Changes in the frequency of theB _u-transition in linear conjugated molecules induced by disturbances in the terminal carbon atoms

Abstract

The theory of the local state spectrum for the Huckel model of heteropolyenes, recently proposed by the authors, is used to examine the possibility that perturbation of the terminal carbon atoms might affect the frequency of theB _u -transition in polyenes of arbitrary length. It is shown for the first time that the position of the Bu band in the spectrum can depend on the extent of the perturbation in a non-monotonic manner. The anomalous behavior of the frequency of the Bu-transition as a function of the heteropolyene parameters is explained by the presence in the π-electron spectrum of in-gap local levels. The results obtained permit the prediction of absorption band positions for the lowest dipole-allowed 0–0 electron transitions in neutral and ionic forms of heteropolyenes of the types X- (CH)_2N-X, X+ -(CH)_2N-X⁻ and X- (CH)_2N- CH₂ and hence to suggest synthetic routes for conjugated molecules and polymeric materials which are optically active in the range required.