ABC System Giving Rise to Bistability

Some ABC systems can give rise to unusual and complex dynamic behavior such as photochemical bistability, corresponding to the possible presence of two different photostationary states for the same irradiation and initial concentration conditions. The system ABC, 2cj)s, 2kc (see Table 3) is bistable under conditions in which ba/ ca 1 and AbsJ, 5. We assume that the molar extinction coefficients of A and B are equivalent (eA eB) and that C does not absorb (e = 0). 

Dynamic analysis of this ideal ABC, 2 j)s, 2kc system gives an estimate of its behavior as a function of Iq. If Iq is low, the photochemical reaction A - B, AB is balanced by the fast thermal back-relation B — A, fcBA B does not accumulate, the photochemical reaction B - C, ()BC does not take place, and the system stabilizes into a steady state enriched in A ([C] 0). On the other hand, if Iq is high, B forms and C is then produced C is relatively stable as the thermal reaction cA is slow, and the system stabilizes into a steady state enriched in C. 

A feature of this ABC, 2 t s, 2kc kinetic scheme is its cyclic nature photoproduct C regenerates the starting compound A. This enables it to operate in a closed system, since it does not photodegrade. Experimental photochromic, photodegradable systems involving similar processes are thus likely to exhibit similar behavior, but they need to be studied in an open system. An illustative case is the photochromism of the triphenylimidazolyl dimer (TPID). 

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