Abstract
Molecular organic fluorophores are currently used in organic light-emitting diodes, though non-emissive triplet excitons generated in devices incorporating conventional fluorophores limit the efficiency. This limit can be overcome in materials that have intramolecular charge-transfer excitonic states and associated small singlet-triplet energy separations; triplets can then be converted to emissive singlet excitons resulting in efficient delayed fluorescence. However, the mechanistic details of the spin interconversion have not yet been fully resolved. We report transient electron spin resonance studies that allow direct probing of the spin conversion in a series of delayed fluorescence fluorophores with varying energy gaps between local excitation and charge-transfer triplet states. The observation of distinct triplet signals, unusual in transient electron spin resonance, suggests that multiple triplet states mediate the photophysics for efficient light emission in delayed fluorescence emitters. We reveal that as the energy separation between local excitation and charge-transfer triplet states decreases, spin interconversion changes from a direct, singlet-triplet mechanism to an indirect mechanism involving intermediate states.
Highlights
Molecular organic fluorophores are currently used in organic light-emitting diodes, though non-emissive triplet excitons generated in devices incorporating conventional fluorophores limit the efficiency
Blue emitters based on the same approach have remained elusive, because the long lifetimes of highenergy triplet excitons lead to organic light-emitting diodes (OLEDs) degradation[12,13]
There is general agreement that Thermally activated delayed fluorescence (TADF) emitters should be designed to have small singlet-triplet exchange energies that promote rapid forward and reverse intersystem crossing (ISC) between singlet and triplet excitons; such small exchange energies can be engineered by spatial separation of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO)[1,2,3,4,5,6]
Summary
Design and detection of multiple triplet excitonic states. Three donor-acceptor emitters sharing a common 4-(3,6-di-tert-butyl-. ZFS parameters (|D | , | E | = 108, 9 mT), the position of the halffield transition (161 mT, Supplementary Fig. 4), and EEEAAA polarisation pattern of DTCz-DPS-1 are all typical of triplet states in organic chromophores[51] These parameters are consistent with the 3LE associated with the carbazole monomer[53], leading us to attribute the trESR signal to a triplet exciton localised on the carbazole donor. Optimised geometries and molecular orbitals were established for the low-lying excitonic states of DTCz-DPS-1, -2, and -3 (Fig. 3a–c) using the range-separated LC-BLYP functional[56] The calculations predicted both 3LE and 3CT in close proximity to the 1CT, supporting our experimental observation of both triplets being populated in trESR and PL measurements in frozen toluene. No significant differences in trESR signal were observed, supporting our interpretation of the mechanisms responsible for triplet state formation as all being intramolecular processes, rather than concentration-dependent aggregate states or intermolecular energy transfer (Supplementary Fig. 5)
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