The dynamics of excited states are critical for grasping the fluorescence characteristics of molecules, which influence their use in various technologies. Recent findings from Shinshu University investigate the impact of molecular structure and geometry on light emission in aggregation-induced emission (AIE) molecules. The research indicates that alterations in molecular shape modify emission behavior in both liquid and solid forms. These revelations are vital for progressing technologies such as organic light-emitting diodes and bioimaging, paving the way for innovations in material development and energy interactions.
The phenomenon of light emission from molecules, especially fluorescence, has intrigued researchers for more than a century and has transformed fields like imaging, sensing, and display technologies. Recent breakthroughs have highlighted aggregation-induced emission (AIE), a distinctive effect where molecules become more efficient at emitting light when in a solid or aggregated state. Understanding the underlying reaction dynamics of this phenomenon is essential to comprehend the structural changes within molecules.
In a recent investigation, Japanese researchers delved into α-substituted dibenzoylmethanatoboron difluoride (BFâ‚‚DBM) complexes to discover how molecular shape and constrained excited state dynamics affect AIE. “Previously, the AIE phenomenon was mainly described through theoretical quantum chemical calculations. However, in our study, we shed light on this phenomenon using two spectroscopic methods for the first time,” states lead author Yushi Fujimoto, a doctoral student in the Department of Chemistry at Shinshu University’s Graduate School of Science and Technology. This research was a collaborative effort with Osaka University and Aoyama Gakuin University, with results featured in Volume 146, Issue 47 of the Journal of the American Chemical Society, published on November 17, 2024.
AIE is a captivating phenomenon that contradicts the typical quenching behavior observed in many materials. Generally, when molecules aggregate, they tend to lose their luminescent properties due to quenching effects. In contrast, certain molecules demonstrating the AIE effect emit light rather than fading under restricted conditions. In a solid state, the mobility of these molecules is limited, which enhances their ability to emit light and reduces energy loss through other mechanisms. This behavior is explained by the restricted access to conical intersections (RACI) model, which illustrates how alterations in a molecule’s structure can influence its light emission capability. The researchers showcased this effect in synthesized BFâ‚‚DBM derivatives, specifically 2aBFâ‚‚ and 2amBFâ‚‚, which are α-methyl-substituted derivatives. “We analyzed the AIE effects of these molecules in both solid and liquid states using sophisticated techniques like steady-state UV-visible and fluorescence spectroscopy, alongside time-resolved visible and infrared spectroscopy to track the molecules’ light emission over time,” explains Prof. Hiroshi Miyasaka, a prominent researcher from Osaka University.
The first molecule, 2aBFâ‚‚, demonstrated robust fluorescence in both solution and solid forms, whereas the second molecule, 2amBFâ‚‚, showed reduced fluorescence in solution yet emitted significantly brighter light in solid state. Co-author Prof. Akira Sakamoto from Aoyama Gakuin University elucidates, “Spectroscopy acts as a communication from molecules. Here, the configuration of the molecules was pivotal. In solution, 2amBFâ‚‚ adopted a bent shape, which caused energy loss through non-radiative processes, resulting in diminished fluorescence. In solid state, this bending was restricted, compelling the molecule to maintain a stable structure that effectively emitted light.” The study also noted that rapid changes occurred within mere trillionths of a second in the solutions, where 2amBFâ‚‚ molecules underwent bending that led to energy loss and reduced fluorescence.
These discoveries hold considerable promise for the future evolution of organic light-emitting diodes (OLEDs) and bioimaging technologies. As co-author Prof. Fuyuki Ito remarks, “Investigating excited state dynamics is vital for improving the properties of luminescent materials, facilitating advancements in OLED applications and bioimaging.” This perspective underscores the importance of understanding molecular behavior in excited states to enhance the performance and efficacy of these cutting-edge technologies. By employing advanced spectroscopy and computational methods, this research illuminates molecular interactions with energy, enriching our comprehension of fluorescence and its practical uses.
