Photochromism of Thiazines

The bleaching process also proceeded on steady-state irradiation with visible light in the presence of moderately reductive compounds.33 Since this oxidation-reduction reaction is not possible in the dark, the reactions have been well studied with a variety of combinations of reductants such as the Fe(II —Fe(III) system in aqueous solutions.1 This section discusses reversible spectral changes using a combination of two functions, i.e., oxidative (thionine) and reductive ones in a polymer matrix.34,35 

The thionine function, possessing a relatively high redox potential, acts as an electron acceptor upon irradiation with visible light, which is considered to first form the electron-donor-acceptor (EDA) complex (exciplex) with the electron donor (D reductant). The electron in the exciplex further added to the thionine function to produce the colorless leucothionine and the electron acceptor (A). The A may be an oxidized form of D. The dark reaction may take place either by back electron transfer to the thionine function via the EDA complex or by air oxidation of the leucothionine when atmospheric oxygen is present. 

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