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...

Development of white organic light-emitting diodes based on purely thermally activated delayed fluorescence with a single-emissive-layer configuration has been a formidable challenge. Here, we report the rational design of a donor-acceptor energy-relaying exciplex and its utility in fabricating single-emissive-layer, thermally activated delayed fluorescence-based white organic light-emitting diodes that exhibit 100% internal quantum efficiency, 108.2 lm W−1 power efficiency, and 32.7% external quantum efficiency. This strategy enables thin-film fabrication of an 8 cm × 8 cm thermally activated delayed fluorescence white organic light-emitting diodes (10 inch2) prototype with 82.7 lm W−1 power efficiency and 25.0% external quantum efficiency. Introduction of a phosphine oxide-based acceptor with a steric group to the exciplex limits donor-acceptor triplet coupling, providing dual levels of high-lying and low-lying triplet energy. Transient spectroscopic characterizations confirm that a ladder-like energy relaying occurs from the high-lying triplet level of the exciplex to a blue emitter, then to the low-lying triplet level of the phosphine oxide acceptor, and ultimately to the yellow emitter. Our results demonstrate the broad applicability of energy relaying in multicomponent systems for exciton harvesting, providing opportunities for the development of third-generation white organic light-emitting diode light sources.

Multiresonance (MR) emitters featuring narrowband emissions and theoretically 100% exciton harvesting are great potential for organic light-emitting diode (OLED) applications. However, how to functionalize MR molecules without scarifying emission color purity is still a key challenge. Herein, we report a feasible strategy for selective optimization of MR molecules, which is demonstrated by a blue MR emitter tCBNDASPO substituted with a diphenylphosphine oxide (DPPO) group. Compared to its DPPO-free parent molecule, tCBNDASPO preserves narrowband feature with full widths at half maximum (FWHM) values of 28 nm in film and 32 nm in OLEDs and achieves 40% increased photoluminescence (92%) and electroluminescence quantum efficiencies (28%). It is showed that insulation effect of P=O effectively confines the singlet excited state on MR core to keep emission color purity, and its induction effect enhances singlet radiation and triplet-to-singlet conversion. This synergism for selective optimization is based on rational linkage between MR core and functional groups.

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