Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors.
Unson, Darcy M L
Conaghan, Patrick J
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Gillett, A. J., Tonnelé, C., Londi, G., Ricci, G., Catherin, M., Unson, D. M. L., Casanova, D., et al. (2021). Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors.. Nature communications, 12 (1) https://doi.org/10.1038/s41467-021-26689-8
Engineering a low singlet-triplet energy gap (ΔE<sub>ST</sub>) 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<sup>5 </sup>cm<sup>-1</sup>) and a relatively large ΔE<sub>ST</sub> 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<sub>ST</sub> in organic DF emitters.
RCUK | Engineering and Physical Sciences Research Council (EPSRC) (EP/M005143/1, EP/M01083X/1)
Simons Foundation (601946)
European Research Council (670405)
External DOI: https://doi.org/10.1038/s41467-021-26689-8
This record's URL: https://www.repository.cam.ac.uk/handle/1810/332387
Attribution 4.0 International
Licence URL: https://creativecommons.org/licenses/by/4.0/