title: Revealing the spin-vibronic coupling mechanism of thermally activated delayed fluorescence.
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.
authors.affiliation: Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Marc K
authors.identifier: 0000-0003-2101-5757
authors.initials: MK
authors.lastname: Etherington
authors.affiliation: School of Chemistry, Newcastle University, Newcastle upon Tyne NE1 7RU, UK.
authors.forename: Jamie
authors.initials: J
authors.lastname: Gibson
authors.affiliation: Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Heather F
authors.initials: HF
authors.lastname: Higginbotham
authors.affiliation: School of Chemistry, Newcastle University, Newcastle upon Tyne NE1 7RU, UK.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Penfold
authors.affiliation: Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Monkman