Mechanistic Insight into Fluoride‐Induced Bathochromic Shifts in Multiple‐Boron‐Embedded π‐Conjugated Systems through Favorable LUMO Localization

Fluoride anion (F − ) detection by fluorescent boron‐containing π‐conjugated compounds has been widely investigated to date. However, a design principle to achieve both a bathochromic shift and bright photoluminescence (PL) remains scarce. Herein, it is demonstrated that B2 , a multiple boron‐embedded π‐conjugated compound, exhibits a PL peak shift from 459 to 495 nm upon F − addition. In sharp contrast to conventional charge‐transfer‐type systems, the photoluminescence quantum yield is up to 0.78 even after F − coordination. Quantum chemical calculations suggest that the anionic character of F − induces a redistribution of frontier molecular orbitals, leading to simultaneous HOMO‐preferential destabilization and a large HOMO–LUMO overlap.