Molecular Design Strategies for Color Tuning of Blue TADF Emitters

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 (ΔE ST ). 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'E ́clairage coordinates of (0.15, 0.18) at 1000 cd m −2 .

Nanosecond-time-scale delayed fluorescence molecule for deep-blue OLEDs with small efficiency rolloff

“…Probably wide recombination and exciton formation width may contribute it. Thus, it should be considered to improve total device performance from the aspect of comprehensive control of delayed lifetime, charge carrier transport, and recombination characteristics 48 .…”

Revealing resonance effects and intramolecular dipole interactions in the positional isomers of benzonitrile-core thermally activated delayed fluorescence materials

“…This is in contrast to more common steric control strategies engineered using repulsive contact forces between neighbouring donors or acceptor substituents, as employed in other TADF materials. 14,[16][17][18]34,42,47 In any case, observation of uniformly orthogonal chromophore arrangements means that the differences in photophysical properties between the DMAC-BZN isomers cannot be attributed to different D-A angles. For further and more detailed structural comparisons the experimentally obtained X-ray crystal structures were taken to represent the ground-state geometries.…”

“…We are therefore able to reattribute the differences in TADF performance to the differences in BZN electron withdrawing strength at the meta-and para-positions, consistent with the different resonance and inductive effects around the acceptor unit aromatic p-system. Previously examined in terms of ''linear'' and ''angular'' isomers, 15,[32][33][34] this newly understood feature of TADF design mirrors the directing rules from organic chemistry, making it a timely and highly appropriate inclusion in this themed collection celebrating the career achievements of Martin Bryce.…”

Bond Rotations and Heteroatom Effects in Donor–Acceptor–Donor Molecules: Implications for Thermally Activated Delayed Fluorescence and Room Temperature Phosphorescence

“…The ideal host for blue materials needs to have a high triplet level (so as not to quench the high triplet emitter) as well as favorable carrier injection and transporting properties. 18 The host also needs to provide an environment of suitable polarity, relaxing the energy of the 1 CT state toward that of the polarityindependent 3 LE state and thus minimizing the energy barrier to rISC. 19 In this way, the rISC rate (k rISC ) is optimized, and improved TADF and triplet harvesting can be achieved.…”