During nuclear waste disposal process, radioactive iodine as a fission product can be released. The widespread implementation of sustainable nuclear energy thus requires the development of efficient iodine stores that have simultaneously high capacity, stability and more importantly, storage density (and hence minimized system volume). Here, we report high I₂ adsorption in a series of robust porous metal–organic materials, MFM-300(M) (M = Al, Sc, Fe, In). MFM-300(Sc) exhibits fully reversible I₂ uptake of 1.54 g g^–1, and its structure remains completely unperturbed upon inclusion/removal of I₂. Direct observation and quantification of the adsorption, binding domains and dynamics of guest I₂ molecules within these hosts have been achieved using XPS, TGA-MS, high resolution synchrotron X-ray diffraction, pair distribution function analysis, Raman, terahertz and neutron spectroscopy, coupled with density functional theory modeling. These complementary techniques reveal a comprehensive understanding of the host–I₂ and I₂–I₂ binding interactions at a molecular level. The initial binding site of I₂ in MFM-300(Sc), I₂^I, is located near the bridging hydroxyl group of the [ScO₄(OH)₂] moiety [I₂^I···H–O = 2.263(9) Å] with an occupancy of 0.268. I₂^II is located interstitially between two phenyl rings of neighboring ligand molecules [I₂^II···phenyl ring = 3.378(9) and 4.228(5) Å]. I₂^II is 4.565(2) Å from the hydroxyl group with an occupancy of 0.208. Significantly, at high I₂ loading an unprecedented self-aggregation of I₂ molecules into triple-helical chains within the confined nanovoids has been observed at crystallographic resolution, leading to a highly efficient packing of I₂ molecules with an exceptional I₂ storage density of 3.08 g cm^–3 in MFM-300(Sc).

One of the long standing debates in actinide chemistry is the level of localization and participation of the actinide 5f valence orbitals in covalent bonds across the actinide series. Here we illuminate the role of the 5f valence orbitals of uranium, neptunium and plutonium in chemical bonding using advanced spectroscopies: actinide M_4,5 HR-XANES and 3d4f RIXS. Results reveal that the 5f orbitals are active in the chemical bonding for uranium and neptunium, shown by significant variations in the level of their localization evidenced in the spectra. In contrast, the 5f orbitals of plutonium appear localized and surprisingly insensitive to different bonding environments. We envisage that this report of using relative energy differences between the 5fδ/φ and 5fπ*/5fσ* orbitals as a qualitative measure of overlap-driven actinyl bond covalency will spark activity, and extend to numerous applications of RIXS and HR-XANES to gain new insights into the electronic structures of the actinide elements.