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)] (RuC8-1, acac = acetylacetonate, acacC8 = 3-(4′-octyloxy-phenylalkynyl)-pentane-2,4-dionato),
[Ru­(acac)­(acacC8)2] (RuC8-2), [Ru­(acacC8)3] (RuC8-3), [Ru­(acacC0)3] (RuC0-3, acacC0 = 3-(phenylalkynyl)-pentane-2,4-dionato),
and [Ru­(acacC24)3] (RuC24-3, acacC24 = 3-(3′,4′,5′-tri­(octyloxy)-phenylalkynyl)-pentane-2,4-dionato).
All the complexes, except for RuC0-3, 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 RuC8-1, RuC8-2, RuC8-3, and RuC24-3 induced
a right-handed (P) helix and the magnitudes of the
HTPs (/μm–1) were determined as follows: RuC8-1 (+60) < RuC8-2 (+109) > RuC8-3 (+78) > RuC24-3 (+41). The HTPs
of the ruthenium dopants were not simply proportional to their size.
The highest HTP observed in biaxial RuC8-2 was attributed to the balance of the molecular
helicity and high ordering in MBBA based on the surface chirality
model.