In the first part of lecture 15 we had a look at this class of molecules whose conductance injunctions was affected by whether or not water was present. Certainly for the longer aliga thiophenes units in the middle of the molecules here.

And in fact for this tour thing, the conductance in the presence of water was 100 times greater.

The nut when water was excluded. Now of course the interaction between a fire femring and water molecules is going to be really quite weak.

Some kind of combination and found the Vols interactions and some sort of Dyett dipole dipole interactions.

And so the question is, if you if one had a stronger interaction between a molecule and some external.

Second molecule or solvent.

Would that result in a larger change in conductance?

And one possible interaction that might be suitable to investigate here is some charge transfer complex station.

Now charge transfer complexes.

I've been known about for a long time, certainly 70 or 80 years at least.

And in fact.

They were right at the heart of the beginning of.

The subject of organic electronics.

One of the first organic materials found.

That had a significant electrical conductivity.

Was first made in the 1930s by some German organic chemists.

They were actually investigating the electrophilic bromination.

Of polyaromatic molecules of various kinds.

And one of them molecules that they tried in their experiments was this, this one here, which is called paroly.

And what they found.

Was that if they took a solution of paroline and benzene?

And they treated it with bromine. Then even before they had the chance to add the appropriate Lewis acid catalyst such as aluminium tribromide or iron tribromide, some kind of reaction seemed to happen between the paroline and the bromine already.

And the pale yellow solution of paroline immediately darkened to a kind of.

Kind of intensely purple Brown color.

When bromine was added and eventually crystals nice shiny, reflective, dark black colored crystals precipitated.

At that time, the only characterizing data that they could get on this substance was some microanalytical data. They analyzed it for carbon, hydrogen and bromine.

And they found that the composition approximated to one, paralleling to three bromine atoms.

And they suggested that what had happened in fact was that the bromine.

Had oxidized the paroline too radical cation, which of course would be delocalized over over all those rings, and the bromine was present as the BR 3 minus anion.

Thus this was assault.

And then.

That didn't really arise. Arouse much interest at the time, but then about 20 years later, a Japanese group who were intrigued by this result and wondered what this black substance really was and why it was so darkly colored.

They remade it.

And they crushed up the crystals and stuck them into a glass tube and put an electrode on either side of the powder and they measured its conductivity.

And they found that the conductivity was really very high indeed for an organic substance it was around 10 to the minus three reciprocal home centimeters.

And that was such an exciting result. Such an unusual result at the time that they've published, they managed to publish that work in the journal Nature.

And that caused.

A lot of interest in organic chemists and the possibility of deliberately making highly conductive organic materials.

By replacing the bromine atoms with something a bit more organic and a bit more interesting, so initially.

What's the synthetic chemist did was they made various highly electron deficient conjugated molecules that were designed to be oxidizing?

To have a tendency to pick up an extra electron. So for example Tetra cyano ethylene.

Was made in the late 1950s and Tetra Seiner quino die methane was also made at about the same time and then the interaction of these.

Electron deficient molecules with other polyaromatic hydrocarbons was investigated.

So for example, in this crystal structure here, which is taken from a paper in the late 1960s.

This is a complex that's made between Tetra Cyano ethylene.

And piring.

And what happens in this compound? Again, it's only intensely purple colored material.

And that's due to charge transfer band, so it's due to.

The promotion of an electron from the **** orbital on the π ring to the LUMO.

On the electron deficient TCNE

and in the solid state, it's got this regular structure where parallel pyrene molecules and T. Cnes.

Alternate along along this direction. Here in the crystal.

And.

Later, the polyaromatic hydrocarbon was replaced by something more electron rich, and so these molecules, like Tetra Fire full valene here.

And this selenium.

Tetramethyl Tetra Cellino full valene were made and the idea here was that these would be more electron rich than the.

Fully automatic hydrocarbons and they would form an even stronger interaction. So what was happening with these?

If these electron deficient moieties were reacted with these very electron rich molecules.

And in fact, what happens in that case quite often is something slightly different.

So if you react one equivalent of TTF.

With one equivalent equivalent of TCNQ.

You gotta you gotta. Crystalline material forming again. But now in the crystal structure.

All of that ETF's.

Π stuck with each other along the crystal axis and the TSN cues all strongly pie stuck with each other.

And within these stacks, there's been quite a significant degree of of charge transfer taken place and along the TTF stacks there are some missing electrons. In fact, there's an average of over half of positive charge for every TTF.

And along the TCNQ stacks, there are extra electrons presence.

