Recent Advances in <i>d</i>‐<i>f</i>  Transition Lanthanide Complexes for Organic Light‐Emitting Diodes: Insights Into Structure–Luminescence Relationships

Abstract Recently, d ‐ f transition lanthanide complexes have emerged as promising emitters with high exciton utilization efficiency (EUE) and short excited state lifetime simultaneously, demonstrating potential applications in organic light‐emitting diodes (OLEDs). First, the d ‐ f transition is parity‐allowed, resulting in short excited‐state lifetimes of the complexes in the nanosecond (ns) scale. Second, the 5 d orbitals are sensitive to ligand‐field environments, and their splitting can be finely tuned by the ligand field, enabling precise control of emission colors. Third, the spin‐allowed single‐electron transitions, such as those in open‐shell Ce(III) and Eu(II) complexes, help address the efficiency limitations arising from singlet and triplet excitons. To date, Ce(III)‐based blue OLEDs have achieved external quantum efficiencies (EQEs) exceeding 20% and brightness levels over 30 000 cd m −2 . However, OLEDs based on d ‐ f transition lanthanide complexes still face significant challenges, including color tunablility, photoluminescence quantum yields (PLQYs), and stability. This review first provides an introduction to OLEDs and luminescent materials. Next, an overview of the ligands used in d ‐ f transition lanthanide complexes is presented, covering four distinct ligands types. Finally, an in‐depth discussion explores the relationship between ligand structures, d ‐ f transition lanthanide complexes, and their photoluminescence (PL) and electroluminescence (EL) performance.