Macrocyclization overrides the polymer effect in the stabilization of liquid crystalline (LC) phases with a novel odd-even alternation. A demonstration with LC crown ethers

The synthesis and characterization of a novel series of crown ethers based on triethylene glycol (3EO) and the conformationally flexible 1-(4-hydroxy-4'-biphenylyl)-2-(4-hydroxyphenyl)butane (TPB), i.e. TPB-(c)3EO(z) (where c stands for cyclic and z = 1-10 is the degree of polymerization (DP) of the cyclic compound), and of the corresponding linear polymer, TPB-(1)3EO (where l stands for linear), are described. The cyclic monomer, the cyclic trimer, the mixture of high molecular weight cyclics, and the linear polymer are amorphous. The cyclic tetramer to decamer display an enantiotropic nematic phase. The isotropization temperatures and the associated entropy and enthalpy changes of these cyclics increase in an odd-even dependence with DP. This unprecedented odd-even effect is determined by the anti and, respectively, gauche conformations of TPB in the supramolecular quasi-rigid rods obtained from collapsed macrocyclics in their nematic phase and contrasts with the continuous increase of the same parameters as a function of DP in linear main-chain LC polymers. These results demonstrate that macrocyclization overrides the well-established polymer effect in the formation and stabilization of liquid crystalline (LC) phases. Therefore, contrary to what has been considered for over 100 years, the cyclic and not the linear architecture is the most powerful in the design of molecular and macromolecular LCs.