10-Methylacridine Derivatives Acting as Efficient and Stable Photocatalysts in Reductive Dehalogenation of Halogenated Compounds with Sodium Borohydride via Photoinduced Electron Transfer

10-Methylacridine derivatives, 9,10-dihydro-10-methylacridine (AcrH₂) and acriflavine (AFH⁺), act as efficient and stable photocatalysts in reductive dechlorination of p-chlorobiphenyl (C1BP) as well as dehalogenation of other halogenated compounds with sodium borohydride (NaBH₄) in a mixture of acetonitrile and H₂O (9:1 v/v) at 298 K. The reductive dechlorination proceeds via the reduction of CIBP by the singlet excited state (¹AcrH₂^⋆) to yield dechlorinated product (biphenyl) and 10-mcthylacridinium ion (AcrH⁺), followed by the facile reduction of AcrH⁺ with NaBH₄ to regenerate AcrH₂. The absence of the primary kinetic isotope effect as well as the comparison of the observed rate constants with those predicted by using the Marcus theory of electron transfer indicates that the reduction of halogenated compounds (RX) by the singlet excited state (¹AcrH₂^⋆) proceeds via photoinduced electron transfer from ¹AcrH₂^⋆ to RX, which results in the cleavage of C-X bonds. In the photocatalytic reductive dehalogenation of o-, m-, and p-bromochlorobenzenes, cleavage of the C-Br bond predominates over that of the C-Cl bond in each case. In the case of halobenzyl halide (X-C₆H₄CH₂X; X = Cl, Br) only the CH₂-X bond is cleaved to yield halotoluene (X-C₆H₄CH}) selectively.