Welcome to the second lecture in Chem 413. So in the first lecture we had a look at.

The historical background to the to the whole area of organic electronics and the discovery of the first conducting polymer, which was polyacetylene.

Now if I'm takes historical kind of.

View of the development of this subject. Then the next thing that.

The next thing to be discovered was really conjugated polymers that were made by electrochemical oxidation of either Aromatic molecules or hetero cyclic molecules. So ultimately will come on to talk about polythiophene because that then opens the way to talking about thinfilm transistors, things made with Poly Thiophenes. But those Poly thiophenes are made by chemical synthesis routes.

So electrochemical syn electrochemically synthesized polymers came first, and then subsequently people.

Optimized the preparation of these polymers by using chemical routes.

So the 1st.

Conducting polymer to be made by an electrochemical.

Oxidation reaction.

Was actually a polymer called Poliana Lynn, and so this is quite an interesting story becausw.

It goes back a long way.

Polyanna Lynn was actually first synthesized as a film.

On a working electrode surface.

By a British pathologist.

In 1861

now.

The reason why pathologist was interested in this.

Well, it was quite strange in the middle Victorian period.

Organic chemistry was making advances and aromatic chemistry.

Was really in the foundations of organic synthetic chemistry. People were learning how to do chemistry on benzene.

Which was obtained of course, from coal tar.

And the nitration of benzene to give nitrobenzene or nitrotoluene things like that.

Became quite prominent.

And people notice that Nitro Aromatics had nice smells and and even nice flavors.

Believe it or not, and.

It got to the stage that Confection is making cheap suites used to put Nitro benzene and Nitro aromatics.

Into their sweets as as a cheap flavoring, even though that was against the law, because these these compounds were known to be toxic and there are cases of poisoning where people died from eating sweets that contained Nitro aromatics. Now what happens to nitrobenzene?

If you ingest, it is that it's reduced in the stomach by bacteria.

To Anna Lynn.

And so it was known that you could test for Anling by treating treating it with an oxidizing agent.

And you got this intensely colored.

Material and the intensity of the color would give you a clue as to how much of the Annalyn had been present.

The problem was that different oxidants it was notest gave different.

Slightly different colors ranging from a sort of greeney blue for a weak oxidant.

Do I almost dark purple color for a stronger oxidant?

An so to get a reliable test for the presence of Anna Lynn.

This doctor

Doctor Letheby.

Use the new invention, the Groves.

The Groves Battery, the Groves cell and he used to platinum.

Working electrode or anode.

And so he connected his electrode to the battery, stuck them in a solution of Anna Lynn.

And got a blue colored film on the electrode and this was this. Became a test for the presence of vanillin and hence.

The implication being that.

The sample contained nitrobenzene.

And this was this. This was what the patient died of poisoning.

So that's way back in 1861. Now today Pollyanna Lynn is still made becausw. It has some technological applications and it's made chemically by oxidizing, a solution of vanillin in.

Two models, sulphuric acid with ammonium persulfate, which is quite a strong oxidant.

And it polymerizes through the NH.

And it's two proton is lost. You form a carbon nitrogen bond and so you have this alternating chain of N with a benzene ring.

And in its partially oxidized state.

There are some imine type units.

In the polymer chain, and when these are protonated, the polymer is conducting.

Highly conducting, and in fact you can make polyaniline if you do it very carefully, and you make a very ordered.

Polymer material you can make a form of Pollyanna. Linda actually behaves as a genuine metal. It's so conductive and not only that, but it's it's conductivity.

Its conductivity increases with decreasing temperature, right? So it's so it's what a physicist would call a metal. It can be made as a metal.

But it's in spite of that. It's not really used in organic electronics, it's it's used in antistatic coatings and.

It's occasionally used to modify the surfaces of electrodes and things, but it's not. It's not really used in organic electronics.

Another polymer discovered a bit later is a Poly at recycle Poly peril, and this was first made back in 1916, or at least it was first described back in 1916, but it probably been made before.

Anne.

