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-C18-1). OPE-C18-1 shows two polymorphs at 123 K (OPE-C18-1′) and 373 K (OPE-C18-1″), whose
crystal structures were characterized via single crystal X-ray diffraction. OPE-C18-1 also exhibits
thermotropic liquid crystalline property revealing a columnar phase.
The inherent π-conjugation of OPE-C18-1 imparts luminescence to the system. Photoluminescence
measurements on the mesophase also reveal similar luminescence as
in the crystalline state. Additionally, OPE-C18-1 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.