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