Electron transfer through norbornadiene and quadricyclane moieties as a model for molecular switching

The bis(pentaammineruthenium(II)) complex of dicyanonorbornadiene has been prepared and characterized. Its photochemical conversion into a dicyanoquadricyclane complex by irradiation in methanol has been followed by UV-visible spectroscopy, IR spectroscopy, H-1 NMR spectroscopy, and cyclic voltammetry. The dicyanonorbornadiene complex exhibits an intervalence transition in the 1000-1600 nm range when oxidized to the ruthenium-(II)-ruthenium(III) state. This allows the determination of the effective coupling V-ab through the dicyanonor-bornadiene ligand, which is found as 0.023 eV. No such band is observed in the case of the dicyanoquadricyclane complex. These results show the possibility to modulate an intramolecular electron transfer process by using a photoisomerizable bridging ligand.