Small molecular substitutions reshape energy pathways behind aggregation-induced emission

Certain chemical substances, called luminogens, naturally emit light. Yet in solid form, they often quench their own glow. These aggregation-induced emission (AIE) luminogens display the opposite effect: they glow weakly in dilute solution but shine bright when aggregated or in a solid state. This peculiar property has made AIE molecules valuable for various applications, such as organic light-emitting diodes, microbial sensors, bioimaging tools, and photodynamic therapy.

What drives the emission tendencies of AIE molecules? Associate Professor Gen-ichi Konishi from Tokyo Institute of Technology and his research team have recently suggested that AIE behavior hinges on significant structural changes within the molecules. In solution, these structural shifts tend to deactivate the excited state without emitting light. However, in a solid state, these alterations are suppressed, permitting the molecules to shine.

Such complex processes can be explored through excited-state potential energy surfaces, particularly through conical intersections (CIs). At these points, excited molecules swiftly revert to the ground state without emitting light. However, calculating and examining these intricate surfaces is computationally intensive, making it challenging to pinpoint straightforward molecular factors that can predict AIE behavior. Historically, many AIE molecules were discovered through trial and error rather than rational design.

In this study, Konishi's team managed to predict AIE behavior using a small set of descriptors derived from quantum chemical calculations. They also discovered that donor and acceptor groups, as well as their placement, are crucial structural factors that mold the excited-state potential energy surface and induce AIE behavior. Their findings, published in Advanced Science, combine quantum chemical calculations with organic synthesis and experimental spectroscopy to devise a computation-guided approach for understanding and investigating AIE materials.

The researchers targeted low-energy conical intersections, which act as energy funnels, allowing excited molecules to rapidly return to the ground state without emitting light. They hypothesized that minor molecular adjustments could regulate the excited-state potential energy surface and determine whether a molecule exhibits AIE.

To validate this hypothesis, the team focused on bridged stilbenes, a well-studied group of light-responsive molecules. They conducted quantum chemical potential energy surface analyses on 30 stilbene derivatives to understand how bridge size, donor, acceptor substitution, and substitution location affect CI accessibility.

The calculations revealed that the introduction and positioning of donor and acceptor groups significantly alter the energetic relationship between the initially excited Franck–Condon state and the CI. This control over the excited-state potential energy surface and regulation of CI accessibility were key to their findings. Using these computational insights, the team synthesized four representative bridged stilbenes.

Their photophysical properties matched the predictions from the computational analysis. Ultrafast transient absorption spectroscopy, conical intersection topology analysis, and additional theoretical calculations supported the notion that donor and acceptor group arrangements modify the CI topology and accessibility, thus controlling nonradiative deactivation.

Konishi emphasized that this work marks a significant shift in luminescent material design, moving from empirical trial and error towards rational, computation-guided molecular exploration. The study provides a mechanistic framework for understanding how molecular structure influences AIE through the excited-state potential energy surface.

Instead of depending solely on empirical structural rules, the research shows that AIE behavior can be evaluated using a limited number of physically meaningful computational descriptors linked to conical intersection accessibility. The proposed framework can potentially guide the development of high-efficiency OLED materials, fluorescent probes for bioimaging and sensing applications.