Primitive molecular recognition effects in electron transfer processes: Modulation of ((trimethylammonio)methyl)ferrocenium/ferrocene self-exchange kinetics via hydrophobic encapsulation

H-1 NMR line broadening measurements show that the electron self-exchange rate constant for ((trimethylamino)-methyl)ferrocenium/ferrocene (TMAFc(2+/+)) in D2O as solvent is decreased by ca. 20-50 fold in the presence of excess beta-cyclodextrin. The rate effect is associated with the selective hydrophobic encapsulation of the ferrocene form of the redox couple (i.e., the ferrocenium form is not significantly encapsulated). Selective encapsulation leads to a coupling of electron transfer to host (cyclodextrin) transfer. Optical intervalence absorption measurements for a closely related mixed-valence system strongly suggest that the coupling decreases the self-exchange rate by increasing the thermal activation barrier-an inference that is corroborated by activation parameter measurements. The barrier increase ultimately can be understood in terms of a redox asymmetry effect upon the isolated electron transfer event, where the overall exchange mechanism likely entails sequential electron and host transfer.