Fluorogenic reactions in which non- or weakly fluorescent reagents produce highly fluorescent products can be exploited to detect a broad range of compounds including biomolecules and materials. We describe a modified dibenzocyclooctine that under catalyst-free conditions undergoes fast strain promoted cycloadditions with azides to yield strongly fluorescent triazoles. The Cycloaddition products are more than 1000-fold brighter compared to the Starting cyclooctyne, exhibit large Stokes shift,. and can be excited above 350 nm, which is required for many applications. Quantum mechanical: calculations indicate that the fluorescence increase upon triazole formation is due to large differences in oscillator strengths of the S-0 <-> S-1 transitions in the planar C-2v-symmetric starting material compared to the symmetry broken and nonplanar.cycloaddition products. The new fluorogenic probe was successfully employed for labeling of proteins modified by an azide moiety.