Orthogonal Photoswitching with Norbornadiene

Abstract

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 red-shifted NBD absorptions.

Keywords: chelates; cycloaddition; isomers; ligand design; photochromism.