A periodic system of polycyclic aromatic hydrocarbons for predictive molecular growth processes

Abstract

Radical–radical reactions of resonantly stabilized species such as propargyl (C3H3˙) and benzyl (C7H7˙) efficiently generate polycyclic aromatic hydrocarbons (PAHs). Key mechanisms—propargyl addition–benzannulation (PABA), methyl addition–ring expansion (MARE), and cyclopentadienyl addition–naphthylization (CPAN)—govern early-stage aromatic growth, but transforming these mechanisms to larger PAHs is computationally challenging due to combinatorial intermediates and reactions on excited state triplet surfaces. Here, we introduce a “periodic system” of PAHs that organizes aromatic growth according to transferable radical–radical motifs, enabling prediction of molecular evolution across the size regime. Using (2-naphthyl)-methyl (2-C11H9˙) and benzyl (C7H7˙) radicals as benchmarks, we experimentally demonstrate the predicted gas-phase formation of four- and five-ring PAHs, including (1) C18H12 isomers: tetracene, benz[a]anthracene, and [4]helicene; (2) C22H14 isomers: benzo[a]tetracene, benzo[a]tetraphene, and [5]helicene. Secondary hydrogen-loss and successive reactions produce even larger 2D and 3D aromatics, such as nanobowls and fullerenes, and their building blocks (corannulene and coronene). This framework provides a unifying and predictive description of PAH growth across combustion and circumstellar environments such as carbon-rich envelopes of late-type Asymptotic Giant Branch (AGB) stars and planetary nebulae.