Exploring excited state dynamics: advances in fluorescence and materials design

Insights into molecular behaviour could unlock potential for organic light-emitting diodes and other bioimaging applications

Researchers at Shinshu University in Nagano, Japan, have advanced the knowledge base on how molecular structure/geometry influences the way light is emitted in aggregation-induced emission molecules.

The study revealed that changes in molecular shape affect emission behaviour 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 been a significant subject of research for more than a century with advanced technologies developed in the areas of imaging, sensing and displays. Recen t advances have brought attention to aggregation-induced emission (AIE) ─ a phenomenon where molecules emit light more efficiently when in a solid or aggregated state.

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,” said lead author Yushi Fujimoto, a doctoral student at the Department of Chemistry, Graduate School of Science and Technology, Shinshu University.

AIE is a fascinating phenomenon that challenges the conventional quenching behaviour 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.

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This behaviour 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 synthesised molecules of BF₂DBM derivatives ─ 2aBF₂ and 2amBF₂ ─ which were α-methyl-substituted derivatives. “We analysed 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 behaviour over time,” explains Professor Hiroshi Miyasaka, from Osaka University ─ the study was conducted in collaboration with Osaka University and Aoyama Gakuin 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.

“Spectroscopy is a message 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,” said co-author Professor Akira Sakamoto from Aoyama Gakuin University.

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.

"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," co-author Professor Fuyuki Ito of Shinshu University added. 

This insight emphasises how understanding the molecular behaviour 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.

For further reading please visit: 10.1021/jacs.4c10277