A true adventure starts when imagination collides with reality. (K. Capek)

What controls selectivity of chemical reactions? Chemistry in solutions is, to a large extent, controlled by entropy and Brownian movement. There are many conformations which are present and, quite often, there are many reactions which are possible. It is suprising how chemists can run chemical transformations in the middle of this chaos! To some extent, selective chemical reactions in solution violate the second order of thermodynamics :-). How one can force molecules to march in a desired direction, if the molecular environment is so disorganized and chaotic? Usually, it is just matter of thermodynamics and statistics to determine the reaction outcome. But what if one wants to perform a reaction which is not favored by thermodynamics and statistics in solution?

The answer is to use external factors. The most popular way to control chemical selectivity is catalysis. The other way, which attracts more and more attention in the last years, is use of a higly organized media, in particularly - crystals.

Crystalline state is a world of perfect order and symmetry. That is why the idea to run chemical reactions in solid state to get dividends from the perfect order of crystal lattice is so attractive. The perfect order in crystals which is analogous to preorganization in many biological systems is decreasing transition state entropy making unlikely possible.

Unfortunately, for many centuries, chemists were hypnotized by the words of great Aristotle:" No reaction proceeds without solvent." Only recently solid state reactions became an object of systematic studies and, to much of their surprise, scientists found that solid state reactions not only proceed but also often have advantages over their solution analogues. In fact, combination of preorganization and selective stabilization / destabilization of alternative transition states in crystals is in many ways similar to enzyme catalysis. The result can be either reversal of regio- and stereoselectivity compared with the reaction in solution or even a totally new reaction!

However, quite often there were no results at all and solid state reactivity does not give an advantage over the solution reactivity. Why? A typical organic compound has only one or two crystal modifications. These modifications can be either photoinert or just show the same selectivity as in solution. This happens quite often. At the first glance, we are at the mercy of Nature which creates crystal lattices without our intervention. We need to have a way to build our own crystals with desired properties if we want to control selectivity of solid state photochemical reactions.

Another problem is that, even if a reaction in crystal is different from its solution counterpart, there is still another caveat. When the reaction proceeds in a crystal, more and more molecules of the starting material are transformed to the product. This process gradually destroys the starting crystal lattice. The initial selectivity is lost and we are falling back to the world of chaos.

Can we solve these problems? Can we create new diverse crystal lattices containing photochemically active compounds? And can we maintain integrity of the crystal lattice in the course of the reaction?

Yes, we can. The solution is to build crystals from two components. One of the components is the photoreagent while the other component can be whatever we want. The only problem is to make the two compounds to crystallize together. It is not an easy problem. Every organic chemist knows that when you crystallize a mixture of different substanses they usually precipitate separately and every organic chemist had used crystallization as powerful purification method. Many organic compounds behave like old bachelors who do not want to get married. They are happy on their own. How can one overcome it and force a compound to a happy marriage (a.k.a. a two-component crystal)?

That is what the field of chemistry called "crystal engineering" is about. It is one of the most beautiful and entertaining parts of modern chemistry. To use symmetry and very subtle molecular forces such as hydrogen bonding and pi-pi interactions for building a periodical 3-dimensional structure is a rewarding and challenging task. These forces and some luck result in one of the greatest nature wonders - self-assembly and formation of a crystal. Zillions of molecules organize themselves in a perfect symmetric structure which we call a crystal lattice. And changing the second component of the system on can create as many new crystal lattices containing the photoactive compound as needed.

Such systems are called inclusion compounds. To build an inclusion compound for small, symmetric and polar molecules such as usual solvents it is not difficult. One can find a lot of examples in the chemical literature. The enthalpy of formation of densely packed crystal lattice is the crucial factor. But to include the molecules which are of interest to us is not so easy. Our molecules are not symmetric, not small, not very polar - typical old "organic bachelors" unwilling to form an inclusion compound.

In my research, I was able to overcome these difficulties and create new crystal lattices containing the interesting photoactive molecules. In other words, I am creating molecular microreactors for photochemical rearrangements.

How they look like? If you want to try the taste of "photochemistry in a sandwich", choose which sandwich you prefer: thickorthin. Or may be you will like the double sandwich? But hurry up, somebody just took a a bite from it.
You are welcome to look at the poster I presented recently (it will look better on a large screen).

Chempointers - good starting point for any chemistry related search. Huge number of references to universities worldwide. Unfortunately, it was not updated since 1996.

One more compilation of chemistry related servers.

Chemistry on the Internet - a professionally made place. A lot of information about chemistry software and other web resources.

WebElements - Fun to browse but at the same time quick and handy resource for the most simple chemical, physical, biological, geological data including electronegativities, bond enthalpies, lattice energies, atom radii, reduction potentials, naturally occurring isotopes , NMR data, electronic configuration, ionization enthalpies, electron affinities etc.

Chemistry information on-line - Databases, on-line journals, journals preview and more...

Education resources - Not only in chemistry. a) 16000+ educational links from all over the world.
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c) Regularly maintained and updated.