Spiropyrans photoreaction

When the initial concentration of the merocyanine form is lower than the CMC of the spiropyran form, the change in surface tension is gradual all through the progression of photoreaction. The value of Ajjq/Acqq remains constant during photoirradiation. Unfortunately, reversibility of this photochromism is poor and the micelle formation/dissociation cycle deteriorates rapidly. 

The structural formula of l,2-dicyano-l,2-bis-(2,4,5-trimethyl-3thienyl)ethene, and 6-nitro-l, 3, 3 -trimethylspiro[2H-l-benzopyrane-2,2 -indoline], referred to here as DE and SP, respectively, and their photochemical isomers are shown in Figure 3.14. The DE and SP chromophores have two photochemical isomers, a stable isomer and a thermally unstable isomer, namely the open-ring and close-ring forms for DE and the spiropyran and photomerocyanine for SP. The stable and thermally unstable isomers are henceforth referred to here as the A and B isomers, respectively- Light irradiation produces photoreaction in both the A—>B and A< B directions, and the thermal reaction proceeds in the A<—B direction. In contrast to the colored photomerocyanine form, which usually fades after several minutes at room temperature, the colored close-ring form of DE is stable for more than three months at Both the A and B isomers of DE and SP can... 

The photoreaction of spiropyrans 4 comprises a bond cleavage resulting in a zwitterionic species, the merocyanine, and subsequent cis to trans isomerization (see Figure 6.4). There are several species in equilibrium, so the chemistry and photochemistry of the spiropyrans are rather complex. The photoreaction of spiropyrans is stericaliy quite demanding and has been reported to be restricted in LBK films. ... 

J.A. Delaire, E. Ishoy K. Nakatani, Photoassisted Poling and Photoswitching of NLO Properties of Spiropyrans and other Photochromic Molecules in Polymers and Crystals, in Z. Sekkat, W. Knoll (eds.), Photoreactive Organic Thin Films, Academic Press, Amsterdam (2002). 

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