Excited state dynamics are essential for understanding fluorescence properties in molecules, impacting their application in technologies. Recent research at Shinshu University explores how molecular structure and geometry influence light emission in aggregation-induced emission molecules. The study reveals that changes in molecular shape affect emission behavior in both solution and solid states. These insights are crucial for advancing applications like organic light-emitting diodes and bioimaging, enabling innovations in material design and energy interactions.
Light emission from molecules, particularly fluorescence, has fascinated scientists for over a century, revolutionizing areas like imaging, sensing, and display technologies. Recent advancements have brought attention to aggregation-induced emission (AIE) -- a unique phenomenon where molecules emit light more efficiently when in a solid or aggregated state. Studying the reaction dynamics underlying this phenomenon, is thus, important for understanding the molecular structural changes.
Now, in a recent study, researchers from Japan explored α-substituted dibenzoylmethanatoboron difluoride (BF₂DBM) complexes to unravel how molecular geometry and restricted excited state dynamics influence AIE. "AIE phenomenon has only been explained by theoretical quantum chemical calculations up till now. However, in our study, we explained this phenomenon by two spectroscopies for the first time," says lead author Yushi Fujimoto, a doctoral student at the Department of Chemistry, Graduate School of Science and Technology, Shinshu University, Japan. The study was conducted in collaboration with Osaka University and Aoyama Gakuin University. The findings were published in Volume 146, Issue 47 of the Journal of the American Chemical Society on November 17, 2024.
AIE is a fascinating phenomenon that challenges the conventional quenching behavior seen 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 phenomena tend to emit light instead of dimming under restricted conditions. This happens because, in solid form, the molecules cannot move freely. These restrictions help them emit light rather than lose energy in other ways. This behavior is explained by the restricted access to conical intersection (RACI) model, which describes how structural changes in a molecule can control its ability to emit light. The researchers demonstrated this effect in synthesized molecules of BF₂DBM derivatives, namely 2aBF₂ and 2amBF₂, which were α-methyl-substituted derivatives. "We analyzed the AIE effects of the molecules in solids and solutions using advanced techniques like steady-state UV-visible and fluorescence spectroscopy and time-resolved visible and infrared spectroscopy to observe the molecule's light emission behavior over time," explains Prof. Hiroshi Miyasaka, a well-known researcher from Osaka University.
The first molecule, 2aBF₂, exhibited strong fluorescence in both solution and solid states, while the second molecule, 2amBF₂, displayed weaker fluorescence in solution but showed much brighter emission in solid form. Co-author Prof. Akira Sakamoto from Aoyama Gakuin University clarifies this saying, "Spectroscopy is a letter sent from molecules. Here, the molecular shape played a crucial role, with 2amBF₂ adopting a bent configuration in solution, causing energy loss through non-radiative processes, leading to weaker fluorescence. In solid form, the bending was restricted, forcing the molecule to maintain a stable structure that emitted light." The study also reveals that rapid changes were observed within a short time frame. In solutions, the 2amBF₂ molecules underwent shape changes within a few trillionths of a second. These quick transitions to bent shapes facilitated energy loss and suppressed fluorescence.
These findings have significant implications for the future development of organic light-emitting diodes (OLEDs) and bioimaging technologies. As co-author Prof. Fuyuki Ito points out, "The exploration of excited state dynamics is crucial for enhancing the properties of luminescent materials, which can lead to advancements in OLED applications and bioimaging." This insight emphasizes how understanding the molecular behavior in excited states can improve the performance and efficiency of these cutting-edge technologies. By leveraging advanced spectroscopy and computational tools, the work sheds new light on how molecules interact with energy, deepening our understanding of fluorescence and its practical applications.
