Our target diazirine (1) was synthesized as shown in Scheme 1. Beginning from the corresponding trifluoromethyl ketone (2, available in 5 steps from commercial 9,9-dimethylfluorene), we generated oxime 3 through the addition of hydroxylamine hydrochloride. The oxime was then activated as the nosylate (4), and gaseous ammonia was added to generate the diaziridine intermediate (5). This in turn could be efficiently oxidized to the desired diazirine using molecular iodine in the presence of triethylamine. With compound 1 in hand, we studied its optical properties by U V/ Vis spectr os copy and its ther mal pr oper ties by differential scanning calorimetry (DSC). In the DSC experiment, we were pleased to note an onset temperature (Tonset) of 89°C for the diazirine activation exotherm, and a peak temperature (Tpeak) of 113°C (Fig. ). The values are significantly lower than those for the unsubstituted trifluoromethyl phenyl diazirine (Tonset = 103°C; Tpeak = 127°C), and are in fact nearly identical to those for the p-methoxyphenyl analogue thereof (T onset = 88°C; T peak = 113°C). We previously showed that the T onset and T peak values shift with the degree of electron-richness in the aromatic ring (see inset data in Fig. ). The activation temperatures that characterize the diazirine activation exotherm for compound 1 thus provided the first experimental verification that the properties of the diazirine were perturbed by the presence of the electron-donating dimethylamino group on the opposite ring of the dimethylfluorene scaffold (although the influence of this strong electron-donating group is no doubt attenuated by being positioned two rings away). As such, the data suggest a significant degree of electronic communication across the molecule. This suggestion was further supported by DFT calculations, which revealed molecular orbitals extending from the electron-donating dimethylamino substituent all the way through the diazirine motif (Fig. ). As with the calculations performed for the unsubstituted trifluoromethyl phenyl diazirine in Fig. , the geometry-optimized structure for compound 1 had the diazirine motif nearly coplanar to the aromatic ring (φCF 3 -C diazirine -C ipso -C ortho = 2.5°), and the calculated CF 3 -C-aryl bond angle was 120.4°. These data once again suggested an sp 2 -hybridization state for the central carbon atom of the diazirine, as did the fact that in the only frontier orbital to which the diazirine carbon made a significant contribution (the HOMO-1), the Cdiazirine atomic orbital contributing to the MO is clearly a p-orbital. As described above, the 9,9-dimethylfluorene scaffold functions as an effective chromophore when the electrondonating group on one of the aromatic rings is paired with a conjugated electron-withdrawing group on the opposite ring. The UV/Vis spectrum for any given fluorene molecule can thus provide a qualitative assessment of the degree of conjugation for a substituent. To probe the electronic communication in diazirine 1, we compared its UV/Vis spectrum to that of trifluoromethyl ketone 2 (a positive control for conjugation) and benzyl alcohol 6 (a negative control for conjugation, available by hydride reduction of the ketone). As expected, ketone 2 behaved as an efficient 'push-pull' fluorene chromophore, resulting in a strong absorbance band centred at 418 nm in methanol, and extending beyond 475 nm (light blue trace in Fig. ). By contrast, negative control 6 (in which the sp -hybridized benzylic centre does not permit conjugation from the electronwithdrawing trifluoromethyl group into the pi system) did not display any absorbance beyond 375 nm (dark blue trace in Fig. DSC data for compound 1, showing the diazirine activation exotherm. Tpeak corresponds to the temperature at which the maximum heat flow is observed. Tonset is determined experimentally from extrapolation of the tangent of the upward slope observed in the DSC experiment (red dotted line), to the fitted baseline of the plot. Inset data taken from reference show the decrease in activation temperature with increasing electron density in the aromatic ring. 8). The UV/Vis spectrum for diazirine 1 (purple trace in Fig. ) had more in common with that of 2 than with 6, displaying a Fig. Geometry-optimized structure and frontier molecular orbitals calculated for compound 1 (M06-2X/6-311+G(2df,2p)).