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 SA (≈657 K) and another smectic, SXA (≈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.