Abstract
New thermally activated delayed fluorescence (TADF) blue emitter molecules based on the known donor–acceptor–donor (D–A–D)-type TADF molecule, 2,7-bis(9,9-dimethylacridin-10-yl)-9,9-dimethylthioxanthene-S,S-dioxide (DDMA-TXO2), are reported. The motivation for the present investigation is via the use of rational molecular design, based on DDMA-TXO2, to elevate the organic light emitting diode (OLED) performance and obtain deeper blue color coordinates. To achieve this goal, the strength of the donor (D) unit and acceptor (A) units have been tuned with methyl substituents. The methyl functionality on the acceptor was also expected to modulate the D–A torsion angle in order to obtain a blue shift in the electroluminescence. The effect of regioisomeric structures has also been investigated. Herein, we report the photophysical, electrochemical, and single-crystal X-ray crystallography data to assist with the successful OLED design. The methyl substituents on the DDMA-TXO2 framework have profound effects on the photophysics and color coordinates of the emitters. The weak electron-donating methyl groups alter the redox properties of the D and A units and consequently affect the singlet and triplet levels but not the energy gap (ΔEST). By systematically manipulating all of the aforementioned factors, devices have been obtained with acceptor-substituted III with a maximum external quantum efficiency of 22.6% and Commission Internationale de l’Éclairage coordinates of (0.15, 0.18) at 1000 cd m–2.
Highlights
The research on materials that exhibit thermally activated delayed fluorescence (TADF) has increased dramatically since the introduction of TADF materials into organic light-emitting diodes (OLEDs) for lighting and display applications.[1−4] highly efficient green TADF organic light emitting diode (OLED) materials have been widely reported,[5−7] research to create efficient and stable red and especially blue TADF OLEDs has not progressed as rapidly.[8,9]By virtue of the key reverse intersystem crossing step being thermally activated, it is understood that the tripletharvesting efficiency of TADF materials depends critically, but not exclusively, on the singlet-triplet energy gap (ΔEST)
Methyl substituents would shift the D−A torsion angle more to the perpendicular on average, which was a pivotal part of the rational molecular design of III
It is important to note that this effect, presumably due to steric interactions of the donor with the acceptor methyl substituents, raises both the CT and the triplet together, maintaining a low ΔEST critical for efficient TADF
Summary
The research on materials that exhibit thermally activated delayed fluorescence (TADF) has increased dramatically since the introduction of TADF materials into organic light-emitting diodes (OLEDs) for lighting and display applications.[1−4] highly efficient green TADF OLED materials have been widely reported,[5−7] research to create efficient and stable red and especially blue TADF OLEDs has not progressed as rapidly.[8,9]. In line with our previous investigations, the transition at around 280−290 nm can be assigned to local excitation of the D unit.[20] Material IV exhibits a shoulder at 325 nm that is not present in the other molecules, this has been shown to arise from its individual D unit (Figure S2) This indicates that the methyl groups substituted on the D unit clearly affect the electronic structure of the donor. It is important to note that this effect, presumably due to steric interactions of the donor with the acceptor methyl substituents, raises both the CT and the triplet together, maintaining a low ΔEST critical for efficient TADF These OLED data for III are very competitive with the recent efficient devices of TADF emitters with similar CIE coordinates. These devices are deeper-blue than III, and they exhibit considerable roll-off to 14.5 and 8.5% at 100 cd m−2
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