Explaining the electroluminescence of single molecules

Experimental studies of electroluminescence from molecules on complex substrates have yielded intriguing relationships between the current-voltage characteristics and optical emission from such systems.\footnote{X. H. Qiu, G. V. Nazin, W. Ho, Science {\bf 299}, 542 (2003), S. W. Wu, N. Ogawa, W. Ho, Science {\bf 312}, 1362 (2006).} In this talk we propose a theoretical model that is able to account for many observed properties of these systems. We obtain distinct photon emission spectra and corresponding I-V curves for different couplings of the electrodes to the molecule, that are consistent with experimental data. We find emission to be highly dependent on the details of the tip/molecule and molecule/substrate coupling, and make further photon emission predictions for systems not yet achieved experimentally.