Rigid Bridge-Confined Double-Decker Platinum(II) Complexes Towards High-Performance Red and Near-Infrared Electroluminescence.

A molecular design to high-performance red and near-infrared (NIR) organic light-emitting diodes (OLEDs) emitters remains demanding. Herein a series of dinuclear platinum(II) complexes featuring strong intramolecular Pt⋅⋅⋅Pt and π-π interactions has been developed by using N-deprotonated α-carboline as a bridging ligand. The complexes in doped thin films exhibit efficient red to NIR emission from short-lived (τ=0.9-2.1 µs) triplet metal-metal-to-ligand charge transfer (3 MMLCT) excited states. Red OLEDs demonstrate high maximum external quantum efficiencies (EQEs) of up to 23.3 % among the best PtII -complex-doped devices. The maximum EQE of 15.0 % and radiance of 285 W sr-1 m-2 for NIR OLEDs (λEL =725 nm) are unprecedented for devices based on discrete molecular emitters. Both red and NIR devices show very small efficiency roll-off at high brightness. Appealing operational lifetimes have also been revealed for the devices. This work sheds light on the potential of intramolecular metallophilicity for long-wavelength molecular emitters and electroluminescence.

Solution-Processed OLEDs Based on Thermally Activated Delayed Fluorescence Copper(I) Complexes with Intraligand Charge-Transfer Excited State.

A new series of tetrahedral heteroleptic copper(I) complexes exhibiting efficient thermally-activated delayed fluorescence (TADF) in green to orange electromagnetic spectral regions has been developed by using D-A type N^N ligand and P^P ligands. Their structures, electrochemical, photophysical, and electroluminescence properties have been characterized. The complexes exhibit high photoluminescence quantum yields (PLQYs) of up to 0.71 at room temperature in doped film and the lifetimes are in a wide range of 4.3-24.1 µs. Density functional theory (DFT) calculations on the complexes reveal the lowest-lying intraligand charge-transfer excited states that are localized on the N^N ligands. Solution-processed organic light emitting diodes (OLEDs) based on one of the new emitters show a maximum external quantum efficiency (EQE) of 7.96%.

Face-to-Face Orientation of Quasiplanar Donor and Acceptor Enables Highly Efficient Intramolecular Exciplex Fluorescence.

Intramolecular through-space charge-transfer (TSCT) excited states have been exploited for developing thermally activated delayed fluorescence (TADF) emitters, but the tuning of excited state dynamics by conformational engineering remains sparse. Designed here is a series of TSCT emitters with precisely controlled alignment of the donor and acceptor segments. With increasing intramolecular π-π interactions, the radiative decay rate of the lowest singlet excited state (S1 ) progressively increased together with a suppression of nonradiative decay, leading to significantly enhanced photoluminescence quantum yields of up to 0.99 in doped thin films. A high-efficiency electroluminescence device, with a maximum external quantum efficiency (EQE) of 23.96 %, was achieved and maintains >20 % at a brightness of 1000 cd m-2 . This work sheds light on the importance of conformation control for achieving high-efficiency intramolecular exciplex emitters.

Lighting Silver(I) Complexes for Solution-Processed Organic Light-Emitting Diodes and Biological Applications via Thermally Activated Delayed Fluorescence.

Luminescent coinage metal complexes have shown promising applications as electroluminescent emitters, photocatalysts/photosensitizers, and bioimaging/theranostic agents, rendering them attractive alternatives to transition metal complexes based on iridium, ruthenium, and platinum that have extremely low earth abundance. In comparison to the widely studied Au(I) and Cu(I) complexes, Ag(I) complexes have seldom been explored in this field because of their inferior emission properties. Herein, we report a novel series of [Ag(N^N)(P^P)]PF6 complexes exhibiting highly efficient thermally activated delayed fluorescence by using easily accessible neutral diamine ligands and commercially available ancillary diphosphine chelates. The photoluminescence quantum yields (PLQYs) of the Ag(I) emitters are ≤0.62 in doped films. The high PLQY with a large delayed fluorescence ratio enabled the fabrication of solution-processed organic light-emitting diodes (OLEDs) with a high maximum external quantum efficiency of 8.76%, among the highest values for Ag(I) emitter-based OLEDs. With superior emission properties and an excited state lifetime in the microsecond regime, together with its potent cytotoxicity, the selected Ag(I) complex has been used for simultaneous cell imaging and anticancer treatment in human liver carcinoma HepG2 cells, revealing the potential of luminescent Ag(I) complexes for biological applications such as theranostics.

Dinuclear ZnII Complexes Exhibiting Thermally Activated Delayed Fluorescence and Luminescence Polymorphism.

ZnII complexes exhibiting strong emission in the solid state remain scarce, and most of them exhibit only prompt fluorescence. Herein the synthesis, structures, and photoluminescence properties of two ZnII complexes containing new donor-acceptor ligands is reported. The new ZnII complexes have dinuclear structures in which each metal ion adopts a distorted square-pyramidal geometry. The ZnII complexes show strong emission in the solid state with quantum yields up to 50 %. Variable-temperature transient photoluminescence studies revealed an emission mechanism involving prompt and thermally activated delayed fluorescence (TADF). DFT calculations showed well-separated HOMO and LUMO in the ground state and small excited singlet-triplet energy splitting, accounting for the TADF. The complexes also exhibit different emission colors in the as-synthesized powder state and in single crystals, that is, they exhibit luminescence polymorphism. The single-crystal emission is responsive to mechanical grinding and was characterized by powder XRD.