Negative fluorescence anisotropy of phosphole oxide-based dyes in nematic liquid crystals

Takuya Ohzono,Shigehiro Yamaguchi,Aiko Fukazawa,Tetsuo Yatabe,Chenguang Wang

doi:10.1038/s42004-018-0055-6

Abstract

It is important to understand how solute molecules align in anisotropic media such as liquid crystals and biological membranes. Alignment of fluorescent probes used in polarised fluorescence microscopy is of interest because anisotropy of the medium is indirectly estimated through the fluorescence intensity originating from the transition dipole moments fixed on the molecule. Here we report fluorescence anisotropies of a series of phosphole oxide-based dyes with an elongated π-conjugated system, along which the transition dipole moment lies, in nematic liquid crystals. We observe that as the number, size and rigidity of substituents protruding from the π-conjugated plane increase, the transition dipole moment tends to tilt away from the nematic director over the magic angle, ~54.7°, showing conspicuous negative fluorescence anisotropy. Analysis of molecular shapes suggests that the substitutions modulate the interaction with the nematic mean field, impacting the direction of the principal molecular axis aligned with the nematic director.

Highlights

It is important to understand how solute molecules align in anisotropic media such as liquid crystals and biological membranes
The negative anisotropy of a phosphole oxide-based dye, C-Naphox[39], in nematic liquid crystals (NLCs) was reported[40]. Since this dye has a high photobleaching resistance, the detailed fluorescence POM (FPOM) study of NLC defects has become possible, eg, uncovering their three-dimensional structures being based on alignment information complementary to that given by the dyes with positive anisotropy
The basal unit vectors of the molecular coordinate on the laboratory one are expressed as x′ 1⁄4 ðcos γ; À cos χ sin γ; sin χ sin γÞ, y′ 1⁄4 ðsin γ cos φ; cos χ cos γ cos φ À sin χ sin φ; À sin χ cos γ cos φ À cos χ sin φÞ and z′ 1⁄4 ðsin γ sin φ; cos χ cos γ sin φ À sin χ cos φ; À sin χ cos γ sin φ À cos χ cos φÞ; ie, e′x;y;z 1⁄4 RxðÀχÞRzðÀγÞRxðÀφÞex;y;z, where Ri(q) is the rotation matrix with respect to the i-axis by angle q

Summary

Introduction

It is important to understand how solute molecules align in anisotropic media such as liquid crystals and biological membranes. The negative anisotropy of a phosphole oxide-based dye, C-Naphox[39], in NLC was reported[40] Since this dye has a high photobleaching resistance, the detailed FPOM study of NLC defects has become possible, eg, uncovering their three-dimensional structures being based on alignment information complementary to that given by the dyes with positive anisotropy. This example demonstrates a new use of fluorescence dyes with negative anisotropy besides guest–host mode displays. There is no principle for the molecular design of such dyes at present because the origin of the negative anisotropy of the phosphole oxide-based dye remains unclear

Methods

Results

Conclusion