Suppressing dimer formation by increasing conformational freedom in multi-carbazole thermally activated delayed fluorescence emitters

Lubna Salah, Marc K. Etherington, Ali Shuaib, Andrew Danos, Ahmed A. Nazeer, Basma Ghazal, Antonio Prlj, Andrew T. Turley, Abhijit Mallick, Paul R. McGonigal, Basile F.E. Curchod, Andrew P. Monkman, Saad Makhseed*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


Ideal emitters for organic light-emitting diodes (OLEDs) are capable of efficiently harvesting non-emissive triplet states, have high colour stabilities, and possess high photoluminescence quantum yields (PLQYs). Maintaining colour stability and PLQY is particularly challenging for multi-carbazole thermally activated delayed fluorescence (TADF) materials that form persistent dimers due to intermolecular interactions of their extended aromatic systems (with altered electronic states). Addressing this challenge, three new emitters are presented, which demonstrate that, somewhat counterintuitively, sterically uncrowded acceptor units can suppress these undesirable interactions. They do so by allowing the surrounding carbazole donors to be arranged with lower dihedral angles, which in turn limits their availability for dimerization. A new pyrazine-centered emitter 4CzPyz is contrasted directly with the cyanopyridine and terephthalonitrile analogues, 4CzCNPy and 4CzTPN respectively. The pyrazine derivative demonstrates enhanced colour stability in the solid-state compared to the cyanopyridine and terephthalonitrile acceptors, which we assign to its absence of intermolecular face-to-face aromatic interactions. This suppression of dimer formation is shared by two cyanopyrazine emitters 2Cz2CNPyz and 3CzCNPyz, each of which feature reduced steric pressure and flatter Cz-Pyz dihedral angles than non-heterocyclic analogues. Flatter dihedral angles consequently lead to C-H bonds of the Cz donors extending outwards at angles that prevent the stacking required for dimerization. This expanded understanding of dimer formation in TADF materials will guide future efforts to maintain colour stability in higher performance TADF materials by curbing the prevalence of face-to-face aromatic interactions.

Original languageEnglish
Pages (from-to)189-198
Number of pages10
JournalJournal of Materials Chemistry C
Issue number1
Publication statusPublished - 7 Jan 2021

Bibliographical note

Funding Information:
The authors would like to thank Dr Dmitry Yufit for solving the crystal structure of 4CzCNPy. L. S. and S. M. would like to thank Kuwait University Research administration (grant no. SC10/18) for funding this work, and Kuwait Foundation for the Advancement of Science-KFAS; RSP unit general facilities of the Faculty of Science GFS (GS 01/01, GS 02/01, GS 03/01, GS 01/03, GS 01/05, GS 02/13). M. K. E., A. D. and A. P. M. would like to thank the EU’s Horizon 2020 research and innovation programme for funding the HyperOLED project under grant agreement No. 732013. A. T. T. acknowledges the EPSRC for a Doctoral Training Grant. A. M. and P. R. M. thank UUKi and BEIS for funding through a Rutherford Strategic Partner Grant.

Publisher Copyright:
© The Royal Society of Chemistry.

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