The most active transporters with the lowest EC50 values were those substituted with the CF3 groups, followed by the NO2 > F > t-Bu-substituted transporters. Extremely high transport activity was achieved by 2c with an EC50 of 0.0000048 mol% followed by 1c with 0.0000088 mol%, 3c at 0.000010 mol%, and finally 4c with an EC50 value of 0.000098 mol% (Table ). The enhanced F -transport under this assay can partially be ascribed to the good size-fit complementarity of the smaller F -anion inside the macrocyclic binding cavity. Valkenier, Butler, and co-workers tested the F -transport properties of two cationic transporters previously reported by Gabbaï and co-workers, the best of which yielded an EC50 value of 0.000014 mol%. Comparing the transport activities of 1a-4c shows that the entire series is capable of highly efficient F -transport. Further comparing the three most active tetralactams 1c, 2c, and 3c with the best previously tested cationic F -transporter displays 1.6-, 2.9-, and 1.4-fold enhancements in transport activity, placing them as the most active F -transporters reported to date using this method.