Orientational Tuning of Monolayers of Amphiphilic Ruthenium(II) Complexes for Optimizing Chirality Distinction Capability

Abstract

The following six types of Ru(II) complexes were synthesized with an aim to develop a monolayer with chiral discrimination: 1 [Ru(bpy)2(4-C11bpy)](ClO4)2 (bpy = 2,2‘-bipyridine, 4-C11bpy= 4-methyl-4‘-undecyl-2,2‘-bipyridine); 2 [Ru(bpy)2(4,4‘-dC11bpy)](ClO4)2 (4,4‘-dC11bpy = 4,4‘-diundecyl-2,2‘ -bipyridine); 3 [Ru(bpy)2(5-C11bpy)](ClO4)2 (5-C11bpy= 5-methyl-5‘-undecyl-2,2‘-bipyridine); 4 [Ru(bpy)2(5,5‘-dC11bpy)](ClO4)2 (5,5‘-dC11bpy = 5,5‘-diundecyl-2,2‘-bipyridine); 5 [Ru(bpy)2(4-C18bpy)](ClO4)2 (4-C18bpy= 4-methyl-4‘-octadecyl-2,2‘-bipyridine); 6 [Ru(bpy)2(4,4‘-dC18bpy)](ClO4)2 (4,4‘-dC11bpy = 4,4‘-dioctadecyl-2,2‘-bipyridine). These complexes possess one or two alkyl chains at 4,4‘- or 5,5‘-positions of the 2,2‘-bipyridine ligand. A traditional amphiphile is a linear molecule with a polar head attached by one or two alkyl chains, whereas the present complexes cover both traditional (1, 2, 5, and 6) and nontraditional amphiphilic molecules (3 and 4). The monolayer behavior of these Ru(II) complexes on a subphase of 0.1 M NaClO4 aqueous solution was compared using surface pressure−molecular area (π−A) isotherm measurements and Brewster angle microscopy (BAM) observations. The vertical structure of monolayers deposited onto a silicon substrate was characterized with X-ray reflectivity (XRR) measurements. Chirality effects were investigated using two procedures: one was to compare the π−A isotherms between a racemic mixture and a pure enantiomer and the other to observe the effect of chiral anions in the subphase. As a result, complex 4 ([Ru(bpy)2(5,5‘-dC11bpy)](ClO4)2), belonging to the nontraditional group, was the only molecule with a distinct chirality discrimination capability toward the chiral anion, bis[(+)-tartrato][diantimonato(III)]dipotassium, K2[Sb2(+)-(C4H2O6)2]. The observed chiral distinction of this molecule was interpreted in terms of a preferential orientation of the polar head with respect to the aqueous phase.