IMM colloquium: Fluorescence spectroscopy with atomic resolution – from fundamental insights toward control of single-molecule electroluminescence

Tuesday 26 March 2024, 4 pm
Preceding lecture by Carmen Grandi - Biophysical Chemistry: 'Tales from the Tails: Deciphering mRNA Kinetics Regulation by Poly(A)-Tails'

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 are often speculative due to the lack of local investigations. The combination of scanning tunneling microscopy (STM) with detection of light emitted from the tunnel junction (referred to as STM-induced luminescence, STML) permits to study single-molecule fluorescence with atomic-scale resolution, while the local environment can be controlled via the utilization of STM manipulation techniques. I will provide two examples to showcase the in-depth fundamental understanding one can gain about excitation and relaxation pathways in the electroluminescent tunnel junction.

I will first present STML results on the fluorescence of individual zink phthalocyanine (ZnPc) molecules [1,2]. We discovered bipolar electrofluorochromism, i.e., depending on the applied junction voltage the molecule can emit light from a neutral, cationic or anionic state. Based on a systematic study that also includes identification of molecular orbitals via scanning tunneling spectroscopy (STS), we resolved a scientific dispute regarding the exciton formation in the molecule. We developed a unified many-body energy diagram that consistently explains all excitation and relaxation pathways of ZnPc leading to electroluminescence and serves as a blueprint for other molecules as well. The results could have relevance beyond STM experiments, for any electroluminescent device.

In my second example, I will show that we can activate fluorescence in a Ni(II) complex via resonant energy transfer from a nearby donor molecule [3]. Usually, most open-shell 3d metal complexes are known to be non-luminescent due to ultrafast intersystem crossing (ISC) and cooling into a dark metal-centered excited state. Our results provide evidence for an ISC activation barrier, which can be utilized to enable fluorescence. This offers new perspectives of utilizing intermolecular interactions in optoelectronic devices, in addition to the established intramolecular chemical design. In my outlook, I will also show that such experiments open possibilities to study the fundamental mechanism of molecular energy transfer with unprecedented detail.

[1]   T.-C. Hung, B. Kiraly, J. H. Strik, A. A. Khajetoorians, and D. Wegner, Plasmon-Driven Motion of an Individual Molecule, Nano Lett. 21, 5006 (2021).

[2]   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).

[3]   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, arXiv:2307.09984.

dr. Daniel Wegner
Tuesday 26 March 2024, 4 pm
Huygens building, HG00.307