Carbon is ubiquitous in space and plays a key role in prebiotic chemistry. Astronomical observations have found interstellar carbon in the form of polycyclic aromatic hydrocarbons (PAHs) as well as carbonaceous dust, confirming its presence in both gaseous and solid phases. The goal of this study is to experimentally investigate low-temperature chemical pathways between these two phases in order to better understand the evolution of cosmic carbon. Cosmic dust analogs are produced in the supersonic expansion of an argon jet seeded with aromatic molecules: benzene, naphthalene, anthracene, phenanthrene, and pyrene. These are prototype aromatic molecules of compact and noncompact structure, and are used to evaluate the effect of precursor structure on the molecular complexity of the resulting grains. The seeded jet is exposed to an electrical discharge and the carbonaceous grains are collected and probed ex situ via laser desorption mass spectrometry. Mass spectra reveal a rich molecular diversity within the grains, including fragments of the parent molecule but also growth into larger complex organic molecules (COMs). In all experiments, the largest number of products is found in the m/z range 200–250, and C16H10 (attributed to pyrene and/or its isomers) is found to be a dominant product, pointing at the formation of this stable PAH as a preferential route in the growth to larger PAHs. Comparison to mass spectra from the Murchison meteorite reveals a similar dominance of compounds related to C16H10 at m/z = 202. Evidence of the methyl-addition-cyclization mechanism in the anthracene experiment is reported. PAH structure is found to impact the dust production yield, as seen by the greater yield for the anthracene compared to the phenanthrene experiment. PAH growth at low temperatures via barrierless routes involving the addition of alkyl- and phenyl-type radicals is suggested as a viable pathway to COMs. These results suggest that PAH growth and dust formation from PAHs are feasible at low temperatures in photon-dominated regions and circumstellar envelopes.

Compositionally similar organic red colorants in the anthraquinone family, whose photodegradation can cause irreversible color and stability changes, have long been used in works of art. Different organic reds, and their multiple chromophores, suffer degradation disparately. Understanding the details of these molecules’ degradation therefore provides a window into their behavior in works of art and may assist the development of improved conservation methods. According to one proposed model of photodegradation dynamics, intramolecular proton transfer provides a kinetically favored decay pathway in some photoexcited chromophores, preventing degradation-promoting electron transfer (ET). To further test this model, we measured excited state lifetimes of substituted gas-phase anthraquinones using high-level theory to explain the experimental results. The data show a general structural trend: Anthraquinones with 1,4-OH substitution are long-lived and prone to damaging ET, while excited state intramolecular proton transfers promote efficient quenching for hydroxyanthraquinones that lack this motif.

Pump–probe experiments and quantum-chemical simulations of UV-excited isoguanine elucidate its tautomer dependent photochemical properties.