Vacuum-ultraviolet irradiation of pyridine:acetylene ices relevant to Titan astrochemistry.

Nitrogen-containing polycyclic aromatic hydrocarbons (NPAHs) are important molecules for astrochemistry and prebiotic chemistry, as their occurrence spans from interstellar molecular clouds to planetary systems. Their formation has been previously explored in gas phase experiments, but the role of solid-state chemical reactions in their formation under cryogenic conditions remains elusive. Here, we explore the formation of NPAHs through vacuum ultraviolet (VUV) irradiation of pyridine:acetylene ices in amorphous and co-crystalline phases, with the aim to simulate conditions relevant to the interstellar medium and Titan's atmosphere. Our results show that the synthesis of ethynylpyridines from VUV-irradiated pyridine:acetylene amorphous ices is achievable at 18 K. In the co-crystal phase, photolysis at 110 K leads to the formation of NPAHs such as quinolizinium+ and precursors, reflecting a dynamical system under our conditions. In contrast, irradiation at 90 K under stable conditions did not produce volatile photoproducts. These results suggest that such chemical processes can occur in Titan's atmosphere and potentially in its stratosphere, where the co-condensation of these molecules can form composite ices. Concurrently, the formation of stable co-crystals can influence the depletion rates of pyridine, which suggests that these structures can be preserved and potentially delivered to Titan's surface. Our findings provide insights into the molecular diversity and chemical evolution of organic matter on Titan, crucial for future space exploration missions, such as the Dragonfly mission, which may uncover higher-order organics derived from pyridine precursors on Titan's surface.