Photochemical bleaching

The third possibility is to design photochromic materials in which the photochemical bleaching reaction of the colored form has an activity energy barrier that could act as a threshold. Picosecond laser photolysis studies on furyl fulgides12 demonstrated that the transformation from the excited state of the colored form to the S o potential surface of furyl fulgides has an activity energy barrier. 

Figure 4. Photochemical bleaching of a film of poly(cyclohexyl methylsilane) upon irradiation at 313 nm. (Reproduced with permission from Ref. 26. Copyright 1984 The International Society for Optical Engineering.)...

D da Silva Perez, A Castellan, S Grelier, MGH Terrones, AEH Machado, R Ruggiero, and AL VUarinho. Photochemical Bleaching of Chemical Pulps Catalyzed by Titanium Dioxide. J. Photochem. Photobiol. A Chem. 115 73-80, 1998. 

The objectives of this chapter are to describe bleach product compositions and their formulation and to review the chemistry of both chemical and photochemical bleaching (i.e., the oxidative degradation of hair pigments and the accompanying oxidative degradation of the proteins of human hair) and to describe the chemical nature of bleached hair. 

Two dyes were separately absorbed at the expanded inner surfaces of a gel. One surface contained the photosensitizer adsorbate, and the other surface the acceptor sorbate. When mixed, the particles are in contact with one another, but the sensitizer and the acceptor are not. The critical experiment for the detection of diffusable, activated oxygen involved irradiating the sensitizer dye, and observing photochemical bleaching of the acceptor dye. The sensitizer was trypaflavine and the acceptor was leuco-malachite green. 

This interpretation is further supported by the observation of a decoupling of the cis-trans-isomerization process in bacteriorhodopsin occurring in the dark as a function of hydration as well as by the fact that the photochemical bleaching of the M-412 component is observed at the lowest humidity state. 

Detailed analyses of the time-dependent emission characteristics of single molecules (fluorescence transient) were used to show that RB molecules entrapped within the dried films are approximately four times more stable than their fresh-film counterparts (Mei, 2000). The transients also showed that the intersystem crossing rate and/or triplet lifetime are also greater in the dried films. Because of the role played by oxygen in the photochemical bleaching of RB, these results likely reflect a decrease in oxygen mobility within the dried films, and/or a reduction in the translational/rotational mobility of the entrapped dye. 

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