BISTABLE PHOTOCHEMICAL-REACTIONS

In photochemistry, under continuous irradiation, kinetic non-linearity results from Beer-Lambert's law. In monophotonic or photoequilibrated systems, non-linearity is too weak in order to exhibit dynamic instab ilities. On the other hand, when there is a biphotonic process, as in the so-called ABC system or a process of auto-inhibition, as in the se lf-quenching model, screen effects (internal filter) are sufficient to give rise to kinetic instabilities such as bistability or oscillation s. Starting from a simple bistable photochemical system, the kinetic s cheme of a sustained photochemical oscillator operating in an isotherm al reactor closed to flux of matter is designed. From the experimental point of view, the photochemistry of the triphenylimidazyl dimer (TPI D) in chloroform is described as an example of a photochemical bistabl e system in a CSTR. The mechanism is interpreted quantitatively by app lication of the laws and concepts of photochemical dynamics. A series of experimental systems are reviewed and discussed in order to highlig ht the non-linear nature of their kinetics. Our objective is to pave t he way for further experimental demonstrations of bistable or oscillat ing systems in photochemistry.