Colour and Constitution of Anthraquinonoid Dyes. Part II–Modified PPP Calculations and Annelation Effects

The Pariser–Parr–Pople (PPP) method with the variable (3, yapproximation has been used to calculate the ^–electronic excitation energy of 1, 4–disubstituted benzo– and dibenzo–anthraquinone dyes. A good linear correlation between the observed and calculated values for the first –* excitation was found for a series of benzo AQ dyes. The effect of annelation on the colour of benzo AQ dyes was studied on the basis of the calculated results. It was concluded that 2, 3–annelation differs from 5, 6– and 6, 7–annelation because the 2, 3–benzo ring is also included in the donor moiety.