What if I told you light could be produced by candies ?
But not any kind of candy :
the one we will present you today has the particularity of being triboluminescent.
Explanations.
First, the term luminescence
refers to a material's ability to emit a radiation
in the visible light range, after being submitted to an excitation.
Several types of luminescence can be distinguished, according to the nature of the excitation
For a photon absorption, it's photoluminescence
for the effect of an electric current, electroluminescence,
and for a biochemical reaction, bioluminescence.
But another form of excitation which can lead to luminescence exists :
the application of a mechanical force on a compound.
This phenomenon is called triboluminescence,
a name which comes from the Greek word "triben", which means "to scrub" or "to crush".
A famous example of triboluminescent material is sugar,
which can produce bluish sparks when broken.
In this video, we will present experiments that we performed
to synthesize a triboluminescent compound and characterize its properties.
We will also try to give an explanation to this phenomenon
Since one picture is worth a thousand words, we thought it would probably be interesting
to show a concrete example that highlights a triboluminescence phenomenon.
To do this, we used a compound that is really easy to get hold of :
A British candy called Wint-O-Green Life Saver.
This candy doesn't exactly shine for its taste
but it has the advantage of having easily observable triboluminescent properties.
Which we tried to highlight.
Several unsuccessful attempts have been required
before finding the right way to shatter the Life Saver.
But finally...
Did you manage to see the bright flash ?
At normal speed, it is difficult to perceive it.
But when we slow things down, we can indeed see
a bluish spark.
Because of technical limitations caused by our equipment,
we could only film at 25 frames per second.
by using more high-performance cameras,
way more impressive results can be obtained !
The following extracts are from the YouTube channel Smarter Every Day
And as you can see,
using a camera able to film at several thousands of frame per second
makes the observation of the triboluminescence phenomenon far easier.
If you were paying enough attention, you may also have noticed
that the flash seems to be brighter
when the Life Saver is broken with a lot of force.
This is explained by the mechanism that causes triboluminescnece
And that we will now try to describe to you.
Before anything else, it is important to note that the exact mechanism causing triboluminescence
has not been completely elucidated yet.
What we will explain is the most commonly accepted hypothesis
among the scientists working on the subject.
But it can't and shouldn't be considered be considered as irrefutable.
So, how does triboluminescence work ?
When a triboluminescent compound undergoes mechanical stress,
for example by being shattered,
a phenomenon called separation of charges takes place.
The negative charges, which are on the electrons,
an the positive charges, which are in what is called electronic gaps,
go on both part of the breaking that spreads.
Because positive and negative charges are separated this way,
a potential difference appears between the two sides of the break and increases as charges are accumulated.
And when this potential difference reaches a critical point...
A shock happens !
This shock is actually nothing more than an electron flow, and as such is not bright by itself.
But it is able to give energy to molecules around the triboluminescent compound !
Receiving this energy will make these molecules switch to an excited state
and they will then try to go back to their fondamental state, which is more stable.
And the easiest way to do so is to simply retransmit the stored energy
as light, by emitting a photon !
This phenomenon has a name that you probably have already heard before :
it simply is fluorescence.
Tu sum things up, triboluminescence happens in three steps :
firstly, a mechanical stress causes a charge separation,
secondly, the potential difference thus created causes a shock,
and thirdly,this shock gives energy to nearby molecules
which retransmit it as light by fluorescence.
Now that we know the mechanism of triboluminescence,
we can go back to what we were talking about earlier :
why does the brightness seem to depend on the strength of the mechanical stress applied to the compound ?
This is actually really easy to explain :
the stronger the applied force, the more the compound's structure is modified
and as such the more charges are separated.
This will result in more shocks, so more nearby molecules being excited,
and finally, a stronger fluorescence.
But now that we are talking about excited molecules, which are those ?
Well... it depends on the situation.
Let's take sugar as an example, since it is probably the most well known triboluminescent compound
if we simply break a piece of sugar,
The only nearby molecules that can be brought to an excited state are those of the air,
and in particular nitrogen, which composes most of it.
So it's the fluorescence of nitrogen which will play a role in this case.
But this molecule mostly emits ultraviolet waves
and only a small part of the emitted radiations can be seen.
This not only explains why the triboluminescence of sugar appears to be a dark blue,
but also why it is rather difficult to observe, as it is not very intense.
Conversely, the light emitted by Life Savers seem way brighter.
Despite this, it is still caused by sugar !
But it happens that in Life Savers,
sugar molecules are surrounded by many other molecules,
including methyl salicylate,
also known as Wintergreen when used as a food aroma.
Methyl salicylate is also fluorescent,
but unlike nitrogen, it mostly emits visible light,
which then cause triboluminescence which is way easier to observe.
And there it is ! If you followed all these explanations,
you now know the basics of triboluminescence.
Now it is time to see how it looks in practice !
Observing the triboluminescence from a candy is fine, but making our own triboluminescent compound is better !
Thus, by following a protocol from the Journal of Chemical Education,
we tried to synthesize a copper coordination complex called :
bipyridi-
...
called : bipyridine-triphenylphosphine-thiocyanate-copper(I)
So, knowing that we had four hours to perform all the experiments shown in this video,
I will let you judge of the feasibility of the synthesis,
which, in the article, had been realized by ten students, working in pairs for two days.
