Effect of Matrices on Photochromism

It is well known that an anion of large molecular size serves as a good nucleophile in polar aprotic solvents and its nucleophilicity decreases with an increase in the protic property of the solvent. Accordingly, polar groups in polymer matrices could affect the photoelectron transfer in viologen molecules. Table 9.2 

CA0 = before, A = after 1-min irradiation from 5 cm with UV lamp. 

The dark reaction takes place either by thermal back electron transfer from the radical cation to a counter radical anion, or oxidation of a radical cation with molecular oxygen when the film is exposed to air. The degree of thermal back reaction or air-oxidation is strongly dependent on the structure of the viologen and the kind of matrix polymer. 

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