Photochromic systems stilbenes

Groups of reportedly photochromic systems which deserve further study include (a) disulfoxides (123,124), (b) hydrazones (125-129), (c) osazones (130-133), (d ) semicarbazones (134-143), (e) stilbene derivatives (144), (/) succinic anhydrides (145-148), and (g) various dyes (149,150). A number of individual compounds also remain unclassified as to their mechanism of photochromic activity. These include o-nitro-benzylidine isonicotinic acid hydrazide (151), 2,3-epoxy-2-ethyl-3-phenyl-1-indanone (152), p-diethyl- and p-dimethyl-aminophenyli-minocamphor (153), brucine salts of bromo- and chloro-nitromethionic acid (154), diphenacyldiphenylmethane (155,156), 2,4,4,6-tetraphenyl-1,4,-dihydropyridine (155,156), 2,4,4,6-3,5-dibenzoyltetrahydropyran (155,156), o-nitrobenzylidenedesoxybenzoin (157), p-nitrobenzylidene-desoxybenzoin (157), N-(3-pyridyl)sydnone (158,159), tetrabenzoyl-ethylene (160), and the oxidation product of 2,4,5-triphenylimidazole (161,162). 

Methylthio)benzthiazole for example gives two products when irradiated in the presence of rraui-stilbene. This reaction was investigated by G. Kaupp and seems to proceed via two different biradical intermediates. Whereas one biradical leads to an azetane simply by radical recombination, the other rearranges to a seven-membered heterocycle. Thermolysis of the latter cycloadduct leads to an 1-azabutadiene which can be subsequently transformed into its geometrical isomer. This isomerization is reversible at another wavelength and thus constitute a simple photochromic system (see also Chapter 8). 

It has long been known that the stilbene-type photoq clization occurs readily with styryl-heteroarenes and 1,2-diheteroaryl ethenes. Such processes not only provide a convenient access to novel as well as known polynuclear heteroarenes, but also in the latter series of these photoactive compounds, have been widely researched for a number of years as photochromic systems that have potential for development as the key element in data storage devices and as molecular switches. ... 

Due to the relatively long lifetime of the sensitiser triplet state and the possibility of integrating data on the stilbene photoisomerisation, the apparent characteristic time of the method can reach hundreds of seconds. This unique property of the cascade system and, therefor triplet-photochrome technique, allows the investigation of slow diffusion processes, including encounters of proteins in membranes using very low concentrations of both the triplet and photochrome probes. 

Another important class of photochromic materials based on reversible triene photocyclization are 1,2-diaiylalkenes, of which the Z-stilbene (175)/9.10-dihydrophenanthrene (176) system is the prototype (equation 66)25b252 fuigije systems, it is nec-... 

A fiuorescent-photochrome method of quantifying the orientation of solid-phase antibodies immobilized on a silica plate was proposed [9]. The method is based on the measurement of fiuorescence quenching by a quencher in solution, rates of trans-cis photoisomerization and photodestruction of a stilbene-labeled hapten in an antibody binding site. These experimental parameters enable a quantitative description of the order of binding sites of antibodies immobilized on a surface and can be used to characterize the micro viscosity and steric hindrance in the vicinity of the binding site. Furthermore, a theoretical method for the determination of the depth of immersion of the fiuorescent label in a two-phase system was developed [9]. 

The other common photo-induced initial step is the cis-trans isomerisation as in the case of stilbenes and azobenzenes. Scheme 2.3. Other systems consist of two steps, a cis-trans isomerisation followed by cyclization, as in diarylethenes. Scheme 2.4, and fulgides. Scheme 2.5. In the case of diarylethenes the photoisomerisation step is avoided by "freezing" the compound in its cis configuration through an appropriate structural modification. This prevents the initial isomerisation process and the photochromism is only controlled by the cyclization reaction. Scheme 2.4, bottom. [4]... 

A strategy to obtain photochromic ionic fiquids was reported by Arai and co-workers and consists on appending an imidazofium substituent to a stilbene. Scheme 5.3. [57] The cis isomers are RTILs except for b) and the trans analogs are solids except for d) and e) which while being IL are not RTILs. Moreover, while the trans isomers are moderately emissive with quantum yields aroimd 0.02, the cis analogs practically do not show fluorescence. This pormits the authors to claim the possibility of switching photochemically from solid to liquid as well as from emissive to non-emissive systems. 

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