Photochromism, bianthrone

Photochromic or phototropic dye stuffs are used as the basis of photochemical high speed memory with an erasable image. They can also be used as automatic variable density filters for example as Q-switches in high intensity lasers. For thermochromic substances colour change is observed on change of temperature (spirans, bianthrones). 

Bianthrone and Analogous Compounds. Bianthrone (VI) and a number of substituted and related compounds (e.g. xanthylidinean-throne, VII) are photochromic at low temperatures. However, Hirsh-... 

Electron paramagnetic resonance (EPR) studies on pyridine solutions of bianthrone by Wasserman (41) suggest that the thermochromic form may have one unpaired electron localized on each half of the molecule. The structure proposed by Woodward and Wasserman (42) is VIII which they consider to be identical to the colored photochromic form. Klochkov (43) has warned that free radicals may be involved in decomposition processes occuring at high temperatures in studies of the bian-... 

A three-level switching device has been demonstrated in which photochromic properties are used to control electrical properties, and vice versa. Such a system has been realized in the form of thiophene bisphenol [90, 91]. Conversion of the open (8a) to the closed (8b) form of the thiophene was achieved by absorption of 312 nm light, and revered by absorption of 600 nm light. The bisphenol oxidation occurs at +0.735 V (vs. SCE), forming the closed-ring bisquinone, compound 8c. This species has large absorptions at 400 and 534 nm. The optical properties of the quinone phenol couple have previously been used in a bianthrone-based system [87]. The bisquinone (8c) cannot be converted to the open thiophene, and locks the system in the closed form. The thiophene has also been incorporated as a component in two-level molecular switches [99, 128] and switchable molecular wires [30]. 

Bianthrones and related bianthrylidene systems undergo a reversible color change induced by light (photochromism), temperature (thermochromism), or pressure (piezochromism). 

R. Korenstein, K. A. Muszkat, and S. Sharafy-Ozeri, Photochromism and thermochromism through partial torsion about an essential double bond. Structure of the B colored isomers of bianthrones, J. Am. Chem Soc. 95, 6177-6181 (1973). 

The photochromism of bianthrone (72) is another example of a formal Gjv reaction. The structure of the colored form (73) has long been a subject of interest. The bond-twisted structure (73) was suggested after extensive NMR, UV-visible, and theoretical studies by Muszkat and his co-workers. Although the photochromism of bianthrone is formally similar to Gj photoreactions like the isomerization of trans-stilbene, the bond-twisted form in this case is a relatively stable nonclassical molecule. 

The photochromism early reported by Hirshberg and co-workers was observed in bianthrones (dianthraquinones). Scheme 2.1, and spiropyrans. Scheme 2.2. Through the years, photochromism of (a few) other families of compounds have been reported, the most relevant are described above. [2]... 

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