Extending Wallach’s Rule: Quasi-Racemate Stability in Liquid Crystalline Metal Complexes

Wallach’s rule states that racemic crystals are generally more dense and thermodynamically stable than enantiopure forms. To test this principle in quasi-racemic liquid crystalline systems, we examined the thermal stability of mixtures of -Ru-C8 and -Ir-C8 across varying apparent enantiomeric excess (Aee). Here, Aee is defined as follows: Aee [%] = (|Ru-C8 - Ir-C8| / Ru-C8 + Ir-C8) × 100.The quasi-racemate (Aee = 0%) exhibited the highest transition temperature and enthalpy, confirming its superior stability. In contrast, mixtures with an excess of either enantiomer lost order and became amorphous. Additionally, mixtures of Ru-C8 and Ir-C8 with the same-chirality, Ru-C8/Ir-C8 and Ru-C8/Ir-C8, crystallized without forming liquid crystalline phases. However, quasi-enantiomers with different alkyl chain lengths (-Ru-C8 and -Ru-C9) still assembled cooperatively, yielding columnar or cubic phases. These results demonstrate that steric recognition between and isomers drives quasi-racemate formation. This extends Wallach’s rule to quasi-racemic liquid crystalline materials and offers new insights into supramolecular assembly and stability.