Mechanism of photocoloration

Quantum mechanical calculations on the 2//-pyran ring opening and closing agree with the different mechanisms for photocoloration and photodecoloration experimentally observed in spiropyrans. The photobleaching proceeds from the 7 state, and mechanistically is very different from thermal bleaching, which proceeds from the S0 state and has a much lower activation of energy barrier. 

B. Ya. Simkin, S. P. Makarov, V. I. Minkin, and V. A. Pichko, Photo- and thermochromic spirans. 18. Theoretical study of the mechanisms of the photocolorization and photodecolorization of 2H-pyrans and their structural analogs. Triplet state, Chem. Heterocycl. Cpds., 1991, 250-255. 

A recent study by laser flash photolysis showed that a triplet state is not involved in the coloration mechanism of spirooxazines when in solution. A similar conclusion was reported, noting that the photocoloration occurs only in the excited singlet state because of the independence of the reaction to the presence of oxygen.29... 

The technical improvements in physicochemical investigations of photochromic behavior combined with the synthesis of new interesting models of photochromic substances are anticipated to lead to new advances in the knowledge of the very complex phenomenon that is photochromism, regardless or independently of the various suggested photophysical mechanisms for photocoloration. 

Another major category of photochromies are those whose photocoloration mechanism involves concerted electrocyclization reactions. Members of this group include the fulgides and the chromenes. 

Ultraviolet Irradiation of the anils of o-hydroxy aldehydes In solid solutions produces a change In color - mostly from yellow to red (5,47-51). In the solid state, a "soft" crystal structure, allowing some molecular movement Is necessary In order to allow photochromlsm (47). The initial form Is regenerated under the action of heat or visible light (eradication). In fluid solutions this process Is so fast that photochromlsm can only be studied by flash techniques (55,57). The nature of the photocolored species and the mechanism by which It Is produced and eradicated has been studied extensively but so far no complete and unambiguous mechanism has been proposed. The following section deals with the spectral properties of the photoproduced species, followed by a discussion of the different mechanisms proposed in the literature. 

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