Spiroindolinobenzopyrans

Figurel.l. Basic skeleton of the colored form in spiroindolinobenzopyran 1.

The numbering of spiropyrans adopted throughout this review is indicated in Figure 1.2. The nomenclature of the spiropyran 1 is given as 1, 3, 3 -trimethyl-spiro[2H-l-benzopyran-2,2 -indoline] it is referred to as spiroindolinobenzopyran and abbreviated as BIPS. 

The spiroindolinobenzopyran 2 is a classical example of spiropyran and is easily prepared by the condensation of l,3,3-trimethyl-2-methyleneindo-line (Fischer s base) and salicylaldehyde in anhydrous ethanol or benzene (Scheme 2).ia The nucleophilic attack of Fischer s base on the carbonyl group (like an enamine) gives an aldol product, which undergoes ring closure followed by dehydration. This condensation is reversible therefore, an exchange of the salicylaldehyde component of spiropyran with a different salicylaldehyde is possible. For example, when a solution of spiropyran 2 (Scheme 2) was refluxed with 3,5-dinitro-substituted salicylaldehyde, the open form of 6,8-dinitro-BIPS was obtained.2... 

The X-ray crystal structures of some spiroindolinobenzopyrans, e.g., 8-N02-BIPS and 6-N02-8-Br-BIPS, have been determined by Russian researchers.6,7 Recently, X-ray crystal structures of the colorless form (7a) and the colored form (7b) of 6,8-dinitro-BIPS have been reported by Nakatsu8 (Figure 1.3). 

Figure 1.3. Structures of colorless form 7a and colored form 7b of spiroindolinobenzopyran 7 determined by X-ray analysis (estimated standard deviation 0.01-0.03).

Table 1. Absorption Maxima of the Colored Form of 6,8-Disubstituted Spiroindolinobenzopyrans in Ethanol1...

Table 2. PPP Calculation ofthe Colored Form of Spiroindolinobenzopyrans... 

In the case of spiroindolinobenzopyrans without a nitro group, the photocoloring reaction generally proceeds via the excited singlet state of the... 

The photocoloring reaction for spiroindolinobenzopyrans with a nitro group proceeds mainly via the formation of the excited triplet state of the molecule. The reaction proceeds partly from the triplet state [(SP )3] of the spiropyran to the triplet state (X)3 of the cis-cisoid isomer which subsequently transforms into the CF and partly from (SP )3 to the CF. This process from (X)3 to the colored form is accelerated by the presence of atmospheric oxygen (Scheme 6).2,28 For the photocoloring reaction, the participation of singlet or triplet state depends not only on the substituent but also on the nature of the heterocyclic component. 

The quantum yields for photocoloration of spirobenzopyrans are collected in Ref. 1. Generally, the coloration quantum yields of spiroindolinobenzopyrans by UV irradiation (366 nm) at room temperature (15-25°C) are 0.1-0.7. Photobleaching quantum yields by visible light are very small (0.1) and less accurate, since both thermal and photobleaching occur simultaneously. 

The photochromisms of spiroindolinobenzopyrans trapped in glass prepared by the sol-gel method have been stuied.36,37... 

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... 

Becker and Kolc first examined the photochromism of spiroindo-linobenzothiopyran 44, a thio analogue of spiroindolinobenzopyran (Scheme 19). The closed form of this spirobenzothiopyran is stable and the photocoloration is very slow, compared with spiropyrans.71... 

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

Figure 1.1 Spiroindolinobenzopyran and ring opened merocyanine quinonoid form.

The synthetic pathway to spiroindolinobenzopyrans (1.8) illustrated in Figure 1.2 is representative of the methodology used to make this class of compounds. The parent spiroindolino compound (1.1) is made by the condensation of the readily available... 

Y. T. Osano, K. Mitsuhashi, S. Maeda, and T. Matsuzaki. Structures of photochromic spiroindo-linobenzoxazines and a spiroindolinobenzopyran, Acta Crystallogr. Sect. C 47, 2137-2141 (1991). 

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