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

Abstract

The two complexes [(bpy)₂Ru(bpy-cyclamH₂)]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 ¹MLCT absorption is blue-shifted, from 535 to 480 nm, and the ³MLCT emission is also blue-shifted, from 790 nm to 680 nm. In addition, the association between the components results in quenching of the characteristic ³MLCT 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 × 10¹¹ 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 ³MLCT luminescence of the Ru-cy unit occurs, most likely by a Förster energy transfer mechanism, with ca. 88% efficiency and a rate constant of k_q = 3.8 × 10⁷ s⁻¹, corresponding to an inter-chromophore separation of ca. 11.5 Å.