Spiropyrans polymer matrix effect

Polymer types and polymer matrix effects will obviously affect stability of both states. Recently, spiropyran has been attached to polystyrene Wang resin, which serves as a solid support, to minimize thermal back reaction and improve bistability (see Fig. 6.3). The resultant photochromic microbeads suspended in toluene greatly... 

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. 

For the w-spiropyranes case, the w-p-xylene merocyanine has the faster fading rate compared with the w-decyl merocyanine. A priori one would expect that the to-decyl merocyanine would have a faster fading rate, taking into account that this molecule is bigger (24.1 A) than the w-p-xylene merocyanine (20.4 A), therefore it will experience more stress in the polymer matrix, so the recovery rate back to the spiropyrane form would be faster. An additional effect is obviously involved. 

Two types of cteviation from the first-orda kinetics are noted for photo-aialthamal isomerization reactions in polymer films. The first is the normal type, in which the reaction rate is the same as or smaller than that in solutions at the initial stage and then progressively becomes smaller. Typical examples are thermal decoloration of the photocolored merocyanine form of spirobenzopyran molecularly dispersed in or chemically bound to a polymer matrix > and photoisomerization of the transazobenzene residue incorporated in polymer main chains The first interpretation for the decoloration of the merocyanine form assumed the existence of different isomers, each of which fades independently following first-order kinetics On the other hand, Kryszewski et al. proposed the kinetic matrix effect, which means that the distribution of free volume may lead to the deviation from first-order kinetics. His idea was based on the finding that deviations from first-order kinetics can be observol even in simple molecules such as azobenzene which has only one trans or cis isomeric form. The effect of free volume distribution on reactivity was further demonstrated by studies of annealed polymer films The distribution function of free volume as well as the critical free volume v were estimated for the merocyanine form of spiropyran in poly(methyl methacrylate) derivatives of azobenzene in polystyrene and azobenzene in polycarbonate The deviation from first-order kinetics was also observed in cyclizing imidization of model poly(amic acid) in a polyamide matrix... 

Figure 9.1 Effect of polymer matrix on the spectra of spiropyran. The absorption peak shifts toward lower wavelength in polar matrix. Larger blueshifts are observed for the poly(n-butyl methacrylate) (PnBMA) compared to poly(methyl methacrylate) (PMMA), and styrene-butadiene-styrene (SBS) matrices.

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