Self-assembled columnar structures of swallow-shaped tetrathiafulvalene- based molecules

A series of swallow-shaped tetrathiafulvalene (TTF)-based liquid crystal (LC) molecules (a-6TTFn, where 6 represents the number of carbon atoms in the two tails of a-6TTFn and n is the even number of carbon atoms in the alkyl chains attached to the side of the asymmetric TTF mesogen, n = 6-12) was designed and synthesized. One of this series of swallow-shaped asymmetric TTF molecules (abbreviated as a-6TTF12) having 12 carbon atoms in the alkyl chains attached to the side of the asymmetric TTF mesogen served as an example in this study. Major phase transitions and their origins in a-6TTF12 were studied with combined techniques of differential scanning calorimetry (DSC), one-dimensional (1D) wide-angle X-ray diffraction (WAXD) and solid-state carbon-13 ( ¹³C) nuclear magnetic resonance (NMR) experiments. a-6TTF12 formed a highly ordered liquid crystalline (Φ_LC) phase at higher temperatures and a crystalline (Φ_Cr) phase at lower temperatures. On the basis of the experimental results, it was found that the highly organized columns in the Φ_LC phase partially lost their long-range positional order compared with the Φ_Cr phase at lower temperatures due to the distinct molecular packing of alkyl chains during the transition from Φ_LC ↔ Φ_Cr. Furthermore, it was realized that there are two main driving forces in the formation of the ordered columnar phases. One was the π-π interaction among the TTF-based mesogens, and the other was the nanophase separation between the rigid TTF-based mesogens and the flexible alkyl chains. The structure, molecular packing symmetry, and morphology of a-6TTF12 in different phases were identified using 2D WAXD together with selected area electron diffraction (SAED) from transmission electron microscopy (TEM). Bright-field TEM images of polyethylene-decorated films of a-6TTF12 allow us to confirm its molecular self-organization which was first identified by SAED as well as by 2D WAXD from oriented films. Morphological observations utilizing cross-polarized optical microscopy (POM) on the micrometer length scale validated the phase transitions and molecular orientation in the macroscopically oriented film.