Influential boron-oxygen interactions for blue emitters with high rate of reverse intersystem crossing Joseph Cameron 1 , Suman Kuila 2 , Andrew P. Monkman 2 , Peter J. Skabara 1 1 University of Glasgow, UK, 2 Durham University, UK email: joseph.cameron@glasgow.ac.uk For the development of efficient organic light-emitting diode (OLED) displays, it is critical that the efficiency roll-off and device lifetime of blue OLEDs is improved. Emissive materials which undergo thermally activated delayed fluorescence (TADF) can efficiently harvest the triplet states that are generated in the emissive layer but devices using these materials typically have a lifetime of 100s of hours at best and exhibit efficiency roll-off. Therefore, it is imperative that new blue emitters are developed. The key to improving device lifetime and roll-off is to increase the rate of reverse intersystem crossing (rISC) and reduce radiative decay lifetimes as the excited state energy levels in blue emitters are close to the energy required for covalent bond dissociation. One method of increasing rISC is to achieve conformational control to avoid the presence of many different conformers, which causes slower radiative decay. 1 Nonetheless, it is also important that donor and acceptor units in emissive materials are not completely fixed to allow for rotational freedom for the vibrational coupling mechanism which is important for TADF. 2 Therefore, there is a balance to be struck between conformational control and constraining the molecule. Non-covalent interactions are a potentially effective means to achieve this difficult balance as such interactions are weak, yet significant enough to dictate conformation. In this work we present new blue emitter materials which feature influential boron-oxygen interactions. The impact of these interactions on molecular structure and the resulting physical properties will be discussed with X-ray crystallography data, cyclic voltammetry and photophysics studies presented. The boron-oxygen interactions strongly influence the photophysical properties, with the compound with most boron-oxygen contacts, CzOMe 2 BMes 2 , exhibiting the highest photoluminescence quantum yield (87%) of the series of emitters, with a rISC rate of ~10 6 s -1 . References
1. Eng J. et al. , J. Mater. Chem. C , 2019, 7 , 12942-12952 2. Ward J. S. et al. , Chem. Commun. , 2016, 52 , 2612-2615
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