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 dibenzocyclooctyne 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 S0 S1 transitions in the planar C2v-symmetric starting material compared to the symmetry-broken and non-planar cycloaddition products. The new fluorogenic probe was successfully employed for labeling of proteins modified by an azide moiety.