Spectral evidence of spontaneous racemic and "pseudoracemic" interactions between optically active poly(pyridyl) metal chelates adsorbed on smectite clays

Abstract

UV-visible absorption and emission spectral studies are reported for enantiomeric(Δ)(-)_D, Λ(+)_D) and racemic(Δ,Λ) Ru(bpy)₃²⁺ (bpy = 2,2'-bipyridine) adsorbed on three naturally occuring smectite clays. The results highlights differences in the binding modes of the two forms at low (1-4%) loading levels. The effect of optical purity, loading level, mode of addition of complex, and clay type are to be discussed. the observed differences in binding state are ascribed to spontaneous interactions betwen optical antipodes(racemic interactions). The quenching effect of coadsorbed Δ,Λ(phen)₃^2+/3+, Λ-(+)_D-Ni(phen)₃²⁺ and Λ-(+)_D-Rh(phen)₃³⁺ (phen = 1,10-phenanthroline) on the luminescece of Δ Λ-Co(phen)₃^2+/3+, Λ-Ru(bpy)₃^2+∗ is also described. the stern-Volmer data are rationalized in terma of differences in electron/energy transfer quenching efficiences between homochiral and heterochiral systems, and luminescene variations arising from binding-state perturbations due to "pseudoracemic" interactions. such an interpretation is supported by studies with Δ, Λ-Zn(phen)₃²⁺, a structurally analogous, "nonquenching" coadsorbate. Finally, the possible factors that might enhance chiral recognition within clay are discussed.