Bianthrones, thermochromism

Figure 1.17 Thermochromic bianthrones folded A-form and twisted B-form.

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). 

In several types of compounds, excited triplet levels lie very close to the ground states, as has been mentioned already. Such thermal population of low-lying triplet levels occurs frequently in inorganic complexes as well as with organic biradicals. Thermochromism of bianthrone has been explained as due to thermal conversion to a triplet state.129... 

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-... 

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

Since the original description of thermochromism of bianthrones at the beginning of the century, this phenomenon has evoked considerable interest and has been the subject of numerous reviews, the most recent being that published by Muszkatin 1988.96... 

The thermochromism of bianthrone in solution has been shown to result from a thermal equilibrium between two distinct and interconvertible isomeric species, A- B. Species A exists at room temperature as a yellow form (kmax 380 nm) upon heating of solutions of A, a significant fraction is converted to the green B form (Xmax Si 680nm), whose enthalpy is 12.5 kJmol-1 greater than that of the A form.97... 

Evans and Fitch102 developed an electrochemical method to determine the thermodynamic equilibrium constant K. The value obtained by this technique (1.9 xlO-3 at 298 K) for bianthrone is in excellent agreement with spectro-photometric measurements.97 This method allows the estimation of the equilibrium constant of 1,1 "-disubstituted bianthrones, even if the substitution prevents any observable thermochromic behavior. 

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). 

Unfortunately we have to end this talk with a sad note. Prof. E. D. Bergmann, one of the foremost specialists in this field, passed away just before this Symposium started. In pioneering studies, in the thirties, he suggested a biradical species as responsible for the thermochromism of bianthrones [13, 14]. These studies were later continued applying LCAO methods [15] and dipole moment measurements [16]. His death is thus a heavy loss to all those working on these subjects. 

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