Nitro-substituted spiropyrans

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. 

Futami Y, Chin MLS, Kudoh S, Takayanagi M, Nakata M (2003) Conformations of nitro-substituted spiropyran and merocyanine studied by low-temperature matrix-isolation infrared spectroscopy and density-functional-theory calculation. Chem Phys Lett 370 460 68... 

Cottone, G., Noto, R., La Manna, G., and Fornili, S.L., Ab initio study on the photoisomers of a nitro-substituted spiropyran, Chem. Phys. Lett., 319, 51, 2000. 

Nitro-substitution especially at the 6-position of BIPS opens up a triplet pathway for photo-isomerization. This pathway runs in parallel to the singlet manifold. This increases the yield and, in turn, may lead to photo-aggregation that is observed for these compounds. Photochemical ring closure to the spiropy-ran form is more efficient for these 6-nitro-substituted compounds. The photochemistry of 6-nitro-BIPS merocyanine is similar to that of unsubstituted BIPS(s) however, the 6,8-dinitro compound efficiently cyclizes upon excitation to form the spiropyran closed form via a singlet manifold. 

In a quantitative study of the photostability of spiropyrans, Malkin et al.11 concluded that the photodegradation quantum yield d is independent of the heterocyclic part and is determined instead by the structure of the chromene part of SPs. According to these authors, photodegradation of nitro-substituted SPs originates from the reaction of the cis-cisoid X isomer with solvent or impurities (Scheme 6) ... 

Spiropyrans, especially the nitro-substituted ones, are in general endowed with good colorability. However, one of the major deficiencies of spiropyrans for many potential applications is their short life cycle because of the extensive thermal and photochemical decomposition that they undergo by repeated and/or continuous light exposure. In 1968 Gautron28 was the first to investigate the photodegradation of 6-nitro-BIPS (6) and four other derivatives. 

The subsequent effects found in this series are very similar to those previously reported.28 In particular, electron-donor groups (e.g., CH30) at C8 increase the photostabihty of C6 nitro-substituted derivatives (216 vs. 217, Table 17). For C8 nitro derivatives (223-230), CH30 and CH3 at C5 decrease photodegradation (224 vs. 223 and 226 vs.227). In general, dilute solutions of these spiropyrans have a better colorability and a poorer photostability (Figure 18). 

Another class of merocyanines consists of ring-opened spiropyrans, such as spiro[2H-l-benzopyran-2,2 -indolines] (BIPS), especially of 6-nitro-substituted derivatives (NOjBIPS). The photochromism of spiro... 

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. 

The third approach is useful when the spiropyran itself (or, more correctly, its intermediates) is easily available, the necessary substituted intermediates are not, and the spiropyran tolerates the chemical transformations involved. Halogenation and nitration of BIPS and spiro(dinaphthopyrans) can be carried out by standard methods bromo substituents replaced by cyano or lithium, nitro substituents reduced to amino, etc. For example, the aldehyde group in 6-formylBIPS can be reduced, oxidized, caused to react with organometallics, condensed with active methylene groups, converted to cyano, and derivatized to give a wide variety of 6-substituted BIPS. These reactions cannot be usefully carried out directly on 5-formylsalicylal-... 

Upon treatment with acid (or on contact with an acidic surface), many spiropyrans give the salt of the open form or the open form itself, depending upon the relative base strengths of the spiro and open forms. Thus, treatment of several BIPS (7-diethylamino, 6-nitro, and 5 -nitro) with trifluoroacetic acid in the nonprotic solvents acetonitrile and chloroform gave the protonated merocyanine form, which upon neutralization with base gave the open colored form.180 This sequence of operations causes coloration by a non-thermal, non-photochemical route the adsorption coloration was utilized in the early applications of spiropyrans in carbonless (pressure-sensitive) copy papers. In this application, dialkylamino-substituted spirodi(benzopyrans) were preferred paper containing BIPS compounds turned pink on storage. 

The reactivities, in terms of photodegradation, of indoline-substituted nitro-spiropyrans are presented in Table 1. By plotting log v for the 5-substituted compounds versus ap, a straight line is obtained (Figure 5). The electron donors CH3 and CH30 have a stabilizing effect (19 and 20 in Table 1) whereas the electron acceptors Cl, F, and Br enhance degradation (9, 17 and 18 in Table 1). 

The results presented in Table 3 show that among the spiropyrans with a nitro substituent at C6, Cl, and C8, the C6-substituted compound is the most stable. OCH3 and CHO have the same effect. Here again, electron-donor substituents, e.g., CH3 and OCH3, stabilize the photochrome against photodegradation. 

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