Dynamics of phototunable two-dimensional polar wetting sheets of a dendritic liquid crystal

Azobenzene-based molecular systems are used as photoreswitchable smart materials with applications in many areas, including designing surface orientational patterning of anisotropic molecules. It is important and challenging to probe the real-time photoresponsive behaviors of these systems, particularly the photodynamic processes from the mesoscopic to microscopic or macroscopic scales near surfaces. We report an unusual reversible interfacial wetting-dewetting switching of a dendrimer incorporating azobenzene moieties. The switching is accompanied by light-driven generation or extinction processes of the surface wetting nanostructures, which are composed of supramolecular polarly layered molecules. The resultant polar-nonpolar switching of the surface allows macroscopic wetting-dewetting and nonlinear optical responses. Our results provide unique insights into controlling transient structures of photoresponsive materials.

Figure 1

(a) Molecular structure of AzD6 and its phase sequence. (b)–(d) Texture variation of 1 wt% AzD6/5CB mixture in an LC cell (b) before and (c) after $3{\mathrm{mW}/\mathrm{mm}}^2$ UV irradiation, and (d) after Vis irradiation. Scale bar is $100\mu \mathrm{m}$. (e) Force curve measurement of AzD6 film left from 1 wt% AzD6/5CB mixture after washing 5CB away with isopropanol. (f)–(h) Corresponding variation of v-GIXRD diffraction patterns for the conditions in (b)–(d).

Figure 2

GIXRD measurement on a spin-coated 100-$\mu \mathrm{m}$ thick AzD6 film. (a) and (b) Small angle GIXRD diffraction patterns of the AzD6 film at conditions of without UV irradiation (a) and upon $3{\mathrm{mW}/\mathrm{mm}}^2$ UV irradiation (b).

Figure 3

Variation of longitudinal correlation length of 5CB and AzD6 calculated from the full width at half maxima of the GIXRD SAD, with UV and Vis light strength.

Figure 4

(a) Contact angle measurement of a water droplet on the corresponding AzD6. The contrast of the pictures was adjusted to improve visibility. (b) Texture variation of a pure AzD6 film on a glass substrate before and upon UV irradiation with schematics. Scale bar is $20\mu \mathrm{m}$.

Figure 5

Snapshots of v-GIXRD upon light irradiation calculated by image subtraction (the computation procedure is given in the main text) at two UV intensities. White regions indicate disappeared signals, and black regions are emerging signals at a given time.

Figure 6

(a) Schematic of SHG measuring system. The labels P, PR and A mean polarizer, polarization rotator and analyzer, respectively. (b) SH signal as a function of time after UV irradiation at different UV intensities. Time 0 is the time the UV irradiation is stopped. (c) Variation of SH intensity under alternating UV light irradiation ($12.73{\mathrm{mW}/\mathrm{mm}}^2$) and thermal relaxation. (d) SH signal at time 0 in (b) as a function of UV strength. The inset shows the relaxation time from the signal level at time 0 to the initial level [saturated level in (b)] as a function of UV strength.

Figure 7

(a) Avrami analysis of SH signals upon relaxing from the signal level at time 0 to the initial level [saturated level in Fig. 6]. This tracks the regeneration and growth processes of the AzD6 SWS. The linear fitting (lines) gives Avrami coefficient $n\sim 1.1$ or 0.5. (b) Avrami coefficients as a function of UV intensity.