Delayed fluorescence from inverted singlet and triplet excited states

Naoya Aizawa (), Yong-Jin Pu (), Yu Harabuchi, Atsuko Nihonyanagi, Ryotaro Ibuka, Hiroyuki Inuzuka, Barun Dhara, Yuki Koyama, Ken-ichi Nakayama, Satoshi Maeda, Fumito Araoka and Daigo Miyajima ()
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Naoya Aizawa: RIKEN Center for Emergent Matter Science (CEMS)
Yong-Jin Pu: RIKEN Center for Emergent Matter Science (CEMS)
Yu Harabuchi: Hokkaido University
Atsuko Nihonyanagi: RIKEN Center for Emergent Matter Science (CEMS)
Ryotaro Ibuka: RIKEN Center for Emergent Matter Science (CEMS)
Hiroyuki Inuzuka: RIKEN Center for Emergent Matter Science (CEMS)
Barun Dhara: RIKEN Center for Emergent Matter Science (CEMS)
Yuki Koyama: RIKEN Center for Emergent Matter Science (CEMS)
Ken-ichi Nakayama: Osaka University
Satoshi Maeda: Hokkaido University
Fumito Araoka: RIKEN Center for Emergent Matter Science (CEMS)
Daigo Miyajima: RIKEN Center for Emergent Matter Science (CEMS)

Nature, 2022, vol. 609, issue 7927, 502-506

Abstract: Abstract Hund’s multiplicity rule states that a higher spin state has a lower energy for a given electronic configuration1. Rephrasing this rule for molecular excited states predicts a positive energy gap between spin-singlet and spin-triplet excited states, as has been consistent with numerous experimental observations over almost a century. Here we report a fluorescent molecule that disobeys Hund’s rule and has a negative singlet–triplet energy gap of −11 ± 2 meV. The energy inversion of the singlet and triplet excited states results in delayed fluorescence with short time constants of 0.2 μs, which anomalously decrease with decreasing temperature owing to the emissive singlet character of the lowest-energy excited state. Organic light-emitting diodes (OLEDs) using this molecule exhibited a fast transient electroluminescence decay with a peak external quantum efficiency of 17%, demonstrating its potential implications for optoelectronic devices, including displays, lighting and lasers.

Date: 2022
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DOI: 10.1038/s41586-022-05132-y

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