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.