Photoinduced Formation of Heterobiradical Species in Crystalline Uranyl–Organic Frameworks Incorporating Cucurbit[8]uril-Viologen [3]Pseudorotaxane

Uranyl (UO₂²⁺) compounds are valued for their unique ligand-to-metal charge transfer (LMCT) states, yet electron transfer (ET) mechanisms within actinide-supramolecular hybrids remain poorly understood. In this study, three photochromic uranyl compounds of cucurbit[8]uril-viologen [3]pseudorotaxane were synthesized from a supramolecular [3]pseudorotaxane ligand and uranyl cations, where two auxiliary electron-rich aromatic dicarboxylic acids, terephthalic acid (H₂TA) and phthalic acid (H₂PA), were also introduced to modulate the coordination environments of the uranyl center and photoinduced electron transfer process. As expected, varying types of radicals are observed in these uranyl compounds, and notably, a special kind of heterobiradical species, i.e., both O-centered radicals and N-centered radicals, emerges in the presence of the auxiliary dicarboxylic acids. A combination of in situ EPR spectroscopy and density functional theory (DFT) calculations reveals that the electron-donating ability of the carboxylate ligand determines the phototriggered electron transfer pathway from the carboxylate group to the uranyl center, thus leading to different photoresponsive behavior of the cucurbit[8]uril-viologen [3]pseudorotaxane moiety and contributing to the formation of these rare heterobiradical species. This work enhances an insightful understanding of electron transfer behavior of photoresponsive actinide-organic hybrid materials, and more importantly, provides a feasible way to tune solid-state uranyl photochemistry through single-crystal-to-single-crystal ligand engineering.