Hybrid Charge-Transfer Semiconductors: (C7H7)SbI4, (C7H7)BiI4, and Their Halide Congeners

Hybrid metal halides yield highly desirable optoelectronic properties and offer significant opportunity due to their solution processability. This contribution reports a new series of hybrid semiconductors, (C7H7)­MX4 (M = Bi3+, Sb3+; X = Cl–, Br–, I–), that are composed of edge-sharing MX6 chains separated in space by π-stacked tropylium (C7H7 +) cations; the inorganic chains resemble the connectivity of BiI3. The Bi3+ compounds have blue-shifted optical absorptions relative to the Sb3+ compounds that span the visible and near-IR region. Consistent with observations, DFT calculations reveal that the conduction band is composed of the tropylium cation and valence band primarily the inorganic chain: a charge-transfer semiconductor. The band gaps for both Bi3+ and Sb3+ compounds decrease systematically as a function of increasing halide size. These compounds are a rare example of charge-transfer semiconductors that also exhibit efficient crystal packing of the organic cations, thus providing an opportunity to study how structural packing affects optoelectronic properties.