Modulation of the Self‐Assembly of π‐Amphiphiles in Water from Enthalpy‐ to Entropy‐Driven by Enwrapping Substituents

Abstract

Depending on the connectivity of solubilizing oligoethylene glycol (OEG) side chains to the π‐cores of amphiphilic naphthalene and perylene bisimide dyes, self‐assembly in water occurs either upon heating or cooling. Herein, we show that this effect originates from differences in the enwrapping capability of the π‐cores by the OEG chains. Rylene bisimides bearing phenyl substituents with three OEG chains attached directly to the hydrophobic π‐cores are strongly sequestered by the OEG chains. These molecules self‐assemble at elevated temperatures in an entropy‐driven process according to temperature‐ and concentration‐dependent UV/Vis spectroscopy and calorimetric dilution studies. In contrast, for rylene bisimides in which phenyl substituents with three OEG chains are attached via a methylene spacer, leading to much weaker sequestration, self‐assembly originates upon cooling in an enthalpy‐driven process. Our explanation for this controversial behavior is that the aggregation in the latter case is dictated by the release of “high energy water” from the hydrophobic π‐surfaces as well as dispersion interactions between the π‐scaffolds which drive the self‐assembly in an enthalpically driven process. In contrast, for the former case we suggest that in addition to the conventional explanation of a dehydration of hydrogen‐bonded water molecules from OEG units it is in particular the increase in conformational entropy of back‐folded OEG side chains upon aggregation that provides the pronounced gain in entropy that drives the aggregation process. Thus, our studies revealed that a subtle change in the attachment of solubilizing substituents can switch the thermodynamic signature for the self‐assembly of amphiphilic dyes in water from enthalpy‐ to entropy‐driven.

Solute–Solvent Interactions in Modern Physical Organic Chemistry: Supramolecular Polymers as a Muse

“…Besides the strong entropy gains of the release of solvated water upon aggregation, the entropic penalty of a decrease in flexibility of the side chains may lead to counterintuitive, strong differences in subtly different monomers. 205 Besides stability, hydrogen bonded water can also influence the dynamic behavior of supramolecular polymers. In sugar-decorated, water-soluble BTA derivatives, hydrogen bonding of water was found to increase the dynamic behavior of the supramolecular polymers.…”

Globular Aggregates Stemming from the Self-Assembly of an Amphiphilic N-Annulated Perylene Bisimide in Aqueous Media

“…Interestingly, heating the diluted aqueous solution of 1 to 95 • C does not result in turbid dispersion due to the release of water molecules interacting by the formation of H-bonds with the polar ethylene glycol side chains. The preliminary results extracted from the VT-UV-Vis experiments, displaying a slight increase in the absorption intensity, suggest the operation of an enthalpically driven self-assembly unlike some reports on the selfassembly of referable amphiphilic PBI-based derivatives [24][25][26]. Some additional studies are ongoing to accurately unravel the thermodynamics of the self-assembly mechanism of compound 1.…”

“…Among all these π-conjugated moieties utilized as scaffolds for the synthesis of non-ionic amphiphiles, perylene bisimides (PBIs) are playing a remarkable role due to their optical and electronic characteristics as well as their trend of self-assembling in an organized manner [23]. Thus, several studies have demonstrated that the self-assembly of amphiphilic PBIs endowed with OEG side chains can be either entropically or enthalpically controlled, depending on the distance from the polar side chains to the central aromatic core [24][25][26].…”

Temperature-dependent modulation by biaryl-based monomers of the chain length and morphology of biphenyl-based supramolecular polymers

“…As B PhePyTA transitions to A PhePyTA , an increase in the polymer length occurs, as has been observed before in several systems containing hydrogen-bond competitors, such as water and alcohols. [59][60][61][62][63][64] In contrast to the previously reported systems, the solvation of the aggregates by water does not lead to strong changes in the aggregates at the molecular level, since the shape of the IR, UV and CD spectra remain the same, but only change in intensity. Upon further heating from 60 to 80 °C, the A PhePyTA aggregates are destabilized and the scattering and CD intensities decrease as the result of the depolymerization of A PhePyTA .…”