Investigation of electronic excited states in single-molecule junctions

The investigation of electronic excited states in single-molecule junctions not only provides platforms to reveal the photophysical and photochemical processes at the molecular level, but also brings opportunities for the development of single-molecule optoelectronic devices. Understanding the interaction mechanisms between molecules and nanocavities is essential to obtain on-demand properties in devices by artificial design, since molecules in junctions exhibit unique behaviors of excited states benefited from the structures of metallic nanocavities. Here, we review the excitation mechanisms involved in the interplay between molecules and plasmonic nanocavities, and reveal the influence of nanostructures on excited-state properties by demonstrating the differences in excited state decay processes. Furthermore, vibronic transitions of molecules between nanoelectrodes are also discussed, offering a new single-molecule characterization method. Finally, we provide the potential applications and challenges in single-molecule optoelectronic devices and the possible directions in exploring the underlying mechanisms of photophysical and photochemical processes.