Specific Supramolecular Interaction Regulated Entropically Favorable Assembly of Amphiphilic Macromolecules

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 Delta S 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.