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.