Spectroscopic Detection of Chiral Aggregation at Liquid-Liquid Interfaces

Introduction Optical activity is a fundamental property, which pro-vides valuable information on the structure of mole-cules and the conformation of their assemblies. There-fore, it is widely used in many fields including organic chemistry, coordination c…

Spectroscopic Detection of Chiral Aggregation at Liquid-Liquid Interfaces

Two new spectroscopic methods to detect the optical activity of liquid-liquid interfaces have been developed. The first one is the centrifugal liquid membrane (CLM) method combined with a conventional cir-cular dichroism (CD) spectropolarimetry and the second one is a more interfacial specific second harmonic generation CD (SHG-CD) spectrometry. In the CLM-CD method, a cylindrical glass cell containing small amounts of organic and aqueous phases was rotated at about 7000 r/min in a sample chamber of a CD spectropolarimeter to generate an interface with a high specific interfacial area between the two-phase liquid membranes. The CD spectra of the J-aggregate of protonated 5,10,15, 20-tetraphenylporphyrin formed at the toluene-sulfuric acid interface have been measured. As for the SHG-CD, a circularly polarized wave-length-variable fs-laser system was constructed to measure the interfacial SHG spectra of a flat liquid-liquid interface. The ion-associated aggregation of a water-soluble anionic porphyrin promoted with a cationic am-phiphile at the heptane-water interface was observed by this technique and the observed SHG-CD spectra proved the generation of a characteristic optical activity accompanied by the formation of the interfacial ag-gregate of inherently achiral porphyrin molecules. These methods will pioneer a new field of interfacial chiral chemistry in the studies of solvent extraction mechanisms.