A Dual-Emissive Phosphine–Borane Lewis Pair with a U‑Shaped Linker: Impact of Methylation and Complexation on Fluoride Binding Affinity

To investigate phosphine to borane through-space charge transfer and its utility in anion sensing and the formation of metal complexes, a phosphine–borane Lewis pair (1) connected by a U-shaped linker has been synthesized. 1 could be readily converted to a phosphonium salt (2) and two 2:1 complexes with Au(I) (1-Au) and Pt(II) (1-Pt). The photophysical properties of the new compounds were examined and compared. Compound 1 displayed an intense P → B CT transition and a bright solvent-dependent dual emission that is switchable by fluoride ions. 2 and 1-Pt showed a turn-off and a turn-on fluorescent response, respectively, toward fluoride ions. The binding constant of 2 with F– was found to be 2 orders of magnitude greater than that of 1. The mechanism that is responsible for the distinct fluorescence response of 1, 2, and 1-Pt toward fluoride ions is proposed.