Probing Up-Conversion Electroluminescence of Decoupled Porphyrin Molecules in a Plasmonic Nanocavity

Molecular triplet states can produce significant phosphorescence and act as a relay state for luminescence, such as in up-conversion processes. While this property makes triplet emitters interesting for organic light-emitting diodes (OLEDs), the study of their luminescence at the single-molecule level in high-resolution scanning tunneling microscopy (STM) is challenging. We investigate individual Pd-octaethylporphyrin (PdOEP) molecules decoupled from Ag(100) and Ag(111) by an ultrathin NaCl layer and observe singlet and triplet emission lines at visible wavelengths, separated by only about 100 nm. This is in stark contrast to metal or free-base phthalocyanines, for which typically the lowest triplet transitions lie in the far red or infrared where the sensitivity of charge-coupled device (CCD) detectors decreases significantly. The singlet S₁ state of PdOEP emits photons even when the photon energy is higher than the energy provided by a tunneling electron, in an energy up-conversion process. This mechanism requires a relay (or shelving) state in which energy is stored in the molecule for the interval between tunneling electrons. By analyzing the energy levels of different molecular states (S₁, D₀, and T₁ states) and fitting the current dependencies of S₁ emission under up-conversion electroluminescence (UCEL) condition for both S₁ and T₁ emission, we verify the validity of a triplet-mediated up-conversion model.