Researchers demonstrate excited state dynamics of BF₂DBM derivatives using structural spectroscopic analysis and computational modelling. Credit: Yushi Fujimoto, Shinshu University (https://doi.org/10.1021/jacs.4c10277). CC BY-NC-ND.

New research from Japan exploring excited state dynamics has provided advancements in fluorescence and material design.

The researchers revealed new insights into molecular behaviour that could unlock potential for organic light-emitting diodes and bioimaging applications.

Excited state dynamics are essential for understanding fluorescence properties in molecules, impacting their application in technologies.

Recent research at Shinshu University, in collaboration with Osaka University and Aoyama Gakuin University, explored how molecular structure and geometry influence light emission in aggregation-induced emission molecules.

The study reveals 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.

Fascination

Light emission from molecules, particularly fluorescence, has fascinated scientists for over a century, revolutionising 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.

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

Phenomenon

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.

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, namely 2aBF₂ and 2amBF₂, which were α-methyl-substituted derivatives.

Professor Hiroshi Miyasaka, a well-known researcher from Osaka University, explained: “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.”

Strong flourescence 

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 Professor Akira Sakamoto from Aoyama Gakuin University said: “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.

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. The findings are published in Journal of the American Chemical Society.