The Effect of Aldehyde and Carboxylic Acid Substitution on the Isomerization of Hemithioindigo Photoswitches

Hemithioindigo (HTI) photoswitches exhibit robust photoisomerization under visible light and relatively high thermal bistability. In this work, we report various modifications of the HTI core, namely the introduction of aldehydes and carboxylic acids at the para position of the stilbene fragment with different oxidation states of the sulfur center, and the incorporation of a Schiff base moiety. These modifications allowed tuning of the absorption properties, quantum yields of isomerization, and thermal stability of the metastable E-isomers. Notably, the formyl- and carboxyl-substituted HTI switches achieved high yields of isomerization under visible light in various solvents, while sulfur oxidation enhanced quantum yields but reduced photochromism. Schiff base formation led to red-shifted absorption and increased thermal stability. Finally, by leveraging the carboxyl substituents, we incorporated an HTI chromophore into the NU-1000 metal-organic framework (MOF), and demonstrated solid-state photoisomerization. These findings highlight key structural modifications that expand the applicability of HTI photoswitches for molecular switching in solution and solid-state environments.