Abstract
Three novel donor-acceptor molecules comprising the underexplored pyridazine (Pydz) acceptor moiety have been synthesized and their structural, electrochemical and photophysical properties thoroughly characterized. Combining Pydz with two phenoxazine donor units linked via a phenyl bridge in a meta configuration (dPXZMePydz) leads to high reverse intersystem crossing rate kRISC = 3.9 · 106 s−1 and fast thermally activated delayed fluorescence (TADF) with <500 ns delayed emission lifetime. Efficient triplet harvesting via the TADF mechanism is demonstrated in OLEDs using dPXZMePydz as the emitter but does not occur for compounds bearing weaker donor units.
Highlights
Electroluminescence (EL) from organic light-emitting diodes (OLEDs) originates from the radiative decay of neutral excited states formed by the recombination of holes and electrons
On the other hand, combining Pydz with phenoxazine resulted in emitter dPXZMePydz with moderate green luminescence with PL of 8.5% and a fast delayed emission lifetime of 470 ns in toluene
Upon deposition of a thin film containing dPXZMePydz, PL increased to 10.9% with an estimated high reverse intersystem crossing rate krISC = 3.9 · 106 s−1 and a small singlet-triplet splitting value EST = 86 meV, which was corroborated by TDA-Density functional theory (DFT) calculations
Summary
Electroluminescence (EL) from organic light-emitting diodes (OLEDs) originates from the radiative decay of neutral excited states (excitons) formed by the recombination of holes and electrons. The nature of the donor has no notable effect on the LUMO levels, with electron affinity values of 1.96 eV, 1.98 eV, and 1.93 eV for dPXZMePydz, dDMACMePydz, and dCzMePydz, respectively These results indicate essentially no electronic coupling between the donor and acceptor moieties. According to DFT calculations this compound should show the strongest PL emission since its oscillator strength for S0−1 transition is an order of magnitude larger than in dPXZMePydz and dDMACMePydz compounds This is corroborated with the absorption spectra (Figure 5) where the low energy absorption of dCzMePydz exhibits the highest molar extinction coefficient values. DFT calculations provide a plausible compelling explanation that intersystem crossing to a higher energy triplet state T4 is the most likely transition due to its close proximity to S1 (Supplementary Figure 20C) and the large exchange integral values due to larger overlap of the corresponding orbitals. PE/lm W−1 d 10.8 ± 0.3 transfer to the emitter is not perfectly efficient, this would lead to a loss in PL that is not present in EL
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