Tunable Physical States and Optical Properties of Bola-Amphiphilic Oligo‑(<i>p</i>‑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-Cgly-A and OPE-Cgly-E, respectively. OPE-Cgly-E and OPE-Cgly-A have the same
π-conjugated backbones with bistriethyleneglycol side chains,
only differing in possessing end-capped ester and acid groups, respectively. OPE-Cgly-E has a less
densely packed structure than OPE-Cgly-A. This leads to exclusive mechanochromic behavior
in OPE-Cgly-E, whereas OPE-Cgly-A generates
reversible polymorphic structures which are not present in OPE-Cgly-E. Density functional theoretical
calculations reveal that breakage of weak supramolecular interactions
in OPE-Cgly-E leads
to the hypsochromic mechanochromic luminescence. Interestingly, both OPE-Cgly-E and OPE-Cgly-A crystals reversibly
melt into an isotropic phase at 90 and 180 °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.