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.

Regio- and conformational isomerization critical to design of efficient thermally-activated delayed fluorescence emitters

“…We have shown that DF reaches a resonant maximum at the point when the CT- 3 LE gap approaches zero, verifying the second-order spin-vibronic mechanism in this system7. The fits of the emission decay curves at different temperatures show that the prompt 1 CT eq lifetime also follows the resonant behaviour, reaching the longest lifetime of 47 ns at the CT- 3 LE zero gap point.…”

“…( c ) Energy positions of the 1 CT and 3 LE states for the molecules 2,8-DPTZ-DBTO 2 (LHS) and 3,7-DPTZ-DBTO 2 (RHS). Changing from MCH to toluene for 2,8-DPTZ-DBTO 2 moves from type II–III TADF, whereas the equatorial conformer D–A pair (eq)3,7-DPTZ-DBTO 2 is type III in MCH and remains so in toluene (see Etherington et al 7. for notation).…”

Thermally Activated Delayed Fluorescence in Polymer–Small-Molecule Exciplex Blends for Solution-Processed Organic Light-Emitting Diodes

“…Previous studies suggested that the ISC and RISC processes due to spin−orbit coupling between pure singlet and triplet charge transfer states is forbidden. 5,47,56,57 However, in the case of intermolecular exciplexes, where negligible energy gap between 1 CT and 3 CT is achieved, hyperfine interactions might become predominant and originate RISC with negligible loss, as no low energy triplet exists below the CT states.…”

Synthesis and Exciton Dynamics of Donor-Orthogonal Acceptor Conjugated Polymers: Reducing the Singlet–Triplet Energy Gap

“…The character of this CT-3 (π-π*) mixture also evolves slightly over its lifetime, which we speculate could be attributed to relaxation of the low-energy vibrational modes coupling the two potential energy surfaces. 34 At no point do we identify a pure, diabatic triplet or CT configuration, but these may be transiently populated, indicating the conventional picture of RISC may not be as simple as suggested. 33,[35][36] It is this mixing with intermediate CT states which enables RISC to occur: the CT component of CT-3 (π-π*) enables a relatively fast (thermally activated) spin flip, resulting in a 1 CT configuration which in turn can emit via mixing with the 1 (π-π*) state.…”