Photochromic reactions, solid state

Photochromism in solution and in the solid state can be the result oi intramolecular changes, e.g. tautomerism, ring opening, cis-trans isomerization, free radical formation, stereoisomenc transition, formation of dimers nd such similar reversible reactions. Some examples are given below ... 

Situations where the absorption spectra of reactants and products are not conducive to high yield solid-state reactions may occur in classic photochromic systems. In a recent study on the crystalline photochromism of l,2-bis(2,4-dimethyl-3-thienyl)perfluorocyclopentane (55, Scheme 35) by Irie et al., it was... 

This chapter relates to the photochromism of viologens and thionine dyes proceeding by electron transfer. The colored species spontaneously disappear in a liquid phase, because the rate of back electron transfer (dotted line in Figure 9.1) is rapid and is accelerated by the existence of oxygen in the system. If the compound is dispersed in a solid-state or layered system, the reversible reaction can be controlled. Since the color development via electron transfer is generally very rapid and the reverse electron transfer can also be carried out electrochemically,4 the combination of both photoreduction and electrooxidation will receive much attention for application in recording devices. 

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. 

Topics of relevance to the content of this chapter which have been reviewed during the year include photoactive [2]rotaxanes and [2]catenanes, photochemical synthesis of macrocycles, phototransformations of phthalimido amino acids, photoaddition reactions of amines with aryl alkenes and arenes, photoreactions between arenenitriles and benzylic donors, photostability of drugs, polycyclic heterocycles from aryl- and heteroaryl-2-propenoic acids, photoreactions of pyrroles, photoamination reactions in heterocyclic synthesis, switching of chirality by light, photochromic diarylethenes for molecular photoionics and solid state bimolecular photoreactions. 

Because photochromic reactions in polymer matrices deviate in general from first-order kinetics of both photochemical and thermal isomerization below the Tg (vide infra), polymers with Tgs lower than room temperature are of interest in order to perform photochromic reactions smoothly even in the solid state. In this respect, polysiloxanes17 and polyphosphazenes18 with flexible main chains are attractive. 

X-ray analysis of structural changes in photochromic salicylideneaniline crystals. Solid-state reaction induced by two-photon excitation. J. Am. Chem. Soc., 121, 5809-5810. 

Phosphorus tribromide stereochemistry, 36 Phosphorus trichloride metal complexes solvolysis, 418 stereochemistry, 36 Phosphorus trifluoride stereochemistry, 36 Phosphorus trihalides stereochemistry, 36 Phosphorus triiodidc stereochemistry, 36 Photoaquation solid state, 471 Photocalorimetry, 410 Photochemical reactions, 397 applications, 408 mechanisms, 385 solid state, 470 Photochromism, 409 Photolysis... 

Many photoreactions on phane skeletons have not been included in this chapter, such as reports related to ring construction via sulfides and esters. Several phanes have been examined as photochromic materials, but these molecules usually do not exhibit reversible photochromicity and decompose on repeated irradiation. It may be possible to carry out some asymmetric-induced formation of phanes. These studies are of value for practical and theoretical reasons. Other work has used a supramolecular catalyst for the preorganization of vinylarenes. The technique could be of considerable value. Some additional photochemical syntheses of phanes have been described for the formation of stericaUy strained systems. In order to make such highly strained phanes, solid-state reactions could play a key role. 

The Mechanism of the Photochromism and Thermochromism of 2,2, 4,4 -Hexa-Phenyl-1,1 -Biimidazoiyi in a Soiid State. Taro Hayashi Koko Maeda, and Takako Kanaji (Ochanomizu Univ., Tokyo). Bull. Chem. Soc. Japan 38 (5), 857 (1965) (Eng.). Pale yellow 2,2, 4,4, 5,5 -hexa-phenyl-l,l -biimidazolyl (I) becomes reddish purple upon irradiation or when heated in the solid state. On the basis of the E.S.R. and absorption spectrum measurements of a sample irradiated at 15°, the mechanism of photochromism of I, in the solid state, was concluded to be the radical dissociation of its N-N bond to triphenylimidazolyl radicals (II). The same mechanism can be applied to the thermochromism in the solid state. The photochromic color gradually reverted to its original pale yellow color at about 40°. The rate of recombination of II in the dark at 46 and 62.5° did not obey the rate expression of the 2nd-order reaction. 

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... 

The photochemistry of aromatic compounds is classified into the same categories adopted in the previous reviews in the series. The photoisomerization of arylalkenes, photoaddition and cycloaddition to aromatic rings, photosubstitution, photorearrangement reactions appear to have received less attention in the period considered than in preceding years (2012-2013). On the other hand, the photochromism including photoisomerization of azobenzenes and intramolecular photocyclization and cycloreversion of l,2-di(hetero)arylethenes (in particular of 1,2-dithienylethenes), and the photodimerization of arenas and aiylalkenes have been largely developed. A further noteworthy aspect is that so many photochemical reactions in solid and/or ciystalline states have been investigated. 

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