Photophysical properties of an assembly containing a [Ru(bpy)3]2+ chromophore and a [Ru(bpy)(CN)4]2- quencher unit linked by a hydrogen-bonded interface based on the [Ru(bpy)(CN)4]2-/aza-crown association

The two complexes [(bpy)(2)Ru(bpy-cyclamH(2))](4+) (Ru-cy; bpy = 2,2'-bipyridine; bpy-cyclam = 1-(2,2'-bipyridin-5-yl-methyl)-1,4,8,11-tetraazacyclotetradecane) and [Ru(bpy)(CN)(4)](2-) (Ru-CN) associate in MeCN solution via a hydrogen-bonding interaction between the externally-directed lone pairs of the cyanide ligands on the Ru-CN unit, and the protonated amine sites of the cyclam macrocycle which is pendant from the [Ru(bpy)(3)](2+) core of the Ru-cy unit. Due to the sensitivity of the Ru-CN chromophore to interactions of the cyanide lone pairs, this association results in characteristic changes in its absorption and emission properties; viz. the (MLCT)-M-1 absorption is blue-shifted, from 535 to 480 nm, and the (MLCT)-M-3 emission is also blue-shifted, from 790 nm to 680 nm. In addition, the association between the components results in quenching of the characteristic (MLCT)-M-3 luminescence of the Ru-cy unit. Analysis of the absorption and emission properties of mixtures of the two components in varying proportions indicates that a 2 : 1 associate Ru-cy:Ru-CN:Ru-cy forms, in which two of the cyanide ligands of Ru-CN interact with each of the Ru-cy units. The overall association constant for formation of this is K-ass = 2.6 x 10(11) M-2; under the dilute conditions necessary for luminescence experiments the association is not complete however and some free Ru-CN and Ru-cy are also present in the equilibrium mixture. Within the hydrogen-bonded associate, quenching of the (MLCT)-M-3 luminescence of the Ru-cy unit occurs, most likely by a Forster energy transfer mechanism, with ca. 88% efficiency and a rate constant of k(q) = 3.8 x 10(7) s(-1), corresponding to an inter-chromophore separation of ca. 11.5 Angstrom.