NSRRC Activity Report 2022

Chemical Science 029 To prepare bioinspired artificial water channels, Chien-Lung Wang (National Yang Ming Chiao Tung University), Kuan- Yi Wu (Chinese Culture University), and Wei-Tsung Chuang (NSRRC) cooperatively developed an amphiphilic discotic molecule (AD) with a hydrophilic core and a hydrophobic shell ( Fig. 1 ). 1 Disk-like AD molecules can form a liquid crystalline (LC) phase of hexagonally packed columns. As water can interact with the hydrophilic pore of the AD molecules via hydrogen bonding, artificial water channels can be formed in the columnar arrays. However, the orientational control of the highly ordered LC phase remains challenging, especially in a large area. They proposed that the dynamic interaction between water and the hydrophilic core of AD enables water to not only act as a morphological modulator that manipulates the phase stability and morphology but also as an orientational director that controls the growth direction and orientational order of the amphiphilic LC. This phenomenon is the so-called water-induced self-assembly (WISA). To confirm the orientational control of artificial water channels by WISA, they designed a cell and then inserted water on the left side of the supercooling melt sample at 57 °C ( Fig. 2(a) ). In Fig. 2(b) , the polarized optical microscopy (POM) image reveals that the LC growth direction is along the normal of the AD–water interface, indicating a guiding field to direct the orientation of LC nucleation and growth. The 2D grazing-incidence X-ray diffraction (GIXRD) patterns ( Fig. 2(c) ) reveal that the hexagonally packed columnar LC phase in the hydrated sample exhibits single-crystal-like orientation and that the column axis is parallel to the substrate according to this method. To change the aligned direction of the artificial water channels, the AD sample was inserted into the grid and then water was allowed to cover the surface ( Fig. 2(d) ). Water induces the column axis to be perpendicular to the substrate, as demonstrated by micro-beam small-angle X-ray scattering (SAXS) at the TPS 25A beamline. Fig. 2 : (a) Illustration of the planar growth of the hydrated LC structure of the AD via directional WISA and (b) the corresponding POM image. (c) Illustration of GIXRD measurement. (d) Illustration of the vertical growth of the artificial water channel array of the AD via the directional WISA and the corresponding microbeam SAXS pattern. (e) Probing depths of photoelectrons from C 1s and O 1s of the LC columnar structure of the AD and relative intensity areas of the hydrophobic (green) and hydrophilic (blue) segments of the AD in C 1s spectra and H 2 O (red) in O 1s spectra as functions of the probing depth. [Reproduced from Ref. 1] Fig. 1 : Schematic of the dual-axis alignment of artificial water channels by WISA. [Reproduced from Ref. 1]