Excited-state acid-base and coordination chemistry of ruthenium(II) diimines: Association with various metal cations.

摘要

Ruthenium(II) diimines have been extensively studied for their remarkable photophysical properties. Of particular interest are compounds with peripheral base sites, such as bis(2,2'-bipyridine) (2,3-bis(2-pyridyl) pyrazine) ruthenium(II), [(bpy)2Ru(dpp)]2+, which upon excitation undergoes a metal-to-ligand-charge transfer out onto the dpp ligand, leading to an emissive state and incredible increases in basicity. Quenching by protonation in acidic aqueous solution most likely involves transfer of the proton from the pyridyl ring of the dpp ligand, the site of higher basicity in the ground state, to the pyrazinyl ring in the excited state, the nearby transient site of higher basicity.; Previous work suggested that such changes in basicity could translate into an excited-state coordination chemistry. For example, the photoinduced reaction of [(bpy)2Ru(dpp)]2+ with PtCl6 2- to form [(bpy)2Ru(dpp)PtCl4]2+ is thought to involve initial exciplex formation. Reexamination of the excited-state acid-base properties of [(bpy)2Ru(dpp)]2+ reveals an inversion in basicity in the asymmetric, distinct imine nitrogens on the dpp ligand upon excitation. This serves as the basis for association between [(bpy)2Ru(dpp)]2+ and various metal cations, in particular the d10 metal cations Ag+, Cd 2+, and Zn2+. Each forms an emissive bimetallic complex where the more well-known quenching mechanisms of energy and electron transfer can be precluded based on energetics and/or the emissivity of the ensuing bimetallic complex. These interactions have been characterized by nuclear magnetic resonance, electronic absorption, and both steady-state and time-resolved emission spectroscopy. Data fit a model based on reversible associational quenching with a degree of diffusional association due to the inversion of imine basicity upon excitation. Thus, distinct, emissive bimetallics are formed, characterized by lower energy transitions and reduced lifetimes, thus strongly implying that in well-chosen systems, excited-state enhanced basicity can lead to heretofore unseen excited-state coordination chemistry.