Excited-state equilibration: a process leading to long-lived metal-to-ligand charge transfer luminescence in supramolecular systems

A recently developed strategy to prolong the luminescence lifetime of metal polypyridine complexes is discussed, and various resulting supramolecular systems are presented. The intervening mechanism demands that certain thermodynamic and kinetic parameters are satisfied, notably rapid and reversible electronic energy transfer between lowest-lying isoenergetic triplet excited states located on an inorganic moiety, e.g., a triplet metal-to-ligand charge transfer state (3MLCT), and localised on a separate organic chromophore, such as a triplet 3π-π* state, although the involvement of other states is also considered. Originally implemented with a pyrenyl-appended Ru(II) polypyridine complex, the generality of this approach has been demonstrated through the judicious combination of different metal complexes having 3MLCT states in conjunction with various matched organic chromophores and thus presents a new tool to instil made-to-order properties in supramolecular systems. Multichromophoric species and systems invoking second-sphere interactions are also considered.