Selenium Substitution Enhances Reverse Intersystem Crossing in a Delayed Fluorescence Emitter

Abstract

Organic emitters exhibiting delayed fluorescence (DF) are promising luminescent materials for next-generation organic light-emitting diodes (OLEDs). Faster intersystem crossing rates and shorter emission lifetimes can be achieved in luminescent molecules through the incorporation of heavy atoms, which enhance spin−orbit coupling and promote intersystem crossing between singlet and triplet states. DF molecules often contain a sulfur atom, and reports of selenium-containing DF OLEDs also exist. However, the literature lacks a direct exploration of the effect of spin−orbit coupling on reverse intersystem crossing in a delayed fluorescence emitter by the substitution of selenium for sulfur. Here we show that substitution of selenium for sulfur in a modified thioxanthenone-triphenylamine analogue increases the rate of forward intersystem crossing by a factor of over 250 and the rate of reverse intersystem by a factor of 22. We attribute the increased rates to enhanced spin−orbit coupling from heavy atom substitution, and computational and electron spin resonance studies support this. This work provides an insight into future molecular design strategies for heavy-atomcontaining, DF emitters.

Enhancement of Reverse Intersystem Crossing in Charge‐Transfer Molecule through Internal Heavy Atom Effect

“…In particular, the atom substitution position (including donor or acceptor moiety) for a heavy atom or controlling the number of halogenated positions at the donor part has been explored. [14,[16][17][18][19] And, the preceding studies about IHA effect on TADF are summarized in Figure S1, Supporting Information. However, the positive IHA effect on TADF luminogens was observed mostly in the solution phase.…”

“…[ 10,11 ] To this end, the internal heavy atom (IHA) effect based on halogenation at the organic molecule (e.g., Cl, Br, and I) previously has been studied for TADF molecules with the enhanced RISC. In most of these previous studies, the shortened delayed lifetime was demonstrated in a solution state, [ 12–14 ] suggesting that inclusion of IHA could be beneficial in reducing the likelihood of bi‐excitonic exothermic annihilation processes such as triplet‐triplet or singlet‐triplet quenching (i.e., TTA or STA), which are major sources of roll‐off behaviors in TADF‐based OLEDs. [ 4–6,15 ] Nevertheless, its influence on OLED device performance is not yet completely clear; roll‐off at high brightness was reportedly reduced in some works on IHA‐based TADF OLEDs, but it was not clearly observed in others, and thus requires further detailed study.…”

Kinetic prediction of reverse intersystem crossing in organic donor–acceptor molecules

“…Figure 5 displays the impact of varying H SO and E A on k RISC . While the existing organic TADF molecules exhibit k RISC smaller than 10 8 s -1 , the theory predicts that even k RISC of 10 9 s -1 , corresponding to a time constant of 1.0 ns, can be achieved with H SO less than 10 cm -1 ; for example, H SO of 7.7 cm -1 for E A of 0.10 eV and H SO of 2.9 cm -1 for E A of 0.05 eV at T of 300 K. These H SO are an order of magnitude smaller than those of iridium-containing phosphors and could be achieved by exploiting heavy atom effects of nonmetals in periods 3 and 4 30,31 . However, we have shown that such heuristic approaches sometimes lead to the retardation of H SO , in part because of their more pronounced effects on the excited-state electronic configurations at the S 1 -T 2 MESX geometries.…”

Promoting Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence via Heavy-Atom Effect

“…Several ways can be considered to boost RISC, such as reduction of ΔEST, [19][20] enhancement of spin-vibronic coupling, 21 and SOC, including heavy-atom effects. 15,20,[22][23][24][25][26][27] When both the S1 and T1 possess charge-transfer (CT) character, RISC can be enhanced by intervening of locally excited (LE) state(s). 28 Dynamic or static fluctuation between donor and acceptor segments also induces RISC.…”

“…For several TADF molecules containing oxygen (O) atoms developed so far, simply replacing O atoms with heavier sulfur (S) or selenium (Se) atoms is an effective approach to enhance SOC and promote RISC. 15,20,[22][23][24][25][26][27] Recently, we have reported on the S-containing TADF molecule 3-(1,3,6,8-tetramethyl-9H-carbazol-9-yl)-9H-thioxanthen-9-one (MCz-TXO, Figure 1a). 18 , where S0 denotes the ground state.…”