And in fact, over half an extra electron per TCNQ.

And because of the Pi stacking and the overlap of the orbitals, these extra electrons or holes and delocalized along the stack over many molecules and the net result is that these materials are electrically conductive.

And the electrons and holes can act as charge as charge carriers.

And this crystal. This substance was actually the first example of an organic metal and by inorganic metal I mean this. This behaved as physicists would define a metal.

I it's electrical conductivity increased with decreasing temperature. This was the first time an organic metal had ever been made.

Incidentally, you can take this further and.

For example, if you take this substance here.

And now instead of complexing it with TCNQ, you just oxidize this Electro chemically.

And in solution electrochemical cell.

You can grow crystals on the anode.

Which consists of.

Two of these molecules to 1 union negative on line. So in a way this is oxidized by half an electron. So what you have in fact is sort of.

Uh.

A pie stacked crystalline lattice weather only holes and anions and this substance turned out to be superconducting, admittedly only at less than one Kelvin.

So only in a helium cryostat and only under the application of.

Very high pressure, but nonetheless this was the first example of an organic superconductor.

So at that time in the late 60s seventies.

There was an awful lot of interest in materials of this kind, and that's really why, for example.

At the dawn of molecular electronics, the other unwrapping, the molecule that I talked about in the first lecture.

This this kind of thought molecule had a TCNQ acting as the electron deficient side, and a TTF unit acting as the electron rich side to make a molecule could behave in theory.

As a diode.

So what about our molecules? Well, firefin rings are very electron rich.

Admittedly not as electron rich as a TTF.

And so they should form a charge transfer interaction with an electron deficient species like Tetris. I know ethylene.

And we can see that they do becausw in solution.

The blue line here is the electronic spectrum of TCNE, which is pale yellow. It's just starting to absorb in the blue part of the visible region.

And of this molecule Mr Thiophenes here, which has a pie pie star interaction that peaks in in the blue part of the spectrum and has its low energy onset somewhere down here at quite low energy.

But if that's combined, if these two are mixed, then you get a new absorption band, so that's the red spectrum.

And that's at about what 11,000 centimeters to the minus one? About 840 nanometers or something like that.

So at very low energy.

And that's the absorption due to the charge transfer band. So the promotion of an electron from the from the **** on that Earth arfin to the LUMO on the TC any.

So the charge transfer interaction is quite strong. In this case. It doesn't take a very energetic photon to kick an electron off the to 13 and onto the TC any.

But if we replace that earthie thing with a less conjugated and less electron rich thing like a benzene ring.

Then that charge transfer band is in is at a much higher energy.

So this compound, this one four diethyl benzene is colorless.

And if you take a solution of this in dichloromethane and you add some, add some TC. Any you get a fluorescent looking orange color developing?

And the absorption band that produces that color is shown here.

It's in the blue part of the visible spectrum, roughly speaking tailing into the green. Hence the orange color.

And.

So that's a higher energy transition because it takes it takes a more energetic photon to kick an electron from the home of the benzene ring to the LUMO of TC any.

And if you study this absorption band intensity as a function of the concentrations of the two components that we see any in the benzene.

You can use this equation called the Benizi Hildebrand equation.

To workout first of all, the molar extinction coefficient of this band.

That's this, this parameter here and Secondly the equilibrium constant for the formation of the one to one complex between the benzene and the T. See any and actually that equilibrium constant in that case is really weak. I mean it's only about one in dichloromethane.

So it's not a very stable species. This charge transfer complex.

In the case of that Earthia scene.

Where the toner is more conjugated and more electron rich. The complexation is stronger.

The equilibrium constants roughly 15.

So what is the effect then are forming that charge transfer complex on the molecular conductance?

Well, these experiments.

Produced rather broad peaks for various reasons that I'm not going to go into here. But what is clear is that if you take this to Thiophenes molecule on its own and you look at a.

You you measure one of these one dimensional conductance histograms using the STM break junction technique, you get a peak.

It's very broad, but you get a peak somewhere around the 10th of the minus 4.2 G nought region.

But if you want to see any and you make the charge transfer complex.

The conductance peak shifts, so that's the orange histogram peak here, which is about 10 to the minus 3G nought.

So a fairly large increase and I should note also that we didn't exclude water here, so this is.

This this green peak is actually the conductance of this species. Even with water present.

No, the benzene molecule.

Doesn't interact with water. We checked that before.

But of course it can interact with TC any.

And the TC. Any boost the conductance of this species as well.

