Particles that need company to shine

Ben Zhong Tang was stumped. A chemist at the Hong Kong University of Science and Technology, he was looking at a powder that glowed bright green under ultraviolet light. But when the powder was dissolved in a clear solution, the glow disappeared.

It was 2001, and Tang’s observation defied everything then known about light-emitting molecules. “I was very excited,” he said in an interview, “but in another way, I was bothered, because I didn’t know what was going on.”

What he had, he later determined, were molecules that lit up only when crowded together — in solid form, for example. Tang’s study of that chemical and its unusual behaviour has led to an emerging class of small, non-metal compounds with applications in unusually diverse arenas, from vastly improving optoelectronic devices like organic light-emitting diode (OLED) televisions to advancing the use of fluorescent technology in the human body.

“For example, they could provide surgeons with better ways to visualise tumours, or enable non-invasive destruction of tumours,” said Richard Conroy, the director of the division of applied science and technology at the National Institute of Biomedical Imaging and Bioengineering.

For years, scientists trying to develop fluorescent molecules grappled with a tricky problem: The molecules’ light went out when they were too crowded. Because fluorescent molecules are almost always used as a group, the problem of quenching, as this phenomenon is known, prevented many otherwise promising compounds from reaching real-world applications. “We hope the fluorescent dye will go to the tumour site so that we’ll see the tumour, but we also hope that not too many go so that it will not quench,” Tang said. “You see the dilemma there.”

For OLED technologies like slim, energy efficient, high-contrast TVs, “we should want organic compounds that emit light in the solid state,” said Masaki Shimizu, a professor at Kyoto Institute of Technology in Japan, who designs light-emitting materials for optoelectronic devices.

That has been hard to achieve. Fluorescence occurs when materials absorb energy and then emit it as light. Conventional fluorescent molecules are flat, and they stack on top of one another like pancakes when they are too close, killing the light. In contrast, the molecule that Tang was studying was propeller-shaped, with five flat “blades” connected to a central ring. “It’s like an electric fan,” he said.

With that molecule, a different mechanism was at work. Together with a postdoctoral fellow, Junwu Chen, Tang reasoned that when the molecules were free to move around, they wasted their energy by rotating their blades, producing heat rather than light. But once space was restricted, they theorised, the molecules would be unable to move, and thus forced to release their energy as light. They named this phenomenon “aggregation-induced emission.”

To prove this, Chen performed a series of experiments. First, he froze a solution of the molecules in liquid nitrogen so they could not move. Next, he added glycerol, a thick liquid, to slow the molecules. Chen left the lab in 2002, but Tang’s students continued investigating. They applied pressure on the solid powder, squeezing the molecules further. They chemically changed the molecules by adding bridges to lock the blades in place. In all the experiments, the molecules shone brighter.

Establishing this mechanism took two years. Then Tang’s group began to look for other molecules with similar shapes that would also undergo aggregation-induced emission — AIE for short. The first AIE molecule, which Tang named “AIE-gen” after mesogens, or liquid crystals, contained silicon and was difficult to prepare. For the next AIE-gen, Tang wanted something easy to make that was carbon-based. He tried tetraphenylethylene (TPE), a hydrocarbon molecule shaped like a dog bone with two big blades at each end.
Under ultraviolet light, TPE produced a disappointingly weak sky-blue glow. What TPE lacked in brilliance, it made up for in versatility. Tang and his team soon realised that they could easily make TPE brighter or change the light’s colour by tweaking its structure or placing it in different environments. “This is a star molecule,” Tang said.

The next major advance came when Tang realised that AIE-gens could be attached to other materials like metal-detecting compounds, proteins or DNA fragments, and still keep their light-emitting properties.

“That’s very powerful,” said Tim Cook, an assistant chemistry professor at the University at Buffalo. Independent of Tang’s research, Cook has prepared self-assembling, light-emitting materials by attaching AIE-gens to compounds already known to self-assemble. “You can basically adapt it to any system,” he said.

Exciting area of research

Living systems are of particular interest. AIE-gens do not contain metals, unlike competing fluorescent materials like quantum dots. This makes them one of the most promising candidates for use in humans, to guide surgeons or deliver light therapy. “That’s probably one of the most exciting areas of research for us,” Conroy said.

In 2011, Bin Liu, a chemical engineering professor at the National University of Singapore, packed a few different AIE-gens from Tang into capsules, creating a series of very bright and robust nanoparticles. In experiments to track cancer cell migration in mice, the nanoparticles were at least 10 times as bright and lasted three times as long as quantum dots developed for the job. Last year, Liu’s spinoff company, LuminiCell, began producing and selling the AIE-gen nanoparticles.

Besides AIE-gens for biological applications, there are AIE-gens that can detect heavy metals, cyanide, explosives or harmful bacteria. Others can help improve OLED displays by eliminating the quenching problem, making OLEDs much easier to manufacture.

But commercialisation of AIE materials may not be so easy. “There are a great many fluorescent technologies,” Chris Geddes, the director of the Institute of Fluorescence at the University of Maryland, said. “Whether the AIE technology will make it, only time will tell.”
Still, 15 years in, Tang and others say that the known AIE-gens may be just the beginning. Now, researchers are reporting that other materials like clusters of gold atoms, not just small organic molecules, can undergo AIE as well. “This opens up huge opportunities,” Tang said. “I am very optimistic.”

International New York Times