Specific Supramolecular Interaction Regulated Entropically Favorable Assembly of Amphiphilic Macromolecules

Abstract

This article reports molecular interaction driven aqueous assembly of supramolecularly engineered amphiphilic macromolecules to cylindrical structure. Each polymer contains a single hydrophobic trialkoxybenzamide-linked naphthalene diimide (NDI) chromophore at the chain terminal as the supramolecular structure directing unit (SSDU). Irrespective of the structure of the appended hydrophilic polymer, H-bonding promoted J-aggregation among the NDI chromophore leads to the formation of thermally stable spherical micelle (critical aggregation concentration: 0.01–0.03 mM) which reorganizes to cylindrical micelle after a few hours. The reorganization time can be regulated by pH in the case of the anionic polymer as it affects the dynamics. Isothermal titration calorimetric (ITC) studies reveal positive ΔS values for assembly of all the polymers, reflecting the self-assembly process is favored by the entropy factor similar to the elegant examples in the biological domain. In contrast, a small molecule analogue of these polymers, having a short hydrophilic wedge (instead of a water-soluble polymer), shows a reverse trend, typically expected in a process of supramolecular organization. This can be attributed to the tightly packed J-aggregation of the NDI chromophore of the SSDU that compels a close packing of the hydrophilic polymer chains in the corona, leading to the release of the surrounding water molecules and causing entropy enhancement.