We report transport measurements on tunable single-molecule junctions of the organic perchlorotrityl radical molecule, contacted with gold electrodes at low temperature. The current−voltage characteristics of a subset of junctions shows zerobias anomalies due to the Kondo effect and in addition elevated magnetoresistance (MR). Junctions without Kondo resonance reveal a much stronger MR. Furthermore, we show that the amplitude of the MR can be tuned by mechanically stretching the junction. On the basis of these findings, we attribute the high MR to an interference effect involving spin-dependent scattering at the metal−molecule interface and assign the Kondo effect to the unpaired spin located in the center of the molecule in asymmetric junctions.

Single-molecule junctions of oligophenyl-diethynyl (OPAn) are studied at low temperature using a mechanically controlled break junction (MCBJ) setup and characterized using inelastic electron tunneling spectroscopy and quantum chemical calculations.