Revealing the spin-vibronic coupling mechanism of thermally activated delayed fluorescence.

Marc K Etherington,Jamie Gibson,Heather F Higginbotham,Andrew P Monkman,Thomas J Penfold

doi:10.1038/ncomms13680

Abstract

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.

Highlights

Knowing the underlying photophysics of thermally activated delayed fluorescence (TADF) allows proper design of high efficiency organic light-emitting diodes
This work shows that the energy separation between the local triplet state 3CT to the local exciton triplet (3LE) and the charge transfer (CT) states of a donor–acceptor– donor (D–A–D) molecule critically controls the efficiency of reverse intersystem crossing (rISC) and the TADF process
This supports the second order coupling responsible for rISC20, which invokes the role of an intermediate triplet state

Summary

Introduction

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. Our recent experimental[19] and quantum dynamics studies[20] suggest that the intersystem crossing (ISC) and reverse intersystem crossing (rISC) between 1CT singlet and 3CT triplet states is a more complicated second order process This second order process is mediated by vibronic coupling of the 3CT to the local exciton triplet (3LE) states to allow spin–orbit coupling to the 1CT state[18,20,21]. This is because, approximation, the spin–orbit within operator thÀe one HSO1⁄4z

Methods

Results

Conclusion