Contradictory Role of Locally-Excited Triplet States in Blue Thermally Activated Delayed Fluorescence of s-Triazine-Based Emitters

In the search of the organic materials with high energies of the lowest excited states for application in highly demanded blue OLEDs, we discovered a tunability of the triplet state localized on phenyl-s-triazines (3LE) by substitution at s- triazine. A widely used three-state model of thermally activated delayed fluorescence (TADF) suggests the key role of energetic closeness of 3LE and charge-transfer states (1CT and 3CT) for achieving fast reverse intersystem crossing (rISC) and high external quantum efficiency (EQE) in OLEDs. Following this model and using phenyl-s-triazines with high 3LE energies in the role of electron acceptors, a series of blue thermally activated delayed fluorescence (TADF) emitters was developed. While emission color shifted from sky blue to deep blue, the photoluminescence quantum yields decreased from 90 to 35%, the rISC rate constant dropped down more than ten times from 4 x 105 to 3 x 104 s-1, while maximum EQE decreased from 14 to 4%. Nevertheless, the designed simultaneous increase of 1,3CT and 3LE energies enabled the analysis of emitters with similar 1CT-3LE energy gaps, but different 1CT-3CT ones. Photophysical and electroluminescence characteristics of emitters investigated in amorphous films of different polarity complemented with quantum-chemical calculations revealed that the enhanced 3LE-1CT interaction is mainly beneficial for ISC, but not rISC, and thus undesired for OLEDs in contradiction to the three-state model prediction. Instead, the direct 3CT -> 1CT interaction was undoubtedly found to be the most crucial factor for rISC and OLED efficiency. We conclude that exploration of the ways to enhance 3CT -> 1CT transition should be a novel design rule for blue TADF emitters.