Ben Zhong Tang shares the story behind aggregation-induced emission and reflects on its impact across chemistry and materials science.
In 2001, Ben Zhong Tang—then an assistant professor at the Hong Kong University of Science and Technology—observed an unexpected reversal.
This behavior, which Tang later termed aggregation-induced emission (AIE), has since grown into a major research field with applications spanning electronics, sensing, and medicine.
In traditional systems, aggregation reduces efficiency; in AIE systems, aggregation improves it.
We are now exploring what we call aggregation-generated functions, where aggregation enables many different properties, not just light emission.
Ben Zhong Tang shares the story behind aggregation-induced emission and reflects on its impact across chemistry and materials science.
For over a century, chemists faced a frustrating limitation: many light-emitting molecules stop glowing when they pack together. This phenomenon, known as aggregation-induced quenching, posed a major challenge for fabricating light-emitting devices, which rely on materials in solid, aggregated forms.
In 2001, Ben Zhong Tang—then an assistant professor at the Hong Kong University of Science and Technology—observed an unexpected reversal. Some molecules were dark in solution yet emitted brightly when aggregated. This behavior, which Tang later termed aggregation-induced emission (AIE), has since grown into a major research field with applications spanning electronics, sensing, and medicine.
Tang, now a professor at the Chinese University of Hong Kong, Shenzhen, reflects on the serendipitous origins of AIE, the science behind it, and how unexpected results can open entirely new directions in research.
For non-experts, how would you explain aggregation-induced emission?
In traditional photophysics, light-emitting molecules behave well when they are isolated. In dilute solutions, they can emit very efficiently — sometimes with nearly 100% quantum yield. But when you bring them together, as in a solid film, their emission weakens or disappears entirely. This is aggregation-caused quenching, and it has been known since the 19th century.
What we observed was the opposite. The molecules themselves were not emissive but, when they aggregated, they became strongly luminescent. That is why AIE is considered counterintuitive — it goes against what people had long believed.
Was this something you were trying to discover?
No, not at all. We were simply trying to make luminescent molecules. But the molecule we synthesized did not emit light in solution, which normally would have been considered a failure.
Then we noticed that in the solid state, it was highly emissive. That contradiction caught our attention.
At first, I thought we had discovered something completely new. But later we realized that similar phenomena had been observed before—people had just not paid much attention to them. That is very common in science. If something does not fit an existing paradigm, it is often ignored. So I prefer to say that we recognized the importance of the phenomenon and gave it a name. That helped others see its value and build on it.
What was the moment when you realized something unusual was happening?
It came from a very simple experiment. A student told me a molecule was not emissive in solution, but when we looked at a thin-layer chromatography plate under UV light, we saw a bright spot. At first, this seemed contradictory. Then we realized that when the solvent evaporates, the molecules form aggregates—and that is when the emission appears.
That was the key insight: the same molecules behave completely differently depending on whether they are isolated or aggregated.
How does AIE work at the molecular level?
In solution, molecules can move freely. They rotate and vibrate, and this motion allows excited-state energy to dissipate as heat instead of light. So you see no emission. When the molecules aggregate, their motion is restricted. They cannot rotate or vibrate as easily, so the energy cannot be lost through these pathways. Instead, it is released as light.
We call this the restriction of molecular motion. Once you understand this, the behavior becomes quite intuitive. Aggregation is not always bad — it can actually enhance emission.
“If you see something unusual, repeat the experiment. If it is reproducible and cannot be explained by current knowledge, then it may be important.”
What kinds of applications does this enable?
The applications are almost endless.
In optoelectronics, AIE materials are very useful for devices such as organic light-emitting diodes. In traditional systems, aggregation reduces efficiency; in AIE systems, aggregation improves it.
In sensing, AIE is especially powerful because you can turn emission on through interaction with a target. For example, if you want to detect a metal ion, you can design a molecule that binds to it. Once binding occurs, molecular motion is restricted, and the system lights up. The same principle can be extended to many other targets. You can detect pollutants in water, ions such as calcium, or even gases like carbon dioxide. If the target interacts with the molecule and restricts its motion, the emission turns on. So, in principle, you can design AIE systems to detect almost anything.
In biomedical applications, the possibilities are especially exciting. We can design AIE systems that accumulate in cancer cells. Once inside, they emit light for imaging, and they can also generate reactive oxygen species or heat to kill the cells. In this way, diagnosis and therapy can be combined in a single system.
Are there still open questions or debates in the field?
Yes, of course. Science is never completely settled.
Most researchers agree on the general mechanism, but there are still debates about the details. That is normal. Even the greatest theories are questioned. Criticism is actually helpful: it forces us to think more deeply and refine our understanding. Over time, the field has expanded beyond emission. We are now exploring what we call aggregation-generated functions, where aggregation enables many different properties, not just light emission.
You’ve also connected AIE to broader philosophical ideas. How so?
AIE challenges the traditional reductionist view that the properties of a system are determined entirely by its individual components. In AIE, the individual molecules are not emissive, but the aggregate is. This is an example of emergence — the whole has properties that the parts do not.
In real life, we do not use single molecules; we use materials, which are aggregates. So we should study aggregates as well, not only molecules. This way of thinking can open new directions in science.
How do you encourage your students to think outside the box?
I tell them that expected results are not enough. If you obtain what you predicted, that is good—but it is not outstanding. You should pay attention to unexpected results. If you see something unusual, repeat the experiment. If it is reproducible and cannot be explained by current knowledge, then it may be important.
Passion is also essential. If you enjoy what you are doing, you will push yourself further. At a university, we are not training technicians—we are training thinkers.
You didn’t originally plan to become a scientist. How did you end up in chemistry?
I did not choose chemistry myself. After I took the national entrance examination, I was assigned to study chemical engineering of polymer materials. Afterwards, I was assigned by China’s Ministry of Education to pursue a PhD in chemistry in Japan.
At that time, opportunities were limited, and entering university at all was very rare. But I believe that if you must do something, you should try to love it. If you enjoy your work, you will do it well, and your life will be happier.
Originally, I wanted to be a writer or an artist. In some sense, science is even more creative than the arts. You are exploring the unknown and trying to understand it.
