The primary concern on high power conservation lead to the development of organic electroluminescent (EL) materials from polycyclic aromatic fluorescence (FL), [3] noble-metal-involved phosphorescence (PH) [4] to donor-acceptor (D-A)-featured thermally activated delayed fluorescence (TADF) molecules. [5] Different to 25% electrogenerated excitons, namely singlet excitons, utilized by FL emitters, both PH and TADF materials can harvest 100% excitons in virtue of mutual singlet-triplet conversion through intersystem crossing (ISC) [6] and reverse ISC (RISC). [7] However, FL molecules characteristic of locally excited (LE) states commonly reveal high chromatic purities, whose FWHM values are less than 50 nm. In contrast, charge-transfer (CT) excitedstate components of PH and TADF molecules markedly broaden their emissions by 50-100 nm.Recently, multi-resonance (MR) featured TADF emitters emerge, because of their potential to overcome the challenge in combining high efficiency and emission color purity. [8] This kind of compound has polycyclic aromatic structures with electron-donating and withdrawing atoms, for example, nitrogen and boron, at ortho-positions to form accordant and amplified resonance effects on electron-cloud distribution. [9] In this case, on the one hand, frontier molecular orbitals (FMOs) of MR molecules are separated to facilitate RISC and achieve TADF characteristics; on the other hand, their fused-ring structures limit vibrational relaxation, leading to unique narrowband emissions with FWHM < 30 nm. [10] To balance optoelectrical properties, MR cores were further substituted with functional groups, [11], for example, cyan, [12] fluorine, [13] carbazole, [14] and diphenylamine, [15] which significantly improved carrier injection and transport, resulting in increased luminance and reduced roll-offs. However, to avert spectral broadening, these functional groups were mostly simple, rigid, and finitely extended, which limit the optimal space of MR materials. [16] Nonetheless, it is noticed that most MR diodes should adopt extremely low doping concentrations (commonly <5%) [17] to avoid bimolecular quenching processes, for example, triplet-triplet annihilation (TTA) and triplet-polaron quenching Emerging multi-resonance (MR) thermally activated delayed fluorescence (TADF) emitters can combine 100% exciton harvesting and high color purity for their organic light-emitting diodes (OLED). However, the highly planar configurations of MR molecules lead to intermolecular-interactioninduced quenching. A feasible way is integrating host segments into MR molecules, namely a "self-host" strategy, but without involving additional charge transfer and/or vibrational components to excited states. Herein, an ambipolar self-host featured MR emitter, tCBNDADPO, is demonstrated, whose ambipolar host segment (DADPO) significantly and comprehensively improves the TADF properties, especially greatly accelerated singlet radiative rate constant of 2.11 × 10 8 s −1 and exponentially reduced nonradiative rate constants. C...
