Highly Fluorinated Chiral Sensors for Enantioselective Fluorescent Recognition

Various kinds of fluorinated organic sensors using 1, 1'-Bi-2-naphthol (BINOL) as the chiral core have been developed for the fluorescent recognition of chiral molecules. This study is potentially applicable for rapid enantiomeric excess determination and high throughput catalyst screening.
A perfluoroalkyl diketone compound is synthesized as the first enantioselective fluorescent sensor in a fluorous solvent. Within a certain concentration range one enantiomer causes 1200-2000 folds fluorescent enhancement to the sensor, while the other one only gives 10-50 folds enhancement. Light scattering studies show the formation of aggregates with different particle sizes when the two enantiomers interact with the sensor. The substantial difference enables visual discrimination of two enantiomers with naked eyes. A series of similar sensors are also prepared and their fluorescence responses in various fluorinated solvents are studied.
A facile amide bond formation is utilized for the enantioselective recognition of various amines for the first time. A dramatic solvent effect is observed for the reaction of amine with the H8BINOL-based perfluoroalkyl ketone. In DMF, cleavage of the perfluoroalkl group to form amides is observed but not in other solvents. This provides a new direction for the design of molecular sensors for amine recognition. A series of other highly fluorinated ketones are also synthesized for the enantioselective sensing of amino acid anions in DMSO. The best enantioselectivity was observed in the case of serine with ef values up to 10 when excess amount of serine is used.
A BINAM-based ketone sensor is found to show ratiometric sensing toward diamines, which enables the simultaneous determination of both concentration and ee values. It also shows moderate enantioselectivity toward amino acids and hydroxyl acids. Moreover, fluorescent dyes have been conjugated to the chiral binaphthyl structure for the sensing of hydroxyl acids and amines at long emission wavelengths.