Chromogens based on non-benzenoid aromatic systems. Part III. Synthesis, spectra, and molecular orbital calculations in the substituted fulvene and 6-azafulvene series

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Abstract
The synthesis and electronic absorption spectra of 6-substituted fulvenes and 6-azafulvenes derived from cyclopentadiene, 1,2,3,4-tetrachlorocyclopenta-1,3-diene, and fluorene have been examined. Some new syntheses of the tetrachlorofulvenes are described. Spectral studies on twenty derivatives show that mesomeric electron-donating groups at position 6 produce a strong bathochromic shift of the long wavelength absorption band. The tetrachlorofulvenes and tetrachloroazafulvenes are appreciably more bathochromic than their fulvene or fluorene counterparts, whereas the fulvenes and fluorenes absorb at similar wavelengths. The PPP molecular orbital method, with limited configuration interaction, has been applied to these compounds, by use of a combination of conventional and empirically modified parameters. The calculated spectra are in excellent agreement with experiment. In all cases a high degree of electron transfer from the donor atom to the five-membered ring was indicated for these transitions.

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