Organic push-pull systems featuring through-space charge transfer (TSCT) excited states have been disclosed to be capable of exhibiting thermally activated delayed fluorescence (TADF), but to realize high-efficiency long-wavelength emission still remains a challenge. Herein, we report a series of strongly emissive orange-red and red TSCT-TADF emitters having (quasi)planar and rigid donor and acceptor segments which are placed in close proximity and orientated in a cofacial manner. Emission maxima (λem) of 594−599 nm with photoluminescence quantum yields (PLQYs) of up to 91% and delayed fluorescence lifetimes of down to 4.9 μs have been achieved for new acceptor-donor-acceptor (A-D-A) molecules in doped thin films. The presence of multiple acceptors and the strong intramolecular π-stacking interactions have been unveiled to be crucial for the efficient low-energy TSCT-TADF emissions. Organic light-emitting diodes (OLEDs) based on the new A-D-A emitters demonstrated electroluminescence with maximum external quantum efficiencies (EQEs) of up to 23.2% for the red TSCT-TADF emitters. An EQE of 18.9% at the brightness of 1000 cd m-2 represents one of the highest values for red TADF OLEDs. This work demonstrates a modular approach for developing high-performance red TADF emitters through engineering through-space interactions, and it may also provide implications to the design of TADF emitter with other colours.

A fast radiative decay process for long-wavelength molecular light-emitters is vital to achieving a high emission efficiency by outcompeting the nonradiative decay imposed by the energy-gap law. An ensuing short emission lifetime is also beneficial for fabricating high-performance organic light-emitting diodes. Herein a series of half-lantern dinuclear platinum(II) complexes is reported, which shows high-efficiency deep red phosphorescence (𝝀 em > 660 nm). The molecules are designed to have a cofacially aligned structure featuring short Pt-Pt distances of 2.80-2.83 Å by using 10H-pyrido[3,2-b][1,4]benzoxazine (PyXZ) as the rigid bridges, which are revealed by single crystal X-ray diffraction analysis. Together with the strong electron-donating property of PyXZ bridges, the metallophilic interaction endows low-energy triplet excited states with mixed metal-metal-to-ligand charge-transfer ( 3 MMLCT) and ligand-to-ligand charge-transfer ( 3 LLCT) characters. The deep red devices based on the diplatinum(II) complexes show maximum external quantum efficiencies (EQEs) up to 21.8%. The EQE of 19.4% and operational lifetime (LT 85 ) of 334 h (the operational time after which the luminance drops to 85% of the initial value) at a luminance of 1000 cd m −2 promise the pratical use of these complexes.

The harnessing of heavy atom effect of chalcogen elements offers a way for boosting the thermally activated delayed fluorescence (TADF) of purely organic luminescent materials that can harvest triplet excitons. However, the conformational and electronic variations induced by the heavy and large atoms may also have adverse effects on the TADF properties. Herein, the design, synthesis, and structures of a new type of through‐space charge transfer (TSCT) emitters containing benzothiazino[2,3,4‐kl]phenothiazine (DPTZ) as the donor unit are reported. The influences of S atoms on the emission properties have been systematically investigated by means of theoretical simulations, electrochemical and spectroscopic studies. Although the presence of π‐stacking interactions and calculated spin‐orbit coupling (SOC) values are beneficial for TSCT‐TADF properties, the triplet TSCT states are uplifted to above the locally excited (LE) state of the acceptor moieties. As a result, the new emitters display longer delayed fluorescence lifetimes (τDF) of 255.0–114.3 μs and lower PLQYs of 45–61 % in comparison with the O‐containing congeners (τDF=26.9–6.8 μs; PLQYs=74–71 %). This work highlights that a full consideration of various effects is essential when making use of heavy chalcogen atoms for the design of TADF emitters.

Increasing the radiative decay rates of triplet excited states to overcome the energy-gap law is important for the development of high-efficiency and short-lived phosphorescent metal complexes in the red to...