Research Interests

We are interested in organic photochemistry, reactive intermediates, environmental photochemistry, 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. We can also use 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.

 

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. Another is trying to use that knowledge to understand how changes in the constitution of the catalyst affect the chemistry it induces.

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 (which are probably bound to the catalyst surface), superoxide, and other related oxygen species. A second very important oxidative pathway is that instead of removing an electron from water, the "hole" can induce single electron transfer (SET) chemistry from an adsorbed organic molecule.

TiO2

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. Many groups, including ours, are interested in producing catalysts that continue to do the oxidative chemistry, but also absorb more visible light. Mostly, this is done by doping the TiO2 with various other elements.

Thus, our recent approach from the organic side has been to use different chemicals as "probes" for different kinds of chemistry that the catalyst can induce. We use molecules that can either undergo SET reactions or hydroxyl radical chemistry in order to understand whether the new catalysts, whose band gaps are obviously lower in energy, can still carry out the degradations while using that visible light.

Degradation paths. Understanding the degradation pathways, then, does provide important information about the general applicability of 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.

A recent move in this chemistry is also a collaboration with people in agricultural engineering, looking at the possibility of using technology related to this for agricultural air purification!

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.

Since then, we have done many experiments on modified DBTO systems that all seem to verify this hypothesis, and have recently improved the reaction's efficiency by using dibenzoselenophene-Se-oxide.

An important recent breakthrough in this chemistry was to extend it from sulfoxides to sulfimines and sulfonium ylides, to make nitrenes and carbenes, respectively. A particularly important result was the direct detection of some of the nitrenes, a feat that had been elusive for the oxgen atom.

 

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 π* 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).

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!