This is actually very easy to make by accident if you open a new bottle of purell.

And you happen to spill some down the side of the bottle.

Which is easily done and you leave the bottle on the shelf for a couple of days. You'll notice that this where where the Pearl was spilled. It goes black.

And that's because peril is oxidized by atmospheric oxygen to a material which is actually a polypyrrole.

So it polymerizes through the.

25 or Alpha carbon positions to give a polymer chain.

And this again is a. It's a conjugated polymer.

And as made by oxidation, it's present in its oxidized form.

Where there are some positive charges along the backbone and so therefore it's got polarons or by polarons within its structure, and so it's it's conductive in that form.

But it wasn't characterized. Its structure wasn't known until really quite recently.

It actually.

Was made 60 years later by some electric chemists. Much more carefully using again a platinum electrode and an aqueous solution.

Of perilin acid.

And in that form, when grown is a film solid state NMR. Carbon 13 NMR.

Help to establish the the structure of the polymer.

And just like.

Like polyacetylene

it can be either doped. I oxidized and containing.

Some bipolar runs with the corresponding counterions taking up from the electrolyte. As the polymer grows.

But then if you change the working electrode potential, you make it less positive. You can re inject the missing electrons and go back to the.

Go to the reduced neutral form of the polymer, and in that case the anions from the electrolyte is spat back out into the electrolyte solution.

And the Poly Pearled is neutral and these two forms have quite different colors and obviously very different conductivities.

So that was first described in 1979 in electrochemical paper.

So this was a gnu conductive polymer material compared with polyacetylene.

And it had some advantages over Poly settling. Obviously it's it's much easier to make than probably settling. You don't need any fancy catalysis.

In its oxidized form.

It's actually stable to oxygen in the air, unlike Poly a settle in.

Although the neutral form is air sensitive, because obviously it's it's oxidized back to the.

Conducting form by by atmospheric oxygen.

It's compatible with aqueous electrolytes, which makes it that's that's another reason why it's particularly easy to make.

Becausw pyrrole is a very electron rich aromatic molecule, and Polypyrrole is therefore very easily oxidized, has a very low oxidation potential, so that's compatible with aqueous electrolytes. In fact, people who did the Nuffield Syllabus a level of in chemistry.

There was actually an experiment. At one time. I don't know if it still exists.

But students couldn't.

Polly perils films for themselves.

Using simply a woman, 1/2 well, a standard one and half Volt battery as the power source.

And a pair of knitting needles as the anode and cathode electrode.

And just bench sulfuric acid as the electrolyte and you can coat the anode knitting needle with a nice purpley black film of oxidized polypyrrole that way.

However, as a polymer, it still has some disadvantages.

It doesn't have any functional groups to help it be soluble, so it's it's totally insoluble in organic solvents once it's made.

It can't be melted, so it can't be melt processed. It's infusible. Is this the correct term?

And obviously it can only be positive as a film on a conducting electrode surface.

It's worth at this point, saying a little bit more about how electric chemical synthesis of these materials works, because we're going to talk about electric chemically generated polythiophene because that's being used in sensing and other applications.

So let's have a look at the the methodology for growing films electric chemically.

For those of you who are doing the electric chemistry course, this might be familiar, but for those of you who are not, then obviously it won't be familiar, so it's worth going through this this methodology.

The standard method for growing conducting polymer films Electro chemically.

Is an electrochemical method called chronoamperometry.

And what that means is.

You're using it.

A piece of equipment called a potential stat which can control the potential the electrical potential of a working electrode surface.

And you step the electrode potential from a value when nothing happens and the monomer is not oxidized.

To a positive value where the electrochemical reaction, the oxidation of the monomer occurs.

As that occurs, you measure the current that flows.

And you do that experiment for a fixed period of time sufficient to grow a film of a desired thickness.

And this is the sort of output that such a such an experiment might give. Now this is.

This is a an example where we're using. We're actually using a platinum microelectrode, so this is a very tiny electrode.

In fact, it's made by sealing a platinum wire.

