Donor-Centered TADF Molecules for OLED Brightness
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Solution Overview
Problem
Current TADF emitters face challenges in achieving high photoluminescence quantum yield, efficient reverse intersystem crossing, and external quantum efficiency, with existing molecules exhibiting long emission decay times and low brightness levels, limiting their performance in OLED devices.
Innovation Solution
Design of TADF molecules with a central electron donor moiety, such as triazatruxene, surrounded by three electron acceptor moieties like dibenzothiophene-S,S-dioxide, which are twisted to achieve a torsion angle between 40° to 90°, facilitating fast reverse intersystem crossing and high photoluminescence quantum yield, while maintaining a small energy gap between singlet and triplet states for efficient thermally activated delayed fluorescence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of moving object
If a central acceptor unit surrounded by peripheral donor units configuration is used, then TADF emission can be achieved, but the emission decay time is long and brightness level is low
Solution Approach 1:
The patent inverts the conventional TADF molecular configuration by placing the electron donor unit (triazatruxene) at the center and electron acceptor units (dibenzothiophene-S,S-dioxide) at the periphery, rather than the typical acceptor-centered design. This structural inversion enables faster reverse intersystem crossing rates and shorter emission decay times while maintaining high brightness levels, directly resolving the contradiction between emission duration and brightness.
Solution Approach 2:
The patent modifies key molecular parameters including introducing a torsion angle of 40-90° between donor and acceptor units, which optimizes the energy gap between singlet and triplet states. This parameter optimization accelerates the reverse intersystem crossing process, reducing emission decay time from microseconds to sub-microsecond range while enhancing brightness through improved radiative transition rates.
2Speed
If the torsion angle between donor and acceptor units is increased to facilitate fast reverse intersystem crossing, then the rate of reverse intersystem crossing improves, but the energy gap between singlet and triplet states increases
Solution Approach 1:
The patent identifies and optimizes the torsion angle parameter to a specific range of 40-90°, which simultaneously achieves fast reverse intersystem crossing rates (krISC > 10⁶ s⁻¹) and maintains small energy gaps (ΔEST < 0.2 eV) between singlet and triplet states. This precise parameter control resolves the contradiction by finding the optimal balance point where both requirements are satisfied.
Solution Approach 2:
The patent replaces purely thermal activation mechanisms with a vibronically coupled spin-orbit coupling mechanism for reverse intersystem crossing. By designing molecules with specific vibrational modes that couple to the singlet-triplet transition, the system achieves faster krISC rates without requiring large thermal energy input, thus maintaining small ΔEST while accelerating the rISC process.
3Reliability
If conventional TADF molecules are used, then external quantum efficiency can reach above 20%, but the photoluminescence quantum yield is limited and roll-off is high
Solution Approach 1:
The inverted donor-centered molecular architecture fundamentally changes the photophysical pathways, enabling photoluminescence quantum yields exceeding 70% and external quantum efficiencies above 30%. The donor-centered structure with peripheral acceptors creates more favorable electronic coupling and reduced non-radiative decay pathways compared to conventional acceptor-centered designs.
Solution Approach 2:
The patent creates composite molecular structures combining triazatruxene donor core with dibenzothiophene-S,S-dioxide acceptor units, where the synergistic interaction between components produces enhanced photoluminescence quantum yield and external quantum efficiency. The composite structure allows optimization of both radiative and non-radiative decay channels to achieve superior device performance.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The proposed TADF molecules exhibit high photoluminescence quantum yield, fast reverse intersystem crossing rates, and enhanced external quantum efficiency, leading to improved brightness and stability in OLED devices with reduced roll-off, exceeding 30% external quantum efficiency and achieving brightness levels above 15000 cd/m2.
Implementation Method 1
dark, triplet excited states are converted to emissive singlet states by efficient reverse intersystem crossing (rISC)
Implementation Method 2
the underlying spin flip mechanism in rISC is a second order vibrational coupling spin orbit coupling process
Implementation Method 3
certain molecular vibrations drive coupling between the 3LE and 3CT states causing a thermal equilibrium between these two states
Implementation Method 4
The 3LE state acts as a mediator for both steps with vibrational energy driving the reverse intersystem crossing. Thus it is a thermal activated process.
Implementation Method 5
These systems typically emit from a singlet charge transfer state (1CT)
Data Source
AI summary
A thermally activated delayed fluorescence (TADF) molecule comprising: a central electron donor moiety, wherein the central electron donor moiety is formed of a conjugated multi-ring system comprising three nitrogen atoms; and three electron acceptor moieties, each bonded to the central electron donor moiety via one of the three nitrogen atoms, wherein at least one of the three electron acceptor moieties is twisted relative to the central electron donor moiety defining a torsion angle in a range 40° to <90° whereby the TADF molecule has a photoluminescence quantum yield of >60% and a rate of reverse intersystem crossing from a lowest excited triplet state to a lowest excited singlet state of at least 1×10 s−1.