Tuning Tetramethoxy-acridiniums for Fluorophores and Organic Photoredox Catalysis

Tetramethoxy substituted alkyl-acridiniums (TMAcr⁺) are readily available by facile nucleophilic aromatic substitution on tris(2,6-dimethoxyphenyl)carbenium, but are non-fluorescent, presumably due to intramolecular photoinduced electron transfer quenching. In this work we introduce electron withdrawing groups by electrophilic aromatic substitution reactions, leading to fluorescence turn-on. The acridiniums are moderately fluorescent (φ_f=20 %) with long fluorescene lifetimes (τ_f=9 ns). The positive excited state reduction potentials (E*_red=+1.6 V) make the TMAcr⁺ excellent electron acceptors in the excited state, and efficient reductive photoredox catalysts able to oxidize a broad range of substrates.