In this work, we presented a complementary set of X-ray structural, computational, spectroscopic, and reactivity data in support of the contention that a diazirine's central carbon atom can be more accurately viewed as being sp 2 -hybridized, and where p-orbital contributions from this atom to the frontier molecular orbitals permit effective conjugation between the diazirine unit and the neighbouring aromatic ring. Indeed, a full molecular orbital analysis of the parent diazirine molecule (H2CN2; Fig. ) shows that the diazirine orbitals can be readily described by considering a linear combination of an sp 2hybridized methylene carbene (H2C:) and a molecule of dinitrogen (N2). This treatment is particularly helpful in illustrating how the diazirine LUMO is derived almost solely from the 𝜋 2𝑝 𝑧 * antibonding orbital of N2, while the HOMO is derived from an in-phase (i.e. bonding) combination of the methylene 𝑝 𝑦 orbital and the dinitrogen 𝜋 2𝑝 𝑥 * orbital. The revised conceptual picture of diazirine bonding suggests the use of extended chromophores to allow longer-wavelength activation of the diazirine group to the corresponding carbene. As a proof of principle, we synthesized a representative diazirine-fluorene conjugate, and showed that this could be activated with >450 nm blue light, or through two-photon excitation using red light. Both of these observations constitute important firsts for the diazirine field, and suggest the development of improved diazirine-containing reagents for carrying out biological target identification in living organisms. Notes and references § The MacMillan and Steele groups have both shown that trifluoromethyl aryl diazirines can be activated via energy transfer from iridium dyes that absorb visible light; this approach, however, requires that the sensitizer and diazirine can come into close contact with another-a requirement that would not be achievable for most biological targeting applications, without adding significant molecular weight to the probe. To the best of our knowledge, there are no previous examples of direct trifluoromethyl diazirine-chromophore conjugates, where photochemical excitation of the chromophore leads to activation of the diazirine motif. 34 † A total of six X-ray structures for trifluoromethyl aryl diazirines have been reported to date. Five of these have the diazirine positioned approximately coplanar to the aromatic ring, and have CF3-C-aryl bond angles of ca. 121°. The sixth structure has the diazirine unit twisted with respect to the aryl group (φCF 3 -C diazirine -C ipso -C ortho = 84°), perhaps due to crystal packing forces, and this molecule displays a smaller CF3-C-aryl bond angle of 118°. These data provide further support for the hypothesis that the degree of orbital conjugation and the bond angle around the diazirine central carbon are coupled to one another. See Table in the ESI for a complete list of structures, together with relevant bond and dihedral angles. ‡ There are six trifluoromethyl aryl diaziridines for which structures have been reported previously in the Cambridge Structural Database, but each of these has additional substituents present on the nitrogen atoms. Regardless, for each of these molecules the diazirine group adopted a twisted conformation (φCF 3 -C diazirine -C ipso -C ortho > 60°) and the CF3-C-aryl bond angle was approximately 114°. See Table in the ESI for a complete list of structures, together with relevant bond and dihedral angles.