Understanding the Effect of Fluorocarbons in Aqueous Supramolecular Polymerization: Ultrastrong Noncovalent Binding and Cooperativity

Abstract

Achieving supramolecular polymerization based on strong yet reversible bonds represents a significant challenge. A solution may be offered by perfluoroalkyl groups, which have remarkable hydrophobicity. We tested the idea that a perfluorooctyl chain attached to a perylene diimide amphiphile can dramatically enhance the strength of supramolecular bonding in aqueous environments. Supramolecular structures and polymerization thermodynamics of this fluorinated compound (<b>1-F</b>) were studied in comparison to its non-fluorinated analogue (<b>1-H</b>). Depending on the amount of organic cosolvent, <b>1-F</b> undergoes cooperative or isodesmic aggregation. The switching between two polymerization mechanisms results from a change in polymer structure, as observed by cryogenic electron microscopy. <b>1-F</b> showed exceptionally strong noncovalent binding, with the largest directly measured association constant of 1.7 × 10⁹ M^–1 in 75:25 water/THF mixture (v/v). In pure water, the association constant of <b>1-F</b> is estimated to be at least in the order of 10¹⁵ M^–1 (based on extrapolation), 3 orders of magnitude larger than that of <b>1-H</b>. The difference in aggregation strength between <b>1-F</b> and <b>1-H</b> can be explained solely on the basis of the larger surface area of the fluorocarbon group, rather than a unique nature of fluorocarbon hydrophobicity. However, differences in aggregation mechanism and cooperativity exhibited by <b>1-F</b> appear to result from specific fluorocarbon conformational rigidity