Exploring excited state dynamics: Advancements in fluorescence and material design

The light emitted by molecules, especially fluorescence, has fascinated scientists for more than a century, revolutionizing areas such as imaging, sensing and display technology. Recent advances have drawn attention to aggregation-induced emission (AIE), a unique phenomenon in which molecules emit light more efficiently when in a solid or aggregated state. Therefore, studying the reaction kinetics behind this phenomenon is important to understand the changes in molecular structure.

Now, in a recent study, Japanese researchers explored α-substituted dibenzoylmethaneboron difluoride (BF2DBM) complexes to reveal how molecular geometry and confined excited-state dynamics influence AIE. “So far, the AIE phenomenon can only be explained through theoretical quantum chemical calculations. However, in our study, we explained this phenomenon through two types of spectroscopy for the first time,” said lead author Yushi, a doctoral student in the Department of Chemistry Fujimoto said, Shinshu University Graduate School of Science and Technology, Japan. The research was conducted in collaboration with Osaka University and Aoyama Gakuin University. The research results were published in the “Journal of the American Chemical Society”, Volume 146, Issue 47, on November 17, 2024.

AIE is a fascinating phenomenon that challenges conventional quenching behavior in many materials. Most of the time, molecules tend to lose their luminescence when aggregated due to quenching effects. Certain molecules that exhibit the AIE phenomenon tend to glow rather than dim under restricted conditions. This happens because, in solid form, the molecules cannot move freely. These limitations help them glow rather than otherwise lose energy. This behavior can be explained through the Restricted Access Cone Intersection (RACI) model, which describes how structural changes in a molecule control its ability to emit light. The researchers demonstrated this effect in synthetic molecules of BF2DBM derivatives, namely 2aBF2 and 2amBF2, which are α-methyl-substituted derivatives. “We analyzed the AIE effect of molecules in solids and solutions using advanced techniques such as steady-state UV-visible spectroscopy and fluorescence spectroscopy, and time-resolved visible and infrared spectroscopy to observe the luminescence behavior of molecules over time,” explained Professor Hiroshi Miyasaka of Osaka University Famous researcher.

The first molecule 2aBF2 exhibits strong fluorescence in both solution and solid state, while the second molecule 2amBF2 exhibits weaker fluorescence in solution but brighter emission in solid form. Co-author Professor Akira Sakamoto of Aoyama Gakuin University clarified this statement: “Spectroscopy is a letter sent by a molecule. Here, the shape of the molecule plays a crucial role, and 2amBF2 adopts a curved configuration in solution, which is transmitted through The study also showed that rapid changes in 2amBF2 were observed over short periods of time in solution due to the loss of energy during non-radiation, which in the solid form is restricted in bending. These rapid transitions into curved shapes promote energy loss and suppress fluorescence.

These findings have important implications for the future development of organic light-emitting diodes (OLEDs) and bioimaging technologies. As co-author Professor Fuyuki Ito points out: “The exploration of excited-state dynamics is crucial to enhance the performance of luminescent materials, which can facilitate advances in OLED applications and bioimaging.” This insight highlights the importance of understanding the dynamics of excited states. How molecular behavior can improve the performance and efficiency of these cutting-edge technologies. By leveraging advanced spectroscopic and computational tools, this work provides new clues about how molecules interact with energy, deepening our understanding of fluorescence and its practical applications.