Donor–acceptor Stenhouse adducts: Exploring the effects of ionic character

The effects of solution‐state dielectric and intermolecular interactions on the degree of charge separation provide a route to understanding the switching properties and concentration dependence of donor–acceptor Stenhouse adducts (DASAs). Through solvatochromic analysis of the open‐form DASA in conjunction with X‐ray diffraction and computational theory, we have analyzed the ionic character of a series of DASAs. First‐ and third‐generation architectures lead to a higher zwitterionic resonance contribution of the open form and a zwitterionic closed form, whereas the second‐generation architecture possesses a less charge‐separated open form and neutral closed form. This can be correlated with equilibrium control and photoswitching solvent compatibility. As a result of the high contribution of the zwitterionic resonance forms of first‐ and third‐generation DASAs, we were able to control their switching kinetics by means of ion concentration, whereas second‐generation DASAs were less affected. Importantly, these results show how the previously reported concentration dependence of DASAs is not universal, and that DASAs with a more hybrid structure in the open form can achieve photoswitching at high concentrations.