Photobleaching visible light

The quantum yields for photocoloration of spirobenzopyrans are collected in Ref. 1. Generally, the coloration quantum yields of spiroindolinobenzopyrans by UV irradiation (366 nm) at room temperature (15-25°C) are 0.1-0.7. Photobleaching quantum yields by visible light are very small (0.1) and less accurate, since both thermal and photobleaching occur simultaneously. 

Fig. 8. ESR spectra of 2-methyltetrahydrofuran containing 0.42 mole-% of styrene irradiated by y-rays to a dose of 6 x 10 rad at 77° K. Solid line, before photo-bleaching dotted line, after photobleaching by visible lights...

Fig. 11a, b. The tj-resolved ESR spectra, recorded with fixed t2 of 1,0 ps, of PMMA y-irradiated to a dose of 5 kGy (a) before and (b) after photobleaching with visible light. All the irradiation, photobleaching and measurements were made at 77 K. 

The spectral change during this progressive warming to room temperature is shown in Fig. 12 for the sample irradiated and photobleached at 77 K. Elimination of the anion radical by photobleaching with visible light makes the analysis of the spectral shape easy. Then the observed spectral change indicates ... 

Fig. 12. Thermal change of the cw-ESR of PMMA y-irradiated to a dose of 5 kGy and photobleached with visible light at 77 K. Each spectrum was recorded after successive warming to temperatures indicated in the figure for 15 min. The spectrum for 295 K was recorded exceptionally after thermo-stating for 24 hours...

Figure 10 shows the photo-induced transformation of radical species in irradiated PMMA [43], The photobleaching with visible light at 77 K causes the liberation of an electron from the anion radical and the splitting of a methyl radical from the ester side group ... 

It should be noted that most of the anion radical simply disappears without giving any new radical species when photobleached with visible light. This observation seems to exclude reaction (26), the key step in the reaction mechanism proposed by Geuskens et al. [31]. The photo-induced disappearance of the anion radical must release an electron, which recombines with some cationic entities. This charge recombination does not actually lead to the formation of the propagating-type radical, in contradiction with the proposed reaction mechanism. 

Thus, the condensation of 2-phenyl-l,3,6-trimethyl-2-azaazulenium perchlorate with 5-nitrosalicylaldehyde gave the cationic styryl dye (A,max 510 nm) in 56% yield.139 This dye is the salt of the open form of the corresponding spiropyran, and upon treatment with base, a reaction which was not originally reported, was found to give the expected dye (126), which absorbs at 733 and 536 nm. This material did not thermally fade or photobleach to any significant extent with visible light in either polar or nonpolar solvents.7... 

Two methods of bonding a spiropyran to the surface of silica gel have already been mentioned the reaction of silica-bound salicylaldehyde with Fischer s base, and the reaction of 6-lithioBIPS with silica. Silica-bonded spiropyrans exhibited properties (reverse photochromism, visible light photobleaching, and color changes upon acid treatment due to salt formation) very similar to those of spiropyrans merely adsorbed onto silica. The differences in behavior were attributed to the different extent of absorption and desorption of atmospheric water caused by the difference in the number of free SiOH groups available in the two cases.86... 

In Figure 17.6, 2b film was irradiated with visible light for photobleach-ing, and then UV light (313 nm) was irradiated until the photostationary... 

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