Influence of organic cation planarity on structural templating in hybrid metal-halides
Controlling the connectivity and topology of solids is a versatile way to target desired physical properties. This is especially relevant in the realm of hybrid halide semiconductors, where the long-range connectivity of the inorganic substructural unit can lead to significant changes in optoelectronic properties such as photoluminescence, charge transport, and absorption. We present the new series of hybrid metal-halide semiconductors, (phen2)BiI5•H2O, (2,2-bpyH2)BiI5, (BrbpyH2)BiI4•H2O, (phen2)2Pb3I10•2H2O, and (2,2-bpyH2)2Pb3I10, where (phenH2)2+ = 1,10-phenanthroline-1,10-diium, (2,2-bpyH2)2+ = 2,2’-bipyridine-1,1’-diium and (BrbpyH2)+ = 6,6′-dibromo-2,2′-bipyridium. These compounds allow us to observe how rigidity of the cation, induced either through structural modification in case of phen or through non-covalent interactions in Brbpy, both relative to (2,2-bpyH2)2+, modifies the inorganic substructural unit. While the Pb2+ series of compounds show minimal changes in inorganic connectivity, we observe large differences in the Bi3+ series, ranging from 0-D dimers to corner- and edge-sharing 1-D chains of octahedra. We find that compounds containing (phenH2)2+ and (BrbpyH2)+ pack more efficiently than those with (2,2-bpyH2)2+ due to their retention of planarity leading to greater inorganic connectivity. Electronic structure calculations and optical diffuse reflectance reveal that the band gaps of these compounds are influenced by the degree of inorganic connectivity as well as the distances of the inorganic substructural unit distances. These results show that the structure and rigidity of organic cations can directly influence both the inorganic connectivity and the optical properties that could be tuned for certain optoelectronic applications.