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.
Highlights
Self-assembly has emerged as an efficient method for the development of functional materials possessing enticing properties, including self-healing and stimuli-responsiveness among many others.[1]
We have previously reported that self-assembly of perylene bisimide dyes, appended with oligoethylene glycol chains, is driven by entropic factors in water.[18a]. While mechanistic studies on the self-assembly process were hampered for this derivative due to its strong aggregation tendency, recently we succeeded in obtaining an understanding of the entropically driven self-assembly for amphiphilic dyes in water utilizing smaller naphthalene bisimide homologues.[18e]. During these investigations, we came across a surprising observation that the glycol chains in these molecules are back-folded to the aromatic core to sequester the hydrophobic surfaces from the surrounding bulk water
The thermodynamic fingerprint of bolaamphiphilic PBIs and NBIs were first probed by temperature-dependent UV/Vis measurements
Summary
Self-assembly has emerged as an efficient method for the development of functional materials possessing enticing properties, including self-healing and stimuli-responsiveness among many others.[1]. We have previously reported that self-assembly of perylene bisimide dyes, appended with oligoethylene glycol chains, is driven by entropic factors in water.[18a] While mechanistic studies on the self-assembly process were hampered for this derivative due to its strong aggregation tendency, recently we succeeded in obtaining an understanding of the entropically driven self-assembly for amphiphilic dyes in water utilizing smaller naphthalene bisimide homologues.[18e] During these investigations, we came across a surprising observation that the glycol chains in these molecules are back-folded to the aromatic core to sequester the hydrophobic surfaces from the surrounding bulk water. Structural insights obtained by 2D NMR and PM7 optimizations point towards the crucial role of the orientation of glycol chains in orchestrating such bias of selfassembly in water
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