Insights into the nanostructuring and phase behaviour of an all-aromatic prototypical nematic liquid crystal

All-aromatic calamitic liquid crystals are an unconventional family of rigid linear mesogens that represents the closest embodiment of the idealised rod-like molecule central to liquid crystal theories. 2,6-biphenyl naphthalene (PPNPP), a prototypical all-aromatic nematogen, has recently been the focus of scientific interest for both fundamental and technological purposes. While it provides a valuable benchmark for classical theories of nematic order, its experimental study presents challenges given the high temperature of the nematic phase. Herein, molecular dynamics (MD) simulations are contrasted with X-ray diffraction (XRD) data to resolve the thermotropic phase behaviour of PPNPP and the main structural features of its liquid crystal order. The observed trend of the molecular conformation with temperature points to an unexpected and significant distortion of the molecular framework in the nematic phase, which demonstrates the substantial non-linearity of the PPNPP molecule. The simulated mesomorphic behaviour and related thermodynamic parameters are in close agreement with the experimental data. The different phases are described in terms of their molecular organisation, pair distribution function, as well as the orientational and positional order parameters. We compare the classical and extended Maier-Saupe nematic theories with the computationally (MD) and experimentally (XRD) determined orientational order parameters. The observed deviation from the theoretical models indicates substantial inadequacies of the classical theories to describe the nematic-isotropic phase transition for this class of compounds. We suggest that modifications of theory should include the effects of short-range positional order, i.e., cybotaxis.