Chiral assemblies of achiral rigid-flexible molecules at the air/water interface induced by silver(I) coordination

Chiroptical Properties in Thin Films of π-Conjugated Systems

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (g_abs and g_lum) and on the application of ECD and CPL to derive structural information on aggregated states.

Towards homochiral supramolecular entities from achiral molecules by vortex mixing-accompanied self-assembly

Achieving homochirality is challenging both at the molecular and the supramolecular levels. While the origin of molecular homochirality still remains mysterious, the fabrication of homochiral assemblies from achiral molecules has attracted considerable interest since it provides many clues to understand the origin of molecular chirality. Here, by using a vortex mixing-accompanied self-assembly strategy, we obtained near-unity homochiral entities with controlled handedness from supramolecular gels that consist of exclusively achiral molecules without any chiral additives. The common supramolecular gelation process via heating and cooling of the achiral molecules only resulted in racemic gels. However, if vortex mixing is applied during the self-assembly, near-unity homochiral assemblies with uncontrolled handedness were obtained. Vortex mixing during the nucleation stage was found to be crucial in this case. On the other hand, if a small amount of the above vortex mixing produced assemblies was added as chiral seeds into the racemic gels, the racemic gels turned into near-unity homochiral suspensions with controlled handedness via a ripening process. Our studies provide an intriguing approach for achieving homochiral supramolecular assemblies from achiral molecules.

Fabrication of Supramolecular Chirality from Achiral Molecules at the Liquid/Liquid Interface Studied by Second Harmonic Generation

We present the investigation into the supramolecular chirality of 5-octadecyloxy-2-(2-pyridylazo)phenol (PARC18) at water/1,2-dichloroethane interface by second harmonic generation (SHG). We observe that PARC18 molecules form supramolecular chirality through self-assembly at the liquid/liquid interface although they are achiral molecules. The bulk concentration of PARC18 in the organic phase has profound effects on the supramolecular chirality. By increasing bulk concentration, the enantiomeric excess at the interface first grows and then decreases until it eventually vanishes. Further analysis reveals that the enantiomeric excess is determined by the twist angle of PARC18 molecules at the interface rather than their orientational angle. At lower and higher bulk concentrations, the average twist angle of PARC18 molecules approaches zero, and the assemblies are achiral; whereas at medium bulk concentrations, the average twist angle is nonzero, so that the assemblies show supramolecular chirality. We also estimate the coverage of PARC18 molecules at the interface versus the bulk concentration and fit it to Langmuir adsorption model. The result indicates that PARC18 assemblies show strongest supramolecular chirality in a half-full monolayer. These findings highlight the opportunities for precise control of supramolecular chirality at liquid/liquid interfaces by manipulating the bulk concentration.

Biomolecules at interfaces: chiral, naturally

Interfaces are a most important environment in natural and synthetic chemistries for a wide variety of processes, such as catalysis, recognition, separation, and so on. Naturally occurring systems have evolved to one handedness and the study of interfaces where biomolecules are located is a potentially revealing pursuit with regard to understanding the reasons and importance of stereochemistry in these environments. Equally, the spontaneous resolution of achiral and chiral compounds at interfaces could lead to explanations regarding the emergence of single handedness in proteins and sugars. Also, the attachment of biomolecules to surfaces leads to systems capable of stereoselective processes which may be useful for the applications mentioned above. The review covers systems ranging from small biomolecules studied under ultrapure conditions in vacuum to protein adsorption to surfaces in solution, and the techniques that can be used to study them.