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

Abstract

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.

Flip-flop dynamics in smectic liquid-crystal organic semiconductors revealed by molecular dynamics simulations

“…1b). 18 However, interlayer sliding cannot explain the flip-flop motion in polar layered structures, where all the molecular long axes have the same orientation, as in p Tol-BTBT-C 10 (Fig. 1a).…”

Structural Order and Thermal Behavior of Ph-BTBT-10 Monolayer Phases

“…Previous works , suggested that the crystal to SmE transition takes place through the interpenetration of neighboring layers by collective interlayer molecular translations. In a recent micro-Raman investigation, we gave clear evidence for such a mechanism, demonstrating that the transition occurs by the intervention of a preparatory stage during which the layer interpenetration is driven by an effective lattice phonon, which softens on approaching the phase boundary . Such a stage is followed by the abrupt in-plane rearrangement of the BTBT cores in the herringbone packing of the mesophase.…”

“…The comparison between the low-frequency Raman spectra of a pristine crystal and thermally treated samples allowed both the identification of the obtained phase and a qualitative assessment of its structural order, as the lattice phonons detected in this range are very sensitive to packing details, representing an excellent tool for investigating polymorphism, crystalline phase purity, and disorder in molecular solids. , In the case of the material obtained with a fast cooling rate (90 K/min), the spectra display a single peak as in the SmE phase, which corresponds to a rotation mode about the long molecular axis . On the other hand, slower cooling rates result in the appearance of three low-frequency bands, marked with a black arrow in Figure , which unequivocally identify the crystal phase.…”

“…Single-crystal domains were selected for the measurements by polarized optical microscopy. The a and b axes were identified as the two in-plane extinction directions, as described in ref . Polarization of the IR beam has been achieved by placing a KRS-5 wire grid polarizer along the optical path and aligning its polarization axis parallel to a or b .…”

Polar Polymorphism: A New Intermediate Structure toward the Thin-Film Phase in Asymmetric Benzothieno[3,2-b][1]-benzothiophene Derivatives

“…Given that the carrier mobility of the films is crucially dependent on the formed polymorph, many studies have focused on establishing strategies to control it via growth and thermal treatments, including recrystallization from the smectic phase, and in gaining a deep understanding of the molecular processes underlying the temperature-induced structural transformations. ,,,− Still a complete perception of the effect of the substrate interface on the origin and variation of polymorphic phases has not yet been obtained. It remains unexplored how the electronic properties of thin films are affected by structural changes.…”