Tris(β-diketonato) Ru(III) Complexes as Chiral Dopants for Nematic Liquid Crystals: the Effect of the Molecular Structure on the Helical Twisting Power

Doping nematic liquid crystals with optically active compounds transforms them into chiral nematic phases with helical structures. In this phenomenon, the chirality of the dopant molecules is transferred or amplified to the bulk of the liquid crystals. Δ-[Ru(acac)3] (acac = acetylacetonato) is known to work as one of the dopants with a strong helical twisting power (HTP). In this study, we have systematically modified [Ru(acac)3] to clarify the correlation between the molecular structure and HTP; we have designed and synthesized five new Ru(III) complexes, [Ru(acac)2(acacC8)] (RuC 8 -1, acac = acetylacetonate, acacC8 = 3-(4′-octyloxy-phenylalkynyl)-pentane-2,4-dionato), [Ru(acac)(acacC8)2] (RuC 8 -2), [Ru(acacC8)3] (RuC 8 -3), [Ru(acacC0)3] (RuC 0 -3, acacC0 = 3-(phenylalkynyl)-pentane-2,4-dionato), and [Ru(acacC24)3] (RuC 24 -3, acacC24 = 3-(3′,4′,5′-tri(octyloxy)-phenylalkynyl)-pentane-2,4-dionato). All the complexes, except for RuC 0 -3, could be separated into Δ, Λ isomers by HPLC on a chiral column and were examined as chiral dopants for a nematic liquid crystal, N-(4-methoxybenzilidene)-4-butylaniline (MBBA). The Δ isomers of RuC 8 -1, RuC 8 -2, RuC 8 -3, and RuC 24 -3 induced a right-handed (P) helix and the magnitudes of the HTPs (/μm–1) were determined as follows: RuC 8 -1 (+60) < RuC 8 -2 (+109) > RuC 8 -3 (+78) > RuC 24 -3 (+41). The HTPs of the ruthenium dopants were not simply proportional to their size. The highest HTP observed in biaxial RuC 8 -2 was attributed to the balance of the molecular helicity and high ordering in MBBA based on the surface chirality model.