Glowing Strong: The Magic of Processable Full-Color Afterglow Glass

Okay, so imagine stuff that glows… not just when the light is on, but after you turn it off! That’s afterglow, and it’s super cool. Think about displays that linger, security features that pop in the dark, or even glowing threads in your clothes. The potential is huge, right?

For a while now, scientists have been playing with organic materials that can do this. They’re lightweight, flexible, and you can design them molecule by molecule. But here’s the tricky part: making them glow brightly, in full color (from deep violet all the way to near-infrared!), making that glow last a really long time, and perhaps most importantly for real-world stuff, making them easy to process. Getting these finicky little molecules into shapes like big sheets, long fibers, or complex 3D objects has been a real headache.

Traditional small organic molecules tend to crystallize, which is great for some things, but makes them brittle and hard to shape. You often have to mix them into other materials like gels or resins, but that can expose the glowing molecules to oxygen and moisture, which totally kills the afterglow. We needed a better way.

Enter the Molecular Glass Hero

This is where our story gets exciting. We decided to tackle this challenge head-on by designing a special kind of material called a molecular glass (MG). Think of a molecular glass as a small organic molecule that, instead of forming neat, ordered crystals when it cools down, gets stuck in a disordered, amorphous state – just like regular window glass, but made of tiny molecules.

The cool thing about molecular glasses is that they can form a viscous, gooey liquid state when heated above a certain temperature, but below their melting point. This “supercooled liquid” state is key because it means we can process them kind of like how glassblowers work with hot glass!

We needed a host material that could do this and also provide a good environment for other molecules to glow. We looked at a known afterglow host, TPPO, but it had issues with mixing well with different glowing molecules. So, we got clever. We modified TPPO by adding methyl groups in just the right places. This little tweak messed up the molecule’s symmetry, making it harder for the molecules to line up and crystallize. The result? A new molecule we call TTPO.

And guess what? TTPO is awesome! It can be made easily on a large scale (we’re talking hundreds of grams!), it stays in that desirable amorphous, glassy state at room temperature, and it has that crucial temperature-dependent viscous supercooled liquid state that’s perfect for processing.

A Rainbow of Light, Locked In

Now that we had our amazing TTPO host, it was time to bring in the color. We used a host-guest strategy, which is basically like having our TTPO “host” matrix and doping it with different “guest” molecules that are designed to emit different colors when they glow. We tried doping TTPO with a whole bunch of different guest molecules, even ones with really different shapes and sizes.

The fantastic news is that TTPO is super tolerant of these different guests. We could uniformly disperse them in the glassy matrix without any annoying phase separation (where the guest molecules clump together instead of spreading out evenly). By choosing different guests, we successfully created systems that glowed in full color, covering the spectrum from violet (around 410 nm) all the way to near-infrared (around 767 nm)!

Not only did we get the full spectrum, but the afterglow also lasted for impressive durations, from a few milliseconds up to an incredible 1695 milliseconds (that’s almost 1.7 seconds!). The TTPO glass provides a really rigid environment for the guest molecules, which is important because it stops them from wiggling around too much. This wiggling (vibration) usually kills the afterglow, so keeping them still helps the glow last longer and be more efficient.

Plus, the glassy TTPO film is transparent and does a great job of protecting the guest molecules from environmental enemies like oxygen and moisture, which are notorious afterglow quenchers. We even dunked a film in water for three days, and it still glowed just fine after being “photoactivated” (basically zapped with UV light for a bit to use up any trapped oxygen). This stuff is tough!

Shaping the Future

But here’s the really wild part that unlocks tons of potential applications: the processability. Because TTPO has that lovely viscous supercooled liquid state, we can shape it just like inorganic glass. This is a game-changer for organic afterglow materials.

We showed that you can easily make large-area films (we demonstrated a 3.0 cm diameter one, which is great for displays) just by melting TTPO with a guest and letting it cool quickly. The transparency is excellent, making it perfect for optical uses.

We also proved you can shape 3D objects. By hot-pressing the viscous supercooled liquid, we successfully created a stamp with a 3D structure that glowed after the light was off. Imagine glowing logos or intricate designs!

And perhaps the most exciting processing feat? We managed to pull meter-long, flexible fibers directly from the viscous supercooled liquid of the TTPO-guest system. As far as we know, this is the longest afterglow fiber made purely from small organic molecules. These fibers are not only long and flexible but also strong – a tiny bit of fiber could hold a weight 1000 times its own!

These fibers also showed waveguiding properties, meaning light can travel along them and come out the ends, even after the excitation is removed. This opens up possibilities for glowing textiles, flexible displays woven into fabrics, or even new ways to transmit light signals.

How Does It Work? (A Little Bit of Science)

So, how does TTPO achieve all this? Beyond providing that rigid cage for the guest molecules, TTPO also acts as an energy transfer bridge. When we excite the material with UV light, energy gets absorbed by both the TTPO host and the guest molecules. TTPO is really good at capturing this energy and then efficiently transferring it to the guest molecules, boosting their ability to get into the “triplet state” – the state from which afterglow happens. This process, called Triplet-Triplet Energy Transfer (TTET), helps make the afterglow brighter and longer-lasting.

The Takeaway

What we’ve got here is a major step forward for organic afterglow materials. We’ve designed a molecular glass host, TTPO, that’s:

• Easy and cheap to make on a large scale.
• Tolerant of a wide variety of guest molecules, enabling full-color afterglow.
• Provides a rigid and protective environment, leading to long-lasting and environmentally stable afterglow.
• Has a viscous supercooled liquid state that allows for unprecedented processability – think large sheets, 3D shapes, and meter-long fibers!
• Actively enhances afterglow through energy transfer.

This work really unlocks the potential of organic afterglow for practical applications. Imagine glowing textiles that don’t need batteries, flexible displays that can be molded into any shape, or smart sensors integrated into everyday objects. The ability to easily process these materials into diverse forms is a game-changer, and we’re excited to see where it leads!

Source: Springer