Spiropyrans photochromic behavior

The photochromism of the spiropyran depends on the structure of heterocyclic parts, the medium such as solvent or plastic films, temperature, and light energy. Though the actual mechanisms may be more complex, a simple photochromic behavior in the spiropyrans is illustrated in Scheme 1. Initially, a spiropyran is excited by photoirradiation, and then a cisoid isomer arises after dissociation of the C—O bond. Finally, the cisoid form changes to the thermodynamically stable transoid form. The equilibrium between the cisoid and transoid forms largely depends on the substituent groups. The reversal of the colored form to the colorless spiropyran occurs by thermal or photochemical energy. More detailed mechanisms will be described in Section 1.2.1.6. 

Poly(L-glutamate)s containing various molar percentages of spiropyran units in the side chains (XV) have been prepared by treating poly(L-glutamic acid) with N-(2-hy-droxyethyl) -spiropyran in the presence of dicyclohexylcarbodiimide and 4-pyrrolidino-pyridine.[60 62] The structure and photochromic behavior of the modified polymers are shown in Figure 8. 

Polymers of L-lysine containing spiropyran units in the side chains (XVI) (Figure 13) were found to show photochromic behavior in HFP analogous to that already... 

The ability to tune the photochromic behavior of crown ether-bound spiropyrans, 4a and 5a, by connecting them to metal ions, was investigated by Kimura and his coworkers [10,11]. This complexation of the crown ether moiety with a metal ion accelerates the transformation from spiropy-ran, 4b to merocyanine, 4c, by light or heat. When this complexation effect is weak, thermodynamic equilibrium favors the spiropyran form, and... 

Spironaphthoxazine with a crown ether, 8, also shows photochromic behavior that is sensitive to the type of alkali and alkaline earth metal ions [14]. Other derivatives of spiropyran and spiro oxazine, 9-12, which can chelate metal ions were synthesized, and the effects of complexation on their photochromic behavior have been investigated [15-17]. 

Because spiropyrans (and especially BIPS) are relatively easy to prepare, a variety of compounds are available from which to deduce correlations (usually semiempirical Hammett-like equations) between structure and photochromic parameters in dilute fluid solutions. The photochromic behavior of a single dye in a variety of solvents can also be correlated with a solvent parameter such as. In applications, however, the dyes are almost always relatively concentrated in a bulk polymer or a polymeric binder film, and various other additives such as antioxidants, surfactants, fire retardants, plasticizers, and other colorants are also present. Under these conditions, the observed photochromic behavior is often very different from that predicted by the dilute fluid solution correlations. 

Little has been reported about the preparation and photochromic behavior of optically active spiropyrans, despite an old suggestion to examine photocoloration with circularly polarized light. 

D. Levy and D. Avnir, Effects of the changes in the properties of silica cage along the gel/xerogel transition on the photochromic behavior of trapped spiropyrans, J. Phys. Chem., 92, 4734-4738 (1988). 

V. Malatesta, L. Longo, R. Fusco, and G. Marconi, Comparison of photochromic behavior between spiroxazines and spiropyrans theoretical calculations of ground and excited states, Mol. Cryst. Liq. Cryst., 246, 235-239 (1994). 

The spiropyrans are not photochromic in the solid state but do exhibit photochromic behavior when dissolved in any of the following solvents, gels, films, plasticized resins, and plastic solids. 

Though the field of molecular electronics is still in its infancy with respect to the porous layered materials many of these phases have yet to be considered as hosts for conducting polymeric materials. The properties of intercalated sufonyl-spiropyran have been examined to determine any novel photochromic behavior. The parent... 

Chapter 7 (Structural Studies by X-ray Diffraction). The geometric parameters of photochromic compounds have been determined by X-ray diffraction on single crystals and yield interesting correlations with photochromic behavior (colorability, thermal bleaching rates, or absorption spectrum of the colored species). More intensive studies have been carried out on members of the spiropyran series (indolinospiropyrans), examining both closed spiranic forms and permanent merocyanines. The latter served as models of open photomer-ocyanines, the transient species produced by UV irradiation of spiropyrans. 

In contrast to the spiropyran-modified polypeptides derived from poly(L-glutamic acid), the poly(L-lysine) analogs have been reported to show no photomodulation of the side-chain conformation in pure HFP, although they exhibited similar photochromic behavior (Figure 2).32,33 However, when appropriate amounts of triethylamine were added to the HFP solution, exposure to visible light resulted in the reversible formation of a helix by the poly(L-lysine) chains.35 Thus, addition of triethylamine to the HFP solution induced the coil - helix transition, but the amount of base necessary was different for the dark-adapted sample as compared with the irradiated one. The authors claimed... 

FIGURE 17. Influence of the temperature on the photochromic behavior of the spiropyran polester VI. 

The coil helix transition proceeds rapidly within seconds, whereas the back reaction requires several hours for full conversion. Notably, in this case, the photochromic behavior of the spiropyran groups is opposite to that observed in other solvents (see example (b) in Table 5-1). The reverse photochromism is due to the high polarity of hexafluoro-2-propanol, which stabilizes the charged mero-cyanine form better than the uncharged spiropyran form. 

Fig. 14 Photochemical reaction responsible for photochromic behavior of spiropyrane-containing ELP. Reproduced with permission from American Chemical Society...

Takagi et al. reported the intercalation of l, 3, 3 -trimethylspiro[2//-l-benzopyran-2,2 -indoline] (H-SP) and its 6-nitto (NO2-SP) and 6-nitro-8-(pyri-dinium)-methyl (Py+-SP) derivatives into montmorillonite their photochromic behavior (the photochromic reaction of spiropyran is shown in Fig. 22) has been studied for colloidal systems (208). The effects of intCTcalation on the rate of thermal coloration and decoloration have been compared with those in other systems, such as colloidal silica, hexadecyltrimethylammonium bromide, and sodium dodecylsulfate (SDS) micelles. 

Spiropyran-modified poly(L-glutamate)s in hexafluoroisopropanol (HFP) exhibit reverse photochromism , that is, a photochromic behavior which is... 

When spiropyrans are treated with acids they are converted into spiropyran salts , which exhibit photochromic behavior differing from that of the parent spiropyran compounds. The gross mechanism proposed is illustrated in Figure 14.9. In the dark at room temperature, the compounds give colored solutions due to the presence of the 0-protonated merocyanine species III. The open form is converted by irradiation with visible light to the iV-protonated spiro form IV. As spiropyrans are fairly strong bases in the open form but very weak bases in the clos spiro form, the charged species IV can lose a proton and the neutral species I can be actually formed. 

The Effect of Tb and Sm Ions on the Photochromic Behavior of Two Spiropyrans of Benzoxazine Series in Solution... 

Following the pioneering work of Hirshberg,[421 the photochromism of spiropyrans has been extensively studied.[43] The photochromic and thermochromic behavior of this class of compounds is due to the interconversion of the closed spiropyran form to the open merocyanine form (Scheme 15). UV irradiation of the closed form 28 results in ring-opening to the zwitterionic form 29, which reverts to the closed form either thermally or on irradiation with visible light. 

---