A Fluorogenic Reaction for Monitoring Cross-Coupling with Turn-On Ratio Greater than a Hundred Thousand.

Fluorogenic reactions convert nonfluorescent reactants into fluorescent products and are ubiquitous in molecular sciences, allowing discovery, quantification, and imaging of a range of chemical and biological processes. However, the "non-fluorescent" reactants typically have substantial residual fluorescence, reducing the turn-on ratio of the reaction. Limits on the turn-on ratio then impose constraints on the fluorogenic reaction's application, such as lowering the maximum allowable concentration of reactants or increasing the minimum detectable amount of product. Here, we report a design scheme producing exceptionally high turn-on ratios of up to 207,000. The scheme relies on a condensation reaction uniting electron-rich and electron-deficient reactants, resulting in a product molecule with "push-pull" character and significantly altered color and photophysical behavior. This approach is demonstrated using the Suzuki cross-coupling to produce a highly fluorescent dicyanomethylenedihydrofuran (DCDHF) product. The product has a peak absorption nearly 1 eV redder than the nearest reactant, while also exhibiting a large Stokes shift of nearly 0.5 eV. As a result, the reactants are negligibly excited, leading to the exceptionally high turn-on ratio, and enabling reactions to be performed at high concentrations of starting material without drowning out the product signal. The reaction is demonstrated in bulk conditions and in microdroplets, where differences in reaction rate are observed.