Unconventional excited-state dynamics in the concerted benzyl (C7H7) radical self-reaction to anthracene (C14H10)

Polycyclic aromatic hydrocarbons (PAHs) are prevalent in deep space and on Earth as products in combustion processes bearing direct relevance to energy efficiency and environmental remediation. Reactions between hydrocarbon radicals in particular have been invoked as critical molecular mass growth processes toward cyclization leading to these PAHs. However, the mechanism of the formation of PAHs through radical - radical reactions are largely elusive. Here, we report on a combined computational and experimental study of the benzyl (C7H7) radical self-reaction to phenanthrene and anthracene (C14H10) through unconventional, isomer-selective excited state dynamics. Whereas phenanthrene formation is initiated via a barrierless recombination of two benzyl radicals on the singlet ground state surface, formation of anthracene commences through an exotic transition state on the excited state triplet surface through cycloaddition. Our findings challenge conventional wisdom that PAH formation via radical-radical reactions solely operates on electronic ground state surfaces and open up a previously overlooked avenue for a more rapid synthesis of aromatic, multi-ringed structures via excited state dynamics in the gas phase. The reaction of benzyl radical self-reaction to anthracene opens-up a previously overlooked avenue for a more efficient synthesis of aromatic, multi-ringed structures via excited state dynamics in the gas phase.

Automated summary

Polycyclic aromatic hydrocarbons (PAHs), prevalent in deep space and on Earth, can be formed more efficiently through excited state dynamics in the gas phase, challenging the conventional wisdom that PAH formation solely occurs on electronic ground state surfaces. This finding opens up new possibilities for the rapid synthesis of aromatic, multi-ringed structures.