Just for information, we were three and had 4h to do everything.
So we invite you to stay until the end of the video to see whether or not we managed to synthesize our compound.
However, we did not loose our motivation and got started.
To do this, we needed :
0,121 g of copper(I) thiocyanate ;
5 mL of pyridine ;
0,262 g de triphenylphosphine.
All of this is put in a round-bottom flask, which gives us this yellow solution,
agitated at about 70°C, with a fluid-cooled condenser,
which makes sure that all the reaction medium isn't evaporated by the heat.
And all of this for about three hours, according to what is described in the article.
In practice, here is what happened :
we realized in the middle of the experiment that the bain-marie no longer was at 70°C
it went down to 25°C, which means we had to turn up the temperature during the reaction.
In any case, we didn't have enough time to heat the solution for three hours,
so we decided to reduce the heating time to only two hours.
In theory, our compound was synthesized in our flask.
But it was scattered in the solvent, which we had to remove.
The goal is to obtain a solid in the end.
To do so, we take the flask to a walk to bring it to a rotary evaporator.
The rotary evaporator is the instrument which you are seeing here.
We simply have to fix the flask to it, make it rotate, and then let it partially in warm water.
Then, we lower the pressure inside the flask.
In theory, solvents evaporate at high temperatures,
but lowering the pressure also lower their boiling point, which let us evaporate them easily.
Once everything has been evaporated, the rotation is stopped and the flask is removed from the device.
A cream coloured produce is obtained as flakes, as we can see here.
A product has indeed been obtained. But how do we check if it is the one we wanted ?
In chemistry, once a synthesis has been realized, analyses are performed
to compare our results to data from the article to check if the synthesis has been realised correctly.
Here, we will get an infrared spectrum, or IR spectrum.
To do this, the product to analyse if put in a spectrometer, as we can see here.
This device will analyse the compound and send the computer it is linked to a spectrum,
which is the curve you can see on the screen.
In an IR spectrum, each peak and each band correspond to a part of our compound,
and appears at a very precise number which we call wavenumber.
For example, this band corresponds to the vibration of the bound "C triple-bound N",
and appears at 2065 cm-1 according to the article.
So here is the spectrum from the article and our own :
Here, we notice that the spectra are strongly similar. We can see, from left to right :
The band at 2065 for the bound "C triple-bound N",
and the one at about 1594 for the pyridine, etc...
So we did mange to synthesise our compound.
Another analysis, the measure of the melting point with a Kofler bench, gave us less convincing results.
We measured for our compound a melting point of 215°C, which is about 50°C above what was expected.
A likely explanation would be the presence of impurities,
in particular of reagents that wouldn't have been completely consumed.
As you can see in this table, the only compound whose presence could make the melting point higher
is CuSCN, so it is the most likely to be responsible.
At first, we mostly wanted to synthesise a triboluminescent compound,
which would be capable of emitting light when undergoing mechanical stress.
So we put our product in a cup, then we scrubbed it with a glass rod.
We went in a dark room to see the phenomenon more easily...
... if the triboluminescence of our compound can indeed be seen.
So, did we succeed ?
Yes, we did succeed !
But how can we explain such a success of the synthesise ?
We theorized that in the end, the copper coordination complex is formed relatively quickly.
A way to check this would be to follow the reaction using thin-layer chromatography.
It happens that the triboluminescence of our compound is quite remarkable :
Indeed, one of our teachers said he rarely saw such an intense triboluminescence.
And we think the use of a rotatory evaporator helped a lot to obtain triboluminescence.
Indeed, to get our compound, we had to choose between two options.
It is possible to use suction filtration :
This consists in cooling down the flask in iced water, forming crystals, and then filtering the solution.
In the end, we obtain something closer to a powder.
For our part, we opted for the evaporator :
After this operation, the product forms fine plates around the flask.
Those are more likely to shatter than an already thin powder,
which makes observing triboluminescence more likely.
Triboluminescence is a nice looking phenomenon, and a good word to use when playing hangman.
But how can it be useful ?
Since its discovery more than 300 years ago, triboluminescence has been stirring up the curiosity of chemists working on it.
For long, the phenomenon stayed a mystery, fascinating to observe but without any known practical use.
It's only in 1999 that a team of researcher proposed a use for triboluminescent compounds
as detectors of structural damages inside of materials.
Let's imagine a material of interest whose integrity we want to monitor as time goes on, like a plexiglas plate for example.
Triboluminescent molecules could be added to it :
This way, if the material starts to shatter, because of an impact, a stress, a pressure...
light is produced by triboluminescence.
It is possible to imagine detecting this light with a device such as a photodiode for example.
The disadvantage would be that we would need triboluminescent materials bright enough
to have detection system sensitive enough, small and relatively cheap.
However, such molecules exist already, and have been tested as detectors of structural damages !
Like this charming molecule whose name I will let you read.
In conclusion, we can say that the observation of triboluminescence, as well as the synthesise of a compound with these properties
has been a success, despite the likely presence of impurities revealed by the measure of the melting point.
In the future, we could look for ways to develop triboluminescent compounds emitting more light
than those studied during this practical work, or simply to look further into the influence of the experimental conditions,
and in particular of the temperature, on the efficiency of the synthesis of our compound.
Thanks for watching our video,
and don't hesitate to keep looking into the subject if it aroused your curiosity !
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