Extremely efficient sky-blue organic electroluminescence with external quantum efficiency of ≈37% is achieved in a conventional planar device structure, using a highly efficient thermally activated delayed fluorescence emitter based on the spiroacridine-triazine hybrid and simultaneously possessing nearly unitary (100%) photoluminescence quantum yield, excellent thermal stability, and strongly horizontally oriented emitting dipoles (with a horizontal dipole ratio of 83%).

Organic light emitting diode (OLED) is a new yet promising technology that is anticipated to replace the liquid crystal display technology in the very near future. The development of both the emitter and host materials for OLEDs is indispensable to realize high device efficiency and optimal performance. Though the presently commercialized OLED panels mostly utilize phosphorescence emitters, the all‐organic thermally activated delayed fluorescence (TADF) emitting materials have some obvious advantages. Considerable progress has been made in search of better performing TADF OLEDs in the past few years. Although major research attention has been drawn toward reporting new TADF emitters, the hosts are equally important in TADF OLEDs, as the doped films of the emitters mostly yield better results than the nondoped films. There are already some good reviews on the TADF emitters in literature. In this review article, the literature data specifically aimed at hosting TADF dopants are carefully selected and comprehensively summarized and categorized into several sub‐groups based on their structural features to draw the attention of the organic electronics research community toward developing new host materials for TADF OLEDs.

Reaction of salicylaldehyde thiosemicarbazone (H 2 L 1 ), 2-hydroxyacetophenone thiosemicarbazone (H 2 L 2 ) and 2-hydroxynaphthaldehyde thiosemicarbazone (H 2 L 3 ) (general abbreviation H 2 L, where H 2 stands for the two dissociable protons, one phenolic proton and one hydrazinic proton) with Na 2 [PdCl 4 ] affords a family of polymeric complexes of type [{Pd(L)} n ]. Reaction of the polymeric species with two monodentate ligands (D), viz. triphenylphosphine (PPh 3 ) and 4-picoline (pic), has yielded complexes of type [Pd(L)(D)]. These mixed-ligand complexes have also been obtained from reaction of the thiosemicarbazones with [Pd(PPh 3 ) 2 Cl 2 ] and [Pd(pic) 2 Cl 2 ]. Crystal structures of [Pd(L 1 )(PPh 3 )] and [Pd(L 2 )(pic)] have been determined. The [Pd(L)(D)] complexes show characteristic 1 H NMR spectra and intense absorptions in the visible and ultraviolet region. They also fluoresce in the visible region at ambient temperature. In vitro cytotoxicity screenings of the complexes along with four human clinical drugs viz. cisplatin, BCNU, 5-fluorouracil (5-FU) and hydroxyurea have been carried out in two human tumor cell lines, namely promyelocytic leukemia HL-60 and histiocytic lymphoma U-937. [Pd(L 2 )(PPh 3 )] shows the lowest IC 50 value and is found to be much more cytotoxic than the reference anticancer drugs in both the cell lines. An apoptosis study in HL-60 with [Pd(L 2 )(PPh 3 )] confirms that at 10 mM concentration it induces apoptosis to a greater extent than cisplatin and camptothecin.

A series of 4,4'-π-conjugated-2,2'-bipyridine chromophores (MS 1-8) were synthesized, and their photophysical and thermal properties were investigated. The title "push-pull' chromophores", except MS 1, were integrated with both alkoxy and alkylamino donor functionalities that differ in their donation capabilities. The oligophenylenevinylene (OPV) chromophores MS 4-8 are associated with a π-extended backbone in which the position and the number of alkoxy donors were systematically varied. All of the studied systems possess a D-π-A-A-π-D dyad archetype in which the A-A is the central 2,2'-bipyridine acceptor core that is electronically attached with the donor termini through π-linkers. The fluorescence quantum yields of the synthesized chromophores are found to be sensitive to the molecular archetype and the solvent medium. Out of the eight fluorescent compounds reported in this article, the compound MS 5 exhibits fluorescence in the solid state also. The modulating effect of the nature, position, and number of donor functionalities on the optical properties of these classes of compounds has further been comprehended on the basis of DFT and TD-DFT computation in a solvent reaction field.

Two isomeric host materials (Sy and Asy) comprising carbazole (donor) and CN-substituted pyrimidine (acceptor) were synthesized, characterized, and utilized as host materials for green and blue thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs). Both molecules have high triplet energy and small energy difference between singlet and triplet states, leading to feasible TADF. The different linking topologies of carbazole and CN groups on the pyrimidine core provide distinct photophysical properties and molecular packing manners, which further influence the efficiency as they served as hosts in TADF OLEDs. As compared to Asy-based cases, the Sy-hosted TADF OLED device gave higher maximum external quantum efficiencies (EQE) of 24.0% (vs 22.5%) for green (4CzIPN as a dopant) and 20.4% (vs 15.0%) for blue (2CzTPN as a dopant) and low efficiency roll-off. The high horizontal dipole ratio (Θ ≈ 88%) for both emitters dispersed in Sy and Asy hosts accounts for the high device efficiency. A clear molecular structure-physical property-device performance relationship has been established to highlight the importance of symmetrical structure in TADF host material design.

