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Red Light-Emitting Thermally-Activated Delayed Fluorescence of Naphthalimide-Phenoxazine Electron Donor-Acceptor Dyad: Time-Resolved Optical and Magnetic Spectroscopic Studies |
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| Authors: | Xue Zhang Xiao Liu Dr Maria Taddei Laura Bussotti Ivan Kurganskii Minjie Li Dr Xiao Jiang Longjiang Xing Prof Shaomin Ji Prof Yanping Huo Prof Jianzhang Zhao Prof Mariangela Di Donato Prof Yan Wan Prof Zujin Zhao Prof Matvey V Fedin |
| Institution: | 1. State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024 P. R. China;2. State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024 P. R. China
These authors contributed equally to this work.;3. LENS (European Laboratory for Non-Linear Spectroscopy), via N. Carrara 1, 50019 Sesto Fiorentino (FI), Italy These authors contributed equally to this work.;4. LENS (European Laboratory for Non-Linear Spectroscopy), via N. Carrara 1, 50019 Sesto Fiorentino (FI), Italy;5. International Tomography Center, SB RAS, and, Novosibirsk State University, 630090 Novosibirsk, Russia;6. College of Chemistry, Beijing Normal University, Beijing, 100875 P. R. China;7. Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian, 116024 P. R. China;8. Light Industry and Chemical Engineering College, Guangdong University of Technology, Guangzhou, 510006 P. R. China;9. State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640 P. R. China |
| Abstract: | We prepared an orthogonal compact electron-donor (phenoxazine, PXZ)-acceptor (naphthalimide, NI) dyad ( NI-PXZ ), to study the photophysics of the thermally-activated delayed fluorescence (TADF), which has a luminescence lifetime of 16.4 ns (99.2 %)/17.0 μs (0.80 %). A weak charge transfer (CT) absorption band was observed for the dyad, indicating non-negligible electronic coupling between the donor and acceptor at the ground state. Femtosecond transient absorption spectroscopy shows a fast charge separation (CS) (ca. 2.02～2.72 ps), the majority of the singlet CS state is short-lived, especially in polar solvents (τCR = 10.3 ps in acetonitrile, vs. 1.83 ns in toluene, 7.81 ns in n-hexane). Nanosecond transient absorption spectroscopy detects a long-lived transient species in n-hexane, which is with a mixed triplet local excited state (3LE) and charge separated state (3CS), the lifetime is 15.4 μs. In polar solvents, such as tetrahydrofuran and acetonitrile, a neat 3CS state was observed, whose lifetimes are 226 ns and 142 ns, respectively. Time-resolved electron paramagnetic resonance (TREPR) spectra indicate the existence of strongly spin exchanged 3LE/3CT states, with the effective zero field splitting (ZFS) |D| and |E| parameters of 1484 MHz and 109 MHz, respectively, much smaller than that of the native 3NI state (2475 and 135 MHz). It is rare but solid experimental evidence that a closely-lying 3LE state is crucial for occurrence of TADF and this 3LE state is an essential intermediate state to facilitate reverse intersystem crossing in TADF systems. |
| Keywords: | charge transfer intersystem crossing naphthalimide thermally activated delayed fluorescence triplet state |