 100 degrees 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.

Automated summary

Lead-free heavy halogenido metalates are being intensively investigated due to their promising semiconducting properties. In this study, we present two isostructural iodido pentelates (Hpyz)(3)E2I9·2H2O (pyz=pyrazine; E=Sb, Bi), which exhibit stable properties up to 100°C. Surprisingly, the antimony compound has a smaller band gap size of 1.91 eV, while the bismuth compound has a slightly larger band gap size of 1.98 eV compared to similar pairs of compounds. DFT calculations confirm that the presence of charge transfer excitations contributes to this finding.