Colour and Constitution of Anthraquinonoid Dyes. Part I – Modified PPP Calculations and Substituent Effects

Calculation of the pi-electronic excitation energies of anthraquinonoid dyes has been carried out by means of the Pariser-Parr-Pople (PPP) method with 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 series of amino- and hydroxy-anthraquinone dyes. The effect of substituents on the colour of different dyes was studied on the basis of these calculated results. The intramolecular charge-transfer character of the first pi-pi transition was supported. The parameters used in this calculation can be adapted to estimate the colour of any anthraquinonoid dye