Photochromic reactions spiropyrans

The reverse reaction, the photochemical ring opening of spiropyranes (22b), takes place by absorption in the short-wave uv region of the spectrum and the merocyanine isomer (22a) is obtained. The electron transition of (22a) is in the visible spectral region, whereas (22b) is colorless. As a result, the dye solution can change from colodess to a colored solution (87,88). These photochromic reactions can be used for technical applications (89). 

Photochemomechanical systems have also been studied using gels and photochemical reactions such as photochromism of spiropyrans [20], trans-ds transition of azobenzene [21] and photo-dissociation of triphenylmethane leuco cyanide [22]. It is an attractive approach to utilize a molecular level conformational change. 

Photoinduced unimolecular reactions often have kinetics of the order of ps. One example of isomerization in ps times is shown in Figure 8.9. This is the photochromic reaction of a spiropyran. The photoinduced process takes place... 

The inclusion complexation of spiropyrans in cyclodextrins has also been explored as a means to control photochromic reactions.1591 Distinct differences in complexation of sulfonic acid-modified spiropyrans to various cyclodextrins were observed and the closed spiropyran form bound to (3-cyclodextrin was stable towards photochemical ring-opening. 

Fig. 12 Photochromic reactions of spiropyrans under acidic conditions.

Fig. 13 Reverse photochromic reactions of spiropyran-modified poly(L-lysine) (XVI) in hexafluoro-2-propanol.

T. Yuzawa and H. Takahashi, Time-resolved resonance Raman and absorption spectroscopies of reaction intermediates in the photochromism of spiropyrans, Mol. Cyst. Liq. Cryst., 246, 279-282 (1994). 

This type of photocyclization is also found in the unimolecular open form/ closed form isomerism of photochromic systems [70] as, for example, spiropyrans and spirodihydroindolazines, fulgides. The chirality in such photochromic reactions will be covered by Chap. 9 of this book an example is 47 in Scheme 9. 

As concerns photochromes in a solid matrix, a question that immediately arises is to what extent the nature of the matrix impedes the photochromic reaction. This problem has been studied in detaih but it is beyond the scope of this review. There is a general rule that states photochromic reactions are sluggish in polymer matrices compared to fluid solutions. This statement is true for some stilbene derivatives, but it is not true for azo derivatives, especially for push-pull azobenzene derivatives like DRl, for which the trans->cis quantum yield equals 0.11 in PMMA at 20°C compared to 0.24 in a liquid hydrocarbon mixture at -110°C. Photochromism of spiropyrans shows an important matrix effect as the quantum yield for the conversion between the spiropyran and the photomerocyanin is equal to 0.8 in ethyl acetate and decreases to 0.102 in PMMA at room temperature. The same decrease is observed for the back photochemical reaction efficiency 0.6 in ethyl acetate, compared with 0.02 in PMMA at room temperature. Conversely, the matrix effect is much less for furylfulgides the quantum yields are almost the same in solutions as in polymer matrices. Although most of photochromic molecules exhibit photochromism in polymers and sol-gels, few of them exhibit this property in the crystalline state, due to topochemical reasons. However, some anils and dithienylethenes are known to be photochromic in the crystalline state. 

The main focus of this chapter is on two well-studied series of photochromic compounds, spiropyrans and spirooxazines, and the comparison of their photo-reactive excited states and molecular structures. The discussion of their photodegradation takes into account the results obtained from spectrophotometric measurements together with the identification of the fragments resulting from photodegradation. Different mechanisms are proposed for the photooxidation reaction. The experimental apparatus and procedures for photodegradation studies are also described. 

Several models have been proposed to explain the properties and photochromic reaction mechanisms of spiropyrans and spirooxazines based on both frequency2,5 and time810 field spectroscopic studies. The photochromic reaction features the dissociation of the Cspiro-0 bond, producing a distribution of isomers3-15 (Scheme 2). 

As briefly outlined above, recent improvements in Raman techniques have opened up new possibilities for the study of the structure and dynamics of species involved in the photochromic reaction. Therefore, the purpose of this chapter is not to cover all aspects of vibrational spectroscopy, particularly Raman spectroscopy, but merely to focus on the new research trends in work on spiropyran and spirooxazine compounds. 

Another application of a photochromic reaction involves the reversibile size control of gold aggregates in nanoparticles by using 5-functionalized spiropyran derivatives. In the dark, the merocyanine form associates with each other, which suppresses the thermal rearrangement. Irradiation, on the contraiy, accurately tunes the dimension of the aggregates due to the different electrostatic properties of spiropyran and merocyanine (Fig. 3). In photocatalysis the effect of changing parameters is often limited, because the process of photon absorption photoexcitation of adsorbate -metal bond occur at some distance. It has recently been shown that when sub 5 nm metal particles are used, the hybridized absorbate-metal ions is excited preferentially. This results is an increased proportion of CO oxidation in H2 rich streams (Fig. 4). ... 

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. 

Another application of a photochromic reaction involves the reversible size control of gold aggregates in nanoparticles by using S-functionalized spiropyran derivatives. In the dark, the merocyanine form associates with each other, which suppresses the thermal rearrangement. Irradiation, on the contrary, accurately tunes the dimension due to the different electrostatic properties of spiropyran and merocyanine (Fig. 11.10) [27]. 

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