Heterocyclic Hemithioindigos: Highly Advantageous Properties as Molecular Photoswitches**

Abstract

A survey of heterocyclic hemithioindigo photoswitches is presented identifying a number of structural motives with outstanding property profiles. The highly sought-after combination of pronounced color change, quantitative switching in both directions, exceptional high quantum yields, and tunable high thermal stability of metastable states can be realized with 4-imidazole, 2-pyrrole, and 3-indole-based derivatives. In the former, an unusual preorganization using isomer selective chalcogen-and hydrogen bonding allows to precisely control geometry changes and tautomerism upon switching. Heterocyclic hemithioindigos thus represent highly promising photoswitches with advanced capabilities that will be of great value to anyone interested in establishing defined and reversible control at the molecular level.

What Happens to a Pyrrole Hemithioindigo Photoswitch Trapped in a Fluorescent Protein?

“…In general, light-driven molecular switches are roughly divided into azobenzenes, spiropyrans, stilbenes, indigoids, and so on. − As is well-known, indigo derivatives are excellent candidates for photoswitches due to their rigidity, higher photostability, absorption in the visible light range, and good quantum efficiency. In 2022, Dube et al reported light-driven molecular switches based on pyrrole-, indole-, and imidazole-substituted heterocyclic hemithioindigos (HTIs) . The electron effect in heterocyclic rings and weak intramolecular interactions such as hydrogen bonding and chalcogen bonding play vital roles in conformation control.…”