A thermally activated delayed fluorescent (TADF) emitter (DMAC-TRZ) was reported either as the emitting dopant in a host or as the non-doped (neat) emitting layer to achieve high EL EQEs of up to 26.5% and 20% in OLEDs, respectively.

A series of twisted D–π–A type emitters based on the acridine donor unit and CN‐substituted pyridine, pyrimidine, and benzene acceptor units are studied. They not only allow one to systematically probe the influence of different acceptor strengths, but also permit one to intriguingly probe the influence of tunable conformations (twist angles) within the acceptor moieties through controlling the orientation of asymmetric heteroaromatic ring relative to the donor component. Intramolecular charge‐transfer transitions are observed in all these compounds and emission wavelengths are widely tunable from deep blue to yellow not only by the general acceptor strength due to the characters of heteroarene and CN‐substitution pattern but also by the subtle control of in‐acceptor conformation (twist angles). Small triplet‐to‐singlet energy gaps (ΔEST) and significant thermally activated delayed fluorescence (TADF) characteristics are obtained in a series of D–π–A compounds with sufficient acceptor strengths and tunable in‐acceptor conformation, yielding a series of efficient blue‐green to yellow TADF emitters with promisingly high photoluminescence quantum yields of 90%–100%. Highly efficient blue‐green to yellow TADF organic light‐emitting diodes (OLEDs) having external quantum efficiencies of up to 23.1%–31.3% are achieved using these efficient TADF emitters, which are among the most efficient TADF OLEDs ever reported.

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Bright & blue: A strategy for reducing metal‐chelate internal strain enables the preparation of blue emitting iridium(III) carbene complexes (see picture; Ir red, N light blue, F green). The phosphorescent OLED fabricated from one of these complexes shows remarkable CIE coordinates of (0.16, 0.13) and peak efficiencies of 6.0 % photons per electron, 6.3 cd A−1, and 4.0 lm W−1.

A new series of charge neutral Os(II) isoquinolyl triazolate complexes (1–4) with both trans and cis arrangement of phosphine donors are synthesized, and their structural, electrochemical and photophysical properties are established. In sharp contrast to the cis‐arranged complexes 2–4, the trans derivative 1, which shows a planar arrangement of chromophoric N‐substituted chelates, offers the most effective extended π‐delocalization and hence the lowest excited state energy gap. These complexes exhibit phosphorescence with peak wavelengths ranging from 692–805 nm in degassed CH2Cl2 at room temperature. Near‐infrared (NIR)‐emitting electroluminescent devices employing 6 wt % of 1 (or 4) doped in Alq3 host material are successfully fabricated. The devices incorporating 1 as NIR phosphor exhibit fairly intense emission with a peak wavelength at 814 nm. Forward radiant emittance reaches as high as 65.02 µW cm−2, and a peak EQE of ∼1.5% with devices employing Alq3, TPBi and/or TAZ as electron‐transporting/exciton‐blocking layers. Upon switching to phosphor 4, the electroluminescence blue shifts to 718 nm, while the maximum EQE and radiance increase to 2.7% and 93.26 (μW cm−2) respectively. Their performances are optimized upon using TAZ as the electron transporting and exciton‐blocking material. The OLEDs characterized represent the only NIR‐emitting devices fabricated using charge‐neutral and volatile Os(II) phosphors via thermal vacuum deposition.