Photodissociation of the diacetylene dimer and implications for hydrocarbon growth in Titan's atmosphere

The surface of Titan is obscured by multiple aerosol layers whose composition and formation mechanism have remained poorly understood. These organic haze layers are believed to arise from photolysis and electron impact triggered chemistry in the dense nitrogen (N₂) and methane (CH ₄) atmosphere involving highly unsaturated hydrocarbon molecules such as acetylene (HCCH), diacetylene (HCCCCH), and triacetylene (HCCCCCCH). Here we show via laboratory studies combined with electronic structure calculations that the photodissociation of the diacetylene dimer ((HCCCCH)₂) readily initiates atomic hydrogen loss and atomic hydrogen transfer reactions forming two prototypes of resonantly stabilized free radicals, C ₄H₃ and C₈H₃, respectively. These structures represent hydrogenated polyynes which can neither be synthesized via traditional photodissociation pathways of the monomer nor via hydrogen addition to the polyynes. The photodissociation dynamics of mixed dimers involving acetylene, diacetylene, and even triacetylene present a novel, hitherto overlooked reaction class and show the potential to synthesize more complex, resonantly stabilized free radicals considered to be major building blocks to polycyclic aromatic hydrocarbons in Titan's low-temperature atmosphere.