Molecular dynamics of pyrene based discotic liquid crystals confined in nanopores probed by incoherent quasielastic neutron scattering

Abstract

Semiconducting nanowires made of discotic columnar liquid crystals can be obtained by impregnation into solid nanoporous templates, and provide new opportunities to tailor devices for organic electronics with promising charge carriers transport properties. These properties are tightly related to the self-assembly and molecular dynamics of the discotic columns inside the nanowires. We recently studied and rationalized the formation of different nanostructures in the columnar phase of pyrene derivative discotics nanoconfined in anodic alumina and porous silicon templates ([Cerclier et al., J. Phys. Chem. C, 2012, 116, 18990–18998, Kityk et al., Soft Matter, 2014, 10, 4522–4534]). We now present the molecular dynamics of nano-confined pyrene derivative mesogenic phases as studied by incoherent quasielastic neutron scattering over a broad range of correlation times. The combination of backscattering and time-of-flight techniques has allowed to describe the nature of the molecular motions at play on the pico to nanosecond time scale. The dynamics of the columnar phase is dominated by fluctuations of the lateral chains, while the onset of larger amplitude motions like whole-body reorientations and slow center-of-mass translational diffusion occurs at high temperature in the isotropic phase. Interestingly, nano-confinement does not qualitatively alter the nature of the molecular dynamics, but essentially blocks the long range translational motions and induces broader distributions of correlation times of the fastest local relaxations.