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Taking a snapshot of the triplet excited state of an OLED organometallic luminophore using X-rays

May 1, 2020

(from PSI news)
Alvra performed it's very first Pilot Experiment in December 2017, where the facility could only reach 2.3 keV and only a very bare-bones installation of equipment was available at the Prime experimental station. The plan was to investigate a Cu-P OLED material from Dr. Matthias Vogt(link is external) (then Uni. Bremen, now Martin-Luther-Universität Halle-Wittenberg) that had been investigated by Dr. Grigory Smolentsev(link is external) at both the ESRF, using wide-angle X-ray scattering (WAXS), and the SuperXAS beamline of the SLS, using X-ray absorption near-edge spectroscopy at the Cu K-edge (9 keV). Both experiments performed at storage rings were looking at the long-lived  triplet excited state of the complex, which is responsible for the energy storage properties of the OLED. This triplet population can then be back-transferred to the luminescent singlet state, resulting in so-called thermally activated delayed fluorescence(link is external) (TADP). TADP is why this material produces this intense green emission, and understanding the interplay between the singlet and triplet states is key to engineering OLED materials. 

Picture showing a liquid jet of the CuPCP OLED molecule in acetonitrile solution being illuminated by 450 nm laser light which causes the OLED to glow an intense green colour.

The results of these X-ray experiments(link is external) has just been published in Nature Communications, where by combining the three different experimental results the structure and electronic character of the triplet excited state has been identified. We wrote up a blog entry(link is external) on the result for the Nature Chemistry Community blogs and a PSI Research Highlight was also written, but perhaps the most exciting outcome was an interview with RSI(link is external) in Italian with Alvra's very own Dr. Claudio Cirelli(link is external). These results provide insight into why this particular molecule is such an efficient OLED, and provides clues as to how such materials can be used in the future for optoelectronic devices.

Reference: Smolentsev, G., Milne, C.J., Guda, A., Haldrup, K., Szlachetko, J., Azzaroli, N., Cirelli, C., Knopp, G., Bohinc, R., Menzi, S., Pamfilidis, G., Gashi, D., Beck, M., Mozzanica, A., James, D., Bacellar, C., Mancini, G.F., Tereshchenko, A., Shapovalov, V., Kwiatek, W.M., Czapla-Masztafiak, J., Cannizzo, A., Gazzetto, M., Sander, M., Levantino, M., Kabanova, V., Rychagova, E., Ketkov, S., Olaru, M., Beckmann, J., and Vogt, M. (2020). Taking a snapshot of the triplet excited state of an OLED organometallic luminophore using X-rays. Nature Commun. 11, 2131. (10.1038/s41467-020-15998-z)

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