Bianthrones twisted form

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. 

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

For appropriately substituted bianthrones, two geometrical isomers are expected the E form and the Z form (Scheme 19). Dynamic NMR studies97 provided information on the pathways for E-Z isomerization and conformational inversion in 2,2 -disubs(i(u(cd bianthrones. These studies indicated that the high-est-energy transition state for these processes is the folded-twisted structure and not the n/2 twisted ethylene (Figure 10). 

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