Design of Hydrazone-Modified 1,8-Naphthalimides as Fluorogenic Click Probes Based on Nitrile Imine-Alkyne Cycloaddition

We designed four 4-hydrazone-1,8-naphthalimides as novel fluorogenic click probes, from which the in situ generated nitrile imines react with bicyclo[6.1.0]nonyne through strain-promoted 1,3-dipolar cycloaddition. The resulting pyrazole cycloadducts were found to be highly fluorescent with compelling photophysical properties, including large stokes shift (up to 152 nm), high fluorescence quantum yields (up to 0.85), and notable fluorescence enhancement (up to 196.9-fold). Density functional theory calculations were carried out to elucidate the relationship between structure modification and fluorescence properties of these probes. Of particular importance, this novel strategy was successfully applied to the fluorogenic labeling of alkyne-modified proteins in situ, providing a significant expansion for the growing chemical biology toolkit.