More Halogen-Localized, Yet More Strongly Electron Bound: Carbon Cage-Mediated Electron Stabilization in C60X– (X = F, Cl, Br, and I) Anions

Carbon–halogen bonds are central motifs in molecular chemistry, yet how a single C–X linkage behaves on an extended electron-accepting carbon framework remains poorly understood. Here, we combine negative ion photoelectron spectroscopy and theoretical calculations to investigate monohalogenated fullerene anions, C60X– (X = F, Cl, Br, and I), as gas-phase models for examining how local C–X bonding and cage-mediated electron stabilization jointly control electron detachment. From F to I, the C60–X interaction weakens systematically: the C–X bond length increases, orbital overlap decreases, and the excess electron evolves from mixed halogen–cage character in C60F–/C60Cl– to more halogen-centered character in C60Br–/C60I–. However, the electron detachment energies of C60X– lie within a narrow energy range of 3.5–3.7 eV, in contrast to isolated halide anions X–, whose electron detachment energies increase from F to Cl and then decrease from Cl to I. Energy decomposition analysis shows that this apparent contradiction arises from different stabilization of the neutral and anionic fullerene–halogen bonded states by the C60 cage. These results highlight fullerene-cage stabilization as a molecular design principle for stabilizing halogen-centered excess electrons and tuning the C–X bond activation in halogen-functionalized π-carbon materials.