Intermolecular charge-transfer aggregates enable high-efficiency near-infrared emissions by non-adiabatic coupling suppression

Harnessing the near-infrared (NIR) light is exceedingly important for biomedical sciences, photovoltaics and optical communications. Though urgently needed for flexible, wearable and implantable applications, NIR organic materials suffer from accelerated non-radiative decay rates (k_nr) and the resulted poor luminous efficiencies as governed by the energy gap law. To date, reported endeavors to decelerate k_nr are mainly focusing on suppressing the electron-vibration coupling with the electronic nonadiabatic coupling assumed as a constant.

Recently, Dr. Juan Qiao and colleagues in Tsinghua University revisited the basic theoretical background of the energy gap law and found the suppression of electronic nonadiabatic coupling is a novel approach to attenuate the limitation of the energy gap law. Also, a feasible strategy was demonstrated for organic materials by forming intermolecular charge-transfer aggregates (CTA) to suppress the electronic nonadiabatic coupling and thus achieve high-efficiency NIR emissions.

Experimental and theoretical investigations expose that the formation of CTA with the involvement of intermolecular CT (xCT) in the excited states can not only reduce the nonadiabatic coupling, which leads to a small k_nr, but also stabilize excited-state energies, which induces strong thermally activated delayed fluorescence (TADF) for high-efficiency NIR electroluminescence. The developed CTA emitter exhibits widely-tunable emissions from 621 to 802 nm in solid films with high photoluminescence quantum yields (PLQY) of over 82% in NIR region.

In comparison to the intramolecular CT (iCT) molecule with similar molecular structure and NIR emission around 760 nm, the CTA emitter displays a 6.5 times higher PLQY contributed by a 3.8 times smaller k_nr. The organic light-emitting diodes (OLEDs) with the CTA emitter achieved very high maximum external quantum efficiencies of 24.8%, 17.0%, 13.0% and 6.8% with emission peaks at 675, 710, 730 and 752 nm, respectively, which are among the best results for deep-red and NIR OLEDs based on TADF emitters. It is anticipated that this work paves a superior way for the rational design of high-efficiency NIR organic materials and lays a foundation for the applications of CTA in a wide variety of fields.

 

See the article: Xue J, Xu J, Ren J, Liang Q, Ou Q, Wang R, Shuai Z, Qiao J. Intermolecular charge-transfer aggregates enable high-efficiency near-infrared emissions by nonadiabatic coupling suppression. Sci. China Chem., Doi: 10.1007/s11426-021-1096-8.

https://link.springer.com/article/10.1007%2Fs11426-021-1096-8

 

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