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

• Published in: Journal of physical chemistry. C Vol. 127; no. 1; pp. 358 - 367
• Main Authors: Serdiuk, Illia E., Jung, Seyoung, Mońka, Michał, Ryoo, Chi Hyun, Park, Soo Young
• Format: Journal Article
• Language: English
• Published: American Chemical Society 12.01.2023
• Subjects: C: Spectroscopy and Dynamics of Nano, Hybrid, and Low-Dimensional Materials
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.2c06445

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 × 105 to 3 × 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.