Proton induced aggregation of water soluble isophthalic acid appended arylene diimides: justification with perylene derivative

We report the self-assembly behaviour of five water soluble arylene diimides based on benzene, naphthalene and perylene moieties, by utilizing the essentiality of two major reversible supramolecular interactions, hydrophobic π-stacking and hydrophilic hydrogen bonding. With modification of their π-core, improved molecules have been designed as well as ensuring their water solubility under mild basic conditions. Experimentally, chaotic and randomly oriented free molecules are induced to assemble at just below pH 7 upon the imposition of suitable external acidic stimuli. Photophysically, with change in the nature of the interacting medium from pH-9 to pH-4, molecules that have benzene or naphthalene core, have revealed the predominant edge on J-type aggregation whereas unsubstituted perylene is showing a dominant as well as rotationally displaced H-type interaction because of its higher π-surface. Self assembly at a higher concentration is manifested in the formation of weak hydrogels. However, a large twist angle (36°) in the bay tetra-chlorinated perylene core hardly shows intentional assembly. Monitoring the changes of the morphologies with respect to major moieties concerned, perylene derivatives are responsible for forming short, super entangled network instead of long fibers and it is a key reason of the decrease in the rheological strength of self assemblies. Correlation of rheological and morphological changes with the mode of assembly in our study successfully points out the fact that in our system, co-facial H-type assembly for unsubstituted perylene diimide tends to disfavor long network formation, as well as the rheological strength of the self-aggregation compared to other small π-aromatic systems like benzene or naphthalene.