Photocoloring reaction

In the case of spiroindolinobenzopyrans without a nitro group, the photocoloring reaction generally proceeds via the excited singlet state of the... 

The photocoloring reaction for spiroindolinobenzopyrans with a nitro group proceeds mainly via the formation of the excited triplet state of the molecule. The reaction proceeds partly from the triplet state [(SP )3] of the spiropyran to the triplet state (X)3 of the cis-cisoid isomer which subsequently transforms into the CF and partly from (SP )3 to the CF. This process from (X)3 to the colored form is accelerated by the presence of atmospheric oxygen (Scheme 6).2,28 For the photocoloring reaction, the participation of singlet or triplet state depends not only on the substituent but also on the nature of the heterocyclic component. 

F. Wilkinson, D. R. Worrall, J. Hobley, L. Janzen, S. L. Williams, A. J. Langley, and P. Matousek, Picosecond time-resolved spectroscopy of the photocoloration reaction of photochromic naphthox-azine-spiro-indolines, J. Chem. Soc., Faraday Trans., 92, 1331-1336 (1996). 

The data currently available on the rates and quantum efficiency of the photocoloration reactions la — lb of perimidinespirocyclohexadienones and their analogs are rather scarce. The values of the quantum yield for the photoconversion of compound la under excitation at its long-wavelength absorption band were determined with the use of the Aberchrome 540 actinometer. These are given in Table 8.3. 

In this photocolor reaction of 2-monosubstituted-amino-5-azidobenzoic acid, the imino and carboxilic groups in the molecules are very important. The compounds (IV and V) which hydrogen of these groups are substituted with alkyl group are very hypsochromic when they are photodecomposed. The reason why the photodecomposed IV and V are hypsochromic may be due to the fact that they can not have inner salt structure like VI. ... 

The photocoloring reaction for spiropyran or spironaphthooxazine generally proceeds via the excited singlet of the molecule to form of the cisoid isomer, and then a cis-trans isomerization gives a transoid colored form (Bertelson, 1971 Gugliemetti, 1990). 

Maurel F, Aubard J, Millie P, Dognon JP, Rajzmann M, Gugl-ielmetti R, Samat A (2006) Quantum chemical study of the photocoloration reaction in the napthoxazine series. J Phys Chem A 110 4759 771... 

With the help of similar molecular design, spirothiopyranonaphtho-pyrans 19 with absorption band in the near IR (692-850nm) on UV irradiation have been prepared.52 As predicted, the photocoloration of these spirothiopyrans occurs, but the reverse reaction to colorless form does not occur in solution as extension of Jt-conjugation increases. 

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... 

A highly efficient photochromic process is essential for organic photochromic compounds used as optical recording materials.39,40 That means that the quantum yields for the photocoloring and bleaching reactions should be high, but the quantum yield for the side reactions should be as low as possible. In order to solve the problems mentioned earlier, extensive studies have been carried out, and some promising results have been obtained so far. 

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

Emeline A. V., Kataeva G. V., Panasuk A. V., Ryabchuk V. K., Sheremetyeva N. V. and Serpone N. (2005b), Effect of surface photoreactions on the photocoloration of a wide band gap metal oxide Probing whether surface reactions are photo-catalytic , /. Phys. Chem. B 109, 5175-5185. 

As with all photochromic polymers, the polymer matrix effect is very important in fulgides and fulgimides as the polymer free volume affects rate of photocoloration and photobleaching. For example, Bahajaj and Asiri recently reported that the rate of photochemical reactions of fulgide doped in a variety of polymer matrices decreased in the order polystyrene > poly(methyl methacrylate) > epoxy resin [37]. Polystyrene possesses a larger free volume than poly(methyl methacrylate) and epoxy resin... 

Two types of cteviation from the first-orda kinetics are noted for photo-aialthamal isomerization reactions in polymer films. The first is the normal type, in which the reaction rate is the same as or smaller than that in solutions at the initial stage and then progressively becomes smaller. Typical examples are thermal decoloration of the photocolored merocyanine form of spirobenzopyran molecularly dispersed in or chemically bound to a polymer matrix > and photoisomerization of the transazobenzene residue incorporated in polymer main chains The first interpretation for the decoloration of the merocyanine form assumed the existence of different isomers, each of which fades independently following first-order kinetics On the other hand, Kryszewski et al. proposed the kinetic matrix effect, which means that the distribution of free volume may lead to the deviation from first-order kinetics. His idea was based on the finding that deviations from first-order kinetics can be observol even in simple molecules such as azobenzene which has only one trans or cis isomeric form. The effect of free volume distribution on reactivity was further demonstrated by studies of annealed polymer films The distribution function of free volume as well as the critical free volume v were estimated for the merocyanine form of spiropyran in poly(methyl methacrylate) derivatives of azobenzene in polystyrene and azobenzene in polycarbonate The deviation from first-order kinetics was also observed in cyclizing imidization of model poly(amic acid) in a polyamide matrix... 

The ring-closure reaction after photocoloration was monitored directly after removal of light at room temperature. First-order rate constants were obtained from the linear In A versus time descending curves. By extrapolation of the obtained In A/t plots to zero time, the absorbance A of the open form or its complexes at t = 0 was related to their colorabilities [18—21] using the expression (A c pb), where Cjp is the initial concentration of SP and b is the optical path length. 

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