A fluorogenic probe for the copper(I)-catalyzed azide-alkyne ligation reaction: modulation of the fluorescence emission via 3(n,pi)-1(pi,pi) inversion.

Chemoselective ligation reactions represent a powerful approach for labeling of proteins or small molecules in a biological environment. We report here a fluorogenic probe that is activated by click chemistry, a highly versatile bio-orthogonal and chemoselective ligation reaction which is based on the azide moiety as the functional group. The electron-donating properties of the triazole ring that is formed in the course of the coupling reaction was effectively utilized to modulate the fluorescence output of an electronically coupled coumarin fluorophore. Under physiological conditions the probe is essentially nonfluorescent and undergoes a bright emission enhancement upon ligation with an azide. Time-resolved emission spectroscopy and semiempirical quantum-mechanical calculations suggest that the fluorescence switching is due to an inversion of the energy ordering of the emissive 1(pi,pi*) and nonemissive 3(n,pi*) excited states. The rapid kinetics of the ligation reaction render the probe attractive for a wide range of applications in biology, analytical chemistry, or material science.