The Growth-Decay Mechanism: A Universal Pathway for Cyclocarbon Fusion

Cyclocarbons constitute a fundamental class of carbon allotropes with unique electronic properties. The recently synthesized C₁₃ provides a crucial model system for understanding odd-numbered cyclocarbons as experimental evidence confirms its dimerization into C₂₆. The underlying mechanism remains unknown, while understanding it is crucial for the synthesis and storage of cyclocarbons. Here, we combine density functional theory with ab initio molecular dynamics simulations to systematically investigate C₁₃ dimerization. Our results identify electronic structures of diverse weakly bound configurations and reveal a nearly barrierless pathway for covalent bond formation of monomers. The reaction proceeds through a general growth-decay mechanism via bridgehead-ring intermediates, ultimately leading to C₂₆. Furthermore, we demonstrate that atomic-scale manipulation of these intermediates may enable the controlled synthesis of various cyclocarbons, including C₇, C₉, C₁₁, C₁₅, C₁₇, and C₁₉. This work provides essential mechanistic insights into cyclocarbon reactivity and establishes a viable bottom-up strategy for the construction of carbon-based nanostructures.