What is TADF? How Does Molecular Shape Affect Light Emission?

You might have seen many glowing displays and lights around you. Many of these utilize “Organic EL” (OLED) technology. Among OLED materials, “TADF (Thermally Activated Delayed Fluorescence)” molecules are particularly noteworthy. TADF molecules are highly efficient at converting electrical energy into light, making them promising key materials for next-generation displays and lighting. However, even slight structural differences in TADF molecules can drastically change how they emit light. Specifically, it’s not well understood how the “substitution pattern” within a molecule—the way atoms or groups of atoms are connected—affects the complex process of light emission, known as “excited-state relaxation.”

Solving the Mystery with “Look-Alike” Molecules

In this research, we focused on two types of TADF molecules: MA-TA and mMA-TA. These two molecules consist of the exact same atoms but differ in their bonding positions, specifically “para-connected” versus “meta-connected.” You can think of them as “look-alike” molecules made from identical materials but assembled slightly differently. We meticulously investigated the changes these molecules undergo in the very short period (picoseconds to nanoseconds) from absorbing light to emitting it. We used specialized experimental techniques like “time-resolved photoluminescence measurements” and “femtosecond transient absorption spectroscopy.” These methods are like using ultra-high-speed cameras to observe the “moment of light emission” in slow motion.

An Unexpected “Intermediate State” Governs Emission

The experiments revealed significant differences in the light emission behavior of these two “look-alike” molecules. The MA-TA molecule, upon absorbing light, underwent a relatively simple transformation, rapidly forming a “charge-transfer-like excited state” within a few picoseconds, which is conducive to light emission. In contrast, the mMA-TA molecule exhibited more complex dynamics. It was found to pass through an additional, temporary “intermediate excited-state species”—a state not clearly observed in MA-TA—during its journey from light absorption to emission. This “intermediate state” is a plausible origin for mMA-TA’s characteristic complex behaviors, such as a time-dependent red shift of the emission band and biexponential photoluminescence decay, and may contribute to its lower emission efficiency.

Designing Molecules to Create the Light of the Future

This study is the first to clearly demonstrate that the “substitution pattern” of TADF molecules strongly influences the early pathways of excited-state relaxation after light absorption. Specifically, understanding and controlling the formation of the “intermediate excited-state species” discovered in mMA-TA will be a crucial key to designing more efficient TADF molecules. For high school students, chemistry isn’t just about mixing substances. By exploring the microscopic world of molecules and understanding their behavior, we can unlock new technologies for future displays and lighting. How about exploring a world of chemistry where you can create the future of light through molecular design?

Reference Paper

• Akira Yamakata, Katsuaki Suzuki, Kosaku Kato, Hironori Kaji (2026) “Substitution-Dependent Excited-State Relaxation Pathways in the TADF Structural Isomers MA-TA and mMA-TA” AIP Publishing. DOI: 10.60893/figshare.jcp.c.8658363

Learn more about Prof. Yamakata → Surface Physical Chemistry