Mechanistic insights into the thermal ground-state recovery of salicylidene aniline photoswitches

Salicylidene aniline (SA) and its derivatives are attractive photochromes for many applications thanks to the environmental sensitivity of their photoswitching. While a large volume of work has been dedicated to understanding the photochemistry of SA, the mechanism by which the trans -keto photoproduct returns to the initial cis -keto form is underexplored in solution. This thermal relaxation has often been previously assumed to proceed via a trans- keto to cis -keto isomerization, as in the solid state. Through a combination of experimental lifetime measurements of such anils with various substituents and theoretical calculations, we show that in solution the thermal recovery is unlikely to occur via direct isomerization of the trans -keto form. Measured lifetimes are on the order of 10 -4 –10 -5 s for all SA derivatives and are incompatible with theoretical predictions. We instead propose three alternative recovery mechanisms, involving an initial proton abstraction, addition, or enolization step before isomerization, which are consistent with experimental lifetimes and further supported by a strong primary isotope effect in deuterated ethanol. Overall, thermal relaxation of SA-based photochromes in solution is shown to be dependent on not only molecular substitution, but also environmental factors, making prediction of the lifetime of the photoproduct a complex problem. • Photochrome lifetimes for a series of salicylidene anilines are measured in solution • Experimental vs. theoretical lifetimes rule out a previously assumed direct mechanism • Photochrome lifetime shows a significant kinetic isotope effect • Alternative mechanisms involving solvent-assisted proton transfer are proposed