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

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.