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.


Determination of thermodynamic parameters Isodesmic model
One dimensional aggregation is assumed to proceed via same binding constant and free energy of aggregation in isodesmic model for each monomer added to the growing chain. [4] The equilibrium between monomer and aggregate for such a system is described as The molar fraction of aggregated species (α agg ) for such a system is expressed as where K is the aggregation constant and T is the total concentration of molecules. agg can be expressed in terms of apparent extinction coefficient at a given dye concentration ( ̅ ( T )) as agg = 1 − ̅( T )− agg mon − agg (2) where mon and agg are the extinction coefficients of the aggregated and monomeric species, respectively. Combining eqn. (1) and (2), the apparent extinction coefficient at a given dye concentration, ̅ ( T ), is denoted as ̅ ( T ) = agg + ( mon − agg ) 2 T +1−√4 T +1 By fitting plot of ̅ ( T ) at a specific wavelength with respect to concentration, T , using eqn (3) corresponding binding constant at a particular temperature can be determined. The Gibbs free energy of association, ΔG ass , is computed using the following equation: S4

Goldstein-Stryer model
The anti-cooperativity process is described using the Goldstein-Stryer model, which combines the formation of a nucleus with size 's' and further aggregation into larger structures. [5] The cooperativity factor σ is defined as the ratio of binding constants for nucleus formation (K s ) and further elongation For σ = 1, the process is considered as isodesmic while for σ < 1 cooperative and σ > 1 anti-cooperative.
An anti-cooperative process can be evaluated by the Goldstein-Stryer model using the following equation. [6] e T = ∑ −1 ( e 1 ) + ∑ −1 ( e 1 ) where K e is the elongation constant, σ is the cooperativity factor, c 1 is the monomeric concentration and T the total concentration of dye molecules.
The Gibbs free energy of association, ΔG ass , is computed using the following equation: where R is the universal gas constant, T is the temperature and e the elongation constant.

3,4,5-Tris((2,5,8,11-tetraoxatridecan-13-yl)oxy)aniline (3)
Compound 2 (250 mg, 0.338 mmol) was added to an aqueous KOH solution (2M, 12 mL) in a roundbottom flask and the mixture was stirred for 15 minutes at 0 °C. Bromine in water solution (35 µL in 5 mL H 2 O, 0.676 mmol) was added slowly to the suspension using a dropping funnel. After the addition was completed, the reaction mixture was stirred for 12 h at 90 °C and then cooled down to room temperature. The product was extracted with dichloromethane (3x50 mL) and washed with water and brine. The organic phase was dried with anhydrous sodium sulphate and the solvent was evaporated under vacuum. The residue was finally purified by silica gel column chromatography (methanol 4%/chloroform) to obtain the compound 3 as a yellow viscous oil (57 mg, 0.080 mmol, 24%