So the green histogram there is for the molecule on its own, about 10 to the minus 5G nought, conductance, and the orange line is the molecule with TC any, and there's a new peak at about 10 to the minus 3 1/2 G nought.

And there's also still a small peak corresponding to the UN complex molecule, because as I said before, the equilibrium constant for the formation of this complex is quite low, and so even in the presence of excess TC, any there will always be a certain amount of molecule in there that's not complex, so you see peaks for both of them.

Under these conditions.

TC any boost the conductance of that Earth I feed molecule by roughly a factor of 20, maybe 25.

And of the benzene molecule, by about a factor of 15.

So.

It's not immediately clear why.

In a single molecule junction, the conductance of the.

Of the molecules should be increased because the electrical properties of the solid state compounds are mentioned before the ones that were metals or superconductors.

Those electrical properties are a function of the whole crystal structure. The solid state arrangement of the molecules.

So why should forming a charge transfer complex boost the conductance of a single molecule?

Well, it's a bit complicated to go into, and I'm not going to explain it in in great depth.

But the theoreticians came up with the answer to this.

And it's the fact that in the charge transfer complex.

We have two alternative paths for the electron to get from one side of the junction to the other.

It can either tunnel essentially using the molecular orbitals of the molecule alone.

Or it can tunnel?

First of all, find the molecule. Then using the loom over the TC any.

And then find the molecule again. So there are two alternative pathways. And because electrons are waves as well as particles.

These pathways can.

Interrupt interfere with each other either constructively or destructively, and what you end up with. Actually in the transmission function.

As you end up with an extra peak, which is an unusual shape.

It's gotta dip, so that's if you like the destructive interference and then it's got a peak, which if you like is the constructive interference.

On this peak shape, this is called a farmer resonance, so you get an extra peak in the transmission function.

But more importantly.

It's where this peak is.

Because the in the charge transfer complex a certain amount of electron density is being lost from the π.

From the **** orbital on that Earth, I think.

And that electron density is being transferred to the lumen of the TC any.

In a way, each of these two parts, each of these units has a partially occupied molecular orbital.

And the laws of physics say that a partially occupied molecular orbital must have the same energy as the Fermi energy.

In an electric device.

Involving two metals, and so. In fact this far no resonance occurs at 0 on this scale. It's where the contact Fermi energy is, and that's why it has such a large effect on the conductance, because the electrons that are crossing the junction.

Can only cross the junction at the farm at the Fermi energy.

So if you change the.

Possibility of them being transferred the transmission by introducing a resonance which is situated at that energy. You're going to have a large effect on the conductance.

And if you translate this.

Transmission function here, which is calculated 0 Kelvin if you multiply by G nought to convert this to conductance and then you allow it to broaden because of thermal effects. To go to room temperature.

You can obtain a plot of the conductance of the junction versus temperature versus energy.

And you find that for this molecule there are the conductances boosted by a factor of 20.

And so this this phone, a resonance is what's responsible for the boosting conductance.

So this is very interesting becausw.

What was shown here is that a chemical effect, a specific chemical effect, can affect the conductance of a single molecule junction.

And so, in principle, that means that if you could make a molecule that acted as a specific sensor or binding site for one target substance, you might expect to be able to affect the conductance of the molecule depending on whether or not the molecule is intercepted its target. So you could in principle you could make a single molecule sensor.

No.

Those TUR fifing and benzene molecules have rather an unusual structure with the flexible alkyl chains.

At either end to connect to the electrodes.

And we did wonder whether more straightforward molecules would behave the same way.

No.

Other workers had shown that fully conjugated.

A legal thiophenes

using thio ether groups as Contacts to the electrodes.

That conductances were not affected significantly by the presence or absence of water.

Anne.

And so therefore they were good targets to try the charge transfer complex interaction with.

So.

I showed part of this data before when I was talking about conjugated molecules acting as molecular wires. So I showed the blue histograms here, which are four.

The molecules here these illegal things with two, three or four siphon rings in the backbone with Thio ether Contacts.

And as expected, the conductance of these molecules decreases exponentially with length, and if you plot the log of the conductance against the length of the molecule, the distance between the two sulfurs at the end here.

You get that kind of straight line and you can calculate the gradients of that and hence extract the beta value. And as we said before, the beta value is actually roughly 5 per nanometer in this case.

But if you then interact these molecules with Tetris, I know ethylene.

Interestingly, they all seem to have really quite similar conductances and quite high conductances.

So.

Um?

TC any boosts the conductance of all of these molecules.

But it does so in an interesting way, in that the longer and less conductive the molecule was to start with.

