  We explore theoretically the principles that govern photon emission from
single-molecule conductors carrying electric currents between metallic
contacts. The molecule and contacts are represented by a generic tight-binding
model. The electric current is calculated using Landauer theory and the photon
emission rate is obtained using Fermi's golden rule. The bias-dependence of the
electronic structure of the molecular wire is included in the theory in a
simple way. Conditions under which significant photon emission should occur are
identified and photon spectra are calculated. We predict the photon emission
rate to be more sensitive than the electric current to coupling asymmetries
between the molecule and contacts. This has important implications for the
design and interpretation of STM experiments searching for electroluminescence
from individual molecules. We discuss how electroluminescence may be used to
measure important characteristics of the electronic structure of molecular
wires such as the HOMO-LUMO gap and location of the Fermi level of the contacts
relative to the HOMO and LUMO. The feasibility of observing photon emission
from Au/benzene-dithiolate molecular wires is also discussed.
