Comprehensive Understanding of Host- and Guest-Dependent Helix Inversion in Chiral Nematic Liquid Crystals: Experimental and Molecular Dynamics Simulation Study.

Abstract

Nematic liquid crystals (LCs) are known to transform into chiral nematic LCs with helical structures upon doping with enantiomeric compounds (called chiral dopants or guests). Here, we investigated the mechanism of host LC- and guest-dependent helix inversion by using octahedral metal complexes as guests. The helical twisting powers (HTPs/μm-1) of eight metal complexes with Δ, Λ chirality were examined in five nematic LC hosts. For example, Δ-[Ru(acac)2(trop)] (Ru-trop, Hacac = 2,4-pentanedione, Htrop = tropolone) induces a left-handed ( M) helix upon doping with N-(4-methoxybenzylidene)-4-butylaniline (MBBA), whereas it induces an opposite right-handed ( P) helix in 4-cyano-4'-pentylbiphenyl (5CB). The monobrominated complex of Ru-trop, Δ-[Ru(acac)2(Brtrop)] (Ru-Br1trop, HBr1trop = 5-bromotropolone), induces P helices in both MBBA and 5CB, whereas the dibrominated complex, Δ-[Ru(acac)2(Br2trop)] (Ru-Br2trop, HBr2trop = 3,7-dibromotroplone) induces M helices in both the media. The molecular dynamics simulation performed in parallel confirmed the drastic effect of the bromo groups on the microscopic ordering direction of guests in nematics. Further, HTPs of all Δ isomers of metal complexes investigated were found to shift in the positive direction as the dielectric constant anisotropy (Δε) of the host LCs increases (and vice versa for Λ isomers). These experimental and calculated results highlight the interplay of steric (excluded volume effect) and electrostatic (dipole-dielectric body) interactions between the host LCs and guest metal complexes.