Electron spin resonance resolves intermediate triplet states in delayed fluorescence

Bluebell H. Drummond, Naoya Aizawa, Yadong Zhang, William K. Myers, Yao Xiong, Matthew W. Cooper, Stephen Barlow, Qinying Gu, Leah R. Weiss, Alexander J. Gillett, Dan Credgington, Yong-Jin Pu, Seth R. Marder and Emrys W. Evans ()
Additional contact information
Bluebell H. Drummond: University of Cambridge
Naoya Aizawa: RIKEN Center for Emergent Matter Science (CEMS)
Yadong Zhang: Georgia Institute of Technology
William K. Myers: University of Oxford, Inorganic Chemistry Laboratory
Yao Xiong: Georgia Institute of Technology
Matthew W. Cooper: Georgia Institute of Technology
Stephen Barlow: Georgia Institute of Technology
Qinying Gu: University of Cambridge
Leah R. Weiss: University of Cambridge
Alexander J. Gillett: University of Cambridge
Dan Credgington: University of Cambridge
Yong-Jin Pu: RIKEN Center for Emergent Matter Science (CEMS)
Seth R. Marder: Georgia Institute of Technology
Emrys W. Evans: University of Cambridge

Nature Communications, 2021, vol. 12, issue 1, 1-11

Abstract: 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.

Date: 2021
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DOI: 10.1038/s41467-021-24612-9

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