Photochromism in polymers

With the polymer chemical industry being the largest of its kind and the enormous impact of polymers on our daily life, the incorporation of photochromic dyes as dopants in polymeric matrices or as integral components in the polymeric structure is highly desirable. In this section, we discuss how photoresponsive dithienylethenes can be integrated with polymer science by briefly describing both polymer structure and materials processing. We will include in our discussions homopolymers, random copolymers and molecular dopants and will start with the last case after a brief mention of some requirements. 

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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. 

This paper reviews experimental work done on the NLO properties of photochromes other than azo derivatives, which have been described in the preceding chapters of this book. In the first part, the molecular second-order NLO polarizabilities will be given the NLO properties of the materials will indeed depend on these values. In the second part, photoassisted poling of different photochromes in polymer matrices will be described. The photo-switching of second-order NLO properties of poled polymers or crystals will then be described in the third section. Both second and third parts will end with some applications of these optical phenomena. We will conclude with the prospectives of these materials in NLO. 

Photoinduced transformations of photochromes (spiropyrans, furan-derived fulgides, dithienylethenes) in polymers 98PAC2157. 

Upon irradiation with UV light, the new absorption band of the closed-ring appeared at 650 nm. The conversion to the closed-ring form was determined to be 92%. The photochromism in the film on a quartz glass reached to 60% conversion at a photostationary state. The quantum yield in toluene was determined to be 0.40. The quantum yield of the polymer in the film was determined to be 0.26. 

In addition, most photochromic reactions are performed in solution. Generally, the practical uses require them to undergo photochromism in solid films or even at the level of single molecules. Therefore, it is desirable to develop new photochromic dithienylethene-phthalocyanines and their analog systems such as single crystals and polymers. 

The methods of synthesis, the spectral and photochromic properties in solution, in polymer film and in vacuum-deposited thin films, and the structural determinations by X-ray diffraction are reviewed as is the electrochemical behavior of this family of switchable materials. 

G. Smets, Photochromic phenomena in the solid phase, in Advances in Polymer Science, Vol. 50, pp. 17-44, Springer-Verlag, Berlin (1983). 

Malatesta35 proposed the key intermediate product (compound 28) of the oxidative degradation of photochromic spirooxazines. These species may result from the photochromic irreversible degradation of the spirooxazines even under conditions of partial or total absence of oxidation as, for example, in polymers coated with thin films of barrier agents such as SiO 2, SiOxCy, AI2O3, and MgO. 

Y. Tateoka, T. Sagawa, and M. Kawasaki, Photochromic materials containing photochromic substance and plasticizer in polymer matrix for quick response, Jap. Pat. Appl. JP 6327,837A to Nissan Motor and Unitika (1988). 

In a polar polymer, i.e., cellulose acetate (CA) or nitrocellulose (NC) 35E, 35Z, and 36 had a relatively longer absorption maximum wavelength than in less polar matrices. In NC the of 36 shifts to 528 nm, which is also longer than in organic solvents. The role of polymer films in the quantum yields of photoreactions is not clear. In a comparison of the photochemical properties of 35 in polymer films and in solvents, it was found that the E c in polymer matrices was substantially smaller than that in the corresponding solvent with similar polarity. However, the decoloration quantum yield Oc e in a polymer film was larger than that in solvents. In conclusion, the polymer matrix properties, such as polarity, viscosity, and glass transition temperature (Tg) are quite important for photochromic reactions and applications. The coloration, E — Z and Z —> E isomerizations were suppressed in polymer matrices. 

K. Horie, K. Hirao, N. Kenmochi, and I. Mita, Photochromic reactions of spirobenzopyran, azo-benzene and fulgide at 4 K in polymer films, Makromol. Chem. Rapid Commun., 9, 267-273 (1988). 

The easiest technique to establish a polymer-photochromic molecule (PC) interaction is to dissolve the photochrome in a polymer solution from which the solvent is evaporated afterwards. DHI 7 has been incorporated by this technique into poly(methyl)- or poly( -butyl methacrylate), vinylidene chloride, acrylonitrile (Saran F), polycarbonate, and polystyrene-butadiene copolymer (Panarez). 

The study of the efficiency of photochromic transformations for a series of aryloxy-substituted anthraquinones in polymer matrices showed that the quantum yields of phototransformations in a polymer were large and somewhat dependent on the compound s structure (Table 7.15).29... 

The insertion of phenoxynaphthacenequinone in the polymeric chains led to an effective decrease in the rate of photochromic transformation.53 The phototransformation rate of this compound in polymers depended on the polymer nature as follows polysiloxane > poly(methyl methacrylate) > polystyrene. This rate was affected moderately by changing the concentration of the photochromic compounds, but it increased with the length of the chain in poly(methyl methacrylate) and... 

An analysis of the kinetic characteristics of photochromic quinones showed that the photochromic transformations of methylnaphthoquinone, alkyl- and acyloxyan-thraquinones in solution at room temperature were observed in the microsecond range. As a consequence, these compounds may be of theoretical interest in the future. The photochromic aryloxyanthraquinones are characterized by a lifetime of the photoinduced form that reaches several minutes in solution and several hours in polymer films at room temperature, which makes them acceptable for a number of applications. In this regard, of special interest are photochromic naphthacenequi-... 

A. Zelichenok, F. Buchholz, E. Fischer, J. Katner, V. Krongauz, H. Anneser, and C. Brauchle, Photochemistry and multiple holographic recording in polymers with photochromic phenoxy-naphthacenequinone side groups, J. Photochem. Photobiol. A Chem. 76, 135-151 (1993). 

As can be seen from Table 9.12, photochromism is also strongly affected by humidity. It is well known that thiazine and its derivatives undergo photoreduction smoothly in the presence of such activated surfaces as silica gel and alumina with water molecules,37 and that methylene blue is also photochemically reduced in acid solution, even with only available water as the reducing agent38 Therefore the water present in polymer films must produce a thionine-water hydration system, which accelerates the rate of the photoreduction of thionine, as well as promoting the contact of reductants by a plasticizer effect. PVA, used as the matrix, can also play the role of reductant, but its extent may be minor as compared with the added reductant. The effect of water is supported by the fact that a less hydrophilic polymer matrix such as poly(methyl methacrylate) does not exhibit photochromism even though the system contains an appreciable amount of reductants. 

Photochromic Organic Compounds in Polymer Matrices, J. C. Crano, C. N. Welch,... 

A third book, edited by C. B. McArdle3, should be added to the list, which focuses on the very important topic of the behavior of photochromic systems in polymer matrices. 

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