Gating Upconversion Electroluminescence in a Single Molecule via Adsorption-Induced Interaction of Unpaired Spin

Molecules with unpaired spins (radicals) offer promising alternatives to closed-shell molecules, as they are less limited regarding the spin statistics in their electroluminescence. Here, we combine scanning tunneling microscopy induced luminescence and density functional theory (DFT) to study single vanadyl phthalocyanine (VOPc) molecules, which are stable neutral radicals. Two distinct adsorption geometries of the molecule on NaCl/Au(111) lead to a difference in the interaction of the unpaired electron with the substrate, which in turn allows us to investigate its effect on the light emission process. We observe that upconversion electroluminescence (UCEL) is gated by the adsorption geometry of the molecule, an effect we attribute to a reordering of excited states and enhanced excited state transition probabilities. The profound influence of the unpaired electron via state reordering enables tuning of not only molecular electroluminescence but also many other spin-dependent phenomena.