Solution‐Processible Brilliantly Luminescent Eu III Complexes with Host‐Featured Phosphine Oxide Ligands for Monochromic Red‐Light‐Emitting Diodes

A series of solution‐processible electroluminescent (EL) Eu 3+ complexes were constructed with a self‐host strategy, in which neutral ligands were employed as functionalized bidentate phosphine oxide (PO) ligands named DPEPOAr n (DPEPO=bis(2‐(diphenylphosphino)phenyl) ether oxide). The solubility of these complexes was dramatically improved owing to the increased ratios of organic components. This further enhanced the antenna effect of these ligands in both singlet and triplet energy‐transfer processes to support high photoluminescent quantum yields (PLQYs) up to 86 % for their Eu 3+ complexes, which is outstanding among conjugated Eu 3+ complexes. Density function theory (DFT) simulations and electrochemical analysis further verified the contributions of DPEPOAr n to the carrier injecting/transporting ability of the complexes. In this sense, these functionalized PO ligands served as hosts in optoelectronic processes, which rendered the self‐host feature of their Eu 3+ complexes. With the enhanced electrical properties, the spin‐coated single‐layer organic light‐emitting diodes (OLEDs) of these complexes achieved improved low driving voltages, such as onset voltages about 6 V, compared to their Eu 3+ ‐contained red‐emitting polymeric analogues. [Eu(DBM) 3 DPEPODPNA 2 ] (DBM=1,3‐diphenylpropane‐1,3‐dione, DPNA=diphenylnaphthylamine) with the most enhanced electrical properties and suitable frontier molecular orbital (FMO) and triplet state locations endowed its devices with the biggest maximum luminance of >90 cd m −2 and the highest EL efficiencies. This work verified the potential of small molecular EL Eu 3+ complexes for solution‐processed OLEDs through rational function integrations.