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Elucidating the electronic structure of a delayed fluorescence emitter via orbital interactions, excitation energy components, charge-transfer numbers, and vibrational reorganization energies
journal contributionposted on 29.03.2021, 13:36 by Zheng Pei, Qi Ou, Yuezhi Mao, Junjie Yang, Aurélien de la Lande, Felix PlasserFelix Plasser, Wanzhen Liang, Zhigang Shuai, Yihan Shao
Recently, Wang and co-workers carried out frontier molecule orbital engineering in the design of m-Cz-BNCz, a thermally activated delayed fluorescence (TADF) molecule that emits pure green light at an external quantum efficiency of 27%. To further understand the underlying molecular design principles, we employed four advanced electronic structure analysis tools. First, an absolutely localized molecular orbitals (ALMO-) based analysis indicates an antibonding combination between the highest occupied molecular orbitals (HOMOs) of the donor 3,6-di-tert-butylcarbazole fragment and the acceptor BNCz fragment, which raises the HOMO energy and red-shifts the fluorescence emission wavelength. Second, excitation energy component analysis reveals that the S1-T1 gap is dominated by two-electron components of the excitation energies. Third, charge transfer number analysis, which is extended to use fragment-based Hirshfeld weights, indicates that the S1 and T1 excited states of m-Cz-BNCz (within time-dependent density functional theory) have notable charge transfer characters (27% for S1 and 12% for T1). This provides a balance between a small single-triplet gap and a substantial fluorescence intensity. Last, a vibrational reorganization energy analysis pinpoints the torsional motion between the BNCz and Cz moieties of m-Cz-BNCz as the source for its wider emission peak than that of p-Cz-BNCz. These four types of analyses are expected to be very valuable in the study and design of other TADF and functional dye molecules.
National Institutes of Health (Grant R01GM135392)
Oklahoma Center for the Advancement of Science and Technology (Grant HR18-130)
Office of the Vice President of Research and the College of Art and Sciences at the University of Oklahoma (OU)
National Natural Science Foundation of China (Grant Nos. 21573177 and 21833006)
National Natural Science Foundation of China (Grant No. 21788102)
Ministry of Science and Technology of China through the National Key R&D Plan (Grant No. 2017YFA0204501)
National Natural Science Foundation of China (Grant No. 22003030)
China Postdoctoral Science Foundation Grant No. 2020M670280
Shuimu Tsinghua Scholar Program