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Chemical and Dynamical Processes in Solution; Polymers, Glasses, and Soft Matter 1
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Suppression of Structural Change upon S-T Conversion Assists Thermally Activated Delayed Fluorescence Process in Carbazole-Benzonitrile Derivatives Masaki Saigo, Kiyoshi Miyata, Sei’ichi Tanaka, Hajime Nakanotani, Chihaya Adachi, and Ken Onda J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00810 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019
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The Journal of Physical Chemistry Letters
Suppression of Structural Change upon S1-T1 Conversion Assists Thermally Activated Delayed Fluorescence Process in Carbazole-Benzonitrile Derivatives Masaki Saigo1, Kiyoshi Miyata1,*, Sei’ichi Tanaka2, Hajime Nakanotani3,4, Chihaya Adachi3,4, Ken Onda1,* 1
Department of Chemistry, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan Department of Chemistry, Tokyo Institute of Technology, O-okayama 2-12-1, H-61, Meguroku, 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 2
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 (ΔEST) 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 derivatives that possess identical ΔEST 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 carbazole-benzonitrile derivatives.
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Molecules that show thermally activated delayed fluorescence (TADF) have the distinctive capability of converting triplet excited states (T1) to singlet excited states (S1) via reverse intersystem crossing (RISC). RISC is potentially useful for organic light-emitting diodes (OLEDs) if the process is quick enough.1 In general, it is believed that two conditions need to be met to accelerate RISC: (1) The energy difference between the S1 and T1 excited states (ΔEST) should be on the same order as thermal energy at room temperature (
