Unexpected quasi-axial conformer in thermally activated delayed fluorescence DMAC-TRZ. Demonstrating how to turn green OLEDs into blue.

Abstract

Although there have been several studies on the mechanism of thermally activated delayed fluorescence (TADF), providing useful models to understand the behaviour of TADF molecules, some photophysical features cannot be explained. Here, we investigate the hidden phenomena taking place in the very well-known TADF emitter, DMAC-TRZ. A molecule that, based on its structure, was considered not to fulfil the required criteria to have more than one ground and excited state conformation. However, based on experimental and computational studies, we have found two conformers, a quasi-axial (QA) and a quasi-equatorial (QE), and explored the effect of their co-existence on both optical and electrical excitation. Although the relative population of the QA conformer appears to be small, its effect is disproportionate because it has high local excited state character. The energy transfer efficiency from the QA to the QE conformer is shown to be high, even at low concentrations, and changes depending on the hosting environment. The currently known triplet energy of DMAC-TRZ quoted from experiment is shown to originate from the QA conformer, completely changing the understanding we have so far for this donor-acceptor molecule. The contribution of the QA conformer in devices has been explored and its presence explains the good performance of the material in neat emissive layer devices. Moreover, hyperfluorescnece (HF) devices, using v-DABNA as the terminal emitter show direct energy transfer from the QA conformer to v-DABNA, explaining the relative improved Förster resonance energy transfer (FRET) efficiency compared to similar HF systems. Using this approach, we demonstrate highly efficient organic light emitting diodes (OLEDs) were green light (TADF only devices) is converted to blue light (HF devices) with the maximum external quantum efficiency remaining high and close to 30%.