Excited-state configuration controls the ability of nitroarenes to act as energy transfer catalysts

Energy transfer (EnT) catalysis enables the selective population of triplet excited states without previous singlet excitation, thus eliminating the need for high-energy irradiation. Traditionally, EnT catalysis has been approached by developing specific photosensitizers with triplet energies (<i>E</i> _T) that match those of the targeted substrates. Here we introduce an alternative approach to EnT using widely available nitroarenes as photocatalysts. Our findings reveal that their catalytic efficiency is governed by the localization of their excited state rather than <i>E</i> _T. Specifically, ³ <i>π</i>,<i>π</i>* nitroarenes, where the excitation is centred on the aromatic core rather than the nitro group, exhibit superior catalytic performance compared with their ³ <i>n</i>,<i>π</i>* counterparts. We have demonstrated the utility of this concept for nitroarene photocatalysis in contra-thermodynamic <i>E</i>-to-<i>Z</i> alkene isomerization and [2 + 2] cycloadditions. Additionally, we use the energetic descriptor Δ<i>E</i> _TT as easy tool to distinguish the preferential population of ³ <i>n</i>,<i>π</i>* versus ³ <i>π</i>,<i>π</i>* triplet states and therefore accelerate the identification of novel photosensitizers.