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

Alexander J. Gillett (), Claire Tonnelé, Giacomo Londi, Gaetano Ricci, Manon Catherin, Darcy M. L. Unson, David Casanova, Frédéric Castet, Yoann Olivier, Weimin M. Chen, Elena Zaborova, Emrys W. Evans, Bluebell H. Drummond, Patrick J. Conaghan, Lin-Song Cui, Neil C. Greenham, Yuttapoom Puttisong (), Frédéric Fages (), David Beljonne () and Richard H. Friend ()
Additional contact information
Alexander J. Gillett: University of Cambridge
Claire Tonnelé: Donostia International Physics Centre (DIPC)
Giacomo Londi: Université de Mons
Gaetano Ricci: Université de Namur
Manon Catherin: Aix Marseille Univ, CNRS, CINaM UMR 7325, AMUtech, Campus de Luminy
Darcy M. L. Unson: University of Cambridge
David Casanova: Donostia International Physics Centre (DIPC)
Frédéric Castet: Université de Bordeaux
Yoann Olivier: Université de Namur
Weimin M. Chen: Chemistry and Biology (IFM) Linköping University
Elena Zaborova: Aix Marseille Univ, CNRS, CINaM UMR 7325, AMUtech, Campus de Luminy
Emrys W. Evans: University of Cambridge
Bluebell H. Drummond: University of Cambridge
Patrick J. Conaghan: University of Cambridge
Lin-Song Cui: University of Cambridge
Neil C. Greenham: University of Cambridge
Yuttapoom Puttisong: Chemistry and Biology (IFM) Linköping University
Frédéric Fages: Aix Marseille Univ, CNRS, CINaM UMR 7325, AMUtech, Campus de Luminy
David Beljonne: Université de Mons
Richard H. Friend: University of Cambridge

Nature Communications, 2021, vol. 12, issue 1, 1-10

Abstract: Abstract Engineering a low singlet-triplet energy gap (ΔEST) 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 × 105 cm−1) and a relatively large ΔEST 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 ΔEST in organic DF emitters.

Date: 2021
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DOI: 10.1038/s41467-021-26689-8

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