Bipolar single-molecule electroluminescence and electrofluorochromism

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. In this paper, we provide 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 organic light-emitting diode devices.This paper is part of the research program of the Foundation for Fundamental Research on Matter, which is financially supported by the Netherlands Organization for Scientific Research (NWO) as well as part of NWO Project OCENW.M20.253. R.R. and N.L. acknowledge financial support from the Spanish State Research Agency grant (Project No. PID2021-127917NB-I00) funded by MCIN/AEI/10.13039/501100011033 as well as Project ESiM 101046364 funded by the European Union.Peer reviewe