Novel blue multiresonance thermally activated delayed fluorescence host materials, including Ge-based bulky groups

We synthesized three materials, namely, TDBA-Ge, mTDBA-Ge, and mTDBA-2Ge, as blue host emitters. These materials incorporate a tetraphenylgermanium (TPG) group with a germanium atom into the main backbone of 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (DOBNA), demonstrating multiple-resonance-induced thermally activated delayed fluorescence (MR-TADF). All three materials exhibited high thermal stability with a glass transition temperature (T_g) exceeding 100 °C. The increased molecular distance also demonstrated the inhibition of self-quenching between molecules in the packing. All three materials showed ΔE_ST values within 0.3 eV, confirming their potential for MR-TADF characteristics. Especially, mTDBA-2Ge exhibited the lowest ΔE_ST value of 0.11 eV among the three materials. When these host materials were doped with the conventional dopant ν-DABNA, efficient energy transfer between the two materials was observed, and the resulting device efficiency was confirmed. In comparison to TDBA-Ph, which lacks the TPG moiety and contains only phenyl groups, an approximately 1.5-1.8-fold increase in external quantum efficiency max (EQE_max) was observed. Among the three materials, mTDBA-2Ge exhibited the highest efficiency, with an EQE_max of 24.41%.