HIGH-INTENSITY LASER PHOTOCHEMISTRY OF ORGANIC-MOLECULES IN SOLUTION

From the account provided above one can see that high-intensity photochemistry provides access to a domain of chemistry not previously available for study. Since this area has just begun to be explored, relatively few examples of high-intensity photochemistry have been described and these are usually in need of much more thorough mechanistic study. In fact, so little is known about the characteristic chemistry of this domain that no serious theoretical studies of the behavior of higher excited states have been reported to date. Thus, virtually every aspect of high-intensity photochemistry provides fertile ground for further study. An effort has been made in this review to emphasize particularly intriguing areas such as intertransient reactions or transient targeting. In this area, potential reactions between any of the classical or nonclassical transient species offer an enormous variety of fascinating possibilities for exotic chemistry. The availability of very high steady-state concentrations of some of these transient species should offer the possibility for their characterization by conventional, nontransient spectroscopic methods. Access to higher excited states in significant concentrations and under controlled conditions makes their study much more practical. In the past, access to these high-energy states was only available by means of vacuum ultraviolet irradiation. Since absorption by solvent in this spectroscopic region usually interferes with absorption by the substrate molecules, the study of these states has had to be conducted in the vapor phase. Now that these highenergy intermediates are accessible with relatively lowenergy photons and can be generated in appreciable quantities in conventional solvent environments, their study becomes much more attractive. This accessibility only serves to emphasize our lack of knowledge of these high-energy species. How might one best view their geometries, charge distributions, spin densities, and associated reactivity patterns? Theoretical models for the correlation of high-energy states would be of great value in developing an understanding of the “fallout” and tandem excited-state processes that follow the multiple-photon absorption events. At its present state of development, high-intensity photochemistry provides one the most fertile areas for observing truly novel chemical reactions. In our experience, more than half of the classical photochemical reactions that have been reexamined under laserjet conditions afford new products, many of which have yet to be characterized. Clearly this area is in a research phase of development. Practical applications of the reactions and processes characteristic of this highintensity domain may be relatively distant. Nevertheless, future chemical applications of lasers will certainly depend upon developments in this area. Finally, in its upper regions the multiple-photon domain merges with the plasma domain and it should be mentioned in this review that this plasma domain can be readily accessed using the argon ion laser-jet technique.136 If the nozzle of the laser-jet apparatus shown in Figure 2 is very briefly exposed to the focal region of the laser beam, a plasma is ignited. This plasma can be maintained indefinitely upon moving the focal region away from the nozzle. Preliminary studies have shown that, if this type of plasma is generated in a benzene-argon atmosphere, polycyclic aromatic hydrocarbons are generated in substantial quantities. These hydrocarbons include anthracene, phenanthrene, pyrene, and a series of other CnH10 hydrocarbons. It is most interesting, that a very similar series of polycyclic aromatic hydrocarbons has recently been produced by the discharge of a carbon arc into a hydrogen-donating atmosphere (propylene). 136 While no fullerenes have yet been detected from these laser plasma experiments, these experiments seem to be one of the first examples of the generation of a sustainable plasma with a CW laser. Thus, the laserjet technique for accessing the multiple-photon domain can be easily extended into the plasma domain. In summary, the tools for investigating high-intensity processes are now in place. Sophisticated spectroscopic techniques when coupled with the ability to obtain and characterize small amounts of high-intensity photoproducts provide a much greater degree of understanding of multiple-photon chemistry than does either of these techniques alone. During the past 10 years, the majority of the work in this Held has been directed toward the development of a basic understanding of the fundamental species involved in high-intensity reactions. More recently, it has become possible to extend these techniques to much more complex molecules. Since reactions in this area can span the full range of available states, they are likely to be rather complex, but the probability of encountering truly unique chemistry in this area is very high, and we suspect will remain so for many years to come. © 1993, American Chemical Society. All rights reserved.