  We develop a quantum master equation (QME) approach to investigate the
electroluminesence (EL) of molecules confined between metallic electrodes and
coupled to quantum plasmonic modes. Within our general state-based framework,
we describe electronic tunneling, vibrational damping, environmental dephasing,
and the quantum coherent dynamics of coupled quantum electromagnetic field
modes. As an example, we calculate the STM-induced spontaneous emission of a
tetraphenylporphyrin (TPP) molecule coupled to a nanocavity plasmon. In the
weak molecular exciton-plasmon coupling regime we find excellent agreement with
experiments, including above-threshold hot luminescence, an effect not
described by previous semiclassical calculations. In the strong coupling
regime, we analyze the spectral features indicative of the formation of
plexcitonic states.
