Collect. Czech. Chem. Commun. 1983, 48, 2825-2839

CNDO/S MO study of the fuchsone derivatives with sterically crowded exocyclic double bond

Lubomír Musil, Bohumír Koutek, Jiří Velek, Josef Krupička and Milan Souček

Institute of Organic Chemistry and Biochemistry, Czechoslovak Academy of Sciences, 166 10 Prague 6


The electronic absorption spectra of 2-methyl-, 3-methyl-, 2,6-dimethyl-, 3,5-dimethyl- and 2,3,5,6-tetramethylfuchsone were investigated. The interpretation was carried out by comparing the spectra with those of the suitable model compounds and by quantum chemical (CNDO/S) analysis of the chromophoric systems of 4-benzhydrylidene-, 4-benzylidene- and 4-(9-fluorenylidene)-2,5-cyclohexadien-1-one. Within this approach also the effects were investigated of torsion and distortion of the double bond on spectral parameters of ethylenic chromophore. The knowledge thus obtained allowed us to suggest a plausible mechanism of the response of fuchsone derivatives to steric crowding of the exocyclic double bond region and to explain in a reasonable way the anomalous bathochromic shift and hypochromic effect exhibited by the long wavelength UV absorption band of 3,5-dimethyl- and 2,3,5,6-tetramethylfuchsone. The suggested interpretation corresponds well also with trends of half-wave potentials of these compounds.