This tutorial review covers recent contributions in the area of hole-transporting hydrazones, which are widely used in optoelectronic devices. It is addressed to students and researchers interested in the synthesis and properties of organic electroactive materials. The thermal, charge transport and other properties of electroactive hydrazones are compared and the relationships between the molecular structures and properties are emphasized. The first part discusses the low-molar-mass hydrazones and presents examples of their synthetic routes and chemical structures. In the second part, polymeric arylaldehyde hydrazones containing hydrazone moieties as the side substituents and in the main-chain are described.

The bicarbazole derivative 4,4′-(9H,9′H-[3,3′-bicarbazole]-9,9′-diyl)bis(3-(trifluoromethyl)benzonitrile), denoted as pCNBCzoCF₃, was synthesized and tested for white OLED applications. pCNBCzoCF₃ demonstrated an extremely small value of the singlet–triplet energy gap that caused intensive thermally activated delayed fluorescence (TADF). In addition, this compound is able to form exciplex-type excited states at the interface with star-shaped 4,4′,4′′-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA). Combining the TADF emission of pCNBCzoCF₃ with the exciplex emission from the pCNBCzoCF₃/m-MTDATA interface, we fabricated a number of highly efficient “warm-white” OLEDs, the electroluminescence of which was close to candle emission. The best device demonstrated a very high brightness of 40 900 Cd m⁻² (at 15 V), current efficiency of 53.8 Cd A⁻¹ and power efficiency of 19.3 lm W⁻¹, while the external quantum efficiency reached 18.8%. The fabricated devices demonstrated high emission characteristics even for the standard test at 1000 Cd m⁻² (current efficiency of 46.2 Cd A⁻¹, power efficiency of 10.6 lm W⁻¹, EQE of 17.0%).

Theoretical and experimental approaches for the development of organic room temperature phosphorescence compounds exhibiting blue, green, red, and white emissions for many potential applications are analysed and classified.