  Understanding the fundamental mechanisms of optoelectronic excitation and
relaxation pathways on the single-molecule level has only recently been started
by combining scanning tunneling microscopy (STM) and spectroscopy (STS) with
STM-induced luminescence (STML). In this paper, we investigate cationic and
anionic fluorescence of individual zinc phthalocyanine (ZnPc) molecules
adsorbed on ultrathin NaCl films on Ag(111) by using STML. They depend on the
tip-sample bias polarity and appear at threshold voltages that are correlated
with the onset energies of particular molecular orbitals, as identified by STS.
We also find that the fluorescence is caused by a single electron tunneling
process. Comparing with results from density functional theory calculations, we
propose an alternative many-body picture to describe the charging and
electroluminescence mechanism. Our study provides aspects toward well-defined
voltage selectivity of bipolar electrofluorochromism, as well as fundamental
insights regarding the role of transiently charged states of emitter molecules
within OLED devices.
