TY  - JOUR
A1  - Syamala, Pradeep P. N.
A1  - Würthner, Frank
T1  - Modulation of the Self‐Assembly of π‐Amphiphiles in Water from Enthalpy‐ to Entropy‐Driven by Enwrapping Substituents
JF  - Chemistry – A European Journal
N2  - 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.
KW  - amphiphilic dyes
KW  - self-assembly
KW  - thermodynamics
KW  - OEG chains
KW  - π-conjugated systems
Y1  - 2020
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-218107
VL  - 26
IS  - 38
SP  - 8426
EP  - 8434
ER  - 
TY  - JOUR
A1  - Quast, Helmut
A1  - Gescheidt, Georg
A1  - Spichty, Martin
T1  - Topological dynamics of a radical ion pair: Experimental and computational assessment at the relevant nanosecond timescale
JF  - Chemistry
N2  - Chemical processes mostly happen in fluid environments where reaction partners encounter via diffusion. The bimolecular encounters take place at a nanosecond time scale. The chemical environment (e.g., solvent molecules, (counter)ions) has a decisive influence on the reactivity as it determines the contact time between two molecules and affects the energetics. For understanding reactivity at an atomic level and at the appropriate dynamic time scale, it is crucial to combine matching experimental and theoretical data. Here, we have utilized all-atom molecular-dynamics simulations for accessing the key time scale (nanoseconds) using a QM/MM-Hamiltonian. Ion pairs consisting of a radical ion and its counterion are ideal systems to assess the theoretical predictions because they reflect dynamics at an appropriate time scale when studied by temperature-dependent EPR spectroscopy. We have investigated a diketone radical anion with its tetra-ethylammonium counterion. We have established a funnel-like transition path connecting two (equivalent) complexation sites. The agreement between the molecular-dynamics simulation and the experimental data presents a new paradigm for ion–ion interactions. This study exemplarily demonstrates the impact of the molecular environment on the topological states of reaction intermediates and how these states can be consistently elucidated through the combination of theory and experiment. We anticipate that our findings will contribute to the prediction of bimolecular transformations in the condensed phase with relevance to chemical synthesis, polymers, and biological activity.
KW  - ion pairing
KW  - radical anion
KW  - kinetics
KW  - thermodynamics
KW  - molecular dynamics
KW  - QM/MM
KW  - EPR
Y1  - 2020
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-285195
SN  - 2624-8549
VL  - 2
IS  - 2
SP  - 219
EP  - 230
ER  -