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.