Suppression of Structural Change upon S1-T1 Conversion Assists the Thermally Activated Delayed Fluorescence Process in Carbazole-Benzonitrile Derivatives

Masaki Saigo; Kiyoshi Miyata; Sei'ichi Tanaka; Hajime Nakanotani; Chihaya Adachi; Ken Onda

doi:10.1021/acs.jpclett.9b00810

Suppression of Structural Change upon S₁-T₁ Conversion Assists the Thermally Activated Delayed Fluorescence Process in Carbazole-Benzonitrile Derivatives

J Phys Chem Lett. 2019 May 16;10(10):2475-2480. doi: 10.1021/acs.jpclett.9b00810. Epub 2019 May 2.

Authors

Masaki Saigo¹, Kiyoshi Miyata¹, Sei'ichi Tanaka², Hajime Nakanotani^{3

4}, Chihaya Adachi^{3

4}, Ken Onda¹

Affiliations

¹ Department of Chemistry , Kyushu University , 744 Motooka , Nishi, Fukuoka 819-0395 , Japan.
² Department of Chemistry , Tokyo Institute of Technology , O-okayama 2-12-1, H-61 , Meguro-ku , Tokyo 152-8550 , Japan.
³ Center for Organic Photonics and Electronics Research (OPERA) , Kyushu University , 744 Motooka , Nishi, Fukuoka 819-0395 , Japan.
⁴ JST, ERATO, Adachi Molecular Exciton Engineering Project , Kyushu University , 744 Motooka , Nishi, Fukuoka 819-0395 , Japan.

PMID: 30973013
DOI: 10.1021/acs.jpclett.9b00810

Abstract

Thermally activated delayed fluorescence (TADF) molecules are gathering attention for their potential to boost the efficiency of organic light-emitting diodes without precious metals. Minimizing the energy difference between the S₁ and T₁ states (Δ E_ST) is a fundamental strategy to accelerate reverse intersystem crossing (RISC). However, the lack of microscopic understanding of the process prevents adequate design strategies for efficient TADF materials. Here, we focused on four carbazole-benzonitrile (Cz-BN) derivatives that possess identical Δ E_ST but distinct TADF activities. We systematically compared their geometrical dynamics upon photoexcitation using time-resolved infrared (TR-IR) vibrational spectroscopy in conjunction with quantum chemical calculations. We found that the most TADF-active molecule, 4CzBN, shows little structural change after photoexcitation, while the TADF-inactive molecules show relatively large deformation upon S₁-T₁ conversion. This implies that the suppression of structural deformation is critical for minimizing the activation energy barrier for RISC in cases of the Cz-BN derivatives.