(Invited) Computational Approach for Materials Design and Comprehensive Understanding of Organic Light-Emitting Diode
Authors: Kaji Hironori 
Published: September 2023

Reverse intersystem crossing (RISC) has been the rate-limiting process in thermally activated delayed fluorescence (TADF)[1,2], and has induced material degradation. Therefore, the acceleration of RISC is important for obtaining further excellent TADF-based organic light-emitting diodes (OLEDs), especially for solving the efficiency roll-off problem and extending the device lifetime. Here, we will present three different ways to accelerate RISC; 1) intervening locally excited (LE) state(s) between charge transfer-type S1 (1CT) and CT-type T1 (3CT) [3], 2) using the heavy atom effect [4], and 3) using fluctuation effect that can achieve effective RISC even between 1CT and 3CT [5].

Next, we will propose a method to quantitatively predict rate constants of all electronic transitions related to emission[6-8]. TADF molecules have been designed through the calculations of ΔE
ST and oscillator strength. However, to develop further excellent emitters, it is desired to precisely calculate the rate constants. Our method based on Fermi’s golden rule allows us to theoretically predict relevant rate constants for all types of electronic transitions in emitter molecules. If the rate constants can be quantified, all quantum yields can be also quantified. The emission mechanism and photophysical performance are also fully understood. We have applied this method to benzophenone and a multiple resonance TADF material, DABNA-1. The calculated rate constants were in quantitative agreement with the experimental ones in both cases. The method was further applied to a very fast RISC molecule, MCz-TXO. Our method quantitatively reproduced the experimentally-obtained very large k
RISC of 108 s-1 caused by the heavy atom effect of sulfur (S). We can expect much larger k
RISC when we use much heavier atoms, such as Se, Te, and Po instead of S. Therefore, we investigated further heavy atom effects by our calculation method. The calculations predicted two orders larger k
RISC of 1010 s-1 for the Se- and Te-containing molecules. Instead, the Po-containing molecule was predicted to show phosphorescence instead of TADF, suggesting that moderate heavy atom effect is favorable for TADF.

Finally, we will present our recent study on inverted S1-T1 (iST) emitters. We have theoretically designed many iST candidates and predicted their photophysical properties. Some of them have been experimentally synthesized and measured their photophysical properties.

We express sincere thanks to my group members. This work was supported by JSPS KAKENHI Grant Numbers JP20H05840 (Grant-in-Aid for Transformative Research Areas, “Dynamic Exciton”). The international Joint Usage/Research Centre at the Institute for Chemical Research, Kyoto University, Japan, supported computation time and NMR measurements.

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[2] H. Kaji, H. Suzuki, T. Fukushima, K. Shizu, K. Suzuki, S. Kubo, T. Komino, H. Oiwa, F. Suzuki, A. Wakamiya, Y. Murata, C. Adachi, Nat. Commun. 6 (2015) 8476.

[3] Y. Wada, H. Nakagawa, S. Matsumoto, Y. Wakisaka, H. Kaji, Nat. Photon. 14 (2020) 643.

[4] Y. Ren, Y. Wada, K. Suzuki, Y. Kusakabe, J. Geldsetzer, H. Kaji, Appl. Phys. Express 14 (2021) 071003.

[5] Y. Wada, Y. Wakisaka, H. Kaji, ChemPhysChem 22 (2021) 625.

[6] K. Shizu, H. Kaji, J. Phys. Chem. A 125 (2021) 9000.

[7] K. Shizu, H. Kaji, Commun. Chem. 5 (2022) 53.

[8] K. Shizu, Y. Ren, H. Kaji, ChemRxiv Doi: 10.26434/chemrxiv-2022-9drhb.