Unexpected Quasi‐Axial Conformer in Thermally Activated Delayed Fluorescence DMAC‐TRZ, Pushing Green OLEDs to Blue

Abstract

Hidden photophysics is elucidated in the very well‐known thermally activated delayed fluorescence (TADF) emitter, DMAC‐TRZ. A molecule that, based on its structure, is considered not to have more than one structural conformation. However, based on experimental and computational studies, two conformers, a quasi‐axial (QA) and a quasi‐equatorial (QE) are found, and the effect of their co‐existence on both optical and electrical excitation isexplored. The relative small population of the QA conformer has a disproportionate effect because of its strong local excited state character. The energy transfer efficiency from the QA to the QE conformer is high, even at low concentrations, dependent on the host environment. The current accepted triplet energy of DMAC‐TRZ is shown to originate from the QA conformer, completely changing the understanding of DMAC‐TRZ. The contribution of the QA conformer in devices helps to explain the good performance of the material in non‐doped organic light‐emitting diodes (OLEDs). Moreover, hyperfluorescence (HF) devices, using v‐DABNA emitter show direct energy transfer from the QA conformer to v‐DABNA, explaining the relatively improved Förster resonance energy transfer efficiency compared to similar HF systems. Highly efficient OLEDs where green light (TADF‐only devices) is converted to blue light (HF devices) with the maximum external quantum efficiency remaining close to 30% are demonstrated.

Facile Functionalization of Ambipolar, Nitrogen-Doped PAHs toward Highly Efficient TADF OLED Emitters

“…This structural feature rationalizes the difference in emissive pathways and the observation of a RTP process. Similar behavior was observed in previous studies, where polymeric hosts allowed for quasi-axial conformation and observation of the RTP process, and it was associated with dopant and host interactions. − In the CBP matrix, likewise to Zeonex, the phosphorescence comes from the LE state, where the fluorescence and DF are from mixed LE&CT states. Different conclusions could be extracted from the time-resolved spectra (Figure ).…”

Azaborine as a Versatile Weak Donor for Thermally Activated Delayed Fluorescence

“…Following from recent reports of unexpected multiple conformers in similar DMAC-based emitters, , the potential energy surfaces (PESs) for the two compounds were analyzed by performing a relaxed dihedral angle scan between the AZB donor and TRZ acceptor. The lowest energy conformer of each compound has a D-A dihedral angle of 90° (Figures S20 and S21).…”

Intramolecular locking and coumarin insertion: a stepwise approach for TADF design

“…The PF among the series shows similar spectral trends as observed in the steady-state measurements, indicating an enhanced acceptor strength for ChromPy-Ph-DMAC and DBChromQx-DMAC. A spectral redshift is also observed within the first 100 nanoseconds for all four emitters, both at room temperature and at 80 K. This spectral shift could be attributed to either aggregation and/or a distribution of dihedral angles between donor and acceptor units, 33,34 each exhibiting a specific emission wavelength and emission lifetime. Furthermore, it can be seen that the coumarin-based compounds exhibit detectable DF throughout the entire time window of the measurement, whereas this is not the case for Qx-Ph-DMAC and BQx-Ph-DMAC where the much weaker DF falls below the noise floor of the instrument in the 1-100 ms range.…”

“…ChromPy-Ph-DMAC shows a red-shifted emission in the early μs DF regime, which can be assigned to various different molecular geometries that result from the rotational freedom of this ‘unlocked’ structure as films are formed. 34 This red-shift is indeed less pronounced for the ‘locked’ analogue DBChromQx-DMAC , with more restricted D–A geometries. These red-shifts are in both cases small though, and the DF observed in the coumarin-based dyads appears to originate from the same excited state as the PF, i.e.…”