Spirooxazines

Spirooxazine is an aza analogue of spiropyran in which the carbon atom at 3-position is replaced by a nitrogen atom. Historically, the photo-chromic phenomenon of spiroindolinooxazine derivatives was found after discovery of photochromic spiroindolinobenzopyran.72... 

Table 5. W1X of the Colored Form of the Spirooxazine Series in Toluene...

Indolines, benzoxazole, and benzothiazole are possible as 2-methylene heterocycles. The number of known spirooxazine derivatives is much less than for the spiropyrans. This may be partly due to lack of many substituted o-nitrosonaphthols and partly due to lack of sufficient stability of spiro-oxazines. The structures of parent spirooxazines and the Xmax of their photomerocyanine forms are listed in Table 5. The Xmax of the colored forms of compounds 41-43 are not described in the literature. 

Unlike spiropyran, 2 -substitution in spirooxazine has no effect on Alkoxy, chloro, and nitro substituents at 5-position, and alkyl substituent at 1-position in the indoline component have small effects on the Xmix of the colored form. 

Like other spiropyrans, the colored form of spirooxazines generated by UV irradiation, reconverts to the colorless form. However, it is possible to measure the thermal decay rates and activation energies at ambient temperature, since this fading reaction obeys first-order kinetics in solution. The thermal decay rate constant for spiroindolinonaphthooxazine has been found to be 0.02-0.15s 1 in ethanol and 0.1-1.4s 1 in toluene, although this may vary according to the substituent groups.72,77 However, these values are smaller than those of the spironaphthopyran series. 

Spirooxazine compounds are useful in the field of plastic lenses, such as sunglasses and ski goggles. The plastic photochromic sunglasses have been in the marketplace since the early 1980s, and their market share is presently ca. 70%. The excellent lightfastness of the spironaphthooxazine series makes such applications possible, compared to other photochromic compounds. 

Other applications of spirooxazine compounds include toys, cosmetics, printing inks, and clothes. 

Scheme 10.4 Reversible photochromism of spirooxazine 247 involving interconversion with...

Hou L., Menning M., Schmidt H., Improvement of photofatigue resistance of spirooxazine entrapped in organic-inorganic composite synthesized via sol-gel process, SPIE Proc. 2288, 328-339, (1994). 

As a separate issue, the ring-opening reaction of the spirooxazine and the spiropy-ran can be sensitized by triplet energy donors[73,113-115]. A typical absorption rise for a benzophenone-sensitized ring-opening reaction is compared to that of the unsensitized fast absorption rise for NOSI3 in Fig. 13 [73,113]. In this case, both solutions were optically matched at the excitation wavelength. 

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). 

The commercially available spirooxazines are based on the spiroindolinonaphthoxazines (1.11) ring structure. The synthetic route to this ring system involves the reaction of a l-hydroxy-2-nitoso bearing aromatic ring with a 2-aUcylidene heterocycle, such as Fischer s base (1.6 R = H). The naphthoxazines are the derivatives of choice... 

Another group of commercially important spirooxazines are those where the naphthalene ring is replaced by quiinoline to give the spiroindolinopyridobenzoxazines (1.17). These are synthesised by reaction of 5-nitroso-6-hydroxyquinoline (1.16) with alkyl substituted 2-methyleneindolines (1.15). ... 

Photochromic systems that have been examined in both of these approaches include spiropyrans, spirooxazines, diarylethenes, dihydroindolizines and azoben-zenes. A schematic of a disk structure is shown in Figure 1.14. To produce the... 

Molecular rearrangement of molecules by the breaking of covalent bonds, e.g. spirooxazines... 

Spiropyrans, and spirooxazines, better known for their photochromic behaviour (see sections 1.22 and 1.23), also exhibit thermochromism. The ring opening to produce the highly coloured merocyanine form is induced by heating either the solid or... 

There have been relatively little ultraviolet-visible (UV-Vis) spectroscopic data for 1,4-oxazines, but selected data are presented in Table 8. UV spectroscopy is important for photochromic compounds, such as spirooxazines. The UV spectra of 33 spirooxazines in five different solvents are collected in a review <2002RCR893>, and the more recently reported examples of photochromic oxazines 65, 66, 101, and 102 are shown here. It can be seen from Table 8 that both adding methoxy substituents to the oxazine and changing to a more polar solvent give a UV maximum at a higher wavelength. This solvent effect can also be seen in the case of 102, which also has important fluorescence properties, discussed in Section 8.06.12.2. 

Solvatochromism and thermochromism are also characteristic of spirooxazines (Scheme 3) <1994RCB780>. The two forms 116 and 117 are in equilibrium in solution and more polar solvents shift the equilibrium more to the colored, acyclic form 117. Higher temperatures have the same effect for both solid spirooxazines and their solutions. A comprehensive review of spirooxazines <2002RCR893> has a collection of the absorption maxima for a large number of spirooxazines and their colored forms, which have their absorption maxima in the visible range at 480-670 nm. 

The formation of the merocyanine form 119 can be induced by addition of heavy metal cations (Pb, La, Eu, Tb ) to a solution of a spirooxazine 118 containing a crown ether group in the B-ring (Equation 1). The chelation occurs first to the crown ether and then to the negatively charged oxygen. In contrast, 118 does not react upon addition of alkaline earth metal cations (Mg, Ca, Ba ) <2005JP0504>. 

The merocyanine form of spirooxazines can react with free radicals, which is important as it causes degradation of the photochromic materials <1995JOC5446>. 

Cycloadditions have also been used to form benzoxazines, especially in the syntheses of photochromic materials. The reactants are typically an alkene such as 296 and a phenanthrenequinone monoxime or a l-nitroso-2-naphthol 295. Scheme 32 shows the synthesis of two photochromic materials 297 and 116 <1981TL3945>. The latter is a spirooxazine, for which a two-step mechanism, also shown in Scheme 32, was later suggested <2004BMC1037>. 

The methods to prepare spirooxazines have been reviewed extensively <2002RCR893>. The method shown in Scheme 32 and Equation (32), Section 8.06.9.3.1 <1981TL3945, 2004SC315, 2005S1876>, has been used most commonly in the synthesis of spirooxazines. This method is not suitable for spiro-l,4-oxazines that are not fused to an aromatic system when the acyclic nitrosoenol 368 is reacted with 298, a 1,2-oxazine 369 is formed instead of the desired 1,4-oxazine (Equation 54) <1996DP(31)155>. 

The medicinal use of the photochromic oxazine 379 was discussed in the previous section. The principle of photochromism was shown in Scheme 3, Section 8.06.5.1. Photochromic spirooxazines 101, 116, and 297 that have appeared in this chapter and their various substituted derivatives have been patented as photochromic materials <2003WO42195> and used as photochromic dyes in a microsphere-based sensor <2005USP19954>. The compound 299 is also used as photochromic material <2005S1876>. More detailed information about the applications of spirooxazines can be found in a review <2002RCR893>. The phenoxazine 380 has also been patented for use in optics <2005SUA2246491>. 

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