| Summary: | 碩士 === 國立中央大學 === 化學研究所 === 95 === In this study, long time-scale all-atom molecular dynamics simulations and density functional
theory calculations were performed to study the structures, interactions and dynamics of a
banana-shaped liquid crystal mesogen. This molecule (see figure 1.16 for details) owns a
flexible alkyl linkage connecting to two rigid cores which is differential from the common
rigid one.
The results show that the intra-molecular hydrogen bond between the Schiff’s base and
benzene’s OH group enhances the π-e resonance between them and integrates these two
moieties into one rigid core. In addition, the characteristics of carbon-oxygen partially double
bond between the pendent chain and benzene limits its conformational space. Thus, the rigid
core extends from the moiety of benzene to the Schiff’s base and to the O-CH2 group of the
alkoxy chain.
The bending angle from the molecular dynamics simulations of the bulk system is ~120o, in
the reasonable range of 110o-140o for banana-shaped mesogens. Moreover, this calculated
bending angle is in good agreement with the observed 128o from the single crystal data.
Arising from the stabilizing energy of intra-molecular π-π stacking, the isolated molecule
prefers to form the hairpin-like conformation with a small bending angle of ~40o. The bending
angle of the isolated molecule is pretty different from the one of bulk system indicating that
the inter-molecular interactions can not only affect the organizations between molecules, but
also their bending angles. Additionally, the networks of inter-molecular hydrogen bonds and
inter-molecular π-π stacking were also observed from the 200 ns long time-scale molecular
dynamics simulations.
The angles between the alkoxy chain and rigid core are dynamical in liquid crystal phase
where the chains have the highest population to align with the same plane of the rigid core.
However, they fluctuate within a magnitude of ±30o up and down of the rigid core plane.
More interesting, our simulations show the alkoxy chain enlarges the rigid core, on the other
hand, it is more flexible than the saturated alkyl chain.
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