Electrofluorochromism at the single molecule level.

The interplay between the oxidation state and the optical properties of molecules plays a key role in photosynthesis and is extremely important for applications in displays, sensors or molecular-based memories. While technological developments essentially focus on new organic compounds presenting higher emissivity and tunable switching rates, the fundamental mechanisms occuring directly at the level of a single-molecule remain hard to probe. In this investigation, we use a scanning tuneling microscopy approach to characterize and control the fluorescence of a single Zn-phthalocyanine radical cation adsorbed on a NaCl covered Au(111) sample. The neutral and oxidized states of the molecule are identified on the basis of their fluorescence spectra that reveals very different emission energies and vibronic fingerprints. The emission of the charged molecule is controled by tuning the thickness of the insulator and the plasmons localized at the apex of the STM tip. In addition, sub-nanometric variations of the tip position are used to investigate the charging and electroluminescence mechanisms.