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 (1-F) were studied in comparison to its non-fluorinated analogue (1-H). Depending on the amount of organic cosolvent, 1-F undergoes cooperative or isodesmic aggregation. The switching between two polymerization mechanisms results from a change in polymer structure, as observed by cryogenic electron microscopy. 1-F showed exceptionally strong noncovalent binding, with the largest directly measured association constant of 1.7 × 10(9) M(-1) in 75:25 water/THF mixture (v/v). In pure water, the association constant of 1-F is estimated to be at least in the order of 10(15) M(-1) (based on extrapolation), 3 orders of magnitude larger than that of 1-H. The difference in aggregation strength between 1-F and 1-H 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 1-F appear to result from specific fluorocarbon conformational rigidity.