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Not AvailThe on-surface synthesis of various organic compounds relies on the self-assembly and subsequent dissociation of halogen-substituted organic molecules for polymerization and functionalization. Here, we demonstrate that the photolytic disassembly and dissociation of bromobenzene molecules within magic-sized tetramer nanoclusters are influenced by halogen bonding on the Cu(111) surface. We explain this phenomenon using a combination of two-photon photoemission spectroscopy, scanning tunneling microscopy, and density functional theory computations. The interactions that determine the preferred cluster sizes of trimers to pentamers arise from a combination of halogen bonding and weak hydrogen bonding. Surface adsorption enhances halogen bonding while weakening the weak hydrogen bonds in the nanoclusters. The most stable tetramers are constructed from a trimer foundation that employs halogen-3 synthons with an exterior fourth molecule. The exterior bromobenzene in this tetramer may detach from the trimer core cluster or undergo dehalogenation before the other bromobenzene molecules under irradiation. The work function of the Cu(111) surface is significantly decreased by the presence of a tetramer. This reduction facilitates the photodissociation of bromobenzene by allowing electrons from the surface to occupy the antibonding molecular orbitals associated with the C–Br bond. The work function increases steadily as smaller clusters and dissociated bromobenzene (phenyl and Br) are formed photolytically. The molecules of the trimers are not photodissociated because the energy levels of the C–Br antibonding orbitals in the trimer core are notably higher in energy than those of the exterior molecule in the tetramer. Our study highlights the potential of weak noncovalent interactions to guide selective photolytic reactions on surfaces.able