Geometry‐Dependent Energy‐Gap Modulation of π‐Conjugated Systems Based on Hypervalent Silicon(IV)‐Fused Azomethine Compounds

Abstract The colors of substances and emissions are determined by the width of the energy gap between frontier molecular orbitals. In general, significant structural transformation or chemical modification is essential to tune the energy gap. Herein, we reveal a hypervalent silicon compound that can form both square pyramidal (SPY) and trigonal bipyramidal (TBPY) geometries and demonstrate a novel technique to modulate the energy gap of the π‐conjugated system. The energy gap in the TBPY geometry is narrower than that in the SPY geometry owing to the stronger contribution of a polarized three‐center four‐electron (3c‐4e) bond and a nitrogen–silicon (N−Si) coordination, and the geometries are changeable by external stimuli such as photoirradiation and temperature variations. Correspondingly, the emission bands in the orange ( λ PL = 640 nm) and yellow ( λ PL = 579 nm) regions were observed from the TBPY geometry at room temperature and the SPY geometry at −196 °C, respectively. Furthermore, the geometry can be fixed to the TBPY geometry by introducing bulky substituents at silicon. These mechanisms are experimentally and theoretically clarified in detail. Our findings described here are expected to be a novel molecular design for creating stimuli‐responsive materials.