Hi guys, I don't know how you feel about those photochromic glasses, but to me, they still feel totally like science fiction. However, they're not fiction at all. They already exist for decades. But how do they work? I think I know exactly who to ask this question. Follow me, it's time to visit the Bergeschon Universität Vopartal.

Hi Nuno, good to see you. Hello Niklas, nice to meet you. Glasses, please. Ah, okay. So I brought these photochromic glasses and you as a photochemist can probably tell me how those work. Yes, I can. There's a photochromic substance in the glass that changes its color when a certain kind of light falls onto it.

It's a kind of a tiny switch, we call them molecular switches. A molecular switch, that sounds super cool. Do you have any others of those? Yes, in fact, I have one film prepared here. This film is a tiny switch embedded. Let's see if we can turn them on and off again.

Okay, so this is a flashlight? This is a UV flashlight. So you're drawing with nothing but light? Only ultraviolet light. Are you sure you haven't hidden a small tiny pen inside of that? You can check the torch if you want. Okay, no, I trust you. So now you've turned those switches on, but can you turn them off again?

I can. I simply use this green torch over here. Okay, and that needs to rest a little bit longer on there. A little bit longer, yes. But I think this is enough. Oh, okay, cool. But does that work with my sunglasses as well? I don't know, let's try it out. Okay, great. So this is the UV flashlight.

Okay, so I'll take this and let's see. Oh, that works very, very well and really fast. That's really cool, but can we switch it off again? Just try it.

So this does not seem to be working, or do I need to keep it on there for a little bit longer? No, it seems this molecular switch doesn't react to green light. But I have a trick. With this film I can use heat to turn it off again.

I can show it to you. This is water. Just warm water. Just warm water. About what temperature? About 60 degrees centigrade. And it's already enough. Oh, that's fast. But does that also work with my glasses? Try it, I don't know.

Ah, it works, and it works really fast. So that might be a cool tip for those of you who have such glasses at home. If you need to turn them transparent again very quickly, you can just put them into warm water. Nice try, but that has nothing to do with photochemistry. That is only thermochemistry.

And here we have to do with photosensitive molecular switches. Okay, are there any other applications for those photosensitive switches except for my sunglasses? There are numerous applications. They are used in medicine to make the drugs work only in certain parts of the body. They are used in car pains, for example. There are a lot of applications.

Okay, but now I'm interested in the science behind all of this. How exactly does this work? Here's a spiroparane molecule. That is a myrosinene molecule. Those are the models of our two isomers. Isomers? What does that mean? Isomers are substances with the same chemical formula but are assembled differently.

And how does that work? They consist out of the same kind and number of atoms but have different connections. So the main difference in the molecules is that the spiroparane molecule

consists of two main parts and each forming one plane. Whereas the myrosinene molecule nearly all parts are in the same plane.

And how does that affect the color? Well, the color is decisively derived from the molecular structure. The myrosinene molecule with its planar structure can absorb photons out of the here black colored region. Okay, so if that is absorbed it's not reflected so we can't see it, right? Correct, and the reflected colors are those here.

And via additive color mixing the blue color we see is created. Okay, and what about the other molecule? The spiroparane molecule cannot absorb photons through visible spectrum. Thus we see no color.

And this is caused by the spirocenter of the here. You just mentioned photons, which is also a very exciting topic. And we've also prepared a different video on that exact topic. You will as always find it linked below. We'll answer the question, what is a photon? Which is a kind of tricky question. You should definitely check that out. And I'll see you there again.

Have a good time. Bye-bye.