A push-pull substituted stiff-stilbene derivative that can be isomerized by visible instead of UV light was prepared. Further, an aryl-oxindole motif was used to achieve double bond isomerization by visible light. These achievements address the issue of using damaging UV light for E/Z carbon-carbon double bond isomerization. Subsequently, different photoswitchable anion receptors based on the stiff-stilbene motif were prepared additionally, either using a molecular tweezer or bridged macrocycle approach. Most notably, the incorporation of a stiff-stilbene bridge to calix[4]pyrrole afforded an 8000-fold affinity difference between photoaddressable isomers, which is two orders of magnitude larger than the highest affinity changes that had been reported thus far. In a later stage, different derivatives of this bridged calix[4]pyrrole with varying linkers and linker lengths were synthesized, providing insight into the origin of the large affinity difference as well as the optimal strap length. Also, a modified design with a dithienylethene instead of stiff-stilbene photoswitch was developed, in which besides binding affinity, selectivity could be controlled.
In addition to the tweezer-type and macrocyclic receptors, [2]rotaxane and [2]catenane receptors were synthesized, which showed a significant affinity change upon isomerization. The same photoswitchable macrocycles could also be used to control pseudorotaxane (de)threading.
The stiff-stilbene based tweezer-type receptors were successfully used to control passive transmembrane transport by light. Interestingly, we observed in our studies that beside binding affinity, other factors such as membrane incorporation and mobility are at least the same important towards controlling bilayer anion transport. We took this to our advantage in modified transporter designs, e.g. based on azobenzene. Unfortunately, the macrocyclic receptors turned out to be inactive or very poorly active in transmembrane transport assays.
The results of this project were disseminated at various national and international conferences, as well as visits to other universities. The dithienylethene-bridged calix[4]pyrrole was highlighted in C2W magazine and chosen as "Molecule of the Year" in 2024 by its readers.
