Photochromic degradation

The merocyanine form of spirooxazines can react with free radicals, which is important as it causes degradation of the photochromic materials <1995JOC5446>. 

The insensitivity of so many of the photophysical and photochemical properties to proteolytic degradation down to about half of the original molecular weight is remarkable. It is particularly noteworthy when the location of the bilatriene chromophore near the N-terminus—one of the sites of preferential endogenous proteolytic attack—is taken into account and also the fact that the photochromic behavior of chromopeptides obtained by further protein degradation progressively deteriorates and is eventually lost [8b],... 

Most photochromic systems are not reversible indefinitely. However, very little careful analytical data have been accumulated to characterize the nature of the degradation products or to specify the degree of quantitative reversibility. The reasons for side reactions are inherent in the high photochemical reactivities of the systems. First of all, there must be an excited state formed by absorption this state is then transformed into other excited states or reactive species. The latter may include triplet states, carbonium ions, carbanions, free radicals, or other highly reactive intermediates. Certain of these are oxygen sensitive so that exclusion of the atmosphere and other potential sources of contaminants during irradiation is necessary. A second major route of degradation involves the excited state of the colored form which may already be... 

In some cases, permanent degradation of the parent molecular system is confused with genuine photochromism. The confusion may be brought about by a number of different mechanisms. Consider the following two hypothetical cases ... 

A crucial point that must be addressed concerns the thermal stability and the fatigue phenomenon observed in the chromophores. It is a fact that many photo-chromic compounds are irreversibly degraded upon long exposure to light, thus limiting their use for various applications. Major advances in the preparation and performance of photochromic materials have been made in the past five years. Irie et al.11271 have recently developed new photochromic compounds, 1,2-diarylethenes, which display photochromic behavior with unchanged intensity even after 104 coloration decoloration cycles. 

Malatesta35 proposed the key intermediate product (compound 28) of the oxidative degradation of photochromic spirooxazines. These species may result from the photochromic irreversible degradation of the spirooxazines even under conditions of partial or total absence of oxidation as, for example, in polymers coated with thin films of barrier agents such as SiO 2, SiOxCy, AI2O3, and MgO. 

V. Malatesta, R. Millini, and L. Montanari, Key intermediate product of oxidative degradation of photochromic spiro-oxazines. X-ray crystal structure and electron spin resonance analysis of its 7,7,8,8-tetracyanoquinodimethane ion-radical salt, J. Am. Chem. Soc., 117, 6258-6264 (1995). 

V. Malatesta, F. Renzi, M. L. Wis., L. Montanari, M. Milosa, and D. Scotti, Reductive degradation of photochromic spiroo-xazines. Reaction of the merocyanine forms with free radicals, J. Org. Chem., 60, 5446-5448 (1995). 

In the literature, the DHLs are claimed to be as efficient a photochromic class as the fulgides and anils. However, DHLs are rapidly degraded by singlet oxygen and their use requires an oxygen barrier of some sort. When dissolved in certain polymers, these materials exhibit photochromism for a duration of 600-5000 cycles [10]. 

Luminescence properties of and phenomena in polymer systems continues to be widely researched in connection with mechanisms of polymer degradation and stabilization, molecular dynamics, solubility, blend miscibility, and solar energy harnessing. A number of interesting reviews have appeared. Molecular dynamics of polymers in solution and in the solid state have been covered, as has excimer formation,photoresponsive polymers,behaviour of polymer gels, and photochromic phenomena. Photoisomerization of enzymes and model compounds has also been discussed in depth, with particular emphasis on proteins and synthetic polymers containing azo-compounds or spirobenzopyrans. ... 

Fundamental questions related to the electronic configuration of the open or colored forms and the number and structures of the photomerocyanine isomers are considered on the basis of the results of continuous-wave (stationary) and time-resolved (picosecond, nanosecond, and millisecond) Raman experiments. For spironaphthoxazine photochromic compounds, the Raman spectra may be attributed to the TTC (trans-trans-cis) isomer having a dominant quinoidal electronic configuration. Surface-enhanced resonance Raman spectroscopy (SERRS) is demonstrated as a new analytical method for the study of the photodegradation process in solution for nitro-BIPS derivatives. The development of this method could lead to the identification of the photoproducts in thin polymer films or sol-gel matrices and ultimately to control of degradation. 

Reactions 1 and 2 describe reversible photochromism, and 3 and 5 irreversible photochemical degradation processes having rate constants of Pad and pDB - The thermal degradation processes are reactions 4 and 6, with the latter having a negligibly small rate constant as SPs do not degrade appreciably in the absence of UV light. 

This study showed37 that irradiation of aerated toluene solutions (10 3 A/) of these three photochromes yielded common products from the heterocyclic moiety, namely, 3,3-dimethyl- and 1,3,3-trimethyloxindole (13 and 14) and 1,2,3,4-tetrahydro-2,3-dioxo-4,4-dimcthylquinolinc (265). From degradation of the chro-mene moiety, three products were formed 5-N02-vanillin (266), 1-formyl-p-napthol (267), and naphth[l,2-d]oxazole (268). 

No emission at 1269 nm was detected, which was taken to mean that no 02 was formed or that its formation quantum yield, fl>A, was lower than the sensitivity limit of the detector (5 x 10-3). In two-laser, two-color experiments, after excitation at 355 nm the merocyanine formed was excited (2-ns delay) with a green layer (532 nm), and again no 02 was detected, a clear indication that the (photo)merocyanine did not participate in the sensitized formation of singlet oxygen, the species that could induce oxidative degradation of the photochromes. The only noticeable exception was found for 11 for which fl>Awas measured to be 0.15. 

To the list of interactions that may lead to degradation must be added the easy reaction of free radicals with the merocyanine forms. We have recently reported48 that photochromic PMMA and polycarbonate cast matrices develop a violet-reddish color on aging, due to the formation of free-radical adducts (FRA),... 

While recent, although not numerous, studies4,33,34 36-38 42 8 of spiropyrans and spirooxazines have addressed the mechanistic aspects of the degradation of these photochromes and elucidated the structure of the intermediates and photooxidation products, only qualitative investigations have been reported regarding the stability of other classes of organic photochromes. 

G. P. Misra, D. Lavabre, and J. C. Micheau, Mechanistic investigations and spectrokinetic parameter determination during thermoreversible photochromism with degradation Example of application to the triphenylimadazolyl dimer (TPID) system, J. Photochem. PhotobiolA 80, 251-256 (1994). 

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