Twisted Hemithioindigo Photoswitches: Solvent Polarity Determines the Type of Light-Induced Rotations

Controlling the internal
motions of molecules by outside stimuli
is a decisive task for the generation of responsive and complex molecular
behavior and functionality. Light-induced structural changes of photo­switches
are of special high interest due to the ease of signal application
and high repeatability. Typically photo­switches use one reaction
coordinate in their switching process and change between two more
or less-defined states. Here we report on new twisted hemi­thio­indigo
photo­switches enabling two different reaction coordinates to
be used for the switching process. Depending on the polarity of the
solvent, either complete single bond (in DMSO) or double bond (in
cyclohexane) rotation can be induced by visible light. This mutually
independent switching establishes an unprecedented two-dimensional
control of intra­molecular rotations in this class of photo­switches.
The mechanistic explanation involves formation of highly polar twisted
intramolecular charge-transfer species in the excited state and is
based on a large body of experimental quantifications, most notably
ultrafast spectroscopy and quantum yield measurements in solvents
of different polarity. The concept of pre-twisting in the ground state
to open new, independent reaction coordinates in the excited state
should be transferable to other photo­switching systems.