ABSTRACT
Over a period of time we have learned to control thermal reactions. This control is often achieved by variation of temperature, pressure, and solvent and with the help of catalysts. Because the activation barrier for photochemical reactions is small, temperature is expected to have only a minor influence. Yet it can be a valuable tool. Solvents, by influencing the stability of products and/intermediates, play an important role in thermal reactions. In addition to such effects, solvents, by influencing the closeness and ordering of excited states, would have a profound effect on photochemical reactions. Solvents with special properties (e.g., with a heavy atom) may also control the spin configuration of the reactive state, a feature we rarely consider in ground-state reactions. Wavelength and the mode of excitation have a significant influence on photoreactions. They help to place molecules in various excited states of different energy and spin. Further, photoreactions are susceptible to control by addends such as cations and Lewis acids that would control the ordering of excited states by interacting with lone pairs present in a carbonyl chromophore. Because most photochemical reactions occur at room temperature, one could preorganize reacting molecules prior to excitation. Such a templating influence is much more difficult to achieve in thermal reactions because temperature has a significant negative effect on the pre-association of molecules. This chapter discusses how one can exploit these features to manipulate the known photochemical behavior of various chromophores. One or two examples are provided for each category to illustrate the concept.
