Ylidenemalononitrile Enamines as Fluorescent “Turn-On” Indicators for Primary Amines

Ylidenemalononitrile enamines undergo
rapid amine exchange followed
by a cyclization with primary amines to yield fluorescent products
with emission intensities as high as 900 times greater than the starting
materials. After identifying the fluorescent species by X-ray crystallography,
we demonstrate that the rate of amine exchange is substrate dependent
and that by simple structural variation the fluorescence can be tuned
over the entire visible spectrum. We further demonstrate their potential
application in biomolecule labeling.