Promoting Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence via the Heavy-Atom Effect

Area of Science:

• Materials Science
• Organic Electronics
• Quantum Chemistry

Background:

• Thermally activated delayed fluorescence (TADF) molecules are key for durable organic light-emitting diodes (OLEDs).
• Fast reverse intersystem crossing (RISC) is crucial for improving OLED device lifetime.
• Current metal-free TADF molecules achieve RISC rate constants (kRISC) up to 10^8 s^-1.

Purpose of the Study:

• Investigate the heavy-atom effect on TADF properties.
• Design TADF molecules with enhanced RISC rates for improved OLED performance.
• Explore the relationship between heavy atom incorporation and TADF/RISC efficiency.

Main Methods:

• Density functional theory (DFT) calculations to reproduce rate constants.
• Theoretical modeling of sulfur, selenium, tellurium, and polonium-containing TADF molecules.
• Analysis of the heavy-atom effect on RISC and TADF mechanisms.

Main Results:

• DFT calculations validated the kRISC of 10^8 s^-1 for a sulfur-containing TADF molecule.
• The heavy-atom effect was clarified to accelerate the RISC process.
• Predicted kRISC exceeding 10^10 s^-1 for selenium- and tellurium-containing TADF molecules.
• A polonium-containing molecule exhibited phosphorescence, not TADF, indicating an optimal heavy-atom effect range.

Conclusions:

• Incorporating heavier atoms like selenium and tellurium can significantly boost RISC rates in TADF emitters.
• An excessively strong heavy-atom effect can disrupt the TADF mechanism, favoring phosphorescence.
• Strategic molecular design leveraging the heavy-atom effect is essential for developing next-generation, high-performance TADF OLEDs.