Fatigue photochromic systems

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). 

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. 

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... 

H. G. Heller and R M Megit, Overcrowded molecules. Part IX. Fatigue-free photochromic systems involving (E)-2-isopropylidene-3-(mesitylmethylene)-succinic anhydride andiV-phenylimide, J. Chem. Soc., Perkin. Trans. 1, 1974, 923—927. 

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 ... 

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]. 

In chapter 7, special emphasis has been placed on the synthesis of representative polycyclic quinones and their photochromic behavior, including the spectral, kinetic, and fatigue characteristics of such systems. Potential applications are focused on recording and multiplication of images, optical memories, and gradation masking. 

As in recent years, the bir-photocyclization and its reversal of 1,2-diheteroarylethene derivatives has again attracted considerable attention. An ab initio molecular orbital study of the photochromic reactions of these systems has been published, and details of their preswitching dynamics have been reported as have those concerned with the ring-opening dynamics. This year there has been an emphasis on the synthesis of novel systems, particularly those incorporating the dithienylperfluorocyclopentene unit, and in many reports their reversibility and fatigue characteristics are issues that are addressed. Such reports... 

The influence on the photochromism in dithienylethene systems of replacing the favoured perfluorocylopentene moiety by other cyclic ethene units has attracted the attention of a number of research groups. Dutch workers have shown that the use of the perhydrocyclopentene unit as in (91) has little effect on the photochromic behaviour of (92) but note that in situations where fatigue resistance and thermal stability are critical, the fluorinated moiety would be preferable. The cyclization quantum yield of (93) is reported to increase in the presence of metal ions, and more detailed accounts have been published of the interesting application of the dithienylcyclopentene-tethered jS-cyclodextrins (94) as photoswitchable hosts for meso tetrakis(4-sulfonatophenyl)porphyrin. ... 

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