Structural and photophysical properties of adducts of [Ru(bipy)(CN)4]2- with different metal cations:: Metallochromism and its use in switching photoinduced energy transfer

We show in this paper how the (MLCT)-M-3 luminescence of [Ru(bipy)(CN)(4)](2-), which is known to be highly solvent-dependent, may be varied over a much wider range than can be achieved by solvent effects, by interaction of the externally directed cyanide ligands with additional metal cations both in the solid state and in solution. A series of crystallographic studies of [Ru(bipy)(CN)(4)](2-) salts with different metal cations Mn+ (Li+, Na+, K+, mixed Li+/K+, Cs+, and Ba2+) shows how the cyanide/Mn+ interaction varies from the conventional end-on with the more Lewis-acidic cations (Li+, Ba2+) to the more unusual side-on interaction with the softer metal cations (K+, Cs+). The solid-state luminescence intensity and lifetime of these salts is highly dependent on the nature of the cation, with Cs+ affording the weakest luminescence and Ba2+ the strongest. A series of titrations of the more soluble derivative [Ru((t)Bu(2)bipy)(CN)(4)](2-) in MeCN with a range of metal salts showed how the cyanide/Mn+ association results in a substantial blue-shift of the (MLCT)-M-1 absorptions, and (MLCT)-M-3 energies, intensities, and lifetimes, with the complex varying from essentially non-luminescent in the absence of metal cation to showing strong (phi = 0.07), long-lived (1.4 mu s), and high-energy (583 nm) luminescence in the presence of Ba2+. This modulation of the (MLCT)-M-3 energy, over a range of about 6000 cm(-1) depending on the added cation, could be used to reverse the direction of photoinduced energy transfer in a dyad containing covalently linked [Ru(bipy)(3)](2+) and [Ru(bipy)(CN)(4)](2-) termini. In the absence of a metal cation, the [Ru(bipy)(CN)(4)](2-) terminus has the lower (MLCT)-M-3 energy and thereby quenches the [Ru(bipy)(3)](2+)-based luminescence; in the presence of Ba2+ ions, the (MLCT)-M-3 energy of the [Ru(bipy)(CN)(4)](2-) terminus is raised above that of the [Ru(bipy)(3)](2+) terminus, resulting in energy transfer to and sensitized emission from the latter.