Organic Photochemistry, Organosulfur Chemistry, Reactive Intermediates.

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Research Interests

We are interested in organic photochemistry, reactive intermediates, and sulfur chemistry.

Photochemistry can be uniquely interesting from a mechanistic-organic or physical-organic perspective, because photochemical reactions allow study not only of starting materials and products, but quite often of the short-lived intermediates that we write to account for reactions. As a result, we can get a terrifically detailed picture of what is going on in a chemical reaction.

Most of the time, these intermediates are characterized by various spectroscopic methods. We use both emission and absorption spectroscopy on the nanosecond to millisecond timescale, for instance. In the last few years, we have also taken to using computational chemistry to characterize certain intermediates or to "measure" certain properties that are difficult to access using experiments. This has broadened our arsenal for the understanding of organic reactions; we believe a working knowledge of computational chemistry is almost a requirement these days for the mechanistic- or physical-organic chemist.

Below is a quick summary of a few things we have done recently or are working on now.

Sulfur chemistry and photochemistry

A major area of our research effort over the last few years has been in the area of sulfur chemistry, particularly the photochemistry of sulfoxides. These are particularly interesting molecules that have unusual bonding properties, are of biological and atmospheric relevance, and are of interest to the synthetic community because of their stereochemical properties.

One of the most interesting reactions we have encountered is the deoxygenation of dibenzothiophene sulfoxide. Originally, we were simply testing a few mechanistic hypotheses, but along the way, we discovered that this reaction produces a powerful oxidizing agent. It converts benzene to phenol, alkanes to alcohols, and olefins to epoxides and allylic alcohols. After carrying out a series of experiments, we determined that the most reasonable explanation was that the deoxygenation takes place simply by cleaving the S-O bond. This directly produces the sulfide, but also makes an unusual reactive intermediate, an oxygen atom! A few of the oxidations we attribute to the oxygen atom are illustrated below.

Another important photochemical reaction of sulfoxides is cleavage of one of the carbon-sulfur bonds to produce a carbon-centered radical and a sulfinyl radical. These have turned out to be very interesting intermediates. Some examples where sulfinyls appear to be involved include biological oxidation of disulfides (as found in proteins, for instance), the oxidation of dimethyl sulfide (the most important source of sulfur in the atmosphere outside of urban areas) to sulfur dioxide, and the reason that so few vic-disulfoxides are known.

So we have looked into the chemistry of these things and continue to do so. Our approaches are a real mix of computation, spectroscopy and wet chemistry here. One of the interesting structural features of the sulfinyl radical is the location of the unpaired spin. Below are two views of the simplest organic sulfinyl radical, MeSO, with the S-O ¹* orbital that contains the unpaired electron illustrated in red and blue.

This arrangement of adjacent atoms with lots of unpaired spin is much more unusual than something like an allyl system, where the spin density "skips" every other carbon. It leads to interesting reactions at BOTH the S and O atom, determined mostly by whether the reagent is nucleophilic (reacts at S) or electrophilic (reacts at O).

Photocatalytic Degradation

We are in the midst of studying the environmentally relevant chemistry of photocatalytic degradations of organic molecules. Our particular niche is trying to understand the fundamental chemistry involved in the oxidative steps.

When aqueous solutions containing organic impurities are treated with light in the presence of titanium dioxide particles, the organic materials are "burned"...generally all the way to carbon dioxide and water. This is a result of the titanium dioxide absorbing light in the presence of molecular oxygen and water. The titanium dioxide particles are otherwise inert and can be removed by filtration at the end or they can be affixed to the walls of the container.

The basic chemistry of the titanium dioxide absorbing light in aqueous solution is illustrated below. The organic contaminants get chewed up because of the formation of the hydroxyl radicals, superoxide, and other related oxygen species. A reaction that is not illustrated below, but can also be important, is the direct one-electron oxidation or reduction of adsorbed organic substrates.

Understanding the mechanisms and pathways of the degradations is important.

In real world applications, degradations are likely to be incomplete on occasion, and it is thus of obvious interest to know what you are likely to release into the environment. Ideally, one would like to know how this goes for any general class of important pollutants. We have, for instance studied chlorophenol derivatives in some detail.

Second, it turns out that while titanium dioxide is an excellent material for this kind of work, it is not ideal because it doesn't utilize sunlight very efficiently. Understanding, for instance, how important direct oxidation might be compared to hydroxyl radical formation may aid others in developing better catalysts. And to be perfectly honest, this is just plain really interesting chemistry...consider for a moment taking a simple molecule like benzene and converting to a bunch of simple molecules of water and carbon dioxide. In between those extremes are some very complex, highly functionalized compounds with a rich organic chemistry!

As an example of the subtleties of this degradation chemistry, let's look at the simple contrast between hydroquinone and 1,2,4-benzenetriol. Despite the similarity of these two compounds, we believe the major oxidation mechanism changes entirely!

We believe that the major oxidizing agent that initiates the reactions depends distinctly on the substrate:

Compare the hydroxyl radical oxidation shown above to the direct electron transfer below.

So why the difference between these two molecules? We suggest two reasons. First, benzenetriol is somewhat easier to oxidize by electron transfer than hydroquinone, though both are rather susceptible. Second, it's been shown that the ortho-hydroxyl groups facilitate a specific strong form of adsorption onto the TiO₂. Perhaps it is simply enough that the benzene triol is specifically bound. A reasonable test of this hypothesis is the oxidation of 4-chlorocatechol (4-chloro-1,2-benzenediol). It turns out that both hydroxylation and C-C cleavage occur. This suggests at least that the oxidation potential is important.

These few paragraphs give a flavor of the sort of things we're interested in, but of course are just excerpts. Feel free to give us a ring or drop us a note!