Twisted acceptors in the design of deep-blue TADF emitters: crucial role of excited-state relaxation in the photophysics of methyl-substituted s-triphenyltriazine derivatives

Abstract

A series of twisted triaryl-s-triazine derivatives are used as acceptor fragments in design of deep-blue TADF emitters for OLED. Comprehensive photophysical investigations indicate high impact of structural relaxation on the TADF color and efficiency.

Vibrationally Assisted Direct Intersystem Crossing between the Same Charge-Transfer States for Thermally Activated Delayed Fluorescence: Analysis by Marcus–Hush Theory Including Reorganization Energy

“…Another important remark: in TADF emitters bearing conjugated fragments such as phenothiazine, phenoxazine, phenyl- s -triazine, phenylpyrimidine, phenazine, benzophenone, naphthalimide, and so forth, as well as some carbazole derivatives and similar aromatic heterocycles, the 3 LE energies are relatively low and, in non-polar medium, represent the T 1 states. Numerous TADF emitters can serve as examples: PTZ-DBTO2, 30 DPTZ-DPTO2, 31 and their derivatives based on which the three-state model was mainly developed, numerous s -triazine derivatives, 14 − 16 , 29 , 32 DBT-BZ-DMAC, 33 NAI, 34 various indolo[3,2- b ]indole derivatives, 35 POZ-DBPHZ, 36 and so on. When polarity increases, the CT states of such emitters are getting energetically closer to 3 LE, but simultaneously, the energy difference between 1 CT and 3 CT states also decreases, which most likely plays the key role in TADF.…”

“…According to the literature reports, within derivatives bearing similar donor and acceptor fragments, better TADF parameters are observed in the emitters with higher donor/acceptor strength. For example, in toluene, DMAC-TRZ 16 bearing strong s -triazine acceptor shows higher rISC rate (7 × 10 5 s –1 ) as compared to its weaker 2-pyrimidine (no TADF) and 4-pyrimidine analogues (6 × 10 4 s –1 ). 37 In the DPEPO host, despite a larger absolute Δ E 1 CT– 3 LE value, CT states of DMAC-TRZ are more stabilized and afford EQE max of 25%, in contrast to the pyrimidine analogues not exceeding 8% and 12%, respectively.…”

“…DMAC-TRZ was synthesized as reported previously 16 and purified by sublimation in vacuum. Solvents for photophysical measurements were of spectroscopic grade or higher.…”

Not the sum of their parts: understanding multi-donor interactions in symmetric and asymmetric TADF emitters

“…Much work has been carried out to discover chemical motifs that minimize the DE ST gap, and correspondingly maximize rISC. [11][12][13][14][15][16] As a result of this multidisciplinary work, generic design rules for successful TADF emitters have emerged. 17 Primarily, bridging of sterically hindered electron donor (D) and acceptor (A) groups in a twisted D-A architecture commonly results in weakly overlapping highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).…”

Low efficiency roll-off blue TADF OLEDs employing a novel acridine–pyrimidine based high triplet energy host

“…Acridine is often used as a donor (D) in emitter molecules because of its rigidity and strong donating feature. [10][11][12][13] Few cases report its use in hosts for red phosphorescence OLEDs. 14,15 Furthermore, what brought our attention to the acridine group is that it has a higher triplet energy than carbazole (due to less conjugation).…”