High-level reverse intersystem crossing of charge transfer compounds: to fluoresce or not to fluoresce?

Thermally activated delayed fluorescence (TADF) has been widely applied to electroluminescent materials to take the best advantage of triplet excitons. For some materials, the TADF originates from high-level reverse intersystem crossing (hRISC), and has attracted much attention due to its high effic...

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Bibliographic Details
Published in:Physical chemistry chemical physics : PCCP 2024-06, Vol.26 (25), p.1789-17816
Main Authors: Deng, Ziqi, Huang, Guanheng, Li, Jiayu, Pang, Junhong, He, Jiaxing, Phillips, David Lee, Li, Ming-De
Format: Article
Language:English
Subjects:
Charge transfer
Deactivation
Excitons
Luminescence
Optical properties
Substitutes
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Summary:Thermally activated delayed fluorescence (TADF) has been widely applied to electroluminescent materials to take the best advantage of triplet excitons. For some materials, the TADF originates from high-level reverse intersystem crossing (hRISC), and has attracted much attention due to its high efficiency for utilizing the triplet excitons. However, reports concerning the mechanistic studies on the hRISC-TADF process and structure-property correlation are sparse. In this study, we prepared three compounds containing triphenylamine and benzophenone with different substitution positions, o -TPA-BP, m -TPA-BP, and p -TPA-BP, in which only p -TPA-BP displays strong luminescence and hRISC-TADF features. To investigate the mechanism of the substituent-position-dependent hRISC-TADF, ultrafast time-resolved spectroscopy was utilized to observe the deactivation pathways with the assistance of theoretical calculations. The results show that o -TPA-BP will not generate triplet species, and the triplet species for m -TPA-BP will rapidly deactivate. Only p -TPA-BP can transition back to the singlet state from the T 2 state effectively and exhibit a large gap between T 1 and T 2 to favor the hRISC route. These results illustrate how the substitution position affects the ISC and further influences the luminescence properties, which can provide new insights for developing new high-efficiency luminescent materials. The mechanism of hRISC-TADF for p -TPA-BP: the delayed PL spectrum on the right side reveals the actual excited-state energy levels of this molecule.
ISSN:1463-9076
1463-9084
1463-9084
DOI:10.1039/d4cp01596g