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

Abstract

The dinuclear complex [(bpy)₂Ru(μ-L¹)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 inter-component 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 ³MLCT 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 Förster energy transfer being the quenching process which is switched on by the change in solvent composition. In contrast, at 77 K (frozen solvent) the ³MLCT 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.