Cyclobis(paraquat-p-phenylene) as a Synthetic Receptor for Electron-Rich Aromatic Compounds: Electrochemical and Spectroscopic Studies of Neurotransmitter Binding

The equilibrium binding constants of the charge-transfer complexes formed by the receptor cyclobis(paraquat-p-phenylene) with four neurotransmitters (dopamine, epinephrine, norepinephrine, and serotonin) and the related aromatic compounds indole and catechol were measured in aqueous media using a spectrophotometric method. The values obtained were in the range 1000-7500 M^-. NMR studies confirmed that the surveyed π-donor guests are included inside the paraquat-lined cavity of the title receptor. Electrochemical studies of the receptor incorporated into thin Nafion films (∼200 nm) cast on the working electrode surface suggested that charge propagation across these films is relatively slow, but the receptor is strongly retained inside the Nafion matrix. This fact was utilized to investigate, using voltammetric techniques, the binding of catechol and indole inside the Nafion polyelectrolyte film. Furthermore, thicker Nafion membranes (0.180 mm) preloaded with the receptor extracted the neurotransmitters, as well as catechol and indole, from aqueous solution, giving rise to charge-transfer absorbances linearly related to the guest concentration in the solution. Overall, the behavior of the title species qualifies it as the first redox-switchable molecular receptor.