Thermally activated delayed fluorescence carbazole‐triazine dendrimer with bulky substituents

Carbazole‐triazine dendrimers with a bulky terminal substituent were synthesized, and the thermally activated delayed fluorescence (TADF) property was investigated. Compared to unsubstituted carbazole dendrimers, dendrimers with bulky terminal substituents showed comparable to better photoluminescence quantum yields (PLQY) in neat films. Phenylfluorene (PF)‐substituted dendrimers showed the highest PLQY of 81%, a small ΔEst of 0.06 eV, and the fastest reverse intersystem crossing (RISC) rate of ∼1 × 105 s−1 compared to other dendrimers. Phosphorescence measurements of dendrimers and dendrons (fragments) indicate that the close proximity of the triplet energy of phenylfluorene‐substituted carbazole dendrons (3LE) to that of phenylfluorene‐substituted dendrimers (1CT, 3CT) contributes to RISC promotion and improves TADF efficiency. Terminal modification fine‐tunes the energy level and suppresses intermolecular interactions, and this study provides a guideline for designing efficient solution‐processable and non‐doped TADF materials. New second‐generation carbazole‐triazine dendrimers were synthesized, and the photoluminescence quantum yield (PLQY) of 81% in the neat film was achieved by substituting the bulky phenylfluorene group at the periphery. Fine‐tuning the locally excited state triplet energy was the key to achieving high PLQY owing to the high reverse intersystem crossing rate (kRISC).