Delayed fluorescence from inverted singlet and triplet excited states

• Published in: Nature (London) Vol. 609; no. 7927; pp. 502 - 506
• Main Authors: Aizawa, Naoya, Pu, Yong-Jin, Harabuchi, Yu, Nihonyanagi, Atsuko, Ibuka, Ryotaro, Inuzuka, Hiroyuki, Dhara, Barun, Koyama, Yuki, Nakayama, Ken-ichi, Maeda, Satoshi, Araoka, Fumito, Miyajima, Daigo
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 15.09.2022; Nature Publishing Group
• Subjects: 140/125; 140/131; 639/624/1020/1091; 639/638/440; Atoms & subatomic particles; Decay rate; Efficiency; Energy; Energy gap; Excitation; Fluorescence; Humanities and Social Sciences; Lasers; multidisciplinary; Nitrogen; Optoelectronic devices; Organic light emitting diodes; Quantum efficiency; Science; Science (multidisciplinary)
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/s41586-022-05132-y

Hund’s multiplicity rule states that a higher spin state has a lower energy for a given electronic configuration 1 . 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. A fluorescent molecule is described that does not follow Hund’s rule and instead shows singlet and triplet excited states with inverted energy levels, leading to high-efficiency OLEDs with potential implications for optoelectronic devices.