Bipolar single-molecule electroluminescence and electrofluorochromism

TL;DR

This study explores the electroluminescence and electrofluorochromism of single zinc phthalocyanine molecules on NaCl/Ag(111), revealing voltage-dependent fluorescence mechanisms and the role of transient charged states through combined experimental and theoretical analysis.

Contribution

It introduces a detailed investigation of bipolar single-molecule electroluminescence using STM-induced luminescence and proposes a new many-body theoretical framework for understanding the charging and emission processes.

Findings

Fluorescence depends on tip-sample bias polarity.

Threshold voltages correlate with molecular orbital energies.

Single-electron tunneling causes fluorescence.

Abstract

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.