Ab initio quantum-mechanical characterization of the electronic states of anthraquinone,quinizarin, and 1,4-diamino anthraquinone

The ground and excited singlet and triplet states of 9,10-anthraquinone and its 1,4-dihydroxy and 1,4-diamino derivatives are investigated by ab initio configuration interaction calculations, using a floating Gaussian basis. For anthraquinone the calculated electronic absorption spectrum is consistent with previous experimental and theoretical results. The possible mechanisms of intersystem crossing and intensity borrowing in phosphorescence are discussed in terms of first- and second-order spin-orbit and vibronic perturbations of the computed singlet and triplet states. The calculated spectra of the 1,4-disubstituted derivatives are more complex than previously suggested from low-resolution polarized absorption studies. The principal effect of 1,4-substitution by electron donating groups is shown to be a powerful conjugative effect which significantly modifies selected molecular orbitals of anthraquinone; the resultant effect on the absorption spectra is the creation of two new substituent-induced π → π* transitions, one of which corresponds to the intense visible absorption band of these systems.