Directed self-assembly of a helical nanofilament liquid crystal phase for use as structural color reflectors

Abstract

The fabrication of molecular structures with a desired morphology, e.g., nanotubes, nanoribbons, nanosprings, and sponges, is essential for the advancement of nanotechnology. Unfortunately, realization of this objective is expensive and complicated. Here, we report that irradiating a film comprising azobenzene derivatives with UV light produces oriented arrays of helical nanofilaments via the photoisomerization-induced Weigert effect. As a result, structural colors are observed due to the extrinsic chiral reflection in the visible wavelength range, and the reflected color can be tuned by adjusting the molecular length of the azobenzene derivative. This simple fabrication method can be used for fabricating large, reversible, and patternable color reflectors, providing a new platform for interference-based structural coloration as it exists in nature, such as morpho butterflies, green-winged teal, and various beetles.

Liquid crystals: putting a new spin on nature-inspired colors

A thin film that produces colors more vibrant and durable than typical pigments has been created with the help of a light-sensitive molecular switch. The rich hues seen in butterfly wings do not arise from light-absorbing pigments. Instead, they are due to complex nanostructures reflecting light, generating fade-free ‘structural’ colors. Dong Ki Yoon from KAIST in Daejeon, South Korea, and co-workers now report that helical nanofilament liquid crystals containing azobenzene, a molecule that changes shape when exposed to strong light, can self-assemble into structural color reflectors. The team used ultraviolet radiation to orient the helical structures into ordered arrays of nanofilaments on a solid surface. Structural colors emitted by these nanofilament films could be tuned by adjusting the length of the azobenzene unit or erased by applying heat.