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

• Published in: Nature communications Vol. 12; no. 1; pp. 6640 - 10
• Main Authors: Gillett, Alexander J., Tonnelé, Claire, Londi, Giacomo, Ricci, Gaetano, Catherin, Manon, Unson, Darcy M. L., Casanova, David, Castet, Frédéric, Olivier, Yoann, Chen, Weimin M., Zaborova, Elena, Evans, Emrys W., Drummond, Bluebell H., Conaghan, Patrick J., Cui, Lin-Song, Greenham, Neil C., Puttisong, Yuttapoom, Fages, Frédéric, Beljonne, David, Friend, Richard H.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 17.11.2021; Nature Publishing Group; Nature Portfolio
• Subjects: 140/125; 639/301/1019/1020/1091; 639/624/1020/1091; Absorption; Absorptivity; Charge transfer; Chemical Sciences; Emitters; Energy gap; Excitons; Fluorescence; Holes (electron deficiencies); Humanities and Social Sciences; Magnetic resonance; Material chemistry; multidisciplinary; or physical chemistry; Orbital resonances (celestial mechanics); Organic semiconductors; Photoluminescence; Photons; Science; Science (multidisciplinary); Semiconductors; Spin-orbit interactions; Theoretical and; Organic LEDs
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-021-26689-8

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. A low singlet-triplet energy gap, necessary for delayed fluorescence organic semiconductors, results in a small radiative rate that limits performance in OLEDs. Here, the authors show that it is possible to reconcile these conflicting requirements in materials that can access both high oscillator strength intramolecular excitations and intermolecular charge transfer states.