[19p-PB8-1~32] 12.4 Organic light-emitting devices and organic transistors

[19p-PB8-8] Suppression of Structural Change in Excited States and Thermally Activated Delayed Fluorescence in carbazole-benzonitrile derivatives

〇Masaki Saigo¹, Umemoto Masakazu¹, Miyata Kiyoshi¹, Nakanotani Hajime^2,3,4, Adachi Chihaya^2,3,4, Onda Ken¹ (1.Kyushu Univ., 2.OPERA, 3.JST-ERATO, 4.WPI-I2CNER)

Keywords:Time-resolved infrared vibrational spectroscopy, Thermally Activated Delayed Fluorescence, Ultrafast spectroscopy

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 (ΔE_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 carbazole-benzonitrile (Cz-BN) derivatives that possess identical ΔE_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 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.