Orthogonal Photoswitching Using Norbornadiene.

Orthogonal photoswitching is a convenient but challenging way of controlling multiple functions in a system by selective photoisomerization of one unit before the other in any arbitrarily chosen sequence. Here we present this concept for the norbornadiene/quadricyclane (NBD/QC) photo/thermo-switch in the presence or absence of a coordinated metal ion. Thus, introducing two pyridyl ligands via ethyne-1,2-diyl bridges provides a system that by chelation of metal ions, such as Pd(II), has altered optical and switching properties. Mixing the Pd(II) complex with its free ligand furnishes a four-state system where NBD-to-QC photoisomerizations for complexed and uncomplexed species are controlled by the irradiation wavelength and can occur orthogonally, that is, the sequence of photoisomerizations can be swapped. Studies on Ag(I) and Pb(II) complexes, being less stable than the Pd(II) complex, are also presented; these exhibit like the Pd(II) complex significantly redshifted NBD absorptions.