Chirality transfer in nematic liquid crystals via octahedral coordination compounds with Δ and Λ chirality

Nematic (N) liquid crystals (LCs), characterized by one-dimensional orientational order, readily transform into chiral nematic (N*) phases upon the incorporation of optically active substances, known as chiral dopants. While the ease of N* induction has driven the extensive development of organic chiral dopants, coordination compounds possessing Δ/Λ chirality have emerged as compelling alternatives, offering distinct structural and functional attributes. A defining feature of these "metallodopants" is their exceptionally high helical twisting power (HTP). For instance, early studies demonstrated that the simple complex Λ-[Ru(acac)3] exhibits a remarkably high HTP of 90 micrometer-1 in MBBA (N-(4-methoxybenzylidene)-4-butylaniline). This efficiency is attributed to the rigid coordination framework, which facilitates robust chirality transfer to the host LC. However, metallodopants often face challenges regarding miscibility with nematic hosts and intrinsic coloration, potentially complicating their integration into optical devices; consequently, systematic investigations have been limited compared to organic dopants. Nevertheless, metallodopants offer unique advantages. In tris(chelate) metal complexes, the sense of the induced helix (P or M) is strongly correlated with ligand orientation; for example, a 90° rotation of a ligand’s elongation axis can reverse the helical sense. This predictable inversion, supported by theoretical models, underscores the critical role of structural rigidity. Furthermore, the intrinsic absorption of these complexes, once viewed as a drawback, is now being harnessed for resonance Raman enhancement and chiroptical modulation. By combining high HTPs, predictable helix inversion, and spectroscopic utility, coordination compounds are positioning themselves as versatile competitors to organic dopants, serving as both functional inducers and optical probes for next-generation photonic and sensing technologies.