Interlayer Sliding Phonon Drives Phase Transition in the Ph-BTBT-10 Organic Semiconductor

In the field of organic electronics, the semiconductor 7-decyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) has become a benchmark due to its high charge mobility and chemical stability in thin film devices. Its phase diagram is characterized by a crystal phase with a bilayer structure that at high temperature transforms into a Smectic E liquid crystal with monolayer structure. As the charge transport properties appear to depend on the phase present in the thin film, the transition has been the subject of structural and computational studies. Here such a process has been investigated by polarized low frequency Raman spectroscopy, selectively probing the intermolecular dynamics of the two phases. The spectroscopic observations demonstrate the key role played by a displacive component of the transition, with the interpenetration of the crystal bilayers driven by lattice phonon mode softening followed by the intralayer rearrangement of the molecule rigid cores into the herringbone motif of the liquid crystal. The mechanism can be related to the effectiveness of thermal annealing to restore the crystal phase in films.