Revealing the spin–vibronic coupling mechanism of thermally activated delayed fluorescence

By Marc K. Etherington, J. Gibson, Heather F. Higginbotham, Thomas J. Penfold, Andy P. Monkman. Nature Communications, 2016

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Knowing the underlying photophysics of thermally activated delayed fluorescence (TADF) allows proper design of high efficiency organic light-emitting diodes. We have proposed a model to describe reverse intersystem crossing (rISC) in donor–acceptor charge transfer molecules, where spin–orbit coupling between singlet and triplet states is mediated by one of the local triplet states of the donor (or acceptor). This second order, vibronically coupled mechanism describes the basic photophysics of TADF. Through a series of measurements, whereby the energy ordering of the charge transfer (CT) excited states and the local triplet are tuned in and out of resonance, we show that TADF reaches a maximum at the resonance point, substantiating our model of rISC. Moreover, using photoinduced absorption, we show how the populations of both singlet and triplet CT states and the local triplet state change in and out of resonance. Our vibronic coupling rISC model is used to predict this behaviour and describes how rISC and TADF are affected by external perturbation. Knowing the photophysics of thermally-activated delayed fluorescence (TADF) is crucial when designing organic light emitting diodes. Here the authors show that spin orbit coupling in TADF materials is described by a second order vibronic coupling mechanism, and demonstrate the importance of resonance effects to achieve efficient TADF.

Subfields: Electrical and Electronic Engineering, Physical and Theoretical Chemistry

View Paper: https://www.semanticscholar.org/paper/8ab2cf7e93a677295f2682a67b3d214a3e45e0d2