Colour and Constitution of 1,4-Naphthoquinonoid Dyes – Modified PPP MO Calculations and Substituent Effects

Calculation of the pi-electron excitation energies of naphthoquinonoid dyes has been carried out by means of the Pariser-Parr-Pople (PPP) method using the variable, beta,gamma-approximation. A good linear correlation between the observed and calculated values for the first pi-pi excitation was found for a new series of naphthoquinonoid dyes. The effect of substituents on the colour of different dyes was studied on the basis of these calculated results, i.e. HOMO-LUMO energy levels and pi-electron-density changes accompanying the excitation. The intramolecular charge-transfer character of the pi-pi transition was shown to be the same as occurs in some anthraquinonoid dyes