Thermal irreversibility

By changing the substituents on the ethylenic linkage and exchanging phenyl rings for heteroaromatic rings, photochromic systems that are thermally reversible are transformed into systems that are thermally irreversible but photochemically reversible. The transition between the ben2othiophene-derivative... 

Among the many unusual properties that the arborescent architecture leads to, most notable is the discovery that block copolymers with a high MW dendritic (arborescent) polyisobutylene core and poly(para-methylstyrene) end blocks can manifest themselves either as a rubber, or as a plastic, depending on their environment (Figures 7.16 and 7.17). The behavior is thermally irreversible. 

Masahiro Irie received his B.S. and M.S. degrees from Kyoto University and his Ph.D. in radiation chemistry from Osaka University. He joined Hokkaido University as a research associate in 1968 and started his research on photochemistry. In 1973 he moved to Osaka University and developed various types of photoresponsive polymers. In 1988 he was appointed Professor at Kyushu University. In the middle of the 1980 s he invented a new class of photochromic molecules - diaryl-ethenes - which undergo thermally irreversible and fatigue resistant photochromic reactions. He is currently interested in developing singlecrystalline photochromism of the diarylethene derivatives. 

It is also possible to use microcalorimetry to obtain useful information about the kinetic processes of the instability (i.e., aggregation, proteolysis) when thermal irreversibility prevails. Scan rates will often distort the onset behavior of the melting transition that can necessarily impose a shift in the Tm, as discussed further in the following text. The scan rate dependence of the Tm may then be used to determine the activation energy of the instability, provided an Arrhenius kinetic model describes the behavior. 

A class of crown spirobenzopyrans 10 exhibit thermally irreversible photochromism only in the presence of alkali-metal cations (Scheme 3). The favored complexation between 10 and lithium cation was thus revealed by the effect of lithium salt on the electronic absorption spectra of 10. Lithium complexation was also corroborated on the basis of FAB mass experiments. Before addition of Lil to 10, ion peaks for MH, [M + Na] and [M + K] were detected after the addition the signals decreased, while a peak for [M + Li]+ appeared (Figure 10) . 

Recently (08JPCA4765), new thermally irreversible photochromes 156-159 containing silicon or phosphorus atoms in the five-membered bridge have been synthesized. They have fluorescence in the closed forms, whereas their open forms do not exhibit fluorescence. Hence, they are highly promising for the nondestructive readout of optical information (07MI1). Their synthesis from 2- and 3-substituted butadienes was documented (09NJC1357). 

Compound 311 underwent a thermally irreversible photochromic reaction in solution (08JCS(CC)335). Upon alternate irradiation with UV and visible light, a hexane solution of 311 reversibly changed its color from colorless to bluish purple due to the isomerization between the open- and closed-ring isomers, 311 and 312. 

The photochromic chromophores can be classified into two categories, depending on the thermal stability of the photogenerated isomers. When photogenerated isomers are unstable and revert thermally to their initial isomer state in the dark, the chromophores are classified as T-type (thermally reversible type). Most photochromic chromophores belong to this type. The photogenerated blue color of 6-nitro-l, 3, 3 -trimethylspiro-[2H-l-benzopyran-2,2 -indoline], for example, disappears in less than half an hour even in high Tg polymer matrices.181 Such thermally unstable photochromic chromophores cannot be applied in photoswitchable molecular systems, because the switched states are unstable. For those applications, the characteristic of persistence, or in other words thermal irreversibility, is indispensable. 

Such thermally irreversible photochromic chromophores represent the other class, classified as P-type (photochemically reversible type). Although many photochromic compounds have been so far reported, P-type chromophores are very rare. Only two families, furylfulgide derivatives and diarylethene derivatives, exhibit this reactivity.19 101 The photogenerated isomers of these derivatives are thermally stable and never revert to their initial isomers even at elevated temperatures (-100 °C). The thermally stable photochromic compounds offer potential for various applications in photoswitching and memory devices. 

Since 1981, when Heller and co-workers synthesized a furylfulgide 2,141 fulgides have included representatives of thermally irreversible photochromic compounds among their number. Fulgides known until then had been thermally reversible. Three major modifications were made to the fulgide structure ... 

The great merit of thermal irreversibility is the permanent nature of the states. Therefore, fulgides have long been viewed as potential candidates for photon-mode optical recording materials. In addition, fulgides have been used as prototypes to demonstrate their potential applicability as photoswitchable functional materials. Those switch models that had appeared up until the end of 1999 are described in this chapter. 

Application of a thermally irreversible photochromic compound in an optical memory medium requires that several conditions should be satisfied. Among them, the development of a non-destructive readout method is a difficult problem. Exposure to light that might be absorbed should be avoided, because it causes destruction of memory. 

The synthesized fulgenates were thermally irreversible photochromic compounds. Their photochromic reaction is shown in Scheme 26. 

Y. Yokoyama, T. Serizawa, S. Suzuki, Y. Yokoyama, and Y. Kurita, Fulgenolides thermally irreversible photochromic lactones with large quantum yields of photoreactions, Chem. Lett., 1995, 17-18. 

I. Cabrera, A. Dittrich, and H. Ringsdorf, Thermally irreversible photochromic liquid crystal polymers, Angew. Chem. Int. Ed. Engl, 30, 76-78 (1991). 

It is worthwhile noting that the closed-ring forms 5b and 6b were found to be thermally irreversible but photochemically reversible. The result indicates that the closed-ring forms of nonsymmetric diarylethenes become thermally stable when at least one of the heterocyclic rings has a low aromatic stabilization energy. 

Diarylethenes having thiazole rings also undergo thermally irreversible photochromic reactions.8 Introduction of a trifluoromethyl group at the 4-position of the thiazole ring is effective in increasing the thermal stability of the closed-ring form. 

M. Irie and M. Mohri, Thermally irreversible photochromic systems. Reversible photocyclization of diarylethene derivatives, J. Org. Chem. 53, 803-805 (1988). 

K. Uchida, T. Ishikawa, M. Takeshita, and M. Irie, Thermally irreversible photochromic systems. Reversible photocyclization of l,2-bis(thiazolyl)perfluorocyclopentenes, Tetrahedron, 54, 6627-6638 (1998). 

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