Bridging Small Molecules to Conjugated Polymers: Efficient Thermally Activated Delayed Fluorescence with a Methyl-Substituted Phenylene Linker.

Based on a "TADF + Linker" strategy (TADF=thermally activated delayed fluorescence), demonstrated here is the successful construction of conjugated polymers that allow highly efficient delayed fluorescence. Small molecular TADF blocks are linked together using a methyl-substituted phenylene linker to form polymers. With the growing number of methyl groups on the phenylene, the energy level of the local excited triplet state (3 LEb ) from the delocalized polymer backbone gradually increases, and finally surpasses the charge-transfer triplet state (3 CT). As a result, the diminished delayed fluorescence can be recovered for the tetramethyl phenylene containing polymer, revealing a record-high external quantum efficiency (EQE) of 23.5 % (68.8 cd A-1 , 60.0 lm W-1 ) and Commission Internationale de l'Eclairage (CIE) coordinates of (0.25, 0.52). Combined with an orange-red TADF emitter, a bright white electroluminescence is also obtained with a peak EQE of 20.9 % (61.1 cd A-1 , 56.4 lm W-1 ) and CIE coordinates of (0.36, 0.51).