Stenhouse Zwitterion Photoswitches as Hydrogen-Bond Acceptors

Forming or disrupting hydrogen-bond pairs is a powerful strategy used to achieve selective chemical transformations and functions. Here, we introduce isolable sulfonamidate-iminium Stenhouse zwitterions, a class of photoswitches featuring hydrogen-bond-accepting capabilities. Unlike prior Stenhouse photoswitches necessitating C2 heteroatomic protonation for open-form stabilization, Stenhouse zwitterions remain in their open form with a C2 sulfonamidate anion when constructed with a sufficiently electron-withdrawing sulfonyl group. The sulfonamidate inverts the traditional C2 hydrogen-bond donor functionality into a hydrogen-bond acceptor, enabling hydrogen-bonding interactions that extend beyond acidochromism. Electronic changes in the sulfonyl group can also be leveraged to alter irradiative and nonirradiative isomeric distributions. Stenhouse zwitterion, hydrogen-bound zwitterion, and Stenhouse salt structural and behavioral patterns are differentiated by X-ray crystallography, NMR, and UV-vis control experiments. The chromophoric hydrogen-bond acceptor isomerizes to a colorless cyclic adduct with up to 96% efficiency upon visible light irradiation, with disruption of the hydrogen-bond interaction when in the hydrogen-bound form. A direct correlation between the triene electronic structure, nonirradiative equilibrium, photoswitching efficiency, and hydrogen-bonding capacity demonstrates the tunability of this platform.