Bottom-Up Formation of Antiaromatic Cyclobutadiene (c-C₄H₄) in Interstellar Ice Analogs

Antiaromatic cyclobutadiene (c-C₄H₄) is the simplest prototype of [n]annulenes and a key reactive intermediate with significant ring strain, serving as the model compound for antiaromatic systems in organic chemistry. Here, we report the first bottom-up formation of cyclobutadiene in low-temperature acetylene (C₂H₂) ices exposed to energetic electrons. Cyclobutadiene was isolated and detected in the gas phase upon sublimation utilizing vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry along with ultraviolet photolysis studies. These findings advance our fundamental understanding of the exotic chemistry and preparation of highly strained antiaromatic cycles through non-equilibrium chemistry in interstellar environments, thus affording a possible route for the formation of highly strained molecules such as the hitherto elusive tetrahedrane (C₄H₄). Because acetylene is a major product of the photolysis and radiolysis of methane (CH₄) ice, an abundant component of interstellar ices, our results suggest that cyclobutadiene can likely be formed in methane-rich ices of cold molecular clouds.