Pyrimidine-based thermally activated delayed fluorescent materials with unique asymmetry for highly-efficient organic light-emitting diodes

We have designed and synthesized three thermally activated delayed fluorescent (TADF) emitters as 2-phenypyrimidine (PPM)-10H-spiro[acridan-9,9′-fluorene] (SPAc) hybrids, collectively named X-SPAc-PPM, featured an asymmetrical D-A-D molecular architecture, through the para-/ortho-, para-/meta-, and para-/ortho- positions of the phenyl spacer in SPAc with PPM unit. The effect of the hybrid connection at ortho-, meta-, or para-positions on the different properties of pyrimidine derivatives is detailed discussed. The investigation suggests that the localization of the HOMO has the preferential choice tendency in asymmetric X-SPAc-PPM, and following the order of para-position arms, ortho-position arms, which adjust effectively the intramolecular charge transfer (ICT) character between SPAc and PPM unit. As a result, three isomers have the smaller ΔE_ST of 0.05–0.16 eV, and exhibit obvious TADF character. In addition, p,m-SPAc-PPM endowed its OLED device with impressive performance, including the maximum external quantum efficiency around 25.1%, and reduced efficiency roll-offs. The smaller ΔE_ST, over 25% EQE, and lower efficiency roll-offs of three isomeric X-SPAc-PPM emitters further prove the great potential of asymmetrical design strategy with the same donor for highly efficient TADF devices.