Molecular Design Strategies for Color Tuning of Blue TADF Emitters

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.