The Current State-of-the-Arts of Redox Chemistry

Introduction to electrochemistry and photochemistry

Nowadays, redox chemistry is becoming more and more prevalent in our daily society. From OLED technology to medicinal treatment advances, the science behind these breakthroughs is becoming fundamental in how we view our world, and how we innovate for the future.

Around 1800 is when the first electroorganic chemistry can be traced back to, and since then, things have progressed at a rate unlike no one anticipated, with greener solutions for energy, as well as advancements in multiple scientific fields, such as electronics and medicine.

Not only have there been advancements in these fields, but the use of electrochemistry has allowed the harsh oxidising and reduction agents regularly used in traditional chemistry to no longer be required in synthesis. This is because electrochemistry allows for a very atom efficient method, compared to using said harsh oxidising and reduction reagents, to oxidise or reduce compounds and molecules selectively. Ackermann and co-workers reported the C-H amination in a renewable solvent, tetrahydro-2H-pyran-2-one, and the amination was done using cyclic secondary amines and aromatic amides. The Lei group on the other hand reported their new electrochemical amination procedure that used a divided cell and was between aromatic amides and secondary amines. The traditional method uses 2.5 equiv. of AgNO3 as the oxidant, and generates a lot of useless waste product during the synthesis. The two electrochemical protocols however, regenerate the cobalt catalyst by anodic oxidation and thus they are much greener than their traditional counterpart.

Not only this but using electrochemistry can permit very useful outcomes during synthesis, as the oxidation is not only selective, but mild, meaning new reaction pathways can be discovered, allowing for remarkable functionalisation of a molecule in the late stage of a synthesis.

The other area of chemistry that redox potentials pertain to heavily, is photochemistry. Photochemistry also allows for access to the radicals, charge-transfer complexes, and radical ions, that are ever growing in importance in chemical synthesis pathways used globally in the current times. Photochemistry usually involves the use of a photocatalyst. A photon will excite the molecule and then this will catalyse the reaction. This is a much newer and exciting way of looking at creating chemical synthesis schemes, in comparison to the traditional methods of oxidation and reduction using stochiometric chemical agents.

This is possible in photochemistry, as like electrochemistry, there is single electron transfer. The most abundant source of photochemistry in the world, is photosynthesis, and for decades scientists have been attempting to link photochemistry to electrochemistry in order to further provide greener forms of oxidative or reductive chemistry that can be used in synthesis on a large scale. Photochemistry works by a molecule of a chemical absorbing photons from visible, ultraviolet, or infrared radiation, and then using the energy from the photons to turn the photocatalyst into an excited state, and in turn this photocatalyst uses SET with the organic substrate to form a highly reactive intermediate that goes on to react further until the desired final product is formed. The creation of such reactive intermediates, is a theme that links photochemistry with electrochemistry, and together they are paving the way for greener chemical synthesis and production the world over. Each technique has its flaws. For instance, electrochemistry has potential generation of radicals that can very quickly and easily over-reduceoxidise a substrate, which could be very undesirable. Alternatively, due to photocatalyst's nature of being net-redox-neutral, stochiometric amounts of oxidantsreductants can be needed, however by utilising the combination of these two techniques, their respective advantages balance the other techniques flaws. This leads to the formation of new reaction pathways, that would be unobtainable by using each technique individually.

In conclusion, the use of electrochemistry and photochemistry, also allows for easy access to radicals, charge-transfer complexes, and radical ions, all of which are some of the most reactive intermediates. These methods have the both pros and cons as mentioned above, however their overall usefulness is immeasurable in terms of their selectivity, greenness, and efficiency, allowing for the potential for old and less efficient methods in synthesis to be discarded in the future.

Electrochemical synthesis

When performing an electrochemical synthesis, the simplified general setup is required for a single chamber reaction, where you have a power supply, a reaction solution and two electrodes. The power supply used for such a simple set up is usually a constant potential power source, such as a battery, like one would use in a high-school level chemistry lab, as the main goal with such a simple set up, is purely to reduce or oxidise a chemical(s), and not quantifiably measure any change in species. In the reaction solution, there is a solvent, the substrate(s), as well as an electrolyte and additives if the reaction requires it. During these reactions, despite both electrodes having their own reaction taking place, there is usually one reaction that is of interest is taking place, and thus the electrode at which this is happening is called the working electrode. A chemical reaction will then occur if the substrates redox potential is lower than the electrode potential generated when turning the power supply on. The final product made during the electrochemical synthesis pathway, undergoes a few key steps. Firstly, the substrate undergoes single electron transfer (SET), and this usually forms an intermediate or radical species. Then this intermediate or radical undergoes further reactions with other intermediates or substrates in the solution, to finally end the synthesis at the final product.

When more advanced experiments are undertaken, a more advanced set-up is required, usually comprising of a potentiostat, which is a power source that has the ability to either control the current or the voltage, making it incredibly versatile. The more advanced set-ups also usually use a 3 electrode set-up, which comprises of a working electrode, reference electrode and counter electrode. The reference electrode never passes any current, and instead it measures and monitors the working electrodes potential so the potentiostat can make adjustments as necessary.

The generalised theme is that most protocols use constant current mode, such as the electrolysis portion of the reaction. As if the current applied is low enough, then the electro-active substrate with the lowest redox potential is transformed, at the working electrode, and the potential at the working electrode is kept roughly around the redox potential of the substrate. The potentiostat in constant current mode, measures the change in potential , which arises from a change in the solution surrounding the electrode due to the flowing current, and alters the voltage accordingly, providing the constant current required for most electrochemical synthesis. This is useful as it means that the substrates used can be changed so that the substrate you are interested in transforming is the one being transformed. Not only this, but using a potentiostat in constant current mode, allows for a constant rate in an electrochemical synthesis.

