  We explore theoretically the electroluminescence of single molecules. We
adopt a local-electrode framework that is appropriate for scanning tunneling
microscopy (STM) experiments where electroluminescence originates from
individual molecules of moderate size on complex substrates: Couplings between
the STM tip and molecule and between the molecule and multiple substrate sites
are treated on the same footing, as local electrodes contacting the molecule.
Electron flow is modelled with the Lippmann-Schwinger Green function scattering
technique. The evolution of the electronic energy levels of the molecule under
bias is modelled assuming the total charge of the molecule to be invariant,
consistent with Coulomb blockade considerations, but that the electronic
occupations of the molecular HOMO and LUMO levels vary with changing bias. The
photon emission rate is calculated using Fermi's golden rule. We apply this
theoretical approach to the STM/Zn-etioporphyrin/Al2O3/NiAl(110) system, and
simulate various configurations of coupling strength between the molecule and
substrate. We compare our results to the experimental observations of Qiu,
Nazin and Ho [Science 299, 542 (2003)] for this system and find that our model
provides a comprehensive explanation of a multitude of previously unexplained
observations. These include the different types of current-voltage
characteristics (CVC's) that are observed experimentally, the observed
association of electroluminescence with some CVC's and not others and key
properties of the observed photon spectra. Theoretical predictions are
presented for further single-molecule electroluminescence experiments.
