Racemic Fluorescence Probe for Enantiomeric Excess Determination: Application of Cononsolvency of a Polymer in Sensing

Both racemic and optically active 3,3'-diformy1-1'-BINOL-based diinitiators (BINOL = 1,1'-bi-2-naphthol) were used to carry out the atom transfer radical polymerization (ATRP) of N-isopropylacrylamide to make polymers rac-I, (R)-1, and (S)-1 which are soluble in water as well as organic solvents. These polymers are found to exhibit a unique cononsolvency property. Mixing an aqueous solution of a polymer and Zn2+ with a dichloromethane solution of (S)- or (R)-amino alcohols led to the formation of polymer films at the interface of the water and dichloromethane phases. The polymer films were separated and showed highly enantioselective fluorescence enhancement in DMSO solution. When the polymers were treated with a mixture of the enantiomers of an amino alcohol, no fluorescence enhancement was observed unless one of the enantiomers was in excess of the other. Such a large nonlinear effect allows the racemic polymer rac-1 to be used to determine the ee of the amino alcohol. This represents the first example to use a racemic fluorescent probe to determine the ee of a chiral molecule.