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

Abstract

Unique flip-flop motion of liquid-crystal organic semiconductor molecules responsible for phase transition was revealed by molecular dynamics simulations.

Accessing the electronic structure of liquid crystalline semiconductors with bottom-up electronic coarse-graining

“…While BTBT molecules form antiparallel cofacial π–π stacking in the smectic A phase, a herringbone-like structure with offset π–π stacking is observed in the smectic E phase, reminiscent of the crystal structure observed in the benzothienobenzothiophene family. 47–55,59–66 These features are consistent with CG structure factor analyses and the characterization of the nematic order parameter as a function of cutoff radius, as depicted in Fig. 2b and c.…”

“…Our simulation results align with these recent findings. 59–66 Specifically, the transition from antiparallel cofacial π–π stacking 133 to offset π–π stacking (also known as nanosegregated stacking) 134,135 within the monolayers from smectic A to smectic E, accompanied by a considerable increase in the tilt angle between the principal director and the layer normal, may serve as a precursor to the formation of the lipid-like bilayer structure observed in the crystal phase. 65,66…”

“…38–43 While the manipulation of liquid crystal (LC) phases in OSCs for enhanced device performance is a common theme throughout the community, 44–46 recent work has explored morphology-dependent conductivity in asymmetric benzothienobenzothiophene-based compounds, a promising type of LC-forming OSCs used in field-effect transistors. 47–66 These compounds exhibit a remarkable combination of attributes, including high charge mobility, superior solubility and processability, robust thermal durability, and the capability to regulate molecular orientation. While layered smectic phases with head-to-head bilayer (lipid-like) structures have been reported in the asymmetric benzothienobenzothiophene family, recent work by Han et al introduced a derivative, 2-(4-methoxyphenyl)-7-octyl-benzothienobenzothiophene (BTBT), 56,57 that exhibited a rare nematic phase (long-range order without layered structure) when inserted into rubbed planar anchoring sandwich cells.…”

“…However, the mechanisms underlying phase transitions between crystals and different smectic phases or the formation of the nematic phase remain unclear. 59–66 Importantly, large changes in the electronic conductivities were observed in transitioning benzothienobenzothiophene-based molecules through different LC phases. For traditional multiscale computational methods, the analysis of such conductivity trends would represent an effort warranting state-of-the-art computing resources; slow relaxation times would necessitate CG modeling, which would need to be connected with all-atom (AA) backmapping and molecular dynamics, followed by ad nauseam QC characterization at the ∼10–100 nm scale to compute morphology dependent electronic properties.…”