Spectra of Colored Forms

The forward reaction can be induced photochemically and the back reaction normally thermally. The products 8, 9 formed in this conversion can only be written in a zwitterionic form. This is the basic difference from spiropyrans or spiroxazines which also undergo a 67t-electrocyclic reaction, yielding, however, an electroneutral molecule. 

The assignment of the betaine bands in 9 is complicated. The long wavelength band is most probably a charge transfer (CT) transition. This has been shown to be 

In fast-bleaching betaines 9, the UV maxima were recorded using flash spectroscopy. A typical setup is described in the following section.42,43 

In a study of the colored forms 9 in various solvents, an interesting effect on the long-wavelength band is observed. Most of the DHI 7 investigated showed a negative solvatochromic effect. This is consistent with earlier reports on spiropy-rans.8 

Thermal Reactions in Solution/Structure - Fading Rate Relationships 

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Figure 6.5. Reflection spectra of colored form of fluoran 50 and 51.

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