Persistent Dimer Emission in Thermally Activated Delayed Fluorescence Materials

Marc K. Etherington*, Nadzeya A. Kukhta, Heather F. Higginbotham, Andrew Danos, Aisha N. Bismillah, David R. Graves, Paul R. McGonigal, Nils Haase, Antonia Morherr, Andrei S. Batsanov, Christof Pflumm, Vandana Bhalla, Martin R. Bryce, Andrew P. Monkman

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


We expose significant changes in the emission color of carbazole-based thermally activated delayed fluorescence (TADF) emitters that arise from the presence of persistent dimer states in thin films and organic light-emitting diodes (OLEDs). Direct photoexcitation of this dimer state in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) reveals the significant influence of dimer species on the color purity of its photoluminescence and electroluminescence. The dimer species is sensitive to the sample preparation method, and its enduring presence contributes to the widely reported concentration-mediated red shift in the photoluminescence and electroluminescence of evaporated thin films. This discovery has implications on the usability of these, and similar, molecules for OLEDs and explains disparate electroluminescence spectra presented in the literature for these compounds. The dimerization-controlled changes observed in the TADF process and photoluminescence efficiency mean that careful consideration of dimer states is imperative in the design of future TADF emitters and the interpretation of previously reported studies of carbazole-based TADF materials.

Original languageEnglish
Pages (from-to)11109-11117
Number of pages9
JournalJournal of Physical Chemistry C
Issue number17
Publication statusPublished - 2 May 2019

Bibliographical note

Funding Information:
We thank W. D. Carswell for obtaining DSC and TGA data, Gary Oswald for collecting the powder X-ray diffraction patterns, and Dr D. S. Yufit for help with the single crystal X-ray study. The Diamond Light Source (RAL) is thanked for the award of instrument time on Station I19 (MT 11145) and the instrument scientists for their kind support. Funding: M.K.E., H.F.H., and A.P.M. acknowledge the EU’s Horizon 2020 research and innovation programme for funding the PHEBE project under Grant No. 641725. M.K.E., A.D., N.H., C.P., A.M., M.R.B., and A.P.M. also acknowledge the EU’s Horizon 2020 research and innovation programme for funding the HyperOLED project under grant agreement No. 732013. A.P.M. and M.R.B. also acknowledge the EPSRC for funding under grant number EP/L02621X/1. A.N.B. is supported by an EPSRC doctoral training grant. P.R.M. and A.P.M. thank BEIS and UUKi for support through a Rutherford Strategic Partner Grant. D.R.G. thanks Merck KGaA for funding his PhD studies.

Publisher Copyright:
© 2019 American Chemical Society.

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