Solvatochromism as a mechanism for controlling intercomponent photoinduced processes in a bichromophoric complex containing [Ru(bpy)3]2+ and [Ru(bpy)(CN)4]2- units

The dinuclear complex [(bpy)(2)Ru(mu-L-1)Ru(CN)(4)] (1) contains {Ru(bpy)(3)}(2+)-type (Ru-bpy) and {Ru(bpy)(CN)(4)}(2-)-type (Ru-CN) chromophores covalently linked by a short, saturated -CH2OCH2CH2OCH2-chain. Since the photophysical properties of the Ru-CN chromophore are strongly solvent-dependent, whereas those of the Ru-bpy chromophore are not, it follows that altering the solvent provides a means of altering the driving force for intercomponent photoinduced energy- or electron transfer processes. At room temperature, in a mixed solvent system varying from pure water to pure dmso, the characteristic luminescence of the excited Ru-bpy unit is progressively quenched as the proportion of dmso in the mixture increases. This behaviour is consistent with both *Ru-bpy --> Ru-CN energy transfer quenching and with Ru-CN --> *Ru-bpy electron transfer quenching, because as the proportion of dmso in the solvent increases, the (MLCT)-M-3 excited state of the Ru-CN unit drops in energy (which facilitates the energy transfer process) and its Ru(III)/Ru(II) reduction potential also becomes less positive (which facilitates the electron transfer process). Consideration of the solvent composition at which luminescence quenching of Ru-bpy by Ru-CN occurs, the saturated nature of the spacer, and the metal-metal separation, collectively point towards Forster energy transfer being the quenching process which is switched on by the change in solvent composition. In contrast, at 77 K (frozen solvent) the (MLCT)-M-3 state of the Ru-CN unit is raised in energy above that of the Ru-bpy unit, such that the energy transfer gradient is reversed and *Ru-CN --> Ru-bpy energy- transfer occurs with strong emission from the Ru-bpy terminus.