A Class of PAH Polymerization Reactions Featured by Radical Growth from Reactants to Products

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous organic molecules and fundamental building blocks of carbonaceous nanoparticles in material synthesis, flames, and extraterrestrial environments, yet their formation reactions remain elusive. Here, we propose a unique PAH growth reaction of ethynyl-substituted aryl radical addition to ethynyl-substituted peri-condensed aromatic hydrocarbons (AEPAH mechanism) by exploring the exemplary reactions of the 2-ethynyl-phenyl radical with phenylacetylene, 1-ethynylnaphthalene, and 9-ethynylphenanthrene. Instead of stable molecules and small radicals as reaction intermediates in conventional mechanisms, the AEPAH mechanism synthesizes PAH radicals, which further grow to large PAHs and incipient carbonaceous nanoparticles. High-level quantum chemistry calculation, particle composition analysis by matrix-assisted laser desorption/ionization mass spectrometry, and product analysis of the elementary reaction by gas chromatography-mass spectrometry and synchrotron radiation photoionization mass spectrometry evidenced the AEPAH mechanism as a direct and efficient PAH growth reaction class under the temperature range of 800-1500 K. The unraveled AEPAH mechanism may help understand the evolution of organic and prebiotic molecules on Earth and in deep space.