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, only about 100 nm apart from each other. This is in stark contrast to the 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 decrease 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. Analyzing the energy levels of different molecular states (S1, D0, and T1 states) and fitting the current dependencies of S1 under up-conversion electroluminescence (UCEL) condition for S1 and T1 emission, we verify the validity of a triplet-mediated up-conversion model.