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

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.