Fatigue, photochromism

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 nitro spiropyrans are susceptible to fatigue which has limited their appHcation. Indolino spiroxa2ines exhibit photochromism by way of a mechanism that is very similar to that of the spiropyrans. 

Masahiro Irie received his B.S. and M.S. degrees from Kyoto University and his Ph.D. in radiation chemistry from Osaka University. He joined Hokkaido University as a research associate in 1968 and started his research on photochemistry. In 1973 he moved to Osaka University and developed various types of photoresponsive polymers. In 1988 he was appointed Professor at Kyushu University. In the middle of the 1980 s he invented a new class of photochromic molecules - diaryl-ethenes - which undergo thermally irreversible and fatigue resistant photochromic reactions. He is currently interested in developing singlecrystalline photochromism of the diarylethene derivatives. 

The colored form of spironaphthopyran 32 absorbs at A,max of ca. 450 m,70 and the closed spiro form is colorless, which has no absorption band above 400nm. Bulky substituent group is especially important for photochromic sunglass. Introduction of the spiroadamantane or spirobi-cyclo[3.3.1]heptane into the 2-position of naphthopyran increases the resistance to photo-fatigue reaction, since endocyclic double bond induced by 1,7-hydrogen shift in the colored form cannot be formed in 2-adamantyl or 2-bicycloheptanyl group. 

Note Mechanical stimuli such as stresses and strains may produce cracks or fracture in a material. Loss of function of a photochromic material resulting from cyclic irradiation may also be considered fatigue. 

The three main classes that have been much studied for ophthalmic applications are spiroindolinonaphthoxazines (see section 1.2.3), diarylnaphthopyrans (see section 1.2.4) and fulgides (see section 1.2.5). How these different photochromic classes match up to the desired criteria for use in lenses is shown in Table 1.9. From this it can be seen that fulgides exhibit only a fair fatigue resistance and spiroindolinonaphthoxazines suffer from a high temperature dependency. Consequently, for this photochromic outlet, naphthopyrans have become the molecules of choice. Typical... 

Both of these mechanisms have been observed either as the major operating mechanism or as side reactions resulting in eventual fatigue of the system. Therefore, great care is necessary to (a) exclude atmospheric contaminants, principally oxygen and water, and (b) to follow the course of the photochromic system by quantitative evaluation of the components of the system. Unfortunately, the qualitative data found in the literature regarding both of these critical factors often creates doubt concerning the reversibility of the system. A few of these examples are cited below. 

Recognizing then that many of the applications which have been proposed still fail because of early fatigue, it is still deemed useful to survey briefly claims of photochromic activity found in the recent patent literature. 

The maximum degree of photoconversion to the cis isomer (85%) is achieved by irradiating at 350-370 nm, whereas the maximum yield of the back-reaction from the cis to the trans isomer (80%) is achieved by irradiating at 450 nm. Using a 200 W lamp, irradiation for 1 or 2 minutes is enough to achieve the photostationary state. The thermal decay in the dark is much slower. At room temperature, it takes more than 200 hours to restore the fully trans isomeric composition. The photochromic cycles are completely reversible and can be repeated at will, without any apparent fatigue 21,22 ... 

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. 

To realize the above-mentioned systems, we carefully chose suitable switching units and radical moieties. As an initial attempt, we employed l,2-bis(2-methyl-l-benzothiophen-3-yl)perfluorocyclopentene (9a) as a photochromic spin coupler (Scheme 9.2). Compound 9a is one of the most fatigue-resistant diarylethenes [21]. Nitronyl nitroxide was chosen for the spin source, because this radical is jr-conjugative. Thus, we designed molecule 10a, which is an embodiment of the simplified model 8a [37, 62]. 

My reservations were based on the fatigue of the HABIs, the weak colors that were formed, and the likelihood that these colors would fade, rather than reverse themselves. I also suspected that the fact that no one had really succeeded in commercializing any photochromic materials was because the fatigue reaction was too considerable an obstacle. I said to some of my friends that the academic scientists who reported in this field were content with a few dozen reversals, which might be achieved by low quantum yields of formation, and so there was an ample reservoir of dimers, provided one did not push too hard. 

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