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Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors

Abstract : Engineering a low singlet-triplet energy gap ( ΔE ST ) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient = 3.8 × 10 5 cm −1 ) and a relatively large ΔE ST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (delayed lifetime = 260 μs), but in aggregated films, BF2 generates intermolecular charge transfer (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a ~24 ns timescale and have an average electron-hole separation of ≥1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states, which is possible even at low BF2 doping concentrations of 4 wt%, resolves the conflicting requirements of fast radiative emission and low ΔE ST in organic DF emitters.
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Submitted on : Friday, November 19, 2021 - 10:24:27 AM
Last modification on : Sunday, June 26, 2022 - 3:20:04 AM

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Alexander Gillett, Claire Tonnelé, Giacomo Londi, Gaetano Ricci, Manon Catherin, et al.. Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors. Nature Communications, Nature Publishing Group, 2021, 12, ⟨10.1038/s41467-021-26689-8⟩. ⟨hal-03436081⟩

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