Sequential Symmetry-Breaking Intercolumnar Transformations of a Conjugated Rod Molecule with a Flexible Coil

Abstract

In this paper, we report distinct symmetry-breaking phase transitions in molecules based on oligo-para-phenylene rods and a poly(ethylene oxide) coil depending on the length of the aromatic rod block The observed phase transitions were characterized by differential scanning calorimetry (DSC) and X-ray scattering methods. Compound 1 with a tetra-para-phenylene rod exhibited a symmetry breaking intercolumnar transition from p2gg to p6mm symmetry. This phase transition was accompanied by an endothermic peak in the DSC profile (first-order transition), suggesting an in-plane twist of the ribbon-like rod domain. On the other hand, 2 with a longer penta-para-phenylene rod exhibited sequential symmetry breaking intercolumnar transitions from c2mm to p2gg to p6mm symmetry. In contrast with 1, the phase transitions were accompanied by no enthalpy change. Therefore, the ribbon-like rod domains were not segmented due to increased inter-rod interactions of the longer aromatic rods. The phase transitions of 2 are attributed to increased thermal motions of the coils, which vary the orientational angle of the aromatic columns in the lattice.