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

Abstract

Moderate variations in the fuel structure cause large changes in the rate of the back and forth motions experienced by a chemically fuelled catenane-based switch.

The Hydrolysis of the Anhydride of 2‐Cyano‐2‐phenylpropanoic Acid Triggers the Repeated Back and Forth Motions of an Acid–Base Operated Molecular Switch

“…The spectrum recorded at t =∞ (3800 min) revealed the complete disappearance of anhydride 5 and the presence of the decarboxylated product 3 as the sole reaction product, besides the catenane in its original neutral state 1 . The spectra recorded in the course of the reaction showed the growth and decay of a transient species, previously shown to be ion pair B“ . The peaks marked with a blue asterisk, with the exception of the peak of the double bond protons at 5.52 ppm, belong to ten of the twelve protons of the two nonequivalent phenanthroline units .…”

“…Signals marked with blue asterisks are related to state B“ , red asterisks to catenane 1 in the neutral state A , green asterisk to decarboxylated product 3 (for a complete assignment of the signals related to state B” , 1 and 3 , see Ref. and ).…”

“…Corroborating evidence came from UV/Vis monitoring of the reaction between 1 and 5 in CH 2 Cl 2 . Ion pair B“ is known to display a strong adsorption centered at 375 nm whereas all of the other species in the reaction mixture, including ion pair B′ , do not absorb, or absorb very little at the given wavelength. The red trace in Figure refers to a comparative experiment in which 2.0 m m 1 was mixed with an equimolar amount of acid 2 .…”

“…Although the rate equation for the hydrolysis reaction is unknown, reliable estimates of the rates v 1 of acid production at the quasi‐stationary state in the different runs could be obtained remembering that v 1 = v 2 (Scheme ) when the concentration of the ion pair in state B“ reaches its maximum value. Since the decay of B” is a first‐order process (k′′=4.8×10 −4 s −1 , see caption to Figure ), and since [ B“ ] max could be estimated from NMR spectra, v 1 values of 4.8×10 −7 , 6.7×10 −7 , and 8.6×10 −7 m s −1 were calculated by using Equation for the runs in which [ 5 ] o =2.0, 4.0, and 8.0 m m in the given order …”

“…Here, we report that the switching motions of catenane 1 could be repeatedly triggered for a large number of cycles through continuous release of acid 2 by the smooth reaction of a molar excess of anhydride 5 with the tiny amount of adventitious water in the reaction medium (Scheme ). Considering that proton transfer from 2 to 1 is instantaneous and quantitative, and that decarboxylation (step B′ → B“ , Scheme ) is complete in less than 2 min, the rate of formation of the ion pair in state B” basically coincides with the rate of formation of acid 2 and, consequently, the rate of the overall process is determined by the rate of the back proton transfer (step B“ → A ). Thus, according to Scheme , the concentration of the ion pair in state B” builds up in the early stages of the reaction, reaches a more or less extended maximum when v 1 = v 2 ( v 1 and v 2 are the rates of the depicted reactions, and eventually dies out.…”