Photochromism and Photodegradation of TPID

The photochemistry of the dimer of the triphenylimidazolyl radical is an example of photochromism whose dynamic behavior resembles that of a bistable ABC system. It is characterized by the formation of colored, relatively persistent radicals (TPI, 5). 

This last process results in degradation of TPID. Furthermore, three additional processes participate in the photodegradation  

FJgur 13. Change in phtnocluomisni of a solution of TPID ([TPICk x HI mol Irtcij ) in chloroform, under continuous imdiarion (366 nm). Tht observation wavelength (354 nm) 

Ed (he absorplun of TPI radical. (X EApcrirrtcnuJ points —, fitted fromIhc model. % = 2.15x 

Dynamic analysis of photochromic systems under continuous irradiation represents a powerful method of investigation of the reaction mechanisms. The characteristic kinetic and spectral parameters such as the quantum yields of the photochemical steps and the molar extinction coefficients of the transient species can be derived using this method. The essence of the method is the inverse treatment based on numerical simulation and fitting of the plots (Abs versus t) obtained under continuous irradiation. This also exploits the information contained in the irradiation kinetics. In order to extract one or more of the relevant parameters of a given process, specially designed experiments need to be carried out in which the effect of the process under consideration is conspicuous. 

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