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.