Electrochemistry is restricted to the surface area of the electrode, making it a heterogenous process. This basic depiction of an electrochemical synthesis really defines how important it is to therefore know the substrates redox potential when designing an experiment, as different electrodes will produce a range of electrode potentials and therefore you need a pair of anode and cathode that will permit the reaction you are interested in to take place.

Using electrochemical synthesis, it is possible to achieve much greater oxidising and reducing potentials over generic oxidising and reducing agents commonly used in synthesis pathways, and the only limit of a reaction is the redox stability of the electrolyte and solvent used in the synthesis pathway. This further backs the claim that traditional oxidising and reducing agents are no longer as necessary as they once were. For an example, traditionally in reducing arenes, aziridines, and ketones, the Birch reaction could be used, whereby the reaction takes place by dissolving metal in liquid ammonia. However, Baran and co-workers used the reality that there was only the redox stability of the solvent and electrolyte limiting a reaction to their advantage, and by using cathodic electrolysis, were able to generate highly reacted solvated electrons and create an electro-reductive Birch reaction.

An overcharge protection agent was introduced to stop the direct reduction of Li , which was unwanted, and surface passivation. The overcharge protection agent stems from their understanding of lithium ion battery research in electronics. In an electronic lithium ion battery, if it is charged too much, it can lead to permanent damage of the battery, and thus an overcharge protection diode is introduced to the circuit. This allows for the potential difference between the anode and cathode of the battery to be monitored (terminal voltage), and charging turned off when it reaches a peak, as to not damage the cell. They therefore realised the same principle applied to this electrochemical Birch reduction, and thus an overcharge protection agent was needed.

The overcharge protection agent used, was tris(pyrrolidino)phosphoramide (TPPA), and came about from Baran and co-workers looking into lithium-ion battery research, another area where redox potentials are incredibly important. TPPA doesn't increase the electron transfer rates that would be expected, and instead appears to be not a part of the mechanistic cycle, but involved in ancillary electrochemical processes. Overall, this newly discovered electro-reductive Birch reaction was much safer and showed the potential of being able to update old methods via the use of electrochemistry.

Using this knowledge, it becomes apparent that a database full of redox potentials would become increasingly useful over the next few years, to save time and energy on the path to a more sustainable future in chemical synthesis, where harsh reagents become used less each time a new reaction pathway is discovered due to these incredible techniques.

Organic electronics

Another field of interest in the redox chemical landscape, is that of organic electronics. There is no doubt that technology has progressed beyond anyone's wildest imagination in the last 30 years, and now what we know as boundaries are being stretched further and further each year, by technology giants, as well as research teams.

Organic electronics is relevant to redox chemistry, as the relationships between substrates and how the react with each other is highly related to reduction and oxidation of said substrates. Everything from excited state compounds transferring electrons and photons, to how organic compounds respond to electronic and other stimuli, all leads back to the fundamentals of redox chemistry. The main link between organic electronics and redox potentials however, is that the redox potential of a compound, allows for information to be gathered about how it will react with the electricity powering the component, and therefore how efficient it will be inside said componentdevice.

Organic light emitting diode's

Organic electronics is largely a fairly new area of redox chemistry, and whilst some polymers have been shown to conduct electricity around the 1860s, the field of study only really started to gain traction after 1987, when C. W. Tang and S. A. Van Slyke from Kodak had a major breakthrough with organic light emitting diode (OLED). They found OLEDs with a low voltage and efficiency, from p-n heterostructure devices, that used thin films of vapour deposited organic materials. After this important discovery, teams around the world and large technology companies realised the potential behind such a finding, such as the Universities of Cambridge's discovery of electroluminescence of polymers in 1990.

OLED screens have a much higher contrast ratio (the difference between the dark and light parts of an image) than their LED counterparts, which makes them much more attractive for those wanting the highest quality screens for the TVs, phones, or monitors. This increased contrast ratio, is due to how OLED screens work versus LED screens. LED screens are a liquid crystal display (LCD) layer with an LED backlight, the LCD layer choses the colour of each pixel and then the screen relies on the backlight to provide the luminance and display what the LCD screen is depicting. On the other hand, OLED technology is incredibly powerful, as unlike regular LEDs, which can only emit light of a single colour and change brightness, OLEDs can change colour and brightness simultaneously, allowing the mitigation of the LED backlight. This means that when the OLED is off, it is completely black, giving a higher contrast ratio and allowing for a much higher quality image. The advancements in this area of organic electronics alone has shaped the way that the highest picture quality screens will be used for the foreseeable future, and with more research being done into organic electronics every day, it is realistic to expect another leap forwards in the future.

Organic semiconductors

Another area of organic electronics that has grown in interest exponentially since the early 90s, is that of organic semiconductors. Organic semiconductors are usually amorphous thin films, or molecular crystals, and the reason there has been so much interest in them, is due to their properties. Whilst regularly, they are electrically insulating, if modified in the correct way, either with charge added from electrodes, or doped, then they become semiconducting. Semiconductors are able to conduct electricity, however fall between something inertly conductive, like glass, or a metal such as copper, which is highly conductive. There are many semiconductors that have been discovered over the years, however pentacene is one that has been used religiously since its discovery, and continues to be used today, primarily as a thin film transistor (TFT). Thin film transistors are types of field-effect transistors, and like the name implies, the transistor is thin in comparison to the device. A common use of thin film transistors is use inside LCD screens. Pentacene is particularly useful however, due to its TFT hole mobilities, which are above 1 cm2Vs.

Summary

Overall, even when looking at a couple of topics out of the many that use redox chemistry, it becomes very clear how much progress is being made each decade with the use of redox chemistry, and these strides forward in science will only grow as our knowledge of redox chemistry does.