Photoskunks: molecules for light-induced thiol generation

Thiols are organosulfur compounds bearing a -SH functionality which have a great importance in organic synthesis, biology, and materials chemistry. The development of approaches towards controllable generation of these species using a light stimulus is an actual and relevant task. The compounds that form thiols in neutral or anionic form under the action of light can be called "photoskunks", and three types of such photofunctional molecules can be identified. The first type of "photoskunks" is based on the reversible phototransformations of the corresponding photoswitches, namely, spirothiopyrans and aza-diarylethenes, which form thiol-containing species under exposure to light. These metastable light-generated thiols return to the initial state thermally or photochemically providing an opportunity of reversible caging/uncaging of the thiol group. The second type of "photoskunks" relies on irreversible phototransformations of organic compounds, such as diarylethenes and o-thiopyrinidylbenzaldehydes, leading to formation of thiols. The third group of "photoskunks" represents photocages containing photocleavable protecting groups. These compounds undergo light-induced scission of the C-S bond yielding thiolate anions. The aim of this review article is to describe the development of molecular tools for light-induced generation of thiols and to outline the current state, challenges and perspectives of this emerging interdisciplinary field.