Adsorption conformation induced kondo switch of fluorenyl radicals on metal surface

Abstract

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(4bromophenyl)-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 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 forms at the radical site, accompanied by a characteristic geometric relaxation of the five-membered ring and complete quenching of the Kondo feature. In both macrocycles and chains, the distribution of Kondoactive 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 surfacesupported π-systems.