Diarylethenes thermally stable

Usually the change in colour in the forward direction is to longer wavelength, i.e. bathochromic, and reversibility of this change is key to the many uses of photo-chromism. In many systems, e.g. spiropyrans, spirooxazines and chromenes, the back reaction is predominantly thermally driven but in others the photochemically induced state is thermally stable and the back reaction must be driven photochemically e.g. fulgides and diarylethenes). The assistance of heat in the reversion of colour can be regarded as an example of thermochromism, but in this text the term is reserved for those systems where heat is the main cause of the colour change (see section 1.3). 

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. 

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. 

Figure 2. Thermally stable and unstable diarylethenes. The lifetimes of the closed-ring isomers of the thermally stable diarylethenes were longer than 12 h at 80 °C.

Y. Li, Q. Li, Photochemically reversible and thermally stable axially chiral diarylethene switches. Org. Lett. 14, 4362-4365 (2012)... 

Among the diarylethene family, the dimethyidihydropyrene (DHP)/cyclophanediene (CPD) system is interesting because it is a rare example of negative photochrome, where the thermally stable DHP closed form is colored. Upon irradiation with visible light, DHP is converted to the colorless, open CPD form, and the reverse reaction occurs either photochemically with UV light or thermally [116, 117]. 

The l,2-bis(5-(4-ethynylphenyl)-2-methylthiophen-3-yl)perfluorocyclopentene diarylethene derivative (1), shown in Fig 4.10, has usefiil spectroscopic properties, since it covers a wide spectral range, and is thermally stable in both isomeric forms [28]. Therefore, the photochemical behaviour as well as the fatigue resistance properties of 1 have been recently studied in detail by us [29] since its absorption spectrum extends in the visible up to about 700 nm and very simple practical procedures are required by photochromic actinometers, this system has also been proposed as a new chemical actinometer [29]. We report here, as an example, the experimental conditions and the actinometers used in this research, the main results obtained and the procedure suggested for the use of this compound as actinometer. 

Thermally irreversible and fatigue-resistant photochromic diarylethene crystals have been developed in the past decade. The colored isomers are stable in the crystals, even at 100 °C and hardly return to the initial colorless isomers in the dark The thermally... 

Diarylethenes are typical photochromic [3] molecules, in which the thermodynamically stable colorless open-ring isomer is converted to the colored closed-ring isomer upon irradiation (see, for example. Fig. 4.10), whereas the reverse reaction is either thermally or photochemicaUy activated [27, 28]. 

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