Intramolecular Dimerization Quenching of Delayed Emission in Asymmetric D-D’-A TADF Emitters

Abstract

Understanding the excited-state dynamics and conformational relaxation in thermallyactivated 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 reorganisation giving rise to multiple charge-transfer excited states. As a result of such reorganization, the TRZ and TPA moieties become co-facially aligned, driven by strong dipole-dipole attraction between the TPA and TRZ units, forming a weakly charge transfer dimer state, in stark contrast to the case of PXZTPA- TRZ where the rigid PXZ bridge only supports a single PXZ-TRZ 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 EQE as high as 22%, more than 2 times higher than that of DMAC-TPA-TRZ-based device showing the impact that such intramolecular reorganization and donor acceptor dimerization has on TADF performance.