Variations in the fuel structure control the rate of the back and forth motions of a chemically fuelled molecular switch

This work deals with the use of 2-cyano-2-arylpropanoic acids as chemical fuels for an acid-base operated molecular switch that consists of a Sauvage-type catenand composed of two identical macrocycles incorporating a phenanthroline unit. When used as a base promoter of the decarboxylation of propanoic acid derivatives, the switch undergoes large amplitude motion from the neutral catenand to a protonated catenate and back again to the neutral state. The rate of back proton transfer, which determines the rate of the overall process, was markedly affected by para-substituents in the order Cl > H > CH₃ > OCH₃ (ρ = +5.2). Thus, the time required to complete a full cycle was almost two days for the OCH₃ derivative and dropped to a few minutes for the Cl derivative. These results show for the first time that the rate of operation of a molecular switch can be regulated by variations in the fuel structure.