The larger the factor by which the conductance is boosted.

So that's why ultimately comparing the conductance histograms for all four, all three molecules even, even though they're quite different in length, they're all quite similar.

So you can plot in this case.

The factor by which the conductors is increased as a log here against molecular length. And that's a straight line as well with the highest value being for the longest molecule.

So what this shows is that although charge transfer complex station like this increases, the conductance of conjugated molecules.

It's only really. It's only actually does it to a limited extent. You won't be able to make.

A molecule that is as conductive as a chain of metal atoms.

By this effect, unfortunately, so you can see that the conductances here in the complexes are all about 10 to the minus 2G nought. But that's still pretty good.

For such quite long molecules, certainly in the case of the flooring molecule, that's quite a high conductance value.

Again.

The theoreticians did their calculations on this and some again they found. So here we're looking at plots. This time of conductance on a log scale as a function of energy.

Looking at either side of the contact Fermi energy.

And you can see that there's the extra resonance or the extra peak here in these plots, which is due to the charge transfer complex forming, and again it's at the Fermi energy.

And so that's why the conductance of the complex is higher than the conductance of the bare molecule.

And notice that the longer the molecule.

The longer the Liga surfing, the larger the apparent increase in that conductances.

I may notice there are two peaks here in this plot, with the complex and that's because these additional calculations showed.

That because electrons have spin as well as charge.

And then the the partial electron that's occupying the loom over the TC. Any could be either spin up or spin down, and that actually makes a difference. Then you have to take that into account in the calculations so you actually get two peaks in these plots.

So this was done by doing what what are called SP independent transmission function calculations and they were done in there fairly sophisticated way where the TC any was allowed to wobble about on the donor molecule and different plots were calculated for each snapshot.

But then the next lesson is that the room temperature conductance.

Is boosted by the by the charge transfer complex formation again, and the explanation is pretty similar. It's a far no resonance.

The shape of the final resonance isn't clear from the conductance plots, but it is clear from the transmission plot, so this is a transmission plot for the longest molecule.

You can see the downward going peak and then the upward going peak there.

So the model in the cartoon for the explanation is shown here.

An in the band molecule is just like any molecule. It conducts by tunneling.

The **** and LUMO of the molecule are either side of the Fermi energy, the Contacts.

And whichever orbital is closest to the Fermi energy dominates the conductance.

But then if we make a complex with the TC, any we have these extra levels which correspond to these farner resonances in there closer in energy to the Fermi energy, the Contacts and so the conductance of the system charge transfer complex is then boosted.

Well, that brings us to the end actually of the molecular electronics part of the course. In dust, the end of the.

Can 413 as a whole.

So to summarize, for the molecular electronics section of the course.

We've seen nuts scanning tunneling microscopy.

Is one of several methods that we can use these days both to make and to measure the electrical properties.

Of single molecules in metal molecule metal junctions.

And looking at structure property relationships in how molecules conduct electricity as wires, if you like, or perhaps it would be more accurate to say as resistors, they aren't terribly good wires.

We've seen that for alkane dye files on gold.

The conductance decays exponentially with length because it's a tunneling mechanism.

It's temperature independent again because it's a tunneling mechanism and the decay constant.

For an alkane is very large.

It's eight or nine per nanometer because those molecules have **** and LUMO which are miles away in energy from the Fermi energy of the Contacts.

The tunneling mechanism also holds good for all other conjugated small molecules tested so far just about so as long as the molecular length is less than about 3 or 4 nanometers, the molecule would conduct, almost certainly by tunneling.

If you build in redox groups into the molecule, you can tune the conductance of molecules by bringing orbitals into resonance.

With the Fermi energy of the metals.

Using electrochemistry, electrochemical control and that gives you an external method of gating the conductance of the molecule. So if you like that's behaving as a molecular transistor.

And finally, we're seeing that in special circumstances with molecules of rather an unusual structure.

The environment can even play a role in determining single molecule conductance.

On going on from that charge transfer complexation.

Can cause quite a large molecular conductance increase owing to the generation of this fahrner resonance.

Due to quantum interference on formation of the charge transfer complex.

And the the percentage boosting conductance.

It is greater if the molecule is longer and therefore less conductive when it's not.

Involved in a charge transfer interaction.

So that brings us to the end of this course. I hope you've enjoyed it reasonably well in spite of the lock down situation and just to re emphasize.

If you have any difficulties in.

In revising this course or going over the material, I can be contacted by email or by teams or some other means like that. Please feel free to get in touch.