In the first part of this Project, our computational investigations showed that the triplet state of nitroarenes can have two distinct electronic configurations, depending on where on the molecule the excitation is localized. This could occur either on the nitro group (what we called a np* state) or on the whole conjugated system (pp* state). This has a noticeable impact on the photophysical properties. All nitroarenes for which the np* state is lower in energy are short-lived and, following UV irradiation, they regenerate the electronic ground state in ultrashort time scales. On the other hand, pp* nitroarenes have much longer-lived excited states, yet they are more reactive due to being antiaromatic, as a direct consequence of the perturbation of the electron density of their phenyl ring. We used this unique property of excited-state antiaromaticity to successfully promote the oxidative cleavage of aromatic compounds; the loss of aromaticity on the substrate, a destabilizing factor, is compensated by the relief of excited-state antiaromaticity on the nitroarene promoter, a stabilizing factor.
Realizing that it is the electronic nature of the excited state what governs reactivity and not simply its energy made us wonder if we could use this to our favor in triplet energy transfer reactions. Indeed, after Stern-Volmer quenching studies and computational modelling of the barriers for energy transfer, we were able to develop a new procedure for the sensitized isomerization of styrenes and the [2+2]-photocycloaddition of remote dienes using pp* nitroarenes. Specifically, the calculations showed that the low geometry deformations that nitroarenes experience following population of the pp* state contribute lowering the energy transfer barriers. In contrast, the low lifetimes of the np* states and their severe distortion of the nitro group raises these barriers to the point that energy transfer becomes unfeasible.
