Excited states of mixed-ligand chelate complexes of ruthenium(II). A re-examination of the evidence for strong interligand coupling

Abstract

An investigation of the effects of the coupling between dissimilar ligands in the excited states of the complexes [Ru(bipy)_n(phen)_3–n]²⁺, [Ru(bipy)_n(4,7-Ph₂phen)_3–n]²⁺ and [Ru(bipy)_n(5-NO₂phen)_3–n]²⁺(n= 0, 1, 2 or 3)(bipy = 2,2′-bipyridine, phen = 1,10-phenanthroline) is presented. Proton magnetic resonance measurements upon the ground states of the complexes [Ru(bipy)_n(phen)_3–n]²⁺ show that the chemical shifts of the 3,3′-protons of bipy and the 4,7-protons of phen do not depend in a significant manner upon the ligand composition. Excited-state lifetime determinations for all three mixed-ligand series at 298 K in aqueous solution suggest that the coupling between dissimilar ligands is strong enough to produce kinetic effects. Simulations of the emission spectra of {[Ru(bipy)_n(phen)_3–n]²⁺}*(n= 1 or 2) show that the shifts in experimental band maxima in this series can be reproduced by a simple weighted addition of the emission profiles of {[Ru(bipy)₃]²⁺}* and {[Ru(phen)₃]²⁺}*, demonstrating that the shifts are not necessarily symptomatic of strong interligand orbital interactions. Finally, the variation of the absorption spectra in the wavelength region 240–300 nm across the series [Ru(bipy)_n(phen)_3–n]²⁺(n= 0, 1, 2 or 3) suggests that the π→π* transitions characteristic of [Ru(bipy)₃]²⁺ and [Ru(phen)₃]²⁺ maintain their discrete identity in the mixed-ligand complexes, an observation from which the presence of ligand-localised excited electronic states may be inferred. These apparently conflicting observations are reconciled through a kinetic model which demonstrates that coupling may occur on a time-scale which is comparable with or significantly shorter than the excited-state lifetimes and still be too weak to cause detectable spectroscopic shifts.