A Rational Approach to Super-Reducing Organophotocatalysts from Fluorene Derivatives

A family of 9-substituted fluorene derivatives has been synthesized, which are potent super-reductants upon photoexcitation at their anionic state. Electronic tunability and consequent modulation of the ground-state redox behavior can be easily achieved for this set of molecules. The entire class of fluorene anions is extremely easy to oxidize at their ground state, showing oxidation in the range of −0.69 to −0.36 V vs SCE, which also makes them super-reductants (Eox* = circa −3 V), once photoexcited. Moreover, these anions are very long-lived in their excited state and display delayed fluorescence. One representative molecule, 9-Ph-fluorene, exhibits a singlet excited-state lifetime of 43.3 ns, while the delayed component is long-lived at 19.2 μs. The molecule also reaches its triplet state as evident from the measured phosphorescence at 77 K. This molecule homolytically cleaves the reductively recalcitrant C(sp2)–Cl bond, generating an aryl radical at RT under white light excitation. The super-reductive power of the class of photocatalysts has been utilized for a set of important chemical transformations, including hydrodehalogenation, C–C cross-coupling, and borylation, from aryl chloride substrates, offering great promise for their use as photocatalysts.