Design of carbazole-based platinum complexes with steric hindrance for efficient organic light-emitting diodes

The construction of platinum complexes with high steric hindrance is expected to suppress triple-triplet annihilation and π-π stacking to achieve high-performance organic light-emitting diodes (OLEDs) with low efficiency roll-off. Herein, two large steric hindrance platinum complexes (N-CzPhPtacac and N-CzCF₃PhPtacac) were prepared by taking advantage of steric hindrance between the phenyl group on carbazole and the functional group (phenyl and trifluoromethyl substituted phenyl) at the 3-position of a pyridine moiety. Due to the similar electron cloud distribution and gap difference between the HOMO and LUMO, the two complexes showed similar orange-red emission peaks at 590 and 596 nm with high PL quantum yields of 90% and 92% and short excited state lifetimes of 2.77 and 3.08 μs in doped films, respectively. Consequently, OLEDs based on N-CzPhPtacac and N-CzCF₃PhPtacac showed maximum external quantum efficiency (EQE_max) values of 15.4% and 18.9%, respectively. Importantly, benefitting from the more stretched spatial configuration from the -CF₃ effect, the corresponding OLED exhibited a lower efficiency roll-off, with an EQE of 18.1% at 1000 cd m^-2.