Interplay of Charge Transfer and Local Triplet States in Donor–Acceptor-Based TADF Compounds

Donor–acceptor-based thermally activated delayed fluorescence (TADF) emitters have a complicated energy level scheme with multiple singlet and triplet energy levels. Among them, TADF compounds with the lowest-energy charge-transfer triplet state (type III compounds) are especially interesting due to their specific emission properties. We show that type III TADF compounds may show dual phosphorescence at low temperatures as well as weak high-energy emission at room temperature, somewhat resembling the phosphorescence spectrum of the local triplet. Such untypical behavior was demonstrated only in compounds with a low energy gap between the triplet states of different character, imposing efficient communication between them through vibronic coupling.