Design and Characterization of Diazabenz[cd]fluoranthenium-Based Donor–Acceptor Organophotoredox Catalysts

The synthesis and characterization of a series of donor-acceptor diazabenz[cd]fluoranthenium derivatives (PC1-PC8) are reported. Spectroscopic and computational studies reveal that compound PC4, which features a carbazole-donor (D) and fluoranthenium-acceptor (A) cationic scaffold with CB₉H₁₀⁻ as the counteranion, exhibits a 3-fold enhancement in the molar extinction coefficient at 400 nm relative to its parent compound PC8, along with a broad absorption across the visible and ultraviolet regions. Furthermore, the pronounced intramolecular charge transfer (ICT) effect induced by this architecture gives rise to a substantial Stokes shift of 7143 cm⁻¹ for the compound. Transient absorption spectroscopy resolves the dynamics of picosecond electron transfer between the *PC4 (*Cz-diNFluo⁺) and DIPEA, evidenced by the rapid decay of the S₁ state and the concomitant formation of the reduced radical species (Cz-diNFluȯ). The application of PC4 in the reductive dehalogenation of organic halides is also disclosed. The reaction proceeds via a consecutive photoinduced electron transfer (ConPET) mechanism, establishing a potent reducing platform far exceeding the thermodynamic limits of a single excitation event. This reducing ability is directly demonstrated by the efficient dehalogenative conversion of diverse substrates, including aryl bromides, alkyl bromides, and even recalcitrant aryl chlorides.