Fluorination in Core-Only Calamitic Liquid Crystals: How Many and Where Should They Go?

Abstract

Introducing fluorine as a lateral substituent within the conjugated cores of conventional, side-chain bearing calamitic liquid crystals has been shown to provide either enhanced or detrimental effects on mesomorphic properties depending on the number and substitution location of the fluorine atom(s) within the mesogenic core. The slightly larger size of fluorine compared to hydrogen combined with its vastly differing electronic properties serve to control molecular packing during mesophase formation that can contribute to stabilizing or selecting for a certain mesophase type, induce new, higher ordered smectic mesophases, or reduce/suppress mesomorphism outright if it is an unfavorable location. Reported here is the systematic investigation of using fluorine as lateral substituents within a core-only (or side-chain free) calamitic structure, where each (hetero)aromatic ring is sequentially fluorinated (partially and/or fully) to see the resulting impact that these groups have on observed properties, such as phase transition temperatures, types of mesophases, and electrooptical behaviour. A comparative analysis was conducted to rationalize resulting trends in mesomorphism, and are based on polarized optical microscopy, thermal analysis, variable temperature powder XRD, single crystal structures, and predictions of geometric/electronic/energetic properties obtained from computational calculations (DFT). The absorption/emission properties of these all-aromatic structures in solution and the solid-state are also reported to determine the effect that fluorine substitution has on the frontier orbitals involved in these processes. Included are variable temperature fluorescence experiments to examine how fluorescence changes with temperature and within different (meso)phases. Reported are the lowest transition temperatures for core-only calamitic liquid crystals with a crystallization temperature of 130 ºC and a clearing temperature of 214 ºC, both of which are suitable for device fabrication.