Geometric Isomers of Bipyridinium in Lead Halide Perovskitoids

Organic-inorganic hybrid lead halides are explored enthusiastically in light of the versatility of their crystal structures. Here, we report lead halide crystals incorporating 2,2'-bipyridinium (bpy), where the various valence states of bpy depending on its degree of protonation influence the structure of inorganic sublattices. We synthesized [bpy⁺]PbCl₃ and [bpy²⁺]₃Pb₃Br₁₀·2Br, together with previously reported [bpy²⁺]₂Pb₃I₁₀. [bpy²⁺]₃Pb₃Br₁₀·2Br and [bpy²⁺]₂Pb₃I₁₀ incorporate <i>trans</i>-isomer divalent [bpy²⁺], consistent with synthetic conditions in strong acids, while [bpy⁺]PbCl₃ exhibits a preference toward <i>cis</i>-isomer monovalent [bpy⁺]. They exhibit distinct Pb-<i>X</i> octahedral stacking patterns, i.e., chained or layered structures. Their bandgaps also depend on the halogen species and inorganic frameworks, while the intramolecular type II band alignment between the inorganic and organic sublattices contributes to the electronic structures in all cases, as suggested by theoretical calculations. This study emphasizes the importance of molecular structures in addition to halide components for crystal structure engineering in perovskitoids.