Cooperative K⁺/CN − ion-pair sensing enabled by a π -conjugated benzo-crown ether–borane system

Abstract π -conjugated molecular systems incorporating a benzo-crown ether and a dimesitylborane unit that enables cooperative recognition of CN − and K + ions were synthesized. UV/Vis spectroscopy and fluorescence measurements revealed contrasting optical responses upon binding with individual ions: CN − coordination quenched fluorescence through deactivation of the intramolecular charge-transfer pathway, whereas K + binding enhanced emission with a blue shift. Notably, sequential addition of K + to the CN − -bound species resulted in fluorescence recovery and up to a fourfold increase in the cation association constants, demonstrating clear positive hetero-ion cooperativity. Nuclear magnetic resonance experiments confirmed stepwise ion association, and theoretical calculations clarified that ion binding induces reorganization of the frontier molecular orbitals and charge compensation across the π -conjugated backbone. These combined results establish an electronic origin for the cooperative ion-pair response and provide a design strategy for developing ion-responsive optical materials based on π -conjugated systems.