Thermally-Induced Mesophase Transitions in Alkyl-Substituted Thienoacenes

Pentathienoacene (T$_{5}$) is an organic molecule---first synthesized in recent years---that is most succinctly described as the thiophene analog of pentacene. In this study, the solid-state structure and phase behavior of dioctyl- and didodecyl-substituted T$_{5}$ were examined \emph{via} differential scanning calorimetry (DSC), variable-temperature, polarized optical microscopy (VT-POM), variable-temperature X-ray diffraction (VT-XRD) and electron diffraction (ED). DSC reveals the presence of a number of phase transitions, while ED, VT-OM and VT-XRD reveal the details of the structural changes of these transitions. The first phase transition exhibited by both materials is a crystal--crystal transformation that involves the contraction of the unit cell along the long axis by nearly 25\%. This change has been attributed to the introduction of numerous gauche defects at elevated temperatures. Further heating causes both molecules to exhibit a Smectic C liquid crystalline phase, identified by VT-OM and VT-XRD. VT-XRD was also utilized to elucidate lattice parameters for these various phases.