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

The Pariser-Parr-Pople (PPP) method with the variable beta, gamma approximation has been used to calculate the pi-electronic excitation energy of 1,4-disubstituted benzo- and dibenzo-anthraquinone dyes (AQ). A good linear correlation between the observed and calculated values for the first pi-pi excitation was found for a series of benzoAQ dyes. The effect of annelation on the colour of benzoAQ 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