2.B.101.  The pH-gated Cl^--transporting Trazine-based Tripodal SemiCage (TTSC) Family

pH-gated chloride transport by a triazine-based tripodal semicage has been reported by Roy et al. 2017; Kumar and Madhavan 2023). Attachment of rigid 3,7-diazabicyclo[3.3.1]nonane arms to a planar triazine core formed a preorganized cavity (Fig. a below). This electron-rich cavity was capable of being protonated at acidic pH. Introduction of benzyl or pentafluorobenzyl groups at the ends of the rigid arms improved anion binding and membrane permeation (Fig. b below). An HPTS assay with vesicles showed that 4a has the highest transport activity. The compounds transport anions via a Cl⁻/OH⁻ antiport mechanism with the anion transport trend being Cl⁻ ≫ SCN⁻ > Br⁻ > F⁻ > OAc⁻ > NO₃⁻ > ClO₄⁻. Mechanistic studies showed that the transport rate for Cl⁻ is higher than that for OH⁻. Maximal chloride efflux was observed at pH 7 for 4a, and at pH 5.3 for 4b. These observations are attributed to the pK_a values of 8.9 and 6.7 for compounds 4a and 4b, respectively (Kumar and Madhavan 2023).