Orientation Tuning of a Polypyridyl Ru(II) Complex Immobilized on a Clay Surface toward Chiral Discrimination

Abstract

The present work reports an attempt to elucidate a stereoselective energy-transfer system by immobilizing a chiral metal complex on a clay surface. The metal complex used was [Ru(bpy)₂L_i]²⁺ with L₁ = bpy (2,2‘-bipyridine), L₂ = 4,4‘-diundecyl-2,2‘-bipyridine, and L₃ = 5,5‘-diundecyl-2,2‘-bipyridine. The adsorption structure of [Ru(bpy)₂L_i]²⁺ was studied by means of electric dichroism measurements on an aqueous dispersion of a colloidal clay. It was found that the molecular orientation of the adsorbed Ru(II) complex was affected remarkably by the positions of the alkyl chains on the bpy ligand; that is, the angle of the 3-fold or pseudo-3-fold symmetry axis of the Ru(II) complex with respect to the surface normal was obtained to be 24°, 30°, and 52° for i = 1, 2, and 3, respectively. The efficiency of the energy-transfer was determined by photoluminescence quenching measurements between the adsorbed Ru(II) complex and [Ru(acac)₃] (acac = acetylacetonate) in solution. As a result, stereoselectivity appeared most for the case of [Ru(bpy)₂L₃]²⁺ in which its two helically twisted bpy ligands were projected in an outward direction.