Unveiling π-Radical Induced Regioselective Pathways in On-Surface Synthesis of Kekulene-Hexaone.

Abstract

On-surface synthesized nanographene and aromatic architectures featuring π-radicals have attracted widespread research interest in recent years. However, the role of delocalized π-electrons in governing the reactivity and regioselectivity of radical-involving reactions, such as cyclodehydrogenation, remains underexplored. In this manuscript, we investigated on-surface reactions of tricyclo(dimethoxy-vinyl-phenanthrene) on Au(111). Following multistep demethylation and cyclodehydrogenation, kekulene-hexaone (KeO) has been successfully synthesized. However, systematic scanning probe microscopy (SPM) and X-ray photoelectron spectroscopy (XPS) revealed an overlap between the thermal reaction windows of the initial demethylation and the first cyclodehydrogenation reactions at a remarkably low temperature of 423 K. Complementary quantum mechanics/molecular mechanics (QM/MM) simulations elucidated a π-radical-assisted mechanism for this low-temperature reaction sequence. The selective first cyclodehydrogenation can be triggered by spin density overlap between two neighboring delocalized π-radicals within a particular demethylated configuration. This configuration-dependent activation provides a fundamental atomistic basis for understanding regioselectivity in on-surface chemistry. Beyond the reaction mechanism, we demonstrated a frustrated assembly of KeO molecules which indicated a tautomerization (C═O⇌C-O·) of ortho-benzoquinone units driven by π-electron resonance. We believe this versatile electronic capability opens a novel avenue for engineering carbon-based quantum materials with exotic correlated electronic states.

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