Intramolecular Dimerization Quenching of Delayed Emission in Asymmetric D–D′–A TADF Emitters

Woon, Kai Lin and Yi, Chih-Lun and Pan, Kuan-Chung and Etherington, Marc K. and Wu, Chung-Chih and Wong, Ken-Tsung and Monkman, Andrew P. (2019) Intramolecular Dimerization Quenching of Delayed Emission in Asymmetric D–D′–A TADF Emitters. The Journal of Physical Chemistry C, 123 (19). pp. 12400-12410. ISSN 1932-7447, DOI

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Understanding the excited-state dynamics and conformational relaxation in thermally activated delayed fluorescence (TADF) molecules, including conformations that potentially support intramolecular through-space charge transfer, can open new avenues for TADF molecular design as well as elucidate complex photophysical pathways in structurally complex molecules. Emissive molecules comprising a donor (triphenylamine, TPA) and an acceptor (triphenyltriazine, TRZ) bridged by a second donor (9,9-dimethyl-9-10-dihydroacridin, DMAC, or phenoxazine, PXZ) are synthesized and characterized. In solution, the flexibility of the sp3-hybridized carbon atom in DMAC of DMAC-TPA-TRZ, compared to the rigid PXZ, allows significant conformational reorganization, giving rise to multiple charge-transfer excited states. As a result of such a reorganization, the TRZ and TPA moieties become cofacially aligned, driven by a strong dipole-dipole attraction between the TPA and TRZ units, forming a weakly charge-transfer dimer state, in stark contrast to the case of PXZ-TPA-TRZ where the rigid PXZ bridge only supports a single PXZ-TRZ charge transfer (CT) state. The low-energy TPA-TRZ dimer is found to have a high-energy dimer local triplet state, which quenches delayed emission because the resultant singlet CT local triplet energy gap is too large to mediate efficient reverse intersystem crossing. However, organic light-emitting diodes using PXZ-TPA-TRZ as an emitting dopant resulted in external quantum efficiency as high as 22%, more than two times higher than that of DMAC-TPA-TRZ-based device, showing the impact that such intramolecular reorganization and donor-acceptor dimerization have on TADF performance. © 2019 American Chemical Society.

Item Type: Article
Funders: MSCA-RISE: for funding the OCTA project under grant agreement No. 778158, Ministry of Science and Technology (MOST) Taiwan (MOST 104-2113-M-002-006-MY3, 105-2221-E-002-162-MY3), University Malaya Research University Grant-Faculty Program (GPF046B-2018)
Uncontrolled Keywords: Charge transfer state; Conformational relaxation; Dipole-dipole attractions; Emissive molecules; Excited-state dynamics; External quantum efficiency; Inter-system crossings; Thermally activated delayed fluorescences
Subjects: Q Science > Q Science (General)
Q Science > QC Physics
Divisions: Faculty of Science > Department of Physics
Depositing User: Ms. Juhaida Abd Rahim
Date Deposited: 28 Nov 2019 08:55
Last Modified: 28 Nov 2019 08:55

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