Borenium Ions as Functional Materials

Borenium ions are tricoordinate boron cations of the type [LBR₂]⁺, where L is a donor ligand. While the inherently reactive nature of these Lewis acidic species has been harnessed to advance bond activation methodologies and catalysis, it has historically limited investigations into the optoelectronic properties of these systems. Borenium ions have the potential to serve as tunable low-energy LUMO materials, but there remains a gap in knowledge concerning the factors that impart stability and the specific optical transitions that mediate their function. This Perspective centralizes the key design principles used to tailor the properties of cyclic borenium ions toward functional luminescent materials applications, with a focus on recent examples from the literature. Design concepts including ligand identity, ring size, heteroatom incorporation, and counteranion selection have been identified as pivotal tools enabling the isolation and discovery of various emissive and stimuli-responsive boron cations. These advancements have resulted in the observation of diverse phenomena, including twisted intramolecular charge transfer (TICT), aggregation-induced emission (AIE), exciton coupling, and thermochromic behavior. The concepts highlighted herein serve as a blueprint for future research in main-group element materials chemistry.