Phase Transition Engineering of Roof-Shaped Nematogens

A library of 20 roof-shaped biaxial nematogens has been synthesized, with two different lateral chains, R1 and R2, and a terminal carboxyl ester linked via spacers of different lengths. Polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray scattering (XRS) studies show that all nematogens exclusively form nematic phases. The chain and spacer lengths influence the melting temperature, which could be decreased by more than 80 K. Some nematogens reveal a second-order glass transition to nematic glass upon cooling. While the short chains R1 are located in the interior of the nematogen, the other aliphatic residues interdigitate with neighboring molecules and generate an aspect ratio close to the optimum molecular biaxiality according to Straley. With XRS (aspect ratio, order parameters) and DSC data, a promising candidate for a comprehensive study on the biaxial nematic phase was identified.