Solvent Geometry Regulated Cooperative Supramolecular Polymerization

Abstract

This article discloses hydrogen-bonding driven supramolecular polymerization of a core-substituted naphthalene-diimide derivative NDI-1, which exhibits J-aggregation in all tested hydrocarbon solvents; however, spontaneous gelation was noticed only in linear alkanes in contrast to free flowing solution in their cyclic analogs. Mechanistic investigation by variable-temperature UV/Vis studies and analyzing the data by using an appropriate model(s) elucidates a highly cooperative self-assembly pathway in linear hydrocarbons (heptane, octane, nonane, or decane) in sharp contrast to cyclohexane or methylcyclohexane in which an ill-defined polymerization was noticed. This is attributed to the direct participation of linear alkanes in the nucleation process by favorable mixing with the peripheral alkyl chains of the NDI monomer, which appears not to be the case for cyclic alkanes owing to geometry mismatch. Although all tested linear hydrocarbons induced nucleation–elongation growth, the thermodynamic parameters were found to depend on the chain length of the alkane. The morphology of the self-assembled polymer was strongly dependent on the growth mechanism as we noticed fibrillar networks and short-length rods, respectively, in decane and methylcyclohexane (MCH). However, in the presence of a small amount of additive (preformed fiber in decane; added in situ or post self-assembly) a fibrillar structure was noticed also in MCH, corroborating the self-assembly in solution, which adopted a cooperative mechanism similar to linear alkanes.