Geometric Isomers of Asymmetric Rigid Four-Membered Chelating Ring Based Deep-Red-Emitting Iridium Complexes Featuring Three Charged (0, -1, -2) Ligands

Geometric isomers are very important and interesting in the field of optical materials. In this work, we have designed and synthesized a new series of geometric isomers of iridium complexes featuring three charged (0, -1, -2) ligands, which contain a rigid asymmetric four-membered Ir-N-C-S chelating ring. The reaction of iridium complex precursors (2a and 2b) with an equal amount of the four-membered ring S<^>N ligand at a low temperature produces the kinetic isomers Ir1K and Ir2K, while a higher temperature leads to the formation of their corresponding thermodynamic isomers Ir1T and Ir2T. The X-ray diffraction analysis shows that the kinetic isomers exhibit a trans-S<^>N configuration, which is in contrast with the trans-N<^>N configuration of the thermodynamic isomers, and their coordination bond lengths, bond angles and packing patterns are also quite different. More importantly, all isomers showed efficient deep-red emission (619-676 nm), and the thermodynamic isomers have shorter emission wavelengths, longer excited state lifetimes and higher luminescent efficiencies than their corresponding kinetic isomers. Theoretical calculations show that the four-membered ring S<^>N ligand in the thermodynamic isomers is more involved in the excited state than that in the kinetic isomers, and the (MLCT)-M-3 effects are more pronounced in the thermodynamic isomers. Notably, OLED devices incorporating both thermodynamic and kinetic isomers (Ir2T and Ir2K) as emitting layers can achieve good maximum external quantum efficiencies (EQEs) (5.0% and 4.6%) peaking at 642 nm and 643 nm of the deep-red region with CIE coordinates (0.675, 0.323) and (0.668, 0.329), respectively, accompanied by a low turn-on voltage (3.0 V). This study provides an important strategy for the design of deep-red emitting geometric isomers of iridium complexes and their photoelectric applications.