Oligo(p-phenylene-ethynylene)-derived super-π-gelators with tunable emission and self-assembled polymorphic structures

Linear π-conjugated oligomers are known to form organogels through noncovalent interactions. Herein, we report the effect of π-repeat units on the gelation and morphological properties of three different oligo(p-phenylene-ethynylene)s: OPE3, OPE5, and OPE7. All of these molecules form fluorescent gels in nonpolar solvents at low critical gel concentrations, thereby resulting in a blue gel for OPE3, a green gel for OPE5, and a greenish yellow gel for OPE7. The molecule-molecule and molecule-substrate interactions in these OPEs are strongly influenced by the conjugation length of the molecules. Silicon wafer suppresses substrate-molecule interactions whereas a mica surface facilitates such interactions. At lower concentrations, OPE3 formed vesicular assemblies and OPE5 gave entangled fibers, whereas OPE7 resulted in spiral assemblies on a mica surface. At higher concentrations, OPE3 and OPE5 resulted in super-bundles of fibers and flowerlike short-fiber agglomerates when different conditions were applied. The number of polymorphic structures increases on increasing the conjugation length, as seen in the case of OPE7 with n=5, which resulted in a variety of exotic structures, the formation of which could be controlled by varying the substrate, concentration, and humidity.