Photochemical fatigue

R. Matsushima, A. Kaneko, A. Tomoda, M. Ishizuka, and H. Suzuki, Photochemical fatigue resistances and thermal stabilities of heterocyclic fulgides in PMMA film, Bull. Chem. Soc. Jpn., 61(10), 3569-3573 (1988). 

Photochemical fatigue of 6-nitro-BIPS (6) depended on the presence in solution of triplet sensitizers or quenchers. The triplet energy Er for 6 is 64.3 kcal mol-1. When benzophenone, a triplet-state sensitizer having a triplet energy Er— 69.2 kcal mol -was added to solutions of 6, a considerable acceleration of the fatigue was observed (Figure 14). On the other hand, 1,1 -binaphthyl ( >=54.3 kcal mol-1) acted as a triplet-state quencher, and its presence in solution decreased the colorability (Figure 15). 

Figure 16. Photochemical fatigue af 6-nilivBIP5 (6) in the presence af IL4-dia]rshicyclfi[2.2.2]oc-ranc DAHt O). Alternating irradiation nas carried out in a acetonitrile as described in 1lK caption io Figure 13. ConectU ration of DAHt fJ O.0 O, J.7 10 W , 2.1 x ID f Af Q 2.7 x 10 3 , 2.1 x lO Vff (Reprinted from Ref 22 iLh permission of the Chemical Society of Japan.)...

The photochemical fatigue (PF) of a photochromic system can be measured with the experimental setup described above by choosing one of the following modes or a combination thereof ... 

Another important concept in the discussion of photochromic systems is fatigue. Fatigue is defined as a loss in photochromic activity as a result of the presence of side reactions that deplete the concentration of A and/or B, or lead to the formation of products that inhibit the photochemical formation of B. The inhibition can result from quenching of the excited state of A or screening of active light. Fatigue, therefore, is caused by the absence of total reversibihty within the photochromic reaction (eq. 2). 

The enantiomers of 8 are stable at ambient temperatures (AGmc = 25.9 kcal mol-1) and fatigue resistant. Upon irradiation at 313 nm, a stereospecific photochemical isomerization process occurs, which converts P-8 into M-8 and vice versa. A fairly efficient, rapid, and selective photoracemization was observed when using unpolarized light (rac = 0.40 n-hexane). Large circular dichroism (CD) absorptions and... 

A recent review discusses the synthesis, spectral properties, and photochemical behavior of novel spirobenzopyrans, Spiro-1,4-oxazines, and 2//-chromenes. It emphasizes the structural features needed for infrared-absorbing photoisomers, luminescent stable forms, improved fatigue resistance, and spiropyrans exhibiting photochromism in vacuum-deposited films.147... 

Practical applications of photochromism at first (ca. 1955-70) concentrated on the spiropyrans, and especially on BIPS compounds, because of their ready availability, photosensitivity, convenient thermal fade rates, and good color contrast when perceived by the human eye. However, applications in which the dye was required to cycle very many times (e g., an optical binary switch for a photochemical computer memory) or be irradiated continuously (e g., sunglasses, vehicle windshields) were impractical because of the rapid fatigue of these dyes.196... 

Spirooxazines have excellent resistance to light-induced degradation. This property, also called fatigue resistance, is considered to be due to photochemical stability of the oxazine molecule framework in the ring-closed form as well as in the open-ring form. The fatigue resistance of the spirooxazines has led to then-successful use in various applications, for example, eyewear. 

The loss of response in a photochromic system (i.e., the irreversible formation of nonphotochromic products) is commonly referred to as fatigue (Scheme 4). Although the open form is most commonly implicated, both the closed and open forms of a photochromic molecule may be capable of undergoing such nondesirable thermal or photochemical reactions. Much like dyes, organic photochromic... 

Poly(4 -vinyl-fraws-benzalacetophenone) was prepared by Unruh 52> by condensation of poly(p-vinyhcetophenone) with benzaldehyde. In his studies Unruh s2> compared the photochemical behavior of poly(4 -vinyl-fraMS-benzalacetophenone) with that of the model compound, 4-ethylbenzalacetophenone. Both the polymer and its model showed typical photoisomerization behavior when irradiated in dilute solution. This behavior of the two species was consistent with the results reported for benzalacetophenone by Lutz, et al. 53<54>. Unruh found, however, that when the trans polymer was photoisomerized to the cis form, the expected complete return to trans form upon addition of acid failed to result. This fatigue or loss was attributed to small amounts of intrachain... 

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