Thermal Decoloration Reaction

A peculiar feature of the photochromic behavior of perimidinespirocyclohex-adienones and their structural analogs is an extremely slow thermal bleaching of the colored quinoneimine photoisomer lb caused by its conversion to the ring-closed form la. The effective lifetimes of the colored isomers formed upon UV irradiation of solutions of perimidinespirocyclohexadienones in nonpolar solvents at room 

Compound Octane Octane + Et3N (40 1) Ethanol Thin solid film 

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When photochromic reactions of spirobenzopyran, azobenzene, and fulgide in polymer films are compared, it is found that the critical free volume n ded fw a reaction is relatively large for spirobenzopyran, but very small for azobenzene and fulgide. The thermal decoloration reaction of spirobenzopyran in a solid film below was much slower than in solutions and was considered diffusion-controlled. Thermal cis to trans isomerization of azobenzene in polycarbonate at room temperature proceeded at a rate comparable to the rate in solutions, except for an anomaly at the initial stage tjjg most part of this reaction can be considered chemically-... 

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... 

The incorporated SP exhibited photochromism in both of the immobilized bilayer complexes with montmorillonite and PSS. Kinetic measurements of the thermal isomerization (decoloration) were carried out for the annealed film. The decoloration reaction rate is dependent on the mobility of the surroundings and, in polymer matrices, is influenced by the glass transition. It was found that the reaction rates abruptly increased near the gel-to-liquid-crystal phase-transition temperature (54°C) of the immobilized bilayer due to increased matrix mobility in this system. The film prepared with montmorillonite gives more homogeneous reaction environments for the chromophore than those with the linear polymer (PSS). This leads to drastic changes in the reaction rate at the crystal-to-liquid-crystal phase transition of the bilayer, showing the effect of the phase transition of immobilized bilayers to be more pronounced than that of the glass transition of amorphous polymer matrices. 

Figure 2 Effect of time of irradiation on the UV/vis absorption spectra of DMF solution of a mixture of 60 pM and 60 pM SP(II) (inset back ring closure reaction and the first-order plots of thermal decoloration of SP...

The mixing time is measured by the schlieren or chemical decolorization method. For short mixing times, consecutive dye reaction can be used. For viscous liquids, the decoloration or thermal methods are recommended. The RTD measurement requires a good choice of tracer. 

The closely related octaphenylmetallocenes of group 14 (Ge, Sn, and Pb), available for comparison, are markedly less thermally [decomposition at about 200°C (43)] and kinetically stable. On contact with thf decomposition is observed within minutes, which can be followed by decolorization and the appearance of a precipitate after the initial dissolution (43). However, this reaction with thf, which also prevents the synthesis of the above octaphenylmetallocenes in this solvent, is not quite understood, especially since normal stannocene can be handled in thf and shows a base-induced polymerization with formation of a Sn(IV) species only after standing for a couple of days (119). 

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. 

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