Consequences of the One-Electron Reduction and Photoexcitation of Unsymmetric Bis-imidazolium Salts

Coupling of uronium salts with in situ generated N‐heterocyclic carbenes provides straightforward access to symmetrical [4]2+ and unsymmetrical bis‐imidazolium salts [6]2+ and [9]2+. As indicated by cyclic and square‐wave voltammetry, [6]2+ and [9]2+ can be (irreversibly) reduced by one electron. The initially formed radicals [6].+ and [9].+ undergo further reactions, which were probed by EPR spectroscopy and density functional calculations. The final products of the two‐electron reduction are the two carbenes. Upon irradiation with UV light both [6]2+ and [9]2+ emit at room temperature in solution but with dramatically different characteristics. The different fluorescence behavior is analyzed by emission spectroscopy and interpreted by using time‐dependent density functional calculations as largely due to different excited‐state dynamics of [6]2+ and [9]2+. The geometries of both radicals [6].+ and [9].+ and excited states {[6]2+}* and {[9]2+}* are substantially different from those of the parent ground‐state molecules. Let's twist again: Unsymmetrical and untethered bis‐imidazolium salts, prepared from uronium salts and in situ generated N‐heterocyclic carbenes, have orthogonal imidazolium moieties in their ground state. Either one‐electron reduction to the radical cations or photoexcitation by irradiation with UV light results in large‐amplitude twisting motions around the central CC bonds (see figure).