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

Wongi Park,Hyungju Ahn,Kyung Ik Sim,Taewoo Ha,Ewa Gorecka,Taek Sun Jung,Dong Ki Yoon,Damian Pociecha,Teun-Teun Kim,Tae Joo Shin,Jae Hoon Kim,Anna Zep

doi:10.1038/s41427-019-0146-6

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.

Highlights

It is known that certain bent-core liquid-crystalline (LC)molecules spontaneously form helical nanofilaments (HNFs; B4 phase) or, more precisely, bundles of twisted crystal layers, usually with a diameter (w) of ~40 nm and a helical pitch (p) 200–300 nm depending on the molecular length (Fig. 1a, b)[1,2,3,4,5]
We describe an efficient method for aligning HNFs by engineering the molecular structure, i.e., by incorporating photochromic azobenzene units
We show that using the Weigert effect, aligned arrays of helical nanostructures can be obtained, which is the basis for patternable and reversible structural color reflectors

Summary

Introduction

It is known that certain bent-core liquid-crystalline (LC)molecules spontaneously form helical nanofilaments (HNFs; B4 phase) or, more precisely, bundles of twisted crystal layers, usually with a diameter (w) of ~40 nm and a helical pitch (p) 200–300 nm depending on the molecular length (Fig. 1a, b)[1,2,3,4,5]. The aromatic cores and aliphatic tails in bent-shaped molecules show a typical bookshelf-like arrangement in the smectic layers, but in-layer mismatch happens upon cooling from the smectic phase to the HNF phase. During this transition, the twisted structures of the HNFs are driven by local saddle-splay deformation of layers, and both right- and left-handed helices are formed because of the absence of molecular chirality. Recently has it been shown that filaments of the B4 phase can be perfectly aligned in a confined environment, e.g., in the cylindrical nanopores of an aluminum oxide matrix[6,7] This method is efficient and simple, its application in optical devices is rather limited owing to the presence of an inorganic scaffold. Azobenzene molecules in their trans-isomer state reveal different UV light absorption efficiencies and have different effective orientation potentials (U(θ)) depending on the angle (θ) between the transition dipole of the azo group and the illuminating linearly polarized light, which can be expressed as

Methods

Conclusion