Electron-Phonon Coupling in Current-Driven Single-Molecule Junctions

Vibrational excitations provoked by coupling effects during charge transport through single molecules are intrinsic energy dissipation phenomena, in close analogy to electron–phonon coupling in solids. One fundamental challenge in molecular electronics is the quantitative determination of charge-vibrational (electron–phonon) coupling for single-molecule junctions. The ability to record electron–phonon coupling phenomena at the single-molecule level is a key prerequisite to fully rationalize and optimize charge-transport efficiencies for specific molecular configurations and currents. Here we exemplarily determine the pertaining coupling characteristics for a current-carrying chemically well-defined molecule by synchronous vibrational and current–voltage spectroscopy. These metal-molecule-metal junction insights are complemented by time-resolved infrared spectroscopy to assess the intramolecular vibrational relaxation dynamics. By measuring and analyzing the steady-state vibrational distribution during transient charge transport in a bis-phenylethynyl-anthracene derivative using anti-Stokes Raman scattering, we find ∼0.5 vibrational excitations per elementary charge passing through the metal-molecule-metal junction, by means of a rate model ansatz and quantum-chemical calculations.