Photochemistry of Halogen-Bonded Complexes: Light-Initiated Electron Transfer from Halide Anions to Bromo-Substituted Electrophiles

Light-initiated reactions involving halogen-bonded (HaB) complexes have attracted increasing attention in recent years as a platform for developing metal-free synthetic procedures. However, mechanistic details and quantitative data on the quantum yields (efficiency) of these photochemical processes remain scarce. Halogen-bonded complexes formed between R–Br electrophiles (R–Br = CBr3NO2, CBr4, CBr3H, or CBr3F) and halide anions, X– (X– = I–, Br–, or Cl–), provided convenient model systems to address these questions. The R–Br···X– complexes exhibit distinct charge-transfer (CT) absorption bands in the 250–400 nm region, which are well separated from the higher-energy intramolecular transitions of the individual HaB donors and acceptors. Irradiation at 365, 302, and 254 nm leads to the formation of X3– anions with quantum yields ranging from about 0.2 to 0.8. Analysis of the wavelength dependence of the quantum yields demonstrates that photochemical reactivity is initiated by excitation of the CT bands rather than by intramolecular transitions. These results highlight the crucial role of the charge-transfer component of halogen bonding in enabling photochemical transformations. In general, higher quantum yields are observed for systems containing stronger electron acceptors (CBr4 or CBr3NO2) and/or stronger electron donors (I–). Since complexes formed by the stronger donors and acceptors show lower-energy CT bands, these findings open a pathway toward the rational design of “green” photochemical processes operating under visible or near-UV irradiation.