In soft glass.

And then cutting the glass and polishing the surface of the end of the.

The end of the glass so that the wire sealed in the glass acts as a sort of tiny little disk electrode.

So this particular platinum microelectrode it's in a solution of pyrrole in aqueous acetic oquist sulfuric acid, and initially the electrode potential is being held at 0 volts versus the saturated calomel reference electrode.

And then nothing happens, because that's not oxidizing.

And then at times 0 here, suddenly the potential increase to plus .7 volts.

And at that point, perils peril molecules in solution near the electrode get oxidized to radical cations and these old couple and make polymer. And because the polymers insoluble, it crashes out on the working electrode surface.

So what that means is that the surface area of the electrode suddenly increases because of all these lumps of polymer, which are conductive in that form growing on it, and so the current goes up.

And if you continue to hold the electrode potential plus .7 volts, you can see that the current carries on rising here, and that's because the area of the electrode is increasing because with this micro electrode actually you have a sort of hemisphere.

Of Polypyrrole stuck to it and it's increasing in radius as the growth goes on and then at 20 seconds.

The electrode potential switched back to 0 Volts, and so the electrochemical growth is terminated.

And So what you're doing here is you're measuring current on this axis as a function of time on this axis, so by integrating.

The area under the electrode current times time is charged.

And so you can measure the charge that's passed to grow the Polypyrrole Film and knowing the charge that's passed, you can work out how much polypyrrole is formed on the electrode surface. So in other words, you can. You can estimate the film thickness.

Of the polypyrrole.

So yeah, I've said all this already, so we measure current against time and by integration we can calculate the charge passed, which in this an experiment here happens to be 60 nano coulombs.

And because of the size of the platinum disk, which is quite very small, the area of the electrode here was 80 square microns, right? So this really is a microelectrode in that its radius is in microns.

So as long as the polymer is totally insoluble, which in the case of polypyrrole it is.

This is a good way of estimating the amount of polymer that's formed by the charge that's passed. So how do we do that?

Well, we know the charge and we know the area of the electrode.

So we can workout the charge pass per unit area by dividing the the 60 nanshoku loans by the.

80 square microns. Remembering that a square Micron is 10 to minus 12 square meters.

So putting this in coulombs per meter squared, it comes out of 750.

Coulombs per meter squared.

We then divide by the Faraday.

To get the number of moles of electrons corresponding to that charge and that comes out as 7.8 * 10 to the minus, 3 moles of electrons per square meter.

Now we have to remember that we have to know the equation for the electrochemical reaction that's actually going on when the polymer forms.

So what we're doing is we're taking peril.

We're oxidizing it to polypyrrole.

And the two hydrogens which were on the Alpha positions of the peril ring 25 positions are lost as Protons.

And.

When we make the polypyrrole film electrochemically, it will be present in its fully oxidized state.

So it will have some missing electrons, and for polypyrrole it's known.

For reasons which I'll show later, it's known that there are about nought .3 electrons missing.

Per monomer ring.

Now we need we need to remove 2 electrons to generate the two new carbon carbon bonds between the pyrrole monomers.

Plus nought .3 electrons to make the each individual monomer have an average charge of plus .3.

So we need to remove 2.3 electrons per pyrrole monomer.

We can calculate the number of moles of monomer from the number of moles of electrons that we measured by simply dividing it by 2.3.

So that gives us about 3.38 * 10 to the minus 3 moles of peril monomer per square meter of area. Or if you want to put it in square centimeters, which in this case is more convenient.

Divide by 10 to the four, so that's 3.38 * 10 to the minus 7 moles of monomer per square centimeter of electrode.

The reason for putting it in in terms of square centimeters.

Is that if we look at our bottle of purell from the chemical supplier?

We see that its molecular weight is 67.1 grams per mole.

And its density is nought. .967 grams per cubic centimeter.

So these numbers are useful because it enables us to calculate the molar volume of the peril.

So that's 67.1 /, .967 or 69.4 centimeters cubed per mole. One mole of peril occupies 69.4 cubic centimeters.

