A novel class of molecular switches has been designed and characterized, based on sterically overcrowded alkenes that respond to multiple stimuli. These switches are synthesized in a modular fashion by combining quinolinium salts and stable carbenes, resulting in folded and twisted tetrasubstituted alkenes. The compounds exhibit multi-stimuli responsive properties: they undergo electrochemical redox-switching, which facilitates C─C bond rotation via a two-electron redox process, and non-symmetrical compounds enable photochemically driven E/Z-switching.
The different oxidation states of these molecular switches can be isolated and fully characterized, with structural changes confirmed by X-ray diffraction. Notably, selected compounds demonstrate excellent photostability for E/Z-switching, supported by UV–vis, NMR, and X-ray data, as well as computations. Time-dependent density functional theory (TD-DFT) calculations accurately predict the absorption properties of the E/Z-photo-switches and the substituent effects governing photo-stationary states and half-lives.
The thermal E/Z-back-switching process occurs via a high-energy barrier featuring a diradical transition state. However, electrochemical switching via the radical cation oxidation state allows instantaneous E/Z-isomerization. The mechanism of the switching process is supported in detail by quantum chemical calculations, including non-adiabatic molecular dynamics simulations.
These multifunctional molecular switches hold promising potential for applications in materials science, catalysis, molecular machines, nanotechnology, and biological systems.