Overcrowded Alkene Photo-Redox-Switches Based on Quinolinium/Carbene Building Blocks

The design and characterization of a novel class of overcrowded alkene-based photo/redox switches are presented. Folded and twisted tetrasubstituted alkenes are prepared in a modular fashion by the combination of quinolinium salts and stable carbenes. The compounds show multi-stimuli responsive properties: besides electrochemical redox-switching, which allows C─C bond rotation via a two-electron redox process, non-symmetrical compounds enable photochemically driven E/Z-switching. The different oxidation states can be isolated and fully characterized including structural changes verified by x-ray diffraction. Excellent photostability for E/Z-switching was observed in selected cases and is supported by UV-vis, NMR, and x-ray data as well as computations. TD-DFT calculations accurately predict the absorption properties of the E/Z-photo-switches and the substituent effects governing photo-stationary states and half-lifes. While the thermal E/Z-back-switching proceeds via a rather high energy barrier featuring a diradical transition state, electrochemical switching via the radical cation oxidation state allows instantaneous E/Z-isomerization, which is catalytic in electrons (electron hole catalysis). The mechanism of the switching process is supported in detail by quantum chemical calculations including nonadiabatic molecular dynamics simulations.