Substituents on the Chromene Moiety of Spirobenzopyrans

The V1/2 values obtained for dioxane solutions (10 4M) of spirobenzopyrans substituted on the chromene moiety are presented in Table 2. In the nitro-substituted spiropyran series (6, 28, and 29), the 6-nitro derivative is usually more stable. OCH3 and CHO substituents are also more stabilizing when on C6 (cf. 6 and 32 30 and 32). Indeed, the electron-donor efficiency of the methoxy group is higher when at C6 as indicated by the Hammett ct constant values, which are -0.76 for the C6 (para) position and -0.39 for the C8 (ortho) position. For the nitro-substituted spiropyrans, the electron-withdrawing character of N02 depends on the substitution position, with Hammett constants of 0.66, 0.78, and 0.80, for N02 at C7, C6, and C8, respectively. 

Having observed that C6 substitution had a major effect on photodegradation, Gautron prepared a series of C6-substituted derivatives and studied their behavior with respect to photoexcitation. The results presented in Table 3 show that indeed electron-donor groups at C6 efficiently stabilize the photochrome against photodegradation (cf. 33, 36, and 3 3 and 33 vs. 34 and 6). 

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