Ordinary and Hot Electroluminescence from Single-Molecule Devices: Controlling the Emission Color by Chemical Engineering

Single-molecule junctions specifically designed for their optical properties are operated as light-emitting devices using a cryogenic scanning tunneling microscope. They are composed of an emitting unit—a molecular chromophore—suspended between a Au(111) surface and the tip of the microscope by organic linkers. Tunneling electrons flowing through these junctions generate a narrow-line emission of light whose color is controlled by carefully selecting the chemical structure of the emitting unit. Besides the main emission line, red and blue-shifted vibronic features of low intensity are also detected. While the red-shifted features provide a spectroscopic fingerprint of the emitting unit, the blue-shifted ones are interpreted in terms of hot luminescence from vibrationally excited states of the molecule.