From S,N-Heteroacene to Large Discotic Polycyclic Aromatic Hydrocarbons (PAHs): Liquid Crystal versus Plastic Crystalline Materials with Tunable Mechanochromic Fluorescence

Abstract

A novel type of discotic polycyclic aromatic hydrocarbon (PAH) based on an enlarged dibenzo[a,c]phenazine core has been developed. The large conjugated mesogenic core with increased dipole moment derived from S,N heteroatoms facilitates the formation of highly ordered columnar superstructures both in solution and bulk. Columnar mesophases, including liquid crystal (LC) and plastic crystal (PC) assemblies could form unprecedentedly based on the same PAH core. The cores are delicately modulated by the peripherical alkoxy chains. Both mesogens have mechanochromic fluorescent (MCF) character, which is also structure dependent and correlated with the different mesophase formation. For the first time, MCF properties can be realized in such a large conjugated mesogenic system.

Keywords: Bischler-Napieralski cyclization; liquid crystal; mechanochromic fluorescence; plastic crystal; polycyclic aromatic hydrocarbons (PAHs).