Non-canonical nucleobases are intriguing, as they would have been present during chemical evolution, and some even play roles in life processes today. For example, D is present in meteorites alongside U, and its extraterrestrial origin is confirmed. When incorporated into DNA, D repairs UV-induced lesions. Nucleoside and nucleotide analogs of D are also products of selective prebiotic syntheses. Remarkably, D is even present in contemporary biology as it completely substitutes A in the genomes of a wide array of siphoviruses. Looking at the molecular level, D contains an additional amino group in the 2 position compared to A and can form up to three intermolecular hydrogen bonds with U (Figures and). On the other hand, Watson-Crick hydrogen bonding between A and U by two intermolecular hydrogen bonds occurs when attached to the sugar-phosphate backbone but is absent on the level of free nucleobases. If available under prebiotic conditions, thermodynamically more favorable D-U base-pairing is important in understanding the chemical evolution of these two nucleobases. For example, the complementary supramolecular assembly of D and U could affect their photostability under UV light. Importantly, solar radiation was likely the primary source of energy available on early Earth and a key factor for chemical selection. In this work, we combine several state-of-the-art experimental and computational techniques to investigate the interaction of D and U, the formation of their assemblies in different aggregate states (crystal, liquid, and gas), and discuss the impact on photochemistry concerning early Earth environments. We reveal the self-assembly of D-U cocrystal hydrate (D-U hyd) and anhydrate (D-U anhyd) under prebiotic reaction conditions of different water availability (bulk water, stoichiometric water, water vapor, and dry state) and temperatures (low and elevated). We monitor the formation of D-U hyd and its wet/dry cycling to D-U anhyd by in situ synchrotron powder X-ray diffraction (PXRD). We elucidate the crystal structure of D-U hyd from single crystal X-ray diffraction (SCXRD) data and D-U anhyd from 3D electron diffraction (ED) data to study the crystal packing. We estimate favorable formation and stabilization energies of D-U base-paired cocrystals by periodic density functional theory (DFT) computation and reveal the formation of Watson-Crick dimers as a driving force for their assembly. To place D-U base-paired cocrystals in the prebiotic context, we study the photostability of its aqueous solution under UV light by in situ UV resonance Raman (UVRR) measurements. The latter shows that it is significantly less photostable than the aqueous solution of pure U. This is surprising since U, and more generally canonical nucleobases, are typically very stable when irradiated by UV light in their free forms, including in aqueous solutions. This behavior is associated with a very short excited-state lifetime at the order of picoseconds. In the case of free U, such rapid radiationless deactivation from ππ * state involves a conical intersection (CI) along a coordinate corresponding to the twist around the double C5C6 bonds. However, as we show in this work, the situation differs when non-covalent interactions between complementary nucleobases, such as D and U, are in place. Furthermore, we perform quantum-chemical and DFT simulations to show that D-U hydrogen bonding may promote photoreaction to form cyclobutane pyrimidine dimers (CPDs) of U. Thus, serving as a possible pathway for the photochemical degradation of U that needs at least two U nucleobases to be in proximity to be effective.