Thiosemicarbazones as versatile photoswitches with light-controllable supramolecular activity

Abstract

Using photoswitchable molecules to manipulate supramolecular interactions under light illumination has driven advancements in numerous fields, allowing for the strategic alteration of molecular systems. However, integrating the moiety responsible for these interactions into the photochromic scaffold can be complex and may hamper the switching efficiency. We thus explored a simple class of organic molecules, namely thiosemicarbazones, featuring both a photoisomerizable CN double bond and a thiourea moiety capable of hydrogen bonding. The scalable two-step synthesis allowed us to prepare 23 thiosemicarbazones to systematically elucidate their optical properties. Attaching various functional groups, extended π-systems, and heterocycles enabled fine-tuning of their absorption profiles. UV light illumination converts thiosemicarbazones from the stable E-isomer to the metastable Z-isomer, exhibiting both negative and positive T-type photochromism, a wide range of thermal half-lives, PSS values up to 92%, and high fatigue resistance. Substituting the CN moiety with a pyridinyl group stabilizes the Z-isomer via intramolecular hydrogen bonding, confirmed by single-crystal X-ray analysis, and transforms thiosemicarbazones into bistable P-type photoswitches. Additionally, thiosemicarbazones dimerize or form aggregates through hydrogen bonding—a process that can be turned off or on with light. Overall, thiosemicarbazones offer tunable photochromic and supramolecular properties, rendering them a promising photoswitch for creating stimuli-responsive systems.