So, knowing that and knowing the number of moles per square centimeter of the electrode.

If we if we.

If we multiply the number of moles per square centimeter of electrode by the molar volume.

Then that gives us the thickness in centimeters.

So the film thickness is given by this this expression here, so it comes out as 2.35 * 10 to the minus 5 centimeters. So the average film thickness in this particular experiment.

Would be 235 nanometers.

Now, that would assume that the film grows uniformly on the electrode, which is probably not true in this experiment. This particular experiment, because we use the microelectrode so it's going to be more like a hemisphere, but at least that gives us some kind of clue as to the amount of peril, no matter polypyrrole on the electrode.

Now this also does make another assumption.

Which is that the density of the part of the pyrrole and the density of the Poly pyrrole?

I'll note that difference, and that's probably not valid because it's known that these Poly heterocycle films are usually a bit denser than the actual monomers.

So that might in that's probably introduces some kind of inaccuracy. So these these calculations are only ever estimates of film things.

How do we know that there are about .3 electrons missing per pyrrole monomer in the fully oxidized form? While that's known? Because if you, once you've grown the polypyrrole film on the electrode, you can take the electrode out of the solution of pyrrole, wash it.

Put it back into a background electrolyte that doesn't contain any monomer.

So any electrochemistry you see is only due to the polymer film stuck to the electrode and then you do an experiment called a cyclic voltammogram.

Now what you're doing here is you're controlling the potential of the working electrode again, but this time instead of stepping it, you're going to smoothly change it.

And you're going to measure again the current flows as a result.

So what's happening here is that we're starting to very negative potential where the film of Polypyrrole is in its fully reduced or neutral state.

And at first nothing happens, and then as we make the Electro potential more positive.

You can see that there's a positive current flowing.

That means an oxidation, so we're oxidizing. At this point, we're oxidizing the film from the neutral to the positively charged state, and at the same time our lines are going into the film from the electrolyte to balance the charge.

And you see a peak in the current and energy continue to increase the.

Increase the potential current falls because you've oxidized all of the Poly pyrrole to Polypro.

Radical cation or bipolar on them or whatever.

And then at some point you reverse the direction of the potential sweep. So you start to decrease the electrode potential, and now you begin to re inject the missing electrons back into the positively charged form of the material, until eventually when we go sufficiently negative, it's in it's fully reduced date. Again assuming that the process is reversible. Now again, this is still a current time experiment because we know that we know the rate at which we changed the electrode potential.

In Volts per second.

So we can measure the. We know the time it takes to do one of these sweeps.

And we're measuring the current as we do it, so again by integrating.

The area under one of these curves, and knowing the sweet rate. For that we can calculate the charge passed.

And.

So by doing this and comparing the charge that's passed to cycle just the polymer on its own with no monomer.

By comparing that with the charge that was required to oxidize the monomer to the polymer.

We can workout how many electrons were missing from the film.

In its fully oxidized state, and that's how we know that in fact, for Poly pyrrole, it's about .3 Electrons Missing, and here are the equations for that. So I've already seen.

In structural form, the equation for the oxidation of the monomer.

While to generate the polymer we removed 2 + X.

Electrons were accessed, the number of missing electrons in the fully oxidized state.

And for the redox cycling experiment and cyclic voltammetry here, all we're doing is taking.

In this case, taking the neutral form of them of the polymer shown here on the on the right.

And we're removing electrons until we have the fully oxidized state of the polymer here and what we want to know is this number X.

So.

What we find is that for polypyrrole, as I say, X is usually about .3, there's about 1/3.

Have a positive charge per monomer unit in the fully oxidized polymer.

Fright for polycythemia, which we'll see in the next part of this lecture. X is actually a little bit smaller. It's usually about 1/4. People say that there's one missing electron per 4th. I have been rings in a fully oxidized polythiophene. OK, so at this point I'm going to stop this first presentation from lecture two and will resume this in the second part of this lecture presentation.