Ingredients to TICT Formation in Donor Substituted Hemithioindigo

Twisted
intramolecular charge transfer (TICT) formation in hemithioindigo
photoswitches has recently been reported and constitutes a second
deexcitation pathway complementary to photoisomerization. Typically,
this behavior is not found for this type of photoswitches, and it
takes special geometric and electronic conditions to realize it. Here
we present a systematic study that identifies the molecular preconditions
leading to TICT formation in donor substituted hemithioindigo, which
can thus serve as a frame of reference for other photoswitching systems.
By varying the substitution pattern and providing an in-depth physical
characterization including time-resolved and quantum yield measurements,
we found that neither ground-state pretwisting along the rotatable
single bond nor the introduction of strong push–pull character
across the photoisomerizable double bond alone leads to formation
of TICT states. Only the combination of both ingredients produces
light-induced TICT behavior in polar solvents.