Conformation-induced Kondo switch of fluorenyl radicals on a metal surface through adsorption

Controlling whether a molecular radical retains its spin on a metal surface is a key prerequisite for building switchable, atomically precise carbon-based spin architectures. Here, we use 3,6-bis(4-bromophenyl)-9H-fluorene to synthesize covalently linked fluorene trimers and oligomeric chains on Au(111) via Ullmann coupling and then generate strongly localized fluorenyl-type radical centers by site-selective tip-induced dehydrogenation. Combining the bond-resolved nc-AFM with scanning tunneling spectroscopy, we identify two interconvertible adsorption configurations: a non-bonded radical state that displays a pronounced zero-bias Kondo resonance and a chemisorbed state in which a local C-Au bond is formed at the radical site, accompanied by a characteristic geometric relaxation of the five-membered ring and complete quenching of the Kondo resonance. In both the macrocycles and chains, the distribution of the Kondo-active sites depends on metastable global adsorption geometries and can be reversibly reconfigured by tip perturbation. These results establish a structure-resolved chemisorption versus physisorption switch as a practical design rule for stabilizing and toggling spins in multi-radical rings and chains directly on metallic substrates, opening opportunities for programmable quantum spin functionalities in surface-supported π-systems.