A computational journey through thermally activated delayed fluorescence materials

Abstract

Thermally Activated Delayed Fluorescence (TADF) process is the new paradigm for Organic Light-Emitting Diodes (OLEDs). Despite all the efforts, a complete mechanistic understanding of TADF materials has not been fully uncovered yet. Part of the complexity arises from the apparent dichotomy between the need for small energy difference between the lowest singlet and triplet excited states (dEST) which has to carry a significant charge transfer (CT) character; and for a significant spin-orbit coupling which according to El-Sayed rules requires the involved singlet and triplet excited states to have very different natures. In this contribution, we will show: (i) How this dichotomy can be resolved once accounting in a fully atomistic model of reference carbazole derivatives for thermal fluctuations of the molecular conformations and discrete electronic polarization effects in amorphous films. Using both computational and experimental techniques, we demonstrate that, electronic excitations involved in the TADF process have a mixed CT-locally excited character being dynamically tuned by torsional vibrational modes. Hence, we will demonstrate that the conversion of triplet-to-singlet and light emission in TADF materials are both electronic processes that are vibrationally-assisted. (ii) How doped triangle-shaped molecules can lead to (i) concomitant narrow emission, high quantum yield of emission and small dEST resulting in a whole new generation of TADF emitters, the multi-resonant TADF emitters and to (ii) a new family of compounds with an inverted singlet-triplet gap and potentially, a downwards energy RISC. To do so, we rely on high level quantum chemical calculations and show that an accurate description of electron correlation effects is key to correctly predict the excited states ordering as well as the optical properties of these compounds. (iii) How the interactions in the solid state can turn RISC from a SOC-driven to a hyperfine interaction (HFI)-driven mechanism. Combining time-resolved and transient electron paramagnetic resonance spectroscopies as well as (time-dependent) density functional theory calculations, we demonstrated that HFI-RISC occurs through delocalized charge transfer states in a curcuminoid derivative.