Energetics of Competing Chiral Supramolecular Polymers

Abstract

Chirality can have unexpected consequences including on properties other than spectroscopic. We show herein that a racemic mixture of bis‐urea stereoisomers forms thermodynamically stable supramolecular polymers that result in a more viscous solution than for the pure stereoisomer. The origin of this macroscopic property was probed by characterizing the structure and stability of the assemblies. Both racemic and non‐racemic bis‐urea stereoisomers form two competing helical supramolecular polymers in solution: a double and a single helical structure at low and high temperature, respectively. The transition temperature between these assemblies, as probed by spectroscopic and calorimetric analyses, is strongly influenced by the composition (by up to 70 °C). A simple model that accounts for the thermodynamics of this system, indicates that the stereochemical defects (chiral mismatches and helix reversals) affect much more the stability of single helices. Therefore, the heterochiral double helical structure predominates over the single helical structure (whilst the opposite holds for the homochiral structures), which explains the aforementioned higher viscosity of the racemic bis‐urea solution. This rationale constitutes a new basis to tune the macroscopic properties of the increasing number of supramolecular polymers reported to exhibit competing chiral nanostructures.

Rationalizing the Extent of the “Sergeants‐and‐Soldiers” Effect in Supramolecular Helical Catalysts: Effect of Copper Coordination

“…[47,48] The extent of the S&S effect is correlated to the number of defects, as quantified by the energetic values associated with helix reversals and chiral mismatches. [49,50] Aggregation of hydrogen-bonded helical structures by complementary interactions has been exploited to construct hierarchical [39,40,[51][52][53] or competitive [54] supramolecular aggregates with tunable chiroptical properties. The orthogonality of hydrogen bonding and metal-ligand interactions also allowed the construction of crosslinked supramolecular polymer networks, yet the impact of chirality was not probed in this system.…”

Chiral supramolecular polymers

“…19(d) and (e)). 91 In this work, they found the existence of two competing helical assemblies, a double helical and a single helical one depending on the temperature. The discrimination between both types of assemblies was assessed by CD spectroscopy, small angle neutron scattering (SANS) and nanodifferential scanning calorimetry (nDSC).…”

Thermo-/Mechano-Chromic Chiral Coordination Dimer: Formation of Switchable and Metastable Discrete Structure through Chiral Self-Sorting

“…Chiral nanoarchitectures have attracted substantial interest due to a broad range of possible applications, − while there has been growing adoption of chiral self-sorting for the design of switchable and metastable supramolecular structures. − For example, Shang et al reported that a mixture of Zn 2+ cations with a chiral naphthalene diimide ligand containing alanine termini forms homochiral and heterochiral coordination polymers, enabling enantioselective sensing of hazardous amines and naproxen (a nonsteroidal anti-inflammatory drug) based on fluorescence and chemoresistive variations . Hecht et al reported the self-assembly of a core-twisted perylene bisimide into a homochiral columnar mesophase when a solution of the monomer is slowly cooled, while the same molecule forms heterochiral lamellar liquid crystals when rapidly cooled based on kinetically controlled chiral self-sorting.…”