Würzburg ToCoTronics Colloquium

In organic light emitting devices, electroluminescence occurs as a consequence of electron and hole transfer into an emitter molecule. The exact excitation and relaxation pathways can be surprisingly complex and often remain a “black box”. The combination of scanning tunnel-ing microscopy (STM) with detection of light emitted from the tunnel junction (STM-induced luminescence, STML) permits to study single-molecule fluorescence with atomic-scale resolu- tion.

Our recent discovery of bipolar electrofluorochromism in the electroluminescence of individual zink phthalocyanine (ZnPc) molecules permitted us to map out what happens inside the black box [1]. Depending on the applied junction voltage the molecule can emit light from a neutral, cationic or anionic state, each characterized by a distinct emission color. Combing these ob- servations with results from scanning tunneling spectroscopy (STS) to identify molecular orbit- als, we develop a unified many-body energy diagram that consistently explains all excitation and relaxation pathways leading to electroluminescence of ZnPc. The results could have rel- evance beyond STM experiments, for any electroluminescent device.

Combining the high spatial resolution with STM manipulation also permits to study molecular interactions with the local environment. As an example, we were able to activate fluorescence in a “dark” Ni(II) complex via resonant energy transfer from a nearby donor molecule [2]. The results allow to map out a detailed potential landscape of the involved many-body states, and provide novel perspectives of utilizing intermolecular interactions in optoelectronic devices. On a fundamental level, STML opens up possibilities to study the fundamental mechanism of mo- lecular energy transfer with unprecedented detail – and with possible surprises.

[1] T.-C. Hung, R. Robles, B. Kiraly, J. H. Strik, B. A. Rutten, A. A. Khajetoorians, N. Lorente and D. Wegner, Bipolar single-molecule electroluminescence and electrofluorochromism, Phys. Rev. Res. 5, 033027 (2023).
[2] T.-C. Hung, Y. Godinez Loyola, M. Steinbrecher, B. Kiraly, A. A. Khajetoorians, N. L. Doltsinis, C. A Strassert and D. Wegner, Activating the fluorescence of a Ni(II) complex by energy transfer, J. Am. Chem. Soc. 146, 8858 (2008).