Activation of diazirine motifs with 365 nm light allows for the convenient labelling of biological targets within cell lysates. However, UV light damages cells, and is not able to penetrate deeply into living tissue, precluding the use of diazirines for identifying protein-binding partners in living organisms. In an effort to find photoactivatable reagents that can be used for target identification in a broader range of contexts, Dai and Yang recently examined the use of a diazoalkyl group (i.e. the linear isomer of a cyclic diazirine) that was conjugated to a coumarin dye. The resulting conjugate could be activated to the corresponding carbene using visible light (430-490 nm), and could even be activated using red light (800 nm) through a twophoton activation process. However, linear diazoalkyl groups are highly electrophilic, and are also prone to engaging in dipolar cycloaddition reactions. This broader reactivity profile makes them useful in organic synthesis (especially when coupled with transition-metal catalysis ), but would be a significant limitation for biological applications. Indeed, linear diazo compounds have never been extensively used for biological target identification. Azido-conjugates of dye molecules-which can be photo-activated to the corresponding nitrene-are better known, but aryl nitrenes are prone to undesirable Büchner ring expansions (which 'uses up' the reactive nitrene), and azides have shorter activation wavelengths than diazirines.