Abstract
Thermally activated delayed fluorescence (TADF) materials, combining high fluorescence quantum efficiency and short delayed emission lifetime, are highly desirable for application in organic light-emitting diodes (OLEDs) with negligible external quantum efficiency (EQE) roll-off. Here, we present the pathway for shortening the TADF lifetime of highly emissive 4,6-bis[4-(10-phenoxazinyl)phenyl]pyrimidine derivatives. Tiny manipulation of the molecular structure with methyl groups was applied to tune the singlet–triplet energy-level scheme and the corresponding coupling strengths, enabling the boost of the reverse intersystem crossing (rISC) rate (from 0.7 to 6.5) × 106 s–1 and shorten the TADF lifetime down to only 800 ns in toluene solutions. An almost identical TADF lifetime of roughly 860 ns was attained also in solid films for the compound with the most rapid TADF decay in toluene despite the presence of inevitable conformational disorder. Concomitantly, the boost of fluorescence quantum efficiency to near unity was achieved in solid films due to the weakened nonradiative decay. Exceptional EQE peak values of 26.3–29.1% together with adjustable emission wavelength in the range of 502–536 nm were achieved in TADF OLEDs. Reduction of EQE roll-off was demonstrated by lowering the TADF lifetime.
Highlights
Molecular structure modification of similar D-s-A-s-D Thermally activated delayed fluorescence (TADF) compounds bearing acridine donor units by methyl units for the minimization of τTADF without the emission redshift was suggested by Komatsu et al.[41]
We presented the synthesis, comprehensive analysis, and device application of a series of TADF compounds based on the phenoxazine−phenylpyrimidine structure with high TADF efficiency and rapid delayed fluorescence decay
The modification of the molecular structure with methyl groups at the acceptor unit was employed to tune the acceptor properties and donor−acceptor interaction, which were shown to have a significant effect on emission properties of TADF compounds
Summary
Electron-accepting (A) units and bounding them in an appropriate molecular geometry.[21] Among the wide variety of possible donor fragments, phenoxazine (PXZ) was shown to be a good candidate for achieving the short TADF lifetime together with a high fluorescence quantum yield[24−32] due to the strong enough electron-donating properties and ability to show large steric hindrance to acceptor units.[21,33] being noncontinuously conjugated, phenoxazine is found in two forms, planar and crooked conformation, and only the planar form shows strong TADF.[34,35] This issue can be solved by weakening the interaction between the phenoxazine and the electron-accepting part of the TADF molecule by introducing the spacer unit, minimizing the population of crooked conformations, and enhancing the number of TADF-active planar orientations.[28,36] The application of such molecular design by using phenoxazine D and suitable pyrimidine (PYR) electron-acceptor (A) together with phenyl spacer units (s) in a D-s-A-s-D layout allowed to achieve highly efficient singleband fluorescence with a TADF lifetime of 2.56 μs together with an EQE value of 19.9%.24. Among the wide variety of possible donor fragments, phenoxazine (PXZ) was shown to be a good candidate for achieving the short TADF lifetime together with a high fluorescence quantum yield[24−32] due to the strong enough electron-donating properties and ability to show large steric hindrance to acceptor units.[21,33] being noncontinuously conjugated, phenoxazine is found in two forms, planar and crooked conformation, and only the planar form shows strong TADF.[34,35] This issue can be solved by weakening the interaction between the phenoxazine and the electron-accepting part of the TADF molecule by introducing the spacer unit, minimizing the population of crooked conformations, and enhancing the number of TADF-active planar orientations.[28,36] The application of such molecular design by using phenoxazine D and suitable pyrimidine (PYR) electron-acceptor (A) together with phenyl spacer units (s) in a D-s-A-s-D layout allowed to achieve highly efficient singleband fluorescence with a TADF lifetime of 2.56 μs together with an EQE value of 19.9%.24 The later modification of a Received: November 25, 2019 Accepted: February 5, 2020 Published: February 5, 2020
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