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

Abstract

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

Solid‐State Light Emission Controlled by Tuning the Hierarchical Superstructure of Self‐Assembled Luminogens

“…Figure c shows a dramatic reduction in peak intensity with increased peak width for the ground sample. This means that the intermolecular interaction of AT 3 EMIS in the ground state is weaker than initial state . However, 1D WAXD pattern of the ground sample still shows a reflection peak at the low‐angle region of 2θ = 2.26°, suggesting the formation of a shear‐induced columnar phase (Col S ).…”

Density functional theory predictions of the mechanical properties of crystalline materials

“…DFT is widely used to study crystal polymorphism, most commonly through the calculation of the ground state energy difference between polymorphic crystal structures, 116 or a simpler analysis of intermolecular interactions (Fig. 5).…”

“…118 For molecular crystals the highest level of theory needed for calculating accurate relative stabilities is still being debated. [118][119][120][121] As with any crystal, DFT-calculated physical properties, be they mechanical, 122 dielectric, 116 or physiochemical 123 can also be used to distinguish and rationalize polymorphic behaviour. Since both polymorphism and distinct mechanical properties arise from differences in crystal packing of molecules (in the case of packing polymorphism), polymorphs can be identified by their unique mechanical properties.…”

Self-assembled organic and hybrid materials derived from oligo-(p-phenyleneethynylenes)

“…64 Investigations into the structure-property correlation of OPE systems have also been carried out by Maji and co-workers. 65 The first molecule probed by the group was OPE-3. Long needle-shaped crystals of the molecule up to 2 mm in length were grown.…”