The tetrahedral (T d ) molecular cation ammonium ion (NH 4 þ ) gets incorporated with the smaller-cavity crown ethers through three N þ -H 3 3 3 O hydrogen bonding interactions, while the fourth N-H bond is projected outward from the crown ether cavity. A hydrogen bonding acceptor, such as a polyoxometalate anion, if placed in suitable position in the crystal lattice, can interact with the ammonium-crown ether supramolecular complex involving the fourth N-H bond donation. This article describes supramolecular association of ammonium-crown ether host-guest complexes with polyoxometalate anions in five crystalline solids, formulated as 5), where B18C6 = benzo-18-crown-6, DB18C6 = dibenzo-18-crown-6, DC18C6 = dicyclohexyl-18-crown-6, and DB30C10 = dibenzo-30-crown-10. Single crystal X-ray structural investigations on these solids confer P1 space symmetry (No. 2) for the compounds 1-2; C2/c space symmetry (No. 15) for the compound 3; P2 1 /n space symmetry (No. 14) for the compound 4, and C2/m (No. 12) space symmetry for the compound 5. Careful examination of the supramolecular interactions between the different molecular fragments show direct contact between the ammonium cation and the polyoxometalate anion (N þ -H 3 3 3 O) in the crystal structure of the compounds 1, 2, and 4. Lattice water molecules have taken the role of hydrogen bonding acceptor (N þ -H 3 3 3 O) in the crystal structure of compound 3, while the larger and flexible crown ether DB30C10 has wrapped around the ammonium cation in the case of compound 5. Thus, there is no direct supramolecular interaction between the ammonium cation and the polyoxometalate anion (N þ -H 3 3 3 O) in the crystal structures of compounds 3 and 5, where the ammonium-crown ether cations are associated with the polyoxometalate anions only via C-H 3 3 3 O hydrogen bonding interactions. Detailed spectroscopic (IR, 1 H NMR, and UV-visible) analyses have been included in comprehending the supramolecular association between the inclusion complexes and polyoxometalate anions. The X-ray powder diffraction analyses have been performed to scrutinize the phase purity of the solids.

This article demonstrates a series of cyclometalated Ir(III) complexes of the type [Ir(III)(C^N)2(N^N)](PF6), where C^N is 2-phenylpyridine, and N^N corresponds to the 4,4'-π-conjugated 2,2'-bipyridine ancillary ligands. All these compounds were synthesized through splitting of the binuclear dichloro-bridged complex precursor, [Ir(C^N)2(μ-Cl)]2, with the appropriate bipyridine ligands followed by the anion exchange reaction. The linear and nonlinear absorption properties of the synthesized complexes were investigated. The absorption spectra of all the title complexes exhibit a broad structureless feature in the spectral region of 350-700 nm with two bands being well-resolved in most of the cases. The structures of all the compounds were modeled in dichloromethane using the density functional theory (DFT) algorithm. The nature of electronic transitions was further comprehended on the basis of time-dependent DFT analysis, which indicates that the origins of various bands are primarily due to intraligand charge transfer transitions along with mixed-metal and ligand-centered transitions. The synthesized compounds are found to be nonemissive at room temperature because of probable nonradiative deactivation pathways of the T1 state that compete with the radiative (phosphorescence) decay modes. However, the frozen solutions of compounds Ir(MS 3) and Ir(MS 5) phosphoresce at the near-IR region, the other complexes remaining nonemissive up to 800 nm wavelength window. The two-photon absorption studies on the synthesized complexes reveal that values of the absorption cross-section are quite notable and lie in the range of 300-1000 GM in the picosecond case and 45-186 GM in the femtosecond case.

Visible electroluminescence (EL) is observed at room temperature by current injection into Eu:CaF2 layers containing 7.5 and 8.0 at. % Eu grown by molecular beam epitaxy on lightly doped (100) p-type silicon. The EL spectra are broad with peaks near 700 and 600 nm, respectively. Room temperature photoluminescence spectra for the same samples exhibited peaks near 420 nm, with higher doped samples showing a more pronounced long wavelength tail. Although both metal and indium–tin–oxide (ITO) contacts were successfully used for current injection, the best EL intensity stability was achieved with contacts made of a 100 Å thick Al layer covered by a 2500 Å thick ITO layer.