Photoreaction photochromism

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. 

Note 1 Examples of the changes in photosensitive polymers are a change in molecular shape (photoresponsive polymer), a change in its constitution (photoreactive polymer), and a reversible change in color (photochromic polymer). 

The initial limited interest can be partially attributed to the complete reversibility of the photoreaction and the consequent fugitive nature of the image. Much attention has been given to stabilizing photochromic images, but often another photographic system is employed to record the image before it fades. On the other hand, the reversibility of the reaction renders photochromic materials re-useable. 

The contribution of fluorescence to the deactivation of the excited singlet state of the phytochromobilin chromophores in Pr and Pfr (see Section II.D) is negligible in quantitative terms. The total fluorescence quantum yield of the photochromic P and P3 components amounts only to from Tables 1 and 4). This means that deactivation proceeds predominantly through nonradiative channels, i.e., via internal conversion back to the electronic ground state of Pr and via primary photoreaction(s). Nevertheless, the fluorescence efficiency suffices to serve as a sensitive tool to monitor certain aspects of the competing primary reaction(s) of P (see Sections III.A and III.C). 

The presence of functional groups in the heterocycle frequently determines the nature of the photoreaction observed. The major product of irradiation of 2-methoxyfuran (191) in the gas phase or in solution is the lactone (192).157 Similarly, 2-nitrofuran (193) undergoes a photoreaction typical of a,f -unsaturated nitro compounds to give the oxime (194) by the pathway shown in Scheme 13.158 A different process is observed, however, on irradiation of thenitrovinylbenzo[6]furan(195) to give the 6-hydroxy-l,2-oxazine(196)159 this transformation is viewed as proceeding via an electrocyclization pathway (Scheme 14) for which there is a precedent in the known photochromism of nitrostyrenes. 

In a polar polymer, i.e., cellulose acetate (CA) or nitrocellulose (NC) 35E, 35Z, and 36 had a relatively longer absorption maximum wavelength than in less polar matrices. In NC the of 36 shifts to 528 nm, which is also longer than in organic solvents. The role of polymer films in the quantum yields of photoreactions is not clear. In a comparison of the photochemical properties of 35 in polymer films and in solvents, it was found that the E c in polymer matrices was substantially smaller than that in the corresponding solvent with similar polarity. However, the decoloration quantum yield Oc e in a polymer film was larger than that in solvents. In conclusion, the polymer matrix properties, such as polarity, viscosity, and glass transition temperature (Tg) are quite important for photochromic reactions and applications. The coloration, E — Z and Z —> E isomerizations were suppressed in polymer matrices. 

Substituent effects on the quantum yield of photoreactions of indolyl fulgide 58 have also been reported by Uchida et al.,15 as shown in Table 4.14. The molecular structure and photochromic reactions are shown in Scheme 18. 

Oxazolyl (64), 4-pyrazolyl (65), 4-thiazolyl (66), and 4-isoxazolyl (67)-substituted fulgides have been prepared and their photochromic reactions investigated by several groups. The absorption spectra data and quantum yields of their photoreactions are listed in Table 4.19. 

Compounds 75, 76, and 77 are photochromic however, fulgenolide 78 has quite poor photochromic properties. The absorption spectra data and quantum yields of the photoreactions of fulgenates and fulgenolides, together with the parent fulgide, are summarized in Table 4.24. 

H. G. Heller and M. Szewczyk, Overcrowded molecules. PartX. Photoreactions of photochromic (a-phenylethylidene) (substituted methylene) succinic anhydrides, J. Chem. Soc., Perkin. Tram. 1, 1974, 1487-1492. 

Y. Yokoyama, T. Serizawa, S. Suzuki, Y. Yokoyama, and Y. Kurita, Fulgenolides thermally irreversible photochromic lactones with large quantum yields of photoreactions, Chem. Lett., 1995, 17-18. 

H. G. Heller, S. N. Oliver, J. Whittall, W. Johncock, P. J. Darcy, and C. Trundle, Photochromic fused-ring organic compounds and their use in photoreactive lenses, U.K. Pat. Appl. G.B. 2146327A (1985). 

The photoreaction of 27 was completely prohibited in cyclohexane. The addition of a very small amount of ethanol to the cyclohexane solution resulted in photochromic reactivity. In the mixed solvents of cyclohexane and ethanol, the... 

Ring opening of the photochromic 5-ring heterocycles can be induced only photochemically. The photoreaction of the colorless form to the betaines involves a singlet species in all cases studied so far. Triplet routes to the photoproducts may be populated with lower efficiency. The reverse process, however, can be brought about thermally or photochemically. 

The well-known photochromic transformations of anthraquinones are closely associated with the photoinduced migration of hydrogen, acyl, or aryl groups. Although photochromism of these compounds fits the reaction shown in Scheme 9, the processes of photochromic transformation exhibit some features related to the nature of the photoreactive state and details of the mechanism of the photochromic transformations. 

Quantum yields of photoproduction of ana-quinones of acetoxy-substituted anthraquinones with amino substituents in the anthraquinone ring as well as with an acetyl group proved to be the lowest (Table 7.2) As in the case of photochromic alkylanthraquinones, the reverse photoreaction, from ana-quinone to para-quinone for derivatives of acetoxyanthraquinone, proved to be impossible.21 The transition from ana-quinone to para-quinone occurred during freezing out of a sample owing to thermal bleaching. 

A comparative study of the photochromic transformations for 6-phenoxy-5,12-naphthacenequinone in toluene and polymer matrix showed that the value of the quantum yields was unchanged with a polymer binder in place of toluene. 61 Quantum yields for direct and back photoreactions were equal to cpB= 0.3 and (Pa = 0.005 under UV light and q>A = 0.005 under visible irradiation. 

The net number density of molecules undergoing a photochro-mic transition depends on the concentration of the photochromic molecules and the fraction that is converted. The maximum achievable conversion (the photostationary state) is determined by the relative absorption cross sections of the two states and the quantum efficiencies for the forward and reverse photoreactions. 

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