Substitution effect in electronic absorption spectra of 2- and 2,6-substituted 4-benzhydrylidene-2,5-cyclohexadien-1-ones

Abstract

Electronic absorption spectra of the seventeen fuchsone derivatives Ia-Iq were recorded in n-hexane. The symmetric Gaussian function could be fitted to absorption profiles within the region of the lowest energy π-π* transition. Bad parameters obtained in this way were utilized in calculation of the corresponding oscillator and dipole strengths. The effect of substitution on positions and intensities of all the observed bands was also investigated. The most sensitive response to substituent change is exhibited by the longest wavelength absorption band. The relation between the substituent nature and band position or oscillator strength may be quantified as P sum from i=1 to n (ρiσi) + P0. It follows from the analysis of variance that both the inductive and steric effects are operating. On the other hand, for the non-symmetrically substituted fuchsones, the band intensity characterized by the molar absorption coefficient is a linear function of the inductive effect only. There is no statistically significant contribution of the resonance effect in any of the quantities investigated. The effect of solvent was followed by the separate evaluation of the wavenumbers related to maxima of the longest wavelength band of fuchsones Ia-Iq in the following series of solvents: n-hexane, cyclohexane, isooctane, tetrahydrofurane, acetone, acetonitrile and ethanol.