Improving Fatigue Resistance and Autonomous Switching of pH Responsive Hydrazones by Pulses of a Chemical Fuel

The chemically triggered reversible switching of pH‐responsive hydrazones involves rotary motion‐induced configurational changes, serving as a prototype for constructing an array of molecular machines. Typically, the configurational isomerization of such switches into two distinct forms (E/Z) occurs through the alteration of the pH the medium, achieved by successive additions of acid and base stimuli. However, this process results in intermittent operation due to the concomitant accumulation of salt after each cycle, limiting switching performance to only a few cycles (5–6). In this context, we introduce a novel strategy for the autonomous E/Z isomerization of hydrazones in acetonitrile using pulses of trichloroacetic acid as a chemical fuel. The use of this transient acid enabled reversible switching of hydrazones even after 50 cycles without causing significant fatigue. To test the broad viability of the fuel, a series of ortho/para‐substituted hydrazones were synthesized and their switching performance was investigated. The analysis of kinetic data showed a strong dependency of switching operations including the lifetime of transient state, on the electronic properties of substituents. Finally, a distinct color change from yellow to orange due to reversible switching of the para‐methoxy substituted hydrazone was employed for the creation of rewritable messages on commercially available paper. We introduce a novel approach for the autonomous E/Z isomerization of pH‐responsive hydrazones using trichloroacetic acid pulses. The use of this transient acid enables reversible switching of hydrazones without apparent fatigue even after 50 cycles. Kinetic analysis indicates a significant dependence of switching operations on the electronic properties of substituents.