Tunable Luminescence From a Single Free-Base Porphyrin Molecule By Controlled Access to Optically Active States

Abstract

Scanning tunneling microscopy-induced luminescence (STML) provides access to optical properties of individual molecules through a cascade of relaxation processes between many-body states. Insufficient charge attachment energies quench the relaxation cascade via optically excited states, causing even intrinsically bright molecules to remain dark in STML. Here, we leverage substrate work function control and tip-induced gating of the double barrier tunnel junction to induce an energy shift of the ionic transition state of a single free-base tetrabenzoporphyrin (H2TBP) to control access to optically excited states and bright exciton emission. The experimental observations are validated by a rate equation and polaron model considering the relaxation energy of the NaCl decoupling layer upon charging of the molecule.