Catalysis of Radical Coupling Reaction via Synergistic Action of Oriented External Electric Field and Light Irradiation.

Radical coupling reactions have been widely used in the synthesis of complex organic molecules, materials science, and drug research. However, restricted conditions or special catalysts are required to overcome the energy barrier and trigger the coupling reaction efficiently. In this study, we provide experimental evidence that the C─N radical coupling reactions can be significantly accelerated by an oriented external electric field (OEEF) under synchronous UV irradiation without a catalyst. By repeatedly forming thousands of single-molecular junctions with reactant molecules in nonpolar solvents under a bias voltage, a new conductance plateau was observed upon UV irradiation, which was completely absent in polar solvents owing to the electrical shielding effect. This finding indicates that the radicals generated under light irradiation can timely participate in the coupling reaction under the in situ action of OEEF, leading to the high-efficiency generation of the C─N radical coupling product, which was confirmed by the measured electron paramagnetic spectrum and high-resolution mass spectrometry. With the assistance of theoretical calculations, the underlying mechanism was further revealed, i.e., OEEF can significantly decrease the reaction barrier and facilitate the intermolecular hydrogen atom transfer (HAT). This study provides a catalyst-free paradigm for high-efficiency radical reactions.