Quinquephenyl: The simplest rigid-rod-like nematic liquid crystal, or is it? an atomistic simulation

We have performed an atomistic molecular-dynamics study on the molecular organization and liquid-crystalline properties of quinquephenyl (P5), a prototypical mesogen that is of interest for organic electronics. The thermotropic behavior reveals different mesophases. When cooling down from the isotropic phase, a transition to nematic (≈715 K) is found, then a smectic S_A (≈657 K) and another smectic, SA_X (≈642 K), before a crystalline phase is recovered (≈617 K). This phase sequence is compared with experimental findings. The different phases are described in terms of their molecular organization, orientational and positional order parameters, and pair distribution functions, as well as of their dynamics properties. In particular, the smectic phases that have not yet been characterized experimentally are discussed. By analyzing the effective shape of P5, it is concluded that its internal torsions and bending make it less rigid than could be expected. Not so rigid or straight after all! The nematic-isotropic and the other transition temperatures of p-quinquephenyl are determined by using atomistic molecular-dynamics simulations. The rigidity of p-quinquephenyl, often taken for granted, is assessed, and an aspect ratio that changes with temperature is found. The distribution is relatively broad due to internal torsion and bending.