Tunable Physical States and Optical Properties of Bola-Amphiphilic Oligo-(p-phenyleneethynylene)-Based Supramolecular Networks Assisted by Functional Group Modulation

Structural understanding and correlation of physical properties of supramolecular networks can lead to improved design strategies for advanced optoelectronic materials. Herein, we demonstrate via single-crystal X-ray diffraction (SCXRD) analysis how end groups (ester and acid) of bola-amphiphilic oligo-(p-phenyleneethynylenes) (OPEs) lead to different supramolecular network formations and also generate selective polymorphism and mechanochromic luminescence in OPE-C-gly-A and OPE-C-gly-E, respectively. OPE-C-gly-E and OPE-C-gly-A have the same pi-conjugated backbones with bistriethyleneglycol side chains, only differing in possessing end-capped ester and acid groups, respectively. OPE-C-gly-E has a less densely packed structure than OPE-C-gly-A. This leads to exclusive mechanochromic behavior in OPE-C-gly-E, whereas OPE-C-gly-A generates reversible polymorphic structures which are not present in OPE-C-gly-E. Density functional theoretical calculations reveal that breakage of weak supramolecular interactions in OPE-C-gly-E leads to the hypsochromic mechanochromic luminescence. Interestingly, both OPE-C-gly-E and OPE-C-gly-A crystals reversibly melt into an isotropic phase at 90 and 180 degrees C, respectively, and recrystallize upon cooling. This difference in melting temperature is due to the variance in molecular packing in both compounds. Thus, this work gives a structural perspective toward the control of physical states and optical properties of OPEs for a better design of bola-amphiphilic supramolecular networks for advanced opto-electronics.