posted on 2022-12-21, 16:09authored byIllia E. Serdiuk, Seyoung Jung, Michał Mońka, Chi Hyun Ryoo, Soo Young Park
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.