Nondoped Deep‐Blue Organic Light‐Emitting Diodes with Color Stability and Very Low Efficiency Roll‐Off: Solution‐Processable Small‐Molecule Fluorophores by Phosphine Oxide Linkage

A series of solution‐processable small molecules PO1 – PO4 were designed and synthesized by linking N‐phenylnaphthalen‐1‐amine groups to a phenyl phosphine oxide core through a π‐conjugated bridge, and their thermal, photophysical, and electrochemical properties were investigated. The phosphine oxide linkage can disrupt the conjugation and allows the molecular system to be extended to enable solution processability and high glass transition temperatures (159–181 °C) while preserving the deep‐blue emission. The noncoplanar molecular structures resulting from the trigonal‐pyramidal configuration of the phosphine oxide can suppress intermolecular interactions, and thus these compounds exhibit strong deep‐blue emission both in solution and the solid state with high photoluminescent quantum yield (PLQY) of 0.88–0.99 in dilute toluene solution. Solution‐processed nondoped organic light‐emitting diodes featuring PO4 as emitter achieve a maximum current efficiency of 2.36 cd A^?1 with CIE coordinates of (0.15, 0.11) that are very close to the NTSC blue standard. Noticeably, all devices based on these small‐molecular fluorescent emitters show striking deep‐blue electroluminescent color stability and extremely low efficiency roll‐off.