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Robust scaffolds were typically applied in thermally activated delayed fluorescence (TADF) molecules to suppress the non‐radiative decay, trigger the fast spin‐flipping, and enhance the light out‐coupling efficiency. Herein, we disclosed for the first time the positive effect of flexible conformation of ancillary groups on the photophysical properties of TADF emitter. The red TADF emitter Ph‐TPA with flexible conformation demonstrated small excited‐state structural distortion and low reorganization energy compared to the counterpart Mc‐TPA with a rigid macrocycle. Consequently, Ph‐TPA showed an excellent photoluminescent quantum yield (PLQY) of 92 % and a state‐of‐the‐art external quantum efficiency (EQE) of 30.6 % at 630 nm. This work could deepen our understanding of structure‐property relationships of organic luminophores and help us to rationalize the design of efficient TADF materials. [ABSTRACT FROM AUTHOR]

White organic light-emitting diode (WOLED) has been recognized as a healthy light source. However, the device performance is still limited by the hosts or emitters. Herein, multiple-exciplex hosts are developed for WOLEDs, in which a novel exciplex host consisting of a thermally activated delayed fluorescent material, triazine–carbazole (Trz-PhCz), and an electron-transport material, 4,6-bis[3,5-(dipyrid-4-yl)phenyl]-2-methylpyrimidine (B4PyMPM), is adopted for green, yellow, orange and red phosphorescent dopants, of which high external quantum efficiencies (EQEs) of 25.0 %, 30.7 %, 32.5 % and 26.2 %, respectively, are achieved. On the other hand, a high-energy exciplex host consisting of 9,9′-biphenyl-3,3′-diylbis-9H-carbazole (mCBP) and B4PyMPM is designed for the blue emitter, iridium(III)bis(4,6-(difluorophenyl)-pyridinato-N,C2') picolinate (FIrpic), which guarantees a maximum EQE of 26.3 %. The small exciton energy difference between the mCBP:B4PyMPM and Trz-PhCz:B4PyMPM hosts can facilitate efficient energy transfer between the hosts. As a result, these exciplex hosts facilitate energy-efficient WOLEDs with a maximum EQE, power efficiency and current efficiency of 36.9 %, 137.4 lm W−1 and 106.7 cd A−1, respectively, without using any optical out-coupling techniques, which provides inspiration for the future design of efficient OLEDs. Highly efficient white light-emitting diodes (WLEDs) enabled by novel exciplex hosts. [Display omitted] • A novel exciplex host, composed of a TADF material, is developed for green, yellow, orange, and red phosphorescent emitters. • In addition, a high-energy exciplex host, combined with mCBP and B4PyMPM, is designed for the blue emitter. • By combining these exciplex hosts, efficient WOLEDs with a maximum EQE of 36.9% and PE of 137.4 lm W−1 are fabricated. [ABSTRACT FROM AUTHOR]

[Display omitted] • A simple strategy for the construction of efficient DR TADFs was developed. • A high-Stokes-shift dye was introduced into a TADF molecular skeleton. • The incorporation of high-Stokes-shift fragment led to dual ICT and geometric distortion. The design and synthesis of efficient deep-red (DR) thermally activated delayed fluorescence (TADF) materials is challenging. In this work, we reported a simple molecular design strategy for constructing DR TADF via embedding a high-Stokes-shift dye in molecular skeleton, which induced dual intramolecular charge transfer and structural relaxation of excited state while achieving remarkable electroluminescence (EL) performance. The proof-of-concept emitter BF 2 -TPA demonstrated longer emission wavelength than the Br/CN-containing counterparts. Organic light-emitting diode (OLED) based on BF 2 -TPA exhibited high external quantum efficiency (EQE) of 28.4 % at 656 nm. This work demonstrates the great potential of utilizing high-Stokes-shift fragment in constructing efficient DR TADF emitters. [ABSTRACT FROM AUTHOR]