Fulgides

Stobbe was the first to observe photochromism in fulgides (1.33) when he synthesised them by the condensation of an arylaldehyde or ketone with a substituted methylene succinate. However, it was not until the 1970s, during the course of the extensive work carried out by Heller and his collaborators, that their chemistry and use in photochromism was truly exploited. Heller s work showed, amongst many other things, the importance of R in (1.33) being a 5-membered ring heterocycle, e.g. furan. 

The reason why the change in absorption spectrum is important is that, when the absorption bands of individual species undergoing irradiation are widely separated, the relative proportions of the species involved can usually be altered markedly. In these cases, the associated properties can be switched , rather than merely changed . 

Fulgides, the common name for bismethylenesuctinic anhydrides possessing at least one aromatic ring on the methylene carbon atoms, such as 1, were first, and extensively, synthesized by Stobbe early in the 20th century.131 

Thanks to the fulgides succinic anhydride moiety, modification to obtain more sophisticated photochromic molecules has been easy. This chapter deals with imides (fulgimides), diesters (fulgenates), and some other derivatives, as well as fulgides themselves. 

Since 1981, when Heller and co-workers synthesized a furylfulgide 2,141 fulgides have included representatives of thermally irreversible photochromic compounds among their number. Fulgides known until then had been thermally reversible. Three major modifications were made to the fulgide structure  

These measures blocked all side reactions, such as oxidative aromatization, hydrogen transfer, and thermal disrotatory back reaction, as well as increasing the proportion of the C form at the UV irradiation pss. 

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The carbonylation of 2-methyl-3-butyn-2-oI (50) in benzene gives teraconic anhydride (51). Fulgide (53) (a dimethylenesuccinic anhydride derivative), which is a photochromic compound, can be prepared by the carbonylation of 2,5-dimethyl-3-hexyne-2,5-diol (52)[21], The reaction proceeds under milder conditions when PdlOAc) is used as a catalyst in the presence of iodine [23],... 

Another class of photochromic compounds, which operate through an electrocychc mechanism, is the fulgides although, with this class, the colored species is formed through a ring formation rather than ring-opening (29,30). The reversion to the colorless species [59000-86-1], does not occur... 

Photoinduced transformations of photochromes (spiropyrans, furan-derived fulgides, dithienylethenes) in polymers 98PAC2157. 

Spiropyrans and fulgides bound with biomolecules as photoswitchable biomaterials (a route to optoelectronic systems) 97ACR347. 

Fuji print films, 19 282 Fujix Pictrography, 19 290 Fujix Pictrography 1000, 19 317-318 Fulgides, photochromic materials, 6 599 Fulgimides, photochromic materials,... 

Aryl-substituted fulgides are the products of condensation of aromatic ketones with succinic anhydride, and form one of the oldest and most important groups of organic photochromic materials241. First discovered in the early part of the last century by Stobbe242, they are now well known to owe their photochromic behavior to reversible (conrotatory)... 

The furyl fulgide 172 has found use as a stable, recyclable actinometer for conventional photochemical experiments in the 313-366 mn wavelength range, where 173 = 0.20 and is independent of temperature and concentration245. It has also been developed as an actinometer in one- and two-laser flash photolysis experiments246. The colored form 173 can be converted back to 172 by simple exposure to visible light (equation 65). 

Usually the change in colour in the forward direction is to longer wavelength, i.e. bathochromic, and reversibility of this change is key to the many uses of photo-chromism. In many systems, e.g. spiropyrans, spirooxazines and chromenes, the back reaction is predominantly thermally driven but in others the photochemically induced state is thermally stable and the back reaction must be driven photochemically e.g. fulgides and diarylethenes). The assistance of heat in the reversion of colour can be regarded as an example of thermochromism, but in this text the term is reserved for those systems where heat is the main cause of the colour change (see section 1.3). 

There are five main classes of compounds which can approach these ideal requirements spiropyrans, specifically spiroindolinobenzopyrans, spironaphthoxazines, naphthopyrans, fulgides and diarylethenes. 

Fulgides can exist as or Z isomers by rotation around the double bonds in (1.33). This is illustrated for the furano derivative in Figure 1.10. Isomerisation of the yellow Z-fulgide (1.34) to the fi-fulgide (1.35) and cyclisation of this to the red coloured photochrome (1.36), designated as C here but often called the P state, occurs on irra-... 

The corresponding imides (1.40), called fulgimides, are most readily synthesised by conversion of the fulgide into the succinamic acids by reaction with amines, followed by dehydration. Functionalisation of the anhydride can also be achieved by reaction of the fulgides with malononitrile in the presence of diethylamine and subsequent recyclisation with acetyl chloride to give (1.41) from the corresponding E-fulgide and (1.42) from the Z-isomer. 

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... 

The use of photochromies in novelty printing is a signiheant commercial outlet. Because of their ability to show resistance to thermal fade rates fulgides have been the class most commonly used for this application. Typical uses are on children s toys and for logos on T-shirts. 

Fulgides such as the diphenyl derivative (144), dithienylethylenes (145), certain 2H-pyrans (146) and 2H-chromenes (147) and their respective thio analogues (148) and (149) also exhibit photochromism by valence tautomerism. 

Figure 7.43 Photochromic reaction (left) and the spectra of a fulgide (right)...

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