Achieving selective photochemistry on a metal surface by tuning the aromatic core

On-surface photochemistry provides a non-thermal strategy that can circumvent the side reactions and structural defects associated with thermal activation, enabling enhanced control over surface reactions. Among these, Ullmann coupling serves as a representative on-surface reaction for fundamental studies of reaction mechanisms and controllability. However, how the aromatic core of molecular precursors governs photoinduced C-Br activation on metal surfaces remains insufficiently understood. In this work, we investigated photoinduced Ullmann coupling on Au(111) using three dibromo molecular precursors with distinct aromatic cores. The three precursors show markedly different photoreactivities under 405 and 532 nm irradiation. Time-dependent density functional theory calculations reveal a photoinduced surface-to-molecule charge-transfer process. Combined with calculations of potential energy surfaces for C-Br bond dissociations, these results provide a qualitative rationale for comparing the relative tendency of photo-triggered C-Br cleavage at the surface. The results establish a direct correlation between photo-selectivity and the molecular aromatic core, providing mechanistic insight into light-controlled on-surface synthesis and offering design principles for tailoring light-responsive precursors toward desired carbon nanostructures.