Reversible Polymorphism, Liquid Crystallinity, and Stimuli-Responsive Luminescence in a Bola-amphiphilic π‑System: Structure–Property Correlations Through Nanoindentation and DFT Calculations

We report the design, synthesis, detailed characterization, and analysis of a new multifunctional π-conjugated bola-amphiphilic chromophore: oligo-(p-phenyleneethynylene)­dicarboxylic acid with dialkoxyoctadecyl side chains (OPE-C 18 -1). OPE-C 18 -1 shows two polymorphs at 123 K (OPE-C 18 -1′) and 373 K (OPE-C 18 -1″), whose crystal structures were characterized via single crystal X-ray diffraction. OPE-C 18 -1 also exhibits thermotropic liquid crystalline property revealing a columnar phase. The inherent π-conjugation of OPE-C 18 -1 imparts luminescence to the system. Photoluminescence measurements on the mesophase also reveal similar luminescence as in the crystalline state. Additionally, OPE-C 18 -1 shows mechano-hypsochromic luminescence behavior. Density functional theory (DFT)-based calculations unravel the origins behind the simultaneous existence of all these properties. Nanoindentation experiments on the single crystal reveal its mechanical strength and accurately correlate the molecular arrangement with the liquid crystalline and mechanochromic luminescence behavior.